Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries

Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries Pengjian Zuo, Yuanheng Wang, Jiaxin Yan, Bingxing Xie, Yan Meng, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5878834/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Nov, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Prussian blue analogues (PBAs) with 3D cyanide-bridged frameworks exhibit significant potential as cathode materials for sodium-ion batteries. However, the dissolution of transition metals and structural distortion often lead to structural instability, causing serious capacity degradation during cycling. Fundamental understanding and tuning the coordination electronic structure to mitigate PBAs instability remain challenging. Herein, we address these challenges by modulating the local electronic structure surrounding high-spin metals to optimize the cyanide coordination environment, enabling a uniform electron distribution within the crystal structure. The resulting uniform electronic structure enhances the reactivity of the transition metals, which helps to achieve 95.7% of the theoretical capacity. More importantly, the regulation of electronic displacement within the cyanide coordination environment significantly improves the crystal structural stability, yielding an impressive capacity retention of 91.7% after 1000 cycles. These findings provide new insights into the coordination structural chemistry of PBAs and offer valuable guidance for the development of advanced cathode materials for sodium-ion batteries. Physical sciences/Chemistry/Electrochemistry/Batteries Physical sciences/Energy science and technology/Energy storage/Batteries Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Prussian blue analogues (PBAs) with open 3D framework structure are promising cathode materials for sodium-ion batteries (SIBs) 1 . In the coordination framework structure of PBAs, the transition metal cations (M HS ) coordinated with nitrogen and Fe cations (Fe LS ) coordinated with carbon are connected by cyanide anions, forming perovskite-type coordination polymers with a formula Na x M HS [Fe LS (CN)₆] y □ 1− y · z H₂O 2,3 . Up to now, the commercialization limitations for PBAs mainly stem from the unsatisfactory cycling performance and the discrepancy between the practical and theoretical capacity, which is mainly related to the poor stability of coordination structure and incomplete activation of metal active sites 4 . Generally, the coordination environment of transition metal cations is affected by their valence and electron configuration as well as the electronegativity of ligand elements. The activity of Fe LS is difficult to be fully activated according to the electronic configuration of Fe LS -C 5 . The electron configurations of Fe LS have filled t 2g orbitals, making it difficult to be oxidated during charging process 6 . In addition, the electron transfer in PBAs during cycling can be also affected by the cyanide electron cloud distribution 7 . The low electronegativity of C atoms and the spin state of Fe LS cations tend to form inner orbital coordination structure, enhancing the interaction of Fe LS to the lone-pair electrons in C and hindering electron transfer process along Fe LS -CN-M HS coordination frameworks 8,9 . Moreover, the degradation of the crystal structure during cycling is mainly determined by the weak interaction between transition metals and cyanide ligands 10,11 . The Fe LS -CN-M HS coordination structure tends to be disrupted during desodiation/sodiation process because of weak coordination interaction of M HS and N with low bond energy 12 . The difference of coordination bond energy between Fe LS -C and M HS -N bond is ascribed to the electron cloud distribution of cyanide ions 13 . The bond energy of inner-orbital Fe LS -C is relatively stronger than that of outer-orbital M HS -N coordination structure, and thus M HS -N bonds prefer to be broken before the Fe LS -C bonds destruction 14,15 . Moreover, some M HS cations (such as Mn 3+ and Cu 2+ ) with asymmetric 3d valance electron orbital configurations lead to the Jahn-Teller effect, triggering crystal structure degradation 16 . The electronic structure of Fe LS -C coordination bonds can be changed by adjusting the electronic distribution of M HS -N coordination bonds, thus the electron transfer energy barrier of Fe LS can be effectively reduced 17 . By selecting transition metals preferring to form stronger M HS -N bonds, the stability of Fe LS -CN-M HS coordination structure will be effectively improved 18,19 . The robust coordination structure of Fe LS -CN-M HS can tolerate the unit cell volume changes and inhibit irreversible phase transition during cycling 20,21 . More importantly, the cyanide as crucial bridge-like functions between Fe LS and M HS plays a vital role in determining the redox activity and structure stability of PBAs 22 . Although it has been proved that the π electron interaction between the cyanide anions and transition metal ions can alleviate the lattice volume change 23,24 , the impact of cyanide coordination electronic structure on cycling stability and redox activity of PBAs has not been investigated systematically. Here we aim to balance reversible capacity and cycling performance by modulating the electronic structure of cyanide coordination frameworks for PBAs. The electronic distribution of Fe LS -CN-M HS coordination structure is homogenized by optimizing the M HS ions, and the designed PBAs not only reduce the capacity loss caused by cyanide electron cloud displacement to Fe LS , but also maintain satisfied stability of Fe LS -CN-M HS coordination structure. The results of density functional theory (DFT) calculations reveal that the uniform-distributed cyanide electronic structure can activate Fe LS and M HS ions simultaneously, thus enhancing the reversible specific capacity of PBAs. In-situ FT-IR and ex-situ EXAFS are used to further confirm that the uniform cyanide electronic distribution between Fe LS and M HS in PBAs helps to maintain the stability of Fe LS -CN-M HS coordination structure during the desodiation/sodiation process. Consequently, the optimized PBA achieves the simultaneous improvement of capacity and cycling lifetime, delivering 95.7% of the theoretical capacity at 0.1 C and retaining 91.7% of the reversible capacity after 1000 cycles at 5 C. Results Material design and characterization To demonstrate the effect of high-spin metals on the electronic structure of cyanide, a series of single high-spin-metal PBAs were modeled and structurally optimized by first principles calculations. The electronic distribution maps of cubic (Fig. 1 a-e) and rhombohedral phase PBAs (Fig. 1 f-j) are obtained. The central position represents the electronic structure of Fe LS , while the four corner positions correspond to the electronic structures of high-spin Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ , respectively. The electronic structure of cyanide is located between Fe LS and M HS . It can be found that the choice of M HS and the differences in phase structure affect the electron distribution of the cyanide coordination structure. As the atomic number increases, the electron cloud of cyanide ions gradually shifts from being biased towards M HS to Fe LS , which is observed from the decreased electronic distribution intensity at the center and the increased intensity at the corner positions (Fig. 1 a-e). Furthermore, when comparing the sodium-rich rhombohedral phase and sodium-poor cubic phase of the same high-spin-site-metal PBAs, the cyanide electron displacement appears to be nearly identical (Fig. 1 f-i). Differently, in the phase transition process of Cu-PBA from cubic (Fig. 1 e) to rhombohedral (Fig. 1 j), the cyanide electrons shift towards Fe LS . Conventionally, as cyanide electrons become more concentrated towards to the transition metal ions, the charge transfer capability of active sites is adversely affected 15 , making redox reactions more challenging and consequently restricting the reversible capacity. Therefore, in order to give full play to the theoretical capacity of PBAs, it is crucial to obtain a uniform distribution of cyanide electrons which can be achieved by regulating the transition metal elements. Three PBA samples were synthesized by using an optimized coprecipitation method. In consideration of the selection number of M HS , the samples are denoted as M2-PBA, M4-PBA, and M5-PBA, respectively. The morphology of the as-prepared PBAs was characterized by SEM, exhibiting cubic morphology with particle sizes of approximately 1.5 µm ( Fig. S1 ). The comparations of high-resolution transmission electronic microscopy (TEM) accompanied by energy dispersive spectroscopy (EDS) mapping were displayed in Fig. S2-S4 . Edge dislocations are detected in the crystals from the TEM testing, which results from the lattice coupling of various PBAs with different M HS . Uniform distribution of corresponding elements is displayed by EDS mapping, revealing all the preseted M HS are present in these PBAs. Full spectroscopy and the corresponding fine spectroscopy of XPS analyses also confirm the exist of preseted transition metals in the crystals ( Fig. S5-S8 ). The g values of ~ 2.03 for three samples from the EPR tests indicates the presence of [Fe(CN)₆]⁴⁻ defect 25 ( Fig. S9 ). The amplitude difference between M4-PBA and M5-PBA in the curves is small. The higher amplitude for M2-PBA indicates more crystal defects of [Fe(CN)₆]⁴⁻ in comparison with M4-PBA and M5-PBA 26 . Thermogravimetric ( Fig. S1 0 ) and ICP-OES tests ( Table S1 ) confirm that the chemical formulas of M2-PBA, M4-PBA, and M5-PBA are Na 1.84 Mn 0.503 Fe 0.497 [Fe(CN) 6 ] 0.892 ·1.633H 2 O, Na 1.927 Mn 0.266 Fe 0.247 Co 0.248 Ni 0.239 [Fe(CN) 6 ] 0.943 ·0.862 H 2 O and Na 1.898 Mn 0.211 Fe 0.202 Co 0.199 Ni 0.198 Cu 0.190 [Fe(CN) 6 ] 0.927 ·0.675H 2 O, respectively. The contents of transition metal elements in the PBAs materials closely match the initial feeding amounts, with slight deviations likely due to the differences in the complexing capabilities between various transition metal elements and chelating agents 18 . The powder X-ray diffraction (PXRD) patterns of the three samples were analyzed through Rietveld refinement (Fig. 1 k-m). Although there are differences in elemental selection and water content for these PBAs, all samples exhibit monoclinic structure 27 . The crystallographic information for the three samples is detailed in Table S2-S4 . The insets show the corresponding charge density distribution diagrams of the (200) crystal planes based on the Rietveld refinement PXRD patterns of these PBAs. In this regard, the cyanide electrons in M2-PBA and M5-PBA respectively skew towards M HS and Fe LS with a pear shape, while M4-PBA displays a uniform cyanide electronic distribution with an ellipsoidal shape, aligning with the intended distribution of cyanide electrons according to the results of first-principles calculations (Fig. 1 a-j). Characterization of the Localized Cyanide Coordination Structure Since Fe 2+ is introduced in both high-spin sites and low-spin sites for all the three samples, various structural characterization techniques focused on the Fe element were employed to investigate the effect of the electronic structural modulation of cyanide on the coordination structure of PBAs. Fe K-edge X-ray absorption spectroscopy (XAS) was utilized to analyze the cyanide electronic environments in the three samples 28,29 . The X-ray absorption near edge structure (XANES) spectra of the three samples show minimal differences, in which M4-PBA exhibits a relatively lower near edge absorption energy, indicating a less oxidation of Fe²⁺ (Fig. 2 a). An increase in the pre-edge peak intensity of M4-PBA suggests a slight deviation from octahedral symmetry due to a higher sodium content 30 . The distinct profiles of extended X-ray absorption fine structure (EXAFS) spectra exhibit different coordination structures of central Fe atom for the three samples (Fig. 2 b), correspondingly revealing the coordination information of C and N atom in cyanide. The splitting of Fe-N peaks in all samples indicates the influence of M HS on the Fe-N coordination bonds. The higher profile intensity for both Fe-C and Fe-N coordination bonds reflects the fine structural symmetry for M4-PBA 31 . Besides, the appearance of shoulder peak at 2061cm − 1 detected by FT-IR test in M2-PBA ( Fig. S11 ) and the blue shift of Raman peak at about 2071cm − 1 representing M HS coordinated with N bonds for M5-PBA ( Fig. S12 ) provide direct evidences for their relatively lower structural symmetry in comparison with M4-PBA 32,33 . The low structure symmetry is affected by the reduced shift of cyanide electron cloud towards the either M HS or Fe 2+ , namely an increased distribution nonuniformity of cyanide electron cloud. However, a more symmetrical localized environment of cyanide electron cloud does not necessarily imply stronger cyanide coordination bonds in the crystals, which is confirmed by the electron binding energy between Fe and cyanide according to the XPS results of Fe 2p. As shown in Fig. 2 c, M4-PBA exhibits the lowest electron binding energy of Fe 2p in comparison with M2-PBA and M5-PBA. The decreased electron binding energy of Fe 2p in M4-PBA is partly due to the strength abatement of cyanide coordination bonds, which reduces the electron binding force on Fe ions. Besides, the uniform and symmetric distribution of the cyanide electron can also reduce the confinement of electrons on Fe ions. Further, the electronic environment and oxidation state of Fe ions were revealed by Mössbauer spectroscopy. According to the fitted Mössbauer spectra of the three samples (Fig. 2 d-f), the red and green doublet peaks represent the deconvolution results for high-spin-site Fe³⁺ and Fe²⁺, respectively, while the blue singlet peak corresponds to Fe LS . The pie charts in the inset reflect the amount of Fe ions with different valence in the three samples, indicating the more oxidation of high-spin Fe ions in M2-PBA. The detailed fitting parameters in Mössbauer spectra are listed in Table S5 , where IS (isomer shift) provides information related to oxidation state and coordination bonds of Fe ions, QS (quadrupole splitting) reflects the nuclear charge distribution of Fe ions at the respective sites, and Γ (half-peak breadth) indicates the degree of disorder in the local coordination environment of Fe ions. The ISs for high-spin-site Fe ions of the three samples display significant differences ( Fig. S13a ). On one hand, the oxidation content of Fe ions in the crystal lead to differences in the IS values. On the other hand, variations in the content and selection of M HS alter the electron environment around Fe ions and the corresponding coordination bond energy, further causing differences in the IS values 10,34 . Regardless of the valence of high-spin-site Fe ions, the IS values in M2-PBA are the highest (1.11 mm/s for Fe 2+ and 0.34 mm/s for Fe 3+ ), indicating that the electron density of high-spin-site Fe ions in M2-PBA is increased due to the shift of the cyanide electron cloud towards the high-spin-site Fe. In contrast, the IS values of the high-spin-site Fe 2+ and Fe 3+ in M4-PBA are 1.06 mm/s and 0.3 mm/s, respectively, in which fall in comparison with those of M2-PBA and M5-PBA (1.02 mm/s and 0.2 mm/s in M5-PBA, respectively). Thus, M4-PBA presents a moderate IS value through the modification of local cyanide coordination structure, which can better keep the even distribution of cyanide electron cloud between high-spin-site metal ions and low-spin-site Fe ions. The QS value reflects changes in the electronic environment around Fe ions in these PBAs ( Fig. S13b ). When the symmetry of the coordination environment around Fe ions decreases, the electric field gradient around the ions increases, leading to an increase in the QS values 35 . Compared to the QS values of high-spin-site Fe ions in M5-PBA (0.95 and 1.18 mm/s for Fe 2+ and Fe 3+ , respectively), the QS values of M2-PBA are significantly higher (1.26 and 1.23 mm/s for Fe 2+ and Fe 3+ , respectively). This phenomenon indicates that the high-spin-site Fe ions are influenced by the shifting of cyanide group electronic structure in M2-PBA, resulting in the symmetry reduction of the electronic environment around the high-spin-site Fe. Correspondingly, the QS value of low-spin-site Fe ions (0.29 mm/s) in M5-PBA is higher than the QS value of M2-PBA (0.17 mm/s), suggesting that the electron cloud of cyanide is more biased towards the low-spin-site Fe ions in M5-PBA. In contrast, the QS values for high-spin-site Fe 3+ and Fe 2+ in M4-PBA (0.59 mm/s and 1.19 mm/s, respectively) and the QS value for Fe LS (0.13 mm/s) are smaller, indicating that the electronic distribution of both high-spin-site and low-spin-site Fe ions are relatively symmetrical. It suggests that the cyanide group electrons are more uniformly distributed in the coordination structure of M4-PBA. Similarly, an increase of Γ values suggests an increased disorder of the coordination structure 36 ( Fig. S13c ). The lower QS and Γ values of M4-PBA indicates that the local electron symmetries of Fe ions are slightly influenced by cyanide electron cloud in comparison with the other two samples. Additionally, the introduction of other M HS somewhat changes the high symmetry of the low-spin-site Fe ions in M5-PBA via the coordination bonds of cyanide. Thus, the results of Mössbauer spectra indicates that the electron cloud of cyanide in M4-PBA well maintained symmetrical and centered between the Fe HS and Fe LS , consistent with the electronic simulation results for the (220) crystal planes in XRD patterns (insets in Fig. 1 k-m). Hereto, by designing the selections and proportions of M HS we can modulate the electronic distribution of M HS and Fe LS , consequently achieving a uniform distribution of cyanide electronic structure between M HS and Fe LS (Fig. 2 g). Electrochemical performances of PBAs Benefited from the uniform distribution of cyanide electrons in PBAs, the electrochemical performances are improved. The galvanostatic charge and discharge curves of the first five times (Fig. 3 a and Fig. S14a-b ) and the corresponding dQ/dV curves were measured at a current density of 0.1C for the three samples ( Fig. S15 ). The first discharge capacities of M2-PBA, M4-PBA and M5-PBA are 142.7, 142.4 and 109.5 mAh·g − 1 , corresponding to 83.9%, 95.7% and 80.5% of their theoretical capacities, respectively. Notably, M4-PBA displays the highest proportion of theoretical capacity, indicating that the modulation of the cyanide electronic structure is more favorable to exert the activity of both M HS and Fe LS . The galvanostatic charge/discharge curves and dQ/dV results further verify the influence of cyanide electron structure on the reversible capacity. According to the previous analysis, the cyanide electrons of M2-PBA and M5-PBA are respectively biased to M HS and Fe LS , inhibiting the redox activity of the corresponding transition metals 37 . Therefore, M2-PBA exhibits greater redox contribution to capacity in the high voltage region (over 3.5V), while M5-PBA shows greater redox contribution to capacity in the low voltage region (below 3.5V), respectively. As for M4-PBA, the dQ/dV curve shows more symmetrically redox doublet peaks, because the cyanide electrons are evenly distributed between M HS and Fe LS . The first charge/discharge and dQ/dV curves greatly differ from the subsequent curves, which is caused by the irreversible decomposition of crystal water during the first charge and discharge process 11,38 . The modified cyanide electronic structure also boosts the cycling stability by providing unobstructed ion transport channels. During the electrochemical reaction, the interaction between guest ions and the coordination framework structure not only affects ion migration, but also has an adverse impact on the stability of the frame structure, resulting in capacity decline 39 . Benefitting from the uniform distribution of cyanide electrons, M4-PBA shows the best cyclic stability (Fig. 3 b). The capacity retention of M2-PBA, M4-PBA and M5-PBA are 21.4%, 91.7% and 69.1% after 1000 cycles at the current density of 5 C, respectively. Similarly, M4-PBA also exhibits a better capacity retention comparing to M2-PBA and M5-PBA in the cyclic tests at 1 C ( Fig. S16 ). In order to further compare the interaction between Na + and coordination frameworks for the three PBAs samples, in-situ electrochemical impedance spectroscopy testing was employed to investigate the Na + transport characteristic during the electrochemical process ( Fig. S17a-c ). The individual electrochemical processes of Na + in PBAs cathodes was revealed by distribution of relaxation times (DRT), considered as a broad range of analysis method without the relatively rigid constraints of equivalent circuits 40,41 . Based on different relaxation times, the electrochemical reaction process of PBAs is divided into four regions, representing the internal electrical resistance of the battery components (τ A = 10 − 5 ~10 − 3 s), the cathode-electrolyte interface (τ B = 10 − 3 ~0.5 s), charge transfer in the electrodes (τ C = 0.5 ~ 10 s), and solid-state diffusion in the electrodes (τ D = 10 ~ 10 3 s) 42,43 . The DRT results of the three samples during the first charge-discharge cycle are shown in Fig. 3 c-e and Fig. S18a-c . The differences in the internal resistance of the battery devices and cathode electrode interface (CEI) resistance are minimal for the three samples. However, there are significant differences in the charge transfer resistance in cathodes and solid-state diffusion resistance. Compared to M2-PBA and M5-PBA, M4-PBA especially exhibits smaller values of solid-state diffusion resistance in the high voltage region (Fig. 3 c-e), indicating a lower Na + migration electrochemical impedance in the M4-PBA lattice. It suggests that Na + migrates more rapidly within the M4-PBA lattice, resulting in a lower polarization and a smaller coordination structural decay during the galvanostatic charge and discharge process. The Na + diffusion ability in the lattice of the three samples with the evolution of Na + diffusion coefficient during charging and discharging process were revealed by galvanostatic intermittent titration technique (GITT), which also reflects the advantageous effect of the modified cyanide electronic distribution in M4-PBA ( Fig. S19 ). During the charging process, the diffusion coefficients of the three samples show a similar trend with voltage change ( Fig. S19a ). The diffusion coefficients significantly decreases in the voltage range of 3.2 to 3.6 V, due to Jahn-Teller effect of Mn 3+ and phase transition in these PBAs 8 . Compared with M4-PBA and M5-PBA, the Na + diffusion coefficient of M2-PBA decreases obviously, suggesting the adverse influence of Jahn-Teller effect and more serious phase transition. During the discharge process ( Fig. S19b ), the Na + diffusion coefficients of M2-PBA and M5-PBA greatly vary with a sharp drop in the range from 3.0 to 2.8 V, while the Na + diffusion coefficients of M4-PBA are well maintained at about 10 − 10 cm 2 ·s − 1 . The uniform distribution of cyanide electrons in M4-PBA can ensure the maintenance of a steady Na + diffusion rate during the discharge process ( Fig. S19c ). Moreover, CV tests at different sweep speeds were also conducted for the three samples with the fitting line ( Fig. S20 ), which was obtained according to linear fitting results and supporting equations ( Equation S1 and Equation S2 ) 19,44 . When the slope value is close to 1, the diffusion of Na + in the crystal is mainly controlled by the capacitive behavior, but when the slope value is closer to 0.5, the migration rate of Na + in the lattice is mainly affected by the diffusion property 7,45 . That is, the higher the slope, the faster the Na + migration through the lattice. The slope values ( b ) of M2-PBA, M4-PBA and M5-PBA during the oxidation process are 0.92, 0.96 and 0.84, respectively, with the corresponding slope values of 0.68, 0.89 and 0.75 during the reduction process. Therefore, M4-PBA with the modified cyanide electronic structure exhibits the highest capacitance contribution in the charging and discharging process, which helps to ensure fast Na + mobility channels. As a result, the rate performance of M4-PBA is significantly improved in comparison with that of M2-PBA and M5-PBA ( Fig. S21 ). The capacity of 85.1 mAh·g − 1 in M4-PBA is emitted at high rate of 20 C (1 C = 148.75 mA·g − 1 ), which is superior to M2-PBA (44.36 mAh·g − 1 ) and M5-PBA (25.81 mAh·g − 1 ). The full cell performances using these PBAs as cathode materials with presodiation of hard carbon as anode material are presented in Fig. S22a . Following the same trend observed in the half-cell electrochemical performance, M4-PBA demonstrates the highest capacity and the best cycling stability in comparison with M2-PBA and M5-PBA (Fig. 3 f-g). Notably, M4-PBA achieves a discharge energy density of 458 Wh·kg − 1 (based on the cathode mass calculation) within the voltage range of 4.0 ~ 1.8 V ( Fig. S22b ). The comparation of electrochemical performance with other research results are list in Table S6 . The excellent electrochemical performance of M4-PBA indicates that the uniform distribution of cyanide electrons for Fe LS -CN-M HS framework not only enables M4-PBA to fully exert its theoretical capacity, but also maintains the localized coordination structural stability in the charge and discharge process. Structural Evolution of PBAs during Desodiation/Sodiation Process In-situ FT-IR and ex-situ XAS testing were employed to investigate the evolution of the Fe LS -CN-M HS electronic structure during the charge and discharge processes 46 . The in-situ FT-IR results of the cyanide in M4-PBA are shown in Fig. 4 a and Fig. S23 . During charging process, the absorption peaks of cyanide near 2075 cm − 1 (denoted as peak 1) gradually show a red-shift to approximately 2050 cm − 1 , which indicates the electron loss process of M HS . With further charging, the single absorption peak of cyanide is split into a doublet contained the newly emerged infrared absorption peak near 2150 cm − 1 , which represents a change of cyanide coordination environment caused by the oxidation of Fe LS (denoted as peak 2). Although some nonlinear changes are observed in peak 1 for M4-PBA during the first charging process, better reversibility is maintained in subsequent cycling (Fig. 4 a). In contrast, significant mutants are observed in peak 1 for both M2-PBA and M5-PBA ( Fig. S24 a-b ). The change of absorption peak 1 is related to the electron interaction of M HS and N. The poor stability of cyanide coordination electronic structure in M2-PBA and M5-PBA is mainly associated with the Jahn-Teller effect of Mn 3+ and Cu 2 + 47 . The negative impact of the Jahn-Teller effect is mitigated due to the modification of cyanide electronic structure in M4-PBA, thus maintaining high cycling stability. The area ratios of peaks 1 and 2 were calculated to reflect the capacity utilization of various active sites ( Fig. S25 ). A higher area ratio indicates a relatively lower redox activity of Fe LS . For M2-PBA, the area ratio at the end of charging is relatively large, which is related to the cyanide electron shifting to M HS . A similar trend is observed for M5-PBA, and the area ratio at the end of charging is relatively small, corresponding to the electron clouds of cyanide skewing towards the Fe LS . However, a linearly varying area ratio throughout cycling is shown in M4-PBA, indicating a stable Na + insertion and extraction process due to the uniform electronic distribution of Fe LS -CN-M HS coordination structure. The calculated results of M4-PBA in the area ratios of peaks 1 and 2 further highlight the relationship between the regulation of the cyanide coordination electronic structure and the full utilization of reversible capacity. Furthermore, when comparing the changes in area ratios of peaks 1 and 2 between the first two cycling processes, M4-PBA shows a more consistent trend, suggesting a better stability of the Fe LS -CN-M HS coordination structure. The dissolution of transition metal elements is one of the primary factors leading to the capacity decay of PBAs 20 , and maintaining the stability of the cyanide coordination electronic structure is beneficial to reduce the capacity loss. The ICP-OES results of the three samples at different cycles show the dissolution of transition metals in the electrolyte ( Fig. S26 a-d ). According to the previous research on Mn-based PBAs, Mn 3+ is considered as the primary cause of electrode material failure due to the Jahn-Teller effect 18 . However, contrary to common understanding, the ICP-OES test results of the three samples reveal that the concentration of dissolved Fe ions is more than ten times that of the dissolved Mn ions during electrochemical process. Combined with the decline in the reversible capacity of the electrode materials (Fig. 3 b), it can be deduced that the dissolution of Fe ions is one of the main factors contributing to the degradation of reversible specific capacity. Ex-situ Fe K-edge XANES and EXAFS testing are further conducted to investigate the changes in the cyanide coordination structure during the Na + insertion and extraction process (Fig. 4 b-c). The XANES results suggest that all three samples maintain high reversibility in the redox behavior of Fe ions. The XANES spectra of Fe element shift to a higher energy area during the Na + extraction process, indicating the oxidation of Fe to higher valence state. During the Na + insertion process, the profiles of Fe finally reverse back to their original positions, manifesting that the valence states of Fe in the PBAs recover to their original states. The shift of XANES profiles after the first cycle for M2-PBA is more pronounced than that of M4-PBA (Fig. 4 b) and M5-PBA ( Fig. S27b ), indicating that the Fe LS -CN-M HS coordination structure is taken apart slightly in M2-PBA ( Fig. S27a ). As to the EXAFS result of M2-PBA, the profiles of Fe-C and Fe-N coordination bonds positively shift at SOC-2, and then negatively shift during the discharge process ( Fig. S28a ). However, both the peak intensity and position undergo significant changes, demonstrating the poor stability of the Fe LS -CN-M HS coordination structure. In the case of M5-PBA, although the EXAFS profile of the Fe-N coordination bond is highly reversible, the shift of the cyanide electron clouds towards to C reduces the stability of Fe LS -C bonds ( Fig. S28b ). In contrast, the stable cyanide coordination electronic structure in M4-PBA is verified by the highly consistent EXAFS profiles of the Fe-C and Fe-N coordination bonds at different sates of charge, indicating the good coordination structural reversibility during cycling (Fig. 4 c). The in-situ XRD patterns of M4-PBA for the first two cycles were measured at a current density of 20 mAh·g − 1 ( Fig. S29 ). The evolution of the characteristic peaks at about 17, 24 and 34° and the corresponding galvanostatic charge/discharge curves of M4-PBA are exhibited in Fig. 4 d. The phase transition from monoclinic to cubic occurs for M4-PBA during the first Na + extraction process, and the material undergoes a phase transition from cubic to rhombohedral during first Na + insertion process. In the second charging process, a reversible phase transition occurs between rhombohedral and cubic phase. A schematic diagram of phase transition processed for M4-PBA during the first two cycles is presented in Fig. 4 e. The reversible structure transition confirmed by in-situ XRD results ensures the excellent electrochemical performance for M4-PBA. A similar phase transition process of M2-PBA is occurred in the ex-situ XRD results for the first two Na + extraction/insertion process ( Fig. S30b-d ). However, there is a significant fade in the intensity of the M2-PBA diffraction peaks in comparison with M4-PBA, indicating that M2-PBA undergoes a structural degradation in the second cycle. The irreversible coordination structural change is consistent with previous EXAFS results ( Fig. S28 ). Different from the PXRD result of monoclinic M5-PBA powder, the pristine electrode of M5-PBA is determined as rhombohedral phase, which is resulted from the loss of some crystal water during the electrode preparation process 48,49 . With low content of crystal water, M5-PBA maintains the rhombohedral-cubic-rhombohedral phase transition throughout the initial two charge/discharge cycles ( Fig. S31b-d ). Intensity fade of Bragg diffraction peaks in M5-PBA suggests unstable structural evolution during electrochemical process. The differences in the irreversible structural evolution among the three samples are attributed to the loss of crystallization water during the first Na + extraction/insertion process, coinciding with the variation trend of initial galvanostatic charge/discharge curves (Fig. 3 a and Fig. S14a-b ). Meanwhile, the Fe LS -CN-M HS coordination electronic structure in PBAs play an important role on the reversibility of the crystal structure changes during cycling. The well-distributed cyanide coordination electronic structure not only ensures stable and reversible structural phase transitions, but also contributes to the enhanced stability of the crystal structure of M4-PBA during the electrochemical reaction process. DFT calculations To investigate the localized electronic structure of the three PBA samples, DFT calculations were employed. According to the partial density of state (PDOS) of PBAs, M HS (such as Fe 2+ , Mn 2+ , and Co 2+ ) tends to be oxidized at low potentials, while Fe LS is oxidized at high potentials (Fig. 5 a-c). The PDOS of Fe LS and M HS in M4-PBA is closer to the Fermi level compared to M2-PBA and M5-PBA, and thus both Fe LS and M HS coordinated with cyanide can be easily activated in M4-PBA, corresponding to higher practical discharge specific capacity. Furthermore, the PDOS of N in M2-PBA shows the highest overlap with the PDOS of M HS , while the PDOS of C in M5-PBA exhibits the highest overlap with the PDOS of Fe LS50 . This indicates that M2-PBA possessed stronger coordination electronic interaction of N and M HS , while stronger coordination electronic interaction between C and Fe LS are also exhibited in M5-PBA. In contrast, M4-PBA has moderate coordination electronic interaction, which not only enhances the stability of Fe LS -CN-M HS chemical bonds but also provides a more uniform distribution of coordination electrons. The computational models of the electron clouds for the three samples and their corresponding cross-sectional charge density distribution diagrams are shown in Fig. 5 d-f. According to the cross-sectional charge density distribution diagrams, M4-PBA exhibits lower charge density at both M HS and Fe LS compared to M2-PBA and M5-PBA, which is consistent with the XRD refinement results (Fig. 1 k-m). Thus, the regulation of cyanide coordination electronic structure effectively reduces the charge density surrounding the transition metal elements of M4-PBA, thereby fully activating the transition metal ions, and improving the practical reversible capacity. The evenly distributed cyanide coordination electronic structure is achieved by regulating the composition of M HS in PBAs (Fig. 5 g). Moreover, the corresponding coordination electron interaction of transition metals and cyanide can be optimized to enhance the stability of Fe LS -CN-M HS frameworks. Consequently, the optimized cyanide coordination electronic structure ultimately balances the reversible capacity and cycling stability of M4-PBAs. Discussion According to the valence bond theory and the DFT calculations results of single high-spin-site-metal PBA (Fig. 1 a-j), the coordination electronic structure of PBAs is affected by the valence electron distribution of transition metals and the electronegativity of ligand elements. Although it has been proved that the stability of Fe LS -CN-M HS coordination frameworks can be improved by selecting transition metal cations with stronger π electron interaction to cyanide anions, the electrochemical performance enhancement mechanism of cyanide coordination electronic structure on cycling stability and redox activity of PBAs remains unclear. The uniform-distributed coordination electronic structure was accomplished for PBAs to achieve both high specific capacity and long cycling life. The optimized Fe LS -CN-M HS electronic structure of M4-PBA is confirmed by the results of Rietveld refinement PXRD patterns and Mössbauer spectroscopy test. Further, linear variation of FT-IR absorption peak area ratios is beneficial to sufficiently activate the theoretical capacity and reduce the Na + migration energy barrier for M4-PBA. As to the cycling performance, the highly reversable structure change is reflected by ex-situ XAS and in-situ XRD, suggesting that the uniform distribution of Fe LS -CN-M HS coordination electrons alleviate the irreversible structural change. The M4-PBA with the optimized coordination electronic structure can release a high capacity of 142.4 mAh·g⁻¹ at 0.1 C and retains 91.7% of its reversible capacity after 1000 cycles at 5 C. In addition, the excellent rate capability of M4-PBA is also demonstrated with a reversible discharge capacity of 85.1 mAh·g − 1 at a high current density of 20 C. This work will provide some reference for studying the structure evolution and failure mechanism of PBAs by homogenizing the electron distribution of coordination structure. Methods Materials The main chemical reagents included Na 4 Fe(CN) 6 ·10H 2 O (AR, ≥ 99%), PVP (K30, ~ 40,000), NaCl (AR, 99.5%), ascorbic acid (ACS, ≥ 99%), MnSO 4 ·H 2 O (AR, 99.0%), FeSO 4 ·7H 2 O (AR, ≥ 99%), CoSO 4 ·7H 2 O (AR, ≥ 99%), NiSO 4 ·6H 2 O (AR, 99.0%) and CuSO 4 ·5H 2 O (AR, 99.0%). All the solid reagents were purchased from Shanghai Aladdin without further purification. PBAs Preparation 5 mmol equimolar amount of MnSO 4 ·H 2 O, FeSO 4 ·7H 2 O, CoSO 4 ·7H 2 O, and NiSO 4 ·6H 2 O in 0.8 mol·L − 1 sodium citrate solution were dissolved to obtain 100 ml solution A. Solution B was prepared by dissolving Na 4 Fe(CN) 6 ·10H 2 O with molar concentration of 0.2 mol·L − 1 in 100 ml deionized water after removal of oxygen. 5 g PVP (K30), 0.4 mol NaCl, and 0.2 g ascorbic acid were dissolved in deoxygenated deionized water to form 200 ml transparent solution C. A white emulsion was evolved by dropwise pumping solution A and B into solution C with a rate of 0.1 ml·min − 1 . After the solutions were fully added, the mixed emulsion was left to stand and aged for additional 12 h. The light green precipitate was centrifuged and transferred into vacuum oven at 120°C for 10 h drying to obtain the prepared Na 2 Mn 0.25 Fe 0.25 Co 0.25 Ni 0.25 [Fe(CN) 6 ] with four M HS (Mn, Fe, Co and Ni), defined as M4-PBA. The designed Na 2 Mn 0.5 Fe 0.5 [Fe(CN) 6 ] with two M HS (Mn, and Fe) was defined as M2-PBA, and Na 2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 Cu 0.2 [Fe(CN) 6 ] with five M HS (Mn, Fe, Co, Ni and Cu) was marked as M5-PBA. A similar synthesis method was used to replace the transition metal salt in the synthesis of M4-PBA with the corresponding equimolecular amount of hydrated transition metal sulfate, with a total molar amount of 20 mmol, according to the chemical formulas of M2-PBA and M5-PBA. Electrochemical measurements The cathode slurry was mixed by active cathode materials, Ketjen black and polyvinylidene fluoride (PVDF) binder (5 wt% in N-methyl pyrrolidone) in a mass ratio of 7:2:1. It was evenly coated on aluminum foil with a thickness of 130 µm and then transferred to a vacuum drying oven at 120 ℃ for overnight. After being cut into a circle with a diameter of 14 mm, it was pressed under 10 MPa for 5 min. The active material loading of each slice was between 1.5 and 2.0 mg. Subsequently, a 2025 button battery was assembled in a glove box with water and oxygen concentrations below 0.01ppm. Sodium metal tablet with a diameter of 15.6 mm was used as counter electrode and reference electrode and 1 mol·L − 1 NaClO 4 was dissolved in the mixture solution of EC, PC and FEC as electrolyte (the volume ratio of EC:PC:FEC was 47.5:47.5:5). The electrochemical performance data of galvanostatic charging and discharging curves were all measured by the Neware battery test system in the voltage range of 2.0-4.1 V. Due to the differences in the theoretical capacity of the three samples, the cells were tested according to the current density of 170 mA·g − 1 ,148.75 mA·g − 1 and 136 mA·g − 1 at 1 C rate, respectively. For the cycling performance test, the three samples were first activated five times at 0.2 C and subsequently subjected to subsequent cycles at different rates. In the rate performance test, the three samples were cycled for 5 times each at the rate of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 C in sequence, followed by 60 cycles at 1 C. GITT testing was conducted after five cycles of the fresh cell at current density of 0.1 C between 2.0 and 4.1 V, in which the cell was alternately charged for 10 min followed by 60 min resting, then discharged in the same way. The cyclic voltammetry (CV) curves and in-situ transient electrochemical impedance spectroscopy (in-situ EIS) were measured by Shanghai Chenhua CHI 760e electrochemical workstation. The scanning speed of CV curves were 0.2, 0.4, 0.6, 0.8 and 1.0 mV·s − 1 , respectively. The in-situ EIS were tested from the open circuit voltages of each sample with the following voltage interval of 0.3V from 2.9V until the end of the first galvanostatic charging and discharging cycle. The current density of galvanostatic charging and discharging process of in-situ EIS testing were 0.1C for each sample. Materials characterization The crystalline structures of the prepared samples were analyzed using laboratory powder X-ray diffraction (PXRD) with a Cu Kα source on an X’Pert3 MRD diffractometer, and in-situ XRD tests were conducted using a Bruker AXS D8 Focus instrument. The microtopography of PBAs were examined via field emission scanning electron microscopy (FE-SEM, VEGA3 TESCAN), while scanning transmission electron microscopy (STEM, JEM-ARM 200F), equipped with energy dispersive spectroscopy (EDS), provided insights of elemental distribution of the as-prepared samples. High-resolution transmission electron microscopy (HR-TEM) measurements were conducted with an FEI Tecnai G2 F30. Surface composition analysis was performed using X-ray photoelectron spectroscopy (XPS, EscaLab 250xi), and electron paramagnetic resonance (EPR) spectra were obtained using a Bruker A300-10/12 spectrometer. Thermogravimetric analysis (TGA) measurements were taken from 25 to 400°C at a heating rate of 5°C min − 1 in nitrogen. FT-IR spectra, both powder and in-situ, were recorded using Bruker FT-IR spectrometers (UK) and Nicolet 6700 FT-IR spectrometers (Thermo Scientific, USA). Raman spectroscopy was performed using a Renishaw inVia system with a 532 nm laser. The 57 Fe Mössbauer spectra were collected using a constant acceleration Halder-type spectrometer in transmission geometry, with a room-temperature 57 Co source embedded in a Rh matrix. The concentrations of sodium and transition metals in the samples and cycled electrolyte were determined using inductively coupled plasma (ICP) analysis (OPTIMA 8000DV Optical Emission). X-ray absorption spectroscopy (XAS) measurements, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were carried out at the BL16U1 beamline of the Shanghai Synchrotron Radiation Facility. Theoretical calculations Theoretical calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab-initio Simulation Package (VASP). The projector augmented wave (PAW) method was employed alongside the Kohn-Sham equations, utilizing the spin-polarized generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional to analyze the electronic interactions between valence electrons and core ions, as well as the exchange-correlation effects. To account for electronic correlation effects of transition metal 3d electrons, Hubbard U corrections were applied for Mn (5.0 eV), Fe (4.3 eV), Co (5.7 eV), Ni (6.0 eV), and Cu (4.0 eV). In the DFT calculations, the kinetic energy cutoff for the electron wave functions was set to 520 eV. Geometry optimization was conducted using the conjugate gradient method, with convergence criteria of 10 − 5 eV for energy and 0.02 eV Å −1 for forces. The Brillouin zone was sampled using a 3×3×3 Monkhorst − Pack k-point grid. Visualization of the electrolyte structures was performed using VESTA. Declarations Data availability The data pertinent to this thesis and its supplementary materials that underpin the study’s findings can be obtained from the corresponding author upon a reasonable request. Author contributions Y. W. and B. X. proposed design concept and complete PBAs material preparation. J. Y. conducted DFT computations. L. W. performed the X-ray absorption spectroscopy test. B. X., L. W., and P. Z. supervised this work and revised the paper. Y. W. and Y. M. carried out the assembly of the full cells and completed the electrochemical performance test. All authors made contribution to the experimental testing, data analysis and discussion. Acknowledgements The authors would like to express their appreciation to the Natural Science Foundation of Heilongjiang Province for Distinguished Young Scholars (JQ2024B001), the National Natural Science Foundation of China (No. 52272241), the Harbin Science and Technology Innovation Talent Project (2023CXRCGD036), Fundamental Research Funds for the Central Universities (No. HIT.DZJJ.2023052), the Key Research and Development Program of Heilongjiang Province (No. GA21A102), Jiangsu Provincial Double-Innovation Doctor Program (No. JSSCBS20220308), the Zhejiang Provincial Natural Science Foundation of China under Grant No. LR24E020001 and the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (No. 2023R01007). We also thank the BL16U beamline scientists of Shanghai Synchrotron Radiation Facility for their assistance with the X-ray absorption spectroscopy experiments. References Wang, L. et al. 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Supplementary Files 03SupplementaryInfo.docx Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries Cite Share Download PDF Status: Published Journal Publication published 18 Nov, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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1","display":"","copyAsset":false,"role":"figure","size":1088316,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDesign of cyanide localized structure and crystal structure characterization. \u003c/strong\u003eCross-sectional charge density distribution diagrams of cubic phase PBAs of various M\u003csup\u003eHS\u003c/sup\u003e calculated via DFT: \u003cstrong\u003ea\u003c/strong\u003e Mn-PBA; \u003cstrong\u003eb\u003c/strong\u003e Fe-PBA; \u003cstrong\u003ec\u003c/strong\u003e Co-PBA; \u003cstrong\u003ed\u003c/strong\u003e Ni-PBA; \u003cstrong\u003ee\u003c/strong\u003e Cu-PBA. Cross-sectional charge density distribution diagrams of rhombohedral phase PBAs: \u003cstrong\u003ef\u003c/strong\u003e Mn-PBA; \u003cstrong\u003eg\u003c/strong\u003e Fe-PBA; \u003cstrong\u003eh\u003c/strong\u003e Co-PBA; \u003cstrong\u003ei\u003c/strong\u003e Ni-PBA; \u003cstrong\u003ej\u003c/strong\u003e Cu-PBA. Rietveld refinement PXRD patterns with the schematic of the cross-sectional electronic structure shown in the inset: \u003cstrong\u003ek\u003c/strong\u003e M2-PBA; \u003cstrong\u003el\u003c/strong\u003e M4-PBA; \u003cstrong\u003em\u003c/strong\u003e M5-PBA.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/50e7f8a0f35b225f5fb24d8f.png"},{"id":75412087,"identity":"97c3c0b2-5b1f-432e-bc12-635aa6ebddd6","added_by":"auto","created_at":"2025-02-04 09:16:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3085694,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCharacterization of cyanide coordination structures.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e EXAFS. \u003cstrong\u003eb\u003c/strong\u003e XANES. \u003cstrong\u003ec\u003c/strong\u003e XPS. Mössbauer spectra fitting results with the proportion of Fe ions in different coordination states shown in the inset: \u003cstrong\u003ed\u003c/strong\u003e M2-PBA; \u003cstrong\u003ee\u003c/strong\u003e M4-PBA and \u003cstrong\u003ef\u003c/strong\u003eM5-PBA; \u003cstrong\u003eg\u003c/strong\u003e diagrammatic drawing of cyanide coordination electronic structure.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/42af1cf71ee8535e8aae7388.png"},{"id":75413154,"identity":"28f803ad-b980-4dd5-9340-c210823eabbe","added_by":"auto","created_at":"2025-02-04 09:24:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4183155,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical characterization of PBAs with modified cyanide electronic structures.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e the first five times galvanostatic charge and discharge curves of M4-PBA, \u003cstrong\u003eb\u003c/strong\u003e comparation of cycling performance. DRT results of \u003cstrong\u003ec\u003c/strong\u003e M2-PBA, \u003cstrong\u003ed\u003c/strong\u003e M4-PBA and \u003cstrong\u003ee\u003c/strong\u003eM5-PBA. Full cell electrochemical performance comparation of \u003cstrong\u003ef\u003c/strong\u003e galvanostatic charge and discharge curves and \u003cstrong\u003eg\u003c/strong\u003e cycling performance of 1 C.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/f58841ef60c7fb2311eeb5ec.png"},{"id":75412098,"identity":"477a2b9a-7b42-4185-af20-1712d0d685be","added_by":"auto","created_at":"2025-02-04 09:16:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5000660,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural evolution of M4-PBA.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e in-situ FT-IR for the first two cycles, ex-situ XAS of different SOCs in the initial cycle:\u003cstrong\u003e b\u003c/strong\u003e XANES and \u003cstrong\u003ec\u003c/strong\u003e EXAFS. (The final state of charge for the first discharge cycle was defined as SOC-1, while the charging and discharging final states of charge for the second cycle were defined as SOC-2 and SOC-3, respectively.) \u003cstrong\u003ed\u003c/strong\u003e in-situ XRD patterns of M4-PBA\u003cstrong\u003e \u003c/strong\u003eand the corresponding \u003cstrong\u003eg\u003c/strong\u003e galvanostatic charge and discharge curves. \u003cstrong\u003ee\u003c/strong\u003e schematic illustration of the phase transition mechanism.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/74a475ac434e59a56c682a34.png"},{"id":75412097,"identity":"7d3a4805-d9ba-471d-a279-8acc62c428d4","added_by":"auto","created_at":"2025-02-04 09:16:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":4549843,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDFT calculations. \u003c/strong\u003ePDOS profiles of \u003cstrong\u003ea\u003c/strong\u003e M2-PBA, \u003cstrong\u003eb\u003c/strong\u003e M4-PBA and \u003cstrong\u003ec\u003c/strong\u003e M5-PBA. Structure model of electronic distribution: \u003cstrong\u003ed\u003c/strong\u003e M2-PBA, \u003cstrong\u003ee\u003c/strong\u003e M4-PBA and \u003cstrong\u003ef\u003c/strong\u003eM5-PBA. \u003cstrong\u003eg\u003c/strong\u003e Schematic diagram of the enhanced electrochemical performance by cyanide electronic structure modification\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/b0a29648a34f129d1dadb50f.png"},{"id":96262874,"identity":"8f080eeb-2469-46d5-ae59-54fffd9e00d8","added_by":"auto","created_at":"2025-11-19 08:12:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":19148741,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/0da74130-bb87-42a6-872e-ddca173edd49.pdf"},{"id":75412093,"identity":"e9fccf80-759f-4a14-9944-ba6888657c73","added_by":"auto","created_at":"2025-02-04 09:16:29","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":31289517,"visible":true,"origin":"","legend":"Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries","description":"","filename":"03SupplementaryInfo.docx","url":"https://assets-eu.researchsquare.com/files/rs-5878834/v1/883f99598abc8ba38e8299a1.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePrussian blue analogues (PBAs) with open 3D framework structure are promising cathode materials for sodium-ion batteries (SIBs)\u003csup\u003e1\u003c/sup\u003e. In the coordination framework structure of PBAs, the transition metal cations (M\u003csup\u003eHS\u003c/sup\u003e) coordinated with nitrogen and Fe cations (Fe\u003csup\u003eLS\u003c/sup\u003e) coordinated with carbon are connected by cyanide anions, forming perovskite-type coordination polymers with a formula Na\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eM\u003csup\u003eHS\u003c/sup\u003e[Fe\u003csup\u003eLS\u003c/sup\u003e(CN)₆]\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e□\u003csub\u003e1\u0026minus;\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e\u0026middot;\u003cem\u003ez\u003c/em\u003eH₂O\u003csup\u003e2,3\u003c/sup\u003e. Up to now, the commercialization limitations for PBAs mainly stem from the unsatisfactory cycling performance and the discrepancy between the practical and theoretical capacity, which is mainly related to the poor stability of coordination structure and incomplete activation of metal active sites\u003csup\u003e4\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eGenerally, the coordination environment of transition metal cations is affected by their valence and electron configuration as well as the electronegativity of ligand elements. The activity of Fe\u003csup\u003eLS\u003c/sup\u003e is difficult to be fully activated according to the electronic configuration of Fe\u003csup\u003eLS\u003c/sup\u003e-C\u003csup\u003e5\u003c/sup\u003e. The electron configurations of Fe\u003csup\u003eLS\u003c/sup\u003e have filled t\u003csub\u003e2g\u003c/sub\u003e orbitals, making it difficult to be oxidated during charging process\u003csup\u003e6\u003c/sup\u003e. In addition, the electron transfer in PBAs during cycling can be also affected by the cyanide electron cloud distribution\u003csup\u003e7\u003c/sup\u003e. The low electronegativity of C atoms and the spin state of Fe\u003csup\u003eLS\u003c/sup\u003e cations tend to form inner orbital coordination structure, enhancing the interaction of Fe\u003csup\u003eLS\u003c/sup\u003e to the lone-pair electrons in C and hindering electron transfer process along Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination frameworks\u003csup\u003e8,9\u003c/sup\u003e. Moreover, the degradation of the crystal structure during cycling is mainly determined by the weak interaction between transition metals and cyanide ligands\u003csup\u003e10,11\u003c/sup\u003e. The Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure tends to be disrupted during desodiation/sodiation process because of weak coordination interaction of M\u003csup\u003eHS\u003c/sup\u003e and N with low bond energy\u003csup\u003e12\u003c/sup\u003e. The difference of coordination bond energy between Fe\u003csup\u003eLS\u003c/sup\u003e-C and M\u003csup\u003eHS\u003c/sup\u003e-N bond is ascribed to the electron cloud distribution of cyanide ions\u003csup\u003e13\u003c/sup\u003e. The bond energy of inner-orbital Fe\u003csup\u003eLS\u003c/sup\u003e-C is relatively stronger than that of outer-orbital M\u003csup\u003eHS\u003c/sup\u003e-N coordination structure, and thus M\u003csup\u003eHS\u003c/sup\u003e-N bonds prefer to be broken before the Fe\u003csup\u003eLS\u003c/sup\u003e-C bonds destruction\u003csup\u003e14,15\u003c/sup\u003e. Moreover, some M\u003csup\u003eHS\u003c/sup\u003e cations (such as Mn\u003csup\u003e3+\u003c/sup\u003e and Cu\u003csup\u003e2+\u003c/sup\u003e) with asymmetric 3d valance electron orbital configurations lead to the Jahn-Teller effect, triggering crystal structure degradation\u003csup\u003e16\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe electronic structure of Fe\u003csup\u003eLS\u003c/sup\u003e-C coordination bonds can be changed by adjusting the electronic distribution of M\u003csup\u003eHS\u003c/sup\u003e-N coordination bonds, thus the electron transfer energy barrier of Fe\u003csup\u003eLS\u003c/sup\u003e can be effectively reduced\u003csup\u003e17\u003c/sup\u003e. By selecting transition metals preferring to form stronger M\u003csup\u003eHS\u003c/sup\u003e-N bonds, the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure will be effectively improved\u003csup\u003e18,19\u003c/sup\u003e. The robust coordination structure of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e can tolerate the unit cell volume changes and inhibit irreversible phase transition during cycling\u003csup\u003e20,21\u003c/sup\u003e. More importantly, the cyanide as crucial bridge-like functions between Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e plays a vital role in determining the redox activity and structure stability of PBAs\u003csup\u003e22\u003c/sup\u003e. Although it has been proved that the π electron interaction between the cyanide anions and transition metal ions can alleviate the lattice volume change\u003csup\u003e23,24\u003c/sup\u003e, the impact of cyanide coordination electronic structure on cycling stability and redox activity of PBAs has not been investigated systematically.\u003c/p\u003e \u003cp\u003eHere we aim to balance reversible capacity and cycling performance by modulating the electronic structure of cyanide coordination frameworks for PBAs. The electronic distribution of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure is homogenized by optimizing the M\u003csup\u003eHS\u003c/sup\u003e ions, and the designed PBAs not only reduce the capacity loss caused by cyanide electron cloud displacement to Fe\u003csup\u003eLS\u003c/sup\u003e, but also maintain satisfied stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure. The results of density functional theory (DFT) calculations reveal that the uniform-distributed cyanide electronic structure can activate Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e ions simultaneously, thus enhancing the reversible specific capacity of PBAs. In-situ FT-IR and ex-situ EXAFS are used to further confirm that the uniform cyanide electronic distribution between Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e in PBAs helps to maintain the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure during the desodiation/sodiation process. Consequently, the optimized PBA achieves the simultaneous improvement of capacity and cycling lifetime, delivering 95.7% of the theoretical capacity at 0.1 C and retaining 91.7% of the reversible capacity after 1000 cycles at 5 C.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterial design and characterization\u003c/h2\u003e \u003cp\u003eTo demonstrate the effect of high-spin metals on the electronic structure of cyanide, a series of single high-spin-metal PBAs were modeled and structurally optimized by first principles calculations. The electronic distribution maps of cubic (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-e) and rhombohedral phase PBAs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef-j) are obtained. The central position represents the electronic structure of Fe\u003csup\u003eLS\u003c/sup\u003e, while the four corner positions correspond to the electronic structures of high-spin Mn\u003csup\u003e2+\u003c/sup\u003e, Fe\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e and Cu\u003csup\u003e2+\u003c/sup\u003e, respectively. The electronic structure of cyanide is located between Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e. It can be found that the choice of M\u003csup\u003eHS\u003c/sup\u003e and the differences in phase structure affect the electron distribution of the cyanide coordination structure. As the atomic number increases, the electron cloud of cyanide ions gradually shifts from being biased towards M\u003csup\u003eHS\u003c/sup\u003e to Fe\u003csup\u003eLS\u003c/sup\u003e, which is observed from the decreased electronic distribution intensity at the center and the increased intensity at the corner positions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-e). Furthermore, when comparing the sodium-rich rhombohedral phase and sodium-poor cubic phase of the same high-spin-site-metal PBAs, the cyanide electron displacement appears to be nearly identical (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef-i). Differently, in the phase transition process of Cu-PBA from cubic (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee) to rhombohedral (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ej), the cyanide electrons shift towards Fe\u003csup\u003eLS\u003c/sup\u003e. Conventionally, as cyanide electrons become more concentrated towards to the transition metal ions, the charge transfer capability of active sites is adversely affected\u003csup\u003e15\u003c/sup\u003e, making redox reactions more challenging and consequently restricting the reversible capacity. Therefore, in order to give full play to the theoretical capacity of PBAs, it is crucial to obtain a uniform distribution of cyanide electrons which can be achieved by regulating the transition metal elements.\u003c/p\u003e \u003cp\u003eThree PBA samples were synthesized by using an optimized coprecipitation method. In consideration of the selection number of M\u003csup\u003eHS\u003c/sup\u003e, the samples are denoted as M2-PBA, M4-PBA, and M5-PBA, respectively. The morphology of the as-prepared PBAs was characterized by SEM, exhibiting cubic morphology with particle sizes of approximately 1.5 \u0026micro;m (\u003cb\u003eFig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e). The comparations of high-resolution transmission electronic microscopy (TEM) accompanied by energy dispersive spectroscopy (EDS) mapping were displayed in \u003cb\u003eFig. S2-S4\u003c/b\u003e. Edge dislocations are detected in the crystals from the TEM testing, which results from the lattice coupling of various PBAs with different M\u003csup\u003eHS\u003c/sup\u003e. Uniform distribution of corresponding elements is displayed by EDS mapping, revealing all the preseted M\u003csup\u003eHS\u003c/sup\u003e are present in these PBAs. Full spectroscopy and the corresponding fine spectroscopy of XPS analyses also confirm the exist of preseted transition metals in the crystals (\u003cb\u003eFig. S5-S8\u003c/b\u003e). The \u003cem\u003eg\u003c/em\u003e values of ~\u0026thinsp;2.03 for three samples from the EPR tests indicates the presence of [Fe(CN)₆]⁴⁻ defect\u003csup\u003e25\u003c/sup\u003e (\u003cb\u003eFig. S9\u003c/b\u003e). The amplitude difference between M4-PBA and M5-PBA in the curves is small. The higher amplitude for M2-PBA indicates more crystal defects of [Fe(CN)₆]⁴⁻ in comparison with M4-PBA and M5-PBA\u003csup\u003e26\u003c/sup\u003e. Thermogravimetric (\u003cb\u003eFig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e0\u003c/b\u003e) and ICP-OES tests (\u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e) confirm that the chemical formulas of M2-PBA, M4-PBA, and M5-PBA are Na\u003csub\u003e1.84\u003c/sub\u003eMn\u003csub\u003e0.503\u003c/sub\u003eFe\u003csub\u003e0.497\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csub\u003e0.892\u003c/sub\u003e\u0026middot;1.633H\u003csub\u003e2\u003c/sub\u003eO, Na\u003csub\u003e1.927\u003c/sub\u003eMn\u003csub\u003e0.266\u003c/sub\u003eFe\u003csub\u003e0.247\u003c/sub\u003eCo\u003csub\u003e0.248\u003c/sub\u003eNi\u003csub\u003e0.239\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csub\u003e0.943\u003c/sub\u003e\u0026middot;0.862 H\u003csub\u003e2\u003c/sub\u003eO and Na\u003csub\u003e1.898\u003c/sub\u003eMn\u003csub\u003e0.211\u003c/sub\u003eFe\u003csub\u003e0.202\u003c/sub\u003eCo\u003csub\u003e0.199\u003c/sub\u003eNi\u003csub\u003e0.198\u003c/sub\u003eCu\u003csub\u003e0.190\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csub\u003e0.927\u003c/sub\u003e\u0026middot;0.675H\u003csub\u003e2\u003c/sub\u003eO, respectively. The contents of transition metal elements in the PBAs materials closely match the initial feeding amounts, with slight deviations likely due to the differences in the complexing capabilities between various transition metal elements and chelating agents\u003csup\u003e18\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe powder X-ray diffraction (PXRD) patterns of the three samples were analyzed through Rietveld refinement (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ek-m). Although there are differences in elemental selection and water content for these PBAs, all samples exhibit monoclinic structure\u003csup\u003e27\u003c/sup\u003e. The crystallographic information for the three samples is detailed in \u003cb\u003eTable S2-S4\u003c/b\u003e. The insets show the corresponding charge density distribution diagrams of the (200) crystal planes based on the Rietveld refinement PXRD patterns of these PBAs. In this regard, the cyanide electrons in M2-PBA and M5-PBA respectively skew towards M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e with a pear shape, while M4-PBA displays a uniform cyanide electronic distribution with an ellipsoidal shape, aligning with the intended distribution of cyanide electrons according to the results of first-principles calculations (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-j).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCharacterization of the Localized Cyanide Coordination Structure\u003c/h3\u003e\n\u003cp\u003eSince Fe\u003csup\u003e2+\u003c/sup\u003e is introduced in both high-spin sites and low-spin sites for all the three samples, various structural characterization techniques focused on the Fe element were employed to investigate the effect of the electronic structural modulation of cyanide on the coordination structure of PBAs. Fe K-edge X-ray absorption spectroscopy (XAS) was utilized to analyze the cyanide electronic environments in the three samples\u003csup\u003e28,29\u003c/sup\u003e. The X-ray absorption near edge structure (XANES) spectra of the three samples show minimal differences, in which M4-PBA exhibits a relatively lower near edge absorption energy, indicating a less oxidation of Fe\u0026sup2;⁺ (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). An increase in the pre-edge peak intensity of M4-PBA suggests a slight deviation from octahedral symmetry due to a higher sodium content\u003csup\u003e30\u003c/sup\u003e. The distinct profiles of extended X-ray absorption fine structure (EXAFS) spectra exhibit different coordination structures of central Fe atom for the three samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), correspondingly revealing the coordination information of C and N atom in cyanide. The splitting of Fe-N peaks in all samples indicates the influence of M\u003csup\u003eHS\u003c/sup\u003e on the Fe-N coordination bonds. The higher profile intensity for both Fe-C and Fe-N coordination bonds reflects the fine structural symmetry for M4-PBA\u003csup\u003e31\u003c/sup\u003e. Besides, the appearance of shoulder peak at 2061cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e detected by FT-IR test in M2-PBA (\u003cb\u003eFig. S11\u003c/b\u003e) and the blue shift of Raman peak at about 2071cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e representing M\u003csup\u003eHS\u003c/sup\u003e coordinated with N bonds for M5-PBA (\u003cb\u003eFig. S12\u003c/b\u003e) provide direct evidences for their relatively lower structural symmetry in comparison with M4-PBA\u003csup\u003e32,33\u003c/sup\u003e. The low structure symmetry is affected by the reduced shift of cyanide electron cloud towards the either M\u003csup\u003eHS\u003c/sup\u003e or Fe\u003csup\u003e2+\u003c/sup\u003e, namely an increased distribution nonuniformity of cyanide electron cloud. However, a more symmetrical localized environment of cyanide electron cloud does not necessarily imply stronger cyanide coordination bonds in the crystals, which is confirmed by the electron binding energy between Fe and cyanide according to the XPS results of Fe 2p. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, M4-PBA exhibits the lowest electron binding energy of Fe 2p in comparison with M2-PBA and M5-PBA. The decreased electron binding energy of Fe 2p in M4-PBA is partly due to the strength abatement of cyanide coordination bonds, which reduces the electron binding force on Fe ions. Besides, the uniform and symmetric distribution of the cyanide electron can also reduce the confinement of electrons on Fe ions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurther, the electronic environment and oxidation state of Fe ions were revealed by M\u0026ouml;ssbauer spectroscopy. According to the fitted M\u0026ouml;ssbauer spectra of the three samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed-f), the red and green doublet peaks represent the deconvolution results for high-spin-site Fe\u0026sup3;⁺ and Fe\u0026sup2;⁺, respectively, while the blue singlet peak corresponds to Fe\u003csup\u003eLS\u003c/sup\u003e. The pie charts in the inset reflect the amount of Fe ions with different valence in the three samples, indicating the more oxidation of high-spin Fe ions in M2-PBA. The detailed fitting parameters in M\u0026ouml;ssbauer spectra are listed in \u003cb\u003eTable S5\u003c/b\u003e, where IS (isomer shift) provides information related to oxidation state and coordination bonds of Fe ions, QS (quadrupole splitting) reflects the nuclear charge distribution of Fe ions at the respective sites, and Γ (half-peak breadth) indicates the degree of disorder in the local coordination environment of Fe ions. The ISs for high-spin-site Fe ions of the three samples display significant differences (\u003cb\u003eFig. S13a\u003c/b\u003e). On one hand, the oxidation content of Fe ions in the crystal lead to differences in the IS values. On the other hand, variations in the content and selection of M\u003csup\u003eHS\u003c/sup\u003e alter the electron environment around Fe ions and the corresponding coordination bond energy, further causing differences in the IS values\u003csup\u003e10,34\u003c/sup\u003e. Regardless of the valence of high-spin-site Fe ions, the IS values in M2-PBA are the highest (1.11 mm/s for Fe\u003csup\u003e2+\u003c/sup\u003e and 0.34 mm/s for Fe\u003csup\u003e3+\u003c/sup\u003e), indicating that the electron density of high-spin-site Fe ions in M2-PBA is increased due to the shift of the cyanide electron cloud towards the high-spin-site Fe. In contrast, the IS values of the high-spin-site Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e in M4-PBA are 1.06 mm/s and 0.3 mm/s, respectively, in which fall in comparison with those of M2-PBA and M5-PBA (1.02 mm/s and 0.2 mm/s in M5-PBA, respectively). Thus, M4-PBA presents a moderate IS value through the modification of local cyanide coordination structure, which can better keep the even distribution of cyanide electron cloud between high-spin-site metal ions and low-spin-site Fe ions.\u003c/p\u003e \u003cp\u003eThe QS value reflects changes in the electronic environment around Fe ions in these PBAs (\u003cb\u003eFig. S13b\u003c/b\u003e). When the symmetry of the coordination environment around Fe ions decreases, the electric field gradient around the ions increases, leading to an increase in the QS values\u003csup\u003e35\u003c/sup\u003e. Compared to the QS values of high-spin-site Fe ions in M5-PBA (0.95 and 1.18 mm/s for Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e, respectively), the QS values of M2-PBA are significantly higher (1.26 and 1.23 mm/s for Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e, respectively). This phenomenon indicates that the high-spin-site Fe ions are influenced by the shifting of cyanide group electronic structure in M2-PBA, resulting in the symmetry reduction of the electronic environment around the high-spin-site Fe. Correspondingly, the QS value of low-spin-site Fe ions (0.29 mm/s) in M5-PBA is higher than the QS value of M2-PBA (0.17 mm/s), suggesting that the electron cloud of cyanide is more biased towards the low-spin-site Fe ions in M5-PBA. In contrast, the QS values for high-spin-site Fe\u003csup\u003e3+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e in M4-PBA (0.59 mm/s and 1.19 mm/s, respectively) and the QS value for Fe\u003csup\u003eLS\u003c/sup\u003e (0.13 mm/s) are smaller, indicating that the electronic distribution of both high-spin-site and low-spin-site Fe ions are relatively symmetrical. It suggests that the cyanide group electrons are more uniformly distributed in the coordination structure of M4-PBA. Similarly, an increase of Γ values suggests an increased disorder of the coordination structure\u003csup\u003e36\u003c/sup\u003e (\u003cb\u003eFig. S13c\u003c/b\u003e). The lower QS and Γ values of M4-PBA indicates that the local electron symmetries of Fe ions are slightly influenced by cyanide electron cloud in comparison with the other two samples. Additionally, the introduction of other M\u003csup\u003eHS\u003c/sup\u003e somewhat changes the high symmetry of the low-spin-site Fe ions in M5-PBA via the coordination bonds of cyanide. Thus, the results of M\u0026ouml;ssbauer spectra indicates that the electron cloud of cyanide in M4-PBA well maintained symmetrical and centered between the Fe\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e, consistent with the electronic simulation results for the (220) crystal planes in XRD patterns (insets in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ek-m). Hereto, by designing the selections and proportions of M\u003csup\u003eHS\u003c/sup\u003e we can modulate the electronic distribution of M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e, consequently achieving a uniform distribution of cyanide electronic structure between M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg).\u003c/p\u003e\n\u003ch3\u003eElectrochemical performances of PBAs\u003c/h3\u003e\n\u003cp\u003eBenefited from the uniform distribution of cyanide electrons in PBAs, the electrochemical performances are improved. The galvanostatic charge and discharge curves of the first five times (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cb\u003eFig. S14a-b\u003c/b\u003e) and the corresponding dQ/dV curves were measured at a current density of 0.1C for the three samples (\u003cb\u003eFig. S15\u003c/b\u003e). The first discharge capacities of M2-PBA, M4-PBA and M5-PBA are 142.7, 142.4 and 109.5 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to 83.9%, 95.7% and 80.5% of their theoretical capacities, respectively. Notably, M4-PBA displays the highest proportion of theoretical capacity, indicating that the modulation of the cyanide electronic structure is more favorable to exert the activity of both M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e. The galvanostatic charge/discharge curves and dQ/dV results further verify the influence of cyanide electron structure on the reversible capacity. According to the previous analysis, the cyanide electrons of M2-PBA and M5-PBA are respectively biased to M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e, inhibiting the redox activity of the corresponding transition metals\u003csup\u003e37\u003c/sup\u003e. Therefore, M2-PBA exhibits greater redox contribution to capacity in the high voltage region (over 3.5V), while M5-PBA shows greater redox contribution to capacity in the low voltage region (below 3.5V), respectively. As for M4-PBA, the dQ/dV curve shows more symmetrically redox doublet peaks, because the cyanide electrons are evenly distributed between M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e. The first charge/discharge and dQ/dV curves greatly differ from the subsequent curves, which is caused by the irreversible decomposition of crystal water during the first charge and discharge process\u003csup\u003e11,38\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe modified cyanide electronic structure also boosts the cycling stability by providing unobstructed ion transport channels. During the electrochemical reaction, the interaction between guest ions and the coordination framework structure not only affects ion migration, but also has an adverse impact on the stability of the frame structure, resulting in capacity decline\u003csup\u003e39\u003c/sup\u003e. Benefitting from the uniform distribution of cyanide electrons, M4-PBA shows the best cyclic stability (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The capacity retention of M2-PBA, M4-PBA and M5-PBA are 21.4%, 91.7% and 69.1% after 1000 cycles at the current density of 5 C, respectively. Similarly, M4-PBA also exhibits a better capacity retention comparing to M2-PBA and M5-PBA in the cyclic tests at 1 C (\u003cb\u003eFig. S16\u003c/b\u003e). In order to further compare the interaction between Na\u003csup\u003e+\u003c/sup\u003e and coordination frameworks for the three PBAs samples, in-situ electrochemical impedance spectroscopy testing was employed to investigate the Na\u003csup\u003e+\u003c/sup\u003e transport characteristic during the electrochemical process (\u003cb\u003eFig. S17a-c\u003c/b\u003e). The individual electrochemical processes of Na\u003csup\u003e+\u003c/sup\u003e in PBAs cathodes was revealed by distribution of relaxation times (DRT), considered as a broad range of analysis method without the relatively rigid constraints of equivalent circuits\u003csup\u003e40,41\u003c/sup\u003e. Based on different relaxation times, the electrochemical reaction process of PBAs is divided into four regions, representing the internal electrical resistance of the battery components (τ\u003csub\u003eA\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e~10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e s), the cathode-electrolyte interface (τ\u003csub\u003eB\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e~0.5 s), charge transfer in the electrodes (τ\u003csub\u003eC\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.5\u0026thinsp;~\u0026thinsp;10 s), and solid-state diffusion in the electrodes (τ\u003csub\u003eD\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;10\u0026thinsp;~\u0026thinsp;10\u003csup\u003e3\u003c/sup\u003e s)\u003csup\u003e42,43\u003c/sup\u003e. The DRT results of the three samples during the first charge-discharge cycle are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-e and \u003cb\u003eFig. S18a-c\u003c/b\u003e. The differences in the internal resistance of the battery devices and cathode electrode interface (CEI) resistance are minimal for the three samples. However, there are significant differences in the charge transfer resistance in cathodes and solid-state diffusion resistance. Compared to M2-PBA and M5-PBA, M4-PBA especially exhibits smaller values of solid-state diffusion resistance in the high voltage region (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-e), indicating a lower Na\u003csup\u003e+\u003c/sup\u003e migration electrochemical impedance in the M4-PBA lattice. It suggests that Na\u003csup\u003e+\u003c/sup\u003e migrates more rapidly within the M4-PBA lattice, resulting in a lower polarization and a smaller coordination structural decay during the galvanostatic charge and discharge process.\u003c/p\u003e \u003cp\u003eThe Na\u003csup\u003e+\u003c/sup\u003e diffusion ability in the lattice of the three samples with the evolution of Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficient during charging and discharging process were revealed by galvanostatic intermittent titration technique (GITT), which also reflects the advantageous effect of the modified cyanide electronic distribution in M4-PBA (\u003cb\u003eFig. S19\u003c/b\u003e). During the charging process, the diffusion coefficients of the three samples show a similar trend with voltage change (\u003cb\u003eFig. S19a\u003c/b\u003e). The diffusion coefficients significantly decreases in the voltage range of 3.2 to 3.6 V, due to Jahn-Teller effect of Mn\u003csup\u003e3+\u003c/sup\u003e and phase transition in these PBAs\u003csup\u003e8\u003c/sup\u003e. Compared with M4-PBA and M5-PBA, the Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficient of M2-PBA decreases obviously, suggesting the adverse influence of Jahn-Teller effect and more serious phase transition. During the discharge process (\u003cb\u003eFig. S19b\u003c/b\u003e), the Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficients of M2-PBA and M5-PBA greatly vary with a sharp drop in the range from 3.0 to 2.8 V, while the Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficients of M4-PBA are well maintained at about 10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The uniform distribution of cyanide electrons in M4-PBA can ensure the maintenance of a steady Na\u003csup\u003e+\u003c/sup\u003e diffusion rate during the discharge process (\u003cb\u003eFig. S19c\u003c/b\u003e). Moreover, CV tests at different sweep speeds were also conducted for the three samples with the fitting line (\u003cb\u003eFig. S20\u003c/b\u003e), which was obtained according to linear fitting results and supporting equations (\u003cb\u003eEquation S1\u003c/b\u003e and \u003cb\u003eEquation S2\u003c/b\u003e)\u003csup\u003e19,44\u003c/sup\u003e. When the slope value is close to 1, the diffusion of Na\u003csup\u003e+\u003c/sup\u003e in the crystal is mainly controlled by the capacitive behavior, but when the slope value is closer to 0.5, the migration rate of Na\u003csup\u003e+\u003c/sup\u003e in the lattice is mainly affected by the diffusion property\u003csup\u003e7,45\u003c/sup\u003e. That is, the higher the slope, the faster the Na\u003csup\u003e+\u003c/sup\u003e migration through the lattice. The slope values (\u003cem\u003eb\u003c/em\u003e) of M2-PBA, M4-PBA and M5-PBA during the oxidation process are 0.92, 0.96 and 0.84, respectively, with the corresponding slope values of 0.68, 0.89 and 0.75 during the reduction process. Therefore, M4-PBA with the modified cyanide electronic structure exhibits the highest capacitance contribution in the charging and discharging process, which helps to ensure fast Na\u003csup\u003e+\u003c/sup\u003e mobility channels. As a result, the rate performance of M4-PBA is significantly improved in comparison with that of M2-PBA and M5-PBA (\u003cb\u003eFig. S21\u003c/b\u003e). The capacity of 85.1 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in M4-PBA is emitted at high rate of 20 C (1 C\u0026thinsp;=\u0026thinsp;148.75 mA\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), which is superior to M2-PBA (44.36 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and M5-PBA (25.81 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The full cell performances using these PBAs as cathode materials with presodiation of hard carbon as anode material are presented in \u003cb\u003eFig. S22a\u003c/b\u003e. Following the same trend observed in the half-cell electrochemical performance, M4-PBA demonstrates the highest capacity and the best cycling stability in comparison with M2-PBA and M5-PBA (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef-g). Notably, M4-PBA achieves a discharge energy density of 458 Wh\u0026middot;kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (based on the cathode mass calculation) within the voltage range of 4.0\u0026thinsp;~\u0026thinsp;1.8 V (\u003cb\u003eFig. S22b\u003c/b\u003e). The comparation of electrochemical performance with other research results are list in \u003cb\u003eTable S6\u003c/b\u003e. The excellent electrochemical performance of M4-PBA indicates that the uniform distribution of cyanide electrons for Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e framework not only enables M4-PBA to fully exert its theoretical capacity, but also maintains the localized coordination structural stability in the charge and discharge process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eStructural Evolution of PBAs during Desodiation/Sodiation Process\u003c/h3\u003e\n\u003cp\u003eIn-situ FT-IR and ex-situ XAS testing were employed to investigate the evolution of the Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e electronic structure during the charge and discharge processes\u003csup\u003e46\u003c/sup\u003e. The in-situ FT-IR results of the cyanide in M4-PBA are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cb\u003eFig. S23\u003c/b\u003e. During charging process, the absorption peaks of cyanide near 2075 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (denoted as peak 1) gradually show a red-shift to approximately 2050 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which indicates the electron loss process of M\u003csup\u003eHS\u003c/sup\u003e. With further charging, the single absorption peak of cyanide is split into a doublet contained the newly emerged infrared absorption peak near 2150 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which represents a change of cyanide coordination environment caused by the oxidation of Fe\u003csup\u003eLS\u003c/sup\u003e (denoted as peak 2). Although some nonlinear changes are observed in peak 1 for M4-PBA during the first charging process, better reversibility is maintained in subsequent cycling (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). In contrast, significant mutants are observed in peak 1 for both M2-PBA and M5-PBA (\u003cb\u003eFig. S24 a-b\u003c/b\u003e). The change of absorption peak 1 is related to the electron interaction of M\u003csup\u003eHS\u003c/sup\u003e and N. The poor stability of cyanide coordination electronic structure in M2-PBA and M5-PBA is mainly associated with the Jahn-Teller effect of Mn\u003csup\u003e3+\u003c/sup\u003e and Cu\u003csup\u003e2\u0026thinsp;+\u0026thinsp;47\u003c/sup\u003e. The negative impact of the Jahn-Teller effect is mitigated due to the modification of cyanide electronic structure in M4-PBA, thus maintaining high cycling stability. The area ratios of peaks 1 and 2 were calculated to reflect the capacity utilization of various active sites (\u003cb\u003eFig. S25\u003c/b\u003e). A higher area ratio indicates a relatively lower redox activity of Fe\u003csup\u003eLS\u003c/sup\u003e. For M2-PBA, the area ratio at the end of charging is relatively large, which is related to the cyanide electron shifting to M\u003csup\u003eHS\u003c/sup\u003e. A similar trend is observed for M5-PBA, and the area ratio at the end of charging is relatively small, corresponding to the electron clouds of cyanide skewing towards the Fe\u003csup\u003eLS\u003c/sup\u003e. However, a linearly varying area ratio throughout cycling is shown in M4-PBA, indicating a stable Na\u003csup\u003e+\u003c/sup\u003e insertion and extraction process due to the uniform electronic distribution of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure. The calculated results of M4-PBA in the area ratios of peaks 1 and 2 further highlight the relationship between the regulation of the cyanide coordination electronic structure and the full utilization of reversible capacity. Furthermore, when comparing the changes in area ratios of peaks 1 and 2 between the first two cycling processes, M4-PBA shows a more consistent trend, suggesting a better stability of the Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure.\u003c/p\u003e \u003cp\u003eThe dissolution of transition metal elements is one of the primary factors leading to the capacity decay of PBAs\u003csup\u003e20\u003c/sup\u003e, and maintaining the stability of the cyanide coordination electronic structure is beneficial to reduce the capacity loss. The ICP-OES results of the three samples at different cycles show the dissolution of transition metals in the electrolyte (\u003cb\u003eFig. S26 a-d\u003c/b\u003e). According to the previous research on Mn-based PBAs, Mn\u003csup\u003e3+\u003c/sup\u003e is considered as the primary cause of electrode material failure due to the Jahn-Teller effect\u003csup\u003e18\u003c/sup\u003e. However, contrary to common understanding, the ICP-OES test results of the three samples reveal that the concentration of dissolved Fe ions is more than ten times that of the dissolved Mn ions during electrochemical process. Combined with the decline in the reversible capacity of the electrode materials (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), it can be deduced that the dissolution of Fe ions is one of the main factors contributing to the degradation of reversible specific capacity. Ex-situ Fe K-edge XANES and EXAFS testing are further conducted to investigate the changes in the cyanide coordination structure during the Na\u003csup\u003e+\u003c/sup\u003e insertion and extraction process (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb-c). The XANES results suggest that all three samples maintain high reversibility in the redox behavior of Fe ions. The XANES spectra of Fe element shift to a higher energy area during the Na\u003csup\u003e+\u003c/sup\u003e extraction process, indicating the oxidation of Fe to higher valence state. During the Na\u003csup\u003e+\u003c/sup\u003e insertion process, the profiles of Fe finally reverse back to their original positions, manifesting that the valence states of Fe in the PBAs recover to their original states. The shift of XANES profiles after the first cycle for M2-PBA is more pronounced than that of M4-PBA (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) and M5-PBA (\u003cb\u003eFig. S27b\u003c/b\u003e), indicating that the Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure is taken apart slightly in M2-PBA (\u003cb\u003eFig. S27a\u003c/b\u003e). As to the EXAFS result of M2-PBA, the profiles of Fe-C and Fe-N coordination bonds positively shift at SOC-2, and then negatively shift during the discharge process (\u003cb\u003eFig. S28a\u003c/b\u003e). However, both the peak intensity and position undergo significant changes, demonstrating the poor stability of the Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination structure. In the case of M5-PBA, although the EXAFS profile of the Fe-N coordination bond is highly reversible, the shift of the cyanide electron clouds towards to C reduces the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-C bonds (\u003cb\u003eFig. S28b\u003c/b\u003e). In contrast, the stable cyanide coordination electronic structure in M4-PBA is verified by the highly consistent EXAFS profiles of the Fe-C and Fe-N coordination bonds at different sates of charge, indicating the good coordination structural reversibility during cycling (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eThe in-situ XRD patterns of M4-PBA for the first two cycles were measured at a current density of 20 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u003cb\u003eFig. S29\u003c/b\u003e). The evolution of the characteristic peaks at about 17, 24 and 34\u0026deg; and the corresponding galvanostatic charge/discharge curves of M4-PBA are exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. The phase transition from monoclinic to cubic occurs for M4-PBA during the first Na\u003csup\u003e+\u003c/sup\u003e extraction process, and the material undergoes a phase transition from cubic to rhombohedral during first Na\u003csup\u003e+\u003c/sup\u003e insertion process. In the second charging process, a reversible phase transition occurs between rhombohedral and cubic phase. A schematic diagram of phase transition processed for M4-PBA during the first two cycles is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. The reversible structure transition confirmed by in-situ XRD results ensures the excellent electrochemical performance for M4-PBA. A similar phase transition process of M2-PBA is occurred in the ex-situ XRD results for the first two Na\u003csup\u003e+\u003c/sup\u003e extraction/insertion process (\u003cb\u003eFig. S30b-d\u003c/b\u003e). However, there is a significant fade in the intensity of the M2-PBA diffraction peaks in comparison with M4-PBA, indicating that M2-PBA undergoes a structural degradation in the second cycle. The irreversible coordination structural change is consistent with previous EXAFS results (\u003cb\u003eFig. S28\u003c/b\u003e). Different from the PXRD result of monoclinic M5-PBA powder, the pristine electrode of M5-PBA is determined as rhombohedral phase, which is resulted from the loss of some crystal water during the electrode preparation process\u003csup\u003e48,49\u003c/sup\u003e. With low content of crystal water, M5-PBA maintains the rhombohedral-cubic-rhombohedral phase transition throughout the initial two charge/discharge cycles (\u003cb\u003eFig. S31b-d\u003c/b\u003e). Intensity fade of Bragg diffraction peaks in M5-PBA suggests unstable structural evolution during electrochemical process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe differences in the irreversible structural evolution among the three samples are attributed to the loss of crystallization water during the first Na\u003csup\u003e+\u003c/sup\u003e extraction/insertion process, coinciding with the variation trend of initial galvanostatic charge/discharge curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cb\u003eFig. S14a-b\u003c/b\u003e). Meanwhile, the Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination electronic structure in PBAs play an important role on the reversibility of the crystal structure changes during cycling. The well-distributed cyanide coordination electronic structure not only ensures stable and reversible structural phase transitions, but also contributes to the enhanced stability of the crystal structure of M4-PBA during the electrochemical reaction process.\u003c/p\u003e\n\u003ch3\u003eDFT calculations\u003c/h3\u003e\n\u003cp\u003eTo investigate the localized electronic structure of the three PBA samples, DFT calculations were employed. According to the partial density of state (PDOS) of PBAs, M\u003csup\u003eHS\u003c/sup\u003e (such as Fe\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, and Co\u003csup\u003e2+\u003c/sup\u003e) tends to be oxidized at low potentials, while Fe\u003csup\u003eLS\u003c/sup\u003e is oxidized at high potentials (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-c). The PDOS of Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e in M4-PBA is closer to the Fermi level compared to M2-PBA and M5-PBA, and thus both Fe\u003csup\u003eLS\u003c/sup\u003e and M\u003csup\u003eHS\u003c/sup\u003e coordinated with cyanide can be easily activated in M4-PBA, corresponding to higher practical discharge specific capacity. Furthermore, the PDOS of N in M2-PBA shows the highest overlap with the PDOS of M\u003csup\u003eHS\u003c/sup\u003e, while the PDOS of C in M5-PBA exhibits the highest overlap with the PDOS of Fe\u003csup\u003eLS50\u003c/sup\u003e. This indicates that M2-PBA possessed stronger coordination electronic interaction of N and M\u003csup\u003eHS\u003c/sup\u003e, while stronger coordination electronic interaction between C and Fe\u003csup\u003eLS\u003c/sup\u003e are also exhibited in M5-PBA. In contrast, M4-PBA has moderate coordination electronic interaction, which not only enhances the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e chemical bonds but also provides a more uniform distribution of coordination electrons. The computational models of the electron clouds for the three samples and their corresponding cross-sectional charge density distribution diagrams are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed-f. According to the cross-sectional charge density distribution diagrams, M4-PBA exhibits lower charge density at both M\u003csup\u003eHS\u003c/sup\u003e and Fe\u003csup\u003eLS\u003c/sup\u003e compared to M2-PBA and M5-PBA, which is consistent with the XRD refinement results (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ek-m). Thus, the regulation of cyanide coordination electronic structure effectively reduces the charge density surrounding the transition metal elements of M4-PBA, thereby fully activating the transition metal ions, and improving the practical reversible capacity. The evenly distributed cyanide coordination electronic structure is achieved by regulating the composition of M\u003csup\u003eHS\u003c/sup\u003e in PBAs (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg). Moreover, the corresponding coordination electron interaction of transition metals and cyanide can be optimized to enhance the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e frameworks. Consequently, the optimized cyanide coordination electronic structure ultimately balances the reversible capacity and cycling stability of M4-PBAs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eAccording to the valence bond theory and the DFT calculations results of single high-spin-site-metal PBA (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-j), the coordination electronic structure of PBAs is affected by the valence electron distribution of transition metals and the electronegativity of ligand elements. Although it has been proved that the stability of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination frameworks can be improved by selecting transition metal cations with stronger π electron interaction to cyanide anions, the electrochemical performance enhancement mechanism of cyanide coordination electronic structure on cycling stability and redox activity of PBAs remains unclear. The uniform-distributed coordination electronic structure was accomplished for PBAs to achieve both high specific capacity and long cycling life. The optimized Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e electronic structure of M4-PBA is confirmed by the results of Rietveld refinement PXRD patterns and M\u0026ouml;ssbauer spectroscopy test. Further, linear variation of FT-IR absorption peak area ratios is beneficial to sufficiently activate the theoretical capacity and reduce the Na\u003csup\u003e+\u003c/sup\u003e migration energy barrier for M4-PBA. As to the cycling performance, the highly reversable structure change is reflected by ex-situ XAS and in-situ XRD, suggesting that the uniform distribution of Fe\u003csup\u003eLS\u003c/sup\u003e-CN-M\u003csup\u003eHS\u003c/sup\u003e coordination electrons alleviate the irreversible structural change. The M4-PBA with the optimized coordination electronic structure can release a high capacity of 142.4 mAh\u0026middot;g⁻\u0026sup1; at 0.1 C and retains 91.7% of its reversible capacity after 1000 cycles at 5 C. In addition, the excellent rate capability of M4-PBA is also demonstrated with a reversible discharge capacity of 85.1 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at a high current density of 20 C. This work will provide some reference for studying the structure evolution and failure mechanism of PBAs by homogenizing the electron distribution of coordination structure.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eThe main chemical reagents included Na\u003csub\u003e4\u003c/sub\u003eFe(CN)\u003csub\u003e6\u003c/sub\u003e\u0026middot;10H\u003csub\u003e2\u003c/sub\u003eO (AR, \u0026ge;\u0026thinsp;99%), PVP (K30, ~\u0026thinsp;40,000), NaCl (AR, 99.5%), ascorbic acid (ACS, \u0026ge;\u0026thinsp;99%), MnSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO (AR, 99.0%), FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO (AR, \u0026ge;\u0026thinsp;99%), CoSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO (AR, \u0026ge;\u0026thinsp;99%), NiSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO (AR, 99.0%) and CuSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;5H\u003csub\u003e2\u003c/sub\u003eO (AR, 99.0%). All the solid reagents were purchased from Shanghai Aladdin without further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePBAs Preparation\u003c/h2\u003e \u003cp\u003e5 mmol equimolar amount of MnSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO, FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO, CoSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO, and NiSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO in 0.8 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e sodium citrate solution were dissolved to obtain 100 ml solution A. Solution B was prepared by dissolving Na\u003csub\u003e4\u003c/sub\u003eFe(CN)\u003csub\u003e6\u003c/sub\u003e\u0026middot;10H\u003csub\u003e2\u003c/sub\u003eO with molar concentration of 0.2 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in 100 ml deionized water after removal of oxygen. 5 g PVP (K30), 0.4 mol NaCl, and 0.2 g ascorbic acid were dissolved in deoxygenated deionized water to form 200 ml transparent solution C. A white emulsion was evolved by dropwise pumping solution A and B into solution C with a rate of 0.1 ml\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. After the solutions were fully added, the mixed emulsion was left to stand and aged for additional 12 h. The light green precipitate was centrifuged and transferred into vacuum oven at 120\u0026deg;C for 10 h drying to obtain the prepared Na\u003csub\u003e2\u003c/sub\u003eMn\u003csub\u003e0.25\u003c/sub\u003eFe\u003csub\u003e0.25\u003c/sub\u003eCo\u003csub\u003e0.25\u003c/sub\u003eNi\u003csub\u003e0.25\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] with four M\u003csup\u003eHS\u003c/sup\u003e (Mn, Fe, Co and Ni), defined as M4-PBA. The designed Na\u003csub\u003e2\u003c/sub\u003eMn\u003csub\u003e0.5\u003c/sub\u003eFe\u003csub\u003e0.5\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] with two M\u003csup\u003eHS\u003c/sup\u003e (Mn, and Fe) was defined as M2-PBA, and Na\u003csub\u003e2\u003c/sub\u003eMn\u003csub\u003e0.2\u003c/sub\u003eFe\u003csub\u003e0.2\u003c/sub\u003eCo\u003csub\u003e0.2\u003c/sub\u003eNi\u003csub\u003e0.2\u003c/sub\u003eCu\u003csub\u003e0.2\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] with five M\u003csup\u003eHS\u003c/sup\u003e (Mn, Fe, Co, Ni and Cu) was marked as M5-PBA. A similar synthesis method was used to replace the transition metal salt in the synthesis of M4-PBA with the corresponding equimolecular amount of hydrated transition metal sulfate, with a total molar amount of 20 mmol, according to the chemical formulas of M2-PBA and M5-PBA.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eElectrochemical measurements\u003c/h2\u003e \u003cp\u003eThe cathode slurry was mixed by active cathode materials, Ketjen black and polyvinylidene fluoride (PVDF) binder (5 wt% in N-methyl pyrrolidone) in a mass ratio of 7:2:1. It was evenly coated on aluminum foil with a thickness of 130 \u0026micro;m and then transferred to a vacuum drying oven at 120 ℃ for overnight. After being cut into a circle with a diameter of 14 mm, it was pressed under 10 MPa for 5 min. The active material loading of each slice was between 1.5 and 2.0 mg. Subsequently, a 2025 button battery was assembled in a glove box with water and oxygen concentrations below 0.01ppm. Sodium metal tablet with a diameter of 15.6 mm was used as counter electrode and reference electrode and 1 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NaClO\u003csub\u003e4\u003c/sub\u003e was dissolved in the mixture solution of EC, PC and FEC as electrolyte (the volume ratio of EC:PC:FEC was 47.5:47.5:5). The electrochemical performance data of galvanostatic charging and discharging curves were all measured by the Neware battery test system in the voltage range of 2.0-4.1 V. Due to the differences in the theoretical capacity of the three samples, the cells were tested according to the current density of 170 mA\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e,148.75 mA\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 136 mA\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 1 C rate, respectively. For the cycling performance test, the three samples were first activated five times at 0.2 C and subsequently subjected to subsequent cycles at different rates. In the rate performance test, the three samples were cycled for 5 times each at the rate of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 C in sequence, followed by 60 cycles at 1 C. GITT testing was conducted after five cycles of the fresh cell at current density of 0.1 C between 2.0 and 4.1 V, in which the cell was alternately charged for 10 min followed by 60 min resting, then discharged in the same way. The cyclic voltammetry (CV) curves and in-situ transient electrochemical impedance spectroscopy (in-situ EIS) were measured by Shanghai Chenhua CHI 760e electrochemical workstation. The scanning speed of CV curves were 0.2, 0.4, 0.6, 0.8 and 1.0 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The in-situ EIS were tested from the open circuit voltages of each sample with the following voltage interval of 0.3V from 2.9V until the end of the first galvanostatic charging and discharging cycle. The current density of galvanostatic charging and discharging process of in-situ EIS testing were 0.1C for each sample.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eMaterials characterization\u003c/h2\u003e \u003cp\u003eThe crystalline structures of the prepared samples were analyzed using laboratory powder X-ray diffraction (PXRD) with a Cu Kα source on an X\u0026rsquo;Pert3 MRD diffractometer, and in-situ XRD tests were conducted using a Bruker AXS D8 Focus instrument. The microtopography of PBAs were examined via field emission scanning electron microscopy (FE-SEM, VEGA3 TESCAN), while scanning transmission electron microscopy (STEM, JEM-ARM 200F), equipped with energy dispersive spectroscopy (EDS), provided insights of elemental distribution of the as-prepared samples. High-resolution transmission electron microscopy (HR-TEM) measurements were conducted with an FEI Tecnai G2 F30. Surface composition analysis was performed using X-ray photoelectron spectroscopy (XPS, EscaLab 250xi), and electron paramagnetic resonance (EPR) spectra were obtained using a Bruker A300-10/12 spectrometer. Thermogravimetric analysis (TGA) measurements were taken from 25 to 400\u0026deg;C at a heating rate of 5\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in nitrogen. FT-IR spectra, both powder and in-situ, were recorded using Bruker FT-IR spectrometers (UK) and Nicolet 6700 FT-IR spectrometers (Thermo Scientific, USA). Raman spectroscopy was performed using a Renishaw inVia system with a 532 nm laser. The \u003csup\u003e57\u003c/sup\u003eFe M\u0026ouml;ssbauer spectra were collected using a constant acceleration Halder-type spectrometer in transmission geometry, with a room-temperature \u003csup\u003e57\u003c/sup\u003eCo source embedded in a Rh matrix. The concentrations of sodium and transition metals in the samples and cycled electrolyte were determined using inductively coupled plasma (ICP) analysis (OPTIMA 8000DV Optical Emission). X-ray absorption spectroscopy (XAS) measurements, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were carried out at the BL16U1 beamline of the Shanghai Synchrotron Radiation Facility.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eTheoretical calculations\u003c/h2\u003e \u003cp\u003eTheoretical calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab-initio Simulation Package (VASP). The projector augmented wave (PAW) method was employed alongside the Kohn-Sham equations, utilizing the spin-polarized generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional to analyze the electronic interactions between valence electrons and core ions, as well as the exchange-correlation effects. To account for electronic correlation effects of transition metal 3d electrons, Hubbard U corrections were applied for Mn (5.0 eV), Fe (4.3 eV), Co (5.7 eV), Ni (6.0 eV), and Cu (4.0 eV). In the DFT calculations, the kinetic energy cutoff for the electron wave functions was set to 520 eV. Geometry optimization was conducted using the conjugate gradient method, with convergence criteria of 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV for energy and 0.02 eV \u0026Aring;\u003csup\u003e\u0026minus;1\u003c/sup\u003e for forces. The Brillouin zone was sampled using a 3\u0026times;3\u0026times;3 Monkhorst\u0026thinsp;\u0026minus;\u0026thinsp;Pack k-point grid. Visualization of the electrolyte structures was performed using VESTA.\u003c/p\u003e \u003c/div\u003e "},{"header":"Declarations","content":"\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe data pertinent to this thesis and its supplementary materials that underpin the study\u0026rsquo;s findings can be obtained from the corresponding author upon a reasonable request.\u003c/p\u003e \u003c/div\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eY. W. and B. X. proposed design concept and complete PBAs material preparation. J. Y. conducted DFT computations. L. W. performed the X-ray absorption spectroscopy test. B. X., L. W., and P. Z. supervised this work and revised the paper. Y. W. and Y. M. carried out the assembly of the full cells and completed the electrochemical performance test. All authors made contribution to the experimental testing, data analysis and discussion.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors would like to express their appreciation to the Natural Science Foundation of Heilongjiang Province for Distinguished Young Scholars (JQ2024B001), the National Natural Science Foundation of China (No. 52272241), the Harbin Science and Technology Innovation Talent Project (2023CXRCGD036), Fundamental Research Funds for the Central Universities (No. HIT.DZJJ.2023052), the Key Research and Development Program of Heilongjiang Province (No. GA21A102), Jiangsu Provincial Double-Innovation Doctor Program (No. JSSCBS20220308), the Zhejiang Provincial Natural Science Foundation of China under Grant No. LR24E020001 and the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (No. 2023R01007). We also thank the BL16U beamline scientists of Shanghai Synchrotron Radiation Facility for their assistance with the X-ray absorption spectroscopy experiments.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e Wang, L. et al. Rhombohedral prussian white as cathode for rechargeable sodium-ion batteries. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cb\u003e137\u003c/b\u003e, 2548\u0026ndash;2554 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e Wang, W. et al. Reversible structural evolution of sodium-rich rhombohedral Prussian blue for sodium-ion batteries. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cb\u003e11\u003c/b\u003e, 980 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e Wang, L. et al. A superior low-cost cathode for a Na-ion battery. \u003cem\u003eAngew. Chem. Int. Ed. 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Mater.\u003c/em\u003e \u003cb\u003e33\u003c/b\u003e, 2215155 (2023).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5878834/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5878834/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePrussian blue analogues (PBAs) with 3D cyanide-bridged frameworks exhibit significant potential as cathode materials for sodium-ion batteries. However, the dissolution of transition metals and structural distortion often lead to structural instability, causing serious capacity degradation during cycling. Fundamental understanding and tuning the coordination electronic structure to mitigate PBAs instability remain challenging. Herein, we address these challenges by modulating the local electronic structure surrounding high-spin metals to optimize the cyanide coordination environment, enabling a uniform electron distribution within the crystal structure. The resulting uniform electronic structure enhances the reactivity of the transition metals, which helps to achieve 95.7% of the theoretical capacity. More importantly, the regulation of electronic displacement within the cyanide coordination environment significantly improves the crystal structural stability, yielding an impressive capacity retention of 91.7% after 1000 cycles. These findings provide new insights into the coordination structural chemistry of PBAs and offer valuable guidance for the development of advanced cathode materials for sodium-ion batteries.\u003c/p\u003e","manuscriptTitle":"Tuning Cyanide Coordination Electronic Structure Enables Stable Prussian Blue Analogues for Sodium-ion Batteries","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-04 09:16:23","doi":"10.21203/rs.3.rs-5878834/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5f309de2-f3b0-4579-b925-403eb7e0ffa0","owner":[],"postedDate":"February 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":43749543,"name":"Physical sciences/Chemistry/Electrochemistry/Batteries"},{"id":43749544,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"}],"tags":[],"updatedAt":"2025-11-19T08:12:02+00:00","versionOfRecord":{"articleIdentity":"rs-5878834","link":"https://doi.org/10.1038/s41467-025-65062-x","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-11-18 05:00:00","publishedOnDateReadable":"November 18th, 2025"},"versionCreatedAt":"2025-02-04 09:16:23","video":"","vorDoi":"10.1038/s41467-025-65062-x","vorDoiUrl":"https://doi.org/10.1038/s41467-025-65062-x","workflowStages":[]},"version":"v1","identity":"rs-5878834","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5878834","identity":"rs-5878834","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}