Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance Sodium-Ion Batteries

Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance 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 Research Article Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance Sodium-Ion Batteries Hao Xie, Pei Ding, Chuxin Cui, ZhengMing Sun This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6950664/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Among sodium-ion battery (SIB) layered cathode materials, O3-type cathodes are attractive due to their high capacity, high initial efficiency, and low cost. However, their commercial application is limited by structural instability, complex phase transitions during cycling, high sensitivity to moisture and CO 2 , and poor rate performance due to sluggish ion diffusion. To overcome these challenges, we designed a multiphase core-shell heterojunction consisting of an O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 core and a P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 shell. This structure effectively isolates the O3 phase from moisture and electrolyte exposure, combining high capacity with structural stability. The half-cells with the optimized heterojunction cathode delivered 129.1 mAh g -1 at 0.1 C and 103.4 mA g -1 at 10 C, with 80.1% capacity retention at high rates. After 300 cycles under ambient air exposure, it maintains 78.2% capacity, compared to only 27.7% for the pure O3 counterpart. The P2 shell enhances structural reversibility, air/thermal stability, electrochemical kinetics, and lowers charge-transfer resistance. This rational design demonstrates a promising strategy for developing high-performance SIB cathodes. Sodium-ion batteries Layered transition metal oxides O3/P2 heterojunction Core-shell Long cycle life High rate Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Rechargeable batteries are the predominant energy storage technology, playing an integral role in enhancing modern life [ 1 ] . Presently, lithium-ion batteries (LIBs) are the preferred power source for a vast array of commercial electronic devices and electric vehicles [ 2 ] . However, the limited lithium reserves, uneven geographical distribution, and high cost of lithium extraction and recycling have raised concerns regarding the sustainability and long-term viability of LIB technology [ 3 ] . Sodium, a geochemically abundant alkali metal with ubiquitous crustal distribution (2.36 wt% vs. 0.0017 wt% for lithium) and inherent cost-effectiveness, has emerged as a promising candidate for next-generation battery systems [ 4 – 6 ] . The performance of SIBs is predominantly determined by the cathode materials. Transition metal layered oxide materials of the form Na x TMO 2 (where TM represents elements such as Ni, Co, Mg, Fe, Mn, etc.) are widely studied owing to their high energy density and high redox potential [ 7 , 8 ] . Based on the coordination environment of sodium ions, these layered oxides are typically categorized as P2-type or O3-type according to the Delmas classification, with the numeral following P or O referring to the repetition period of the transition metal layer formed by edge-sharing MO6 octahedra. [ 9 ] In O3-type structures, sodium ions reside in octahedral sites between MO 6 slabs, allowing for high theoretical capacity [ 10 ] . However, sodium ions need to overcome high energy barriers when passing through tetrahedral interstices, and most O3-type cathodes exhibit poor electrochemical kinetics and complex phase transition processes, leading to rapid capacity decay [ 11 – 13 ] . In contrast, P2-phase Na x TMO 2 possesses open prismatic channels, which contribute to fast ion conductivity and greater structural stability. Nevertheless, sodium deficiency may lead to abnormal initial coulombic efficiency, necessitating anode pretreatment or the use of sodium supplements, which limits practical applications [ 14 , 15 ] . To overcome the individual limitations of P2 and O3 phases, recent studies have focused on constructing multiphase P2/O3 composite cathodes. These hybrid structures aim to combine the high capacity of O3-type materials with the superior structural stability and ion conductivity of P2-type phases. [ 16 – 18 ] . For instance, Guo et al. [ 19 ] synthesized a P2/O3-Na 0.76 Ni 0.33 Mn 0.5 Fe 0.1 Ti 0.07 O 2 composite by mechanically milling and sintering P2-Na 2/3 Ni 1/3 Mn 0.57 Ti 0.1 O 2 with O3-NaNi 1/3 Fe 1/3 Mn 1/3 O 2 . The resulting material showed mitigated phase transitions and improved cycling performance. Similarly, Xiao et al. [ 20 ] tuned sintering temperatures of a sol-gel-derived P2-type precursor to create a P2/P3 composite, achieving enhanced cycling stability. Wang et al. [ 21 ] developed multi-element-doped P2/O3 cathodes with prolonged lifespan over 500 cycles, highlighting the benefits of phase integration and cation substitution. Despite these advances, one critical challenge remains: the intrinsic instability of O3-type materials in ambient and electrochemical environments. Within P2/O3 composites, the non-uniform distribution of phases often leaves O3 domains exposed to atmospheric moisture and reactive electrolytes. This exposure accelerates cathode degradation via (i) spontaneous surface reactions with H₂O/CO₂, forming sodium hydroxide and carbonate species, and (ii) transition metal dissolution during repeated sodiation/desodiation cycles [ 22 ] . These synergistic degradation pathways substantially compromise the structural integrity and electrochemical performance of O3-type cathodes while simultaneously increasing material storage requirements and associated costs. To mitigate these interfacial instabilities, surface modification techniques have been widely explored. Atomic-layer coatings, such as AlPO 4 [ 23 ] , NaPO3 [ 24 ] , and Al2O3 [ 25 ] , have proven effective in protecting O3-type cathodes by serving as barriers that isolate active materials from air and electrolyte contact while simultaneously providing mechanical support during volume changes. Additionally, nanoscale structural engineering—such as particle downsizing and morphology control—can further enhance material durability by shortening diffusion paths and relieving strain accumulation. [ 26 ] . Despite these promising strategies, most efforts have focused on physical mixtures or simple coatings of multiphase materials. Few studies have explored rationally designed core–shell heterostructures that spatially organize P2 and O3 phases to synergistically optimize both electrochemical performance and environmental stability. Leveraging the synergistic benefits of multiphase architecture and interfacial engineering, we propose a novel core-shell heterostructured cathode design through phase-selective surface modification. An O3-type NaNi 0.33 Fe 0.33 Mn 0.33 O 2 (O3-NNFM) core with high theoretical capacity was synthesized, followed by in situ growth of a P2-type Na 0.67 Ni 0.3 Mn 0.7 O 2 (P2-NNM) protective layer through controlled thermal treatment. This hierarchical O3/P2 configuration establishes a chemically stabilized heterointerface while maintaining ionic transport channels. Structural and morphological evolution was systematically investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses, confirming the synergistic relationship between the O3 core and P2 shell. Electrochemical evaluation revealed remarkable performance enhancement: the optimized O3-NNFM@P2-NNM composite delivered a specific discharge capacity of 130 mAh g − 1 at 0.1C (94% of the initial capacity of the pristine O3-NNFM was 138.42 mAh g − 1 ), demonstrating effective surface protection without significant capacity sacrifice. More critically, the heterostructured cathode exhibited a superior capacity retention of 82.4% after 300 cycles (vs. 57.5% for pure O3-NNFM). Through further rate performance tests, they exhibit reversible capacities of 77.4 mAh g − 1 and 103.4 mAh g − 1 at 10 C, which are 56.1% and 80.1% of the discharge capacity at 0.1 C ( [email protected] mAh g − 1 and [email protected] mAh g − 1 ). This represents a 25% improvement in cycle life and a 24% enhancement in high-rate performance compared with the unmodified counterpart. 2. Experimental Section 2.1 Synthesis of O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 The precursor for the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 cathode was synthesized via a coprecipitation method. Initially, a multimetal solution containing 2 M NiSO 4 ·6H 2 O, FeSO 4 ·7H 2 O, and MnSO 4 ·H 2 O in a 1:1:1 ratio was obtained. Subsequently, NaOH solution (4 M) and NH 3 ·H 2 O solution (10 M) were introduced into a 10 L reactor protected by N 2 . The precipitate, Ni 0.33 Fe 0.33 Mn 0.33 (OH) 2 , denoted as NFM(OH) 2 , formed. NFM(OH) 2 was then mixed with Na 2 CO 3 at a molar ratio of 1:1.02 and calcined at 850 ℃ for 24 hours to yield the product powder NaNi 0.33 Fe 0.33 Mn 0.33 O 2 , designated NNFM. 2.2 Synthesis of O3/P2 multiphase material The NNFM cathode material was coated with P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 via a wet chemical method. The prepared O3-NNFM particles were mixed with the required Ni(CH 3 COO) 2 , Mn(CH 3 COO) 2 , and CH 3 COONa in a specific ratio in an ethanol solvent and magnetically stirred for 30 minutes to obtain a homogeneous mixture. The molar ratio of the mixture to O3-NNFM particles was 5, 10 and 20 mol% with reference to the total mass of the NNFM. Then, oxalic acid solution was added dropwise, and the mixture was transferred into a Teflon-lined stainless-steel autoclave. The autoclave was heated at 180 ℃ for 12 hours to form an oxalate coprecipitate. The solvent was evaporated overnight at 100 ℃, followed by precalcination in an oxygen atmosphere at 500 ℃ for 8 hours and calcination at 850°C for 15 hours to produce P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 -coated (designated NNM) O3/P2 multiphase material. According to the difference in the P2 phase molar ratio, the prepared O3/P2 multiphase materials were denoted as NNFM5NNM, NNFM10NNM and NNFM20NNM. 2.3 Characterizations The products were analyzed via a D/Max-RA X-ray diffractometer with a Cu Kα radiation source (λ = 1.5418 Å) at a scanning rate of 2°/min for X-ray diffraction (XRD) analysis. The morphology and size of the products were examined via field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) at 10 kV and transmission electron microscopy (TEM, JEM-2100, JEOL) at an acceleration voltage of 200 kV. 2.4 Electrochemical measurements The prepared electrodes consisted of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) mixed mechanically with the solvent N-methyl-2-pyrrolidone (NMP) and then coated onto aluminum foil. The electrodes were subsequently dried overnight in a vacuum oven at 60 ℃. After drying, the electrodes were calendered and cut into discs with a diameter of 12 mm. Electrochemical tests were conducted using CR2032 coin-type cells assembled in an argon-filled glove box. Sodium metal served as the anode, and the electrolyte was 1.0 M NaPF6 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) with a volume ratio of 98:2. Charge/discharge measurements were performed at different rates on a LAND battery test system between 2–4 V (vs. Na + /Na). Electrochemical impedance spectroscopy (EIS) tests were conducted on the same workstation over a frequency range of 100 kHz to 10 MHz with a sinusoidal amplitude of 5 mV. 3. Results and Discussion The crystalline phase evolution and structural characteristics of the multiphase composites were systematically investigated through XRD analysis. As illustrated in Fig. 1 , the diffraction patterns of the NNFM, NNFM5NNM, NNFM10NNM, and NNFM20NNM samples exhibit sharp Bragg peaks, indicating high crystallinity and low defect density. The pristine NNFM sample has a single-phase O3-type layered structure, as evidenced by its characteristic diffraction peaks matching the trigonal R3m space group (JCPDS No. 54–0887) [ 27 ] . This α-NaFeO 2 -type configuration confirms the successful synthesis of the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 core material with well-ordered Na + intercalation pathways. Controlled surface modification via P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 coating introduces distinct structural changes. With the designed Na 0.67 Ni 0.3 Mn 0.7 O 2 coating on the core part of NaNi 0.33 Fe 0.33 Mn 0.33 O 2 , P2 phases (JCPDS NO. 54–0894, P63/mmc space group) are observed, indicating that the P2 phase material has been successfully coated onto the substrate surface [ 28 ] . Furthermore, as the P2-NNM content of the heterojunction materials increases, the intensity of the O3 phase peaks decreases. In contrast, the additional diffraction peaks assigned to the P2-type structure of NNM start to become stronger. This variation not only demonstrates the successful coating of the P2 phase material but also reflects the controllability of the coating amount. The (003) peak of the O3 phase and the (002) peak of the P2 phase both shift in the lower angle direction, which is related mainly to the reduction in sodium storage within the material, implying an increase in the interlayer spacing of the transition metals and an increase in the lattice parameter c [ 29 ] . Notably, when the coating layer molar ratio reached 10%, the sample exhibited a significant shift, further validating the impact of the coating effect on the structure and lays the foundation for enhanced electrochemical performance. FE-SEM images of NNFM and NNFM10NNM samples (Fig. 2 a-d) reveal that both materials consist of spherical secondary particles with average diameters of 3–5 µm, indicating that the P2-phase coating does not alter the overall particle morphology. However, clear distinctions in surface texture are observed: the pristine O3-NNFM displays a smooth and dense surface (Fig. 2 a-b), while NNFM10NNM exhibits a wrinkled surface morphology (Fig. 2 c-d). This morphological evolution originates from the conformal coating of the P2-NNM phase on the O3-NNFM core, which generates interfacial strain during thermal treatment and induces surface reconstruction. [ 30 ] The intentionally constructed wrinkled architecture significantly amplifies the effective contact area at the cathode‒electrolyte interface (CEI), which facilitates homogeneous charge distribution and reduces the local current density during high-rate operation. [ 31 ] Moreover, the integrated P2-phase coating establishes continuous Na + diffusion paths along the O3/P2 heterojunction [ 32 ] . This synergistic combination of an enlarged CEI contact and optimized bulk transport pathways collectively contributes to the superior rate capability compared with that of pure O3-NNFM, as quantitatively verified by subsequent electrochemical analysis. To verify the heterostructure, TEM images (Fig. 2 e-f) confirm that the particle surface is uniformly coated with a P2-rich layer. A distinct and coherent interface between the P2 shell and the O3 core is clearly observed, supporting the formation of a core-shell heterojunction architecture. The electrochemical performance of the O3/P2 core-shell heterojunction cathode material was systematically evaluated through galvanostatic charge-discharge measurements in Na half-cells cycled between 2.0 and 4.0 V (vs. Na + /Na). Figure 3 (a) shows the evolution of the long-term cycling performance, revealing a critical dependence on the P2-phase coating ratio. The initial discharge capacities follow a controlled decreasing trend: 138.42 (pristine O3-NNFM), 136.5 (5% P2-NNM), 130 (10% P2-NNM) and 121.58 mAh g − 1 (20% P2-NNM), demonstrating a deliberate capacity sacrifice for structural stabilization. After 300 cycles at 0.1 C, the optimized NNFM10NNM cathode delivers a remarkable reversible capacity of 107.1 mAh g − 1 with 82.4% retention, outperforming both the lower-order coating counterparts (86.33 mAh g − 1 /63.2% for 5% P2) and the unmodified O3-NNFM baseline (79.65 mAh g − 1 /57.5% retention). Notably, the NNFM10NNM cathode maintains an exceptionally stable median discharge voltage of 3.1 V throughout 300 cycles (ΔV = 0.13 V), as shown in Fig. 3 (b). In stark contrast, the pure O3-NNFM suffers from severe voltage decay (2.92 V → 2.33 V), accompanied by rapid capacity fading from 138.42 to 79.65 mAh g − 1 . These results indicate that the O3/P2 core-shell heterojunction modification of multiphase composites significantly enhances the cycling retention rate, demonstrating that the protective P2 phase NNM coating layer plays a crucial role in preserving the structural integrity of the material surface and reducing the leaching of transition metals, thereby greatly improving the cycling stability of the modified material [ 18 , 33 ] . These improvements are attributed to the conformal P2-phase layer, which enhances surface integrity and suppresses transition metal dissolution. The open prismatic channels of the P2 phase are known to facilitate rapid Na + diffusion with minimal phase transitions, contributing to improved kinetics and long-term stability. [ 34 ] . To verify the effect of the O3/P2 core-shell heterojunction on the rapid charging and discharging capability of multiphase composites, we conducted rate performance tests on the materials. As the current rate gradually increased, as shown in Fig. 3 (c), the unmodified sample NNFM exhibited a rapid decline in reversible capacity, with reversible capacities of 138.0, 127.7, 120.0, 109.8, 101.8 and 77.4 mAh g − 1 at 0.1, 0.5, 1, 3, 5 and 10 C, respectively. At 10 C, the discharge capacity was only 56.1% of that at 0.1 C, as shown in Fig. 3 (d). In contrast, the modified NNFM10NNM demonstrated superior rate electrochemical performance, as shown in Fig. 3 (d), with reversible capacities of 129.1, 124.8, 118.8, 113.9, 108.8 and 103.4 mAh g − 1 at 0.1, 0.5, 1, 3, 5 and 10 C, respectively. At 10 C, the discharge capacity was 80.1% of that at 0.1 C, indicating fast sodium ion transport kinetics. This is primarily attributed to the P2 phase material on the surface of the O3/P2 core-shell heterojunction cathode, which, compared with the internal O3 phase material, has direct sodium ion transport characteristics and can act as a fast ion conductor between the internal O3 phase material and the electrolyte [ 25 , 35 ] . The superior long-term rapid charging and discharging performance of the typical NNFM10NNM cathode was further demonstrated by comparative charge-discharge profiles. As shown in Fig. 4 (a), NNFM10NNM displays smoother initial voltage curves than the pure O3-NNFM, indicating that the degree of complex Na-vacancy ordering of the material is suppressed after surface modification with the P2 phase coating, which promotes the diffusive migration of sodium ions, reduces the charge transfer resistance, and enhances the diffusion kinetics performance of the multiphase material [ 36 ] . In addition, in Fig. 4 (b), the charge-discharge curves of pure O3-NNFM show a voltage drop of 790 mV after 300 cycles, and the voltage drop of NNFM10NNM is not very obvious and still maintains a shape similar to the initial charge-discharge curve, demonstrating low polarization and excellent structural reversibility during the cycling process [ 37 , 38 ] . This may be related to the fast ion channels provided by the P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 on the surface of the multiphase material. To elucidate the sodium-ion transport kinetics and interfacial characteristics of the materials during prolonged cycling, electrochemical impedance spectroscopy (EIS) measurements were conducted on NNFM and NNFM10NNM electrodes before and after 300 cycles at 1C within the voltage window of 2.0–4.0 V (vs. Na⁺/Na), as shown in Fig. 5 . The Nyquist fitting equivalent circuit mainly consists of the high-frequency intercept R S (representing the ohmic resistance, arising from the electrolyte resistance and cell components), the high-frequency semicircle R SEI (representing the solid electrolyte interphase film resistance, which is related to the surface film resistance associated with sodium ion transport through the CEI film), the mid-frequency semicircle R ct (representing the charge transfer resistance during charging and discharging, related to redox kinetics) and the low-frequency slope of the Warburg impedance (representing the diffusion resistance, related to the sodium ion transport within the material, reflecting the sodium ion diffusion coefficient) [ 16 , 22 ] . Additionally, considering the effects of the particle size distribution, surface roughness, and electrode bending, the constant phase elements CPE1 and CPE2, which represent capacitors, were also included in the Nyquist fitting equivalent circuit. Before cycling, NNFM10NNM exhibits a notably lower R SEI than pristine NNFM, indicating a thinner and more stable cathode-electrolyte interphase (CEI) layer. This can be attributed to the conformal P2-phase coating, which serves as a protective barrier against direct contact between the O3 core and the electrolyte, effectively suppressing side reactions and electrolyte decomposition. [ 21 ] After 300 cycles, the differences between the two samples become more pronounced. As summarized in Table 1 , the R SEI and R ct values of NNFM are 2 and 3 times greater than those of NNFM10NNM, respectively. This significant difference originates from the continuous deintercalation and intercalation of sodium ions during charging and discharging. The uniform coating layer of the P2 phase on the surface of NNFM10NNM blocks direct contact between the internal active components and the electrolyte, reducing the dissolution of transition metals and the decomposition of the electrolyte [ 39 ] . This also slows the formation of the surface CEI film and inhibits irreversible phase changes on the material surface, which cause structural collapse and block sodium ion transport channels [ 40 ] . This synergistic effect accounts for the superior cycling durability and rate performance of the NNFM10NNM sample, as demonstrated in previous electrochemical tests. Table 1 Electrochemical impedance results for the NNFM10NNM and pure O3-NNFM cathodes. Sample Cycle number R s , Ω R SEI , Ω R ct , Ω O3-NNFM 0 4.6 / 202.1 300 5.2 310.7 551.6 NNFM10NNM 0 3.5 / 145.3 300 4.3 105.1 273.3 To address the critical issue of air instability in O3-type layered cathodes [ 5 , 11 , 24 ] , we evaluated the environmental tolerance of pristine O3-NNFM and P2-phase-coated NNFM10NNM through controlled air-aging tests. Although previous studies have reported O3/P2 composite structures, the random phase distribution in such systems leaves O3 regions susceptible to moisture-induced degradation. [ 41 , 42 ] In this work, we employed a conformal P2-phase coating to construct a core–shell heterostructure aimed at mitigating ambient degradation pathways. The O3-NNFM and NNFM10NNM samples were placed in an environment containing water and carbon dioxide with a humidity of 55% for 24 hours to evaluate the electrochemical performance of the aged cathodes and verify the improvement in the air stability of the NNFM10NNM cathode. As shown in Fig. 6 (a), the reversible capacity of the aged O3-NNFM cathode at 0.1 C was 112.4 mA hg − 1 , which was equivalent to only 81.2% of the original O3-NNFM. In contrast, owing to the protective effect of the P2-NNM coating layer, the aged NNFM10NNM cathode provided a high discharge capacity of 126.1 mA hg − 1 , equivalent to 97% of that provided by the original electrode, confirming the improvement in its air stability. Similarly, the aged NNFM10NNM exhibited good rate performance (98.5 mA g − 1 at 10C). In comparison, the aged O3-NNFM showed poor Na storage performance. Figure 6 (b) shows the long-term cycling performance of the O3-NNFM and NNFM10NNM materials at a 1C rate. After being exposed to a humid environment, the capacity retention rate of O3-NNFM after 300 cycles was only 27.7%. However, after a P2-type coating protective layer is constructed, the capacity retention rate of NNFM10NNM reaches 78.2% after 300 cycles, which is much greater than that of the O3-NNFM material, demonstrating excellent air stability. To elucidate the structural evolution responsible for these differences, ex situ XRD analysis was conducted on the aged samples. For O3-NNFM, the (003) peak of the initial hexagonal O3 structure split into two diffraction peaks, corresponding to the O3 phase with an expanded c-axis and the monoclinic O’3 phase structure with structural distortion (Fig. 6 (c)) [ 43 ] . The instability of O3-NNFM is due to the reaction between sodium ions and the intercalated water/carbon dioxide molecules during the exposure process, resulting in the formation of a hydrated phase and sodium carbonate between the layers, which leads to an increase in the interlayer spacing [ 34 ] . These alkaline compounds do not possess electrochemical activity. This structural transformation and the formation of surface sodium carbonate impurities lead to severe structural degradation and rapid capacity decay during the cycling process, hindering the practical application of the O3-NNFM cathode. In contrast, in Fig. 6 (d), air aging did not significantly change the structure of NNFM10NNM, indicating that the surface-coated P2 phase has a protective effect. Together, these results demonstrate that the designed core–shell O3/P2 heterostructure provides a robust pathway to simultaneously improve the air stability and long-term electrochemical performance of sodium layered cathode materials. 4. Conclusions In this work, a core-shell heterostructured cathode material (NNFMxNNM) composed of O3 and P2 phases was successfully synthesized via coprecipitation. The enhanced Na storage performance of the heterojunction cathode is attributed to the synergistic effect of the multiphase structure, where the O3 phase provides adequate initial Na storage and the P2 phase acts as a protective layer. This unique architecture not only mitigates phase transitions and lattice stress but also promotes rapid sodium-ion diffusion and efficient charge transfer. Electrochemical tests revealed a significant improvement in high-rate performance, with the 10C discharge capacity increasing from 77.4 mAh g − 1 (pure O3-NNFM) to 103.4 mAh g − 1 after P2-phase coating. Additionally, the P2 layer effectively protects the O3 phase from air and electrolyte exposure, contributing to long-term cycling durability. As a result, the optimized cathode retained 82.4% of its initial capacity after 300 cycles at 0.1 C. These findings highlight the potential of core-shell O3/P2 heterojunction engineering for high-performance and air-stable sodium-ion battery cathodes. Declarations Availability Our study is not a clinical trial. Consent to Publish declaration: not applicable. Consent to Participate declaration: not applicable. Ethics declaration: not applicable. Data availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. Acknowledgments The authors kindly acknowledge the support by the DMHL20230512SIBC of Hualu New Materials Technology Co., LTD. Author Contribution Hao Xie: Conceptualization, Funding acquisition, Writing – original draft. Hao Xie, Pei Ding, Chuxin Cui: Investigation, Methodology, Data curation. ZhengMing Sun: Supervision, Writing – review & editing. All authors discussed the results and approved the final manuscript. References BAO Z, LU C, LIU Q et al. An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries [J]. Nat Commun, 2024, 15(1): 1934. EL AFIA S, CANO A, ARéVALO P et al. Rechargeable Li-Ion Batteries, Nanocomposite Materials and Applications [J]. Batteries, 2024, 10(12). USISKIN R, LU Y, POPOVIC J, et al. Fundamentals, status and promise of sodium-based batteries [J]. Nat Reviews Mater. 2021;6(11):1020–35. YANG H, WANG D, LIU Y, et al. Improvement of cycle life for layered oxide cathodes in sodium-ion batteries [J]. Energy Environ Sci. 2024;17(5):1756–80. XIAO, Y, ABBASI N M, ZHU Y F et al. Layered Oxide Cathodes Promoted by Structure Modulation Technology for Sodium-Ion Batteries [J]. Adv Funct Mater, 2020, 30(30). HWANG J Y, MYUNG S T, SUN Y K. Sodium-ion batteries: present and future [J]. Chem Soc Rev. 2017;46(12):3529–614. HASA I, BUCHHOLZ D. PASSERINI S, High Performance Na0.5[Ni0.23Fe0.13Mn0.63]O2 Cathode for Sodium-Ion Batteries [J]. Adv Energy Mater, 2014, 4(15). WANG PF, YOU Y, YIN Y X et al. Layered Oxide Cathodes for Sodium-Ion Batteries: Phase Transition, Air Stability, and Performance [J]. Adv Energy Mater, 2017, 8(8). DELMAS C, FOUASSIER C. HAGENMULLER P. Structural classification and properties of the layered oxides [J]. Physica B + c, 1980, 99(1–4): 81 – 5. YUAN X G, GUO Y J GANL et al. A Universal Strategy toward Air-Stable and High‐Rate O3 Layered Oxide Cathodes for Na‐Ion Batteries [J]. Adv Funct Mater, 2022, 32(17). ZHAO A, FANG Y, AI X et al. Mixed polyanion cathode materials: Toward stable and high-energy sodium-ion batteries [J]. J Energy Chem, 2021, 60(635 – 48). GONZALO E, ZARRABEITIA M. Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries [J]. Energy Storage Mater. 2021;34:682–707. ZHANG L, WANG C, LIU Y et al. Suppressing interlayer-gliding and Jahn-Teller effect in P2-type layered manganese oxide cathode via Mo doping for sodium-ion batteries [J]. Chem Eng J, 2021, 426. BERTHELOT R, CARLIER D. Electrochemical investigation of the P2-NaxCoO2 phase diagram [J]. Nat Mater. 2011;10(1):74–80. TANG K, WANG Y, ZHANG X, et al. High-performance P2-Type Fe/Mn-based oxide cathode materials for sodium-ion batteries [J]. Electrochim Acta. 2019;312:45–53. ZHOU D, HUANG W, LV X et al. A novel P2/O3 biphase Na0.67Fe0.425Mn0.425Mg0.15O2 as cathode for high-performance sodium-ion batteries [J]. J Power Sources, 2019, 421(147 – 55). WANG JE, KIM H, JUNG Y H, et al. Designing High Energy Sodium-Ion Battery Cathodes by Utilizing P2/O3 Biphasic Structure and Lithium Honeycomb Ordering [J]. Small. 2021;17(30):e2100146. KATCHO N A, CARRASCO J. Origins of bistability and Na ion mobility difference in P2-and O3‐Na2/3Fe2/3Mn1/3O2 cathode polymorphs [J]. Adv Energy Mater. 2017;7(1):1601477. CHENG Z, FAN X Y, YU L, et al. A Rational Biphasic Tailoring Strategy Enabling High-Performance Layered Cathodes for Sodium-Ion Batteries [J]. Angew Chem Int Ed Engl. 2022;61(19):e202117728. JI H, ZHAI J, CHEN G et al. Surface Engineering Suppresses the Failure of Biphasic Sodium Layered Cathode for High Performance Sodium-Ion Batteries [J]. Adv Funct Mater, 2021, 32(12). WANG H, LIU Q, LIU Y. Multi-ion doped P2/O3 biphasic layered oxides cathodes NaxNi0.2Mn0.55Cu0.14TiyZnzO2 for sodium-ion batteries [J]. J Alloys Compd, 2023, 962. JIN J, LIU Y, PANG X, et al. A comprehensive understanding of the anionic redox chemistry in layered oxide cathodes for sodium-ion batteries [J]. Sci China Chem. 2020;64(3):385–402. ZHANG Q, GU Q F, LI Y et al. Surface Stabilization of O3-type Layered Oxide Cathode to Protect the Anode of Sodium Ion Batteries for Superior Lifespan [J]. iScience, 2019, 19(244 – 54). LAMB J. Surface-modified Na (Ni0. 3Fe0. 4Mn0. 3) O2 cathodes with enhanced cycle life and air stability for sodium-ion batteries [J]. ACS Appl Energy Mater. 2021;4(10):11735–42. ZHANG B, ZHAO Y, LI M, et al. Amorphous Aluminum Oxide-Coated NaFe(0.33)Ni(0.33)Mn(0.33)O(2) Cathode Materials: Enhancing Interface Charge Transfer for High-Performance Sodium-Ion Batteries [J]. ACS Appl Mater Interfaces; 2023. LIU Y, FANG X, ZHANG A, et al. Layered P2-Na2/3[Ni1/3Mn2/3]O2 as high-voltage cathode for sodium-ion batteries: The capacity decay mechanism and Al2O3 surface modification [J]. Nano Energy. 2016;27:27–34. KIM D, LEE E, SLATER M et al. Layered Na[Ni1/3Fe1/3Mn1/3]O2 cathodes for Na-ion battery application [J]. Electrochem Commun, 2012, 18(66 – 9). WANG X, LUO J, DOU W et al. Stimulating the redox capacity by multi-ion substitution for P2-type sodium-ion battery cathodes [J]. Mater Today Energy, 2024, 43. CHEN C, HUANG W, LI Y et al. P2/O3 biphasic Fe/Mn-based layered oxide cathode with ultrahigh capacity and great cyclability for sodium ion batteries [J]. Nano Energy, 2021, 90. HOU P, DONG M, LI F et al. Recent Advances in Cathode Materials with Core–Shell Structures and Concentration Gradients for Advanced Sodium-Ion Batteries [J]. Adv Funct Mater, 2024, 34(49). ZHU W, XU S, YANG S et al. Advanced Electrode Materials for Low-Temperature Na Storage [J]. Adv Funct Mater, 2024, 35(14). GABRIEL E, MA C, GRAFF K et al. Heterostructure engineering in electrode materials for sodium-ion batteries: Recent progress and perspectives [J]. eScience, 2023, 3(5). DUAN Y, MA Z H, HUANG Y Y et al. Synergistic effect of P2, O3 phase on biphasic layered oxide with enhanced electrochemical performance for sodium storage [J]. J Colloid Interface Sci, 2025, 682(715 – 24). ZUO W, QIU J, LIU X, et al. The stability of P2-layered sodium transition metal oxides in ambient atmospheres [J]. Nat Commun. 2020;11(1):3544. ZUO W, REN F, LI Q et al. Insights of the anionic redox in P2–Na0.67Ni0.33Mn0.67O2 [J]. Nano Energy, 2020, 78. YANG Q, WANG P F, GUO J Z, et al. Advanced P2-Na(2/3)Ni(1/3)Mn(7/12)Fe(1/12)O(2) Cathode Material with Suppressed P2-O2 Phase Transition toward High-Performance Sodium-Ion Battery [J]. ACS Appl Mater Interfaces. 2018;10(40):34272–82. YUAN M, LIU H, RAN F. Fast-charging cathode materials for lithium & sodium ion batteries [J]. Mater Today, 2023, 63(360 – 79). DENG YP, WU Z G, LIANG R et al. Layer-Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries [J]. Adv Funct Mater, 2019, 29(19). LIANG X, YU T-Y, RYU H-H et al. Hierarchical O3/P2 heterostructured cathode materials for advanced sodium-ion batteries [J]. Energy Storage Mater, 2022, 47(515 – 25). JU J-H, RYU K-S. Synthesis and electrochemical performance of Li(Ni0.8Co0.15Al0.05)0.8(Ni0.5Mn0.5)0.2O2 with core–shell structure as cathode material for Li-ion batteries [J]. J Alloys Compd. 2011;509(30):7985–92. SUN Y-K. Direction for Commercialization of O3-Type Layered Cathodes for Sodium-Ion Batteries [J]. ACS Energy Lett. 2020;5(4):1278–80. HWANG J-Y, MYUNG S-T, CHOI J U, et al. Resolving the degradation pathways of the O3-type layered oxide cathode surface through the nano-scale aluminum oxide coating for high-energy density sodium-ion batteries [J]. J Mater Chem A. 2017;5(45):23671–80. YAO H R, WANG P F, GONG Y, et al. Designing Air-Stable O3-Type Cathode Materials by Combined Structure Modulation for Na-Ion Batteries [J]. J Am Chem Soc. 2017;139(25):8440–3. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted 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. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6950664","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":478412165,"identity":"8c5c55d8-4d26-48ce-9d51-55426e79651d","order_by":0,"name":"Hao Xie","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Hao","middleName":"","lastName":"Xie","suffix":""},{"id":478412166,"identity":"a98c5efb-0d3d-4fb9-8a14-6ee81d25137c","order_by":1,"name":"Pei Ding","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Pei","middleName":"","lastName":"Ding","suffix":""},{"id":478412167,"identity":"a2d2d96c-02a8-45eb-9146-6ec1a667e6e6","order_by":2,"name":"Chuxin Cui","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Chuxin","middleName":"","lastName":"Cui","suffix":""},{"id":478412168,"identity":"4acb00ef-ed8b-41d0-9a63-15145095de8b","order_by":3,"name":"ZhengMing Sun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtUlEQVRIiWNgGAWjYNCCCgglQYKWMyRrYWwjRYv8tOPPpAvn1ckZHGA+eJuHwS6PsAWzc8ykZ247bGxwgC3ZmochuZigFmbpHDZp3m0HEjcc4DGT5mE4kNhASAubdPozad45dfUbDvB/I04Lj3SCmTRvA3OCwQEeNuK0SEjnGFvzHDtsOPMwm7HlHINkwlrkZ6c/vM1TUyfPd7z54Y03FXaEtSAAM4gwIF79KBgFo2AUjAI8AADvQzIYwjv2WQAAAABJRU5ErkJggg==","orcid":"","institution":"Southeast University","correspondingAuthor":true,"prefix":"","firstName":"ZhengMing","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2025-06-22 16:38:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6950664/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6950664/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85862235,"identity":"ac6a1c32-66ea-47c0-a6aa-4d106c2f323d","added_by":"auto","created_at":"2025-07-02 12:26:41","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":173270,"visible":true,"origin":"","legend":"\u003cp\u003eStructural characterization of the as-prepared cathode materials and powder XRD patterns of pure O3-NNFM, NNFM5NNM, NNFM10NNM and NNFM20NNM.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/bac8b6e1fff39c5e378b96d4.png"},{"id":85860623,"identity":"4d927a0e-2329-42f7-ae97-6d03ab1120da","added_by":"auto","created_at":"2025-07-02 12:18:41","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":694673,"visible":true,"origin":"","legend":"\u003cp\u003eMorphology and surface characteristics of the as-prepared cathode materials: SEM images of (a, b) pure O3-NNFM and (c, d) NNFM10NNM. TEM images of (e,f) NNFM10NNM.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/f169bc7e2d7e0f799a9265b3.png"},{"id":85859279,"identity":"fde40fb1-9ce2-4b7c-a38a-4252ff15cb72","added_by":"auto","created_at":"2025-07-02 12:02:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":384191,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of half-cells with pure O3-NNFM, NNFM5NNM, NNFM10NNM and NNFM20NNM (1C=120 mAh g\u003csup\u003e-1\u003c/sup\u003e, 2.0-4.0 V, 25 ℃). (a) Long-term cycle life achieved by half cells at 0.1 C. (b) The variation trend of the midpoint voltage. (c) Rate capabilities of half cells with various cathodes. (d) Normalized capacities at different rates.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/a29be5df1277ac4a0717aafb.png"},{"id":85860187,"identity":"d9da204b-e681-4574-9a72-59320e95b2d7","added_by":"auto","created_at":"2025-07-02 12:10:41","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":168157,"visible":true,"origin":"","legend":"\u003cp\u003eCharge/discharge curves for (a) NNFM10NNM and (b) pure O3-NNFM (1C=120 mAh g\u003csup\u003e-1\u003c/sup\u003e, 2.0-4.0 V, 25℃) at different numbers of cycles: 1st, 100th, 200th, and 300th.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/153f6597197fd295c19c7348.png"},{"id":85859273,"identity":"f4bbd563-4f0e-4822-bebf-1dce5d5da9a7","added_by":"auto","created_at":"2025-07-02 12:02:41","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":126716,"visible":true,"origin":"","legend":"\u003cp\u003eNyquist plots of cells with (a) NNFM10NNM and (b) pure O3-NNFM cathodes before cycling and after 300 cycles.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/97fde7b672f05b5ec411d9ad.png"},{"id":85859280,"identity":"38129d3c-a9d8-4aa7-bd80-c327f8802905","added_by":"auto","created_at":"2025-07-02 12:02:41","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":316515,"visible":true,"origin":"","legend":"\u003cp\u003eStability of the electrochemical performance in humid air with a humidity of 55% for 24 hours. (a) Rate capabilities of NNFM10NNM and pure O3-NNFM at 0.1C, 1C, 5C and 10C. (b) Long-term cycle life achieved by half cells at 1 C. XRD pattern variations of the pure (c) O3-NNFM and (d) NNFM10NNM cathodes before and after exposure to air.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/697e9a57d6e945df0a82aa14.png"},{"id":90967874,"identity":"de444655-024f-4a1b-8afa-2ea3efee8ab0","added_by":"auto","created_at":"2025-09-10 06:54:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2381213,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6950664/v1/66f160bc-b761-43ee-a281-e2e302c28835.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance Sodium-Ion Batteries","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRechargeable batteries are the predominant energy storage technology, playing an integral role in enhancing modern life \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. Presently, lithium-ion batteries (LIBs) are the preferred power source for a vast array of commercial electronic devices and electric vehicles \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. However, the limited lithium reserves, uneven geographical distribution, and high cost of lithium extraction and recycling have raised concerns regarding the sustainability and long-term viability of LIB technology \u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSodium, a geochemically abundant alkali metal with ubiquitous crustal distribution (2.36 wt% vs. 0.0017 wt% for lithium) and inherent cost-effectiveness, has emerged as a promising candidate for next-generation battery systems \u003csup\u003e[\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. The performance of SIBs is predominantly determined by the cathode materials. Transition metal layered oxide materials of the form Na\u003csub\u003ex\u003c/sub\u003eTMO\u003csub\u003e2\u003c/sub\u003e (where TM represents elements such as Ni, Co, Mg, Fe, Mn, etc.) are widely studied owing to their high energy density and high redox potential \u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Based on the coordination environment of sodium ions, these layered oxides are typically categorized as P2-type or O3-type according to the Delmas classification, with the numeral following P or O referring to the repetition period of the transition metal layer formed by edge-sharing MO6 octahedra. \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e In O3-type structures, sodium ions reside in octahedral sites between MO\u003csub\u003e6\u003c/sub\u003e slabs, allowing for high theoretical capacity \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. However, sodium ions need to overcome high energy barriers when passing through tetrahedral interstices, and most O3-type cathodes exhibit poor electrochemical kinetics and complex phase transition processes, leading to rapid capacity decay \u003csup\u003e[\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. In contrast, P2-phase Na\u003csub\u003ex\u003c/sub\u003eTMO\u003csub\u003e2\u003c/sub\u003e possesses open prismatic channels, which contribute to fast ion conductivity and greater structural stability. Nevertheless, sodium deficiency may lead to abnormal initial coulombic efficiency, necessitating anode pretreatment or the use of sodium supplements, which limits practical applications \u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo overcome the individual limitations of P2 and O3 phases, recent studies have focused on constructing multiphase P2/O3 composite cathodes. These hybrid structures aim to combine the high capacity of O3-type materials with the superior structural stability and ion conductivity of P2-type phases. \u003csup\u003e[\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. For instance, Guo et al. \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e synthesized a P2/O3-Na\u003csub\u003e0.76\u003c/sub\u003eNi\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.5\u003c/sub\u003eFe\u003csub\u003e0.1\u003c/sub\u003eTi\u003csub\u003e0.07\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e composite by mechanically milling and sintering P2-Na\u003csub\u003e2/3\u003c/sub\u003eNi\u003csub\u003e1/3\u003c/sub\u003eMn\u003csub\u003e0.57\u003c/sub\u003eTi\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e with O3-NaNi\u003csub\u003e1/3\u003c/sub\u003eFe\u003csub\u003e1/3\u003c/sub\u003eMn\u003csub\u003e1/3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The resulting material showed mitigated phase transitions and improved cycling performance. Similarly, Xiao et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e tuned sintering temperatures of a sol-gel-derived P2-type precursor to create a P2/P3 composite, achieving enhanced cycling stability. Wang et al. \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e developed multi-element-doped P2/O3 cathodes with prolonged lifespan over 500 cycles, highlighting the benefits of phase integration and cation substitution.\u003c/p\u003e \u003cp\u003eDespite these advances, one critical challenge remains: the intrinsic instability of O3-type materials in ambient and electrochemical environments. Within P2/O3 composites, the non-uniform distribution of phases often leaves O3 domains exposed to atmospheric moisture and reactive electrolytes. This exposure accelerates cathode degradation via (i) spontaneous surface reactions with H₂O/CO₂, forming sodium hydroxide and carbonate species, and (ii) transition metal dissolution during repeated sodiation/desodiation cycles \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. These synergistic degradation pathways substantially compromise the structural integrity and electrochemical performance of O3-type cathodes while simultaneously increasing material storage requirements and associated costs. To mitigate these interfacial instabilities, surface modification techniques have been widely explored. Atomic-layer coatings, such as AlPO\u003csub\u003e4\u003c/sub\u003e \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e, NaPO3 \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e, and Al2O3 \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e, have proven effective in protecting O3-type cathodes by serving as barriers that isolate active materials from air and electrolyte contact while simultaneously providing mechanical support during volume changes. Additionally, nanoscale structural engineering\u0026mdash;such as particle downsizing and morphology control\u0026mdash;can further enhance material durability by shortening diffusion paths and relieving strain accumulation. \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. Despite these promising strategies, most efforts have focused on physical mixtures or simple coatings of multiphase materials. Few studies have explored rationally designed core\u0026ndash;shell heterostructures that spatially organize P2 and O3 phases to synergistically optimize both electrochemical performance and environmental stability.\u003c/p\u003e \u003cp\u003eLeveraging the synergistic benefits of multiphase architecture and interfacial engineering, we propose a novel core-shell heterostructured cathode design through phase-selective surface modification. An O3-type NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (O3-NNFM) core with high theoretical capacity was synthesized, followed by in situ growth of a P2-type Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (P2-NNM) protective layer through controlled thermal treatment. This hierarchical O3/P2 configuration establishes a chemically stabilized heterointerface while maintaining ionic transport channels. Structural and morphological evolution was systematically investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses, confirming the synergistic relationship between the O3 core and P2 shell. Electrochemical evaluation revealed remarkable performance enhancement: the optimized O3-NNFM@P2-NNM composite delivered a specific discharge capacity of 130 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 0.1C (94% of the initial capacity of the pristine O3-NNFM was 138.42 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), demonstrating effective surface protection without significant capacity sacrifice. More critically, the heterostructured cathode exhibited a superior capacity retention of 82.4% after 300 cycles (vs. 57.5% for pure O3-NNFM). Through further rate performance tests, they exhibit reversible capacities of 77.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 103.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 10 C, which are 56.1% and 80.1% of the discharge capacity at 0.1 C ([email protected] mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and [email protected] mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). This represents a 25% improvement in cycle life and a 24% enhancement in high-rate performance compared with the unmodified counterpart.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Synthesis of O3-NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eThe precursor for the O3-NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e cathode was synthesized via a coprecipitation method. Initially, a multimetal solution containing 2 M NiSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO, and MnSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO in a 1:1:1 ratio was obtained. Subsequently, NaOH solution (4 M) and NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO solution (10 M) were introduced into a 10 L reactor protected by N\u003csub\u003e2\u003c/sub\u003e. The precipitate, Ni\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e, denoted as NFM(OH)\u003csub\u003e2\u003c/sub\u003e, formed. NFM(OH)\u003csub\u003e2\u003c/sub\u003e was then mixed with Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e at a molar ratio of 1:1.02 and calcined at 850 ℃ for 24 hours to yield the product powder NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, designated NNFM.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of O3/P2 multiphase material\u003c/h2\u003e \u003cp\u003eThe NNFM cathode material was coated with P2-Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e via a wet chemical method. The prepared O3-NNFM particles were mixed with the required Ni(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e, Mn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e, and CH\u003csub\u003e3\u003c/sub\u003eCOONa in a specific ratio in an ethanol solvent and magnetically stirred for 30 minutes to obtain a homogeneous mixture. The molar ratio of the mixture to O3-NNFM particles was 5, 10 and 20 mol% with reference to the total mass of the NNFM. Then, oxalic acid solution was added dropwise, and the mixture was transferred into a Teflon-lined stainless-steel autoclave. The autoclave was heated at 180 ℃ for 12 hours to form an oxalate coprecipitate. The solvent was evaporated overnight at 100 ℃, followed by precalcination in an oxygen atmosphere at 500 ℃ for 8 hours and calcination at 850\u0026deg;C for 15 hours to produce P2-Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e-coated (designated NNM) O3/P2 multiphase material. According to the difference in the P2 phase molar ratio, the prepared O3/P2 multiphase materials were denoted as NNFM5NNM, NNFM10NNM and NNFM20NNM.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterizations\u003c/h2\u003e \u003cp\u003eThe products were analyzed via a D/Max-RA X-ray diffractometer with a Cu Kα radiation source (λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;) at a scanning rate of 2\u0026deg;/min for X-ray diffraction (XRD) analysis. The morphology and size of the products were examined via field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) at 10 kV and transmission electron microscopy (TEM, JEM-2100, JEOL) at an acceleration voltage of 200 kV.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Electrochemical measurements\u003c/h2\u003e \u003cp\u003eThe prepared electrodes consisted of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) mixed mechanically with the solvent N-methyl-2-pyrrolidone (NMP) and then coated onto aluminum foil. The electrodes were subsequently dried overnight in a vacuum oven at 60 ℃. After drying, the electrodes were calendered and cut into discs with a diameter of 12 mm. Electrochemical tests were conducted using CR2032 coin-type cells assembled in an argon-filled glove box. Sodium metal served as the anode, and the electrolyte was 1.0 M NaPF6 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) with a volume ratio of 98:2. Charge/discharge measurements were performed at different rates on a LAND battery test system between 2\u0026ndash;4 V (vs. Na\u003csup\u003e+\u003c/sup\u003e/Na). Electrochemical impedance spectroscopy (EIS) tests were conducted on the same workstation over a frequency range of 100 kHz to 10 MHz with a sinusoidal amplitude of 5 mV.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003eThe crystalline phase evolution and structural characteristics of the multiphase composites were systematically investigated through XRD analysis. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the diffraction patterns of the NNFM, NNFM5NNM, NNFM10NNM, and NNFM20NNM samples exhibit sharp Bragg peaks, indicating high crystallinity and low defect density. The pristine NNFM sample has a single-phase O3-type layered structure, as evidenced by its characteristic diffraction peaks matching the trigonal R3m space group (JCPDS No. 54\u0026ndash;0887) \u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. This α-NaFeO\u003csub\u003e2\u003c/sub\u003e-type configuration confirms the successful synthesis of the O3-NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e core material with well-ordered Na\u003csup\u003e+\u003c/sup\u003e intercalation pathways. Controlled surface modification via P2-Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e coating introduces distinct structural changes. With the designed Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e coating on the core part of NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, P2 phases (JCPDS NO. 54\u0026ndash;0894, P63/mmc space group) are observed, indicating that the P2 phase material has been successfully coated onto the substrate surface \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Furthermore, as the P2-NNM content of the heterojunction materials increases, the intensity of the O3 phase peaks decreases. In contrast, the additional diffraction peaks assigned to the P2-type structure of NNM start to become stronger. This variation not only demonstrates the successful coating of the P2 phase material but also reflects the controllability of the coating amount. The (003) peak of the O3 phase and the (002) peak of the P2 phase both shift in the lower angle direction, which is related mainly to the reduction in sodium storage within the material, implying an increase in the interlayer spacing of the transition metals and an increase in the lattice parameter c \u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Notably, when the coating layer molar ratio reached 10%, the sample exhibited a significant shift, further validating the impact of the coating effect on the structure and lays the foundation for enhanced electrochemical performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFE-SEM images of NNFM and NNFM10NNM samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-d) reveal that both materials consist of spherical secondary particles with average diameters of 3\u0026ndash;5 \u0026micro;m, indicating that the P2-phase coating does not alter the overall particle morphology. However, clear distinctions in surface texture are observed: the pristine O3-NNFM displays a smooth and dense surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-b), while NNFM10NNM exhibits a wrinkled surface morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec-d). This morphological evolution originates from the conformal coating of the P2-NNM phase on the O3-NNFM core, which generates interfacial strain during thermal treatment and induces surface reconstruction. \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe intentionally constructed wrinkled architecture significantly amplifies the effective contact area at the cathode‒electrolyte interface (CEI), which facilitates homogeneous charge distribution and reduces the local current density during high-rate operation. \u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e Moreover, the integrated P2-phase coating establishes continuous Na\u003csup\u003e+\u003c/sup\u003e diffusion paths along the O3/P2 heterojunction \u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. This synergistic combination of an enlarged CEI contact and optimized bulk transport pathways collectively contributes to the superior rate capability compared with that of pure O3-NNFM, as quantitatively verified by subsequent electrochemical analysis. To verify the heterostructure, TEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-f) confirm that the particle surface is uniformly coated with a P2-rich layer. A distinct and coherent interface between the P2 shell and the O3 core is clearly observed, supporting the formation of a core-shell heterojunction architecture.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical performance of the O3/P2 core-shell heterojunction cathode material was systematically evaluated through galvanostatic charge-discharge measurements in Na half-cells cycled between 2.0 and 4.0 V (vs. Na\u003csup\u003e+\u003c/sup\u003e/Na). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows the evolution of the long-term cycling performance, revealing a critical dependence on the P2-phase coating ratio. The initial discharge capacities follow a controlled decreasing trend: 138.42 (pristine O3-NNFM), 136.5 (5% P2-NNM), 130 (10% P2-NNM) and 121.58 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (20% P2-NNM), demonstrating a deliberate capacity sacrifice for structural stabilization. After 300 cycles at 0.1 C, the optimized NNFM10NNM cathode delivers a remarkable reversible capacity of 107.1 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 82.4% retention, outperforming both the lower-order coating counterparts (86.33 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e/63.2% for 5% P2) and the unmodified O3-NNFM baseline (79.65 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e/57.5% retention). Notably, the NNFM10NNM cathode maintains an exceptionally stable median discharge voltage of 3.1 V throughout 300 cycles (ΔV\u0026thinsp;=\u0026thinsp;0.13 V), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). In stark contrast, the pure O3-NNFM suffers from severe voltage decay (2.92 V \u0026rarr; 2.33 V), accompanied by rapid capacity fading from 138.42 to 79.65 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These results indicate that the O3/P2 core-shell heterojunction modification of multiphase composites significantly enhances the cycling retention rate, demonstrating that the protective P2 phase NNM coating layer plays a crucial role in preserving the structural integrity of the material surface and reducing the leaching of transition metals, thereby greatly improving the cycling stability of the modified material \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThese improvements are attributed to the conformal P2-phase layer, which enhances surface integrity and suppresses transition metal dissolution. The open prismatic channels of the P2 phase are known to facilitate rapid Na\u003csup\u003e+\u003c/sup\u003e diffusion with minimal phase transitions, contributing to improved kinetics and long-term stability. \u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. To verify the effect of the O3/P2 core-shell heterojunction on the rapid charging and discharging capability of multiphase composites, we conducted rate performance tests on the materials. As the current rate gradually increased, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c), the unmodified sample NNFM exhibited a rapid decline in reversible capacity, with reversible capacities of 138.0, 127.7, 120.0, 109.8, 101.8 and 77.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 0.1, 0.5, 1, 3, 5 and 10 C, respectively. At 10 C, the discharge capacity was only 56.1% of that at 0.1 C, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d). In contrast, the modified NNFM10NNM demonstrated superior rate electrochemical performance, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d), with reversible capacities of 129.1, 124.8, 118.8, 113.9, 108.8 and 103.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 0.1, 0.5, 1, 3, 5 and 10 C, respectively. At 10 C, the discharge capacity was 80.1% of that at 0.1 C, indicating fast sodium ion transport kinetics. This is primarily attributed to the P2 phase material on the surface of the O3/P2 core-shell heterojunction cathode, which, compared with the internal O3 phase material, has direct sodium ion transport characteristics and can act as a fast ion conductor between the internal O3 phase material and the electrolyte \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe superior long-term rapid charging and discharging performance of the typical NNFM10NNM cathode was further demonstrated by comparative charge-discharge profiles. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), NNFM10NNM displays smoother initial voltage curves than the pure O3-NNFM, indicating that the degree of complex Na-vacancy ordering of the material is suppressed after surface modification with the P2 phase coating, which promotes the diffusive migration of sodium ions, reduces the charge transfer resistance, and enhances the diffusion kinetics performance of the multiphase material \u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. In addition, in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), the charge-discharge curves of pure O3-NNFM show a voltage drop of 790 mV after 300 cycles, and the voltage drop of NNFM10NNM is not very obvious and still maintains a shape similar to the initial charge-discharge curve, demonstrating low polarization and excellent structural reversibility during the cycling process \u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e. This may be related to the fast ion channels provided by the P2-Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e on the surface of the multiphase material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the sodium-ion transport kinetics and interfacial characteristics of the materials during prolonged cycling, electrochemical impedance spectroscopy (EIS) measurements were conducted on NNFM and NNFM10NNM electrodes before and after 300 cycles at 1C within the voltage window of 2.0\u0026ndash;4.0 V (vs. Na⁺/Na), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The Nyquist fitting equivalent circuit mainly consists of the high-frequency intercept \u003cem\u003eR\u003c/em\u003e\u003csub\u003eS\u003c/sub\u003e (representing the ohmic resistance, arising from the electrolyte resistance and cell components), the high-frequency semicircle \u003cem\u003eR\u003c/em\u003e\u003csub\u003eSEI\u003c/sub\u003e (representing the solid electrolyte interphase film resistance, which is related to the surface film resistance associated with sodium ion transport through the CEI film), the mid-frequency semicircle \u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e (representing the charge transfer resistance during charging and discharging, related to redox kinetics) and the low-frequency slope of the Warburg impedance (representing the diffusion resistance, related to the sodium ion transport within the material, reflecting the sodium ion diffusion coefficient) \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Additionally, considering the effects of the particle size distribution, surface roughness, and electrode bending, the constant phase elements CPE1 and CPE2, which represent capacitors, were also included in the Nyquist fitting equivalent circuit. Before cycling, NNFM10NNM exhibits a notably lower \u003cem\u003eR\u003c/em\u003e\u003csub\u003eSEI\u003c/sub\u003e than pristine NNFM, indicating a thinner and more stable cathode-electrolyte interphase (CEI) layer. This can be attributed to the conformal P2-phase coating, which serves as a protective barrier against direct contact between the O3 core and the electrolyte, effectively suppressing side reactions and electrolyte decomposition. \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e After 300 cycles, the differences between the two samples become more pronounced. As summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the \u003cem\u003eR\u003c/em\u003e\u003csub\u003eSEI\u003c/sub\u003e and \u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e values of NNFM are 2 and 3 times greater than those of NNFM10NNM, respectively. This significant difference originates from the continuous deintercalation and intercalation of sodium ions during charging and discharging. The uniform coating layer of the P2 phase on the surface of NNFM10NNM blocks direct contact between the internal active components and the electrolyte, reducing the dissolution of transition metals and the decomposition of the electrolyte \u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e. This also slows the formation of the surface CEI film and inhibits irreversible phase changes on the material surface, which cause structural collapse and block sodium ion transport channels \u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. This synergistic effect accounts for the superior cycling durability and rate performance of the NNFM10NNM sample, as demonstrated in previous electrochemical tests.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElectrochemical impedance results for the NNFM10NNM and pure O3-NNFM cathodes.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCycle number\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e, Ω\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003eSEI\u003c/sub\u003e, Ω\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e, Ω\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eO3-NNFM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e202.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e310.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e551.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNNFM10NNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e145.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e105.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e273.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo address the critical issue of air instability in O3-type layered cathodes \u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e, we evaluated the environmental tolerance of pristine O3-NNFM and P2-phase-coated NNFM10NNM through controlled air-aging tests. Although previous studies have reported O3/P2 composite structures, the random phase distribution in such systems leaves O3 regions susceptible to moisture-induced degradation. \u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e In this work, we employed a conformal P2-phase coating to construct a core\u0026ndash;shell heterostructure aimed at mitigating ambient degradation pathways. The O3-NNFM and NNFM10NNM samples were placed in an environment containing water and carbon dioxide with a humidity of 55% for 24 hours to evaluate the electrochemical performance of the aged cathodes and verify the improvement in the air stability of the NNFM10NNM cathode. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), the reversible capacity of the aged O3-NNFM cathode at 0.1 C was 112.4 mA hg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was equivalent to only 81.2% of the original O3-NNFM. In contrast, owing to the protective effect of the P2-NNM coating layer, the aged NNFM10NNM cathode provided a high discharge capacity of 126.1 mA hg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, equivalent to 97% of that provided by the original electrode, confirming the improvement in its air stability. Similarly, the aged NNFM10NNM exhibited good rate performance (98.5 mA g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 10C). In comparison, the aged O3-NNFM showed poor Na storage performance. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) shows the long-term cycling performance of the O3-NNFM and NNFM10NNM materials at a 1C rate. After being exposed to a humid environment, the capacity retention rate of O3-NNFM after 300 cycles was only 27.7%. However, after a P2-type coating protective layer is constructed, the capacity retention rate of NNFM10NNM reaches 78.2% after 300 cycles, which is much greater than that of the O3-NNFM material, demonstrating excellent air stability.\u003c/p\u003e \u003cp\u003eTo elucidate the structural evolution responsible for these differences, ex situ XRD analysis was conducted on the aged samples. For O3-NNFM, the (003) peak of the initial hexagonal O3 structure split into two diffraction peaks, corresponding to the O3 phase with an expanded c-axis and the monoclinic O\u0026rsquo;3 phase structure with structural distortion (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c)) \u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e. The instability of O3-NNFM is due to the reaction between sodium ions and the intercalated water/carbon dioxide molecules during the exposure process, resulting in the formation of a hydrated phase and sodium carbonate between the layers, which leads to an increase in the interlayer spacing \u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. These alkaline compounds do not possess electrochemical activity. This structural transformation and the formation of surface sodium carbonate impurities lead to severe structural degradation and rapid capacity decay during the cycling process, hindering the practical application of the O3-NNFM cathode. In contrast, in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d), air aging did not significantly change the structure of NNFM10NNM, indicating that the surface-coated P2 phase has a protective effect. Together, these results demonstrate that the designed core\u0026ndash;shell O3/P2 heterostructure provides a robust pathway to simultaneously improve the air stability and long-term electrochemical performance of sodium layered cathode materials.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this work, a core-shell heterostructured cathode material (NNFMxNNM) composed of O3 and P2 phases was successfully synthesized via coprecipitation. The enhanced Na storage performance of the heterojunction cathode is attributed to the synergistic effect of the multiphase structure, where the O3 phase provides adequate initial Na storage and the P2 phase acts as a protective layer. This unique architecture not only mitigates phase transitions and lattice stress but also promotes rapid sodium-ion diffusion and efficient charge transfer. Electrochemical tests revealed a significant improvement in high-rate performance, with the 10C discharge capacity increasing from 77.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (pure O3-NNFM) to 103.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e after P2-phase coating. Additionally, the P2 layer effectively protects the O3 phase from air and electrolyte exposure, contributing to long-term cycling durability. As a result, the optimized cathode retained 82.4% of its initial capacity after 300 cycles at 0.1 C. These findings highlight the potential of core-shell O3/P2 heterojunction engineering for high-performance and air-stable sodium-ion battery cathodes.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAvailability\u003c/strong\u003e\u003c/p\u003e\n\u003col\u003e\n\u003cli\u003eOur study is not a clinical trial.\u003c/li\u003e\n\u003cli\u003eConsent to Publish declaration: not applicable.\u003c/li\u003e\n\u003cli\u003eConsent to Participate declaration: not applicable.\u003c/li\u003e\n\u003cli\u003eEthics declaration: not applicable.\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors kindly acknowledge the support by the DMHL20230512SIBC of Hualu New Materials Technology Co., LTD.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eHao Xie: Conceptualization, Funding acquisition, Writing \u0026ndash; original draft. Hao Xie, Pei Ding, Chuxin Cui: Investigation, Methodology, Data curation. ZhengMing Sun: Supervision, Writing \u0026ndash; review \u0026amp; editing. All authors discussed the results and approved the final manuscript.\u003c/p\u003e\n\u003ch2\u003e\u0026nbsp;\u003c/h2\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBAO Z, LU C, LIU Q et al. An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries [J]. Nat Commun, 2024, 15(1): 1934.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEL AFIA S, CANO A, AR\u0026eacute;VALO P et al. Rechargeable Li-Ion Batteries, Nanocomposite Materials and Applications [J]. Batteries, 2024, 10(12).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eUSISKIN R, LU Y, POPOVIC J, et al. Fundamentals, status and promise of sodium-based batteries [J]. Nat Reviews Mater. 2021;6(11):1020\u0026ndash;35.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYANG H, WANG D, LIU Y, et al. Improvement of cycle life for layered oxide cathodes in sodium-ion batteries [J]. Energy Environ Sci. 2024;17(5):1756\u0026ndash;80.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXIAO, Y, ABBASI N M, ZHU Y F et al. Layered Oxide Cathodes Promoted by Structure Modulation Technology for Sodium-Ion Batteries [J]. Adv Funct Mater, 2020, 30(30).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHWANG J Y, MYUNG S T, SUN Y K. Sodium-ion batteries: present and future [J]. Chem Soc Rev. 2017;46(12):3529\u0026ndash;614.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHASA I, BUCHHOLZ D. PASSERINI S, High Performance Na0.5[Ni0.23Fe0.13Mn0.63]O2 Cathode for Sodium-Ion Batteries [J]. Adv Energy Mater, 2014, 4(15).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWANG PF, YOU Y, YIN Y X et al. Layered Oxide Cathodes for Sodium-Ion Batteries: Phase Transition, Air Stability, and Performance [J]. Adv Energy Mater, 2017, 8(8).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDELMAS C, FOUASSIER C. HAGENMULLER P. Structural classification and properties of the layered oxides [J]. Physica B\u0026thinsp;+\u0026thinsp;c, 1980, 99(1\u0026ndash;4): 81\u0026thinsp;\u0026ndash;\u0026thinsp;5.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYUAN X G, GUO Y J GANL et al. A Universal Strategy toward Air-Stable and High‐Rate O3 Layered Oxide Cathodes for Na‐Ion Batteries [J]. Adv Funct Mater, 2022, 32(17).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHAO A, FANG Y, AI X et al. Mixed polyanion cathode materials: Toward stable and high-energy sodium-ion batteries [J]. J Energy Chem, 2021, 60(635\u0026thinsp;\u0026ndash;\u0026thinsp;48).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGONZALO E, ZARRABEITIA M. Sodium manganese-rich layered oxides: Potential candidates as positive electrode for Sodium-ion batteries [J]. Energy Storage Mater. 2021;34:682\u0026ndash;707.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHANG L, WANG C, LIU Y et al. Suppressing interlayer-gliding and Jahn-Teller effect in P2-type layered manganese oxide cathode via Mo doping for sodium-ion batteries [J]. Chem Eng J, 2021, 426.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBERTHELOT R, CARLIER D. Electrochemical investigation of the P2-NaxCoO2 phase diagram [J]. Nat Mater. 2011;10(1):74\u0026ndash;80.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTANG K, WANG Y, ZHANG X, et al. High-performance P2-Type Fe/Mn-based oxide cathode materials for sodium-ion batteries [J]. Electrochim Acta. 2019;312:45\u0026ndash;53.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHOU D, HUANG W, LV X et al. A novel P2/O3 biphase Na0.67Fe0.425Mn0.425Mg0.15O2 as cathode for high-performance sodium-ion batteries [J]. J Power Sources, 2019, 421(147\u0026thinsp;\u0026ndash;\u0026thinsp;55).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWANG JE, KIM H, JUNG Y H, et al. Designing High Energy Sodium-Ion Battery Cathodes by Utilizing P2/O3 Biphasic Structure and Lithium Honeycomb Ordering [J]. Small. 2021;17(30):e2100146.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKATCHO N A, CARRASCO J. Origins of bistability and Na ion mobility difference in P2-and O3‐Na2/3Fe2/3Mn1/3O2 cathode polymorphs [J]. Adv Energy Mater. 2017;7(1):1601477.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCHENG Z, FAN X Y, YU L, et al. A Rational Biphasic Tailoring Strategy Enabling High-Performance Layered Cathodes for Sodium-Ion Batteries [J]. Angew Chem Int Ed Engl. 2022;61(19):e202117728.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJI H, ZHAI J, CHEN G et al. Surface Engineering Suppresses the Failure of Biphasic Sodium Layered Cathode for High Performance Sodium-Ion Batteries [J]. Adv Funct Mater, 2021, 32(12).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWANG H, LIU Q, LIU Y. Multi-ion doped P2/O3 biphasic layered oxides cathodes NaxNi0.2Mn0.55Cu0.14TiyZnzO2 for sodium-ion batteries [J]. J Alloys Compd, 2023, 962.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJIN J, LIU Y, PANG X, et al. A comprehensive understanding of the anionic redox chemistry in layered oxide cathodes for sodium-ion batteries [J]. Sci China Chem. 2020;64(3):385\u0026ndash;402.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHANG Q, GU Q F, LI Y et al. Surface Stabilization of O3-type Layered Oxide Cathode to Protect the Anode of Sodium Ion Batteries for Superior Lifespan [J]. iScience, 2019, 19(244\u0026thinsp;\u0026ndash;\u0026thinsp;54).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLAMB J. Surface-modified Na (Ni0. 3Fe0. 4Mn0. 3) O2 cathodes with enhanced cycle life and air stability for sodium-ion batteries [J]. ACS Appl Energy Mater. 2021;4(10):11735\u0026ndash;42.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHANG B, ZHAO Y, LI M, et al. Amorphous Aluminum Oxide-Coated NaFe(0.33)Ni(0.33)Mn(0.33)O(2) Cathode Materials: Enhancing Interface Charge Transfer for High-Performance Sodium-Ion Batteries [J]. ACS Appl Mater Interfaces; 2023.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLIU Y, FANG X, ZHANG A, et al. Layered P2-Na2/3[Ni1/3Mn2/3]O2 as high-voltage cathode for sodium-ion batteries: The capacity decay mechanism and Al2O3 surface modification [J]. Nano Energy. 2016;27:27\u0026ndash;34.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKIM D, LEE E, SLATER M et al. Layered Na[Ni1/3Fe1/3Mn1/3]O2 cathodes for Na-ion battery application [J]. Electrochem Commun, 2012, 18(66\u0026thinsp;\u0026ndash;\u0026thinsp;9).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWANG X, LUO J, DOU W et al. Stimulating the redox capacity by multi-ion substitution for P2-type sodium-ion battery cathodes [J]. Mater Today Energy, 2024, 43.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCHEN C, HUANG W, LI Y et al. P2/O3 biphasic Fe/Mn-based layered oxide cathode with ultrahigh capacity and great cyclability for sodium ion batteries [J]. Nano Energy, 2021, 90.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHOU P, DONG M, LI F et al. Recent Advances in Cathode Materials with Core\u0026ndash;Shell Structures and Concentration Gradients for Advanced Sodium-Ion Batteries [J]. Adv Funct Mater, 2024, 34(49).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZHU W, XU S, YANG S et al. Advanced Electrode Materials for Low-Temperature Na Storage [J]. Adv Funct Mater, 2024, 35(14).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGABRIEL E, MA C, GRAFF K et al. Heterostructure engineering in electrode materials for sodium-ion batteries: Recent progress and perspectives [J]. eScience, 2023, 3(5).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDUAN Y, MA Z H, HUANG Y Y et al. Synergistic effect of P2, O3 phase on biphasic layered oxide with enhanced electrochemical performance for sodium storage [J]. J Colloid Interface Sci, 2025, 682(715\u0026thinsp;\u0026ndash;\u0026thinsp;24).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZUO W, QIU J, LIU X, et al. The stability of P2-layered sodium transition metal oxides in ambient atmospheres [J]. Nat Commun. 2020;11(1):3544.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZUO W, REN F, LI Q et al. Insights of the anionic redox in P2\u0026ndash;Na0.67Ni0.33Mn0.67O2 [J]. Nano Energy, 2020, 78.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYANG Q, WANG P F, GUO J Z, et al. Advanced P2-Na(2/3)Ni(1/3)Mn(7/12)Fe(1/12)O(2) Cathode Material with Suppressed P2-O2 Phase Transition toward High-Performance Sodium-Ion Battery [J]. ACS Appl Mater Interfaces. 2018;10(40):34272\u0026ndash;82.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYUAN M, LIU H, RAN F. Fast-charging cathode materials for lithium \u0026amp; sodium ion batteries [J]. Mater Today, 2023, 63(360\u0026thinsp;\u0026ndash;\u0026thinsp;79).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDENG YP, WU Z G, LIANG R et al. Layer-Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries [J]. Adv Funct Mater, 2019, 29(19).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLIANG X, YU T-Y, RYU H-H et al. Hierarchical O3/P2 heterostructured cathode materials for advanced sodium-ion batteries [J]. Energy Storage Mater, 2022, 47(515\u0026thinsp;\u0026ndash;\u0026thinsp;25).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJU J-H, RYU K-S. Synthesis and electrochemical performance of Li(Ni0.8Co0.15Al0.05)0.8(Ni0.5Mn0.5)0.2O2 with core\u0026ndash;shell structure as cathode material for Li-ion batteries [J]. J Alloys Compd. 2011;509(30):7985\u0026ndash;92.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSUN Y-K. Direction for Commercialization of O3-Type Layered Cathodes for Sodium-Ion Batteries [J]. ACS Energy Lett. 2020;5(4):1278\u0026ndash;80.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHWANG J-Y, MYUNG S-T, CHOI J U, et al. Resolving the degradation pathways of the O3-type layered oxide cathode surface through the nano-scale aluminum oxide coating for high-energy density sodium-ion batteries [J]. J Mater Chem A. 2017;5(45):23671\u0026ndash;80.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYAO H R, WANG P F, GONG Y, et al. Designing Air-Stable O3-Type Cathode Materials by Combined Structure Modulation for Na-Ion Batteries [J]. J Am Chem Soc. 2017;139(25):8440\u0026ndash;3.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Sodium-ion batteries, Layered transition metal oxides, O3/P2 heterojunction, Core-shell, Long cycle life, High rate","lastPublishedDoi":"10.21203/rs.3.rs-6950664/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6950664/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAmong sodium-ion battery (SIB) layered cathode materials, O3-type cathodes are attractive due to their high capacity, high initial efficiency, and low cost. However, their commercial application is limited by structural instability, complex phase transitions during cycling, high sensitivity to moisture and CO\u003csub\u003e2\u003c/sub\u003e, and poor rate performance due to sluggish ion diffusion. To overcome these challenges, we designed a multiphase core-shell heterojunction consisting of an O3-NaNi\u003csub\u003e0.33\u003c/sub\u003eFe\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e core and a P2-Na\u003csub\u003e0.67\u003c/sub\u003eNi\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e shell. This structure effectively isolates the O3 phase from moisture and electrolyte exposure, combining high capacity with structural stability. The half-cells with the optimized heterojunction cathode delivered 129.1 mAh g\u003csup\u003e-1\u003c/sup\u003e at 0.1 C and 103.4 mA g\u003csup\u003e-1\u003c/sup\u003e at 10 C, with 80.1% capacity retention at high rates. After 300 cycles under ambient air exposure, it maintains 78.2% capacity, compared to only 27.7% for the pure O3 counterpart. The P2 shell enhances structural reversibility, air/thermal stability, electrochemical kinetics, and lowers charge-transfer resistance. This rational design demonstrates a promising strategy for developing high-performance SIB cathodes.\u003c/p\u003e","manuscriptTitle":"Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance Sodium-Ion Batteries","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-02 12:02:36","doi":"10.21203/rs.3.rs-6950664/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"346b0da3-bd84-4122-a7e6-23f1cc83c51a","owner":[],"postedDate":"July 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-10T06:53:54+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-02 12:02:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6950664","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6950664","identity":"rs-6950664","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}