Synergistic O2/O3 Phase Engineering Suppresses Voltage Fade and Enhances Cycling Stability in Lithium-Rich Layered Oxides

Synergistic O2/O3 Phase Engineering Suppresses Voltage Fade and Enhances Cycling Stability in Lithium-Rich Layered Oxides | 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 Synergistic O2/O3 Phase Engineering Suppresses Voltage Fade and Enhances Cycling Stability in Lithium-Rich Layered Oxides Yufang Chen, Jinhui Wang, Yanshuang Zhao, Wenjin Song, Peitao Xiao, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7677792/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 Lithium-rich manganese-based layered oxides are considered next-generation cathode materials due to their ultra-high capacity and voltage plateau. However, the single O3 phase is prone to lattice oxygen loss and irreversible phase transitions during high-voltage cycling, leading to rapid capacity and voltage decay. In this study, a molten salt ion exchange method was used to quantitatively convert the P2 phase into the O2 phase, creating a composite cathode with a precisely adjustable O2/O3 ratio. The effects of phase ratio on microstructure, crystal structure, surface chemistry, and electrochemical performance were systematically investigated. The results show that the introduction of the O2 phase enhances the structural stability of the cathode material. Specifically, the 0.3O2/0.7O3 sample, after 200 cycles at 4.6 V and 1 C, maintained a capacity of 159.8 mAh/g with a capacity retention of 98.8%, a voltage decay rate of only 2.43 mV/cycle, and the smallest shift in the oxidation peak, showing the best overall performance. This work elucidates the synergistic stabilization mechanism of different O2/O3 ratios and provides a reference for phase engineering in lithium-rich manganese-based cathodes. ionic replacement O2/O3 biphasic layered oxides structural stability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Lithium-rich manganese-based layered oxide cathode materials benefit from additional oxygen-related redox reactions and typically exhibit high capacity (> 250 mAh/g) and high operating voltage (> 4.5 V).[ 1 – 3 ] Compared with commercial lithium-rich nickel cathodes LiNi 1 − x−y Mn x Co y O 2 (x + y < 0.4), this significantly enhances the energy density of current battery systems, making them one of the most promising next-generation cathode materials. However, traditional single O3 phase lithium-rich manganese-based cathode materials are prone to lattice oxygen release, transition metal migration and dissolution during high-voltage cycling, accompanied by phase transformations from layered to spinel structures, leading to the collapse of the layered structure and irreversible phase changes, which cause capacity loss and voltage decay.[ 4 – 7 ] This severely restricts the development of lithium-rich manganese-based cathode materials. To break through the structural bottleneck of the single O3 phase, researchers have proposed the novel idea of introducing the O2 phase to construct O2/O3 composite cathodes.[ 8 ] The O2 phase has a larger alkali metal interlayer spacing and more open ion diffusion channels, exhibiting higher structural reversibility, which can effectively alleviate the lattice stress in the O3 phase during deep delithiation.[ 9 ] Meanwhile, the O3 phase provides high initial capacity, and the combination of the two phases is expected to balance high energy density and structural stability. Chen et al.[ 10 ] synthesized a lithium-rich manganese-based cathode material with an equal mixture of O2 and O3 phases, and the O2/O3 composite structure significantly improved the capacity and voltage stability of the cathode. Cao et al.[ 11 ] synthesized a biphasic O2/O3 layered cathode material Li 0.9 [Li 0.3 Mn 0.7 ]O 2 , with a composition of approximately 81% O2 phase and 19% O3 phase. The presence of the O2 phase reduced the volume strain of the O3 phase during ion intercalation and deintercalation and inhibited the formation of the spinel phase. However, existing studies have mainly focused on synthetic methodologies, and there is still a lack of systematic and quantitative correlation research between “O2/O3 phase ratio-microstructure-crystal structure-electrochemical performance,” which leads to a lack of theoretical guidance for performance optimization. Clarifying the influence of the O2/O3 phase ratio on the morphology, crystal structure, surface chemistry, and lithium storage performance of lithium-rich manganese-based oxide particles is not only conducive to revealing the O2/O3 synergistic stabilization mechanism but also provides a theoretical basis for designing cathode materials with high capacity, high rate capability, and long cycle life. In addition, the precise conversion of Na + to Li + by molten salt ion exchange can provide a reference for the design of high-performance composite cathodes in other layered systems. In this study, a series of P2/O3 samples were first obtained by adjusting the precursor ratio, followed by the conversion of the P2 phase into the O2 phase via a molten salt ion-exchange method to construct composite cathodes with tunable O2/O3 ratios. The evolution of phase composition, microstructure, and chemical states was systematically characterized using SEM, XRD, XPS, and ICP techniques. By evaluating the initial Coulombic efficiency, rate performance, and long-term cycling over 200 cycles, the influence of the O2/O3 ratio on lithium-ion diffusion kinetics, structural reversibility, and cycling stability was thoroughly investigated. The optimal O2/O3 phase ratio was ultimately determined based on comprehensive electrochemical performance. Results and Discussion The preparation process of the O2/O3 composite cathode is shown in Fig. 1 . After adding appropriate amounts of sodium and lithium sources to the P2 and O3 phase precursors, the P2 and O3 phase cathodes are converted through solid-state sintering. The P2 and O3 phase cathodes are then mixed in proportion, and the Na + are replaced by Li + using molten salt in a muffle furnace. Scanning electron microscope (SEM) analysis revealed the microstructural evolution of different P2/O3 and O2/O3 ratio samples (Fig. 2 (a)-(j) ). The study shows that the control of the P2/O3 ratio significantly influences the particle morphology characteristics of the cathode materials. All samples exhibited multiple particle agglomerates, with uniform distribution of primary particles. As the P2 phase ratio increased, the primary particle size of the cathode material showed a noticeable increase. Notably, when the P2 phase ratio reached 0.7 or higher, flake-like primary particles began to appear in the sample. Specifically, a regular flaky structure was observed in the 0.9P2/0.1O3 sample. This morphological evolution trend was also observed in the O2/O3 system samples after sodium ions were replaced by lithium ions. These results clearly demonstrate that the P2/O3 phase ratio is a key factor in regulating the microstructure of layered oxide cathode materials. The X-ray diffraction (XRD) patterns of the composite configurations, as shown in Fig. 2 (k)(l) , reveal significant crystal structure differences in samples with different P2/O3 and O2/O3 ratios. The characteristic diffraction peaks of the P2 phase and O3 phase clearly indicate the coexistence of both phases, and their relative proportions change significantly with increasing P2 phase content. Specifically, in the (002), (004), and (006) diffraction peaks, the increase in P2 phase content results in a shift of these peaks to lower angles, indicating that the interlayer spacing of the P2 phase is larger than that of the O3 phase.[ 12 ] This shift further validates the characteristic of the P2 phase with a larger interlayer spacing in its layered structure. In contrast, the diffraction peaks of the O3 phase become less prominent at higher P2 ratios, which may be due to the covering effect of the P2 phase or crystal defects in the O3 phase. Furthermore, the intensity of the diffraction peaks of the O3 phase gradually decreases with the increase of P2 content, which aligns with the actual P2/O3 ratio in the samples, suggesting that XRD analysis effectively characterizes the phase composition of the samples. Upon ion exchange, the P2 phase was transformed into the O2 phase (Fig. 2 (l) ). The (002) diffraction peak of the P2 phase disappeared, and a corresponding (002) diffraction peak for the O2 phase appeared at 18.19°.[ 13 ] Meanwhile, the (003) diffraction peak of the O3 phase at 18.71° did not show significant changes before ion exchange. Compared to conventional O3 phase lithium-rich manganese-based cathode materials, the ion-exchanged O2 phase retains a small amount of sodium ions in the alkali metal layer as structural support. The interlayer spacing of the alkali metal layer is relatively larger, leading to the O2 phase diffraction peak being at a smaller angle than that of the O3 phase. The characteristic diffraction peaks of the O2 and O3 phases clearly indicate a coexistence of both phases, and their ratio changes significantly with the increasing O2 phase content. The XRD Rietveld refinement results for the 0.1O2/0.9O3 and 0.3O2/0.7O3 samples are shown in Fig. 2 (m)(n) , while the Rietveld refinement results for the 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3 samples are shown in Figure S1 ,S2,S3 . The atomic position information is provided in Table S1 ,S2 . The refinement results indicate that these five materials are composed of both O2 and O3 phases. The space group of the O2 phase is P6 3 mc, and the space group of the O3 phase is R-3m. The O2/O3 phase ratio is consistent with the mixing ratio of P2/O3 phases. The chemical composition of the samples with different P2/O3 and O2/O3 ratios was systematically characterized using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) analysis (Fig. 3 ). The full XPS spectra show that the P2/O3 system samples primarily consist of seven elements: Li, C, O, Mn, Co, Ni, and Na (Fig. 3 (a) ).[ 14 , 15 ] After ion exchange, where Na⁺ in the P2 phase is converted to Li⁺, the O2/O3 system samples only show the presence of Li, C, O, Mn, Co, and Ni (Fig. 3 (c)), indicating complete substitution of Na⁺. The ICP quantitative analysis further confirms this result. As the initial P2 phase content increases, the Na content in the P2/O3 samples gradually increases (Fig. 3 (b) ), whereas in the ion-exchanged O2/O3 samples, Na is almost undetectable (Fig. 3 (d) , with specific data in Table S3 ). The high consistency between the XPS and ICP results verifies the successful substitution of Na⁺ and the controlled regulation of material composition. The Mn 3s spectra can intuitively represent the surface oxidation states of manganese, as shown in Figure S4 . The Mn 3s multiplet splitting peak distances for the 0.1O2/0.9O3, 0.3O2/0.7O3, 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3 samples are 4.44 eV, 4.40 eV, 4.34 eV, 4.31 eV, and 4.30 eV, respectively. The calculated AOS (average oxidation state) values for these five samples are 3.96, 4.00, 4.07, 4.10, and 4.11.[ 16 ] These results indicate that the O2 phase significantly enhances the surface manganese oxidation state of the lithium-rich cathode. Increasing the O2 phase ratio helps alleviate the Jahn-Teller effect of low-valent manganese and the MnO 6 octahedral distortion, thereby suppressing the leaching of surface elements and structural degradation. The first cycle charge-discharge curves of O2/O3 systems are shown in Fig. 4 (a) The results indicate that the phase composition significantly affects the electrochemical performance. The O3-phase-dominated 0.1O2/0.9O3 sample shows a low first cycle Coulombic efficiency of 71.3%, with a charge capacity of 235.2 mAh/g and a discharge capacity of 167.7 mAh/g. The difference between these values suggests significant irreversible capacity loss during cycling, possibly due to the formation of a larger solid electrolyte interface (SEI) layer involving more lithium ions, linked to its complex surface structure. As the O2 phase content increases to 30% (0.3O2/0.7O3), the first cycle Coulombic efficiency rises to 85.1%, indicating that the O2 phase helps to reduce side reactions. Notably, when the O2/O3 ratio reaches 1:1, the material exhibits the best balance, with a charge capacity of 223.6 mAh/g and a discharge capacity of 197.0 mAh/g (Coulombic efficiency of 88.1%), indicating that the synergistic effect between the two phases significantly improves the reversible lithium ion intercalation and de-intercalation efficiency. When the O2 phase content increases to 90% (0.9O2/0.1O3), the first cycle Coulombic efficiency further improves to 96.1%, but the charge capacity of 187.8 mAh/g is notably lower than that of the high O3 content sample. This confirms that although the O2 phase has higher structural stability, its intrinsic capacity contribution is lower. Differential capacity versus voltage (dQ/dV) analysis (Fig. 4 (b) ) further reveals that the oxidation peak for 0.5O2/0.5O3 is at 4.59V, lower than the oxidation peaks of other samples, indicating that the composite cathode facilitates charge transfer during cycling. Rate performance tests reveal the critical influence of the O2/O3 phase ratio on electrochemical performance. The results show that the material with high O3 content (0.1O2/0.9O3) exhibits a large capacity difference between low and high rates. Its discharge capacity drops sharply from 207.7 mAh/g at 0.2 C to 44.3 mAh/g at 5 C, suggesting that the O3 phase cannot maintain effective Li + diffusion channels at high current densities. As the O2 phase ratio increases, the rate performance improves gradually. The 0.5O2/0.5O3 sample shows more balanced electrochemical performance, maintaining discharge capacities of 217.5 mAh/g at 0.2 C and 48.1 mAh/g at 5 C. The 0.9O2/0.1O3 sample demonstrates the best performance at high rates (5 C), with a discharge capacity of 64.2 mAh/g, though it shows slightly lower capacity at low rates compared to the O3-phase-dominated samples. These results confirm that the O2 phase significantly enhances lithium ion diffusion and that proper O2/O3 phase ratio control is crucial for achieving high-rate performance. Electrochemical performance analysis of different O2/O3 phase ratio cathode materials after 200 cycles at a 1 C rate (Fig. 4 (d)(e) , Figure S5 ) reveals the key influence of phase composition on cycling stability. The O3-phase-dominated 0.1O2/0.9O3 sample shows a relatively high capacity retention rate (95.9%), but its median discharge voltage declines at a rate of 2.95 mV/cycle, leading to an energy retention rate of 85.6%, indicating significant structural degradation during cycling. In contrast, the 0.3O2/0.7O3 sample exhibits optimal overall performance with a capacity retention rate of 98.8%, energy retention rate of 89.2%, and a slowed voltage decay rate of 2.43 mV/cycle, confirming that the O2 phase can effectively stabilize the O3 phase’s crystal structure under high voltage. However, when the O2 phase content is too high (0.9O2/0.1O3), although the voltage decay rate (2.61 mV/cycle) is slightly improved compared to the O3 phase, both the capacity retention rate (81.8%) and energy retention rate (73.6%) significantly decrease. This phenomenon suggests that excessive O2 phase, while enhancing structural stability, weakens the electrochemical reversibility of the material. Thus, the best cycling performance requires a balance between the two phase ratios. Figure 5 (a)(b)(c) show the charge-discharge curves of 0.1O2/0.9O3, 0.3O2/0.7O3, and 0.9O2/0.1O3 after 3, 50, 100, 150, and 300 cycles at 1 C. The charge-discharge curves for 0.5O2/0.5O3 and 0.7O2/0.3O3 are shown in Figure S6(a)(c) . After 200 cycles, the 0.3O2/0.7O3 sample has the highest specific capacity, at 159.8 mAh/g. Figure 5 (d)(e)(f) show the corresponding dQ/dV curves. From the figures, it can be seen that the oxidation peaks of different composite ratio cathode materials shift to the right to varying degrees. A greater rightward shift of the oxidation peak usually indicates that the battery material’s structure is unstable during charging. A comparative analysis of the oxidation peak shifts and the capacity retention after cycling reveals that 0.3O2/0.7O3 has the smallest oxidation peak shift, with the two oxidation peaks shifting by 0.111 V and 0.246 V. This indicates that 0.3O2/0.7O3 maintains both its electrochemical performance and structure relatively stably during cycling. In contrast, the oxidation peaks of 0.9O2/0.1O3 shift by 0.311 V and 0.412 V, while 0.1O2/0.9O3 only has one oxidation peak with a shift of 0.289 V. The oxidation peak shift of 0.5O2/0.5O3 and 0.7O2/0.3O3 is also higher than that of 0.3O2/0.7O3 ( Figure S6(b)(d) ). Conclusion This study successfully converted the P2 phase into the O2 phase via an ion-exchange method, and constructed composite cathodes with different O2/O3 phase ratios. The results demonstrate that the regulation of the O2/O3 phase ratio significantly influences the electrochemical performance of lithium-rich manganese-based cathode materials. The 0.3O2/0.7O3 composite exhibited superior cycling stability, retaining a capacity of 159.8 mAh·g⁻¹ (98.8% capacity retention) after 200 cycles at 4.6 V and 1 C. Additionally, it showed the smallest oxidation peak shifts (0.111 V and 0.246 V), indicating that the O2 phase helps maintain the reversibility of the layered structure during prolonged cycling, effectively suppressing voltage decay (2.43 mV/cycle). In contrast, materials with a higher proportion of the O3 phase exhibited a significant decline in discharge capacity and cycling performance due to increased interlayer slippage and irreversible phase transitions under high-voltage conditions, leading to poor structural stability. Experimental Materials synthesis This study uses Mn 2/3 Co 1/6 Ni 1/6 CO 3 as a precursor to prepare a series of P2/O3 composite-type cathode materials by precisely controlling the synthesis process. The specific preparation process is as follows: First, the precursor is mixed with Li 2 CO 3 and Na 2 CO 3 at a molar ratio of (Mn + Co + Ni):Na = 0.8:0.6:0.2 and ball-milled. After pre-firing at 500°C for 5 hours, the mixture is sintered at 850°C in an oxygen atmosphere for 12 hours to obtain P2-type Na 0.6 Li 0.2 M 0.8 O 2 (M = Mn 2/3 Co 1/6 Ni 1/6 ) material. The O3-type material is obtained by mixing the precursor with Li 2 CO 3 at a 1:1.5 ratio, followed by the same thermal treatment process. By adjusting the raw material ratio, composite cathode materials with P2/O3 ratios of 0.1:0.9, 0.3:0.7, 0.5:0.5, 0.7:0.3, and 0.9:0.1 were synthesized using the above method. To obtain O2/O3 composite-phase materials, the aforementioned P2/O3 samples were heat-treated with a mixed molten salt of LiCl:LiNO 3 = 12:88 (mass ratio) at 280°C for 4 hours. After washing with deionized water and vacuum drying at 100°C for 8 hours, the final series of samples—0.1O2/0.9O3, 0.3O2/0.7O3, 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3—were obtained. This preparation method achieves the structural transformation from P2 phase to O2 phase via molten salt ion exchange, while preserving the composite phase ratio characteristics of the original materials. Materials characterizations X-ray diffraction (XRD) measurements were conducted using a Rigaku X-ray diffractometer (Cu-Kα radiation, λ = 1.542 Å) in the 2θ range of 10–80°. Rietveld refinement data was obtained through GASA-II software. The precise content of dissolved transition metals after cycling was tested by inductively coupled plasma atomic emission spectrometry (ICP-OES, Agilent 5110). The morphology and elements distribution of the samples were obtained with scanning electron microscopy (SEM, Regulus 8100). The valence of the elements in the samples was determined through X-ray photoelectron spectroscopy (XPS, PHI VersaProbe 4). Electrochemical measurements In the electrochemical experiments, the cathode paste was uniformly scraped onto the aluminum foil using a 100 µm scraper. The slurry consisted of 80 wt% cathode powder, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) binder mixed with a certain amount of n-methyl-2-pyrrolidone (NMP). After drying in a vacuum oven at 110°C for 12 h, the positive electrode was stamped into a circular battery pole piece with a diameter of 12 mm. The loading of active substance per pole piece was about 2 mg. Then CR-2032-type coin cells were assembled in an argon-filled glove box with sodium metal as negative electrode, Celgard 2400 and 1.2M LiPF6 EC:EMC = 3:7 vt% 1% LiDFOB as electrolyte. The electrochemical tests were conducted at 25°C. The galvanostatic charge–discharge tests were examined at 1 C (1 C = 250 mA/g) on a LAND battery test system (LAND, CT2001A). Declarations Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgements Yufang Chen, Jinhui Wang, Yanshuang Zhao contribute equally. This work was supported by the National Natural Science Foundation of China (No. 51902343) and Natural Science Foundation of Hunan Province (No. 2025JJ20041). References J. Zhao, Y. Su, J. Dong, X. Wang, Y. Lu, N. Li, Q. Huang, J. Hao, Y. Wu, B. Zhang, Q. Qi, F. Wu, L. 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University","correspondingAuthor":false,"prefix":"","firstName":"Xueyi","middleName":"","lastName":"Guo","suffix":""},{"id":525266009,"identity":"964cf481-682b-4dcf-aa1c-96af23a56b23","order_by":8,"name":"Hongbo Shu","email":"","orcid":"","institution":"National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Xiangtan University","correspondingAuthor":false,"prefix":"","firstName":"Hongbo","middleName":"","lastName":"Shu","suffix":""},{"id":525266010,"identity":"b0417cb5-d7e5-4bf7-b979-83c8083c1c91","order_by":9,"name":"Di Lu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAArklEQVRIiWNgGAWjYBACPgYGgwMfGBgYG4AcCaK0sAG1HJwB1NJDkhZmHtK0SCRvPGy747Dsfgbmg7d5GOzyiNCSVnA498xh4x4GtmRrHobkYiK05Bgczm07nNjDwGMmzcNwILGBKC2WYC3830jQwgixhY1ILTzPCg72tqUb9xxmM7acY5BMWAs/e/LmDz/brGXb25sf3nhTYUdYCwIwgwgD4tWPglEwCkbBKMADACO6NolE/PyBAAAAAElFTkSuQmCC","orcid":"","institution":"National University of Defense Technology","correspondingAuthor":true,"prefix":"","firstName":"Di","middleName":"","lastName":"Lu","suffix":""}],"badges":[],"createdAt":"2025-09-22 14:08:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7677792/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7677792/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":93040035,"identity":"37c9f9d1-2911-4f2f-a92e-4c66f51dd0ad","added_by":"auto","created_at":"2025-10-08 12:10:56","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2488325,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.docx","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/b8cab701c7c7f24a1e880fe9.docx"},{"id":93040230,"identity":"e6295332-f8c2-4b84-a128-0de75f3f3bff","added_by":"auto","created_at":"2025-10-08 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12:10:59","extension":"html","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":76924,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/8830be8895ccf5802778cadf.html"},{"id":93040301,"identity":"a7396be8-dc9a-4107-afaa-7f816f675124","added_by":"auto","created_at":"2025-10-08 12:11:10","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":135254,"visible":true,"origin":"","legend":"\u003cp\u003eO2/O3 composite cathode preparation process flowchart.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/ca385cd103d39c8e1763afb0.png"},{"id":93039930,"identity":"a5791e9a-7572-44ad-bb31-2ab785fb82e9","added_by":"auto","created_at":"2025-10-08 12:10:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":224193,"visible":true,"origin":"","legend":"\u003cp\u003e(a)(b)(c)(d)(e) Morphology of the P2/O3 phase composite configuration. (f)(g)(h)(i)(j) Morphology of the O2/O3 phase composite configuration. (k) XRD of the P2/O3 phase composite configuration. (l) Morphology of the O2/O3 phase composite configuration. (m)(n) XRD Rietveld refinement results for 0.1O2/0.9O3 and 0.3O2/0.7O3.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/c1ab921aa66860eff27cfe92.png"},{"id":93039907,"identity":"dafdbd84-3ee7-4705-9f92-0997d3e61fea","added_by":"auto","created_at":"2025-10-08 12:10:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":172292,"visible":true,"origin":"","legend":"\u003cp\u003e(a)(c) Full XPS spectra and (b)(d) element composition analysis of samples with different P2/O3 and O2/O3 ratios.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/e810a136f13877cd1eabc3d1.png"},{"id":93040212,"identity":"f6d8b5db-2c64-4277-a88e-cb192dcdc693","added_by":"auto","created_at":"2025-10-08 12:11:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":84592,"visible":true,"origin":"","legend":"\u003cp\u003e(a) First cycle charge-discharge curves and corresponding (b) dQ/dV curves and (c) rate performance curves of samples with different O2/O3 ratios at 0.1 C. (d) Capacity retention curves and corresponding (e) median voltage of samples with different O2/O3 ratios at 1 C.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/f87f8a5954c1f2477c0260b6.png"},{"id":93040185,"identity":"9d832c0b-b4e2-47e6-b9ae-b9d0a363757d","added_by":"auto","created_at":"2025-10-08 12:11:02","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":82162,"visible":true,"origin":"","legend":"\u003cp\u003e(a)(b)(c) Charge-discharge curves and corresponding (d)(e)(f) dQ/dV curves of samples with different O2/O3 ratios at 1 C.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/9f40bc3cc6396a40e15c76d5.png"},{"id":93092528,"identity":"b1949665-6d55-4860-a93b-1d1898c9f3af","added_by":"auto","created_at":"2025-10-09 02:52:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1108287,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/e297fef3-e195-452a-a8df-3f6b963a31fc.pdf"},{"id":93040154,"identity":"e7882b96-399a-4ec7-ad68-a2a20593a674","added_by":"auto","created_at":"2025-10-08 12:11:00","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":2256812,"visible":true,"origin":"","legend":"","description":"","filename":"supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7677792/v1/2a304f3c8d504803df4df3e8.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synergistic O2/O3 Phase Engineering Suppresses Voltage Fade and Enhances Cycling Stability in Lithium-Rich Layered Oxides","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLithium-rich manganese-based layered oxide cathode materials benefit from additional oxygen-related redox reactions and typically exhibit high capacity (\u0026gt;\u0026thinsp;250 mAh/g) and high operating voltage (\u0026gt;\u0026thinsp;4.5 V).[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] Compared with commercial lithium-rich nickel cathodes LiNi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u0026minus;y\u003c/sub\u003eMn\u003csub\u003ex\u003c/sub\u003eCo\u003csub\u003ey\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e(x\u0026thinsp;+\u0026thinsp;y\u0026thinsp;\u0026lt;\u0026thinsp;0.4), this significantly enhances the energy density of current battery systems, making them one of the most promising next-generation cathode materials. However, traditional single O3 phase lithium-rich manganese-based cathode materials are prone to lattice oxygen release, transition metal migration and dissolution during high-voltage cycling, accompanied by phase transformations from layered to spinel structures, leading to the collapse of the layered structure and irreversible phase changes, which cause capacity loss and voltage decay.[\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] This severely restricts the development of lithium-rich manganese-based cathode materials.\u003c/p\u003e\u003cp\u003eTo break through the structural bottleneck of the single O3 phase, researchers have proposed the novel idea of introducing the O2 phase to construct O2/O3 composite cathodes.[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] The O2 phase has a larger alkali metal interlayer spacing and more open ion diffusion channels, exhibiting higher structural reversibility, which can effectively alleviate the lattice stress in the O3 phase during deep delithiation.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] Meanwhile, the O3 phase provides high initial capacity, and the combination of the two phases is expected to balance high energy density and structural stability. Chen et al.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] synthesized a lithium-rich manganese-based cathode material with an equal mixture of O2 and O3 phases, and the O2/O3 composite structure significantly improved the capacity and voltage stability of the cathode. Cao et al.[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] synthesized a biphasic O2/O3 layered cathode material Li\u003csub\u003e0.9\u003c/sub\u003e[Li\u003csub\u003e0.3\u003c/sub\u003eMn\u003csub\u003e0.7\u003c/sub\u003e]O\u003csub\u003e2\u003c/sub\u003e, with a composition of approximately 81% O2 phase and 19% O3 phase. The presence of the O2 phase reduced the volume strain of the O3 phase during ion intercalation and deintercalation and inhibited the formation of the spinel phase. However, existing studies have mainly focused on synthetic methodologies, and there is still a lack of systematic and quantitative correlation research between \u0026ldquo;O2/O3 phase ratio-microstructure-crystal structure-electrochemical performance,\u0026rdquo; which leads to a lack of theoretical guidance for performance optimization.\u003c/p\u003e\u003cp\u003eClarifying the influence of the O2/O3 phase ratio on the morphology, crystal structure, surface chemistry, and lithium storage performance of lithium-rich manganese-based oxide particles is not only conducive to revealing the O2/O3 synergistic stabilization mechanism but also provides a theoretical basis for designing cathode materials with high capacity, high rate capability, and long cycle life. In addition, the precise conversion of Na\u003csup\u003e+\u003c/sup\u003e to Li\u003csup\u003e+\u003c/sup\u003e by molten salt ion exchange can provide a reference for the design of high-performance composite cathodes in other layered systems.\u003c/p\u003e\u003cp\u003eIn this study, a series of P2/O3 samples were first obtained by adjusting the precursor ratio, followed by the conversion of the P2 phase into the O2 phase via a molten salt ion-exchange method to construct composite cathodes with tunable O2/O3 ratios. The evolution of phase composition, microstructure, and chemical states was systematically characterized using SEM, XRD, XPS, and ICP techniques. By evaluating the initial Coulombic efficiency, rate performance, and long-term cycling over 200 cycles, the influence of the O2/O3 ratio on lithium-ion diffusion kinetics, structural reversibility, and cycling stability was thoroughly investigated. The optimal O2/O3 phase ratio was ultimately determined based on comprehensive electrochemical performance.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eThe preparation process of the O2/O3 composite cathode is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. After adding appropriate amounts of sodium and lithium sources to the P2 and O3 phase precursors, the P2 and O3 phase cathodes are converted through solid-state sintering. The P2 and O3 phase cathodes are then mixed in proportion, and the Na\u003csup\u003e+\u003c/sup\u003e are replaced by Li\u003csup\u003e+\u003c/sup\u003e using molten salt in a muffle furnace. Scanning electron microscope (SEM) analysis revealed the microstructural evolution of different P2/O3 and O2/O3 ratio samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(a)-(j)\u003c/b\u003e). The study shows that the control of the P2/O3 ratio significantly influences the particle morphology characteristics of the cathode materials. All samples exhibited multiple particle agglomerates, with uniform distribution of primary particles. As the P2 phase ratio increased, the primary particle size of the cathode material showed a noticeable increase. Notably, when the P2 phase ratio reached 0.7 or higher, flake-like primary particles began to appear in the sample. Specifically, a regular flaky structure was observed in the 0.9P2/0.1O3 sample. This morphological evolution trend was also observed in the O2/O3 system samples after sodium ions were replaced by lithium ions. These results clearly demonstrate that the P2/O3 phase ratio is a key factor in regulating the microstructure of layered oxide cathode materials.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe X-ray diffraction (XRD) patterns of the composite configurations, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(k)(l)\u003c/b\u003e, reveal significant crystal structure differences in samples with different P2/O3 and O2/O3 ratios. The characteristic diffraction peaks of the P2 phase and O3 phase clearly indicate the coexistence of both phases, and their relative proportions change significantly with increasing P2 phase content. Specifically, in the (002), (004), and (006) diffraction peaks, the increase in P2 phase content results in a shift of these peaks to lower angles, indicating that the interlayer spacing of the P2 phase is larger than that of the O3 phase.[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] This shift further validates the characteristic of the P2 phase with a larger interlayer spacing in its layered structure. In contrast, the diffraction peaks of the O3 phase become less prominent at higher P2 ratios, which may be due to the covering effect of the P2 phase or crystal defects in the O3 phase. Furthermore, the intensity of the diffraction peaks of the O3 phase gradually decreases with the increase of P2 content, which aligns with the actual P2/O3 ratio in the samples, suggesting that XRD analysis effectively characterizes the phase composition of the samples.\u003c/p\u003e\u003cp\u003eUpon ion exchange, the P2 phase was transformed into the O2 phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(l)\u003c/b\u003e). The (002) diffraction peak of the P2 phase disappeared, and a corresponding (002) diffraction peak for the O2 phase appeared at 18.19\u0026deg;.[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] Meanwhile, the (003) diffraction peak of the O3 phase at 18.71\u0026deg; did not show significant changes before ion exchange. Compared to conventional O3 phase lithium-rich manganese-based cathode materials, the ion-exchanged O2 phase retains a small amount of sodium ions in the alkali metal layer as structural support. The interlayer spacing of the alkali metal layer is relatively larger, leading to the O2 phase diffraction peak being at a smaller angle than that of the O3 phase. The characteristic diffraction peaks of the O2 and O3 phases clearly indicate a coexistence of both phases, and their ratio changes significantly with the increasing O2 phase content. The XRD Rietveld refinement results for the 0.1O2/0.9O3 and 0.3O2/0.7O3 samples are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(m)(n)\u003c/b\u003e, while the Rietveld refinement results for the 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3 samples are shown in \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e,S2,S3\u003c/b\u003e. The atomic position information is provided in \u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e,S2\u003c/b\u003e. The refinement results indicate that these five materials are composed of both O2 and O3 phases. The space group of the O2 phase is P6\u003csub\u003e3\u003c/sub\u003emc, and the space group of the O3 phase is R-3m. The O2/O3 phase ratio is consistent with the mixing ratio of P2/O3 phases.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe chemical composition of the samples with different P2/O3 and O2/O3 ratios was systematically characterized using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The full XPS spectra show that the P2/O3 system samples primarily consist of seven elements: Li, C, O, Mn, Co, Ni, and Na (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e).[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] After ion exchange, where Na⁺ in the P2 phase is converted to Li⁺, the O2/O3 system samples only show the presence of Li, C, O, Mn, Co, and Ni (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c)), indicating complete substitution of Na⁺. The ICP quantitative analysis further confirms this result. As the initial P2 phase content increases, the Na content in the P2/O3 samples gradually increases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e), whereas in the ion-exchanged O2/O3 samples, Na is almost undetectable (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e, with specific data in \u003cb\u003eTable S3\u003c/b\u003e). The high consistency between the XPS and ICP results verifies the successful substitution of Na⁺ and the controlled regulation of material composition.\u003c/p\u003e\u003cp\u003eThe Mn 3s spectra can intuitively represent the surface oxidation states of manganese, as shown in \u003cb\u003eFigure S4\u003c/b\u003e. The Mn 3s multiplet splitting peak distances for the 0.1O2/0.9O3, 0.3O2/0.7O3, 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3 samples are 4.44 eV, 4.40 eV, 4.34 eV, 4.31 eV, and 4.30 eV, respectively. The calculated AOS (average oxidation state) values for these five samples are 3.96, 4.00, 4.07, 4.10, and 4.11.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] These results indicate that the O2 phase significantly enhances the surface manganese oxidation state of the lithium-rich cathode. Increasing the O2 phase ratio helps alleviate the Jahn-Teller effect of low-valent manganese and the MnO\u003csub\u003e6\u003c/sub\u003e octahedral distortion, thereby suppressing the leaching of surface elements and structural degradation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe first cycle charge-discharge curves of O2/O3 systems are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e The results indicate that the phase composition significantly affects the electrochemical performance. The O3-phase-dominated 0.1O2/0.9O3 sample shows a low first cycle Coulombic efficiency of 71.3%, with a charge capacity of 235.2 mAh/g and a discharge capacity of 167.7 mAh/g. The difference between these values suggests significant irreversible capacity loss during cycling, possibly due to the formation of a larger solid electrolyte interface (SEI) layer involving more lithium ions, linked to its complex surface structure. As the O2 phase content increases to 30% (0.3O2/0.7O3), the first cycle Coulombic efficiency rises to 85.1%, indicating that the O2 phase helps to reduce side reactions. Notably, when the O2/O3 ratio reaches 1:1, the material exhibits the best balance, with a charge capacity of 223.6 mAh/g and a discharge capacity of 197.0 mAh/g (Coulombic efficiency of 88.1%), indicating that the synergistic effect between the two phases significantly improves the reversible lithium ion intercalation and de-intercalation efficiency. When the O2 phase content increases to 90% (0.9O2/0.1O3), the first cycle Coulombic efficiency further improves to 96.1%, but the charge capacity of 187.8 mAh/g is notably lower than that of the high O3 content sample. This confirms that although the O2 phase has higher structural stability, its intrinsic capacity contribution is lower.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDifferential capacity versus voltage (dQ/dV) analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e) further reveals that the oxidation peak for 0.5O2/0.5O3 is at 4.59V, lower than the oxidation peaks of other samples, indicating that the composite cathode facilitates charge transfer during cycling.\u003c/p\u003e\u003cp\u003eRate performance tests reveal the critical influence of the O2/O3 phase ratio on electrochemical performance. The results show that the material with high O3 content (0.1O2/0.9O3) exhibits a large capacity difference between low and high rates. Its discharge capacity drops sharply from 207.7 mAh/g at 0.2 C to 44.3 mAh/g at 5 C, suggesting that the O3 phase cannot maintain effective Li\u003csup\u003e+\u003c/sup\u003e diffusion channels at high current densities. As the O2 phase ratio increases, the rate performance improves gradually. The 0.5O2/0.5O3 sample shows more balanced electrochemical performance, maintaining discharge capacities of 217.5 mAh/g at 0.2 C and 48.1 mAh/g at 5 C. The 0.9O2/0.1O3 sample demonstrates the best performance at high rates (5 C), with a discharge capacity of 64.2 mAh/g, though it shows slightly lower capacity at low rates compared to the O3-phase-dominated samples. These results confirm that the O2 phase significantly enhances lithium ion diffusion and that proper O2/O3 phase ratio control is crucial for achieving high-rate performance.\u003c/p\u003e\u003cp\u003eElectrochemical performance analysis of different O2/O3 phase ratio cathode materials after 200 cycles at a 1 C rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(d)(e)\u003c/b\u003e, \u003cb\u003eFigure S5\u003c/b\u003e) reveals the key influence of phase composition on cycling stability. The O3-phase-dominated 0.1O2/0.9O3 sample shows a relatively high capacity retention rate (95.9%), but its median discharge voltage declines at a rate of 2.95 mV/cycle, leading to an energy retention rate of 85.6%, indicating significant structural degradation during cycling. In contrast, the 0.3O2/0.7O3 sample exhibits optimal overall performance with a capacity retention rate of 98.8%, energy retention rate of 89.2%, and a slowed voltage decay rate of 2.43 mV/cycle, confirming that the O2 phase can effectively stabilize the O3 phase\u0026rsquo;s crystal structure under high voltage. However, when the O2 phase content is too high (0.9O2/0.1O3), although the voltage decay rate (2.61 mV/cycle) is slightly improved compared to the O3 phase, both the capacity retention rate (81.8%) and energy retention rate (73.6%) significantly decrease. This phenomenon suggests that excessive O2 phase, while enhancing structural stability, weakens the electrochemical reversibility of the material. Thus, the best cycling performance requires a balance between the two phase ratios.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(a)(b)(c)\u003c/b\u003e show the charge-discharge curves of 0.1O2/0.9O3, 0.3O2/0.7O3, and 0.9O2/0.1O3 after 3, 50, 100, 150, and 300 cycles at 1 C. The charge-discharge curves for 0.5O2/0.5O3 and 0.7O2/0.3O3 are shown in \u003cb\u003eFigure S6(a)(c)\u003c/b\u003e. After 200 cycles, the 0.3O2/0.7O3 sample has the highest specific capacity, at 159.8 mAh/g. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(d)(e)(f)\u003c/b\u003e show the corresponding dQ/dV curves. From the figures, it can be seen that the oxidation peaks of different composite ratio cathode materials shift to the right to varying degrees. A greater rightward shift of the oxidation peak usually indicates that the battery material\u0026rsquo;s structure is unstable during charging. A comparative analysis of the oxidation peak shifts and the capacity retention after cycling reveals that 0.3O2/0.7O3 has the smallest oxidation peak shift, with the two oxidation peaks shifting by 0.111 V and 0.246 V. This indicates that 0.3O2/0.7O3 maintains both its electrochemical performance and structure relatively stably during cycling. In contrast, the oxidation peaks of 0.9O2/0.1O3 shift by 0.311 V and 0.412 V, while 0.1O2/0.9O3 only has one oxidation peak with a shift of 0.289 V. The oxidation peak shift of 0.5O2/0.5O3 and 0.7O2/0.3O3 is also higher than that of 0.3O2/0.7O3 (\u003cb\u003eFigure S6(b)(d)\u003c/b\u003e).\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study successfully converted the P2 phase into the O2 phase via an ion-exchange method, and constructed composite cathodes with different O2/O3 phase ratios. The results demonstrate that the regulation of the O2/O3 phase ratio significantly influences the electrochemical performance of lithium-rich manganese-based cathode materials. The 0.3O2/0.7O3 composite exhibited superior cycling stability, retaining a capacity of 159.8 mAh\u0026middot;g⁻\u0026sup1; (98.8% capacity retention) after 200 cycles at 4.6 V and 1 C. Additionally, it showed the smallest oxidation peak shifts (0.111 V and 0.246 V), indicating that the O2 phase helps maintain the reversibility of the layered structure during prolonged cycling, effectively suppressing voltage decay (2.43 mV/cycle). In contrast, materials with a higher proportion of the O3 phase exhibited a significant decline in discharge capacity and cycling performance due to increased interlayer slippage and irreversible phase transitions under high-voltage conditions, leading to poor structural stability.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eMaterials synthesis\u003c/h2\u003e\u003cp\u003eThis study uses Mn\u003csub\u003e2/3\u003c/sub\u003eCo\u003csub\u003e1/6\u003c/sub\u003eNi\u003csub\u003e1/6\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e as a precursor to prepare a series of P2/O3 composite-type cathode materials by precisely controlling the synthesis process. The specific preparation process is as follows: First, the precursor is mixed with Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e at a molar ratio of (Mn\u0026thinsp;+\u0026thinsp;Co\u0026thinsp;+\u0026thinsp;Ni):Na\u0026thinsp;=\u0026thinsp;0.8:0.6:0.2 and ball-milled. After pre-firing at 500\u0026deg;C for 5 hours, the mixture is sintered at 850\u0026deg;C in an oxygen atmosphere for 12 hours to obtain P2-type Na\u003csub\u003e0.6\u003c/sub\u003eLi\u003csub\u003e0.2\u003c/sub\u003eM\u003csub\u003e0.8\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (M\u0026thinsp;=\u0026thinsp;Mn\u003csub\u003e2/3\u003c/sub\u003eCo\u003csub\u003e1/6\u003c/sub\u003eNi\u003csub\u003e1/6\u003c/sub\u003e) material. The O3-type material is obtained by mixing the precursor with Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e at a 1:1.5 ratio, followed by the same thermal treatment process. By adjusting the raw material ratio, composite cathode materials with P2/O3 ratios of 0.1:0.9, 0.3:0.7, 0.5:0.5, 0.7:0.3, and 0.9:0.1 were synthesized using the above method. To obtain O2/O3 composite-phase materials, the aforementioned P2/O3 samples were heat-treated with a mixed molten salt of LiCl:LiNO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;12:88 (mass ratio) at 280\u0026deg;C for 4 hours. After washing with deionized water and vacuum drying at 100\u0026deg;C for 8 hours, the final series of samples\u0026mdash;0.1O2/0.9O3, 0.3O2/0.7O3, 0.5O2/0.5O3, 0.7O2/0.3O3, and 0.9O2/0.1O3\u0026mdash;were obtained. This preparation method achieves the structural transformation from P2 phase to O2 phase via molten salt ion exchange, while preserving the composite phase ratio characteristics of the original materials.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMaterials characterizations\u003c/h3\u003e\n\u003cp\u003eX-ray diffraction (XRD) measurements were conducted using a Rigaku X-ray diffractometer (Cu-Kα radiation, λ\u0026thinsp;=\u0026thinsp;1.542 \u0026Aring;) in the 2θ range of 10\u0026ndash;80\u0026deg;. Rietveld refinement data was obtained through GASA-II software. The precise content of dissolved transition metals after cycling was tested by inductively coupled plasma atomic emission spectrometry (ICP-OES, Agilent 5110). The morphology and elements distribution of the samples were obtained with scanning electron microscopy (SEM, Regulus 8100). The valence of the elements in the samples was determined through X-ray photoelectron spectroscopy (XPS, PHI VersaProbe 4).\u003c/p\u003e\n\u003ch3\u003eElectrochemical measurements\u003c/h3\u003e\n\u003cp\u003eIn the electrochemical experiments, the cathode paste was uniformly scraped onto the aluminum foil using a 100 \u0026micro;m scraper. The slurry consisted of 80 wt% cathode powder, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) binder mixed with a certain amount of n-methyl-2-pyrrolidone (NMP). After drying in a vacuum oven at 110\u0026deg;C for 12 h, the positive electrode was stamped into a circular battery pole piece with a diameter of 12 mm. The loading of active substance per pole piece was about 2 mg. Then CR-2032-type coin cells were assembled in an argon-filled glove box with sodium metal as negative electrode, Celgard 2400 and 1.2M LiPF6 EC:EMC\u0026thinsp;=\u0026thinsp;3:7 vt% 1% LiDFOB as electrolyte.\u003c/p\u003e\u003cp\u003eThe electrochemical tests were conducted at 25\u0026deg;C. The galvanostatic charge\u0026ndash;discharge tests were examined at 1 C (1 C\u0026thinsp;=\u0026thinsp;250 mA/g) on a LAND battery test system (LAND, CT2001A).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYufang Chen,\u0026nbsp;Jinhui Wang, Yanshuang Zhao contribute equally. This work was supported by the National Natural Science Foundation of China (No. 51902343) and\u0026nbsp;Natural Science Foundation of Hunan Province (No.\u0026nbsp;2025JJ20041).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJ. Zhao, Y. Su, J. Dong, X. Wang, Y. Lu, N. Li, Q. Huang, J. Hao, Y. Wu, B. Zhang, Q. Qi, F. Wu, L. Chen, Enhanced structural stability and durability in lithium-rich manganese-based oxide via surface double-coupling engineering, Journal of Energy Chemistry 98 (2024) 274\u0026ndash;283. https://doi.org/10.1016/j.jechem.2024.06.047.\u003c/li\u003e\n\u003cli\u003eY. Jin, Z. Zhao, P. Ren, B. Zhang, Z. Chen, Z. Guo, F. Ren, Z. Sun, S. Liu, P. Song, H. Yang, K. Xu, X. Li, Recent Advances in Oxygen Redox Activity of Lithium‐Rich Manganese‐Based Layered Oxides Cathode Materials: Mechanism, Challenges and Strategies, Advanced Energy Materials 14 (2024). https://doi.org/10.1002/aenm.202402061.\u003c/li\u003e\n\u003cli\u003eZ. Wu, C. Li, P. Gao, X. Zhang, Y. Lin, X. Yu, Y. Liu, W. Sun, Y. Jiang, M. Gao, H. Pan, Y. Yang, Nitrogen-based redox couple regulated anionic redox to long-term cycling stability of Li and Mn-rich layered oxide cathode for Li-Ion batteries, Journal of Materials Science \u0026amp;amp; Technology (2024). https://doi.org/10.1016/j.jmst.2024.05.083.\u003c/li\u003e\n\u003cli\u003eZ. Xu, X. Guo, X. Zeng, J. Liu, J. Yin, M. Ren, J. Wang, T. Qin, Z. Zhang, L. Li, K. Amine, Y. Yuan, T. Liu, Coherent Strain-Inhibiting Phase Construction of Lithium-Rich Manganese-Based Oxide Toward High Mechanochemical Stability., Journal of the American Chemical Society (2025). https://doi.org/10.1021/jacs.4c11385.\u003c/li\u003e\n\u003cli\u003eJ. Zhao, Y. Su, J. Dong, X. Wang, Y. Lu, N. Li, Q. Huang, J. Hao, Y. Wu, B. Zhang, Q. Qi, F. Wu, L. Chen, Enhanced structural stability and durability in lithium-rich manganese-based oxide via surface double-coupling engineering, Journal of Energy Chemistry (2024). https://doi.org/10.1016/j.jechem.2024.06.047.\u003c/li\u003e\n\u003cli\u003eY. Jin, Z. Zhao, P. Ren, B. Zhang, Z. Chen, Z. Guo, F. Ren, Z. Sun, S. Liu, P. Song, H. Yang, K. Xu, X. Li, Recent Advances in Oxygen Redox Activity of Lithium‐Rich Manganese‐Based Layered Oxides Cathode Materials: Mechanism, Challenges and Strategies, Advanced Energy Materials 14 (2024). https://doi.org/10.1002/aenm.202402061.\u003c/li\u003e\n\u003cli\u003eW. Hua, S. Wang, M. Knapp, S. Leake, A. Senyshyn, C. Richter, M. Yavuz, J. Binder, C. Grey, H. Ehrenberg, S. Indris, B. Schwarz, Structural insights into the formation and voltage degradation of lithium- and manganese-rich layered oxides, Nature Communications 10 (2019). https://doi.org/10.1038/s41467-019-13240-z.\u003c/li\u003e\n\u003cli\u003eT. Li, Y. Xiao, T. Zhu, Y. Li, W. Wang, Mitigating Capacity and Voltage Decay in Li‐Rich Cathode Via Dual‐Phase Design, Small Methods 9 (2024). https://doi.org/10.1002/smtd.202401206.\u003c/li\u003e\n\u003cli\u003eT. Li, Y. Xiao, T. Zhu, Y. Li, W. Wang, Mitigating Capacity and Voltage Decay in Li‐Rich Cathode Via Dual‐Phase Design, Small Methods 9 (2024). https://doi.org/10.1002/smtd.202401206.\u003c/li\u003e\n\u003cli\u003eY. Chen, Y. Liu, J. Zhang, H. Zhu, Y. Ren, W. Wang, Q. Zhang, Y. Zhang, Q. Yuan, G.-X. Chen, L.C. Gallington, K. Li, X. Liu, J. Wu, Q. Liu, Y. Chen, Constructing O2/O3 homogeneous hybrid stabilizes Li-rich layered cathodes, Energy Storage Materials 51 (2022) 756\u0026ndash;763. https://doi.org/10.1016/j.ensm.2022.07.016.\u003c/li\u003e\n\u003cli\u003eX. Cao, J. Sun, Z. Chang, P. Wang, X. Yue, J. Okagaki, P. He, E. Yoo, H. 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Chen, Study on the impact of cutoff voltage on structural and electrochemical stability of sodium-ion layered cathodes, Chemical Engineering Journal 500 (2024) 157032. https://doi.org/10.1016/j.cej.2024.157032.\u003c/li\u003e\n\u003cli\u003eY. Sun, L. Zhang, S. Dong, J. Zeng, Y. Shen, X. Li, X. Ren, L. Ma, C. Hai, Y. Zhou, Improving the electrochemical performances of Li-rich Li1.2Ni0.13Co0.13Mn0.54O2 through cooperative doping of Na+ and Mg2+, Electrochimica Acta (2022). https://doi.org/10.1016/j.electacta.2022.140169.\u003c/li\u003e\n\u003cli\u003eJ. Cerrato, M. Hochella, W. Knocke, A. Dietrich, T. Cromer, Use of XPS to identify the oxidation state of Mn in solid surfaces of filtration media oxide samples from drinking water treatment plants., Environmental Science \u0026amp; Technology 44 15 (2010) 5881\u0026ndash;6. https://doi.org/10.1021/es100547q.\u003c/li\u003e\n\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":"ionic replacement, O2/O3 biphasic, layered oxides, structural stability","lastPublishedDoi":"10.21203/rs.3.rs-7677792/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7677792/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLithium-rich manganese-based layered oxides are considered next-generation cathode materials due to their ultra-high capacity and voltage plateau. However, the single O3 phase is prone to lattice oxygen loss and irreversible phase transitions during high-voltage cycling, leading to rapid capacity and voltage decay. In this study, a molten salt ion exchange method was used to quantitatively convert the P2 phase into the O2 phase, creating a composite cathode with a precisely adjustable O2/O3 ratio. The effects of phase ratio on microstructure, crystal structure, surface chemistry, and electrochemical performance were systematically investigated. The results show that the introduction of the O2 phase enhances the structural stability of the cathode material. Specifically, the 0.3O2/0.7O3 sample, after 200 cycles at 4.6 V and 1 C, maintained a capacity of 159.8 mAh/g with a capacity retention of 98.8%, a voltage decay rate of only 2.43 mV/cycle, and the smallest shift in the oxidation peak, showing the best overall performance. This work elucidates the synergistic stabilization mechanism of different O2/O3 ratios and provides a reference for phase engineering in lithium-rich manganese-based cathodes.\u003c/p\u003e","manuscriptTitle":"Synergistic O2/O3 Phase Engineering Suppresses Voltage Fade and Enhances Cycling Stability in Lithium-Rich Layered Oxides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-08 11:55:52","doi":"10.21203/rs.3.rs-7677792/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":"7afeaa93-c713-427b-9c92-a95a23a48f7f","owner":[],"postedDate":"October 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-09T02:44:14+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-08 11:55:52","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7677792","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7677792","identity":"rs-7677792","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}