View PDF Version

 

Tuning the rate performance in O2-type layered manganese-based oxides through cobalt doping

Junda Li^a, Xiaoxia Yang*^bc, Guanjie Yan*^d, Jilu Zhang^b, Qin Wang^be, Chunliu Li^d, Laijun Liu*^a and Weibo Hua*^b
aKey Laboratory of New Processing Technology for Nonferrous Metal & Materials, Ministry of Education/Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China. E-mail: ljliu2@163.com
^bState Key Laboratory of Electrical Insulation and Power Equipment, School of Chemical Engineering and Technology, Xi'an Jiaotong University, No. 28, West Xianning Road, Xi'an 710049, China. E-mail: weibo.hua@xjtu.edu.cn
^cSchool of Energy and Materials Engineering, Taiyuan University of Science and Technology, No. 66, Waliu Road, Taiyuan 030024, China. E-mail: yangxiaoxia1@tyust.edu.cn
^dSouth Manganese Building, No. 18 Zhujin Road, Nanning 530028, China. E-mail: yanguanjie@soutnmn.com
^eSchool of Chemical Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China

First published on 5th August 2025

According to the arrangement of oxygen, lithium-rich manganese-based oxides are generally classified into O2 and O3 structures.^5,6 In detail, the O3-type layered structure possesses an ABCABC oxygen stacking sequence, and LiO₆ octahedra share only edges with TMO₆ octahedra, where TM ions can migrate to the nearest neighboring tetrahedral site of the Li layer and readily to adjacent octahedral Li sites owing to the thermodynamic preference for octahedral sites upon the charge process.^6,7 And this migration process is irreversible. They are faced with issues such as severe voltage decay, poor cycling stability and inferior rate performance. Therefore, it is necessary to implement thermodynamic or kinetic roadblocks that inhibit intra-layer movements of TM ions. Researchers have proposed various improvements to maintain the stability of the structure, including lattice doping,⁸ surface coating,⁹ design of the heterostructure,¹⁰ and microstructure engineering.^11–13 However, the intrinsic O3-type configuration framework limits the effects.¹⁴

In the O2-type LLOs with ABCBA oxygen stacking, LiO₆ octahedra share faces with TMO₆ octahedra. Benefiting from the unique coordination environment of the O2-type structure, TM migration from the intermediate sites to adjacent Li sites was impeded by the strong electrostatic repulsion between face-shared TMO₆ octahedra. By streamlining the path for return, this blockage of face-shared sites makes it easier for TM ions to come back during the discharge process, which alleviates the formation of spinel phase and voltage attenuation.^15,16

Many researchers have conducted studies on O2-type layered manganese-based oxides. Xia's group¹⁷ synthesized a novel Li-Mn–O cathode material, Li_0.700[Li_0.222Mn_0.756]O₂ with a single layer Li₂MnO₃ superstructure through a molten salt ion exchange method, with Rm symmetry, displaying ultra-high specific capacity and good cycling stability. Eum et al.⁶ prepared O2-phase Li_x(Li_0.2Ni_0.2Mn_0.6)O₂ (x ≈ 0.83), which allows reversible intra-cycle TM migration, thus delivering outstanding voltage retention over extended cycling and far outperforming their O3-phase counterparts and other lithium-rich layered 3d metal oxides. Feng et al.¹⁸ investigated O2-Li(Li_0.25Mn_0.75)O₂ by Li/Na ion exchange, and the results show that the regulation of Li can improve the cycling stability of Li-rich materials and suppress voltage fading. Nevertheless, despite these applications, O2-type layered manganese-based oxides still suffer from poor rate capability.

Herein, we introduce cobalt into the TM layer of the O2-type layered oxide, Li_0.80[Ni_0.25Mn_0.66Co_0.02□_0.07]O₂, through a low-temperature Li⁺/Na⁺ ion exchange method. The incorporation of Co improves Li⁺ diffusion kinetics and ionic conductivity,¹⁹ exhibiting higher D_Li⁺ and σ_Li⁺ values (3.94 × 10⁻¹² cm⁻² s⁻¹ and 1.23 × 10⁻⁵ S cm⁻¹) than those of Li_0.86[Ni_0.25Mn_0.66□_0.09]O₂ (O2-LNMO) (2.90 × 10⁻¹² cm⁻² s⁻¹ and 1.05 × 10⁻⁵ S cm⁻¹), which showcases a significant improvement in the rate performance. Specifically, O2-LNMCO can deliver 145 mAh g⁻¹ at 5 C and 111.6 mAh g⁻¹ at 10 C, higher than those of O2-LNMO, 102.3 mAh g⁻¹ at 5 C and 17.9 mAh g⁻¹ at 10 C. Importantly, there are no cracks in the fatigued O2-LNMCO cathode, exhibiting superior structural stability. And the O2-LNMCO cathode displays excellent cycling performance with a capacity retention of 85.4% after 50 cycles at 0.1 C. Significantly, the voltage decay of O2-LNMCO was restrained with the attenuation rate of 2.23 mV per cycle.

The precursors for the O2-type Li_0.80[Ni_0.25Mn_0.66Co_0.02□_0.07]O₂ and O2-type Li_0.86[Ni_0.25Mn_0.66□_0.09]O₂ are prepared by the carbonate coprecipitation method, as illustrated in Fig. 1a. The precursors were respectively mixed well with sodium carbonate followed by high-temperature sintering at 800 °C for 12 h to prepare intermediates of sodium ion layered manganese-based oxides, Na_0.80[Ni_0.25Mn_0.66Co_0.02□_0.07]O₂ (NNMCO) and Na_0.86[Ni_0.25Mn_0.66□_0.09]O₂ (NNMO). An ion exchange method was conducted by respectively heating NNMCO and NNMO together with an appropriate amount of LiNO₃ and LiCl (88:12 mol%) in a muffle furnace at 280 °C for 4 h followed by vacuum filtration. The molar ratio of Na⁺ and Li⁺ is 1:4. The detailed experimental procedures are described in the SI. The atomic ratios of Li, Ni, Mn and Co in LNMCO are approximately 0.80:0.25:0.66:0.02 according to inductively coupled plasma mass spectrometry (ICP-MS) results, which is consistent with the theoretical value, as shown in Table S1 in the SI.

	Fig. 1 Structural characterizations of the P2-NNMO, P2-NNMCO, O2-LNMO, and O2-LNMCO materials. (a) Schematic illustration for the preparation of O2-LNMCO and O2-LNMO; XRD patterns of (b) P2-NNMO and P2-NNMCO, and (c) O2-LNMO and O2-LNMCO; SEM images of (d) O2-LNMO and (e) O2-LNMCO.

The phase structures of the samples were investigated by X-ray diffraction (XRD) measurements. As depicted in Fig. 1b, the XRD results indicate that the prepared sodium ion layered oxide intermediates, NNMCO and NNMO, both possess hexagonal P2-type layered structures, with the space group of P6₃mmc, and there are no impurity phases.^20,21 In order to confirm whether the ion exchange is complete, the structures of the two samples after the exchange were analyzed. As shown in Fig. 1c, all reflections of the two samples can be assigned to hexagonal O2-type layered phase with the P6₃mc space group. No other reflections are observed, signifying the generation of a pure layered phase. Rietveld refinement against powder X-ray diffraction was performed. As shown in Tables S2 and S3, the small weighted profile R-factors (R_wp) of both samples indicate that the fitting results are reliable. And the resulting lattice parameters (a, b, c, and V) align well with the previously reported values of O2-LNMO.⁵ Fig. S1–S3 depicts the XRD patterns of the materials with different contents of Co.

The morphological evolutions from the carbonate precursor to the P2-type intermediate, and to O2-type oxides were studied by scanning electron microscopy (SEM). Fig. S4 in the SI exhibits the morphology of the carbonate precursors for two samples. The result shows that the precursor of O2-LNMCO is composed of spherical particles with an average particle size of approximately 5.5 μm. The morphology of the precursor for O2-LNMO is similar to that of O2-LNMCO, whose average particle size is about 6 μm. After further solid-state reaction with Na₂CO₃, the precursors were transformed into P2-NNMCO and P2-NNMO, which retained the spherical morphology of the particles, as shown in Fig. S5. The subsequent ion exchange process in the LiNO₃/LiCl molten salt had little effect on the overall morphology of the particles. Both O2-LNMO and O2-LNMCO inherited the morphology of the P2-type oxide intermediates, possessing spherical particles. The SEM images of O2-LNMO and O2-LNMCO are shown in Fig. 1d and e, respectively. Furthermore, the elemental distributions of the two samples were investigated by SEM-energy dispersive spectroscopy (EDS) mapping. As illustrated in Fig. S6a and b, both O2-LNMO and O2-LNMCO display homogeneous distributions of Ni, Mn, and O or Ni, Mn, Co and O, respectively.

In order to evaluate the influence of cobalt on the electrochemical performance, a series of electrochemical measurements were carried out in CR2032-type coin cells at 25 °C, using Li metal as the anode and a LiPF₆-based electrolyte. The initial charge/discharge curves of the O2-LNMO and O2-LNMCO cathodes at 0.1 C (1 C = 280 mA g⁻¹) within a voltage range of 2.0 to 4.8 V are illustrated in Fig. 2a. Both cathodes show an inclined slope below 4.6 V and a small voltage plateau at around 4.6 V, corresponding to the oxidation processes from Ni²⁺/Co³⁺ to Ni⁴⁺/Co⁴⁺ and lattice oxygen, respectively. The initial discharge capacity of O2-LNMCO is 233.5 mAh g⁻¹, higher than that of O2-LNMO (206.2 mAh g⁻¹), which can be attributed to the additional redox of Co³⁺. Significantly, both cathodes display an initial coulombic efficiency beyond 100%, assigned to the presence of around 0.20 and 0.14 vacancies in O2-Li_0.80[Ni_0.25Mn_0.66Co_0.02□_0.07]O₂ (130%) and Li_0.86[Ni_0.25Mn_0.66□_0.09]O₂ (120%), respectively. Importantly, the average discharge voltage of O2-LNMCO is higher than that of O2-LNMO, which suggests that the energy levels of the energy bands formed by the O 2p orbitals and Co 3d orbitals in LNMCO are smaller than those of the two bands in LNMO. This difference in energy levels results in a lower energy requirement for the redox process of LNMCO, which may endow LNMCO with superior electrochemical activity.²⁰ The cycling stabilities of the two cathode materials are further investigated at 0.1 C between 2.0 and 4.8 V. As shown in Fig. 2b and Fig. S7, notably, O2-LNMCO demonstrates superior cycling stability, maintaining 85.4% of its initial capacity after 50 cycles. In stark contrast to O2-LNMCO, over long-term cycling, the discharge capacity of the O2-LNMO electrode exhibits serious decline, retaining 74.7% of the initial capacity. In addition, O2-LNMCO delivers negligible voltage decay, holding steady at nearly 96.8% (decay rate of 2.23 mV per cycle) during cycling, exceeding the values observed in O2-LNMO (94.9%, decay rate of 3.40 mV per cycle), as depicted in Fig. 2c. These results suggest that O2-LNMCO exhibits superior structural stability during prolonged cycling. The rate properties of the O2-LNMCO and O2-LNMO cathodes are presented Fig. 2d and Fig. S8. O2-LNMCO yields the discharge capacities of 163.4, 145.5 and 111.6 mAh g⁻¹ at 3 C, 5 C, and 10 C, respectively, which are superior to O2-LNMO (121.5, 102.3, and 17.9 mAh g⁻¹) and other Li-rich cathode materials (see Tables S4 and S5, SI). The Li-ion (de)intercalation kinetics and electronic conductivity of the two cathodes were determined by electrochemical impedance spectroscopy (EIS) tests. The EIS data were analyzed by Zview software, and the corresponding equivalent circuit is shown in the inset of Fig. 2e. Ex situ EIS impedance spectra of the O2-LNMCO cathode at various charge/discharge states are displayed in Fig. S9. Tables S6 and S7 present the corresponding fitting results. The O2-LNMCO cathode yields lower R_ct (64 Ω), higher D_Li⁺ (3.94 × 10⁻¹¹ cm⁻² s⁻¹) and higher electronic conductivity (1.23 × 10⁻⁵ S cm⁻¹) than those of the O2-LNMO electrode (168 Ω, 2.90 × 10⁻¹² cm⁻² s⁻¹, 1.05 × 10⁻⁵ S cm⁻¹), demonstrating more facile Li-ion transport kinetics in the O2-LNMCO electrode.

	Fig. 2 Electrochemical performance of O2-LNMCO and O2-LNMO at 25 °C. (a) Initial charge/discharge voltage curves at 0.1 C; (b) cycling and (c) average voltage stability for two electrodes at 0.1 C within 2.0–4.8 V; (d) rate performance and (e) EIS impedance spectra with an equivalent circuit inset of two cathodes; and (f) the DLi+ values calculated from GITT during the second cycle within 2.0–4.8 V.

The electrochemical redox responses of O2-LNMCO and O2-LNMO were evaluated by cyclic voltammetry (CV) measurements within 2.0–4.8 V. As shown in Fig. S10, two oxidation peaks at around 4.0 and 4.6 V (vs. Li⁺/Li) in the anodic scan of both electrodes can be assigned to the oxidation of Co³⁺/Ni²⁺ and O²⁻ ions. During the reversal scan, two reduction peaks emerge at approximately 2.7 and 3.7 V, corresponding to the reduction of Mn⁴⁺ and Ni^3+/4+, respectively. Notably, the positions of the CV peaks of O2-LNMCO occur earlier with higher intensity than those of O2-LNMO, which suggest faster Li⁺ migration dynamics and stronger reaction activity of O2-LNMCO after cobalt incorporation. Furthermore, the galvanostatic intermittent titration technique (GITT) was conducted at 28 mA g⁻¹ to investigate Li⁺ diffusion kinetics during the electrochemical reaction. The Li-ion diffusion coefficient was calculated according to the equation in the SI. As depicted in Fig. 2f and Fig. S11, the D_Li⁺ of O2-LNMCO is higher than that of O2-LNMO, corresponding to faster Li⁺ transport kinetics.

In order to figure out the underlying cause of voltage and capacity fading, the structural and morphological alterations of the fatigued electrodes were examined by SEM and XRD analyses. The morphological alterations in O2-LNMO and O2-LNMCO are displayed in Fig. 3a and b. Distinctly, after 50 cycles, the particle cracks are clearly observed in cycled O2-LNMO, suggesting substantial structural variations during cycling. In contrast, O2-LNMCO displays no cracks, again verifying its excellent structural stability. Furthermore, XRD was carried out to analyze structural changes of the two cathodes before and after cycling. As illustrated in Fig. 3c and d, compared to the cycled O2-LNMCO cathode, all reflections of the fatigued O2-LNMO electrode progressively broadened and weakened upon cycling, demonstrating severe structural degradation. Furthermore, compared with O2-LNMCO, all the reflections in the XRD patterns of the O2-LNMO cathode shift towards higher scattering angles after 50 cycles, indicating a contrast of unit cell volume. Ex situ XRD patterns of the O2-LNMCO cathode upon cycling illustrate superior structural stability, as shown in Fig. S12. These findings underline the favorable role of Co in improving the property of O2-type layered manganese-based oxides during long-term cycling.

	Fig. 3 Structural and morphological evolutions of the O2-LNMO and O2-LNMCO cathodes after cycling. SEM images of the (a) O2-LNMO and (b) O2-LNMCO cathodes after cycling; a comparison of the XRD patterns for the (c) O2-LNMO and (d) O2-LNMCO electrodes before and after cycling.

In summary, a low-temperature Li⁺/Na⁺ ion-exchange strategy is developed by synthesizing O2-LNMCO from P2-type NNMCO. XRD Rietveld refinement confirms lattice expansion upon Co doping due to the increased ionic radius. EIS and GITT analyses reveal that O2-LNMCO exhibits lower charge transfer resistance and faster Li⁺ diffusion kinetics compared to O2-LNMO. Electrochemical measurements demonstrate superior rate performance. Specifically, O2-LNMCO delivers discharge capacities of 145 mAh g⁻¹ at 5 C and 111.6 mAh g⁻¹ at 10 C, higher than those of O2-LNMO (102.3 and 17.9 mAh g⁻¹). Notably, the fatigued O2-LNMCO cathode maintains structural integrity, showing 83.2% capacity retention after 50 cycles at 0.1 C, higher than the 73.9% of O2-LNMO. The voltage decay of O2-LNMCO is reduced to 2.23 mV per cycle, lower than that of O2-LNMO (3.40 mV per cycle). This strategy provides a feasible approach to enhance rate capability and mitigate voltage fading in the lithium-rich cathodes for high-energy lithium-ion battery material design.

This work was financially supported by the National Natural Science Foundation of China (Grant No. 20A20145 and 22108218), and the Shaanxi Natural Science Basic Research Program (Grant No. 2025JC-QYCX-011), and the Scientific and Technological Innovation Project of CITIC Group (No. M2030). W. H. acknowledges “Young Talent Support Plan” of Xi’an Jiaotong University (HG6J016) and Qinchuangyuan Innovative Talent Project (QCYRCXM-2022-137), and Shaanxi Sanqin Talent Special Support Program: Young Top-Notch Talent Project. We greatly appreciate Zhongzhu Liu from CITIC Metal Co., Ltd. for his guidance. The authors wish to acknowledge the Instrument Analysis Center of Xi'an Jiaotong University for the assistance test.

Conflicts of interest

Data availability

Experimental sections, SEM images, Rietveld refinement results of XRD data, crystallographic parameters, ex situ XRD patterns of the samples, EIS results. See DOI: https://doi.org/10.1039/d5cc03661e

1. W. Hua, S. Wang, M. Knapp, S. J. Leake, A. Senyshyn, C. Richter, M. Yavuz, J. R. Binder, C. P. Grey, H. Ehrenberg, S. Indris and B. Schwarz, Nat. Commun., 2019, 10, 5365 CrossRef PubMed .
2. X. Li, S. Yu, J. Peng, L. Liang, Q. Pan, F. Zheng, H. Wang, Q. Li and S. Hu, Small, 2025, e2500940 CrossRef PubMed .
3. Q. Wang, M. Yao, A. Zhu, Q. Wang, H. Wu and Y. Zhang, Angew. Chem., Int. Ed., 2023, 62, e202309049 CrossRef CAS PubMed .
4. Y. Li, S. Xu, W. Zhao, Z. Chen, Z. Chen, S. Li, J. Hu, B. Cao, J. Li, S. Zheng, Z. Chen, T. Zhang, M. Zhang and F. Pan, Energy Storage Mater., 2022, 45, 422–431 CrossRef .
5. X. Yang, T. Zhao, X. Zhai, J. Zhang, S. Wang and W. Hua, Ind. Eng. Chem. Res., 2024, 63, 4197–4204 CrossRef CAS .
6. D. Eum, B. Kim, S. J. Kim, H. Park, J. Wu, S. P. Cho, G. Yoon, M. H. Lee, S. K. Jung, W. Yang, W. M. Seong, K. Ku, O. Tamwattana, S. K. Park, I. Hwang and K. Kang, Nat. Mater., 2020, 19, 419–427 CrossRef CAS PubMed .
7. X. Yang, S. Wang, H. Li, J. Tseng, Z. Wu, S. Indris, H. Ehrenberg, X. Guo and W. Hua, Electron, 2024, 2, e18 CrossRef CAS .
8. F. Cao, W. Zeng, J. Zhu, J. Xiao, Z. Li, M. Li, R. Qin, T. Wang, J. Chen, X. Yi, J. Wang and S. Mu, Small, 2022, 18, 2200713 CrossRef CAS PubMed .
9. T. Zhao, J. Zhang, K. Wang, Y. Xiao, Q. Wang, L. Li, J. Tseng, M. C. Chen, J. J. Ma, Y. R. Lu, I. Hirofumi, Y. C. Shao, X. Zhao, S. F. Hung, Y. Su, X. Mu and W. Hua, Angew. Chem., Int. Ed., 2025, 64, e202419664 CrossRef CAS PubMed .
10. S. Wang, W. Hua, A. Missyul, M. S. D. Darma, A. Tayal, S. Indris, H. Ehrenberg, L. Liu and M. Knapp, Adv. Funct. Mater., 2021, 31, 2009949 CrossRef CAS .
11. J. Zhang, X. Zhai, T. Zhao, X. Yang, Q. Wang, Z. Chen, M.-C. Chen, J.-J. Ma, Y.-R. Lu, S.-F. Hung and W. Hua, J. Mater. Chem. A, 2025, 13, 1181–1190 RSC .
12. J. Ge, C. Ma, Y. Zhang, P. Ma, J. Zhang, Z. Xie, L. Wen, G. Tang, Q. Wang, W. Li, X. Guo, Y. Guo, E. Zhang, Y. Zhang, L. Zhao and W. Chen, Adv. Mater., 2024, 2413253 CrossRef PubMed .
13. J. Luo, K. Yang, J. Gai, X. Zhang, C. Peng, C. Qin, Y. Ding, Y. Yuan, Z. Xie, P. Yan, Y. Cao, J. Lu and W. Chen, Angew. Chem., Int. Ed., 2025, 64, e202419490 CrossRef CAS PubMed .
14. J. Feng, Y.-S. Jiang, F.-D. Yu, W. Ke, L.-F. Que, J.-G. Duh and Z.-B. Wang, J. Energy Chem., 2022, 66, 666–675 CrossRef CAS .
15. C. Cui, X. Fan, X. Zhou, J. Chen, Q. Wang, L. Ma, C. Yang, E. Hu, X.-Q. Yang and C. Wang, J. Am. Chem. Soc., 2020, 142, 8918–8927 CrossRef CAS PubMed .
16. C. Heubner, B. Matthey, T. Lein, F. Wolke, T. Liebmann, C. Lämmel, M. Schneider, M. Herrmann and A. Michaelis, Energy Storage Mater., 2020, 27, 377–386 CrossRef .
17. Y. Zuo, B. Li, N. Jiang, W. Chu, H. Zhang, R. Zou and D. Xia, Adv. Mater., 2018, 30, 1707255 CrossRef PubMed .
18. J. Feng, Y. Jiang, F. Yu, W. Ke, L. Que, J. Duh and Z. Wang, J. Energy Chem., 2022, 66, 666–675 CrossRef CAS .
19. X. Duan, S. Wu, J. Zheng, L. Hua, C. Lou, M. Tang, X. Deng, J. Lv, J. Fang, L. Liu and J. Xu, Acta Mater., 2024, 280, 120345 CrossRef CAS .
20. X. Yang, S. Wang, H. Li, J. Peng, W. J. Zeng, H. J. Tsai, S. F. Hung, S. Indris, F. Li and W. Hua, ACS Nano, 2023, 17, 18616–18628 CrossRef CAS PubMed .
21. L. Wen, J. Zhang, J. Zhang, L. Zhao, X. Wang, S. Wang, S. Ma, W. Li, J. Luo, J. Ge and W. Chen, eScience, 2025, 5, 100313 CrossRef .