English

• 1. INTRODUCTION

  Recently, the low available specific capacity of traditional cathode materials such as LiCoO₂, LiMn₂O₄, and LiFePO₄ is becoming a bottleneck to the increase of energy densities of the lithium-ion batteries (LIBs) for electric vehicles and electricity grid. Layered Li-rich Mn-based oxide cathode (LMC) materials, a solid solution of Li₂MnO₃ and LiMO₂ (M = Ni, Mn, Co), are considered as potential candidate cathodes for the next-generation LIBs due to its high specific capacity (> 250 mAh g^-1), high average discharge voltage (> 3.5 V) and low cost^{[1, 2]} (Fig. 1a and 1b). Nevertheless, commercial applications of Li-rich Mn-based cathode materials are challenged by three main drawbacks. First, a large irreversible capacity loss happens in the first charge-discharge process^[3] (Fig. 1c). Second, significant voltage decay occurs during cycling^[4] (Fig. 1d). Third, their cyclability and rate capability are not sufficient^{[5, 6]} (Fig. 1e).

  Figure 1

  

  Figure 1.  (a) Structure units of Li-rich Mn-based cathodes (LMC); From ref. [1], copyright of ACS; (b) Capacity and operation voltage comparisons of the present common cathodes for LIBs; From ref. [2], copyright of ACS; (c) Charge-discharge curves showing the irreversible first-cycle charge-discharge process of LMC; From ref. [3], copyright of nature; (d) Charge-discharge curves at different cycle number showing the voltage decay of LMC; From ref. [4], copyright of ACS; (e) The poor cyclability and rate capability of bare LMC; From ref. [5, 6], copyright of Wiley

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  To solve such problems, many researches have been done, including the mechanism study and attempt to improve the electrochemical performance. This article comprehensively presents the challenge and recent progress of high-capacity Li-rich Mn-based cathodes from a fundamental perspective and will also provide suggestions for future research of high-energy cathodes.

  2. THE FIRST IRREVERSIBLE CHARGE-DISCHARGE PROCESSES

  Li₂MnO₃ activation is the key and most attractive reaction of Li-rich oxides in the first charge-discharge process, which is the origin of its extra-high capacity^[30]. Many studies have been done to understand the activation process of Li₂MnO₃. Bruce's group firstly observed the O₂ release during the first charge process, and proposed an O loss accompanied with Li⁺ extraction mechanism for Li₂MnO₃ activation^[31] (Fig. 2a). Then they re-determined that the O₂ release is from the electrolyte decomposition at high voltage after ten years^[30]. Formation of localized electron holes on O atoms (O₂^2-) was experimentally observed instead at the Li₂MnO₃ activation region, and ex-situ resonant inelastic X-ray scattering (RIXS) was used as detect tool; The O₂^2- localized electron holes can reversibly redox during the charge-discharge process^[30], and such a behavior was called as "oxygen redox". The reversible oxygen redox phenomenon was supported by Ceder's group using the first-principle calculations^[7]. Afterwards, Chueh's group adopted ex-situ scanning transmission X-ray microscope (STXM) and obtained a coupling phenomenon between oxygen redox and transition metal cation migration^[8]. However, the limitation of these results is that they are achieved using ex-situ methods^{[3, 8]} or calculations^[7]. While in-situ Raman was applied for Li₂MnO₃ activation study, entirely different Li₂O evolution^[9] (Fig. 2b) and O^2-/O^- redox observation were acquired^[10]. Besides, Raman and RIXS (100 nm) can only obtain the surface information of Li-rich materials. To get the real-time bulk information during charge-discharge process, Sun's Group adopted in-situ X-ray diffraction (XRD) and electrochemical quartz crystal microbalance (EQCM) to investigate the first-cycle behavior of Li-rich oxides, which can get the bulk structure evolution and mass change^{[11, 12]}. β-MnO₂ product and phase transformation of β-MnO₂ to layered Li_0.9MnO₂ were observed using in-situ XRD^[11]. In-situ EQCM results demonstrate that both Li₂O evolution and oxygen redox exist in bare Li₂MnO₃ and LMC materials, while Li₂O evolution is dominated in bare Li₂MnO₃ and oxygen redox in LMC cathode^[12] (Fig. 2c). This explains why the Li-rich material (0.5Li₂MnO₃·0.5LiNi_0.3Co_0.3Mn_0.4O₂) has a higher capacity than the mean value of Li₂MnO₃ and LiNi_0.3Co_0.3Mn_0.4O₂. However, the real species of Li₂O and O₂^2- were not observed using other direct tools. From above, there is still controversy about the first charge-discharge process of LMC cathodes. One possible reason for the inconsistency of different groups' work is the composition difference of LMC cathodes they used. And in-situ methods development, which can get both the surface and bulk information, is vital to unravel the internal first charge-discharge mechanism. This is further benefit to reduce the irreversible capacity in the first cycle.

  Figure 2

  

  Figure 2.  (a) Operando mass spectrometry showing the first-cycle oxygen release at high voltage; From ref. [31], copyright of Nature; (b) Scheme of the Li₂O evolution during the first cycle; From ref. [9], copyright of ACS; (c) Scheme of different activation ways of bare Li₂MnO₃ (Li₂O evolution domination) and Li₂MnO₃ component in LMC (oxygen redox domination); From ref. [12], copyright of ACS; (d) Scheme of the voltage decay mechanism showing metal cations migration for layered-to-spinel conversion; From ref. [1], copyright of ACS; (e) EDS mapping showing the different Mn/Ni distribution in Li-rich particles obtained by different synthesis methods; From ref. [16], copyright of ACS; (f) TEM image showing the guar gum (GG) binder coated on LMC particle; From ref. [19], copyright of RSC; copyright of ACS; (g) Cyclability and rate performance of crystal habit-tuned LMC nanoplates; SEM image of the LMC nanoplates is insert; From ref. [21], copyright of Wiley; (h) Structure-dependent cyclability and rate capability of LMC cathodes; From ref. [22], copyright of Elsevier; (i) Initial charge-discharge profiles and Coulombic efficiencies of Li-rich, spinel, Li-rich layered/spinel composite and physical mixture of Li-rich and spinel; From ref. [25], copyright of ACS

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  3. VOLTAGE DECAY

  The Li-rich cathodes suffer voltage decay during cycling, which means average discharge voltage decreases with the cycling number. The voltage decay mechanism was always attributed to layered-to-spinel phase transformation and migration of Mn and Li between transition metal layer and lithium layer^{[1, 4]} (Fig. 2d), which are obtained using ex-situ transmission electron microscope (TEM) and neutron powder diffraction. Some more detailed work revealed that the layered-to-spinel phase transformation prefers to take place in lattice defect^[13]; In-situ XRD and Mass spectroscopy (MS) results demonstrated that a low Co/Ni ratio LMC released no lattice oxygen at the first cycle, which leads to a milder voltage decay behavior^[14]. Based on this mechanism, it is reasonable that doping and structure modulation can remit voltage decay, e.g. Al³⁺ doping^[15] and metal ions distribution regulation^[16] (Fig. 2e). The conventional oxides coatings, such as Al₂O₃, LiAlO_x, ZrO₂ and so on have been proved that they have little-to-zero effect on voltage decay mitigation^[17]. Some functional coatings (e.g. LiFePO₄^[18]) partly suppress voltage decay, but the reason was ascribed to bulk metal ion doping of core Li-rich materials caused by surface modification. But when organic guar gum (GG), which only includes C, H, O elements and lack of metal ions, was applied as the binder of LMC cathode electrode, a coating layer on LMC particles was observed after cycling^[19]. Such a coating layer is composed of GG and solid electrolyte interphase (SEI), leading to an abnormal voltage fade remission^[19]. To the best of our knowledge, there are no reports that electrolyte decomposition produced SEI can induce bulk doping of particles^{[26, 27]}. And it is infeasible that an organic GG binder can induce bulk doping. The extraordinary voltage decay mitigation by GG binder indicates an in-depth mechanism existing. But Sun's work doesn't reveal the specific role of water-soluble GG binder^[19]. Further studies still need to be done to reveal the root origin of voltage decay, especially using in-situ methods, and strategies to eliminate voltage decay of LMC cathodes are urgent to develop.

  4. CYCLABILITY AND RATE CAPABILITY IMPROVEMENT

  Li-rich cathode materials suffer capacity fade during cycling, and its rate capability is still not enough for commercial applications due to its structure instability, side reactions with electrolyte and low Li⁺ conductivity^[20]. To improve the cyclability and rate capability of Li-rich materials, the most common methods were doping, surface modification, structure regulation and composition adjustment^[20-25]. Doping and surface modifications always need to introduce inert metal ions or coatings, such as Mg²⁺, Ti³⁺, ZrO₂, etc.^{[20, 28]}, which results in capacity decrease of Li-rich materials. In comparison, structure regulation and composition adjustment only simply require synthesis condition and raw materials ratio controlling, such as tuning the thermodynamic and kinetic conditions^[29], optimizing the particle shape, particle size, crystal orientation, morphology and layered/spinel ratio, and so forth^[21-25]. And such simple structure regulation and composition adjustment methods can improve the cyclability, rate capability, and initial Coulomb efficiency while keeping capacity^[21-25].

  For example, Sun's group prepared crystal habittuned nanoplates with increased (010) nanoplate, which exhibit excellent cyclability and rate performance^[21] (Fig. 2g); High rate-capability Li-rich cathodes with hierarchical micro/nanostructure were synthesized by solvothermal and coprecipitation methods^[22]; The relationship between the morphologies and electrochemical properties were also revealed^[23] (Fig. 2h). Layered/spinel composite materials with spinel Li₄Mn₅O₁₂ and LiNi_0.5Mn_1.5O₄ composition were obtained using one-step solvothermal and Pechini methods, respectively^{[24, 25]}; The cyclability, rate capability and first-cycle Coulomb efficiency were improved in the same time, while the capacity was well kept, compared to the bare layered Li-rich cathodes^{[24, 25]}. A comparison of bare Li-rich, spinel, Li-rich layered/spinel, physical mixture of Li-rich and spinel indicate the positive role of integrated spinel in electrochemical performance improvement (Fig. 2i).

  5. SUMMARY AND PERSPECTIVE

  In summary, three main drawbacks hindering the commercial applications of Li-rich Mn-based cathode materials for LIBs were briefly reviewed. Based on the existing experimental and theoretical results, some solid conclusions and perspectives are as follows: 1) Developing in-situ research techniques, which can obtain both surface and bulk information, is vital to clarify the unclear first charge-discharge process and voltage fade mechanism, especially the origin of extra-high capacity of LMC cathodes and the function of organic coating layer on voltage decay mitigation; 2) The different compositions of Li-rich materials used in different groups' research may be part of the reason why different results were obtained, even using the same characterization method; It is urgent to build a quantitative composition-mechanism-performance relationship of Li-rich cathodes, which can guide the future research and applications; 3) Structure regulation and composition adjustment can provide a simple and valid way to achieve efficient cyclability, rate capability and the first-cycle Coulombic efficiency; Unclear first charge-discharge process and voltage decay are the present key issues hindering the applications of LMC cathodes; Overall, we believe Li-rich Mn-based cathodes will play a vital role in next-generation LIBs due to its unique advantages of high capacity, high operation voltage and low cost. And further studies need to be focused on the charge-discharge mechanism and elimination of voltage fade during cycling.

  
  
• 1. [1]

     Mohanty, D.; Li, J.; Abraham, D. P.; Huq, A.; Payzant, E. A.; Wood III, D. L.; Daniel, C. Unraveling the voltage-fade mechanism in high-energy-density lithium-ion batteries: origin of the tetrahedral cations for spinel conversion. Chem. Mater. 2014, 26, 6272–6280. doi: 10.1021/cm5031415

  2. [2]

     Liu, C.; Neale, Z. G.; Cao, G. Understanding electrochemical potentials of cathode materials in rechargeable batteries. Mater. Today 2016, 19, 109–123. doi: 10.1016/j.mattod.2015.10.009

  3. [3]

     Shunmugasundaram, R.; Senthil Arumugam, R.; Dahn, J. R. High capacity Li-rich positive electrode materials with reduced first-cycle irreversible capacity loss. Chem. Mater. 2015, 27, 757–767. doi: 10.1021/cm504583y

  4. [4]

     Gu, M.; Belharouak, I.; Zheng, J.; Wu, H.; Xiao, J.; Genc, A.; Amine, A.; Thevuthasan, S.; Baer, D.; Zhang, J.; Browning, N. D.; Liu, J.; Wang, C. (2012). Formation of the spinel phase in the layered composite cathode used in Li-ion batteries. ACS Nano 2013, 7, 760–767. doi: 10.1021/nn305065u

  5. [5]

     Liu, W.; Oh, P.; Liu, X.; Myeong, S.; Cho, W., Cho, J. Countering voltage decay and capacity fading of lithium-rich cathode material at 60 ℃ by hybrid surface protection layers. Adv. Energy Mater. 2015, 5, 1500274. doi: 10.1002/aenm.201500274

  6. [6]

     Yu, X.; Lyu, Y.; Gu, L.; Wu, H.; Bak, S. M.; Zhou, Y.; Amine, K.; Ehrlich, S.; Li, H.; Nam, K.; Yang, X. Q. Understanding the rate capability of high-energy-density Li-rich layered Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ cathode materials. Adv. Energy Mater. 2014, 4, 1300950. doi: 10.1002/aenm.201300950

  7. [7]

     Seo, D. H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials. Nat. Chem. 2016, 8, 692–697. doi: 10.1038/nchem.2524

  8. [8]

     Gent, W. E.; Lim, K.; Liang, Y.; Li, Q.; Barnes, T.; Ahn, S. J.; Chueh, W. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat. Commun. 2017, 8, 2091. doi: 10.1038/s41467-017-02041-x

  9. [9]

     Hy, S.; Felix, F.; Rick, J.; Su, W. N.; Hwang, B. J. Direct in situ observation of Li₂O evolution on Li-rich high-capacity cathode material, Li[Ni_xLi_(1-2x)/3Mn_(2-x)/3]O₂ (0≤x≤0.5). J. Am. Chem. Soc. 2014, 136, 999–1007. doi: 10.1021/ja410137s

  10. [10]

      Li, X.; Qiao, Y.; Guo, S.; Xu, Z.; Zhu, H.; Zhang, X.; Yuan, Y.; He, P.; Ishida, M.; Zhou, H. Direct visualization of the reversible O²⁻/O⁻ redox process in Li-rich cathode materials. Adv. Mater. 2018, 30, 1705197. doi: 10.1002/adma.201705197

  11. [11]

      Shen, C. H.; Wang, Q.; Fu, F.; Huang, L.; Lin, Z.; Shen, S. Y.; Su, H.; Zheng, X.; Xu, B.; Li, J. T.; Sun, S. G. Facile synthesis of the Li-rich layered oxide Li_1.23Ni_0.09Co_0.12Mn_0.56O₂ with superior lithium storage performance and new insights into structural transformation of the layered oxide material during charge–discharge cycle: in situ XRD characterization. ACS Appl. Mater. Inter. 2014, 6, 5516–5524. doi: 10.1021/am405844b

  12. [12]

      Yin, Z. W.; Peng, X. X.; Li, J. T.; Shen, C. H.; Deng, Y. P.; Wu, Z. G.; Zhang, T.; Zhang, Q.; Mo, Y.; Wang, K.; Huang, L.; Zheng, H.; Sun, S. G. Revealing of the activation pathway and cathode electrolyte interphase evolution of Li-Rich 0.5Li₂MnO₃·0.5LiNi_0.3Co_0.3Mn_0.4O₂ cathode by in situ electrochemical quartz crystal microbalance. ACS Appl. Mater. Inter. 2019, 11, 16214–16222. doi: 10.1021/acsami.9b02236

  13. [13]

      Shen, C. H.; Shen, S. Y.; Fu, F.; Shi, C. G.; Zhang, H. Y.; Pierre, M. J.; Su, H.; Wang, Q.; Xu, B.; Huang, L.; Li, J. T.; Sun. S. G. New insight into structural transformation in Li-rich layered oxide during the initial charging. J. Mater. Chem. A 2015, 3, 12220–12229. doi: 10.1039/C5TA01849H

  14. [14]

      Shen, S.; Hong, Y.; Zhu, F.; Cao, Z.; Li, Y.; Ke, F.; Fan, J.; Zhou, L.; Wu, L.; Dai, P.; Cai, M.; Huang, L.; Zhou, Z.; Li, J.; Wu, Q.; Sun. S. G. Tuning electrochemical properties of Li-rich layered oxide cathodes by adjusting Co/Ni ratios and mechanism investigation using in situ X-ray diffraction and online continuous flow differential electrochemical mass spectrometry. ACS Appl. Mater. Inter. 2018, 10, 12666–12677. doi: 10.1021/acsami.8b00919

  15. [15]

      Nayak, P. K.; Grinblat, J.; Levi, M.; Levi, E.; Kim, S.; Choi, J. W.; Aurbach, D. Al doping for mitigating the capacity fading and voltage decay of layered Li and Mn-rich cathodes for Li-Ion batteries. Adv. Energy Mater. 2016, 6, 1502398. doi: 10.1002/aenm.201502398

  16. [16]

      Zheng, J.; Gu, M.; Genc, A.; Xiao, J.; Xu, P.; Chen, X.; Zhu, Z.; Zhao, W.; Pullan, L.; Wang, C.; Zhang, J. G. Mitigating voltage fade in cathode materials by improving the atomic level uniformity of elemental distribution. Nano Lett. 2014, 14, 2628–2635 doi: 10.1021/nl500486y

  17. [17]

      Bloom, I.; Trahey, L.; Abouimrane, A.; Belharouak, I.; Zhang, X.; Wu, Q.; Lu, W.; Abraham, D.; Bettge, M.; Elam, J.; Meng, X.; Burrell, A.; Ban, C.; Tenent, R.; Nanda, J.; Dudney, N. Effect of interface modifications on voltage fade in 0.5Li₂MnO₃·0.5LiNi_0.375Mn_0.375Co_0.25O₂ cathode materials. J. Power Sources 2014, 249, 509–514. doi: 10.1016/j.jpowsour.2013.10.035

  18. [18]

      Zheng, F.; Yang, C.; Xiong, X.; Xiong, J.; Hu, R.; Chen, Y.; Liu, M. Nanoscale surface modification of lithium-rich layered-oxide composite cathodes for suppressing voltage fade. Angew. Chem. Int. Edit. 2015, 54, 13058–13062. doi: 10.1002/anie.201506408

  19. [19]

      Zhang, T.; Li, J. T.; Liu, J.; Deng, Y. P.; Wu, Z. G.; Yin, Z. W.; Guo, D.; Huang, L.; Sun, S. G. Suppressing the voltage-fading of layered lithium-rich cathode materials via an aqueous binder for Li-ion batteries. Chem. Commun. 2016, 52, 4683–4686. doi: 10.1039/C5CC10534J

  20. [20]

      Hu, S.; Pillai, A. S.; Liang, G.; Pang, W. K.; Wang, H.; Li, Q.; Guo, Z. Li-rich layered oxides and their practical challenges: recent progress and perspectives. Electrochem. Energy Rev. 2019, 1–35.

  21. [21]

      Wei, G. Z.; Lu, X.; Ke, F. S.; Huang, L.; Li, J. T.; Wang, Z. X.; Zhou, Z. Y.; Sun, S. G. Crystal habit-tuned nanoplate material of Li[Li_1/3-2x/3Ni_xMn_2/3-x/3]O₂ for high-rate performance lithium-ion batteries. Adv. Mater. 2010, 22, 4364–4367. doi: 10.1002/adma.201001578

  22. [22]

      Fu, F.; Deng, Y. P.; Shen, C. H.; Xu, G. L.; Peng, X. X.; Wang, Q.; Xu, Y.; Fang, J.; Huang, L.; Sun, S. G. A hierarchical micro/nanostructured 0.5Li₂MnO₃·0.5LiMn_0.4Ni_0.3Co_0.3O₂ material synthesized by solvothermal route as high rate cathode of lithium ion battery. Electrochem. Commun. 2014, 44, 54–58. doi: 10.1016/j.elecom.2014.04.013

  23. [23]

      Fu, F.; Yao, Y.; Wang, H.; Xu, G. L.; Amine, K.; Sun, S. G.; Shao, M. Structure dependent electrochemical performance of Li-rich layered oxides in lithium-ion batteries. Nano Energy 2017, 35, 370–378. doi: 10.1016/j.nanoen.2017.04.005

  24. [24]

      Deng, Y. P.; Fu, F.; Wu, Z. G.; Yin, Z. W.; Zhang, T.; Li, J. T.; Huang, L.; Sun, S. G.; Layered/spinel heterostructured Li-rich materials synthesized by a one-step solvothermal strategy with enhanced electrochemical performance for Li-ion batteries. J. Mater. Chem. A 2016, 4, 257–263. doi: 10.1039/C5TA06945A

  25. [25]

      Yin, Z. W.; Wu, Z. G.; Deng, Y. P.; Zhang, T.; Su, H.; Fang, J. C.; Xu, B.; Wang, J.; Li, J. T.; Huang, L.; Zhou, X.; Sun, S. G. A synergistic effect in a composite cathode consisting of spinel and layered structures to increase the electrochemical performance for Li-ion batteries. J. Phys. Chem. C 2016, 120, 25647–25656. doi: 10.1021/acs.jpcc.6b07169

  26. [26]

      Huang, W.; Xing, L.; Wang, Y.; Xu, M.; Li, W.; Xie, F.; Xia, S. 4-(Trifluoromethyl)-benzonitrile: a novel electrolyte additive for lithium nickel manganese oxide cathode of high voltage lithium ion battery. J. Power Sources 2014, 267, 560–565. doi: 10.1016/j.jpowsour.2014.05.124

  27. [27]

      Li, J.; Zhang, L.; Yu, L.; Fan, W.; Wang, Z.; Yang, X.; Lin, Y.; Xing, L.; Xu, M.; Li, W. Understanding interfacial properties between Li-rich layered oxide and electrolyte containing triethyl borate. J. Phys. Chem. C 2016, 120, 26899–26907. doi: 10.1021/acs.jpcc.6b09097

  28. [28]

      Kong, D.; Hu, J.; Chen, Z.; Song, K.; Li, C.; Weng, M.; Li, M.; Wang, R.; Liu, T.; Liu, J.; Zhang, M.; Xiao, Y.; Pan, F. Ti-Gradient doping to stabilize layered surface structure for high performance high-Ni oxide cathode of Li-ion battery. Adv. Energy Mater. 2019, 1901756.

  29. [29]

      Zheng, J.; Ye, Y.; Liu, T.; Xiao, Y.; Wang, C.; Wang, F.; Pan, F. Ni/Li disordering in layered transition metal oxide: electrochemical impact, origin, and control. Accounts Chem. Res. 2019, 52, 2201–2209. doi: 10.1021/acs.accounts.9b00033

  30. [30]

      Luo, K.; Roberts, M. R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y. S.; Edstrom, K.; Guo, J.; Chadwick, A. V.; Duda, L. C.; Bruce, P. G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nat. Chem. 2016, 8, 684–691. doi: 10.1038/nchem.2471

  31. [31]

      Armstrong, A. R.; Holzapfel, M.; Novák, P.; Johnson, C. S.; Kang, S. H.; Thackeray, M. M.; Bruce, P. G. Demonstrating oxygen loss and associated structural reorganization in the lithium battery cathode Li[Ni_0.2Li_0.2Mn_0.6]O₂. J. Am. Chem. Soc. 2006, 128, 8694–8698. doi: 10.1021/ja062027+

• 

  Figure 1  (a) Structure units of Li-rich Mn-based cathodes (LMC); From ref. [1], copyright of ACS; (b) Capacity and operation voltage comparisons of the present common cathodes for LIBs; From ref. [2], copyright of ACS; (c) Charge-discharge curves showing the irreversible first-cycle charge-discharge process of LMC; From ref. [3], copyright of nature; (d) Charge-discharge curves at different cycle number showing the voltage decay of LMC; From ref. [4], copyright of ACS; (e) The poor cyclability and rate capability of bare LMC; From ref. [5, 6], copyright of Wiley

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  Figure 2  (a) Operando mass spectrometry showing the first-cycle oxygen release at high voltage; From ref. [31], copyright of Nature; (b) Scheme of the Li₂O evolution during the first cycle; From ref. [9], copyright of ACS; (c) Scheme of different activation ways of bare Li₂MnO₃ (Li₂O evolution domination) and Li₂MnO₃ component in LMC (oxygen redox domination); From ref. [12], copyright of ACS; (d) Scheme of the voltage decay mechanism showing metal cations migration for layered-to-spinel conversion; From ref. [1], copyright of ACS; (e) EDS mapping showing the different Mn/Ni distribution in Li-rich particles obtained by different synthesis methods; From ref. [16], copyright of ACS; (f) TEM image showing the guar gum (GG) binder coated on LMC particle; From ref. [19], copyright of RSC; copyright of ACS; (g) Cyclability and rate performance of crystal habit-tuned LMC nanoplates; SEM image of the LMC nanoplates is insert; From ref. [21], copyright of Wiley; (h) Structure-dependent cyclability and rate capability of LMC cathodes; From ref. [22], copyright of Elsevier; (i) Initial charge-discharge profiles and Coulombic efficiencies of Li-rich, spinel, Li-rich layered/spinel composite and physical mixture of Li-rich and spinel; From ref. [25], copyright of ACS

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