Improving the oxygen redox reversibility of Li-rich battery cathode materials via Coulombic repulsive interactions strategy

The oxygen redox reaction in lithium-rich layered oxide battery cathode materials generates extra capacity at high cell voltages (i.e., >4.5 V). However, the irreversible oxygen release causes transition metal (TM) dissolution, migration and cell voltage decay. To circumvent these issues, we introduce a strategy for tuning the Coulombic interactions in a model Li-rich positive electrode active material, i.e., Li1.2Mn0.6Ni0.2O2. In particular, we tune the Coulombic repulsive interactions to obtain an adaptable crystal structure that enables the reversible distortion of TMO6 octahedron and mitigates TM dissolution and migration. Moreover, this strategy hinders the irreversible release of oxygen and other parasitic reactions (e.g., electrolyte decomposition) commonly occurring at high voltages. When tested in non-aqueous coin cell configuration, the modified Li-rich cathode material, combined with a Li metal anode, enables a stable cell discharge capacity of about 240 mAh g−1 for 120 cycles at 50 mA g−1 and a slower voltage decay compared to the unmodified Li1.2Mn0.6Ni0.2O2.

Also, the authors chose perform NMR measurements of the 7Li to provide evidence of oxygen vacancy. On page 7, line 132, the authors state: "The oxygen vacancy can be illustrated by the widened sideband pattern in M-LRO sample". This data analysis is rather superficial; what proof do the authors have?

Comment 2
In the data analysis shown in Figure 4 a) and b) (on page 17, lines 347-353), the authors compare the eg peak intensity from the ex-situ O K-edge soft XAS for the both samples (pristine and M-LRO). However, the current form of spectra does not allow to the reader to properly observe eg peak differences between the both samples. Could the authors apply a more thorough data analysis? I suggest to use two figures showing the spectra of the pristine and M-LRO samples overlapped, for the first at 4.55V and for the second one at 4.88V (between 525eV and 534eV). Also I want to add that the Figure 4c) is not sufficient to demonstrate eg peak variations. Figure 8, the PITT spectra of both samples during charging process are presented. This data analysis is superficial. The authors need to clearly discuss the result, especially, the higher value obtained for the M-LRO sample near 2.3V, and the Li diffusion coefficient variation for both samples which are different. The authors state: "electrochemical performance is in good agreement with the result of the PITT". What is the reason for this in regard to this figure?

Comment 3 In Supplementary
Comment 4 On page 5, line 83/84, the authors state "It is worth noting that the LMRO with oxygen vacancies and/or spinel phase can improve the properties of cathode materials". LMRO is not define in the main text. Could the authors specify this term? If it is the abbreviation of Li-rich Mn-based oxides, modify LMRO by LRMO in the whole manuscript.

Comment 5
In the data analysis shown in Figure 4f) and h) (on pages 18-19), the authors state: "the peak intensity of M-LRO sample can revert back to the original state compared to the pristine sample" (lines 382-383), and "the d-d excitation peak of M-LRO sample can again revert back to the original intensity at OCV state, in contrast, the peak of pristine sample almost disappeared" (lines 394-396) I highly recommend to add "in the limit of the resolution of the ex situ Mn L-edge RIXS spectra" because, the pristine spectrum obtained at D2.0V present a low signal to noise ratio. Comment 6 On page 9, line 184/185, the authors state: "To reveal the mechanisms of the mitigated voltage decay and increased capacity. In-situ XRD is conducted to monitor the crystal structure change in real time". I suggest to replace "capacity. In-situ" by "capacity, in-situ". Comment 7 On page 18, lines 371-373, the authors state: "To further illustrate the nature of enhanced reversible oxygen redox due to the oxygen vacancy and reduced Mn. The Mn L-edge RIXS of both samples are collected as Fig. 4f, h shown". I suggest to replace "reduced Mn. The Mn" by "reduced Mn, the Mn".
Reviewer #3 (Remarks to the Author): Non-hysteretic anionic redox reactions are important for designing high energy density cathode materials for lithium ion batteries. Authors reports on the reversible oxygen capacity in lithium rich LiMnO2 cathode materials. Although the substitute of TM and O atom enhanced anion redox, those strategies have issues for practical usages, whereas the oxygen vacancy controlling d-d Coulomb interaction can improve the anion redox reversibility cost-effectively. This work attempted to demonstrate the fundamental understands of the oxygen redox. However, there are contradictions between this work and those published in the literature. It should be further clarified carefully. Overall the manuscript is well written, pleasant to read and well supported by a large panel of characterization. As a results the authors clearly demonstrate that the strategy they propose works to reduce O2 release during the anionic redox process and enhance the reversibility. To my point of view it can be of interest for Li-ion battery community.
However several points might be addressed.

Reply:
We would like to sincerely thank the reviewer for the positive comments. We will answer the following questions one by one. Reply: Thanks a lot. This is really a good question. We have reviewed lots of literatures, and we found that there is no specific report on the relationship between oxygen vacancy/Mn 3+ and superlattice diffraction peaks. According to our observation, when the content of oxygen vacancies is ~5%, they have no obvious change on the superlattice peaks, which can be seen in the normalized  In the process of lithiation and delithiation, the crystal structure of the material will shrink and expand, which can be seen from the change of peaks in in-situ XRD (Figure 3a and b). This continuous shrinkage and expansion will cause irreversible damage and transition metal migration in the crystal structure, and when the damage accumulates to a certain degree, the crystal structure will collapse. In this study, the oxygen vacancy and reduced Mn soften the crystal structure, which can be verified by the smaller change of peaks in in-situ XRD and can be verified by dislocation as shown in aberration-corrected scanning transmission electron microscopy (Supplementary Figure 2). This soften structure can withstand crystal changes and can limit the migration of transition metals in 20 the form of MO6 octahedral reversible distortion, thereby stabilizing the crystal structure, which can also be proved by the Mn K-edge XAS (Figure 3c-f). In addition, the oxygen vacancy can mitigate oxygen release during charging and discharging, which can be verified by the DEMS (Supplementary Figure 16).

Specific Comment
On the one hand, less oxygen release reduces the collapse of crystal structure due to the lack of coordinated oxygen. On the other hand, more oxygen can react more completely with electrolyte, the by-products (HF and oxygen radicals etc.) produced in this process will corrode and attack the material crystal, so the damage to the crystal structure is more serious. In summary, the soften structure and reduced oxygen release decrease the crystal change in the modified sample.