Improved Cycling Performance of Li‐Excess Cation‐Disordered Cathode Materials upon Fluorine Substitution

The recent discovery of Li‐excess cation‐disordered rock salt cathodes has greatly enlarged the design space of Li‐ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materia...

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Bibliographic Details
Published in:Advanced energy materials 2019-01, Vol.9 (2), p.n/a
Main Authors: Lun, Zhengyan, Ouyang, Bin, Kitchaev, Daniil A., Clément, Raphaële J., Papp, Joseph K., Balasubramanian, Mahalingam, Tian, Yaosen, Lei, Teng, Shi, Tan, McCloskey, Bryan D., Lee, Jinhyuk, Ceder, Gerbrand
Format: Article
Language:English
Subjects:
Cathodes
Cations
cyclability
Cycles
DFT
Electrode materials
Fluorination
Fluorine
Halites
Jahn-Teller effect
Jahn–Teller distortion
Li‐excess cation‐disordered cathodes
Oxyfluorides
Oxygen
Substitutes
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Summary:The recent discovery of Li‐excess cation‐disordered rock salt cathodes has greatly enlarged the design space of Li‐ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn3+–Nb5+‐based cation‐disordered oxyfluorides, Li1.2Mn3+0.6+0.5xNb5+0.2−0.5xO2−xFx (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition‐metal redox (Mn3+/Mn4+) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn3+‐induced Jahn–Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn‐redox capacity to the overall capacity. A class of Li‐excess cation‐disordered Mn3+–Nb5+‐based oxyfluorides are designed and electrochemically tested. Fluorination is found to improve the cycling performance, especially the retention in a very significant way, by reducing the Mn3+‐induced Jahn–Teller distortion. This structural optimization raises some Mn3+ energy levels so that more of them can be oxidized before irreversible oxygen oxidation sets in.
ISSN:1614-6832
1614-6840
DOI:10.1002/aenm.201802959