Elucidating\nthe Structural Evolution of O3-type NaNi<sub>1/3</sub>Fe<sub>1/3</sub>Mn<sub>1/3</sub>O<sub>2</sub>: A Prototype\nCathode for Na-Ion Battery

Extending\nthe depth-of-charge (DoC) of the layered oxide cathode\npresents an essential route to improve the competitiveness of the\nNa-ion battery versus the commercialized LiFePO<sub>4</sub>-based\nLi-ion battery (0.8 CNY/Wh). However, the DoC-dependent boundary between\ndetrimental/irreversible structural distortion and neutral/reversible\nstructure interconversion cannot be clearly distinguished, which is\nattributed to the ambiguous recognition of correlation among the complex\nphase transition, local covalent environment evolution, and charge\ncompensation. Herein, to bridge the above gap, we employed O3-NaNi<sub>1/3</sub>Fe<sub>1/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> as the prototype\ncathode and extended the target DoC from typical Na0.4 (∼125\nmAh/g, 4.0 V cutoff) to Na0.2 (∼180 mAh/g, 4.3 V cutoff). Regarding\nphase transition and charge compensation, the O3-to-P3 phase transition\noccurs before moderate Na<sub>0.4</sub>-DoC (Fe/Mn redox silence,\nNi oxidation dominated), while further desodiation (start from Na<sub>0.4</sub>) induces a P-to-O slab transition, resulting in the coexistence\nof P3 and OP2 phases and subsequent OP2/O3 intergrowth phases at higher\nDoC (Na<sub>0.2</sub>), upon which the Fe<sup>3+</sup>-to-Fe<sup>4+</sup> oxidation is activated for capacity contribution. The local covalent\nenvironment presents severer deviation at high DoC (merely 0.2 mol\ndesodiation from Na<sub>0.4</sub> to Na<sub>0.2</sub>), which can\nbe attributed not only to the slab gliding induced by the P-to-O slab\ntransition but also to the further aggravation caused by the Jahn–Teller\ndistortion of the FeO<sub>6</sub> octahedron. Such irreversible distortion\nof the local covalent environment would be accumulated and evolved/deteriorated\ninto structural degradation during long-term cycling. Furthermore,\nthe rate-dependent artificial regulation of redox process has been\ndemonstrated and the doping strategy toward structural stabilization\nhas been proposed.