Structural\nDisorder of a Layered Lithium Manganese\nOxide Cathode Paving a Reversible Phase Transition Route toward Its\nTheoretical Capacity

Layered lithium manganese oxides\nsuffer from irreversible phase\ntransitions induced by Mn migration and/or dissolution associated\nwith the Jahn–Teller effect (JTE) of Mn<sup>3+</sup>, leading\nto inevitable capacity fading during cycling. The popular doping strategy\nof oxidizing Mn<sup>3+</sup> to Mn<sup>4+</sup> to relieve the JTE\ncannot completely eliminate the detrimental structural collapse from\nthe cooperative JTE. Therefore, they are considered to be impractical\nfor commercial use as cathode materials. Here, we demonstrate a layered\nlithium manganese oxide that can be charged and discharged without\nany serious structural collapse using metastable Li-birnessite with\ncontrolled structural disorder. Although Li-birnessite is thermodynamically\nunstable under ambient conditions, Li ion exchange into Na-birnessite\nfollowed by an optimal dehydration resulted in a disordered Li-birnessite.\nThe control over crystal water in the interlayer provides intriguing\nshort-range order therein, which can help to suppress parasitic Mn\nmigration and dissolution, thereby ensuring a reversible electrochemical\ncycling. The Mn redox behavior and local structure change of the Li-birnessite\nwere investigated by <i>ex situ</i> soft X-ray absorption\nspectroscopy (sXAS) and X-ray pair distribution function (PDF) analysis.\nThe combined sXAS and PDF with electrochemical analyses disclosed\nthat the reversible Mn redox and suppressed phase transitions in Dh\nLi-birnessite contribute to dramatically improving its electrochemical\nreversiblity during cycling. Our findings underscore the substantial\neffects of controlled static disorder on the structural stability\nand electrochemical reversibility of a layered lithium manganese oxide,\nLi-birnessite, which extends the practical capacity of layered oxides\nclose to their theoretical limit.