Structural Disorder of a Layered Lithium Manganese Oxide Cathode Paving a Reversible Phase Transition Route toward Its Theoretical Capacity.

Layered lithium manganese oxides suffer from irreversible phase transitions induced by Mn migration and/or dissolution associated with the Jahn-Teller effect (JTE) of Mn, leading to inevitable capacity fading during cycling. The popular doping strategy of oxidizing Mn to Mn to relieve the JTE cannot completely eliminate the detrimental structural collapse from the cooperative JTE. Therefore, they are considered to be impractical for commercial use as cathode materials. Here, we demonstrate a layered lithium manganese oxide that can be charged and discharged without any serious structural collapse using metastable Li-birnessite with controlled structural disorder. Although Li-birnessite is thermodynamically unstable under ambient conditions, Li ion exchange into Na-birnessite followed by an optimal dehydration resulted in a disordered Li-birnessite. The control over crystal water in the interlayer provides intriguing short-range order therein, which can help to suppress parasitic Mn migration and dissolution, thereby ensuring a reversible electrochemical cycling. The Mn redox behavior and local structure change of the Li-birnessite were investigated by soft X-ray absorption spectroscopy (sXAS) and X-ray pair distribution function (PDF) analysis. The combined sXAS and PDF with electrochemical analyses disclosed that the reversible Mn redox and suppressed phase transitions in Dh Li-birnessite contribute to dramatically improving its electrochemical reversiblity during cycling. Our findings underscore the substantial effects of controlled static disorder on the structural stability and electrochemical reversibility of a layered lithium manganese oxide, Li-birnessite, which extends the practical capacity of layered oxides close to their theoretical limit.