Understanding of Surface Redox Behaviors of Li2MnO3 in Li-Ion Batteries: First-Principles Prediction and Experimental Validation.

Critical degradation mechanism of many cathode materials for Li-ion batteries is closely related to phase transformations at the surface/interface. Li2MnO3 in x Li2MnO3 ⋅(1-x) LiMO2 (M=Ni, Co, Mn) provides high capacity, but the Li2MnO3 phase is known to degrade during cycling through phase transformation and O2 evolution. To resolve such degradation problems, it is critical to develop a fundamental understanding of the underlying mechanism. Using first-principles calculations, we identified the surface delithiation potential (<4.5 V vs. Li/Li(+) ) of Li2MnO3, which is significantly lower than the bulk redox potential. A lower Mn oxidation state at the surface would reduce the delithiation potential compared with the fully oxidized Mn(4+) in the bulk. As a result, the delithiation would be initiated from the surface, which induces a phase transformation of Li2MnO3 into a spinel-like structure from the surface. These theoretical findings have been confirmed by experimental analyses. Based on these detailed mechanistic understanding, it would be possible to develop rational approaches to modify and coat the surface to suppress degradation mechanisms.