Manganese-based layered oxides for electrochemical energy storage: a review of degradation mechanisms and engineering strategies at the atomic level

The ever-increasing demand for high-energy-density electrochemical energy storage has been driving research on the electrochemical degradation mechanisms of high-energy cathodes, among which manganese-based layered oxide (MLO) cathodes have attracted high attention thanks to their low cost and eco-friendliness. More importantly, MLO materials with large and tunable interlayer spacing are ideal candidates for the insertion of (monovalent, divalent, trivalent) alkaline ions, such as Li+, Na+, K+, Zn2+, Mg2+, and Al3+, enabling impressive electrochemical performance. Nevertheless, the local MnO6 octahedron distortion induced by the Jahn-Teller (J-T) effect can lead to irreversible phase transformation, dissolution/disproportionation reactions, interfacial degradation arising from Mn2+, and crack formation, which significantly impact the electrochemical stability of MLO materials. Hence, in this review, we discuss the various degradation processes caused by J-T distortion in MLO cathodes at the atomic level. Advances in the atomic-level structure and property optimizations of MLO materials and in-depth structure-function-property correlations are also systematically reviewed. Finally, we provide our perspectives on the future development of MLO materials. The integration of high-performance MLO cathodes in energy storage devices has great potential to address growing global energy demands.