The Roles of Ni and Mn in the Thermal Stability of Lithium-Rich Manganese-Rich Oxide Cathode

The pursuit of high-energy-density lithium-ion batteries (LIBs) has brought extensive research on the high-capacity lithium-rich manganese-rich oxide cathode (LRMO). However, practical applications of LRMO require a comprehensive understanding of its thermal stability, which determines the boundary for the safe use of LIBs. Here, systematic investigations of the thermal stability of LRMO are carried out by using in situ X-ray diffraction and full-field transmission X-ray microscopy combined with X-ray absorption near edge structure. The roles of Ni and Mn in affecting the thermal stability of LRMO are uncovered. It is surprising that Ni, despite being in the minority, acts as a key factor that governs the onset temperature of thermal decomposition. Unlike in lithium nickel-cobalt-manganese oxide cathodes where a higher content of Mn is believed to stabilize the structure with reduced heat release, in LRMO it causes more heat release which can be attributed to the lithium excess environment around Mn. In addition, it is revealed that the incomplete coverage of solid polymer electrolytes over the LRMO particle surface may lead to the deterioration of thermal stability. These findings provide mechanistic insights into the thermal behavior of LRMO cathodes for developing high-capacity cathodes with improved safety, particularly, for future applications in solid-state batteries.