Unveiling the Role of Transition‐Metal Ions in the Thermal Degradation of Layered Ni–Co–Mn Cathodes for Lithium Rechargeable Batteries

Abstract The need for batteries with high energy density and safety has motivated the development of Ni‐rich layered cathodes with high thermal stability, requiring a revisit of the role of the transition‐metal ion in the phase transition accompanying the oxygen evolution of highly charged cathodes. Here, the role of the transition‐metal ion in Li_xNi_0.5Co_0.2Mn_0.3O₂ (x = 0.5, 0.33) is revealed in the phase transition and O₂ evolution occurring at high temperatures using combined in situ high‐temperature neutron diffraction (ND) and gas analyses. The thermal migration of each transition‐metal ion upon heating is directly visualized at different states of charge using Rietveld refinement of ND patterns as well as the maximum entropy method. The oxygen evolution observed for the highly charged state at low temperature is accompanied by M₃O₄‐type spinel (M = Ni, Co, and Mn) phase formation with preferential occupation of Co in the tetrahedral site. Co³⁺/Co²⁺ reduction accompanying the oxygen evolution rather can mitigate and delay the formation of the rock‐salt phase. The findings provide insight into the manipulation of the composition of Ni‐rich layered cathode for the design of cathodes with high energy density and safety.