Inhibiting the Structural Degradation of P2-Type Cobalt-Free Layered Transition Metal Oxides by Chromium Doping for Improved Lithium-Ion Batteries

The conventional P2-type cathode material suffers from irreversible structural degradation, lattice-oxygen release, and severe capacity fading at the high-charge state during cycling. This study systematically investigates the failure mechanisms associated with the structural evolution and oxygen loss of LiNi0.5Mn0.5O2 in the high-charge state while also revealing the positive impact of Cr doping. Cr substitution in the lithium layer increases the proportion of lattice-oxygen content, lowers the oxygen activation plateau during the first cycle, and restrains structural degradation during cycling. These improvements are attributed to the high degree of delocalized Mn–O bonds caused by robust Cr–O covalency. As a result, an extraordinarily stable voltage (decay rate <0.76 mV per cycle) and a high capacity-retention rate are achieved. Insights into the structural stability of LiNi0.5Mn0.5O2 in a high-charge state provide valuable guidance for designing and optimizing Co-free layered cathode materials with enhanced performance.