Unveiling charge compensation mechanisms in Na2/3MgxNi1/3-xMn2/3O2 cathode materials: insights into cationic and anionic redox

Abstract

P2-type Na2/3Ni1/3Mn2/3O2 cathode material for sodium-ion batteries exhibits negligible voltage hysteresis during charge and discharge cycles, with oxygen redox mechanisms playing a pivotal, non-negligible role. Conversely, for the Na2/3Mg1/3Mn2/3O2 material featuring Mg substitution for Ni, O redox largely governs the entire charge and discharge process, accompanied by a pronounced voltage hysteresis. Upon combining these two materials – Na2/3MgxNi1/3-xMn2/3O2 – the transition metal and O redox processes, constituting the charge compensation mechanism, become paramount. These phenomena underscore the transition metal's influence on anionic redox, a topic with limited in-depth exploration in the existing literature. Within this study, we synthesized two materials, Na2/3Mg1/9Ni2/9Mn2/3O2 and Na2/3Mg2/9Ni1/9Mn2/3O2 (Mg-0.11, Mg-0.22), subsequently probing their transition metal and oxygen redox contributions to electrochemical capacity during the initial and 10th cycle charge and discharge processes by synchrotron-based mapping of resonant inelastic X-ray scattering and soft X-ray Absorption Spectroscopy. Our findings corroborate the presence of similar O redox contributions in the Mg-substituted materials. Intriguingly, the Mg-0.11 material exhibits a substantial enhancement in O redox following ten cycles, warranting further investigation due to its exceptional multiplicity, cycling performance, minimal voltage hysteresis, and potential impact on advancing sodium-ion battery commercialization.