Coupling the Electronic Distribution and Oxygen Redox Potential via Cu Substitution of Layered Oxide Cathodes for Sodium-Ion Batteries

Anionic redox chemistry is extensively studied by virtue of the promising strategy for appealing to the large capacity in sodium-ion batteries (SIBs). However, stimulating the lattice oxygen activity generally occurs at high voltage suffering from irreversible oxygen release, transition metal ion migration, and even structural distortion, which is a critical limitation for their further application. In this study, a series of Na_yLi_0.1Cu_xMn_0.9–xO₂ cathode materials are developed by introducing Cu²⁺ into the transition metal layer. Theoretical calculations disclosed that the electron agglomeration around the O atom has been enhanced with the increase of Cu content, leading to a strong covalent bonding between Cu and O and contributing to a more stable structure. The local structure facilitates the loss of electrons, thus reducing the O redox potential along the charging process, which is further confirmed by an experimental study. Besides, the Cu substitution introduced a whole solid solution reaction during the charge–discharge process with small volume change, hence greatly improving the cycling performance of NLCMO systems. Our research provides unique insights into the activation mechanism of anionic redox reactions and provides critical guidance in further designing anionic redox-based layered oxide systems.