Role of Cu Dopant in Improving the Structural Stability of O3-Type Sodium-Ion Battery Cathode: Mitigating Oxidation State Variation toward Reinforcing Aqueous Stability

We propose a mechanism of how Cu substitution in the O3-type sodium-ion battery cathode enhances its aqueous stability compared to the Co-doped O3-type cathode. The aqueous stability is closely associated with the crystal structural stability. Cu dopant allows electronic redistribution, leading to maintaining its oxidation state upon desodiation. It also mitigates changes in polyhedral volume of transition metal-O₆, resulting in high structural stability upon desodiation. In contrast, the Co dopant is oxidized upon desodiation, inducing larger polyhedral volume variations. It finally entails structural instability of the Co-doped cathode, which can be exacerbated during vigorous desodiation. Therefore, in an aqueous environment, the Co-doped cathode experiences severe structural degradation to form OP4, Fd3̅m, and C2/m phases, retaining only 19% of Na-storing capability. In contrast, the Cu-doped cathode retains the original structure, thereby preserving 82% of its Na-storing capability. Our findings provide invaluable insights on resolving storage and handling issues of sodium-ion battery cathodes induced by their moisture sensitivity.