Achieving structurally stable O3-type layered oxide cathodes through site-specific cation-anion co-substitution for sodium-ion batteries

Abstract

O3-type layered oxides have garnered great attention as cathode materials for sodium-ion batteries because of their abundant reserves and high theoretical capacity. However, challenges persist in the form of uncontrollable phase transitions and intricate Na+ diffusion pathways during cycling, resulting in compromised structural stability and reduced capacity over cycles. This study introduces a special approach employing site-specific Ca/F co-substitution within the layered structure of O3-NaNi0.5Mn0.5O2 to effectively address these issues. Herein, the strategically site-specific doping of Ca into Na sites and F into O sites not only expands the Na+ diffusion pathways but also orchestrates a mild phase transition by suppressing the Na+/vacancy ordering and providing strong metal–oxygen bonding strength, respectively. The as-synthesized Na0.95Ca0.05Ni0.5Mn0.5O1.95F0.05 (NNMO-CaF) exhibits a mild O3 → O3 + O'3 → P3 phase transition with minimized interlayer distance variation, leading to enhanced structural integrity and stability over extended cycles. As a result, NNMO-CaF delivers a high specific capacity of 119.5 mA h g−1 at a current density of 120 mA g−1 with a capacity retention of 87.1% after 100 cycles. This study presents a promising strategy to mitigate the challenges posed by multiple phase transitions and augment Na+ diffusion kinetics, thus paving the way for high-performance layered cathode materials in sodium-ion batteries.