Multicomponent high-entropy assisted high-rate and air-stable layered cathode for sodium ion batteries

Sodium-based O3-type layered oxide materials are attractive for Sodium-ion batteries (SIBs) due to their simple synthesis, affordability, and high capacity. However, challenges remain, including limited reversible capacity and poor cycling stability caused by detrimental phase transitions during cycling and the tendency to form sodium carbonate upon air exposure. In this study, based on O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NNFM), a high-entropy strategy was introduced to successfully synthesize O3-type NaNi_0.25Fe_0.21Mn_0.18Co_0.21Ti_0.1Mg_0.05O₂ (HE-NNFM). The introduction of Co, Ti, and Mg ions increases the system's disorder, highlighting the synergistic interactions among inert atoms. The delayed phase transformation effect in high-entropy materials alleviates the destruction of the O3 structure by the insertion and extraction of sodium ions. Simultaneously, the narrower sodium layer in HE-NNFM acts as a physical barrier, effectively preventing adverse reactions with H₂O and CO₂ in the air, resulting in excellent reversibility and air stability of the HE-NNFM material. Consequently, the HE-NNFM material exhibits a reversible capacity of 110 mAh g^-1 with a capacity retention of 97.3% after 200 cycles at 1 C. This work provides insights into the design of high-entropy sodium layered oxides for high-power density storage systems.