High-entropy configuration of O3-type layered transition-metal oxide cathode with high-voltage stability for sodium-ion batteries

The O3-type cathode of NaNi_0.4Fe_0.2Mn_0.4O₂ stands out for its remarkable theoretical capacity, straightforward production process, affordability, and ecological compatibility for sodium-ion batteries. Nonetheless, the cycle stability of the cathode material is suboptimal. The slippage of the transition metal layers during the charging and discharging process leads to structural instability, and the elevated charging voltages (notably around 4.2 V) initiate the oxygen redox activities, leading to a sustained loss of oxygen ions and irreversible structural degradation. This work pioneers the utilization of a co-substitution strategy with titanium and antimony, fostering a high-entropy configuration of a new O3-type layered oxide cathode of NaNi_0.35Fe_0.2Mn_0.3Ti_0.1Sb_0.05O₂. It meticulously examines the potential effects of Ti⁴⁺ and Sb⁵⁺ substitution on the electrochemical performance of the cathode, including the role of the high-entropy configuration in modulating the phase transitions throughout the charging and discharging processes. Importantly, the designed cathode of NaNi_0.35Fe_0.2Mn_0.3Ti_0.1Sb_0.05O₂ showcased superior discharge-specific capacity (183.3 mA h g⁻¹ at 0.1C in the voltage range of 1.9-4.1 V), improved cycle stability (with a capacity retention of 76.0% after 200 cycles at 1C), robust rate capability (76.3 mA h g⁻¹ at 10C) and exceptional high-voltage resilience (213 mA h g⁻¹ at 0.1C and capacity retention of 74.0% after 100 cycles at 1C in the voltage range of 1.7-4.3 V). This work articulates a strategic approach for the deliberate design of entropy-regulated cathode materials, aiming at the enhancement of the high voltage stability of cathode materials for sodium-ion batteries.