Improvement of Electrochemical Performance in O3-Type NaNi_0.4Fe_0.25Mn_0.35O₂ Cathodes for Sodium-Ion Batteries Using a High-Entropy Approach

The O3-type layered cathodes for sodium-ion batteries demonstrate significant theoretical capacity and cost-effectiveness. However, their widespread application is hindered by inadequate cycling stability, attributed to unfavorable phase transitions and structural degradation during charge-discharge cycles. To address these challenges, we employed a high-entropy approach that involves incorporating Cu²⁺, Al³⁺, and Ti⁴⁺ ions into the NaNi_0.4Fe_0.25Mn_0.35O₂ (NFM) framework, resulting in a novel cathode composition of NaNi_0.3Fe_0.1Mn_0.3Cu_0.1Al_0.05Ti_0.15O₂ (NFMCAT). Electrochemical assessments indicate that NFMCAT delivers an initial discharge capacity of 134.6 mAh g^-1 at a rate of 0.1 C, with 88% capacity retention after 200 cycles at 2 C. Enhanced sodium ion diffusion is verified through GITT, CV, and EIS techniques, while in situ X-ray diffraction reveals diminished lattice strain during phase transitions. Comprehensive evaluations of full cells with a hard carbon anode demonstrate outstanding rate capability and longevity, achieving roughly 73.2% capacity retention after 500 cycles at 2 C. This study underscores the effectiveness of high-entropy strategies in developing stable, high-performance cathode materials, significantly enhancing the functionality of sodium-ion batteries.