Exploration of High-Entropy Layered Oxides with Ultrahigh Rate Performance for Sodium-Ion Batteries

O3-type layered oxides that are of the type of O3 serve as attractive cathode materials for sodium-ion batteries due to their facile production and elevated sodium content. Nonetheless, their practical application is impeded by intricate phase transitions and inadequate air stability. This paper presents a sodium alginate sol-gel approach that utilizes sodium alginate as both a sodium supply and a chelating agent. This method creates a 3D mesh structure through the binding of transition metal salts, facilitating the synthesis of O3-type high-entropy layered oxides. The high-entropy design improves the reversibility of the O3-P3 phase transition, inhibits Na⁺/H⁺ exchange to decrease air reactivity, and stabilizes the material's structure, thus minimizing electrochemical deterioration during cycling. DFT calculations demonstrate that the increased Li elements in the high-entropy oxides Na_0.9Ca_0.05Fe_0.2Mn_0.2Ni_0.2Ti_0.2Li_xCo_0.2-xO₂ increase the local bonding strengths of TM-O near the Li doping sites and improve the structural stability. The optimized cathode preserves 71% capacity after 200 cycles at 10 C (starting capacity: 117 mAh g^-1) and sustains 115 mAh g^-1 for 100 cycles at 1 C (85.2% retention) after 10 days of exposure to air with 40-50% relative humidity. This study promotes a high-entropy approach for the development of high-performance, air-stable O3-type cathodes for sodium-ion batteries.