A High-Entropy Approach to Activate the Oxygen Redox Activity and Suppress the Phase Transition of P2-Type Layered Cathode for Sodium-Ion Batteries

Introducing electrochemically active or inactive metal ion substitution is a well-known modification strategy in the layered transition-metal oxide cathode materials for sodium ion batteries. However, the introduction of active or inactive metal ions into the transition-metal layer often triggers the redox reaction of anionic oxygen. The charge compensation induced by the redox reaction of anionic oxygen can improve the specific capacity of the cathode material, whereas it also brings problems, such as voltage hysteresis and attenuation and sluggish reaction kinetics. Here, we propose a high-entropy strategy using Li, Cu, and Ti, and we find that the synergistic effect of these elements can stimulate the redox reaction of oxygen and prevent the adverse effects of anionic oxygen. The incorporation of Li⁺ can increase Na content and stimulate the oxygen redox reaction, leading to increased theoretical capacity and disrupted Na⁺/vacancy ordering. The incorporation of Cu²⁺ can stabilize the environment of the oxygen and reduce the O loss. The incorporation of Ti⁴⁺ can stabilize the transition-metal layer framework. As a result, the reversible capacity of the optimized P2-type cathode of Na_0.73Ni_0.21Mn_0.6Li_0.06Cu_0.06Ti_0.07O₂ was 128.12 mAh/g, which also delivers an excellent capacity retention of 79.21% after 500 cycles and an excellent rate performance with a capacity of 85.6 mAh/g at 10 C. At the same time, it exhibits the smallest voltage attenuation and the highest Na⁺ diffusion coefficient. By stimulating and regulating the redox reaction of oxygen, this work provides new insights into the design of high-performance and practical P2-type cathode materials for sodium-ion batteries.