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 Na0.73Ni0.21Mn0.6Li0.06Cu0.06Ti0.07O2 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.