Exploring the Stability Effect of the Co-Substituted P2-Na0.67[Mn0.67Ni0.33]O2 Cathode for Liquid- and Solid-State Sodium-Ion Batteries.

Pursuing high-performance cathode materials for sodium-ion batteries (SIBs) has great significance in the modern green energy world. The P2-type sodium-based layered oxide Na0.67[Mn0.67Ni0.33]O2 with high operating potential upon 4.3 V and high theoretical capacity has emerged as the most promising cathode. However, the material suffers from severe capacity decay during the electrochemical reaction process. Herein, the P2-Na0.67[Mn0.67Ni0.21Li0.06Zn0.06]O2 cathode is gained by moderately substituting lithium/zinc for the nickel sites. The inactive Li/Zn co-substitution is endowed with the ability to stabilize the crystal structure, resulting in enhanced electrochemical kinetics and remarkable long cyclic performance in liquid- and solid-state electrolytes. Thus, the Li/Zn co-substituted cathode presents a specific capacity of 154 mAh g-1 at the first discharge process, excellent rate capability with 77 mAh g-1 at a high current density of 5 C, and long cyclic stability in liquid-state batteries. Excitingly, it is also endowed with a high capacity retention of 85% after 500 cycles in solid-state batteries. Furthermore, ex situ XRD, TEM, and ex situ XPS are applied to reveal the structural evolution and charge compensation mechanism of P2-Na0.67[Mn0.67Ni0.21Li0.06Zn0.06]O2, allowing a deep insight into the great significance of structural stability.