Stimulating the redox capacity by multi-ion substitution for P2-type sodium-ion battery cathodes

Layered cathode materials based on oxygen redox are of great significance for the realization of high energy density sodium-ion batteries (SIBs) that meet the demands of future-oriented energy storage technic. However, the conventional P2-type natrium-poor cathode Na0.67Ni0.3Mn0.7O2 is unable to perform oxygen redox at high voltage. In this work, the multiple substitution of Mg, Ti, and Li in the layered cathode of P2-type is investigated. It has been proved experimentally and theoretically that Li balances the charge imbalance caused by Ti substitution, reduces the c-axis spacing and improves the structure and cycle stability. Meanwhile, Mg and Ti activate and promote the redox reaction of lattice oxygen, stabilize the material structure, and inhibit the loss of active substances caused by transition metal dissolution. In-situ X-ray diffraction analysis shows that the O2 phase does not appear in the whole range of de-/sodiation, and the volume change is small. After 100 cycles of 2-4.5 V, the capacity is maintained to 93.6%, and the initial capacity of 0.1 C is 159.3mAh g-1. Taking a broader view, this study provides a possible design thought for the development of SIB cathode materials with optimized anionic activity as well as excellent structural stability.