Na+/vacancies Promise Excellent Electrochemical Properties for Sodium Ion Batteries

Sodium ion batteries (SIBs) are an attractive option for energy storage because of their low cost and extensive Na resources. A novel Sr-doped P-type Na0.57Sr0.05Mn0.6Ni0.2Co0.2O2 (NSMNC) consisting of multiple-layer oriented stacking nanoflakes is constructed via a sol-gel method. It is the first time that high-valence metal ions with large radius are introduced into the sodium ionic layer to optimize its structure. The substitution of bivalent strontium for monovalent sodium leads to the formation of equivalent Na+/vacancies. Meanwhile, the intercalation of strontium enlarges the cell parameters, providing wide channels for ionic transport. The electrode exhibits a reversible capacity of 120.9 mAh g(-1) at 0.1C with retention up to 85.6% after 200 cycles. Besides, it still keeps a capacity of 101.1 mAh g(-1) at 1C with high retention of 81.5% after 200 cycles. Owing to the Na+/vacancies and wide ionic channels, NSMNC shows excellent ionic diffusion ability and low transport energy barrier. This work offers a feasible strategy for the development of promising layered oxide cathode materials for SIBs.