Tunable Electrochemical Activity of P2–Na_0.6Mn_0.7Ni_0.3O_2–xF_x Microspheres as High-Rate Cathodes for High-Performance Sodium Ion Batteries

As an important cathode candidate for the high-performance sodium ion batteries (SIBs), P2-type oxides with layered structures are needed to balance the specific capacities and cycling stability. As a result, a cation and anion codoped strategy has been adopted to tune the electrochemical activity of the redox centers and modulate the structure properties. Herein, a series of P2–Na0.6Mn0.7Ni0.3O2–xFx (x = 0, 0.03, 0.05, and 0.07) cathodes with microsphere structures are synthesized, using a solid-state reaction in the presence of MnO2 microsphere self-templates. Compared with the cation-doped Na0.6Mn0.7Ni0.3O2, additional F-doping can affect the lattice parameters and redox centers of Na0.6Mn0.7Ni0.3O2–xFx. Comprehensively considering the specific capacities, cycling stability, and rate capability, the optimized x value in Na0.6Mn0.7Ni0.3O2–xFx is determined to be 0.05. In the half cells, Na0.6Mn0.7Ni0.3O1.95F0.05 (F-0.05) maintains a capacity of 90.5 mA h g–1 in the first cycle at 1.0 A g–1, giving a capacity retention of 78% within 900 cycles. The superior rate capability of F-0.05 is guaranteed by the larger diffusion coefficient of Na+ (DNa) combined with higher charge transfer speed. In addition, when coupled with MoSe2/PC anodes, the full cells also exhibit impressive electrochemical performance.