Optimizing quasi-solid-state sodium storage performance of Na3V2(PO4)2F2.5O0.5 cathode by structural design plus nitrogen doping

Abstract

Sodium vanadium fluorophosphate has been extensively explored for the cathode of sodium ion batteries (SIBs), which is known for its high voltage platform, three-dimensional framework structure and minor volume deformation. However, low intrinsic electronic conductivity restricts its practical charge/discharge performances. Accelerating the electron/ion transport process of sodium vanadium fluorophosphate is the vital point to realize high performances of SIBs. Herein, a well-designed nitrogen doped three-dimensional (3D) nanosheet flower-like Na3V2(PO4)2F2.5O0.5 composite (N-NVPFO) is proposed by optimizing nanostructure plus heteroatom doping. The 3D flower-like structure of N-NVPFO provides a great deal of active sites and guarantees the structure stability. In addition, the N-doping in Na3V2(PO4)2F2.5O0.5 bulk phase could enhance the intrinsic electronic conductivity by narrowing the energy gap, which is confirmed through the density functional theoretical (DFT) calculation. The N-NVPFO electrode delivers excellent rate capability (84 mAh g−1 at 10C) and long cycling life (86% capacity retention up to 2000 cycles). Furthermore, the quasi-solid-state SIBs using NaTi2(PO4)3 anode also exhibits good electrochemical performances.