Surface Crystal Modification of Na3 V2 (PO4 )3 to Cast Intermediate Na2 V2 (PO4 )3 Phase toward High-Rate Sodium Storage.

The two-phase reaction of Na3 V2 (PO4 )3 - Na1 V2 (PO4 )3 in Na3 V2 (PO4 )3 (NVP) is hindered by low electronic and ionic conductivity. To address this problem, a surface-N-doped NVP encapsulating by N-doped carbon nanocage (N-NVP/N-CN) is rationally constructed, wherein the nitrogen is doped in both the surface crystal structure of NVP and carbon layer. The surface crystal modification decreases the energy barrier of Na+ diffusion from bulk to electrolyte, enhances intrinsic electronic conductivity, and releases lattice stress. Meanwhile, the porous architecture provides more active sites for redox reactions and shortens the diffusion path of ion. Furthermore, the new interphase of Na2 V2 (PO4 )3 is detected by in situ XRD and clarified by density functional theory (DFT) calculation with a lower energy barrier during the fast reversible electrochemical three-phase reaction of Na3 V2 (PO4 )3 - Na2 V2 (PO4 )3 - Na1 V2 (PO4 )3 . Therefore, as cathode of sodium-ion battery, the N-NVP/N-CN exhibited specific capacities of 119.7 and 75.3 mAh g-1 at 1 C and even 200 C. Amazingly, high capacities of 89.0, 86.2, and 84.6 mAh g-1 are achieved after overlong 10000 cycles at 20, 40, and 50 C, respectively. This approach provides a new idea for surface crystal modification to cast intermediate Na2 V2 (PO4 )3 phase for achieving excellent cycling stability and rate capability.