Engineering Crystal Growth and Surface Modification of Na 3 V 2 (PO 4 ) 2 F 3 Cathode for High-Energy-Density Sodium-Ion Batteries.

Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) is a suitable cathode for sodium-ion batteries owing to its stable structure. However, the large radius of Na + restricts diffusion kinetics during charging and discharging. Thus, in this study, a phosphomolybdic acid (PMA)-assisted hydrothermal method is proposed. In the hydrothermal process, the NVPF morphologies vary from bulk to cuboid with varying PMA contents. The optimal channel for accelerated Na + transmission is obtained by cuboid NVPF. With nitrogen-doping of carbon, the conductivity of NVPF is further enhanced. Combined with crystal growth engineering and surface modification, the optimal nitrogen-doped carbon-covered NVPF cuboid (c-NVPF@NC) exhibits a high initial discharge capacity of 121 mAh g -1 at 0.2 C. Coupled with a commercial hard carbon (CHC) anode, the c-NVPF@NC||CHC full battery delivers 118 mAh g -1 at 0.2 C, thereby achieving a high energy density of 450 Wh kg -1 . Therefore, this work provides a novel strategy for boosting electrochemical performance by crystal growth engineering and surface modification.