Crystallinity Tuning of Na3 V2 (PO4 )3 : Unlocking Sodium Storage Capacity and Inducing Pseudocapacitance Behavior.

ABSTRACT

As a promising cathode material of sodium-ion batteries, Na3 V2 (PO4 )3 (NVP) has attracted extensive attention in recent years due to its high stability and fast Na+ ion diffusion. However, the reversible capacity based on the two-electron reaction mechanism is not satisfactory limited by the inactive M1 lattice sites during the insertion/extraction process. Herein, self-supporting 3D porous NVP materials with different crystallinity are fabricated on carbon foam substrates by a facile electrostatic spray deposition method. The V5+ /V4+ redox couple is effectively activated and the three-electron reactions are realized in NVP for sodium storage by a proper crystallinity tuning. In a disordered NVP sample, an ultra-high specific capacity of 179.6 mAh g-1 at 0.2 C is achieved due to the coexistence of redox reactions of the V4+ /V3+ and V5+ /V4+ couples. Moreover, a pseudocapacitive charge storage mechanism induced by the disordered structure is first observed in the NVP electrode. An innovative model is given to understand the disorder-induced-pseudocapacitance phenomenon in this polyanion cathode material.