Crystallinity Tuning of Na V (PO ) : Unlocking Sodium Storage Capacity and Inducing Pseudocapacitance Behavior.

As a promising cathode material of sodium-ion batteries, Na V (PO ) (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 V /V 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 at 0.2 C is achieved due to the coexistence of redox reactions of the V /V and V /V 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.