Heterogeneous NASICON-Type Cathode With Reversible Multielectron Reaction for High-Performance Sodium-Ion Batteries

Na superionic conductor (NASICON)-structured compounds demonstrate great application potential by their robust framework and compositional diversity. However, they are blamed for the mediocre energy density, and achieving both multielectron reaction and good cycling stability simultaneously is challenging. Herein, a novel heterogeneous Na₄Fe₃(PO₄)₂(P₂O₇)/Na₂VTi(PO₄)₃ (NFPP/NVTP) material with stable multielectron reaction is constructed by spray drying technology. The mutual promotion effect of intergrowth structures effectively improves the purity and the crystallization of NFPP/NVTP during the fabrication process, which is beneficial to the high capacity and cycling stability. As a result, the optimized NFPP/NVTP demonstrates a high reversible capacity of 155.3 mAh g^-1 at 20 mA g^-1 and outstanding cycling stability with 82.9% capacity retention over 2500 cycles at 1 A g^-1, which are much superior to those of NFPP and NVTP individually. Even in full cell configuration, the energy density remains high at ≈380 Wh kg^-1 based on the cathode mass. The high capacity of NFPP/NVTP is also attributed to the successive reduction/oxidation mechanism involving the introduction of Ti³⁺ and interfacial charge redistribution effect between the heterogeneous phases, which greatly improve the electronic and ionic conductivity. Moreover, high reversible structural evolution during the multisodium storage process further guarantees excellent cycling stability.