Achieving a Rapid Na + Migration and Highly Reversible Phase Transition of NASICON for Sodium-Ion Batteries with Suppressed Voltage Hysteresis and Ultralong Lifespan

Sodium ion batteries have attracted great attention for large scale energy storage devices to replace lithium ion batteries. As a promising polyanionic cathode material of sodium-ion batteries, Na₃V₂(PO₄)₂F₃ (NVPF) belonging to NASICON exhibits large gap space and excellent structural stability, leading to a high energy density and ultralong cycle lifespan. To improve its stability and Na ion mobility, K⁺ cations were introduced into NVPF crystal as in situ partial substitution for Na⁺. The influence of K⁺ in situ substitution on crystal structure, electronic properties, kinetic properties and electrochemical performance of NVPF were investigated. Through ex-situ examination, it turned out that K⁺ occupied Na1 ion, in which the K⁺ did not participate in the charge-discharge process and played a pillar role in improving the mobility of Na⁺. Moreover, the doping of K⁺ cation can reduce the band gap energy and improve the electronic conductivity. Besides, the optimal K⁺ doping concentration in N_0.92K_0.08VPF/C was found so as to achieve rapid Na⁺ migration and reversible phase transition. The specific capacity of N_0.92K_0.08VPF/C was as high as 128.8 mAh g^{− 1} at 0.2 C, and at 10 C its rate performance was excellent, which showed a capacity of 113.3 mAh g^{− 1}.