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, NaV(PO)F (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 are 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 are investigated. Through ex situ examination, it turns out that K occupied Na1 ion, in which the K does not participate in the charge-discharge process and plays a pillar role in improving the mobility of Na. Moreover, the doping of K cation can reduce the bandgap energy and improve the electronic conductivity. Besides, the optimal K doping concentration in NKVPF/C is found so as to achieve rapid Na migration and reversible phase transition. The specific capacity of NKVPF/C is as high as 128.8 mAh g at 0.2 C, and at 10 C its rate performance is excellent, which shows a capacity of 113.3 mAh g.