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

Abstract 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, Na ₃ V ₂ (PO ₄ ) ₂ F ₃ (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 N _0.92 K _0.08 VPF/C is found so as to achieve rapid Na ⁺ migration and reversible phase transition. The specific capacity of N _0.92 K _0.08 VPF/C is as high as 128.8 mAh g ⁻¹ at 0.2 C, and at 10 C its rate performance is excellent, which shows a capacity of 113.3 mAh g ⁻¹ .