Air Stability and Na Site Activation of NASICON-Based NFPP/NVOPF/C Heterostructured Cathode via Surface Reconstruction

Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) is a promising cathode material for sodium-ion batteries due to its low cost, high safety, and long cycle life. However, its poor electronic conductivity and air instability hinder practical application, and the issue of air stability has received limited attention. Herein, the hydrolysis behavior of NFPP/C in humid air is systematically investigated. Based on the identified degradation mechanism, a surface reconstruction strategy is proposed via in situ liquid-phase encapsulation of nanostructured Na 3 (VO) 2 PO 4 F (NVPOF). During heat treatment, VO x in NVPOF catalyzes carbon ordering and enables partial V doping into the NFPP surface lattice, thereby modulating surface chemistry, crystal and electronic structure of NFPP. Additionally, the aligned NVPOF/NFPP heterostructure with minimal lattice mismatch forms a built-in electric field, reducing Na⁺ diffusion barriers and interfacial stress. As a result, the Na3 site is activated, leading to significantly enhanced electrochemical performance. The optimized NFPP/NVPOF/C composite delivers excellent rate capability, with a discharge capacity of 126.55 mAh g −1 at 0.1C and 114.40 mAh g −1 at 10C. This work addresses the overlooked air stability issue of NFPP and highlights the importance of surface reconstruction coupled with interface engineering.