An Asymmetric Sodium Extraction/Insertion Mechanism for the Fe/V-Mixed NASICON Na4FeV(PO4)3

Vanadium multi redox reactions of V4+/3+ and V5+/4+ can contribute to significant capacity gain in positive electrodes for Na-Ion batteries, but the activation of the V5+/4+ redox couple is often accompanied by asymmetric and irreversible electrochemical reactions that are very important to understand for developing high capacity materials while maintaining the structural stability. Here, the asymmetric Na+ extraction/insertion process in Fe/V-mixed NASICON Na4FeV(PO4)3 electrode material is thoroughly investigated from both local and bulk perspectives. X-ray diffraction analysis demonstrates that the Na(1) site is depopulated at the third charge domain when the V5+/4+ redox couple is activated. Local environments of Fe and V were investigated through Mössbauer and X-ray absorption spectroscopies. The results point toward the existence of a distorted VO6 octahedral environment, including a short V–O bond, when the V5+/4+ redox couple is activated at high voltage. This study presents valuable aspects of the asymmetric behavior during multielectron reactions that are commonly observed in V-based NASICON materials.