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

Vanadium multi redox reactions of V4+/3+andV5+/4+can contribute to significant capacity gain in positiveelectrodes for Na-Ion batteries, but the activation of the V5+/4+redox couple is often accompanied by asymmetric and irreversibleelectrochemical reactions that are very important to understand fordeveloping high capacity materials while maintaining the structuralstability. Here, the asymmetric Na+extraction/insertion process inFe/V-mixed NASICON Na4FeV(PO4)3electrode material isthoroughly investigated from both local and bulk perspectives. X-ray diffraction analysis demonstrates that the Na(1) site isdepopulated at the third charge domain when the V5+/4+redoxcouple is activated. Local environments of Fe and V wereinvestigated through Mo??ssbauer and X-ray absorption spectros-copies. The results point toward the existence of a distorted VO6octahedral environment, including a short V-O bond, when theV5+/4+redox couple is activated at high voltage. This study presents valuable aspects of the asymmetric behavior during multielectronreactions that are commonly observed in V-based NASICON materials