Fe-57-Mossbauer and XAFS Studies of Conductive Sodium Phospho-Vanadate Glass as a Cathode Active Material for Na-ion Batteries with Large Capacity

A relationship between local structure and electrical property of xNa(2)O center dot 10P(2)O(5)center dot(90-x)V2O5 glass (x = 5, 25 and 45 mol%, abbreviated as xNPV) was investigated by means of Fe-57-Mossbauer spectroscopy(FeMS), X-ray absorption fine structure (XAFS), X-ray diffractometry (XRD), impedance spectroscopy (IS), and cathode active performance test of Na-ion battery (SIB). XAFS of xNPV glasses showed that the oxidation number of the vanadium in xNPV glasses was reduced after the heat treatment. FeMS of xNa(2)O center dot 10P(2)O(5)center dot(89-x)V2O5 center dot(Fe2O3)-Fe-57 glass showed a decrease in the isomer shift and quadrupole splitting with an increase of Na2O content. DC conductivity was decreased with increasing Na2O content and the glass samples exhibit electronic conduction which is explained by the polaronic conduction mechanism. The charge-discharge capacity of xNPV glasses showed a larger initial capacity than that of heat-treated samples. A large initial capacity of 184 mAh g(-1) was recorded for 25NPV glass, suggesting that the sodium phospho-vanadate glass can be used as a good cathode of SIB.