Structural Behavior of Vanadate Alkali Phosphate Glass upon Electrochemical Lithiation and Sodiation

Vanadate phosphate glasses constitute a promising class of high-energy-density materials for Li-ion or Na-ion batteries due to their multielectron redox transitions and reversible cycling; however, their electrochemically induced structural transformations have not been elucidated. This study evaluates the melt-quenching fabrication and performance of Vanadate alkali phosphate (VO 2.5:LiPO 3 and VO 2.5:NaPO 3 ) glasses from 90 to 70 mol% VO 2.5 in Li-ion and Na-ion half-cells. All glasses showed electrochemical reactivity, with the 9VO 2.5:1LiPO 3 and 9:VO 2.5:1NaPO 3 glasses having the best insertion/extraction reversibility and cycling stability, reaching 300 and 140 mAh/g at C/10 from 1.5 to 3.8 V vs Li/Li + and 1.2–3.5 V vs Na/Na +, respectively. An increased voltage hysteresis is observed upon increasing the alkali phosphate content from 10% to 30%. Ex situ X-ray pair distribution function (PDF) and Raman spectroscopy analyses reveal that ion insertion induces an increase in the length of V=O bonds and increases the fraction of V–O bonding in the glass. At >1 Li/V, the V=O bonds are converted to longer V–O bonds, and the original corner-sharing VO 5 network is replaced by a V–V edge-sharing local arrangement similar to a rock-salt structure. Galvanostatic intermittent titration technique (GITT) studies indicate that higher lithium phosphate content tends to increase the voltage polarization, which we attribute to alkali-ion diffusion limitations and frustration of V network restructuring. These results demonstrate that the electrochemical behavior of Vanadate-phosphate glasses is governed by kinetic rather than structural reaction differences when changing the alkali phosphate content.