Polymorphic Purity and Structural Charge–Discharge Evolution of β-LiVOPO4 Cathodes

Lithium vanadyl phosphate, LiVOPO4, holds interest as a Li-ion battery electrode due to the multiple active redox states of vanadium (V5+ ⇋ V4+ ⇋ V3+) and the high redox potential induced by vanadyl phosphate. The orthorhombic β-LiVOPO4 polymorph is of special interest owing to its low Li-ion diffusion barrier and high stability. In this work, we synthesize β-LiVOPO4 and investigate the polymorphic purity under different synthesis conditions and show that high-energy ball milling without postsintering can alter the sample morphology and thereby dramatically increase the electrochemical performance of β-LiVOPO4. Through operando powder X-ray diffraction (PXRD), the structural evolution of β-LiVOPO4 during charge and discharge is unraveled and unexpected solid solution behavior in the Li-poor β-VOPO4 is revealed.