Role of disorder in limiting the true multi-electron redox in ε-LiVOPO₄

Recent advances in materials syntheses have enabled ε-LiVOPO₄ to deliver capacities approaching, and in some cases exceeding the theoretical value of 305 mA h g⁻¹ for 2Li intercalation, despite its poor electronic and ionic conductivity. However, not all of the capacity corresponds to the true electrochemical intercalation/deintercalation reactions as evidenced upon systematic tracking of V valence through combined operando and rate-dependent ex situ X-ray absorption study presented herein. Structural disorder and defects introduced in the material by high-energy ball milling impede kinetics of the high-voltage V⁵⁺/V⁴⁺ redox more severely than the low-voltage V⁴⁺/V³⁺ redox, promoting significant side reaction contributions in the high-voltage region, irrespective of cycling conditions. The present work emphasizes the need for nanoengineering of active materials without compromising their bulk structural integrity in order to fully utilize high-energy density of multi-electron cathode materials.