Structural, Morphological and Interfacial Investigation of H₂V₃O₈ upon Mg²⁺ Intercalation

Abstract

Magnesium-ion batteries (MIBs) are a promising alternative to lithium-ion batteries due to their higher theoretical energy densities and lower cost. However, fundamental limitations such as the sluggish diffusion of Mg²⁺ ions into cathode materials have hindered their practical implementation. In this work, the structural, morphological and interfacial changes of H₂V₃O₈, a promising cathode material for MIBs, are elucidated upon Mg²⁺ intercalation in TFSI-based electrolytes. Post-mortem analysis revealed a shrinkage of the interlayer distance, an increase in cell volume, and a decrease of the surface V⁵⁺/V⁴⁺ ratio during discharge. These changes were only partially reversible upon subsequent charge due to Mg trapping. A cathode electrolyte interphase (CEI) formed mainly of TFSI− anions, its decomposition products, and MgF₂ was detected on the surface of H₂V₃O₈. The CEI thickness and chemical composition vary upon charge and discharge as well as during prolonged cycling, and its presence correlates with the additional capacity recorded during discharge. Our study aims to contribute to a better understanding of Mg²⁺ intercalation and cathode-electrolyte interaction in high voltage cathode materials for MIBs.