H₂V₃O₈ as a High Energy Cathode Material for Nonaqueous Magnesium-Ion Batteries

Magnesium-ion batteries (MIBs) suffer from a low energy density of cathode materials in a conventional nonaqueous electrolyte, contrary to the expectation due to the divalent Mg ion. Here, we report H₂V₃O₈, or V₃O₇·H₂O, as a high-energy cathode material for MIBs. It exhibits reversible magnesiation-demagnesiation behavior with an initial discharge capacity of 231 mAh g^-1 at 60 °C, and an average discharge voltage of ∼1.9 V vs Mg/Mg²⁺ in an electrolyte of 0.5 M Mg(ClO₄)₂ in acetonitrile, resulting in a high energy density of 440 Wh kg^-1. The structural water remains stable during cycling. The crystal structure for Mg_0.97H₂V₃O₈ is determined for the first time. Bond valence sum difference mapping shows facile conduction pathways for Mg ions in the structure. The high performance of this material with its distinct crystal structure employing water-metal bonding and hydrogen bonding provides insights to search for new oxide-based stable and high-energy materials for MIBs.