Ammonium Vanadium Bronze (NH₄V₄O₁₀) as a High-Capacity Cathode Material for Nonaqueous Magnesium-Ion Batteries

Magnesium-ion batteries (MIBs) offer improved safety, lower cost, and higher energy capacity. However, lack of cathode materials with considerable capacities in conventional nonaqueous electrolyte at ambient temperature is one of the great challenges for their practical applications. Here, we present high magnesium-ion storage performance and evidence for the electrochemical magnesiation of ammonium vanadium bronze NH₄V₄O₁₀ as a cathode material for MIBs. NH₄V₄O₁₀ was synthesized via a conventional hydrothermal reaction. It shows reversible magnesiation with an initial discharge capacity of 174.8 mAh g^-1 and the average discharge voltage of ∼2.31 V (vs Mg/Mg²⁺) using 0.5 M Mg(ClO₄)₂ in acetonitrile as the electrolyte. Cyclic voltammetry, galvanostatic, discharge-charge, FTIR, XPS, powder XRD, and elemental analyses unequivocally show evidence for the reversible magnesiation of the material and suggest that keeping the ammonium ions in the interlayer space of NH₄V₄O₁₀ could be crucial for the structural stability with a sacrifice of initial capacity but much enhanced retention capacity. This is the first demonstration of electrochemical magnesiation with a high capacity above 2 V (vs Mg/Mg²⁺) using a conventional organic electrolyte with a relatively low water concentration.