Towards Solid‐State Magnesium Batteries: Ligand‐Assisted Superionic Conductivity

Abstract Solid‐state inorganic magnesium batteries are considered as potential high energy storage devices of the future. Here we present a series of magnesium borohydride tetrahydrofuran (THF) composites, Mg(BH₄)₂ ⋅ xTHF(−MgO), 0≤x≤3, as solid‐state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH₄)₂ ⋅ 2/3THF (Cc), α‐Mg(BH₄)₂ ⋅ 2THF (P2₁/c) and β‐Mg(BH₄)₂ ⋅ 2THF (C2), and the detailed structures of α‐ and β‐Mg(BH₄)₂ ⋅ 2THF are presented. The magnesium ionic conductivity of composites formed by these compounds were several orders of magnitude higher than that of the distinct compounds, x=0, 2/3, 2, and 3. The nanocomposite stabilized by MgO nanoparticles (∼50 nm), Mg(BH₄)₂ ⋅ 1.5THF−MgO(75 wt%), displayed the highest Mg²⁺ conductivity, σ(Mg²⁺)∼10⁻⁴ S cm⁻¹ at 70 °C, a high ionic transport number of t_ion=0.99, and cyclic voltammetry revealed an oxidative stability of ∼1.2 V vs. Mg/Mg²⁺. The electrolyte was stable towards magnesium electrodes, which allowed for stable Mg plating/stripping for at least 100 cycles at 55 °C with a current density of 0.1 mA cm⁻². Finally, a proof‐of‐concept rechargeable solid‐state magnesium battery was assembled with a magnesium metal anode and a TiS₂ cathode. A maximum discharge capacity of 94.2 mAh g⁻¹ was displayed, which corresponds to y=0.2 in Mg_yTiS₂.