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 tion=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 MgyTiS2.