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)center dot xTHF(-MgO), 0 <= x <= 3, as solid-state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH4)(2)center dot 2/3THF (Cc), alpha-Mg(BH4)(2)center dot 2THF (P2(1)/c) and beta-Mg(BH4)(2)center dot 2THF (C2), and the detailed structures of alpha- and beta-Mg(BH4)(2)center dot 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 (similar to 50 nm), Mg(BH4)(2)center dot 1.5THF-MgO(75 wt%), displayed the highest Mg2+ conductivity, sigma(Mg2+)similar to 10(-4) S cm(-1) at 70 degrees C, a high ionic transport number of t(ion)=0.99, and cyclic voltammetry revealed an oxidative stability of similar to 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 degrees 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.