Towards Solid‐State Magnesium Batteries: Ligand‐Assisted Superionic Conductivity
Lasse N. Skov, Jakob B. Grinderslev, Asger W. Rosenkranz, Young‐Su Lee, Torben R. Jensen
Abstract
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(BH 4 ) 2 ⋅ x THF(−MgO), 0≤ x ≤3, as solid‐state electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH 4 ) 2 ⋅ 2/3THF ( Cc ), α‐Mg(BH 4 ) 2 ⋅ 2THF ( P 2 1 /c ) and β‐Mg(BH 4 ) 2 ⋅ 2THF ( C 2), and the detailed structures of α‐ and β‐Mg(BH 4 ) 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(BH 4 ) 2 ⋅ 1.5THF−MgO(75 wt%), displayed the highest Mg 2+ conductivity, σ (Mg 2+ )∼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/Mg 2+ . 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 TiS 2 cathode. A maximum discharge capacity of 94.2 mAh g −1 was displayed, which corresponds to y =0.2 in Mg y TiS 2 .