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 • xTHF(À MgO), 0 � x � 3, as solidstate electrolytes for magnesium batteries. Three new monoclinic compounds were identified, Mg(BH 4 ) 2 • 2/3THF (Cc), α-Mg(BH 4 ) 2 • 2THF (P2 1 /c) and β-Mg(BH 4 ) 2 • 2THF (C2), and the detailed structures of αand 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 • 1.5THFÀ MgO(75 wt%), displayed the highest Mg 2 + conductivity, σ(Mg 2 + ) ~10 À 4 S cm À 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 À 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 À 1 was displayed, which corresponds to y = 0.2 in Mg y TiS 2 .

Entropy-driven formation of high entropy alloy (HEA) nanoparticles from metal precursors requires high temperatures and controlled cooling rates. However, several proposed HEA nanoparticle synthesis strategies avoid the high-temperature regime. In our work, we address the question of how single-phase HEA nanoparticles can form at low temperatures. Investigating a system of five noble metal single source precursors, we combine in situ X-ray powder diffraction with multi-edge X-ray absorption spectroscopy to demonstrate that the formation of single-phase nanoparticles is governed by stochastic principles and the inhibition of precursor mobility during the formation process. The proposed formation principle is supported by simulations of the nanoparticle formation in a randomized process, rationalizing the experimentally found differences between two-element and multi-element metal precursor mixtures.