Magnesium batteries are considered promising solutions for future energy storage beyond the lithium-ion battery era. However, the development of magnesium batteries is hindered by the lack of suitable electrolytes. Here we present solid Mg 2+ electrolytes based on ammine magnesium borohydride composites, Mg(BH 4 ) 2 •xNH 3 , which have conductivities ca. three orders of magnitude higher than the parent compounds (x = 1, 2, 3, and 6). A nanocomposite formed by the Mg(BH 4 ) 2 •xNH 3 composite and MgO nanoparticles exhibits outstanding Mg 2+ conductivity of the order of 10 −5 S cm −1 at room temperature and around 10 −3 S cm −1 at moderate temperature (ca. 70 °C), with an activation energy for Mg 2+ conduction of E a ∼108 kJ/mol (1.12 eV) and high thermal stability (T dec = 120 °C). Characterization using solid-state nuclear magnetic resonance, powder X-ray diffraction, and transmission electron microscopy reveals that the high Mg 2+ conductivity is attributed to amorphization of Mg(BH 4 ) 2 •xNH 3 resulting in a highly dynamic state. This nanocomposite is compatible with a Mg metal anode and allows stable Mg plating/stripping (at least 100 cycles) in a symmetric cell. The results represent a major advancement of solid-state multivalent ion conductors here demonstrated for Mg 2+ .

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Fast Li-ion conductivity at room temperature is a major challenge for utilization of all-solid-state Li batteries. Metal borohydrides with neutral ligands are a new emerging class of solid-state ionic conductors, and here we report the discovery of a new mono-methylamine lithium borohydride with very fast Li + conductivity at room temperature. LiBH 4 •CH 3 NH 2 crystallizes in the monoclinic space group P2 1 /c, forming a twodimensional unique layered structure. The layers are separated by hydrophobic À CH 3 moieties, and contain large voids, allowing for fast Li-ionic conduction in the interlayers, σ(Li + ) = 1.24 × 10 À 3 S cm À 1 at room temperature. The electronic conductivity is negligible, and the electrochemical stability is � 2.1 V vs Li. The first allsolid-state battery using a lithium borohydride with a neutral ligand as the electrolyte, Li-metal as the anode and TiS 2 as the cathode is demonstrated.

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 .

The development of efficient storage systems is one of the keys to the success of the energy transition. There are many ways to store energy, but among them, electrochemical storage is particularly valuable because it can store electrons produced by renewable energies with a very good efficiency. However, the solutions currently available on the market remain unsuitable in terms of storage capacity, recharging kinetics, durability, and cost. Technological breakthroughs are therefore expected to meet the growing need for energy storage. Within the framework of the Hydrogen Technology Collaboration Program – H2TCP Task-40, IEA's expert researchers have developed innovative materials based on hydrides (metallic or complex) offering new solutions in the field of solid electrolytes and anodes for alkaline and ionic batteries. This review presents the state of the art of research in this field, from the most fundamental aspects to the applications in battery prototypes.

Metal closo-borates are attractive electrolytes for solid state batteries. Here we present a detailed investigation of the polymorphism and thermal and electrochemical properties of Li2B10H10 and Li2B12H12 and their composites,...

New materials for the next generation of electrochemical energy storage devices such as batteries are of extreme importance. Here we investigate the structure, ionic conductivity and thermal properties of isopropylamine...

Solid-state magnesium electrolytes may pave the way for novel types of rechargeable, sustainable, and cheap batteries with high volumetric and gravimetric capacities. There are, however, currently no solid-state magnesium electrolytes that fulfill the requirements for solid-state battery applications. Here, we present the synthesis, structure, and properties of six new methylamine magnesium borohydride compounds, α- and β-Mg(BH4)2·6CH3NH2, Mg(BH4)2·3CH3NH2, and α-, α′- and β-Mg(BH4)2·CH3NH2. The β-Mg(BH4)2·CH3NH2 polymorph displays a record high Mg2+ ionic conductivity of σ(Mg2+) = 1.50 × 10–4 S cm–1 at room temperature. The high Mg2+ conductivity of β-Mg(BH4)·CH3NH2 is facilitated by a one-dimensional chain-like structure interconnected by weak dihydrogen bonds and dispersion interactions, forming a migration pathway across the chains. The oxidative stability of Mg(BH4)2·CH3NH2 is ∼1.2 V vs Mg/Mg2+, and the reversible plating and stripping were confirmed by cyclic voltammetry and symmetric cell cycling, revealing high stability toward magnesium electrodes for at least 50 cycles at 60 °C.