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.

Li2B12H12 is reported in the molten state for the first time, which enables a range of new research opportunities.

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.

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