Polymorphism and solid solutions of trimethylammonium monocarboranes.

Metal closo-boranes have recently received significant attention as solid-state electrolytes due to their high thermal and electrochemical stability, and the weak interaction between the cat- and anion, facilitating fast ionic conductivity. Here we report a synthesis method for obtaining a novel mixed closo-carborane compound, [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08]. The crystal structures are investigated for [NH(CH3)3][CB9H10] and [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08], revealing that the latter forms a solid solution isostructural to [NH(CH3)3][CB9H10]. The compounds exhibit polymorphism as a function of temperature, and we report the discovery of four polymorphs of [NH(CH3)3][CB9H10] and four isostructural solid solution [NH(CH3)3][(CB8H9)0.26(CB9H10)0.66(CB11H12)0.08], along with a high-temperature decomposition intermediate of the latter. The α-polymorph is an ordered structure, with increasing amounts of disorder for the ß- and γ-polymorphs, while the high temperature δ- and ε-polymorphs at T > 476 K are fully disordered on both the cation and anion site. These new compounds may be used as precursors for new types of solid-state ionic conductors.

Fast magnesium ion conducting isopropylamine magnesium borohydride enhanced by hydrophobic interactions.

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 magnesium borohydride based composites with different compositions, Mg(BH4)2·x(CH3)2CHNH2, x = 0.5, 0.9, 1.25, 1.5, 1.75, 2.5, 3.1. Three new compounds are discovered, x = 1, 2, and 3 and the monoclinic structure of Mg(BH4)2·2(CH3)2CHNH2 (P21/c) is investigated in detail. This structure consists of neutral complexes [Mg(BH4)2((CH3)2CHNH2)2] di-hydrogen bonded to form layers and these layers are connected by hydrophobic interactions via the isopropyl moieties. The orthorhombic unit cell of Mg(BH4)2·(CH3)2CHNH2 was also determined, a = 9.78, b = 12.17 and c = 17.24 Å. In general, the samples are thermally stable up to 50 °C where they started to become softer, and at 70 °C isopropylamine release and melting started. The highest Mg2+ ionic conductivity was that of Mg(BH4)2·1.5(CH3)2CHNH2, σ(Mg2+) = 2.7 × 10-4 S cm-1 at 45 °C, with an activation energy of EA = 1.22 eV. Furthermore, reversible stripping/plating of Mg was displayed at 45 °C, with an oxidative stability of 1.2 V vs. Mg/Mg2+. The addition of MgO nanoparticles (75 wt%) improves the mechanical and thermal stability, and decreases the activation energy, to EA = 0.56 eV. Thereby the Mg2+ conductivity is increased at low temperature. This suggests that the hydrophobic interactions contribute to the high ionic conductivity in the solid state, which opens a new avenue for design and discovery of electrolyte materials.

Methylamine Lithium Borohydride as Electrolyte for All-Solid-State Batteries.

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. LiBH4 ⋅CH3 NH2 crystallizes in the monoclinic space group P21 /c, forming a two-dimensional unique layered structure. The layers are separated by hydrophobic -CH3 moieties, and contain large voids, allowing for fast Li-ionic conduction in the interlayers, σ(Li+ )=1.24×10-3  S cm-1 at room temperature. The electronic conductivity is negligible, and the electrochemical stability is ≈2.1 V vs Li. The first all-solid-state battery using a lithium borohydride with a neutral ligand as the electrolyte, Li-metal as the anode and TiS2 as the cathode is demonstrated.