Hemi-methylamine lithium borohydride as electrolyte for all-solid-state batteries

Utilization of next-generation all-solid-state lithium batteries require new fast Li-ion conducting solid electrolytes. LiBH4-based materials have emerged as a promising class of Li+-conductors, and recent advancements show sufficiently high ionic conductivity for battery operation at room temperature. In this work we report a new compound, hemi-methylamine lithium borohydride, and the crystal structure of LiBH4 & BULL;1/2CH(3)NH(2) is solved in the orthorhombic space group Pnma. The structure is built from two-dimensional layers consisting of alternating [Li(BH4)(4)] and [Li(CH3NH2)(BH4)(3)] tetrahedral units, and voids in the interlayers allows for two potential conduction pathways for an interstitial Li+. This results in a high lithium ion conductivity of & sigma;(Li+) = 1.88 x 10(-3) S cm(-1) at T = 31 & DEG;C. The electrochemical stability of LiBH4 & BULL;1/2CH(3)NH(2) is similar to that of LiBH4 (about 2.2 V vs. Li+/Li) and the electrolyte appear to form a favorable interface towards Li-metal with a very low overpotential of 0.1 mV at 30 & DEG;C. A full cell battery was tested operando with simultaneous collection of diffraction- and electrochemical data using a Li-metal anode and a layered TiS2 cathode, revealing a spontaneous discharge to LixTiS2 (x > 0.85). The battery could be cycled with an initial discharge of 105 mA h g(-1) (& UDelta;x = 0.44), but a sidereaction occurring at & SIM;1.8 V prevents full charging and suggests that LiBH4 & BULL;1/2CH(3)NH(2) is incompatible with layered TiS2.