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.

Automated summary

Utilization of next-generation all-solid-state lithium batteries requires new fast Li-ion conducting solid electrolytes. LiBH4-based materials have shown promising high ionic conductivity at room temperature. A new compound, hemi-methylamine lithium borohydride (LiBH4 & BULL;1/2CH(3)NH(2)), with a crystal structure consisting of two-dimensional layers, has been discovered. This compound exhibits high lithium ion conductivity and electrochemical stability, making it suitable for battery operation. However, it is incompatible with layered TiS2 cathode, limiting its full charging potential.