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

Abstract

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 LiBH₄·1/2CH₃NH₂ 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 σ(Li⁺) = 1.88 × 10⁻³ S cm⁻¹ at T = 31 °C. The electrochemical stability of LiBH₄·1/2CH₃NH₂ 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 °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 Li_xTiS₂ (x > 0.85). The battery could be cycled with an initial discharge of 105 mA h g⁻¹ (Δx = 0.44), but a sidereaction occurring at ∼1.8 V prevents full charging and suggests that LiBH₄·1/2CH₃NH₂ is incompatible with layered TiS₂.