Polymorphism, ionic conductivity and electrochemical properties of lithium closo-deca- and dodeca-borates and their composites, Li2B10H10–Li2B12H12

Abstract

Metal closo-borates are attractive electrolytes for solid state batteries. Here we present a detailed investigation of the polymorphism and thermal and electrochemical properties of Li₂B₁₀H₁₀ and Li₂B₁₂H₁₂ and their composites, (1 − x)Li₂B₁₂H₁₂–xLi₂B₁₀H₁₀. A new polymorph, β-Li₂B₁₀H₁₀, is identified with a cubic structure, a = 9.567(1) Å (Fmm), with dynamically disordered B₁₀H₁₀²⁻ anions. A sub-stoichiometric compound denoted as γ-Li₂B₁₀H_10−y is prepared by thermal treatment (380 °C, 1 hour) in hydrogen and can be indexed to a cubic face-centered unit cell with a = 9.9224(5) Å. The ⁷Li MAS NMR spectra along with spin–lattice relaxation (T₁) measured by ⁷Li saturation-recovery NMR experiments clearly reveal a high degree of dynamics assigned to increasing amounts of γ-Li₂B₁₀H_10−y, which is in accordance with the measured Li⁺ ionic conductivity. Thermal treatment (380 °C, 1 hour) of Li₂B₁₂H₁₂ in argon reveals the highest degree of dynamics and Li⁺ conductivity. The (1 − x)Li₂B₁₂H₁₂–xLi₂B₁₀H₁₀ composites are found to be physical mixtures of γ-Li₂B₁₀H_10−y and Li₂B₁₂H₁₂ with minor amounts of α-Li₂B₁₀H₁₀, and their Li⁺ conductivities are proportional to the amount of γ-Li₂B₁₀H_10−y. The highest Li⁺ conductivity is observed for γ-Li₂B₁₀H_10−y: σ(Li⁺) = 7.6 × 10⁻⁶ S cm⁻¹ at 30 °C and 7.2 × 10⁻³ S cm⁻¹ at T = 330 °C. Cyclic voltammetry of γ-Li₂B₁₀H_10−y reveals an oxidative stability up to 2.8 V vs. Li/Li⁺, and a stable plating and stripping of lithium.