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Polymorphism, ionic conductivity and electrochemical properties of lithium closo-deca- and dodeca-borates and their composites, Li 2B 10H 10-Li 2B 12H 12

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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 Li 2B 10H 10 and Li 2B 12H 12 and their composites, (1 − x)Li 2B 12H 12-xLi 2B 10H 10. A new polymorph, β-Li 2B 10H 10, is identified with a cubic structure, a = 9.567(1) Å (Fm3̄m), with dynamically disordered B 10H 10 2− anions. A sub-stoichiometric compound denoted as γ-Li 2B 10H 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 (T 1) measured by 7Li saturation-recovery NMR experiments clearly reveal a high degree of dynamics assigned to increasing amounts of γ-Li 2B 10H 10−y, which is in accordance with the measured Li + ionic conductivity. Thermal treatment (380 °C, 1 hour) of Li 2B 12H 12 in argon reveals the highest degree of dynamics and Li + conductivity. The (1 − x)Li 2B 12H 12-xLi 2B 10H 10 composites are found to be physical mixtures of γ-Li 2B 10H 10−y and Li 2B 12H 12 with minor amounts of α-Li 2B 10H 10, and their Li + conductivities are proportional to the amount of γ-Li 2B 10H 10−y. The highest Li + conductivity is observed for γ-Li 2B 10H 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-Li 2B 10H 10−y reveals an oxidative stability up to 2.8 V vs. Li/Li +, and a stable plating and stripping of lithium.

Original languageEnglish
JournalJournal of Materials Chemistry A
Pages (from-to)16137-16151
Number of pages15
Publication statusPublished - Aug 2022

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