Hydrated lithium nido-boranes for solid–liquid hybrid batteries

Abstract

Hydridoborate salts are considered as promising solid-state electrolyte candidates for the development of solid-state batteries (SSBs). The presence of coordinated water in the crystal structure may facilitate the migration of the cation, yielding compounds with high ionic conductivity. In the present study, two samples of hydrated LiB₁₁H₁₄, here called LiB₁₁H₁₄·2H₂O and a-LiB₁₁H₁₄·(H₂O)_n (n < 2), demonstrate remarkably different properties as solid-state electrolytes. LiB₁₁H₁₄·2H₂O is identified as a new class of ionic liquid, as it melts at ≈70 °C, whereas the sample a-LiB₁₁H₁₄·(H₂O)_n undergoes a polymorphic phase transition close to this temperature, reaching the liquid-like ionic conductivity of 3.2 × 10⁻² S cm⁻¹ at 70 °C and an oxidative stability limit of 2.8 V against Li⁺/Li. Galvanostatic cycling and battery tests were conducted with a-LiB₁₁H₁₄·(H₂O)_n as the solid-state electrolyte (SSE) at 60 °C with the addition of traces of either the ionic liquid (IL) LiB₁₁H₁₄·2H₂O or the liquid electrolyte (LE) 1.0 M LiPF₆ EC/DMC (v/v = 50/50) at their interfaces. Galvanostatic experiments for the cell Li/IL/SSE/IL/Li showed an overpotential of only 21 mV after 9 days cycling (48 h at 25 μA cm⁻² and 168 h at 50 μA cm⁻²), and the battery Li/LE/SSE/SSE + TiS₂ retained 83% of its capacity shown in the first cycle at 0.4C after 50 cycles. LiB₁₁H₁₄·2H₂O and 1.0 M LiPF₆ EC/DMC work effectively as wetting agents to improve SSE/Li contact.