Filling the Gaps in the LiBr-LiOH Phase Diagram: A Study on the High-Temperature Li₃(OH)₂Br Phase

In this paper, we build on previous work to characterize a phase with stoichiometry Li₃(OH)₂Br existing between ∼225 and ∼275 °C in the LiBr-LiOH phase diagram. Diffraction studies indicate that the phase takes a hexagonal unit cell, and theoretical modeling is used to suggest a possible crystal structure. Nuclear magnetic resonance spectroscopy and electrochemical impedance spectroscopy measurements demonstrate excellent lithium-ion dynamics in this phase, with an ionic conductivity of 0.12 S cm^-1 at 250 °C. Initial attempts to stabilize this phase at room temperature through quenching were not successful. Instead, a metastable state demonstrating poor ionic conductivity is found to form. This is an important consideration for the synthesis of Li₂OHBr solid-state electrolytes (also found in the LiBr-LiOH phase diagram) which are synthesized by cooling through phase fields containing Li₃(OH)₂Br, and are hence susceptible to these impurities.