Phase Stability and Fast Ion Conductivity in the Hexagonal LiBH4-LiBr-LiCl Solid Solution

This study shows a flexible system that offers promising candidates for Li-based solid-state electrolytes. The Br- substitution for BH4- stabilizes the hexagonal structure of LiBH4 at room temperature (RT), whereas Cl- is soluble only at higher temperatures. Incorporation of chloride in a hexagonal solid solution leads to an increase in the energy density of the system. For the first time, a stable hexagonal solid solution of LiBH4 containing both Cl- and Br-halide anions has been obtained at RT. The LiBH4-LiBr-LiCl ternary phase diagram has been determined at RT by X-ray diffraction coupled with a Rietveld refinement. A solubility of up to 30% of Cl- in the solid solution has been established. The effect of halogenation on the Li-ion conductivity and electrochemical stability has been investigated by electrochemical impedance spectroscopy and cyclic voltammetry. Considering the ternary samples, h-Li(BH4)0.7(Br)0.2(Cl)0.1 composition showed the highest value for conductivity (1.3 × 10-5 S/cm at 30 °C), which is about 3 orders of magnitude higher than that for pure LiBH4 in the orthorhombic structure. The values of Li-ion conductivity at RT depend only on the BH4- content in the solid solution, suggesting that the Br/Cl ratio does not affect the defect formation energy in the structure. Chloride anion substitution in the hexagonal structure increases the activation energy, moving from about 0.45 eV for samples without Cl- ions in the structure up to about 0.63 eV for h-Li(BH4)0.6(Br)0.2(Cl)0.2 compositions, according to the Meyer-Neldel rule. In addition to increasing Li-ion conductivity, the halogenation also increases the thermal stability of the system. Unlike for the Li-ion conductivity, the Br/Cl ratio influences the electrochemical stability: a wide oxidative window of 4.04 V versus Li+/Li is reached in the Li-Br system while further addition of Cl is a trade-off between oxidative stability and weight reduction. The halogenation allows both binary and ternary systems operating below 120 °C, thus suggesting possible applications of these fast ion conductors as solid-state electrolytes in Li-ion batteries.