Stability and conductivity of cation- and anion-substituted LiBH4 -based solid-state electrolytes

The high-temperature phase of LiBH4 (HT-LiBH4) exhibits a promisingly high lithium ion conductivity but is unstable at room temperature. We use density functional theory (DFT) calculations to investigate the stabilization effect of halogen and alkali cation/anion substitutions on HT-LiBH4 as well the underlying mechanism for the high lithium ion conductivity. We find that increasing the substituent concentration enhances the stabilization of HT-LiBH4 (i.e., the DFT energy difference between the low- and high-temperature forms of substituted LiBH4 is reduced). Cation/anion substitution also leads to a higher Li defect (vacancy, interstitial, and Frenkel) formation energy, thereby reducing the Li defect (vacancy, interstitial, and Frenkel) concentrations. Using DFT migration barriers input into kinetic Monte Carlo simulations and the Materials INTerface (MINT) framework, we calculate the room-temperature lithium ion conductivities for Li(BH4)1-xIx (x=0.25 and 0.5) and Li1-yKyBH4 (y=0.25). Our calculations suggest that the lower I concentration leads to a higher lithium ion conductivity of 5.7×10-3 S/cm compared with that of Li(BH4)0.5I0.5 (4.2×10-5 S/cm) because of the formation of more Li-related defects. Based on our findings, we suggest that the stabilization of HT-LiBH4-based lithium ion conductors can be controlled by carefully tuning the cation/anion substituent concentrations to maximize the lithium ionic conductivities of the specific system.