Stability and conductivity of cation- and anion-substituted ${\mathrm{LiBH}}_{4}$-based solid-state electrolytes

Stability and conductivity of cation- and anion-substituted ${LiBH}_{4}$ -based solid-state electrolytes

Zhenpeng Yao, Soo Kim, Kyle Michel, Yongsheng Zhang, Muratahan Aykol, and Chris Wolverton

Phys. Rev. Materials 2, 065402 – Published 19 June 2018

Abstract

The high-temperature phase of ${LiBH}_{4}$ (HT- ${LiBH}_{4}$ ) 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- ${LiBH}_{4}$ as well the underlying mechanism for the high lithium ion conductivity. We find that increasing the substituent concentration enhances the stabilization of HT- ${LiBH}_{4}$ (i.e., the DFT energy difference between the low- and high-temperature forms of substituted ${LiBH}_{4}$ 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 $Li {(B H_{4})}_{1 - x} I_{x}$ ( $x = 0.25$ and 0.5) and $L i_{1 - y} K_{y} B H_{4}$ ( $y = 0.25$ ). Our calculations suggest that the lower I concentration leads to a higher lithium ion conductivity of $5.7 \times 10^{- 3}$ S/cm compared with that of $Li {(B H_{4})}_{0.5} I_{0.5}$ ( $4.2 \times 10^{- 5}$ S/cm) because of the formation of more Li-related defects. Based on our findings, we suggest that the stabilization of HT- ${LiBH}_{4}$ -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.

Received 18 March 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.065402

Physics Subject Headings (PhySH)

Ionic transport

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zhenpeng Yao^*, Soo Kim^†, Kyle Michel^‡, Yongsheng Zhang^§, Muratahan Aykol^∥, and Chris Wolverton^¶

Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, USA

^*Present address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, USA.
^†Present address: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
^‡Present address: Citrine Informatics, Inc., 1741 Broadway #300, Redwood City, CA 94063, USA.
^§Present address: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China.
^∥Present address: Toyota Research Institute, 4440 El Camino Real, Palo Alto, CA 94022, USA.
^¶Corresponding author: c-wolverton@northwestern.edu