Understanding Ionic Conductivity Trends in Polyborane Solid Electrolytes from Ab Initio Molecular Dynamics

Published

December 22, 2016

Author(s)

Joel B. Varley, Kyoung Kweon, Prateek Mehta, Patrick Shea, Tae Wook Heo, Terrence J. Udovic, Vitalie Stavila, Brandon C. Wood

Abstract

Polyborane salts based on B₁₂H^2-₁₂, B₁₀H^2-₁₀, and their carboborane counterparts CB₁₁H^- 12^-₁₂ and CB₉H^u-₁₀ demonstrate extraordinary Li and Na superionic conductivity that have made them attractive as electrolytes in all-solid-state batteries. The rich chemical and structural diversity of these materials creates a versatile design space that could in principle be used to optimize materials with even higher conductivity at lower temperatures. Unfortunately, many mechanistic details of cation conduction in these materials remain enigmatic, including reasons why certain modifications lead to improved performance. Here, we present a systematic study based on extensive ab initio molecular dynamics simulations that broadly explores the dependence of ionic conductivity on cation/anion pair combinations for Li and Na polyborane salts. Further simulations based on Li₂B₁₂H₁₂ as a model system are used to probe the additional influence of local perturbations, including modifications to chemistry, stoichiometry, and composition. Carbon doping, anion alloying, and cation off-stoichiometry are found to be favorable because they introduce intrinsic disorder, which facilitates local deviations from the expected cation population. Anion reorientations are also discovered to be critical for conduction, with benefits associated with lattice expansion traceable to the facilitation of anion rotation at larger volumes. Implications for engineering polyboranes for improved ionic conductivity are discussed.

Citation

ACS Energy Letters

Volume

2

Pub Type

Journals

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Keywords

ab initio molecular dynamics, anion rotational dynamics, carboranes, closo-boranes, density functional theory, ionic conductivity, molecular dynamics

Citation

Varley, J. , Kweon, K. , Mehta, P. , Shea, P. , Heo, T. , Udovic, T. , Stavila, V. and Wood, B. (2016), Understanding Ionic Conductivity Trends in Polyborane Solid Electrolytes from Ab Initio Molecular Dynamics, ACS Energy Letters, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=922285 (Accessed May 8, 2024)