Fluorinated Boron-Based Anions for Higher Voltage Li Metal Battery Electrolytes.

Jonathan Clarke-Hannaford,Thomas Rüther,Michelle J S Spencer,Michael Breedon

doi:10.3390/nano11092391

Abstract

Lithium metal batteries (LMBs) require an electrolyte with high ionic conductivity as well as high thermal and electrochemical stability that can maintain a stable solid electrolyte interphase (SEI) layer on the lithium metal anode surface. The borate anions tetrakis(trifluoromethyl)borate ([B(CF3)4]−), pentafluoroethyltrifluoroborate ([(C2F5)BF3]−), and pentafluoroethyldifluorocyanoborate ([(C2F5)BF2(CN)]−) have shown excellent physicochemical properties and electrochemical stability windows; however, the suitability of these anions as high-voltage LMB electrolytes components that can stabilise the Li anode is yet to be determined. In this work, density functional theory calculations show high reductive stability limits and low anion–cation interaction strengths for Li[B(CF3)4], Li[(C2F5)BF3], and Li[(C2F5)BF2(CN)] that surpass popular sulfonamide salts. Specifically, Li[B(CF3)4] has a calculated oxidative stability limit of 7.12 V vs. Li+/Li0 which is significantly higher than the other borate and sulfonamide salts (≤6.41 V vs. Li+/Li0). Using ab initio molecular dynamics simulations, this study is the first to show that these borate anions can form an advantageous LiF-rich SEI layer on the Li anode at room (298 K) and elevated (358 K) temperatures. The interaction of the borate anions, particularly [B(CF3)4]−, with the Li+ and Li anode, suggests they are suitable inclusions in high-voltage LMB electrolytes that can stabilise the Li anode surface and provide enhanced ionic conductivity.

Highlights

Li metal is an ideal anode material for rechargeable Li-based batteries due to its high theoretical gravimetric capacity (3862 mAh g−1 ) that can enable much greater capacities than current state-of-the art Li-ion batteries [1]
Density functional theory (DFT) calculations are used to determine the binding energy, charge distribution, dipole moment, chemical hardness, and electrochemical stability limits of the Li-salt systems. These findings are discussed relative to a series of common sulfonamide anions reported in reference [40]: [TFSI]−, bis(flurosulfonyl)imide ([FSI]− ), and(trifluoromethanesulfonyl)imide ([FTFSI]− )
We have shown that the borate anions [B(CF3 )4 ]−, [(C2 F5 )BF3 ]−, and [(C2 F5 )BF2 (CN)]−

Summary

Introduction

Li metal is an ideal anode material for rechargeable Li-based batteries due to its high theoretical gravimetric capacity (3862 mAh g−1 ) that can enable much greater capacities than current state-of-the art Li-ion batteries [1]. One strategy that can prevent dendrite growth and prolong the cycle life of LMBs is the formation of a stable (in situ) solid electrolyte interphase (SEI) layer, which consists of a thin film of reaction products immediately formed after contact between the electrolyte and the Li anode [2,8,9,10]. Traditional organic carbonate-based electrolytes have high volatility, flammability, and will form an SEI layer on Li metal with poor chemical stability and mechanical strength [2]. These electrolytes possess poor stability above 4.3 V vs

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