Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases

21 November 2022, Version 3
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Electrolyte decomposition constitutes an outstanding challenge to long-life Li-ion batteries (LIBs) as well as emergent energy storage technologies, contributing to protection via solid electrolyte interphase (SEI) formation and irreversible capacity loss over a battery's life. Major strides have been made to understand the breakdown of common LIB solvents; however, salt decomposition mechanisms remain elusive. In this work, we use density functional theory to explain the decomposition of lithium hexafluorophosphate (LiPF6) salt under SEI formation conditions. Our results suggest that LiPF6 forms POF3 primarily through rapid chemical reactions with Li2CO3, while hydrolysis should be kinetically limited at moderate temperatures. We further identify selectivity in the proposed autocatalysis of POF3, finding that POF3 preferentially reacts with highly anionic oxygens. These results provide a means of interphase design in LIBs, indicating that LiPF6 reactivity may be controlled by varying the abundance or distribution of inorganic carbonate species or by limiting the transport of PF6- through the SEI.

Keywords

lithium-ion battery
LIB
solid-electrolyte interphase
SEI
electrolyte
electrolyte decomposition
electrochemistry
lithium carbonate
Li2CO3
LiPF6
hexafluorophosphate
lithium fluoride
LiF
autocatalysis
homogeneous catalysis
thermal decomposition
selectivity
battery chemistry
density functional theory
reaction mechanism
computational chemistry
theoretical chemistry
quantum chemistry
DFT
transition-state theory

Supplementary materials

Title
Description
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Supporting Information
Description
Data availability statement; computational methods; discussion of additional reaction mechanisms for LiPF6 decomposition.
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Supplementary weblinks

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