Stability of Solid Electrolyte Interphase Components on Lithium Metal and Reactive Anode Material Surfaces

TL;DR

This study models the stability and reactions of key SEI components on lithium metal and silicon anodes, revealing their thermodynamic instability and suggesting a stable Li2O layer as the innermost SEI component.

Contribution

It provides a thermodynamic and kinetic analysis of SEI component stability and reactions on reactive anode surfaces, highlighting the instability of Li2CO3 and LEDC.

Findings

Li2CO3 and LEDC are thermodynamically unstable near equilibrium potentials.

Both SEI components exhibit exothermic reactions on lithium metal surfaces.

Li2O is identified as the stable innermost SEI layer.

Abstract

Lithium ion batteries (LIB) can feature reactive anodes that operate at low potentials, such as lithium metal or silicon, passivated by solid electrolyte interphase (SEI) films. SEI is known to evolve over time as cycling proceeds. In this modeling work, we focus on the stability of two main SEI components, lithium carbonate (Li2CO3) and lithium ethylene dicarbonate (LEDC). Both components are electrochemically stable but thermodynamically unstable near the equilibrium Li+/Li(s) potential. Interfacial reactions represent one way to trigger the intrinsic thermodynamic instability. Both Li(2)CO(3) and LEDC are predicted to exhibit exothermic reactions on lithium metal surfaces, and the barriers are sufficiently low to permit reactions on battery operation time scales. LEDC also readily decomposes on high Li-content Li(x)Si surfaces. Our studies suggest that the innermost SEI layer on lithium metal surfaces should be a thin layer of Li(2)O -- the only thermodynamically and kinetically stable component (in the absence of a fluoride source). This work should also be relevant to inadvertant lithium plating during battery cycling, and SEI evolution on Li(x)Si surfaces.