Electrochemical kinetics of SEI growth on carbon black, II: Modeling

TL;DR

This paper develops a theoretical electrochemical model for SEI growth on carbon black electrodes, capturing nonlinear voltage dependence and charge-discharge asymmetry, thereby improving understanding of capacity fade in lithium-ion batteries.

Contribution

The model introduces a novel hypothesis that the initial SEI's electronic conductivity depends on lithium concentration, explaining asymmetric growth during lithiation and delithiation.

Findings

Model accurately predicts experimental SEI growth behavior.

Incorporates lithium concentration-dependent conductivity in SEI.

Explains capacity fade mechanisms during battery cycling.

Abstract

Mathematical models of capacity fade can reduce the time and cost of lithium-ion battery development and deployment, and growth of the solid-electrolyte interphase (SEI) is a major source of capacity fade. Experiments in Part I reveal nonlinear voltage dependence and strong charge-discharge asymmetry in SEI growth on carbon black negative electrodes, which is not captured by previous models. Here, we present a theoretical model for the electrochemical kinetics of SEI growth coupled to lithium intercalation, which accurately predicts experimental results with few adjustable parameters. The key hypothesis is that the initial SEI is a mixed ion-electron conductor, and its electronic conductivity varies approximately with the square of the local lithium concentration, consistent with hopping conduction of electrons along percolating networks. By including a lithium-ion concentration dependence for the electronic conductivity in the SEI, the bulk SEI thus modulates the overpotential and exchange current of the electrolyte reduction reaction. As a result, SEI growth is promoted during lithiation but suppressed during delithiation. This new insight establishes the fundamental electrochemistry of SEI growth kinetics. Our model improves upon existing models by introducing the effects of electrochemical SEI growth and its dependence on potential, current magnitude, and current direction in predicting capacity fade.