Thermodynamic Origin of Reaction Non-Uniformity in Battery Porous Electrodes and its Mitigation

TL;DR

This paper investigates how the thermodynamic properties of electrode materials influence reaction non-uniformity in porous battery electrodes, revealing that equilibrium potential slopes significantly affect current distribution and proposing mitigation strategies.

Contribution

It uncovers the thermodynamic origin of reaction non-uniformity and introduces a reaction uniformity number to predict and mitigate inhomogeneity in battery electrodes.

Findings

Reaction non-uniformity increases as the slope of the equilibrium potential curve decreases.

The reaction uniformity number can predict the degree of inhomogeneity and capacity loss.

Mitigation strategies include matching resistances, grading conductivity, and reducing surface kinetics.

Abstract

The development of non-uniform reaction current distribution within porous electrodes is a ubiquitous phenomenon during battery charging / discharging and frequently controls the rate performance of battery cells. Reaction inhomogeneity in porous electrodes is usually attributed to the kinetic limitation of mass transport within the electrolyte and/or solid electrode phase. In this work, however, we reveal that it is also strongly influenced by the intrinsic thermodynamic behavior of electrode materials, specifically the dependence of the equilibrium potential on the state of charge: electrode reaction becomes increasingly non-uniform when the slope of the equilibrium potential curve is reduced. We employ numerical simulation and equivalent circuit model to elucidate such a correlation and show that the degree of reaction inhomogeneity and the resultant discharge capacity can be predicted by a dimensionless reaction uniformity number. For electrode materials that have equilibrium potentials insensitive to the state of charge and exhibit significant reaction non-uniformity, we demonstrate several approaches to spatially homogenizing the reaction current inside porous electrodes, including matching the electronic and ionic resistances, introducing graded electronic conductivity and reducing the surface reaction kinetics.