  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.
