Thermodynamic Origin of Reaction Inhomogeneity in Thick Battery Electrodes

Abstract

As battery electrodes continue to be made thicker to improve energy density at the cell level, reaction non-uniformity within the electrodes during charge/discharge becomes increasingly severe, which degrades rate performance and increases the risk of overcharge / overdischarge. A thorough understanding of the origins of such phenomenon is important for developing its mitigation strategies. Prediction of reaction inhomogeneity in battery cells traditionally relies on numerical simulations based on the Newman-type model. Here we present an alternative approach using “reduced-order” theoretical models, which allow for faster and yet reliable predictions and also provide a more transparent understanding on the dependence of reaction non-uniformity on material properties, cell geometry and (dis)charge conditions. An analytical mass transport model is developed to predict reaction distribution and discharge performance in thick electrode systems that are kinetically limited by electrolytic transport. A circuit model is further proposed to analyze the interplay between surface reaction, electronic conduction and electrolytic transport in controlling the degree of reaction non-uniformity. One significant insight from this study is that inhomogeneous reaction is not only kinetically induced but also influenced by the intrinsic thermodynamic properties of the electrode materials, specifically their SOC dependence of the equilibrium potential. Electrodes that undergo first-order phase transitions upon cycling and exhibit a flat voltage curve against SOC, such as LiFePO4 and Li4Ti5O12, produce “move-zone” reaction behavior with strongly non-uniform Li intercalation flux. In contrast, battery compounds with sloped voltage curves such as layered transition metal oxides inherently favor uniform reaction within electrodes. Counter-intuitively, our theoretical model and numerical simulation suggest reducing the electronic conductivity and surface reaction kinetics as an effective strategy to improve the reaction uniformity in thick electrodes.