Understanding the Reaction Uniformity during Discharge of Thick Electrodes

Abstract

The use of thick electrodes in Li-ion batteries can enhance cell-level energy density and lower the cost by reducing the fraction of inactive components. However, as the electrode thickness increases, reaction inhomogeneity along the thickness direction becomes a critical issue, which leads to not only inferior rate performance but also localized heat generation, overcharge or overdischarge, causing degradation and even failure of batteries. In this work, we analyze the discharge kinetics of thick electrodes in the framework of the porous electrode theory. Two characteristic reaction behaviors are identified, i.e. the uniform reaction behavior, which is exhibited in solid-solution-type electrode materials such as transition metal layered oxides, and the moving-zone reaction behavior, which applies to battery compounds that undergo first-order phase transformation(s) upon cycling such as LiFePO4 and Li4Ti5O12. A general analytical model is developed to describe the two types of reaction behaviors with good agreement with numerical simulations. We further elucidate the effect of various factors on controlling the reaction uniformity including current density, equilibrium potential, ionic and electronic conductivities and surface reaction kinetics through a simplified circuit model. Based on the understanding obtained from this model, we propose that intentionally reducing the electronic conductivity and surface reaction rate provide two counter-intuitive strategies to homogenize the non-uniform reaction distribution and improve the discharge performance of thick electrodes.