Electrolyte movement-induced salt inhomogeneity in lithium-ion batteries

Inhomogeneous electrolyte concentration in lithium-ion batteries has been implicated in performance loss and degradation, yet the mechanism that drives inhomogeneity remains poorly understood. Here, we identify a generic physical mechanism by which electrolyte flow, induced by stresses created by cyclic expansion and contraction of active particles that alter the porosity in the electrodes and separator, progressively redistributes electrolyte salt within a cylindrical battery. We derive a reduced mathematical model that couples electrode/separator deformation, fluid flow and salt transport. We show that concentration inhomogeneities arise generically from asymmetries in porosity, permeability and swelling between the electrodes. We derive analytical expressions and numerical results for the evolution of the average salt concentration, enabling us to quantify the effect via a single key dimensionless parameter. The model allows us to predict the bulk accumulation/depletion characteristics of parametrised batteries in the literature, and can guide design rules to reduce electrolyte inhomogeneity.