Numerical simulation of porous networks in relation to battery electrodes and separators

Numerical simulation is used to explore the influence of particle shape and overall porosity on the liquid phase conductivity, σeff, inside porous networks, such as electrodes or separators used for lithium-ion batteries. Such battery components are often modelled by a power law, σeff=var epsilonασ0, which relates electrolyte bulk conductivity σ0 and void volume fraction var epsilon via a Bruggeman exponent α. Frequently, a value of 1.5 is assumed for α. In this work, theoretical and experimental evidence is presented to show that a Bruggeman exponent of 1.5 is often not valid for real electrodes or separator materials. It is found that only idealized morphologies, based on spherical or slightly prolate (i.e. rod-type) ellipsoids, are expected to give rise to a Bruggeman law with an exponent of about 1.3. Porous networks based on other particle morphologies such as oblate (i.e. disk-type) ellipsoids or lamellar or flaky materials increase the tortuous path for ionic conductivity and result either in a significant increase of the exponent α, or in a complete deviation from the power law. These models imply that spherical or slightly prolate ellipsoidal particles should be preferred for batteries where high-rate performance is required and that future separators could be designed with higher ionic conductivity.