Conversion cathode materials are gaining interest for secondary batteries due
to their high theoretical energy and power density. However, practical
application as a secondary battery material is currently limited by practical
issues such as poor cyclability. To better understand these materials, we have
developed a pseudo-two-dimensional model for conversion cathodes. We apply this
model to FeS2 - a material that undergoes intercalation followed by conversion
during discharge. The model is derived from the half-cell Doyle-Fuller-Newman
model with additional loss terms added to reflect the converted shell
resistance as the reaction progresses. We also account for polydisperse active
material particles by incorporating a variable active surface area and
effective particle radius. Using the model, we show that the leading loss
mechanisms for FeS2 are associated with solid-state diffusion and electrical
transport limitations through the converted shell material. The polydisperse
simulations are also compared to a monodisperse system, and we show that
polydispersity has very little effect on the intercalation behavior yet leads
to capacity loss during the conversion reaction. We provide the code as an
open-source Python Battery Mathematical Modelling (PyBaMM) model that can be
used to identify performance limitations for other conversion cathode
materials