Modeling a High-Energy, High-Rate Li//CFx Battery with a Capacity-Contributing Electrolyte

Abstract

Li//CFx cells have achieved the highest specific energy of commercial batteries, but new applications requiring higher rates (e.g., C/3) and pulsing (e.g., at 5 C/3 rate for 1 min) drive the push for higher energy and power densities. A capacity-contributing electrolyte (CCE) can provide additional capacity at a slightly lower potential than the CFx reaction, increasing cell specific energy. In this work we present a 0D transient model of a primary Li//CFx cell with a CCE composed of both a salt and solvent that provide capacity with a focus on a C/3 rate and pulsing. Novel aspects of our model, in addition to the two CCE reactions, include a variable cathode thickness and porosity (CFx cathode thickness has been measured to expand by >40% at 25 °C) and a detailed presentation of the transient evolution of all species and terms that contribute to cell potential (including how salt and solvent reactions affect ionic polarization and the growth of solid-phase product resistances). Our work quantifies the delicate balance of thermodynamic, kinetic, and transport processes and properties that is needed to obtain specific energy enhancements from CCE reactions, and how changing cathode thickness and porosity affect the mechanisms that cause the end of discharge.