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

The Li/CFx primary cell has the highest specific energy of any commercialized cell, but recent funding programs (e.g., the IARPA RESILIENCE program) seek significant increases in performance beyond that achieved with current cell formulations, including both higher average rates than are typical for Li/CFx (e.g., C/3 for the RESILIENCE program) and high-rate pulses (e.g., 5C/3, also for the RESILIENCE program).1 One method to raise the specific energy beyond that of current commercial cells is to also obtain capacity from the electrolyte (i.e., to use a capacity-contributing electrolyte).2,3 This should occur after the CFx has been mostly consumed. The present talk will briefly introduce experimental results demonstrating the operation of a capacity-contributing electrolyte in a Li/CFx cell, and then focus on our 0-D modeling framework that includes three reactants (CFx, salt, and solvent), a cathode that undergoes volume expansion during discharge (based on the molar volumes of the reactants and products), and the growth of resistive electrolyte products.4 Our base case cell design is for a ~750 Wh/kg cell. We demonstrate the behavior of our model formulation for a C/3 average rate with two 5C/3 one-minute pulses (one at 99% and one at 20% SOC, based on the capacity of the CFx and capacity-contributing electrolyte). We find that there is significant cathode swell during discharge (i.e., 10s of %) that affects the cell polarization, and the electrochemical behavior of the 20% SOC pulse is highly dependent on the thermodynamics, kinetics, and transport properties of the salt and solvent reactants and products. Our modeling framework can be applied to additional chemistries with multiple active materials, significant electrode thickness changes, and resistive electrolyte products. 1. https://www.iarpa.gov/research-programs/resilience 2. J. Am. Chem. Soc. 2014, 136, 19, 6874–6877 3. Adv. Mater., 2015, 27, 3473–3483 4. Journal of The Electrochemical Society, 158 (5) A504-A510 (2011)