Evaluation of Matching Ratio Impact on Reduced Charging Time

Abstract

As electric vehicles (EV) show an increased penetration into the transportation market, a challenging hurdle to overcome is a vehicle “refuel” time similar to that of refilling a gasoline vehicle tank. One pathway to accomplish this is fast charging lithium-ion cells. The majority of commercial Li-Ion cells manufactured today contain various positive electrode active materials, coupled with a graphite negative electrode. An advantage of graphite electrodes is that the lithiated graphite voltage lies close to that of Li/Li+, resulting in in increased cell voltage. This benefit also has a detrimental effect on fast charging cells with graphite electrodes, where a small polarization of the negative electrode can result in a condition favorable to lithium metal plating on the negative graphite electrodes1.While modified high rate charging procedures for commercial lithium-ion cells with conventional graphite negative electrodes, such as multistage constant current charging and boost charging, have been implemented. These procedures rely on reducing charging currents when cell voltage limits are reached 2,3. Following the development of an accelerated charging procedure based solely on negative electrode lithiation rate and polarization 4, the presented work evaluates the effect of negative to positive (N:P) matching ratio on cell fast charge performance. Two varying N:P matching ratios of 1.2:1 and 1.7:1 are evaluated and the resulted impact on fast charge performance will be presented. Utilizing an accelerated charging procedure based solely on the limiting negative electrode results in an identical charging procedure being used for multiple cell designs when charging to the same degree of negative electrode lithiation. Theoretically the applied procedure should result in a reduction in recharge time to 80% state of charge from 58 minutes to 34 minutes by altering the cell N:P ratio. Actual cell performance is altered as increased irreversible capacity loss occurs in cells with increased N:P rations, but improvements in cell charge times can be accomplished from design changes, namely cell N:P ratios. References C. Uhlmann, J. Illig, M. Ender, R. Schuster, and E. Ivers-Tiffée, Journal of Power Sources, 279, 428–438 (2015).D. Anseán et al., Journal of Power Sources, 321, 201–209 (2016).P. H. L. Notten, J. H. G. O. het Veld, and J. R. G. van Beek, Journal of Power Sources, 145, 89–94 (2005).W. Yourey, Y. Fu, N. Li, V. Battaglia, and W. Tong, J. Electrochem. Soc., 166, A1432–A1438 (2019).