Effects of Fast Charging on Lithium-Ion Cells

Abstract

As electrified vehicles become widely accepted, consumers expect the characteristics to be similar to those with a conventional, internal-combustion engine. Among these is the ability to ‘refuel’ quickly. For example, current lithium-ion battery technology takes an hour or more to recharge (refuel) whereas a conventional car can take 5 minutes or less. Some work has been does to understand the electrical effect fast charging lithium-ion cells has. However, no one understands what faster charging does to internal cell components. We cycled commercially-available 18650 cells at four different charge C-rates (0.7, 2, 4, 6) and two different charge-returned windows (40% and 100%); the discharge rate was C/3. The cells were opened and both electrodes as well as separator were analysed in an argon-environment glove box using a range of surface-sensitive, analytical techniques. The binder material at the negative electrode decomposed at higher C-rates causing delamination from the copper current collector. Surprisingly, cells cycled within a smaller charge-returned window completely delaminated irrespective of C-rate, suggesting there are at least two different mechanisms responsible for electrode delamination in the cells. Analysis of surface film chemistry and thickness showed a dependence on C-rate. Cells cycled at higher C-rates had a thicker surface film and a more complex chemistry. Gel permeation - liquid chromatography showed that the surface film of the cells was extremely complex, with a large range of molecular weights (MW). As the C-rate increased, the average MW of the oligomers from the surface film also increased. These findings indicate that an increased charging rate affects the negative electrodes surface film and its adhesion to the current collector. Although a smaller SoC window caused less resistance increase and capacity losses in 18650 cells it was detrimental to the lamination properties. This could have significant implications for unconstrained pouch cells.