Optimum fast charging of lithium-ion pouch cells based on local volume expansion criteria

Fast charging of high-energy lithium-ion cells is desired in many applications but there is a lack of simple methods to determine the maximum charging current a cell will accept without sustaining damage. Here we explore the link between local volume expansion and lithium plating on the graphite anode of lithium-ion cells and propose a method that makes use of this link to design fast and gentle charging protocols. The expansion across the entire surface of a commercial nickel-manganese-cobalt-lithium oxide/graphite pouch cell is measured in short intervals using laser triangulation while operating the cell under various conditions. Irreversible expansion and expansion overshoots are correlated to voltage relaxation curves, post-mortem findings and capacity loss. Using the proposed method, a fast charging protocol is derived and validated in a cycle study covering 1000 cycles. The fast charging protocol enables both a 11% charging time reduction and a 16% reduction of capacity loss per cycle as compared to 1C constant current - constant voltage charging.