A study of the relationship between coulombic efficiency and capacity degradation of commercial lithium-ion batteries

High coulombic efficiency (CE) usually indicates a long battery cycle life. However, the relationship between long-term CE evolution and battery degradation is not fully understood yet. This paper explores the behavior of long-term CE and clarifies its relationship with capacity degradation. Cycle life tests are conducted on two types of mainstream commercial lithium-ion batteries. An incremental capacity (IC) analysis is then employed to identify battery aging mechanisms. Experimental observations along with in-depth discussions are presented regarding battery degradation, aging mechanisms, and CE evolution. From the experimental results, two typical degradation patterns are recognized. From the IC analysis, we observed that, in addition to a loss of lithium inventory, a loss of active material accelerates battery degradation and brings down CE values. From an electrochemical perspective, this paper establishes the relationship between CE evolution and capacity degradation. This relationship can help develop battery degradation models, estimate battery health states, and provide early failure warnings for a battery management system.