Programming galvanostatic rates for fast-charging lithium ion batteries: a graphite case

Abstract

Galvanostatically induced lithiation of graphite, as a cathodic process of lithium ion batteries during charging, was investigated in situ by galvanostatic electrochemical impedance spectroscopy (GS-EIS). When lithiation is driven by charge rates slow enough for kinetics of the lithiation process to be considered relatively sluggish, charge transfer resistance (R_CT) is slightly reduced as lithium ion intercalation proceeds from the dilute stage to stage 2L. Subsequently, R_CT begins to increase during transformation of stage 2L to stage 2, followed by an abrupt increase in R_CT observed during transition from stage 2 to stage 1, or after the inter-space of graphites is fully filled with lithium ions. As the ratio of charge rate to lithiated graphite increases, the potential responsible for the transition from stage 2L to stage 2 is shifted to more negative values due to significant polarization. Simultaneously, cells reach cut-off potentials before the transition from stage 2 to stage 1 proceeds. Based on the information regarding R_CT profiles obtained by galvanostatic charging processes, a charging strategy is programmed with several different charge rates (C-rates). The capacity of lithiation is significantly enhanced by a C-rate switching (CRS) strategy. As a representative example, 75% of available capacity is charged for 50 minutes by a combination of 2 C, 1 C, and 0.5 C. However, only 12% and 51% of graphite is lithiated within the same time duration by a single charge rate of 0.1 C and 0.5 C, respectively.