Mechanistic Understanding of Lithium-Ion Adsorption, Intercalation, and Plating during Charging of Graphite Electrodes

Low-temperature and fast-charging lithium (Li)-ion batteries remain challenging due to the undesirable Li plating on graphite anodes under these conditions. Here, we present a kinetic mechanism that underpins electrochemical Li+ cation intercalation and Li metal plating reactions on graphite electrodes at low temperatures and fast rates. Variable-temperature (30 °C to -40 °C) and variable-rate (0.1 to 10 mA/cm2) constant-current measurements were conducted on three-electrode cells comprised of Li metal counter, graphite working, and Li metal reference electrodes, as well as two-electrode cells. The local minima in the potential profiles, often associated with the nucleation overpotential for Li metal plating on graphite, must be disentangled from contributions from Li metal stripping at the counter electrode. Differential capacity analyses of three-electrode measurements of graphite potential show that the extent of electrochemical Li+ cation intercalation drops precipitously as temperature decreases below -20 °C. The temperature dependence of empirically defined rate constants for Li+ cation intercalation and Li plating determined from constant-current measurements revealed non-Arrhenius behavior for Li+ cation intercalation that suggests a two-step pre-equilibration mechanism, while typical Arrhenius behavior for Li plating suggests a unimolecular single-step process. A kinetic model based on Langmuir adsorption shows that the interfacial concentration of Li+ cations adsorbed on graphite active sites is critical in dictating the kinetics of the charging process. We show that rate limitations, either adsorption-limited or surface reaction-limited, manifest at different temperatures and rates during the charging process. The results yield new mechanistic understanding of how Li+ cations electrochemically compete for intercalation into and plating on graphite electrodes, as a function of temperature and charge rate.