(413d) Mechanisms of Lithium and Zinc Electrodeposition on Graphite and Chevrel Phase Intercalation Electrodes at Low Temperatures and Fast Rates

Low-temperature charging of lithium (Li)-ion batteries remains a challenge due to the undesirable Li plating that occurs on graphite anodes under these conditions. Here, we yield new insights into the mechanistic processes underpinning electrochemical Li-ion intercalation and Li metal plating reactions on graphite anodes at low temperatures and fast rates. Variable-temperature (30 °C to -40 °C) galvanostatic 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 results establish that the local minima in the voltage 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 the graphite electrode potential show that the extent of electrochemical Li⁺ cation intercalation drops precipitously as temperature decreases below -20 °C, suggesting that graphite itself limits the amount of charge that can be stored. The possibility of electrochemically intercalating Li⁺ cations into graphite at temperatures below -20 °C was further investigated through asymptotically slower (≪ 0.1 mA/cm²) charging rates. The temperature-dependence of empirically defined rate constants for electrochemical Li⁺ cation intercalation and Li plating determined from constant-current measurements revealed non-Arrhenius behavior for electrochemical Li⁺ cation intercalation that suggests a multi-step process, while typical Arrhenius behavior for Li plating suggests a unimolecular single-step process. A kinetic model based on Langmuir adsorption shows how the interfacial concentration of Li⁺ cations adsorbed on graphite active sites and the availability of active sites affect the kinetics of Li-ion battery charging. Specifically, we show that either ion adsorption-limited or reaction-limited regimes manifest at different temperatures and charging rates for electrochemical Li⁺ cation intercalation and Li plating. The results yield a mechanistic understanding of how Li⁺ cations electrochemically compete for intercalation and plating into graphite electrodes as a function of temperature and charge rate.

Furthermore, we extend these results by investigating zinc (Zn) metal plating on a model multivalent-ion intercalation electrode material, chevrel-phase Mo₆Se₈, to demonstrate the universal behavior of metal electroplating on intercalation electrodes. Two- and three-electrode cells using Zn metal as a counter/reference electrode and chevrel-phase Mo₆Se₈ as a working electrode were used to demonstrate for the first time that Zn can electrochemically plate on Mo₆Se₈ in aqueous 1 M ZnSO₄ electrolyte. Under a constant-current, Zn²⁺ cations can either electrochemically intercalate into the chevrel-phase or reduce to form metallic Zn, as evidenced by the measurement of the nucleation overpotential at potentials below 0 V. The results are analyzed with respect to lithium cation intercalation versus lithium metal plating on graphite anodes to yield a general understanding of how ion charge density and size affect these competing electrochemical processes at variable-temperature and current density.