(435i) Mechanistic Understanding of Li+ Adsorption, Intercalation, and Plating in Graphite Anodes Elucidated Using Variable-Temperature Two- and Three-Electrode Measurements

Lithium (Li)-ion batteries power the technology we use to communicate, work, and drive. Li-ion batteries possess high energy densities that can be charged and discharged for hundreds of cycles due to the highly reversible Li⁺ (de)intercalation processes in the electrode materials. Fast-rate and low-temperature charging of Li-ion batteries remains a challenge due to the undesirable Li plating on the graphite anodes that occurs under these conditions. Here, we establish the mechanistic processes underpinning electrochemical Li⁺ cation intercalation versus Li nucleation & plating in graphite anodes. Galvanostatic measurements were conducted on both two-electrode and three-electrode cells comprised of Li metal as a counter electrode and graphite as a working electrode. Variable-temperature (-40 to +60 C) galvanostatic measurements on two-electrode Li metal half-cells were used to measure the temperature-dependence of the nucleation overpotential, a measured parameter used commonly to mark the onset of metal electrodeposition on a working electrode. The temperature-dependence of the nucleation overpotential suggests the existence of a two-step, pre-equilibriation mechanism, wherein Li⁺ cations reversibly adsorb and desorb on the graphite surface followed by irreversible charge transfer to form Li metal. We hypothesize that Li⁺ cations can adsorb on the graphite surface spontaneously at lower temperatures due to the exothermic nature of the adsorption process, resulting in an inherent build-up of available Li⁺ cations for Li plating. Variable-temperature three-electrode galvanostatic measurements were conducted which showed that as temperature decreased, there was also a significant increase in overpotential associated with the electrostripping of Li on the Li metal counter electrode. The temperature-dependence on both the measured Li metal counter electrode and graphite working electrode potentials revealed the coupled electrochemical dynamics of both electrodes in dictating the Li nucleation process on graphite. The results yield mechanistic understanding into why Li⁺ cations can electrochemically plate on, versus intercalate into, graphite electrodes in Li-ion batteries, particularly at low temperatures.