(680g) Mechanistic Understanding of Lithium-Ion Adsorption, Intercalation, and Plating in Graphite Anodes Down to -40 °C

Low-temperature and fast 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 (-40 to 30 °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 without the reference electrode. 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 analysis enables intercalation and plating processes to be further distinguished, revealing temperature regimes where the reactions either occur sequentially (e.g., near ambient temperature) or simultaneously (e.g., below -20 °C). The temperature dependence of overpotentials and characteristic reaction time scales were analyzed, suggesting that a two-step, pre-equilibration mechanism occurs prior to either intercalation or plating, wherein Li⁺ cations reversibly adsorb on the graphite surface followed by irreversible charge transfer to form either Li metal or Li_xC₆. Variable-rate galvanostatic measurements on two-electrode cells show that the voltage profiles at faster rates exhibit key similarities with those at lower temperatures. The results yield mechanistic understanding into how Li⁺ cations electrochemically intercalate and plate into graphite electrodes, as well as their competition at low temperatures and fast rates.