Quantification of Inactive Lithium and Solid Electrolyte Interphase (SEI) Species on Graphite Electrodes After Fast Charging

Rapid charging of Li-ion batteries is limited by lithium plating on graphite anodes, whereby Li⁺ ions are reduced to Li metal on the graphite particle surface instead of inserting between graphitic layers. Plated Li metal not only poses a safety risk due to dendrite formation, but also contributes to capacity loss due to the low reversibility of the Li plating/stripping process. Understanding when Li plating occurs and how much Li has plated is therefore vital to remedying these issues. We demonstrate a titration technique with a minimum detection limit of 20 nmol (5×10^-4 mAh) Li which is used to quantify inactive Li that remains on the graphite electrode after fast charging. Additionally, the titration is extended to quantify the total amount of solid carbonate species and lithium acetylide (Li₂C₂) within the solid electrolyte interphase (SEI). Finally, electrochemical modeling is combined with experimental data to determine the Li plating exchange current density (10 A/m²) and stripping efficiency (65%) of plated Li metal on graphite. These techniques provide a highly accurate measure of Li plating onset and quantitative insight into graphite SEI evolution during fast charge.