Surprising Dependence of the Exfoliation of Graphite During Formation on Electrolyte Composition

Abstract

Graphite is the most used lithium intercalation host for the negative electrode of the lithium-ion battery. Extensive research has been carried out to achieve high Coulombic Efficiency (CE) and long cycle life for the graphite anode. Here, LFP/graphite (graphite from Vendor 1) cells that undergo formation at 40 °C with either 1.2 M LiPF6 dissolved in ethylene carbonate:dimethyl carbonate (EC:DMC), or ethylene carbonate:ethylmethyl carbonate (EC:EMC) have excellent first cycle efficiency (FCE). However, when the formation is done at 20 °C, EC:EMC and ethylene carbonate:diethyl carbonate (EC:DEC) cells show much reduced FCE while EC:DMC cells retain high FCE. We prove by a variety of experiments that the reduced FCE is caused by solvent co-intercalation. We explore the impact of temperature, different graphites, electrolyte additives, and varied salt content on this effect. We show that basic additives, such as vinylene carbonate, are sufficient to eliminate the co-intercalation. With a well-designed electrolyte system containing additives, graphites that show co-intercalation in the absence of additives perform equivalently or better than graphites that do not show co-intercalation in the absence of additives.