γ-Valerolactone: An Alternative Solvent for Manufacturing of Lithium-Ion Battery Electrodes

Currently, cathode manufacturing for lithium-ion batteries requires N-methyl-2-pyrrolidone (NMP) as a coating solvent. With concerns over its petrochemical origins and increasing scrutiny due to its undesirable toxicological profile, there is market demand for application-specific, less-regulated alternatives. Here, we evaluate γ-valerolactone (GVL), a promising green-candidate based on its closeness to NMP in the Hansen Solubility Space. It has successfully dissolved polyvinylidene fluoride (PVDF) at a temperature of 60 °C. We also show that the handling of lithium nickel cobalt manganese oxide (NCM 111) coating slurry prepared using GVL at 60 °C is comparable to NMP-based slurry at 25 °C. Cathode sheets (with dry mass loadings: 18 and 25 mg/cm2) were manufactured by blade coating and drying (gas temperatures: 80 and 120 °C) in a custom-built batch coater with a convective drying unit. Adhesion and cell tests were performed to compare the resulting sheets. NMP-based films generally have higher adhesion strength than their GVL counterpart. We also report comparable resistance and columbic efficiency (after 1 cycle) for all cathodes, with slightly higher resistance for GVL dried harshly (i.e., 120 °C). Also, thinner NMP-based cathodes had better specific capacity during cycling than their GVL counterparts, irrespective of the drying temperature. However, thicker cathodes dried under higher throughput conditions (temperature of 120 °C) have comparable specific capacity after 100 cycles. Through this study, we highlight that GVL is a promising candidate currently available that merits further investigation.