Residual NMP and Its Impacts on Performance of Lithium-Ion Cells

Abstract

N-methyl-2-pyrrolidone (NMP) is the most common solvent used in coating positive electrode materials on aluminum foil during the manufacturing of lithium-ion batteries. NMP is a strongly polar aprotic solvent that effectively dissolves the polyvinylidene difluoride binder. While the majority of NMP typically evaporates during the electrode baking process, trace amounts may persist, particularly in positive electrodes containing nano-sized and highly-porous active materials. We noted residual NMP in the positive electrodes of Li-ion pouch cells containing LiMn0.8Fe0.2PO4 due to the extremely high surface area of the material and we wanted to determine the impact of this residual NMP. Therefore, a control electrolyte was purposely spiked with varying amounts of NMP and used in NMC532/graphite pouch cells to investigate the impact of residual NMP on lithium-ion battery performance. Experimental results indicate that NMP has the potential not only to neutralize the electrolyte additive ethylene sulfate but also to independently increase cathode impedance, leading to a higher rate of capacity loss during charge-discharge cycling. It is crucial to establish the appropriate procedure for baking electrodes containing NMP, both in laboratory and industrial settings, to mitigate these effects.