Contamination in LIB Pouch Cells Promoting Self-Discharge and Crosstalk

Abstract

Storage studies of lithium-ion battery electrolyte within bags made of commercial pouch foils, commonly used as encasing material of battery cells, revealed the presence of contamination leaching from the pouch foil material into the electrolyte. By analyzing the stored electrolyte via GC-MS the appearing compound was identified as 2,4-di-tert-butylphenol (2,4-DTBP). To investigate the influence of DTBP on the battery cell performance, full cells employing commercial LiNi_1/3Mn_1/3Co_1/3O₂ based cathodes and graphite-type anodes were assembled using 1 M LiPF₆ in ethylene carbonate/ dimethyl carbonate mixture as the electrolyte with/out the intentional addition of either 2,4-DTBP or its constitutional isomer 2,6-DTBP. Furthermore, dimethyl terephthalate (DMT), a literature known redox shuttle triggering impurity leaching from PET-based fixing tape used in LIBs, was added to compare the effect of DMT to DTBPs. It was revealed that either DTBP contaminations have a significant impact on the self-discharge behavior of the studied cells, which exceed the effect of present DMT. Moreover, all contaminants heavily increase transition metal dissolution-migration-deposition (TM DMD) processes and irreversible capacity loss. When vinylene carbonate, an SEI forming additive, is added to the electrolyte mixtures self-discharge, as well as TM DMD are suppressed to a different degree depending on the type of contaminant added.