Improving Lithium-ion Cells by Replacing Polyethylene Terephthalate Jellyroll Tape

Abstract

Polyethylene terephthalate (PET) tape is widely used by well-known lithium-ion battery manufacturers to prevent electrode stacks from unwinding during assembly. PET tape is selected since it has suitable mechanical and electrical properties, but its chemical stability has been largely overlooked. In the absence of effective electrolyte additives, PET can depolymerize into its monomer dimethyl terephthalate (DMT), which is an unwanted redox shuttle that induces significant self-discharge in a lithium-ion cell. This study presents a chemical screening experiment to probe the PET decomposition mechanism involving in situ generated methanol and lithium methoxide from dimethyl carbonate, one of the most common electrolyte solvents in lithium-ion cells. By screening other polymers, it is found that polypropylene (PP) and polyimide (Kapton) are stable in the electrolyte. Finally, it is demonstrated that reversible self-discharge of LiFePO4/graphite cells can be virtually eliminated by replacing PET jellyroll tape with chemically stable PP tape.

Understanding the Self-Discharge Redox Shuttle Mechanism of Dimethyl Terephthalate in Lithium-Ion Batteries

“…This is consistent with Adamson et al, who showed that DMT stems from the lithium alkoxide catalyzed decomposition of PET tape with methanol. 10 Neither of the reactants comes from the conducting salt. Effect of DMT on ultra-high precision coulometry.-Figure 11 shows ultra-high precision coulometry data for LFP/graphite pouch cells with CTRL + LiPF 6 (upper panels) and CTRL + LiFSI electrolyte (lower panels).…”

“…8 DMT can cause shuttling currents at an order of magnitude that can explain the complete reversible self-discharge of LFP/graphite cells with additive-free alkyl carbonate electrolyte and LiPF 6 within 500 h. 4,9 Adamson et al showed that DMT is the monomer and decomposition product of polyethylene terephthalate (PET), a common polymer for the adhesive tape used in commercial battery cells to secure the wound or stacked electrodes (see Scheme 1). 10 They showed that the decomposition reaction of PET requires methanol (MeOH) and lithium methoxide (LiOMe); the former is a product of dimethyl carbonate (DMC) hydrolysis, the latter is formed from DMC reduction at the graphite negative electrode. 10 The PET decomposition reaction is accelerated at elevated temperatures between 55 and 70 °C.…”

“…10 They showed that the decomposition reaction of PET requires methanol (MeOH) and lithium methoxide (LiOMe); the former is a product of dimethyl carbonate (DMC) hydrolysis, the latter is formed from DMC reduction at the graphite negative electrode. 10 The PET decomposition reaction is accelerated at elevated temperatures between 55 and 70 °C. 10,11 Prior to these studies, supposedly inactive cell components like PET were not considered to participate in parasitic reactions.…”

“…10 The PET decomposition reaction is accelerated at elevated temperatures between 55 and 70 °C. 10,11 Prior to these studies, supposedly inactive cell components like PET were not considered to participate in parasitic reactions. While DMT was unambiguously identified as the shuttle molecule responsible for the self-discharge of these cells, the exact redox potential and electrochemical stability of DMT, as well as its shuttling mechanism across electrodes passivated with a solid-electrolyte interphase (SEI) were not understood.…”