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

Abstract

Recent studies showed that LFP/graphite cells without additives show higher reversible self-discharge after 500 h of open-circuit voltage (OCV) storage at 40 or 60°C than NMC811/graphite cells.1 Furthermore, cells with lithium hexafluorophosphate (LiPF6), the most commonly used conducting salt in lithium-ion batteries, show higher self-discharge than cells with lithium bis(fluorosulfonyl)imide (LiFSI), an alternative salt with higher temperature stability.2 Recently, dimethyl terephthalate (DMT) was identified as the redox shuttle molecule responsible for the unwanted self-discharge of these lithium-ion batteries.3 In a lithium-ion cell, a reversible shuttle can gain an electron at the negative electrode by reduction, diffuse to the positive electrode, lose the electron by oxidation, and then repeat the process many times (see Figure 1). Adamson et al.4 proved that DMT is created in-situ as a breakdown product of polyethylene terephthalate (PET), which is a surprisingly common polymer for the adhesive tapes found in commercial batteries. The exact redox potential and electrochemical stability of DMT, as well as its shuttling mechanism across electrodes passivated with a solid-electrolyte interphase (SEI) however were not understood.Based on an optimized coin cell cyclic voltammetry setup, ultra high precision coulometry measurements, OCV storage, and GC-MS experiments, we present new insights on the self-discharge redox shuttle mechanism of DMT in LFP/graphite pouch cells, investigate if this redox shuttle can account for the self-discharge differences in LiPF6 and LiFSI cells, and explore the stability of DMT in pouch cells.