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

Dimethyl terephthalate (DMT) is a redox shuttle molecule that leads to unwanted self-discharge of lithium-ion cells. It can be created in situ as a breakdown product of polyethylene terephthalate (PET), which is a surprisingly common polymer for the adhesive tapes found in commercial cells. This study investigates the shuttling mechanism and electrochemical stability of DMT, as well as its impact on the performance of LFP/graphite pouch cells with LiFSI and LiPF6 conducting salts. Cyclic voltammetry shows that DMT has a redox potential of 1.5 V vs Li+/Li and is redox active in the full voltage range of LFP/graphite cells. Ultra-high precision coulometry and open-circuit storage experiments show that DMT lowers the coulombic efficiency, increases the charge endpoint capacity slippage, and dramatically accelerates the reversible self-discharge of LFP/graphite pouch cells. Gas chromatography-mass spectrometry shows that DMT is stable over weeks in cells with LiPF6, but only for several days in cells with LiFSI. A well-insulating solid-electrolyte interphase layer derived from vinylene carbonate can prevent DMT from shuttling. However, VC can be consumed, and passivation layers can deteriorate in aged cells, so the best way to prevent DMT-induced self-discharge of lithium-ion batteries is to eliminate PET components.