A team from Dalhousie University in Nova Scotia has demonstrated that the reversible self-discharge of LiFePO₄–graphite (LFP) cells can be virtually eliminated by replacing the polyethylene terephthalate (PET) jellyroll tape commonly used to prevent electrode stacks from unwinding during assembly with chemically stable polypropylene (PP) tape.

A paper on their study is published in Nature Materials.

Whether stacked or wound, Li-ion batteries use tape to hold the assembled stack or jellyroll together before insertion into the cell casing during manufacturing. Many manufacturers use polyethylene terephthalate (PET) tape in their cells for this purpose. After manufacturing the tape is generally regarded as an inactive cell component; however, the team noted, up to now, the chemical stability of PET has not been investigated.

Recent studies from the Dalhousie group have shown that LIBs with LiFePO₄ (LFP) or LiNi_1-x-yMn_xCo_yO₂ (NMC) positive electrodes and graphite negative electrodes are prone to rapid self-discharge at elevated temperature unless effective electrolyte additives are used.

The researchers found an unwanted redox shuttle molecule that is generated in situ during formation, cycling or storage of some battery cells at elevated temperature.

A redox shuttle can diffuse between the positive and negative electrode of a battery and transport electrons from one side to the other through a reversible redox reaction, accepting an electron from the negative electrode and donating it to the positive electrode. This results in lithium ions being transferred from the negative to the positive electrode without charge being drawn from the cell to power a device in the external circuit. This self-discharge reaction is (in principle) non-damaging and reversible since the lithium inventory is unaffected, but it is of concern in many applications, for example, consumer electronics.

In addition, it is of great concern for cells that are connected in series in a battery module. If the self-discharge rates of such cells differ, the cell balancing algorithm of the battery management system may fail to safely charge the module.

Büchele et al. identified the shuttle molecule as dimethyl terephthalate (DMT). This study will demonstrate that DMT is the product of PET depolymerization with methanol and lithium methoxide, two reactants that can be generated in lithium-ion cells from dimethyl carbonate (DMC), a ubiquitous electrolyte solvent. In addition, this study will show how reversible self-discharge of LIBs can be virtually eliminated by replacing PET tape with polypropylene (PP), an alternative polymer with far superior chemical stability.

… Replacing PET with PP jellyroll tape resulted in slightly better capacity retention, higher Coulombic efficiency and almost a complete elimination of reversible self-discharge, even in additive-free cells. Cell manufacturers should replace PET tape with PP tape to eliminate any parasitic reactions from this long overlooked, supposedly inactive cell component.

—Adamson et al.

Resources

• Adamson, A., Tuul, K., Bötticher, T. et al. (2023) “Improving lithium-ion cells by replacing polyethylene terephthalate jellyroll tape.” Nat. Mater. doi: 10.1038/s41563-023-01673-3