Improving Lithium-Ion Cells by Mitigating the Degradation of Electrolyte and Inactive Plastic Components

Abstract

The demand for portable electronics and electric vehicles is increasing year-over-year. This means we need more battery storage. Li-ion batteries have unparalleled energy density and lifetime when compared to any other battery type currently on the market. It is essential to extend the battery lifetime to alleviate the pressure on limited resources like nickel, cobalt, and copper required for battery production. This will help ensure that these critical materials are utilized to their fullest potential. To extend battery life, it is essential to understand why batteries fail. First, this thesis identifies a cause of reversible self-discharge in commercial Li-ion and Na-ion cells. It was found that polyethylene terephthalate (PET) tape, which is commonly used as mechanical support in commercial Li-ion cells, is not chemically stable during cycling. In some cases during cell lifetime, this tape can produce a dimethyl terephthalate (DMT) redox shuttle, leading to self-discharge of Li- and Na-ion cells. Additionally, the thesis identifies other potential sources of self-discharge from common polymer components in Li-ion cells and offers alternative design recommendations for battery manufacturers. Second, this work investigates the consumption of electrolyte additives in Li-ion cells with different graphite negative electrodes, after which the stability of PET jellyroll tape can be compromised. Additionally, it is shown that redox shuttles produced from ester-based polymers can also have unexpected benefits in LiMn2O4/graphite cells. This work shows that nearly every cell, regardless of chemistry, cycling temperature, or cutoff voltages, can produce the reagents needed to depolymerize PET over long cycling times, in some cases exceeding 8 years. Therefore, it is recommended that battery manufacturers use polypropylene (PP) instead of ester-based polymers whenever possible.