Enzymatically demethylated pectins: from fruit waste to an outstanding polymer binder for silicon-based anodes of Li-ion batteries

Pectin is a polysaccharide frequently found in large amounts in the peel and seeds of many fruits and therefore represents very common food industry waste. In this study, we investigate demethylated citrus and apple pectins with a methylation degree ranging from 76% to 3%, as polymer binders for silicon-based anodes of lithium-ion batteries. Both chemical and enzymatic pectin demethylations were considered. First, the usual aggressive saponification reaction was carried out leading to 24% and 28% methylesterified pectins from citrus and apple, respectively, but leading at the same time to unavoidable strong pectin depolymerization, as shown by SEC studies. In a second approach, the methylesterase enzyme was used to catalyze citrus pectin demethylation leading to a similar methylesterification degree (24%) but drastically minimizing polymer chain degradation. Our best-demethylated pectin was compared with the standard polymer binder for Li-ion batteries i.e. carboxymethyl cellulose (CMC) inside a composite silicon anode for their effect on silicon electrochemical capacity retention. The 24% enzymatically demethylated citrus pectin achieved here a remarkable capacity of 2275 mA h g-1 after 49 cycles with a load of 1.6 mg cm-2 compared to 245 mA h g-1 measured for CMC. These demethylated pectins have a buffering effect on the silicon particles' volume change during discharge/charge cycles. The increase of interactions between silicon and pectin, probably due to the presence of numerous carboxylic acid functions in this demethylated pectin, is hypothesized to be, at least partly, responsible for these enhanced electrochemical performances. In addition, the existing type of glycosidic linkage (& alpha; in pectins and & beta; in CMC) can also be responsible for these enhanced results. Enzymatically demethylated citrus pectins: an efficient polymer binder in Si-based anodes of Li-ion batteries.