Fibroin: A Multi-Functional Bio-Derived Binder for Lithium–Sulfur Batteries

Traditionally, binders such as poly-(vinylidene fluoride) (PVDF) have been used within lithium-sulfur (Li-S) batteries, but these present environmental and recyclability challenges and have little to no impact on the processes that drive degradation in the cell's chemistry. Ideally, a Li-S battery binder would contribute to the mitigation of the polysulfide shuttle effect and negate the impacts of positive electrode volume expansion while being compatible with aqueous ink preparation and low-energy, low-toxicity recycling processes. In this work, we demonstrate that fibroin, an economical and sustainable biological polymer with an abundance of functional groups, can effectively trap polysulfides while still offering the durability, cyclability, and ease of use offered by the current state-of-the-art binder (PVDF). In Li-S coin and pouch cells, fibroin-based electrodes with a high sulfur loading are shown to offer high capacities and Coulombic efficiencies, which in situ and operando analysis shows is due to its beneficial properties as a binder. Importantly, fibroin's ability to be denatured under mildly acidic conditions is shown to make electrodes significantly easier to recycle than ones prepared using PVDF, which require energy-intensive mechanical processes for recycling. Hence, overall, this study highlights fibroin as a promising ecofriendly alternative binder that not only enhances Li-S battery performance but also offers significant advantages for sustainability and recyclability.