Silk fibroin-based biopolymer composite binders with gradient binding energy and strong adhesion force for high-performance micro-sized silicon anodes

Micro-sized silicon anodes have shown much promise in large-scale industrial production of high-energy lithium batteries. However, large volume change (>300%) of silicon anodes causes severe particle pulverization and the formation of unstable solid electrolyte interphases during cycling, leading to rapid capacity decay and short cycle life of lithium-ion batteries. When addressing such issues, binder plays key roles in obtaining good structural integrity of silicon anodes. Herein, we report a biopolymer composite binder composed of rigid poly(acrylic acid) (PAA) and flexible silk fibroin (SF) tailored for micro-sized silicon anodes. The PAA/SF binder shows robust gradient binding energy via chemical interactions between carboxyl and amide groups, which can effectively accommodate large volume change of silicon. This PAA/SF binder also shows much stronger adhesion force and improved binding towards high-surface/defective carbon additives, resulting in better electrochemical stability and higher coulombic efficiency, than conventional PAA binder. As such, micro-sized silicon/carbon anodes fabricated with novel PAA/SF binder exhibit much better cyclability (up to 500 cycles at 0.5 C) and enhanced rate capability compared with conventional PAA-based anodes. This work provides new insights into the design of functional binders for high-capacity electrodes suffering from large volume change for the development of next-generation lithium batteries.