Stress-Dissipative Elastic Waterborne Polyurethane Binders for Silicon Anodes with High Structural Integrity in Lithium-Ion Batteries

Silicon (Si) has been recognized as a promising anode material for lithium-ion batteries (LIBs) owing to its high theoretical capacity and cost efficiency. However, conventional poly(vinylidene fluoride) (PVDF) binders have been revealed to be inadequate in withstanding the significant volume changes that occur during repeated lithiation/delithiation processes, leading to the degraded cycling performance of LIBs. To address this problem, polymer binders containing polar functional groups and elastic properties have been developed. However, the use of toxic organic solvents to prepare polymeric binders poses a challenge. In this study, we prepare an eco-friendly polyurethane binder using water as the dispersion solvent by incorporating nonionic hydrophilic poly(ethylene glycol monomethyl ether)-based trimethylolpropane (MPEG-TMP) chains into the polyurethane structure (designated as POWPU). The POWPU binder is designed to remain insoluble in the electrolyte and retain its mechanical stretchability and elasticity at the Si anode. In the Si/binder system, the polar urethane groups form hydrogen bonds with the silanol groups on the Si particles, while the stretchable and elastic poly(propylene oxide) (PPO) chains are able to dissipate stress caused by volume changes. Besides, the high electrolyte absorption of POWPU and the coordination between the PEO segments of the MPEG-TMP chains and Li+ facilitated Li+ transports, which in turn enhances the ionic conductivity of the POWPU binder. As a result of these advantages, the rate capability and cycling performance are enhanced compared with those of the conventional PVDF binder. These results suggest that the sustainable and environmentally benign POWPU binder could be an attractive choice for fabricating eco-friendly and stable Si anodes.