Balancing Rigidity and Elasticity in Polymer Binders for Long-Life Silicon Anodes

Silicon (Si) is a promising anode material for high-energy-density lithium-ion batteries, but its severe volume change during repeated lithiation and delithiation causes particle pulverization, electrode delamination, and unstable solid electrolyte interphase (SEI) formation. Here, we report a mechanically robust, elastic, and stress-relaxable polymer binder constructed by interweaving poly(acrylic acid) (PAA) with an epoxy-telechelic poly(urea-urethane) (PUU) elastomer through thermal cross-linking. By systematically tuning the molecular weight of the PUU soft segment and the structure of the hard segment, we elucidate how binder elasticity, toughness, and stress relaxation govern the cycling stability of Si anodes. The optimized binder, xPUU1k, combines the structural rigidity of PAA with the elasticity and hydrogen-bonding capability of PUU, resulting in enhanced electrode adhesion, rapid stress dissipation, and reversible interfacial interactions with the silanol-rich Si surface. The xPUU1k electrode exhibits a toughness of 4.9 MJ m⁻³, an elastic recovery ratio of 0.54, and an adhesion force of approximately 7.5 gf mm⁻¹, all substantially higher than those of the PAA electrode. Consequently, the xPUU1k-based Si anode retains 61% of its capacity after 270 cycles at 0.5 C, compared with only 7% for the PAA-based electrode. Operando Raman spectroscopy, electron microscopy, and X-ray photoelectron spectroscopy further reveal that the elastic and adhesive binder preserves particle integrity and promotes the formation of a thin, uniform, and stable SEI layer. These results demonstrate that balancing mechanical robustness, elasticity, toughness, and stress relaxation is critical for designing long-lasting polymer binders for high-capacity Si anodes.