In situ cross-linked poly(ether urethane) (PEU) is used as a binder for silicon (Si) anodes of lithium-ion batteries. Methylene di-p-phenyl diisocyanate and poly(ethylene glycol) are polymerized to form cross-linked PEU, while gallic acid is used as a cross-linker. The PEU binders are characterized by infrared spectroscopy, solid-state 13C nuclear magnetic resonance spectroscopy, and gel permeation chromatography. The results of differential scanning calorimetry measurements show that the glass transition temperature of PEU is −23.2 °C, confirming that the binder is an elastomer. Moreover, 180° peeling tests show a dramatic increase in the adhesion force of the Si anode with the cross-linked binder. The charge capacities of the Si anode with the cross-linked binder at C-rates of 0.1, 0.2, 0.5, 1, and 2 C are found to be 3628, 3490, 3156, 2838, and 2491 mAh g−1, respectively, which are higher than those of the Si electrode with a non-cross-linked PEU binder. Furthermore, the Si anode with the in situ cross-linked PEU binder exhibits good cycle-life performance with a specific charge capacity of 2414 mAh g−1 after 100 cycles at 0.2 C.