Phase-Separated Ionogel Binders for Durable High-Capacity Silicon Anodes

Silicon is a promising anode material for next-generation lithium-ion batteries because of its high theoretical capacity; however, large volume changes during cycling lead to particle fracture, loss of electrical contact, and unstable interfaces. Although advanced polymer binders improve electrode integrity, balancing mechanical robustness and ion transport remains challenging in homogeneous networks.

Herein, we report a phase-separated ionogel binder that regulates the local microenvironment of silicon electrodes. The binder is formed by in situ crosslinking acrylamide and acrylic acid in the presence of EMIMBF4, generating interconnected polymer-rich and ionic-liquid-rich domains. Small-angle X-ray scattering reveals correlated nanostructures with a characteristic spacing of about 10.5 nm. The resulting architecture combines strong adhesion and mechanical compliance with efficient lithium-ion transport.

The ionogel binder exhibits a tensile strain above 470% and a peel force of about 6.5 N. The activation energy for lithium-ion transport decreases from 19.15 to 17.04 kJ mol-1 compared with the ionic-liquid-free counterpart. Mechanical simulations, in situ impedance measurements, and post-cycling analyses indicate reduced stress concentration, suppressed electrode swelling, slower impedance growth, and improved interfacial stability.

Consequently, silicon electrodes using the ionogel binder deliver 1,827 mAh g-1 after 300 cycles at 1 A g-1 and retain 703 mAh g-1 after 1,000 cycles at 5 A g-1. They also achieve an areal capacity of 3.71 mAh cm-2, while Si-LiFePO4 full cells retain 108 mAh g-1 after 200 cycles. These findings demonstrate that phase-separated ionogel binders provide an effective route to durable, high-performance silicon anodes.