Highly Ion-Conductive Si Anode Binder with Optimized Tensile Strength and Large Deformation for Lithium-Ion Batteries

Silicon (Si) anodes exhibit exceptional theoretical capacity but suffer from structural pulverization caused by dramatic volume changes derived from oxidation-reduction reactions during lithiation/delithiation cycles. Despite progress in binder development, integrating robust mechanical properties with high ionic conductivity in a single binder system remains a critical challenge due to the difficulty in optimal functional monomer sequence architecture. Herein, a PAA-based comb copolymer combining acrylic acid (AA), 2-carboxymethyl acrylate (CEA), and 3-sulfopropyl acrylate lithium salt (SPALi) was synthesized to be used as the Si anode binder. In the formulation, AA, CEA, and SPALi serve as rigid monomer, flexible monomer, and ion-conductive monomer, respectively, and the synergistic effect of the three functional monomers fulfills the integrated design of an ion-conductive rigid-flexible copolymer and thus enables the accommodation of external stress and high rate capacity of Si anodes. The ingenious combination of AA, CEA, and SPALi endows the binder with a tensile strength as high as 23.1 MPa, an elongation at break as high as 196.0%, and a high ionic conductivity reaching 5.7 × 10^-4 S cm^-1. The electrochemical performances of the Si anodes constructed with the p(AA-<i>co</i>-CEA-<i>co</i>-SPALi) binder are stabilized, and a retention capacity of 2120.4 mAh g^-1 at 840 mA g^-1 after 200 cycles together with a rate capacity of 1740.6 mAh g^-1 at 2100 mA g^-1 after 200 cycles is obtained. These results indicate that the Si anode with the aqueous p(AA-<i>co</i>-CEA-<i>co</i>-SPALi) binder has promising prospects for practical application, and this design also provides a reference for solving the expansion problem of the Si materials.