Rigid-Flexible Integrated Polymer Binder via In Situ Esterification Cross-Linking for Si/C Anodes in Li-Ion Batteries

Conventional binders fail to mitigate the severe volume expansion (>300%) in silicon-based anodes, resulting in electrode pulverization and rapid capacity decay. In this work, we synthesized a binder with an integrated rigid-flexible structure by copolymerizing with a flexible monomer on the main chain and cross-linking with an elastomer via side groups. The synthesis process involves first copolymerizing acrylic acid (AA) and hydroxyethyl acrylate (HEA), followed by in situ esterification cross-linking with carboxylated nitrile butadiene rubber (XNBR). Through a systematic investigation, we discovered a crucial synergy between the cross-linking density, which is controlled by the AA/HEA mass ratio, and the chain flexibility in maintaining the electrochemical integrity of the electrode. The optimized PAH_8:2-XNBR binder reaches an ideal balance between the elastic modulus and adhesion strength. It can effectively adapt to the expansion of silicon while keeping the conductive pathways intact. Electrochemical evaluations have shown remarkable cycling stability. The Si/C600 anodes using this binder retain a capacity of 465.20 mAh g^-1 after 300 cycles at 0.5 C and 466.51 mAh g^-1 after 200 cycles at 2 C. The binder network, with the synergistic interaction of its rigid and flexible chain segments, effectively suppresses the expansion of the silicon anode. After 300 cycles at 0.5 C, scanning electron microscopy revealed only 10.93% electrode thickening. This work offers valuable insights for the design of efficient binders for silicon-based anodes that undergo significant volume changes.