Facile synthesis of polynorbornene-based binder through ROMP for silicon anode in lithium-ion batteries

Although binders are a minor component in battery electrodes, they can improve the electrochemical performance considerably, particularly in conversion-type electrodes, such as the silicon (Si) anode, which has volume expansion problems. Although numerous binders are reported for Si anode, less attention has been paid to those prepared through controlled polymerization, which can allow the preparation of well-defined polymers. Herein, we report the facile synthesis of carboxylic acid-functionalized polynorbornene (CA-PNB) via ring-opening metathesis polymerization (ROMP) and apply this as the binder for Si anode. Owing to the ultrafast polymerization kinetics of ROMP, a high molecular weight polymer (similar to 279 kDa) with narrow dispersity (D=1.07) was readily prepared within 30 min. The resulting CA-PNB was used as the binder for a Si nanoparticle anode, which exhibits an initial coulombic efficiency of 81% and a specific capacity of 1,654 mAh g(-1) after 100 cycles at 0.5 A g(-1). These values outperform the Si anodes prepared using conventional polyvinylidene difluoride and carboxymethyl cellulose binders, probably due to the abundant carboxylic acid groups in the CA-PNB that offer stronger interactions with the Si surface. Since ROMP is a highly efficient polymerization tool to produce polymers with tailored architectures and controlled monomer sequences, this method will be valuable for the rational molecular design of high-performance binders for battery electrodes.