Exploring the optimal molecular weight of polyacrylic acid binder for silicon nanoparticle anodes in lithium-ion batteries

Abstract

Polyacrylic acid (PAA)-based binders have been demonstrated to significantly enhance the cycling stability of pure silicon (Si) anodes compared to other binder types. However, there is a notable lack of systematic and in-depth investigation into the relationship between the molecular weight (MW) of PAA and its performance in pure Si anodes, leading to an absence of reliable theoretical guidance for designing and optimizing of PAA-based binders for these anodes. Herein, we select a series of PAA with varying MWs as binders for Si nanoparticle (SiNP) anodes to systematically identify the optimal MW of PAA for enhancing the electrochemical performance of SiNP anodes. The actual MWs of the various PAA were confirmed by gel permeation chromatography to accurately establish the relationship between MW and binder performance. Within an ultrawide weight average molecular weight (M_w) range of 35.9–4850 kDa, we identify that the PAA binder with a M_w of 1250 kDa (PAA125) exhibits the strongest mechanical strength and the highest adhesion strength, attributed to its favorable molecular chain orientation and robust interchain interactions. These characteristics enable the SiNP anodes utilizing PAA125 to maintain the best interfacial chemistry and bulk mechanical structure stability, leading to optimal electrochemical performance. Notably, the enhancement in cycling stability of SiNP anode by PAA125 under practical application conditions is further validated by the 1.1 Ah LiNi_0.8Co_0.1Mn_0.1O₂/SiNP@PAA125 pouch cell.