Silicon (Si) has been regarded as the next generation anode material for the lithium-ion batteries due to its high theoretical specific capacity about 4200 mAh/g and low costs. However, the Si nanoparticle (NP) experiences repeated volume changes in size up to 300 % during the lithiation/delithiation processes, which leads to a series of problems like particle pulverization, electrode delamination, and destabilization of solid-electrolyte interface (SEI) layer. The conventional polyvinylidene fluoride (PVDF) binder does not work on the Si anode and by far quite a few polymer binders have been studied and some have shown promising results, such as carboxymethyl cellulose (CMC), alginate, and poly(acrylic acid) (PAA), and some synthesized conductive polymers. While some impressive improvements have been obtained, none of those binders could afford comparable cycling performance to that of carbonaceous anode cells, and the deep understanding of the degradation mechanism of Si NPs is not fully established. Therefore, it is of paramount importance and practical interest to conduct a more systematic study to carefully investigate the relationship between the polymer structures and cycling performance and seek opportunities for next generation binder development. Here linear polysiloxanes (PSs) polymers were chosen as the polymer backbone that can allow us to precisely engineer different properties into the binder polymers. PSs can afford some desirable properties as silicon anode compatible binders, such as elasticity and durability, which were evidenced by previous studies in graphite anodes.^[1] On the other hand, PSs do possess several weak spots as a binder material, such as low molecular weight, lack of adhesion to the Si particles, and non-conductivity. Fortunately, it is convenient to utilize hydrosilylation reaction to introduce desirable properties into the PS polymer backbones. For instance, the molecular weight and mechanical stiffness could be easily improved by incorporation crosslinking ability into PSs. The adhesion and conductivity can also be tuned via introductions of groups with high affinity to silicon particles and large conjugation groups that can prompt electron hopping via Pi-Pi stacking (Scheme 1).^[2] Those choices of properties could be unlimited as long as we know what properties we are looking for. Moreover, the added properties can be precisely tuned by controlling the ratio of different functional groups, which, combined with the cell testing data, provides an excellent system to investigate the relationship of the binder properties and cell performance.

References

1. Xue, Z.; Zhang, Z.; Hu, L.; Amine, K. J. Electrochem. Soc. 2013, 160, A1819-A1823.
2. Zhao, H.; Wei, Y.; Qiao, R.; Zhu, C.; Zheng, Z.; Ling, M.; Jia, Z.; Bai, Y.; Fu, Y.; Lei, J.; Song, X.; Battaglia, V. S.; Yang, W.; Messersmith, P. B.; Liu, G. Nano Lett. 2015, 15, 7927-7932.

Scheme 1. Illustration of synthesis of comb-like polysiloxanes with different functional groups to increase the flexibility, adhesion, conductivity as well as crosslinking ability.