Silicon-based electrodes offer charge capacities much greater than graphite, however, short-cycle life stymies commercialization of Si in Li-ion batteries (LIBs). Binding agents plays a crucial role in Si-electrode performance; binder composition induces interactions/reactions with Si and electrolytes that greatly influence cycling performance of Si-based systems. Thus, elucidation of such interactions and reactions is paramount to extend cycle life and increase capacity. We previously reported on usage of guar gum (GG) as a novel binding agent for Si nanoparticle (SiNP) electrodes [1]. Electrodes using GG binders retain >70% capacity after 250 cycles and outperform carboxymethyl-bound Si electrodes. Nevertheless, shortcomings in SiNP/GG system include 1) low ionic and electronic conductivity and 2) fractured, and possibly delaminated, composite electrodes. To overcome these shortcomings, we developed a novel gel binder in which a conductive additive was reinforced within the binder.

The novel gel binder comprises crosslinked GG/carbon black (CB) dispersions. We first probed network formation via dynamic rheology, which revealed greater moduli for the GG/CB system than in the absence of CB, and suggests that CB particles enhance network strength through physical crosslinks (Figure 1a). In attempt to elucidate the role of the binder in Si-based electrodes, we investigated the influence of reaction time (or crosslinking density) on physical properties such as elasticity, electrolyte uptake, ionic conductivity, and adhesion. Physical properties could then be correlated with electrochemical performance in Li-ion half-cells. We found that marginally-crosslinked GG/CB binders in SiNP electrodes led to enhanced cycling performance, whereas capacity was limited in SiNP electrodes using densely-crosslinked GG/CB binders. Symmetrical charge/discharge cycling at 3.6 A g-1 revealed SiNP electrodes with marginally-crosslinked GG/CB binders have larger capacities than non-crosslinked binders (Figure 1b), which may be attributed to electronically conductive networks of CB. Additionally, capacity retention increased with crosslinking of binders (Figure 1b), which may be attributed to preservation of initial electrode microstructure. In attempt to enhance Si electrode feasibility in commercial LIBs, the ensuing study provides fundamental relations between crosslinked-biopolymer binders and SiNP electrode performance.

Figure 1. (a) Evolution of storage modulus, G’, in GG systems with CB (blue) and without (black) using measurements from dynamic stress sweeps; (b) Capacities over 200 cycles of SiNP electrodes using various GG/CB-binder crosslinking densities at 3.6 A g^-1

Reference

[1] J. Mater. Chem. A, 2015, 3, 12023-12030