Crucial Role of Volumetric Variation Tolerable Single-Walled Carbon Nanotube/Polymer Binder Network for Stabilizing Bulk Silicon Anode

Water-soluble polymer binders have shown better affinity to silicon-based anodes of lithium-ion batteries than the traditionally used polymer binder poly(vinylidene difluoride), but their performance is still far from desired while their ultimate capacity to improve the stability of a silicon-based anode is unclear. In this work, the composition of anodes with 80% weight fraction of pristine silicon nanoparticles, which can sufficiently embody giant volumetric variation of silicon and growth of the surface solid electrolyte interface (SEI), is set for investigation of the role of binders. Carboxymethyl cellulose (CMC) and the single-walled carbon nanotube (SWCNT) are hybridized, forming easily processed dispersions of SWCNT–CMC that make the industrial manufacturing process favorable. The ex-situ impedance under different working potentials, lithium-ion diffusion coefficient, and polarization potential of anodes with CMC only and SWCNT–CMC are systematically studied. Furthermore, the role of SWCNT in CMC binder bulk film along with binder sponge, the latter of which simulates the binder network in the anode, has been studied via multiple mechanical tests. It is found that the SWCNT–CMC hybrid binder in the anode provides a dense network that a normal conducting agent cannot form, which highly effective delivers electrons to the anode, reducing polar potential, depressing overgrowth of SEI, and improving rate performance of the anode. More importantly, the dense network of SWCNT–CMC is mechanically more stable than CMC. Even though an anode with SWCNT–CMC cracks after 100 cyclic charge/discharge tests, SWCNT–CMC still can bridge Si nanoparticles in the anode, preserving the bulk anode electrical conductance.