Effect of Binder on the Formation and Depth Distribution of SEI in Silicon Anodes

Silicon alloying anodes form an attractive technology in the ongoing evolution of Li-ion batteries. Their easy of availability, low cost, large theoretical specific capacities and low alloying potentials versus lithium (Li) would allow easy commercial adoption with ~10-15 wt% silicon already in use in the industry. To increase the available areal energy density, the mass loading of silicon needs to be increased. In this respect, the volume expansion in silicon (of ~ 300%) during cycling needs to be mitigated, to prevent mechanical failure and loss of electrical contact, both of which severely impede the capacity retention. While traditional techniques such as the use of intermetallics, conformal carbon coatings and tailoring the morphology, shape and size of the silicon anodes have attained reasonable success, polymeric organic binders and ionic liquid electrolyte have recently gained popularity. Polyacrylic acid-based binders (PAA) are ideal due to their high mechanical flexibility while ionic liquids form more stable SEI species compared to carbonate-based electrolytes. However, interactions between the binder and the ionic liquid salts as well as its influence on the SEI has not been considered, due to the traditional held view of the binder being a soft backbone matrix. Herein we report on the effect of PAA and CMC binder on the rate, composition and depth distribution of the SEI species in silicon anodes. We use STEM-EDX (scanning transmission electron microscope-energy dispersive x-ray spectroscopy), EELS (electron energy loss spectroscopy) amd XPS (X-ray photoelectron spectroscopy), to study and observe the distribution of LiF and sulphides/sulphates in the SEI. Based on our observations we propose a mechanism for better capacity retention in electrodes with PAA binder, due to formation of passivating sulphides close to the silicon surface which is a direct consequence of faster LiFSI decomposition to LiF in presence of PAA binder.