Role of Polyacrylic Acid (PAA) Binder on the Solid Electrolyte Interphase in Silicon Anodes

To obtain high-energy density Li-ion batteries for the next generation storage devices, silicon anodes provide a viable option because of their high theoretical capacity, low operating potential versus lithium (Li), and environmental abundance. However, the silicon electrode suffers from large volume expansion (similar to 300%) that leads to mechanical failure, cracks in the SEI (solid electrolyte interphase), and loss of contact with the current collector, all of which severely impede the capacity retention. In this respect, the choice of binders, carbon, electrolyte, and the morphology of the silicon itself plays a critical role in improving capacity retention. Of specific mention is the role of binders where a carboxylic acid-heavy group, PAA (polyacrylic acid), has been demonstrated to have better cycling capacity retention as compared to CMC (carboxy methyl cellulose). Traditionally, the role of binders has been proposed as a soft matrix backbone that allows volume expansion of the anode while preserving its morphology. However, the effect of the binder on both the rate of formation of SEI species across cycles and its distribution around the silicon nanoparticles has not been completely investigated. Herein, we use two different binders (PAA and CMC) coupled with LiFSI (lithium bis(fluorosulfonyl)imide)/EMI-FSI (1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide) ionic liquid as the electrolyte to understand the effect of binder on the SEI. Using STEM-EDX (scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy), EELS (electron energy loss spectroscopy), and XPS (X-ray photoelectron spectroscopy), we discuss the evolution of the SEI on the Si electrode for both binders. Our results indicate that a faster decomposition of FSI- with a PAA binder leads to LiF (lithium fluoride) formation, making F- unavailable for subsequent SEI formation cycles. This allows further decomposition of the LiFSI salt to sulfates and sulfides which form a crucial component of the SEI around silicon nanoparticles after 100 cycles in the PAA binder-based system. The dual effects of faster consumption of to form LiF together with the distribution of passivating sulfides in the SEI could allow for better capacity retention in the PAA binder system as compared to that with CMC.