Article Cite This: Chem. Mater. 2019, 31, 2535−2544 pubs.acs.org/cm Role of Polyacrylic Acid (PAA) Binder on the Solid Electrolyte Interphase in Silicon Anodes † † † † † Pritesh Parikh, Mahsa Sina, Abhik Banerjee, Xuefeng Wang, Macwin Savio D’Souza, † † † † ‡ ‡ Jean-Marie Doux, Erik A. Wu, Osman Y. Trieu, Yongbai Gong, Qian Zhou, Kent Snyder, † and Ying Shirley Meng*, † Department of NanoEngineering, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States ‡ Energy Storage Research Department, Ford Motor Company, 2101 Village Road, Dearborn, Michigan 48124, United States *S Supporting Information ABSTRACT: 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 (∼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 F− 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. Downloaded via UNIV OF CALIFORNIA SAN DIEGO on April 23, 2019 at 20:39:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 1. INTRODUCTION Since its introduction by Sony in 1991, the cathode consisting The ability to engineer a sustainable and green future presents a of LCO (lithium cobalt oxide) has seen improvements with different chemistries based on Ni, Co, and Mn to enable high- formidable challenge to governments and industries alike. voltage operation (>4.3 V), higher specific capacity (>200 mA Regular reports of Antarctic ice melts,1 increase in respiratory h/g), and greater capacity retention.5 Currently, NCM (lithium issues in both developing and developed countries,2 and the nickel manganese cobalt oxide) and NCA (lithium nickel cobalt alarming increase in the rate of global greenhouse gas 3,4 aluminum oxide) are the two cathode options with continuous emissions all point to the gravity of the situation at hand. research and development toward novel chemistries and optimal Photovoltaics, wind, tidal, and geothermal energy all provide Ni/Co and Co/Mn ratios. On the anode side, graphite renewable sources with minimal carbon footprints but the continues to be the material of choice, providing 372 mA h/g intermittency in the generation warrants the need for energy theoretical specific capacity. However, to develop the next storage solutions. Li-ion batteries have evolved to form a pivotal generation of batteries, the specific capacity should be increased role in this ever-changing energy landscape. With its foray into the electric car and the grid storage markets, in the span of a Received: January 4, 2019 decade, Li-ion batteries have taken the center stage in energy Revised: March 19, 2019 storage. Published: March 19, 2019 © 2019 American Chemical Society 2535 DOI: 10.1021/acs.chemmater.8b05020 Chem. Mater. 2019, 31, 2535−2544 Chemistry of Materials Article to 1000 mA h/g.6 To this effect, research on anodes has explored Hence, the ability to form a matrix that can effectively buffer hard carbons, oxides, sulfides, phosphides, nitrides, alloy anodes, the volume expansion associated with silicon and still be and more recently even graphene/rGO (reduced graphite commercially adaptable has still not been completely explored. oxide) and CNTs (carbon nanotubes).5,7 For the silicon electrode, the binder can thus play this very Although hard carbon, due to the presence of nanograins and critical role of forming a soft matrix that can provide both the voids, allows a theoretical capacity larger than graphite, it also required mechanical and electrochemical stability/electronic leads to greater solid electrolyte interphase (SEI) formation, conductivity. In this respect, engineering a binder which forms a limiting the first cycle Coulombic efficiency. Oxides such as small portion of the total cost and fabrication process provides a LTO (lithium titanium oxide) provide high thermal stability and very reliable strategy to improve the Si anode performance. The high safety but suffer from low-energy density. Sulfides, exploration of different branched and co-polymers as binders for phosphides, and nitrides all provide high specific capacity and silicon anodes is now an active and burgeoning field of research. low operation potential versus Li but suffer from poor capacity Choi et al. demonstrated stable capacity over 3000 mA h/g with retention and high cost of production.8 micro-silicon particles and a novel polyrotaxane-based binder.19 In this respect, alloy anodes provide high specific and Liu et al. also prepared a polyfluorene (PF) based conductive volumetric energy densities amongst all anode chemistries. binder to overcome the volume expansion and resulting Among alloy anodes, Si, Ge, Al, Sn, and Sb are all potential electrical isolation.20 Wang et al. prepared a self-healing polymer candidates to replace graphite, but they all suffer from severe based on dynamic hydrogen-bonding sites to allow the micro- volume expansion that is currently limiting their commercial silicon particles to obtain a stable capacity over 2000 mA h/g adoption. Si, due to its abundance (leading to potentially low after 130 cycles.21 Wu et al. have also proposed the components production costs), environmentally benign nature at the required for the design of an optimized ideal binder with PF macroscale, high specific capacity (∼4200 mA h/g theoretical), groups for electronic conductivity and methyl benzoate esters and low operating voltage versus Li (compared to other alloy for mechanical strength.22 Amongst the traditional binders anodes), forms a natural choice of anode material.7,9 However, though, polyacrylic acid (PAA)-derived polymers have shown the volume expansion and subsequent contraction present a host enhanced capacity retention over 100 cycles as compared to that of different issues: (a) the SEI layer undergoes continuous for carboxy methyl cellulose (CMC), alginate, and PVDF fracture during cycling, exposing fresh silicon surface to the (polyvinylidene fluoride) binders because of their superior − electrolyte and formation of new SEI. Thus, silicon sees a mechanical strength.23 25 The PAA binder contains a larger gradual increase in the SEI layer thickness and electrolyte number of −COOH (carboxylic acid) groups that are proposed consumption which is detrimental to capacity retention. (b) The to have a positive effect on the silicon electrode through volume expansion also creates large stresses on the silicon hydrogen bonding with the SiOx terminated surface, thus particle itself that leads to pulverization. This can result in a loss allowing the pulverized Si particles to still maintain contact with of usable active material, trapping of Li+ in the anode host due to the current collector. While the role of the binder from a reduced diffusion across different particles, a thicker SEI layer, mechanical aspect and its effect on electrochemical performance and increased possibility of reaction with surface oxides. (c) The are largely well explored through peel strength tests, adhesion pulverization and volume expansion/contraction further con- tests, and stress−strain studies,19,26,27 its interaction with the tribute to delamination and/or loss of electrical contact with the electrolyte and ultimately on the formation of the SEI has not current collector leading to a rapid capacity drop after a few been well understood to date. For the silicon electrodes, the SEI cycles.9 chemical composition and its spatial distribution are important To address these issues, a variety of different solutions have