(49b) High-Performance Silicon Active Materials from Biorenewable Resources

Nancy Chen, Morteza Sabet, Craig M. Clemons, Apparao M. Rao, and *Srikanth Pilla

Ph.D. Candidate, Clemson University, Research Assistant Professor, Clemson University, Materials Research Engineer, USDA Forest Products Laboratory, Professor, Clemson University, and Professor, Clemson University

Presentation Keywords: lithium-ion batteries, anode, silicon nano-quill, sol-gel, cellulose nanocrystal

Abstract

The escalating global population and surging energy demands are straining our fossil fuel reserves and raising concerns about resource availability for future generations. As the transportation sector rapidly electrifies, rechargeable lithium-ion batteries (LIBs) are emerging as a viable alternative to conventional fuel-based technologies. The quest for higher energy density and longer cycling life continues to drive advancements in LIBs. Among the recent innovations in anode electrode materials, silicon (Si) is considered the most promising candidate to replace graphite, boasting a theoretical specific capacity of ~4200 mAh g^-1, over ten times greater than that of graphite anodes (~372 mAh g^-1). Despite these advancements, persistent challenges obstruct the commercialization of anodes based on pristine Si active material. Si-based electrodes are prone to rapid degradation due to the substantial volume change of Si particles (approximately 400%) during lithium insertion and extraction. This repeated volume change results in the pulverization of Si material, ultimately causing decreased cycling stability due to the loss of contact with the current collector. To overcome this obstacle, porous and hollow nanostructures have been employed to provide sufficient void space to accommodate the volume change during electrochemical cycling. However, with the demands for material cost-reduction from industry, Si structures' strategic engineering must be cost-effective for commercial viability. To this end, we developed a low-cost process in which we leverage the 1-dimensional (1D) bio-renewable templates to synthesize a 3D porous Si architecture called silicon nano-quills (SiNQs). We innovated a two-step, cost-effective process that yields SiNQs with a porous morphology and hollow interior structure. First, in a scalable sol-gel process, silica gel particles were prepared using low-cost chemicals. A unique mesoporous morphology was engineered using surfactant-modified cellulose nanocrystals as a sacrificial template. The templates were removed via thermal treatment to form silica nano-quills (SilicaNQs), which possess a 3D bulk structure comprised of hollow quill-like arms and a high degree of porosity. In the second step, we employed a low-temperature magnesiothermic reduction method to convert SilicaNQs into SiNQs with a relatively large surface area.

The electrochemical cycling performance of SiNQs was determined using half-cell testing. A water-based slurry was prepared using a combination of SiNQ and graphite as the active materials. The slurry with 73 wt% MCMB graphite, 15 wt% SiNQ, 2 wt% carbon black, and 10 wt% LiPAA binder was cast onto the commercial copper foil. After room-temperature drying, the electrode was calendered and vacuum dried. We also prepared identical electrodes using commercial Si nanoparticles (100 nm, solid spherical). The 2032-type coin half cells were assembled for battery testing using SiNQ-graphite and Si-graphite electrodes (with an active mass loading of 3 mg cm^-2). The coin cells underwent a formation cycle at a current rate of 0.05C followed by continuous cycling at a current rate of 0.1C (90 mA g^-1) over the potential range of 0.005 – 1.5 V at room temperature. The SiNQ-graphite anode offered an initial reversible capacity of 587 mAh g^-1 and a superior capacity retention of 70% after 200 cycles. In comparison, the commercial Si-graphite battery exhibited an initial capacity of 375 mAh g^-1 and capacity retention of 41% after 200 cycles. The SiNQs possess a BET surface area of 399 m² g^-1 and a total pore volume of 0.64 cm³ g^-1. The superior performance of SiNQs is due to their unique morphology that offers high surface area and porosity for effective diffusion of lithium ions and their electrochemical interactions with NQs, leading to a higher reversible capacity. Moreover, the porous architecture of SiNQs can effectively mitigate the volume change during lithiation and delithiation, thus ensuring excellent cycling performance.