Porous Silicon Nano-Quill Anodes for Lithium-Ion Batteries

The growing population and increasing energy demands are overwhelming our fossil fuel supply and limiting future generations' availability. With the rapid electrification of the transportation sector, rechargeable lithium-ion batteries (LIBs) are emerging to replace conventional fuel-based technologies. LIBs continuously evolve due to demands for higher energy density and long cycling life. Among the recent developments in anode electrode materials, silicon (Si) is regarded as the most promising replacement for graphite due to its high theoretical specific capacity (~4200 mAh g^-1), which is over 10 times greater than that of graphite anodes (~372 mAh g^-1). Despite the advancements, persisting challenges hinder the commercialization of Si for LIB anodes. Si-based electrodes are susceptible to rapid degradation due to the large volume change (approx. 400%) of Si particles during lithium insertion and extraction. The repeated volume change leads to the pulverization of the Si material, ultimately leading to decreased cycling stability from the loss of contact with the current collector. To overcome this obstacle, 3-dimensional (3D) 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.<br/>Our team has developed a patent-pending methodology for utilizing bio-renewable templates to synthesize a 3D Si architecture called Si 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. 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 batteries using commercial Si nanoparticles (100nm, spherical) with the same slurry composition. 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 superior capacity retention of 91% after 90 cycles. In comparison, the commercial Si-graphite battery exhibited an initial capacity of 375 mAh g^-1 and capacity retention of 61% after 90 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 providing a good cycling performance.