Description:
<jats:p> One of the biggest challenges of the near future is how to meet the increasing energy demand without any adverse environmental impact. Lithium-Ion batteries (LIBs) are one of the promising alternative energy storage systems to replace conventional non-rechargeable batteries. LIBs are becoming one of the most widely used energy storage devices because of their relatively high specific energy density (~300 Wh/kg), excellent stability over a wide temperature range, and lower cost than other battery systems. Intense research is in progress to increase the capacity of anode electrodes for realizing high-energy LIBs. In this context, silicon (Si) has a great potential to replace commercial graphitic anode active materials mainly due to its high theoretical specific capacity (4200 mAh/g) and low working potential (0.37 to 0.45 V vs. Li/Li<jats:sup>+</jats:sup>). To harness and maintain a high capacity from Si-based anodes, we must deal with two main challenges: (i) volume changes of Si (>300%) during charging and discharging, which cause pulverization of the material and loss of electrical contact, and (ii) unstable growth of solid-electrolyte interphase (SEI) layer, which can cause early degradation of performance in Si-anode batteries. To overcome these challenges, different approaches have been taken. This includes developing graphite/Si anodes with limited amounts of Si (<30 wt.%), conformal coating of Si active materials (e.g., CVD carbon coating), etc. However, these challenges still could not be fully overcome.</jats:p>
<jats:p>To advance the use of Si materials in LIB anodes, we developed a viable technology to create a hybrid silicon-carbon material, called Si@C, in which a soybean-derived carbon cloud protects Si nanoparticles during the battery operation. We employed soybean oil in a scalable oil-in-water emulsion polymerization technique to produce Si-containing polymeric particles. In this method, we emulsified two immiscible solutions. One contains a homogeneous Si mixture in epoxidized soybean oil (ESO), and another contains a uniform dispersion of ball-milled lignin or soyhull powders in water. Citric acid, a crosslinking agent, was used to help polymerize the ESO and integrate carbon-rich lignin/soyhull with polymerized particles. The final Si@C product was achieved by carbonizing the polymerized solid product at 500 °C (under argon) and ball milling to get a fine powder. Several Si@C hybrid materials containing 20 wt.% to 50 wt.% Si were successfully prepared and utilized for anode preparation. Electrodes were made by coating a slurry of Si@C active material, carbon black, and binder with a mass ratio of 60:20:20 onto an ion-permeable Bucky Paper (BP, a flexible and conductive paper made of carbon nanotubes). Anodes with different binding systems were prepared to determine an appropriate binder for Si@C based batteries. The 2032-type coin cells were assembled for battery testing using 1M LiPF<jats:sub>6</jats:sub> in EC:DMC in a volume ratio of 1:1 as the electrolyte, Li chips as the counter electrode, and Celgard 2325 as the separator. The coin cells were cycled at a current rate of 0.1C (420 mA/g<jats:sub>Si</jats:sub>) over the potential range of 0.01 – 1.0 V at room temperature.</jats:p>
<jats:p>Battery results showed that Si@C hybrid materials increased the capacity of Si anodes by a factor of >2. At Si mass loading of 1.0 mg/cm<jats:sup>2</jats:sup>, implementing our carbon cloud approach led to an increase in the discharge capacity of anodes from 0.5 mAh (corresponding to anode with bare Si) to >1.0 mAh (corresponding to anode with Si@C hybrids). Results from battery cycling at 0.1C demonstrated excellent capacity retention of >95% after 130 cycles for anodes prepared using our Si@C active materials. The Si content of Si@C hybrid particles was also found to be an influential factor in the cycling performance of anodes. Finally, we observed that the use of water-based polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) binders improve the electrochemical performance of Si@C based anodes. These water-based binders are ideal for preparing Si-based slurries, and the need for using toxic solvents (e.g., NMP) to prepare slurries can be averted. In conclusion, we innovated a viable technology that uses biomass (soybean oil and soyhulls) to enhance Si-based batteries' performance. We demonstrated that our Si@C materials with a carbon cloud protecting the Si nanoparticles are promising active materials to improve the capacity and cycling stability of LIB anodes. </jats:p>