This Small Business Innovation Research Phase 1 project will introduce a novel method of manufacturing a high capacity Si nanowire anode for Li-ion battery applications. Si has the highest lithium absorption capacity of any anode material (4200 mAh/g) whereas the incumbent anode material in today's Li-ion batteries has a capacity of only 372 mAh/g. The adoption of Si anodes however, is limited by the fact that bulk Si pulverizes due to volume expansion during cycling. Nanostructured Si does not pulverize, however, the approaches to make nanostructured Si anodes are either expensive, or not suitable for mass manufacturing, or limited by specific capacity. The proposed method is estimated to be low cost, scalable for manufacturing, and does not suffer from any of the technical limitations of current technologies used to fabricate anodes based on nanostructured Si. The researchers also propose to optimize the nanowire morphology and architecture for maximizing the anode capacity and enabling it to withstand more than 1,000 cycles with sufficient capacity retention. Furthermore, the researchers propose to integrate an ionic liquid electrolyte with the Si nanowire anode, which is expected to show high voltage stability.
The broader impact / commercial potential of this project is truly transformational. This research will lead to a better understanding of electrochemical reactions in cells utilizing Si nanowire anodes. The novel approach to fabricate the anode may find use in other technologies such as in thermoelectric devices, gas / chemical sensors, and biomedical devices. The commercial impact of this low-cost technology is also expected to be highly significant. The batteries with Si nanostructures reported to-date have used expensive methods to create nanowires and current collectors. The researchers envision that their unique and disruptive approach will make ultra-high capacity anodes (~3000 mAh/g) possible on a large scale. The increase in anode capacity will result in an estimated 50%-60% increase in energy density for the total battery pack. Since Li-ion batteries are expected to play an increasingly larger role in consumer device storage as well as in industrial and electric vehicle (EV) applications, this would have a tremendous social and environmental impact. For one, the range that EVs can travel without a charge will be extended, thereby leading to widespread adoption of EVs. The increased battery capacity will also lead to solutions for residential and grid storage wherever renewable energy is used.
Silexta demonstrated the feasibility of developing low cost, high capacity Li-ion battery anodes based on nanostructured silicon during Phase 1 of an NSF-funded SBIR project. The company successfully developed a proprietary method to fabricate low cost silicon nanowire anodes using electrochemical etching followed by direct transfer of silicon nanowire arrays to a current collector foil. Figure 1 shows silicon nanowires fabricated on an n type silicon wafer and then transferred to a metal foil to form the silicon anode. During Phase 1 of this project, the company demonstrated nanowire formation on 156 x 156 mm size silicon wafers. The company also demonstrated anode formation from 100 mm diameter silicon wafers. Coin cells fabricated from these anodes showed over 1000 mAhg-1 capacity retention after 1000 cycles at high charge/discharge rates. It was also shown that further improvements in cell performance are possible with optimization of cycling conditions and with optimization of additives in the electrolyte. Full cells were fabricated with these silicon anodes and testing begun on these. The high capacity cycling results demonstrated in this study show that these silicon anodes have the potential to store over 10x the energy than that stored using the conventional graphite anodes used in today’s lithium-ion batteries. This would enable next generation batteries that would have over 35% more storage capacity than today’s lithium ion batteries. The company’s technology is projected to be low cost and scalable to high volume production. The company also demonstrated that thermal conductivity of these silicon nanowires is almost two orders of magnitude lower than their bulk Si value, indicating their potential usefulness for thermoelectric applications. The technology developed in this project can therefore be used not only for energy storage applications but also for power generation applications using low cost thermoelectric devices.
