Inverse Vulcanized Sulfur Cathode for Low Cost, High Energy, and Long Lasting EV Battery

As transportation systems rely more on electrical power, specific energy of batteries must increase and their cost must decrease. Moreover, current high energy density battery technology heavily relies on critical minerals (e.g. nickel and cobalt) and cell production from foreign countries, especially China. Lithium-sulfur (Li-S) batteries are a promising cell chemistry that uses low cost materials, none of which are rare earth metals. They have a theoretical specific energy of 2600 Wh/kg, with demonstrated specific capacity between >300 Wh/kg. However, Li-S batteries suffer from rapid capacity fade due to ôpolysulfide shuttleö and irreversible deposition of solid lithium sulfides (Li2S or Li2S2). Functionalizing sulfur to polymers such as sulfurized polyacrylonitrile (SPAN) has been show to increase cycle life to several hundreds of cycles by anchoring polysulfides and forming reversible Li2S. While these polymers have high sulfur utilization (>1000 mAh/g), they have low overall sulfur concentration (typically 30-50%), which drops the energy density, typically < 250 Wh/kg. A recent discovery in 2013 demonstrated that organic crosslinkers are able to stabilize and polymerize molten sulfur chains via inverse vulcanization. These sulfur polymers utilize vinylic crosslinker that covalently bond sulfur together and can therefore have extremely high sulfur concentration (e.g. 70-90%). Moreover, by covalently crosslinking the sulfur, polysulfides are chemically bound in the cathode preventing them from migrating and attacking the anode. Therefore, Li-S cell utilizing sulfur polymers can have extremely high energy density and long- term cycling. However, sulfur polymers are still in their infancy and the knowledge of their types, structure, charge/discharge mechanism and properties are still deficient. Additionally, this technology has not be scaled and requires additional support to transition into commercial use. The proposed project will develop a lithium sulfur cell chemistry which will have higher energy density and lower cost compared to current high nickel cells. Moreover, utilizing sulfur as the cathode active material, which is an under-utilized byproduct of gas and petroleum refineries, will help reduce the United Statesĺ dependency on critical minerals from china. The lithium sulfur cell technology will leverage a novel inverse volcanized sulfur cathode active material developed. In Phase I, the sulfur polymer chemistry will be refined in double layer pouch cells to improve performance. In Phase II, the technology will be scaled to full size pouch cells. Additionally, a transition path will be developed to scale production using semi-automated pilot production equipment, including a 1000 ft2 dry room. Cornerstone Research Group (CRG) proposes to develop a novel sulfur polymer cell chemistry which will have higher energy density (>250 Wh/kg), lower cost compared to next generation lithium ion cells (~30%), and long cycle life (>1000 cycles). CRG will accomplish this goal by developing a unique sulfur polymer chemistry. In Phase I, CRG examined several promising sulfur polymer chemistries, one of which increased cycle life by 300% and increase the sulfur specific capacity to >1000 mAH/g. Moreover, CRG demonstrated they could functionally modify the sulfur polymer to include heteroatom oxygen groups that intermolecularly attracted and sequester polysulfides from the electrolyte. CRG demonstrated this work with a >500 mAh/g Li-S pouch cell prototype. In Phase II CRG will optimizing the inverse vulcanization process and sulfur polymer chemistry using a three-prong approach (computational, analytical, and experimental). CRG will to create a quantum-mechanical atomistic simulation that models and calculates the binding energy of the polysulfides. CRG will scale the fabrication and production of the cell and demonstrate its performance in moderately sized pouch cells (>10 Ah). The developed Li- S cell chemistry will provide a low cost, high energy density, and high cycle life cell for not only electric vehicles, but also man portable power, grid storage, and UAVs. Moreover, this cell chemistry will help reduce the USĺs dependency on critical minerals from China (graphite and rare earth metals) .