|A lithium-sulfur cell. Source: Sion Power. Click to enlarge.|
Sion Power Corporation has received a three-year, $800,000 research grant from the US Department of Energy (DOE) to support Sion’s ongoing work to develop a new class of electrolytes used in lithium sulfur (Li-S) batteries for electric vehicle (EV) applications. Sion Power will provide matching funds for this three-year effort.
The project objective is to increase performance of very-high-energy lithium metal anodes used in rechargeable battery systems. Sion Power will complete development of its unique electrolyte system employing multiple components. While improving lithium conductivity, one component will be optimized to enhance metallic lithium anode performance; another will enhance cathode functionality.
|Ragone plot of current Sion Power cells and other chemistries. Sion believes that its current development work will push the specific energy to 550 Wh/kg. Source: Sion Power. Click to enlarge.|
The multi-component electrolyte system will enable Sion Power to improve chemical stability leading to improved safety and abuse tolerance.
In May Sion Power and BASF SE signed a Joint Development Agreement (JDA) to accelerate the commercialization of Sion Power’s proprietary Li-S battery technology for the electric vehicle (EV) market and other high-energy applications. (Earlier post.)
Lithium-sulfur batteries. Lithium Sulfur batteries (LSBs) use a lithium metal anode and a soluble polysulfide cathode. Lithium ions are stripped from the anode during discharge and form lithium polysulfides in the cathode. Li2S in the cathode is the result of complete discharge. On recharge, the lithium ions are plated back onto the anode as the lithium polysulfides in the cathode move towards S8. High order Li-polysulfides (Li2S3 to Li2S8) are soluble in the electrolyte and migrate to the anode, scrubbing off any dendrite growth.
The theoretical specific energy of a lithium-sulfur battery chemistry is in excess of 2,500 Wh/kg with a theoretical energy density of 2,600 Wh/L.
|Current status of Li-S development compared to USABC baselines. Source: Sion Power. Click to enlarge.|
However, LSBs have a number of issues, including cycle life and operation at higher temperatures. Among the limiting mechanisms, according to Sion, are the rough lithium surface on the anode during cycling and Li/electrolyte depletion. Lithium roughness leads to generation of porous “mossy” Li deposits, absorption of electrolyte by porous deposits and premature Li anode disintegration. Li/electrolyte depletion leads to loss of the solvent necessary for proper functioning of the cathode. The products of these Li-solvent reactions also increase cell impedance and the rate of capacity fade.
Sion’s collaboration with BASF is pursuing solutions to those issues, including a proprietary anode design to reduce lithium roughness; development of structurally sable cathodes; and new materials for multi-functional membrane assemblies for the physical protection of lithium.
Sion Power’s Li-S technology already provides rechargeable cells with a specific energy of more than 350 Wh/kg, which is 50% greater than the currently commercially available rechargeable battery technologies.
The reduction of lithium surface roughness with new anode design, and better cathode structure demonstrate shows a substantially increased sulfur utilization to more than 1.45 Ah/g of specific capacity. With this improved specific capacity and reduced cell mass (resulting from the utilization of thin membrane protection) and, with improved cell design, Sion believes that the energy density of the Li-S cell can be increased from the present value of 350 Wh/kg to 550 Wh/kg.
The new approach is also resulting in a recharge time reduced to less than 3 hours, and a substantial cycle life increase. It is also eliminating the potential for thermal runaway, according to Sion.
High Energy Rechargeable Li-S Cells for EV Application. Status, Challenges and Solutions. (Sion Power presentation at ECS)
High Energy Rechargeable Li-S Cells for EV Application. Status, Remaining Problems and Solutions. (Abstract)
V. S. Kolosnitsyn and E. V. Karaseva (2008) Lithium-sulfur batteries: Problems and solutions. Russian Journal of Electrochemistry Volume 44, Number 5, pp 506-509 doi: 10.1134/S1023193508050029