Chemical transformation research is focused on replacing conventional intercalation of the working ion at the cathode with a true chemical reaction. On the basis of techno-economic modeling of the possible battery systems, we have chosen to focus our efforts on a battery with a lithium metal anode and sulfur cathode.
Although the theoretical energy densities of the active materials alone are exceptional for lithium-sulfur batteries, there are significant challenges in reversing the downhill discharge reaction, which can contain up to six or eight separate reaction steps for lithium and sulfur. Also, the discharge products are notoriously unstable and often insulating, precluding investigation by conventional electrochemistry.
Chemical transformation presents special problems for the electrolyte as well, because the electrolyte must not react with any of the many intermediate reaction products, which would remove active material and rapidly degrade battery capacity.
Techno-economic modeling indicated the need for high sulfur concentration compared to the volume of electrolyte in the lithium-sulfur system (a “lean” electrolyte) to achieve JCESR cost and energy storage targets. Reducing the electrolyte percentage from the commonly employed value of 90% to the desired leaner value of 50% will require new methods for preventing unwanted side reactions of the electrolyte that prematurely consume it while ensuring that it retains its ability to transport lithium ions between anode and cathode. Several promising methods are under investigation.