Molten Lithium Battery for Space Applications
Authors: Estevez Luis , Gogia Ashish , Kumar Jitendra 
Published: May 2020

Solid-state electrolytes (SEs) have gained much interest lately as a potential for mitigating the various issues inherent in a most energy dense lithium metal (anode) based batteries such as lithium ion, lithium sulfur (Li-S), and lithium oxygen. Notable high energy next generation cathodes such as sulfur have their own issues with a liquid electrolyte based system, where the polysulfides discharge product dissolve into the electrolyte causing both: a loss of cathode active material and an unwanted shuttling of the polysulfides over the polymer separator to the lithium anode limiting their practical use. Moreover, the formation of lithium dendrites in lithium batteries is yet another challenging issue that not only limits battery cycle-life but also poses safety risks. The use of SEs can potentially address most of the issues related to next generation lithium batteries such as Li-S batteries, but getting rid of dendrite issues may not be addressed completely. One way to eliminate the dendrite issue is to use molten lithium as an anode.
The National Aeronautics and Space Administration (NASA) has an ambitious plan to explore one of the most challenging planets in our solar system: Venus. The surface temperature of Venus can reach as high as 465°C and at same time, the Venus atmosphere is filled with corrosive gases such as carbon dioxide, sulfur dioxide, carbon monoxide, etc. A battery for such a challenging space mission can be built only if battery components (especially the electrolyte) is stable beyond 500°C and must be stable with molten lithium and sulfur electrodes for at least hours or days if not months—longer is better. Last but not the least, yet another challenge is containment of gaseous sulfur inside the battery container at such a high temperature. The potential to replace sulfur with selenium may mitigate the issue but at cost of reduced energy density.
Under a NASA funded project, the University of Dayton Research Institute (UDRI) is developing molten lithium sulfur (Li-S) or lithium-selenium (Li-Se) batteries with the help of a high temperature solid ceramic electrolyte (SCE) that can enable a operating temperature >465°C for several hours. UDRI has been working on a high temperature SCE such as phosphate-based Li1+
xAlxGe2-
x(PO4)3 (LAGP) ceramic electrolyte material1 that is stable beyond 500°C and also shows broad electrochemical window (up to 5V) that is sufficient for a molten Li-S or Li-Se battery operation; provided LAGP is stable with Li and S or Se in such harsh conditions and temperature. Unfortunately, LAGP is not stable with Li (forms a highly resistive interface layer between LAGP and electrode) and its stability with sulfur or Se, needs to be determined.
Under the NASA project, UDRI has been developing interface materials between LAGP and electrodes for Li-S and Li-Se cells that can be stable at wide temperature range (room temperature to 500°C). The presentation will include progress on high temperature (up to 465°C) i) SCE, ii) SCE/Li interface materials, iii) SCE/S interface material, iv) SCE/Se interface material, v) impedance and open circuit voltage of Li-S or Li-Se cells at 465°C, and vi) cycle-life of Li-S or Li-Se cells at 465°C. To the authors knowledge, there is no such report (molten Li-S or Li-Se cells that can function at 465°C) that exists in literature.
B. Kumar, D. Thomas, J. Kumar, “Space-Charge-Mediated Superionic Transport in Lithium Ion Conducting Glass–Ceramics,” J. Electrochem. Soc. 156, A506 (2009)