Cathode Design for High Energy Molten Salt Lithium-Oxygen Batteries
Authors: Tozier Dylan , Giordani Vincent , Uddin Jasim , Tan Hongjin , McCloskey Bryan D , Greer Julia R , Chase Gregory V , Addison Dan 
Published: February 2020

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support.

While the lithium-oxygen battery is promising with regard to its energy density, there are many practical challenges that remain to be solved. For instance, traditional organic electrolytes decompose in the presence of superoxide anions, intermediates in the growth of the lithium peroxide discharge product. However, by replacing the organic electrolyte with a molten salt, we can inherently avoid this organic electrolyte decomposition. In addition, the use of a molten salt electrolyte results in a system operating at elevated temperature with a large concentration of lithium ions, encouraging faster diffusion and kinetics. The morphology of the lithium peroxide grown in this molten salt lithium-oxygen battery is notably different from that previously observed in literature. While previous works have observed thin films, platelets, and “toroids” on the order of several hundred nanometers, we observe much larger (several micron) structures which appear to be stacks of hexagonal layers. We believe these stacks to be a new morphology of lithium peroxide growth. These new lithium peroxide morphologies are characterized with SEM, EDS, and XRD.

In addition to the lithium-oxygen system described above, we introduce another phase-forming chemistry, whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts have been previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate converts to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of large (several micron) lithium oxide crystals in this system, as substantiated by SEM, XRD, and TEM.

After introducing these molten salt lithium batteries, we address the effect of cathode geometry on the discharge capacity. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. For instance, it is not just the surface area of the catalyst support that determines the discharge capacity, but also the amount of usable pore volume to allow this solid phase discharge product to form. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area. Such cathode design principles could be extended to other phase-forming chemistries.