Consider a similar phase-forming conversion chemistry[1], 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 reduces 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 this molten salt lithium battery, we address the effect of cathode geometry on the electrochemical performance of this model system. 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 which can accommodate this solid phase discharge product. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area, comparing it with the more typical geometries of thin films and nanoparticles. These electrode design principles can be extended to other phase-forming conversion chemistries.
[1] Addison, D.; Bryantsev, V.; Chase, G. V.; Giordani, V.; Uddin, J.; Walker, W Rechargeable Batteries Employing Catalyzed Molten Nitrate Positive Electrodes. US Patent 2016/0204418, August 8, 2013.