Cathode Design for a Molten Salt Lithium-Oxygen Battery

The rechargeable lithium-oxygen battery has attracted attention due to its large theoretical energy density compared to modern lithium-ion batteries. This large energy density is attributed to the reaction of lithium with molecular oxygen to form lithium peroxide, which grows on the surface of the cathode. While this is a promising chemistry, there are many practical challenges that remain to be solved, such as the decomposition of organic electrolyte in the presence of superoxide anions and large overpotentials on charge. Additionally, the mechanism of lithium peroxide growth and its resulting morphologies is not fully understood.

Here we propose a system which inherently avoids many of the issues associated with organic electrolyte decomposition, while also forming lithium peroxide with a unique morphology. By using a molten salt (Li/K nitrate) in place of a conventional solvent/salt electrolyte, solvent decomposition is obviated. In addition, the elevated temperature of the molten salt as well as the large concentration of lithium ions encourage faster diffusion and kinetics.

In literature, the three commonly observed morphologies of electrochemically grown lithium peroxide are thin films, platelets, and “toroids” which are small stacks of platelets. While we do observe the platelet style growth of lithium peroxide, we also see much larger structures which appear to be stacks of hexagonal layers (see attached figure). We believe these stacks to be a new morphology of lithium peroxide growth. To substantiate this claim, we note that the Wulff construction for lithium peroxide is a short hexagonal prism, while also confirming the reaction product using XRD. This new morphology could be attributed to the fact that our cell operates with elevated temperature and large concentration of lithium and superoxide ions, making it easier to achieve an equilibrium (Wulff) structure.

We have shown a lithium-oxygen battery chemistry that produces a new morphology of lithium peroxide, and begun to develop a mechanism for why it forms. In addition, we have explored the effect of various cell parameters such as discharge rate on the morphology of the resulting lithium peroxide.