Atomic-Scale Cryo-TEM Studies of the Electrochemistry of Redox Mediator in Li-O₂ Batteries

Rechargeable aprotic lithium (Li)-oxygen battery (LOB) is a potential next-generation energy storage technology because of its high theoretical specific energy. However, the role of redox mediator on the oxide electrochemistry remains unclear. This is partly due to the intrinsic complexity of the battery chemistry and the lack of in-depth studies of oxygen electrodes at the atomic level by reliable techniques. Herein, cryo-transmission electron microscopy (cryo-TEM) is used to study how the redox mediator LiI affects the oxygen electrochemistry in LOBs. It is revealed that with or without LiI in the electrolyte, the discharge products are plate-like LiOH or toroidal Li₂O₂, respectively. The I₂ assists the decomposition of LiOH via the formation of LiIO₃ in the charge process. In addition, a LiI protective layer is formed on the Li anode surface by the shuttle of I₃ ^-, which inhibits the parasitic Li/electrolyte reaction and improves the cycle performance of the LOBs. The LOBs returned to 2e^- oxygen reduction reaction (ORR) to produce Li₂O₂ after the LiI in the electrolyte is consumed. This work provides new insight on the role of redox mediator on the complex electrochemistry in LOBs which may aid the design LOBs for practical applications.