Lithium-air batteries have the potential to outperform conventional lithium-ion batteries by storing significantly more energy at the same weight. However, their high performance has remained theoretical, and their lifespan too short. Now, a Chinese team has proposed adding a soluble catalyst to the electrolyte. This acts as an oxidation-reduction mediator, facilitating charge transport and counteracting electrode passivation.
Unlike lithium-ion batteries, where lithium ions “move” between two electrodes, lithium-air batteries (Li-O2) use a lithium metal anode. During battery operation, positively charged lithium ions dissolve and move to a porous cathode, through which air flows. Oxygen is oxidized and bonds with lithium to form lithium peroxide (Li2O2). During charging, oxygen is released, and lithium ions are reduced back to lithium metal, which settles back on the anode. Unfortunately, the theoretically high performance of these batteries has yet to materialize.
In practice, an effect known as overvoltage slows down the electrochemical reactions: the formation and breakdown of insoluble Li2O2 are slow, and its conductivity is also very low. Additionally, the pores of the cathode tend to clog, and the high potential needed for oxygen formation decomposes the electrolyte and promotes unwanted side reactions. As a result, batteries lose much of their performance after just a few charge/discharge cycles.
The team led by Zhong-Shuai Wu from the Dalian Institute of Chemical Physics CAS, in collaboration with Xiangkun Ma from Dalian Maritime University, proposed adding a new iodide salt of imidazole (1,3-dimethylimidazolium iodide, DMII) to act as a catalyst and oxidation-reduction mediator to improve performance and lifespan of the batteries.
Iodide ions (I−) in the salt can easily react with the I3− form and then return back (oxidation-reduction pair). In this process, they transfer electrons to oxygen (during discharge) and take them back (during charging). This facilitates charge transport, accelerates reactions, reduces cathode overvoltage, and increases the discharge capacity of the electrochemical cell. The DMI salt ions contain a ring made of three carbon atoms and two nitrogen atoms.
This ring has freely moving electrons and can “capture” lithium ions during discharge and effectively transfer them to oxygen at the cathode. Additionally, DMI ions form an ultra-thin but very stable interface film on the anode that prevents direct contact of the electrolyte with the lithium surface, minimizing electrolyte decomposition and preventing side reactions. This stabilizes the anode and extends the battery’s lifespan.
The electrochemical test cells made by the team showed very good results, demonstrating a very low overvoltage (0.52 V), high cycle stability for 960 hours, and highly reversible formation/decomposition of Li2O2 without side reactions.