A group of researchers headed by Zhong-Shuai Wu from the Dalian Institute of Chemical Physics of CAS, working with Xiangkun Ma from the Dalian Maritime University, has suggested adding a new imidazole iodide salt (1,3-dimethylimidazolium iodide, DMII) to improve the lifespan and performance of Lithium-air batteries by acting as a redox mediator and catalyst. The journal Angewandte Chemie International Edition published the study.
Image Credit: Angewandte Chemie
Lithium-air batteries have the potential to surpass conventional lithium-ion batteries by storing significantly more energy at the same weight. However, their high-performance potential has remained largely theoretical, and their lifespan is still too short.
The research team has suggested adding a soluble catalyst to the electrolyte. This catalyst is a redox mediator, enhancing charge transport and preventing electrode passivation.
Lithium-air batteries (Li-O2) employ an anode composed of metallic lithium instead of lithium-ion batteries, which use lithium ions that are “pushed” back and forth between two electrodes. Positively charged lithium ions dissolve during battery operation and migrate to the porous cathode, which is exposed to air.
Lithium peroxide (Li2O2) is formed when oxygen is oxidized. The oxygen is expelled during charging, and the lithium ions undergo reduction to metallic lithium, which subsequently re-deposits on the anode. Such batteries' high performance, in theory, has not materialized.
An overpotential effect slows down electrochemical reactions, resulting in slow formation and breakdown of insoluble Li2O2 and extremely low conductivity. Furthermore, the cathode's pores tend to clog, and the high potential needed to form oxygen breaks down the electrolyte and encourages unwanted side reactions. As a result, the batteries lose most of their performance after just a few cycles of charging and discharging.
It is easy for the salt's iodide ions (I−) to react to form I3− and then back again (redox pair). They discharge electrons to oxygen during this process and then recharge them. This enhanced charge transport speeds up the reactions, lowers the cathode's overpotential, and expands the electrochemical cell's discharge capacity. The salt's DMI+ ions have a ring composed of two nitrogen and three carbon atoms.
Due to its freely moving electrons, this ring can “capture” lithium ions during discharge and efficiently move them to the cathode's oxygen. Furthermore, by preventing direct contact between the electrolyte and the lithium surface, the DMI+ ions create an extremely thin but stable interface film on the anode, reducing electrolyte breakdown and averting adverse reactions. This prolongs the battery's life and stabilizes the anode.
The team's electrochemical test cells were extremely promising, with a very low overpotential (0.52 V), high cycle stability over 960 hours, and highly reversible formation/decomposition of Li2O2 with no side reactions.
Journal Reference:
Liu, J., et al. (2024) A Bifunctional Imidazolyl Iodide Mediator of Electrolyte Boosts Cathode Kinetics and Anode Stability Towards Low Overpotential and Long‐Life Li‐O2 Batteries. Angewandte Chemie International Edition. doi.org/10.1002/anie.202421107.