Development of Redox Mediators for High-Energy-Density and High-Efficiency Lithium-Oxygen Batteries

Encountering energy and environmental issues such as the heavy use of fossil fuel and relating pollution problem, human society necessitates the development of sustainable energy resources such as solar and wind energy. In this regard, energy storage systems to store electricity is a key technology to realize renewable future energy society. After continued development during the last decades, Li ion battery is currently widely used as the most promising energy storage system. However, the further development of Li ion battery’s energy density is limited by its redox chemistry exploiting heavy transition metal as redox center, which triggers the extensive research efforts to develop next-generation energy storage systems. Among them, Li–O₂ battery is considered the most promising candidate owing to its extremely high theoretical energy density. For the development of Li–O₂ battery, exploring catalysts to facilitate its electrochemical reaction is inevitable because of its sluggish cathode reaction. The introduction part provides the basics of Li–O₂ battery and redox mediators that are required to understand the following sections.

In this part, the research to develop novel RM for oxygen reduction reaction (ORR) capable of promoting the solution-phase formation of lithium peroxide (Li₂O₂) will be introduced. The new RM is derived from mimicking the biological redox mediation in cell respiration system, where vitamin K2 mediates the electron transfer from flavin mononucleotide to cytochrome b in the cell membrane. The redox potential of vitamin K2 is shown to satisfy the suitable ORR potential range in aprotic solvent, thereby enabling its functioning as a RM promoting the solution-based discharge. The use of vitamin K2 suppresses the growth of film-like Li₂O₂ even in ether electrolytes, which have been reported to drive surface-based discharge and early passivation of the electrode, thereby boosting the discharge capacity by ~30 times. The similarity of the redox mediation in the biological cell respiration and lithium–oxygen ‘cell’ inspires the exploration of redox active bio-organic molecules for high-performance RMs for lithium–oxygen batteries.

In this part, a comparative kinetic study for several RMs for OER by investigating the RM-assisted charging focusing on the chemical decomposition rate of discharge product and the RM diffusivity in the controlled lithium–oxygen cells. It was revealed that the overall RM kinetics have a positive correlation with the RM redox potential, and, the RM with multi-redox capability can display kinetic properties depending on its oxidation states. Among the investigated RMs, DMPZ²⁺ (5,10-dihydro-5,10-dimethylphenazine) exhibits the highest lithium peroxide decomposition rate, while TEMPO⁺ (2,2,6,6-tetramethyl-1-piperidinyloxy) shows the highest mass diffusion rate. Furthermore, the selection of electrolytes is observed to greatly influence the rate capability of the RM-assisted charge, and thereby be carefully considered.

Although the use of RMs is considered an effective approach to reduce the large overpotential in lithium–oxygen batteries, the mobility of RMs triggering the detrimental shuttle effect hinders the sufficient enhancement of the cyclability. In this part, the research to address shuttle effect by anchoring the RMs in polymer form and simultaneously maintaining charge-carrying property will be introduced. Exploting PTMA (2,2,6,6–tetramethyl–1–piperidinyloxy–4–yl methacrylate) as a model polymer system, it is observed that PTMA has the capability to function as a stationary RM, while preserving the redox activity. Due to the prevention of shuttle effect, the consumption of oxidized RMs or lithium anode degradation was significantly suppressed, and at the same time, the efficiency of Li₂O₂ decomposition by RMs remains remarkably stable, resulting in the remarkable improvement of lithium–oxygen cell performance.

In this thesis, multifaceted research strategy, including (1) exploring new materials, (2) studying fundamental aspect, and (3) addressing practical issues with the aim of developing and designing high performance redox mediator was introduced. This study will found a solid basis for developing catalysts for Li–O₂ battery and contributes to realizing practically feasible Li–O₂ battery. Furthermore, it will provide stong research insight in developing various next-generation energy storage systems that necessitates the development of optimal catalysts.