Redox Mediators: A Solution for Advanced Lithium–Oxygen Batteries

The introduction of redox mediators to lithium–oxygen batteries has resulted in a recent breakthrough in overcoming their limitations, hastening the realization of future battery systems. Redox mediators differ from conventional solid catalysts in that they convert the series of Li2O2-related electrochemical reactions into simple chemical reactions, thus circumventing the issues associated with side reactions. Remarkable improvements in the electrochemical properties such as energy efficiency, power capability, and coulombic efficiency have been lately demonstrated in lithium–oxygen battery performance with the aid of redox mediators. Optimizing catalytic performance and stability of redox mediators in the cell systems are issues remaining to be addressed. Despite the exceptionally large theoretical energy density of lithium–oxygen batteries, their high charging overpotential and poor cycle life are critical limitations preventing their commercialization. To overcome these bottlenecks, redox mediators (i.e., soluble catalysts) that facilitate the electrochemical reaction between lithium and oxygen have attracted tremendous research interest. A wide variety of materials have been reported as promising redox mediators for lithium–oxygen batteries, successfully enhancing energy efficiency and cycle stability. However, their overall performance still requires further improvement. Herein, recent progress on the use of redox mediators for lithium–oxygen batteries are reviewed, with a particular focus on improvements in energy efficiency, power capability, and coulombic efficiency. In addition, the aspects of redox mediators requiring immediate optimization are discussed together with future research directions. Despite the exceptionally large theoretical energy density of lithium–oxygen batteries, their high charging overpotential and poor cycle life are critical limitations preventing their commercialization. To overcome these bottlenecks, redox mediators (i.e., soluble catalysts) that facilitate the electrochemical reaction between lithium and oxygen have attracted tremendous research interest. A wide variety of materials have been reported as promising redox mediators for lithium–oxygen batteries, successfully enhancing energy efficiency and cycle stability. However, their overall performance still requires further improvement. Herein, recent progress on the use of redox mediators for lithium–oxygen batteries are reviewed, with a particular focus on improvements in energy efficiency, power capability, and coulombic efficiency. In addition, the aspects of redox mediators requiring immediate optimization are discussed together with future research directions. a quantitative measure for the tendency to accept electron pairs from donors or electrophilic properties of a solvent, whose counterpart is donor number. the ratio of available discharge capacity to charge capacity. the ratio of the electric permeability of the material to the electric permeability of free space, a parameter of a solvent regarding its capability to dissolve ionized solutes. the percentage ratio of energy recovered during discharge to energy input during charge of batteries. the interaction of a sigma orbital with an adjacent empty or partially filled nonbonding or antibonding σ or π orbital, which results in an increased stability of the molecule. an electron transfer during which electron donor and acceptor are chemically bonded (e.g., absorption, ligand bridging). The behavior of chemical bonding significantly affects the electron transfer rate. the minimum energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule. a theory to explain the rate of electron transfer reaction between electron donor and electron acceptor. A major aspect is that the electron transfer rate is dependent on the thermodynamic driving force (difference in redox potentials). an electron transfer during which electron donor and acceptor remains separate and intact. The electron transfer rate is dependent on the thermodynamic driving force. a collective term for certain phenomena by which electrode potential deviates from equilibrium potential. a capability of the energy storage system operating at high current rates. a measure of the potential of chemical species to extract electrons (oxidation) or acquire electrons (reduction). In general, redox potential of species is measured against a reference electrode. a technique for measuring a behavior of a local electrochemical reaction occurring at interfaces; liquid/gas, liquid/liquid, and liquid/solid.