|Proposed catalytic cycle for the electrochemical charging of Li-O2 cells with TEMPO. Credit: ACS, Bergner et al.Click to enlarge.|
One of the major challenges with the realization of commercial Li-air batteries and their promise of ultra-high energy densities is the reduction of the high charge overpotential. The high potential gap leads to a low round-trip efficiency of the cell and causes electrochemical decomposition of other cell constituents. (Earlier post.)
In a new paper in the Journal of the American Chemical Society a team from Justus-Liebig-Universität Gießen (JLU) in Germany reports that adding the oxidation catalyst TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), homogeneously dissolved in the electrolyte to function as a mobile redox mediator, provides a distinct reduction of the charging potentials by 500 mV. Moreover, adding TEMPO enabled a significant enhancement of the cycling stability leading to a doubling of the cycle life (from 27 to 55).
Li−O2 batteries are based on the oxidation of lithium at the (lithium) metal electrode and reduction of oxygen at the air electrode to induce current flow. During discharge, lithium peroxide (Li2O2) is formed by the reduction of oxygen at around 2.7 V (vs Li+/Li); however, its electrochemical decomposition during the charge process requires potentials up to 4.5 V, leading to low round-trip efficiency.
Further, the carbon cathode has been identified as a center of parasitic reactions during charging, including the oxidative decomposition of the cathode at high potentials. The blocking of the surface by decomposition products is seen as as the major reason for the poor cycle life of Li-O2 batteries. Thus, reducing the overvoltage upon charging is a key step in the further development of Li-O2 batteries, for both higher efficiency but also better cycling stability.
Various approaches have been pursued, including the use of heterogeneous catalysts like metal nanoparticles, metal nitrides, and also various classes of metal oxides. Researchers at the Ohio State University reported developed a novel strategy of integrating a dye-sensitized photoelectrode into a lithium-oxygen battery along with the oxygen electrode to enable “photo-assisted charging” of the Li-air cell.
The basic concept of their integrated solar battery is to use the contribution of the photovoltage to reduce greatly the charging overpotential caused by the difficulty in efficiently electrochemically decomposing lithium peroxide (Li2O2). (Earlier post.)
An alternative approach comprises dissolved redox mediators (RM), which act as mobile charge carriers between the electrode surface and Li2O2. The charge transfer is based on the reversible redox pair RM ⇌ RM+ + e−. While heterogeneous catalysts influence the oxygen evolution reaction (OER) only at the limited and rigid contact surface to the depleting Li2O2, dissolved redox mediators provide oxidative attack at the much larger and dynamic interphase between Li2O2 and the liquid electrolyte.
Up to now two possible redox mediators have been investigated extensively in Li-O2 cells, while additional couples were proposed in patent applications. … Herein we propose the chemical class of nitroxides as dissolved redox mediators with very favorable properties for the OER in Li-O2 cells. We investigated TEMPO (2,2,6,6- tetramethylpiperidinyloxyl) exemplarily, and the efficiency might be further improved by using other nitroxide compounds.—Bergner et al.
The team investigated the (electro)chemical stability, redox potential, diffusion coefficient and the influence on the oxygen solubility of adding TEMPO as well as the charging mechanisms of Li-O2 cells with and without TEMPO.
TEMPO exhibits high electrochemical stability, fast diffusion kinetics, an appropriate redox potential and enables a sufficient oxygen solubility. The use of TEMPO in Li-O2 cells leads to a significantly reduced charging voltage and, hence, to a distinctly higher round-trip efficiency. The observed charging plateau is associated with a parallel gas evolution indicating at most a minor contribution of a parasitic shuttle to the anode, which might be overcome by a proper cathode design. However, a parasitic shuttle was identified by changing the ratio of the competitive diffusion paths.
Furthermore, TEMPO provides a significantly enhanced cycle life primarily based on the reduced charging voltages. The catalytic activity of TEMPO allows a wide range of current densities and different carbon based cathode materials. Since TEMPO is only one typical representative for the chemical class of nitroxides, modification of the chemical substituents may lead to a further decrease of the charging potential and to an enhancement of the efficiency.—Bergner et al.
Benjamin J. Bergner, Adrian Schürmann, Klaus Peppler, Arnd Garsuch, and Jürgen Janek (2014) “TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries” Journal of the American Chemical Society doi: 10.1021/ja508400m