Dynamic Modeling of the Reaction Mechanism in a Li/O2 Cell: Influence of a Redox Mediator

Interest and research in metal-air batteries is ongoing on a high level due to their high theoretical energy density and potential low-cost materials. Most research activities are performed in the field of lithium-oxygen chemistry with organic electrolytes. This system suffers high overpotentials during charge, indicating asymmetrical charge/discharge reaction mechanisms. High charge overpotentials low the energy efficiency and can increase potential-driven side reactions (e.g., electrolyte decomposition). In order to reduce charge overpotentials, the use of redox mediators has been proposed. Different redox mediators have been demonstrated, such as, TTF [1], LiI [2] and FePc [3]. Bergner et al. introduced 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) as redox mediator [4], which will be further investigated in the present study.

In order to further understand the mechanism of redox mediators, we present a continuum modeling study of a lithium-oxygen cell. The one-dimensional computational domain consists of a gas reservoir filled with oxygen, a porous carbon cathode flooded with an organic electrolyte, a porous separator, and a lithium metal anode. The model includes a detailed description of the electrochemistry in the cathode. During discharge, gaseous oxygen dissolves at the interface between the closed gas reservoir and the cathode (O₂,gas ⇄ O₂,dissolved). Following, a multi-step mechanism is implemented involving lithium superoxide as intermediate (Li⁺ + O₂ + e^– ⇄ LiO₂), followed by the formation Li₂O₂ by chemical disproportion of LiO₂ (2LiO₂ ⇄ Li₂O₂ + O₂). In the anode the metallic lithium reacts to lithium ions (Li ⇄ Li⁺ + e^–). This mechanism is complemented by charge-transfer of the TEMPO redox mediator (TEMPO ⇄ TEMPO⁺ + e^–), allowing for direct reduction of the lithium peroxide upon charge (Li₂O₂ + 2 TEMPO⁺ ⇄ 2 Li⁺ + O₂+ TEMPO). The model is parameterized using experimental data [1]. Galvanostatic discharge and charge simulations reveal the advantage of using a redox-mediator. The electrochemical reaction mechanism and the crystal growth of the sodium superoxide within the cathode is quantified.

Figure 1: Discharge (left) and charge mechanism involving a redox-mediator (right) in a Li-O₂ cathode.

[1] Y. Chen, S. A. Freunberger, Z. Peng, O. Fontaine, P. Bruce, Nature Chemistry 5, 489 (2013).

[2] H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J. Hong, H. Kim, T. Kim, Y. H. Kim, X. Lepró, R. Ovalle-Robles, R. H. Baughman, and K. Kang, Angew. Chem. 126, 4007–4012 (2014).

[3] D. Sun, Y. Shen, W. Zhang, L. Yu, Z. Yi, W. Yin, D. Wang, Y. Huang, J. Wang, D. Wang, and J. B. Goodenough, J. Am. Chem. Soc. 136, 8941–8946 (2014).

[4] B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch, and J. Janek, “TEMPO: A Mobile Catalyst for Rechargeable Li-O₂ Batteries,” J. Am. Chem. Soc. 136, 15054–15064 (2014).