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

Abstract

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). Figure 1