Extensive research efforts on metal-air batteries are being carried out due to their high theoretical energy density. Specifically, the lithium-oxygen (Li/O₂) based batteries have drawn attention of the researchers as a promising alternative to Li-ion batteries for long range electric vehicles. However, this system suffers from high overpotentials during charge, indicating asymmetrical charge/discharge reaction mechanisms. High charge overpotentials lower the energy efficiency and can increase potential-driven side reactions (e.g., electrolyte decomposition). Furthermore, the discharge product forms as a new solid phase in the cathode by a nucleation and growth process, which is in general different from lithium-ion battery technologies. Thus, a detailed understanding of the reaction mechanism is crucial for enhancing battery performance and durability.

Our approach towards understanding the charge/discharge reaction mechanism is by development and exploitation of a continuum model of a lithium-oxygen cell [1]. 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. We present a multi-step reaction mechanism of the electrochemistry in the cathode, consisting of O₂ dissolution, O₂ reduction, LiO₂ precipitation, LiO₂ disproportionation during discharge (Figure 1, left), as well as Li₂O₂ electrostripping during charge. In particular, the mechanism is asymmetric with respect to discharge and charge. The model is parameterized by experimental Swagelok cells [2] and galvanostatic cycling simulations are compared to experimental data. Furthermore, possible reasons for the high charge overpotential are analyzed, options to reduce the charge overpotential (e.g. redox mediator) are discussed, and a charge reaction mechanism including a surface-mediated Li₂O₂ dissolution is proposed (Figure 1, right). 

[1] D. Grübl, B. J. Bergner, J. Janek and W. G. Bessler, “Dynamic Modeling of the Reaction Mechanism in a Li/O₂Cell: Influence of a Redox Mediator,” ECS Transactions 69(19), 11-21 (2015).

[2] 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).