Abstract
Li–air or Li–text{O}_2 batteries are a promising energy storage technology due to the potentially high energy density. However, significant challenges related to reversible charge/discharge of these cells need to be solved. The discharge reaction is generally agreed to proceed via two main routes, which may occur simultaneously. These are the surface mechanism, leading to text{Li}_2text{O}_2 product formation as surface films, or the solution mechanism, with solid particles formed in the pore structure of the cathode. A detailed understanding of the reaction mechanisms and the dynamic performance of the electrodes is key to further improvements. Here, we present a mathematical model for the discharge process, based on porous electrode theory, including effects of reactant transport and kinetic limitations, as well as the continuous change of properties due to the formation of reaction products via the solution mechanism and the surface mechanism. The model describes the dynamic change in the ratio of the surface and solution mechanism as a function of growth of film thickness, in line with recent findings. The model is able to predict the differences in experimentally obtained discharge curves between dimethyl sulfoxide and tetra ethylene glycol dimethyl ether solvents with 1M LiTFSI, with a minimum of free parameters. The model parameters are based on physical characterization of the materials and the electrodes, or determined by fitting to impedance spectra recorded during the discharge. The developed model and the methodology will provide a powerful tool for optimization of such electrodes.Graphic abstract
Highlights
A battery system which has attracted much attention as a possible alternative for energy storage is the lithium–air (Li–air) battery
One of the main challenges for a successful development of a practical Li–air battery is the search for an electrolyte that is stable in the reactive environment of the cathode with simultaneous support of the electrochemical reactions
Good progress has been made in the understanding of the discharge reaction mechanism of these batteries, and the role of the electrolyte; see i.e. Ref. [2]—for a comprehensive review
Summary
A battery system which has attracted much attention as a possible alternative for energy storage is the lithium–air (Li–air) battery. Li2O2 is formed via two main reaction pathways: the surface-based mechanism forming a film on the electrode surface and the solution-based mechanism forming crystalline toroids in the electrolyte, which are attached to the electrode surface [9]. The product formation causes a gradually increasing overpotential and local current density, which again leads to a shift in the reaction mechanism towards the surface pathway, and eventually passivation due to blocking of the active surface. We have developed a continuum type model for the Li–air cathode, in order to describe the discharge reaction, incorporating both mechanisms (i.e. the surface and the solution pathway) as simultaneously occurring processes in the electrode. An escape parameter was introduced, taken to be a function of the resulting film thickness, in line with recent findings in the field This allowed for a dynamic description of the discharge process. A good agreement with experimentally obtained discharge curves for LiTFSI/DMSO and LiTFSI/ TEGDME electrolytes could be achieved with a minimum of free parameters
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