In this paper we present a mesoscopic model of the transport and electrochemical processes inside a Lithium-O 2 battery cathode pore. The model dynamically resolves both Oxygen Reduction Reaction (ORR) thin film and solution phase mechanisms together with the transport of O 2 , Li + and LiO 2 in the electrolyte. It is supported on an extension to three dimensions of our Kinetic Monte Carlo (KMC) Electrochemical Variable Step Size Method (E-VSSM) recently published by our group in [M. A. Quiroga and A. A. Franco, J. Electrochem. Soc., 162, E73 (2015)]. The model allows predicting porosity evolution as a function of multiple operational, physical and geometrical parameters including the pore size and inlet/outlet channel size, O 2 and Li + concentration, the property of the solvent as well as the applied overpotential. The investigation of the impact of these different aspects reveals that at the mesoscale level, the overall ORR kinetics and the discharge mechanism strongly depend on a balance between the geometrical features of the pore and the transport as well as the electrochemical properties of the system. The escalating demand for energy and the depletion of fossil fuel resources create an urge to find alternative methods to convert the available energy on Earth into useful energy. Because of their ecofriendly character, renewable energy devices undergo a tremendous development.1-3 However, the intermittent nature of renewable energy harvest as well as the hourly fluctuation of energy consumption highlight the importance of developing advanced energy storage systems.4,5 Lithium Ion Batteries (LIBs) have already dominated the market of electronics, but for other applications such as electric vehicles, their further enhancement in energy density is still requested.The Li-O 2 battery, especially the non-aqueous type, attracted much attention in the last decade due to its high theoretical specific energy density. In spite of tremendous efforts, the performance of nonaqueous Li-O 2 batteries in the state-of-art is still far from expectations in view of its unsatisfactory discharge capacity, high overpotential and severe parasitic reactions. All the above deficiencies are partially, if not all, due to the insulating and insoluble nature of Li 2 O 2 formed during discharge.Johnson et al. 6 reported a two-step discharge process with a dual mechanism as shown in the following reactions:where the subscript "sol" stands for solution phase, while the star sign stands for species that are adsorbed on the electrode surface .The first step is the reduction of O 2 with the existence of Li + to form LiO 2 ion pairs (Reaction 1). Then, the LiO 2 ion pairs can either move into the electrolyte and be disproportionated, generating Li 2 O 2 of toroidal shape and O 2 (Reaction 2a), or they can stay on the electrode surface and be reduced electrochemically, forming a passivation layer (Reaction 2b). The former path is the "solution phase mechanism" but the origin of the LiO 2 solubility, whether from the high donor number solvent ...
We investigated the lithium peroxide (Li 2 O 2 ) and pore size distribution in lithium−O 2 battery electrodes at different states of charge using transmission X-ray microscopy coupled with Zernike phase contrast to carry out nanocomputed tomography. We report that such a technique enables us, at the nanoscale, to distinguish light elements such as carbon and Li 2 O 2 in Li− O 2 battery cathode electrodes. We verified by wave-propagation simulation that this approach efficiently improves the contrast of images in comparison with pure absorption. The Li 2 O 2 distribution and thickness, interphases, and pore network are visualized and quantified, giving a valuable insight into our cathode architecture. From this 3D analysis, we highlight modifications of the air-cathode morphology and the Li 2 O 2 spatial organization as well as their potential implication in terms of carbon surface passivation and pore-clogging. After the full recharge process, this technique can also reveal the spatial distribution of the residual Li 2 O 2 and other byproducts.
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