Changing the Cathode Microstructure to Improve the Capacity of Li-Air Batteries: Theoretical Predictions

Abstract

The transport equations in Li-air batteries are revisited and modified to account for different pore microstructures, pore size distribution effects and electron transport through the discharge product. Different material microstructures are analyzed including structures made of spherical and cylindrical pores, nanoparticles, carbon nanotubes and nanofibers, and a new hybrid model is proposed to describe the deposition of discharge product in the cathode. It is shown that although the different microstructures result in different dynamics in which the pores are being filled, they lead to relatively similar values of the energy and power densities. Microstructures based on carbon nanotubes, nanofibers, and spherical particles tend to increase the effective size of the fibers and particles during the first part of the discharge process and result in a slight increase of the cell voltage at the beginning of the discharge. The power density of Li-air batteries decreases with the variance of the pore size distribution function, while the capacity shows a slight increase with the variance of this distribution. In general, the resistivity of the deposit layer (e.g. Li 2 O 2 , Li 2 O, etc.) has a negative effect on the performance of Li-air batteries. However, it is shown that, as long as this resistivity is not too large, the deposit layer tends to increase the capacity of the battery by approximately 10%. This rather unexpected phenomenon is attributed to the fact that the voltage drop across the deposit layer reduces the reaction rate at the air side of the cathode and, in this way, delays the formation of the deposit product in this region. Growing greenhouse gas emissions and degrading air quality due to electric power generation by fossil fuels have resulted in a shift in interest toward less-polluting and sustainable energy sources such as wind and solar energy.1 Heat-trapping greenhouse gasses resulting from human activity, such as burning fossil fuels, contribute to climate change around the world including rising sea levels, above average temperatures, heavy precipitations and droughts in certain regions, dwindling ecosystems, and others.2-4 There is, therefore, an increasing pressure on governments and industry worldwide to move to safer, environmentally friendly, renewable sources. The penetration of renewable energy sources in electric grids is however expected to produce large power fluctuations that are detrimental to the security and safety of the grid. To smooth out these power fluctuations it is important to utilize storage systems that are charging when there is less demand for power in the grid and discharging during high-demand periods. Among the existing energy storage systems, electrochemical systems based on metal-air batteries can potentially provide compact, high energy density, and environmentally friendly energy storage devices for the electric grid.5 In addition to grid applications, metal-air systems also have great potential to be used in the electric vehicle industry. The theoretical spec...

Boosting Rechargeable Batteries R&D by Multiscale Modeling: Myth or Reality?

“…The model highlighted the importance of designing electrodes having an appropriate compromise between high surface area (small pore size) and large pore radius (slow degradation rate of active surface) (Figure ). The model results and conclusions were later on reproduced by other authors. , The authors proposed an extension of the model accounting for both the formation of Li 2 O 2 as a thin film (surface limited reaction mechanism) and the formation of Li 2 O 2 in the solution phase (solution phase reaction mechanism) . The relative impact of the two mechanisms on the overall discharge performance was captured through an escape function, quantifying the extent to which the reaction takes place in the solution phase.…”

Metal–Air Batteries: From Static to Flow System

“…Modeling investigations on batteries can provide in-depth information about the mass transfer properties and the electrochemical reactions during the discharge and charge processes. For Li-air static batteries, the existing theoretical modeling including continuum modelling techniques, [194] ab-initio computation methods, [195] and compact (analytical) models [196] are employed to study the properties and reaction dynamics in the optimization of the power and energy density of Li-air static batteries. For instance, a diffusion-limited model for Li-air static batteries using continuum methods was developed based on abundant experimental results, [197,198] in which many factors such as oxygen diffusion in an organic electrolyte, variation in cathode pore radius and Tafel reaction kinetics were considered.…”

Multistep Reaction Mechanisms in Nonaqueous Lithium–Oxygen Batteries with Redox Mediator: A Model-Based Study

“…Different redox mediators have been successfully applied, such as tetrathiafulvalene (TTF), 26 lithium iodide (LiI), 27 lithium bromide (LiBr), 28,29 iron phthalocyanine (FePc), 30 and nitroxides 31 like 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO). 12 Recent modeling studies on Li/O 2 cells focus mainly on the discharge mechanism, for example, the rate-dependent formation of the Li 2 O 2 , 32 precipitation mechanisms, 33 influence of selected parameters on the cathode design, 34 development of cell design concepts, 35 influence of cathode microstructure and pore size distribution on discharge capacity and power, 36 analytical prediction of the discharge voltage characteristics, 37 or a reaction pathway analysis of surface and solution-mediated reaction. 38 The charge mechanism has, to the best of our knowledge, only been the subject of very few modeling and simulation studies 39,40 lacking detailed investigations on the reaction mechanism.…”