A Simple Model for Interpreting the Reaction–Diffusion Characteristics of Li-Air Batteries

Abstract

With the goal of creating a model of a Li-air battery that is consistent with voltammetry data, we develop a full battery model capable of giving insight into details of cell operation otherwise inaccessible to common experimental techniques. With this model, we investigate the dependence of the current on: the diffusion characteristics of the electrolyte, the solubility of the ambient oxygen, the structure of the cathode, and aspects of the primary surface reaction. We explore modifications to a basic reaction-diffusion model of a full cell that bring better agreement with experimental data, including gas-electrolyte surface limited diffusion and a partially active cathode. We discuss how the basic form of the model and the simulated reaction and concentration profiles affect cell dynamics. Non-aqueous Li-air/Li-O 2 batteries have the highest theoretical energy density of any existing battery technology, 1-3 yet the practical application of these systems is hampered by electrolyte-related phenomena including material degradation and mass transport limited behavior. Ideally, electrolytes for Li-air batteries should exhibit high ionic conductivity, resistance to decomposition, high oxygen solubility and fast oxygen diffusion rates. While many groups have focused on the degradation aspects and a few groups have investigated oxygen mass transport as a function of electrolyte chemistry, 4-8 fewer have deeply explored the impact of oxygen diffusion on cell performance (see Ref. 9 for a recent review of Li-air models). Here we use a computational approach with experimental validation to model a full Li-air battery under transport-limited conditions and extract the influence of reaction-diffusion parameters.In designing Li-air battery systems voltammetry experiments play an important role in providing estimates of the reaction-diffusion properties which influence power density, energy density and cell lifetime. Voltammetry, a category of electroanalytic methods in which potential is varied linearly with time, has become a standard experimental tool to discern thermodynamics, kinetics and transport related to electrochemical processes. Randles and Sevcik derived a widely-used voltammetric correlation in which the peak current is proportional to the square root of the voltage scan rate for a reversible reaction at a flat electrode surface, 10-12 which was subsequently modified for semiirreversible electrochemical processes (e.g. oxygen reduction in Li-air batteries) by Delahay, 13,14 and Nicholson and Shain. 15,16 Despite the ability to estimate the diffusion coefficient with a simple algebraic equation, these semi-empirical correlations are inappropriate for application in devices with complex (i.e. porous) electrode geometries, as in Li-air batteries.Modeling electrochemical processes under practical device conditions, such as in a coin cell, provides insight into phenomena not easily accessible by experimentation. Generally speaking, a number of physical mechanisms are important in a complete battery model such as th...

Strategy to enhance selectivity in a two-step reaction using the R1-R2-D reactions-diffusion phenomena

“…The internal mass transfer has been considered in a vast number of heterogeneous catalysis studies in different applications ranging from bio-applications 1,2 and biomass upgrading 3 to thermal catalysis [4][5][6][7][8] and energy materials [9][10][11][12] . The internal transport effect in porous catalytic materials is studied in a reaction-diffusion phenomenon where it is coupled with the intrinsic kinetics of the catalysts 13 .…”

Linking the Performances of Li–O2 Batteries to Discharge Rate and Electrode and Electrolyte Properties through the Nucleation Mechanism of Li2O2

“…Due to its high gravimetric theoretical capacity up to ∼3000 Wh/kg, the Li–O 2 battery is regarded as a promising candidate to meet the demands of high endurance electrical vehicles. , However, the practical discharge capacity of state-of-the-art Li–O 2 batteries (about 500 Wh/kg) is still far from the theoretical value due to several performance limiting factors, such as the electrode surface passivation by the Li 2 O 2 formed during discharge, the O 2 diffusion in the electrolyte and the electrolyte stability. − Moreover, it is found that the performance of a Li–O 2 cell and the morphology of Li 2 O 2 depend on multiple factors including the discharge rate, the type of electrolyte, and the surface properties of the electrode. − Besides, it has been reported that strategies like preseeding the electrode by adding seed crystals or predischarging the cell at low potential have significant impacts on the discharge capacity. More fundamental knowledge of the discharge mechanism is then required to enhance the performance of Li–O 2 batteries, and multiscale modeling is revealed as a useful tool to rationalize the experimental observations. − However, mathematical models reported in the literature consider oversimplified discharge mechanisms, and therefore, there is a lack of versatile models able to predict the discharge trends with respect to multiple experimentally controllable parameters.…”

Assessing Electrolyte Transport Properties with Molecular Dynamics

“…Through a combination of techniques, e.g. ab initio calculations to provide fundamental parameters to molecular dynamics which, in turn, provides transport coefficients to a reaction-diffusion model of the full-scale battery, 2 we can predict battery performance. These predictions can be compared with limited experiments for validation before selecting the best candidates for intensive development.…”