Pore-Scale Transport Resolved Model Incorporating Cathode Microstructure and Peroxide Growth in Lithium-Air Batteries

Monday, 6 October 2014: 13:00
Sunrise, 2nd Floor, Galactic Ballroom 1 (Moon Palace Resort)
L. Zhang, C. Andersen, G. Qiu, V. Kalra, and Y. Sun (Drexel University)
The lithium-air (Li-air) battery, shown schematically in Fig. 1, with its usable energy density close to 1,700Wh/kg has captured worldwide attention as a promising battery solution for electric vehicles. However, a major hurdle facing the development of Li-air systems is their poor round-trip efficiency owing to the formation of electrically insulating lithium peroxide (Li2O2) at the cathode surface. Although it is well known that Li2O2 inhibits the reduction process during cell charging, how the deposition morphology of Li2O2 affects the cell performance is poorly understood. Ex-situ imaging has shown that the Li-air cathodes have highly intricate structures and the deposition of Li2O2 is equally complex. However, all existing simulations thus far treat the electrode/Li2O2 matrix as a homogenous continuum and utilize simply-shaped electrode morphologies, such as spheres and rods, to construct volume-averaged expressions (see Fig. 2a) for porosity and surface area. The formation of Li2O2 is then modeled as growing uniformly above these simplified electrodes, and manifests merely as a change in bulk porosity. Such assumptions are insufficient in characterizing the precise effect of the complex electrode/Li2O2microstructure on cell performance.

    In this paper, we present a pore-scale transport resolved model (see Fig. 2b) of the Li-air battery that fully accounts for the electrode microstructure and peroxide growth. This approach requires no empirical correlations regarding the electrode morphology. Additionally, the growth of Li2O2 is modeled locally and no longer requires the assumption of uniform deposition. Utilizing this pore-scale transport resolved model, the complex electrode and Li2O2 morphologies can be directly incorporated into the numerical model and their effects on system-level performance can be evaluated. Incorporating the thickness-dependent electron resistivity and rate-dependent growth morphology of Li2O2, results obtained from our pore-scale model agree well with experiments. The validated model is then used to predict the galvanostatic discharge behavior of a Li-air cell for a variety of electrode/Li2O2 morphologies. The effects of pore-size (e.g., macro-, meso-, and micro-pores), electrode structure, discharge/charge rate, and oxygen solubility on Li2O2 growth and cell performance are presented. Figure 3 shows the cell voltage versus specific  capacity curves for electrodes with different microstructures. The model presented here will be a valuable tool for rational design of electrode microstructures for improved cell performance.