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

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.