Elucidating Li/O2 Battery Performance By Comparing Cell-Level Models with Experiments
Jing Liu, Lucas D. Griffith, and Charles W. Monroe
Chemical Engineering Department, University of Michigan
A viable rechargeable lithium/oxygen battery would impact transportation markets dramatically, by expediting the development of highway-capable full-electric vehicles with ranges comparable to gasoline-powered automobiles. Even conservative estimates show that Li/O2 electrochemistry could support batteries with energy densities four times greater than present-day lithium-ion technology . This talk will focus on theoretical and experimental research we have undertaken to elucidate design factors and reaction mechanisms that control the rate capabilities and energy capacities of Li/O2 cells.
In a metal/oxygen battery cell, the positive electrode comprises a liquid-permeated, electronically conductive porous solid, which poses many of the main barriers that limit its performance. During discharge, lithium ions and dissolved O2 diffuse through the liquid phase to meet at the solid surface, where they receive an electron and precipitate the Li2O2 discharge product. We have performed systematic experiments to illustrate how Li/O2 cell capacities, voltage responses, and discharge-product morphologies vary with respect to discharge current. Some representative first-discharge voltage vs. capacity data is shown in Figure 1; scanning electron microscopy and X-ray diffraction were also performed to provide more detailed characterization. Experimental results suggest several possible mechanisms that could limit capacity and affect the rate capability of the system. For instance, discharge-product formation may shrink – or even block – liquid-filled pores, decrease the surface area available for charge transfer, or introduce interfacial resistances to charge (electronic) or material (ionic/molecular) exchange.
The relative importance of hypothesized performance-limiting processes can be illustrated by multicomponent, multiphase transport modeling. We therefore developed a PDE-based cell model that accounts for the three distinct phases comprising the positive electrode, as well as the individual components that constitute each phase. Figure 1 shows that simulations can produce discharge-voltage curves that agree both qualitatively and quantitatively with experiments, displaying a ‘voltage plateau’ before ‘sudden death’ of the cell. The oxygen, porosity and reaction distributions in the positive electrode at various discharge levels reveal key characteristics of electrodes and electrolytes that could be tailored to improve the capacities and power capabilities of Li/O2 batteries.
The model is used to probe three hypothetical reaction mechanisms, which involve distinct product morphologies and electron-transfer sites. To achieve consistency with experimental data, it appears that the discharge-product layer must have electronic conductivity several orders of magnitude higher than bulk lithium peroxide, supporting the notion that the discharge product does not wholly prevent electrons from reaching dissolved reactants . The product also appears to allow charge transport over length scales longer than the proposed electron-tunneling mechanism  permits. At high rates, the ‘sudden death’ of voltage in Li/O2 cells is explained by macroscopic oxygen-diffusion limitations in the positive electrode; at low rates, sudden death occurs as a consequence of pore clogging by the discharge product.
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