Characterizing Metal/Oxygen Batteries with Multiphase Continuum-Scale Models.

[摘要] This dissertation focuses on the development of theory and continuum-scale models to characterize transport and kinetics in metal/oxygen batteries.Newman’s concentrated-solution theory is extended to elucidate two transport mechanisms associated with the volumes dissolved electrolytes occupy: the ;;excluded-volume effect’ and ;;Faradaic convection’. Both mechanisms can be accounted for in the concentrated-solution theory by incorporating a thermodynamic state equation. Two dimensionless parameters quantify the importance of these phenomena, which prove relevant when modeling nonaqueous electrolytes.Accurately measured material properties are of great significance in modeling battery systems. An analysis of binary electrolytic solutions in planar electrochemical cells that support symmetric electrode reactions is performed to serve as a foundation for experimental measurements of diffusivities and transference numbers. Prior theory is extended to include a nonlinear relationship between concentration polarization and cell voltage, as well as accounting for solute-volume effects. The extended theory provides significant corrections when concentration polarization is very large or when electrolytes are very concentrated, rationalizing unexpected voltage responses that have been observed during prior transport-property measurements.The modified concentrated-solution theory is next incorporated into a porous-electrode theory, which is used to model the positive electrodes in metal/oxygen batteries. Continuum simulations of a discharging lithium/oxygen cell are implemented and compared with experimental data to examine how cell capacity is controlled by macroscopic mass transfer, interfacial kinetics, and electronic conduction through the discharge product. The model accounts for the three-phase nature of the positive electrode, including an explicit discharge-product layer whose volume distribution depends on the local depth of discharge. Three hypothetical deposition mechanisms involving different product morphologies and electron-transfer sites are studied. ;;Sudden death’ of lithium/oxygen cells is explained by macroscopic oxygen-diffusion limitations in the positive electrode, which are exacerbated by pore clogging as the discharge product forms.Finally, the multicomponent, multiphase continuum model is applied to simulate the first discharge/charge cycle of a sodium/oxygen battery. Simulated discharge/ recharge curves are compared with experiments. Unlike the lithium/oxygen cell, the sodium/oxygen system exhibits low total overpotentials for both discharge and charge, suggesting that the positive-electrode reaction mechanism may follow a reversible pathway.

[发布机构] University of Michigan

[效力级别] solute-volume effects

[关键词] continuum-scale models;solute-volume effects;transport-property measurements;multiphase modeling;metal/oxygen batteries;discharge/charge mechanisms;Chemical Engineering;Engineering (General);Engineering;Chemical Engineering