Charge-Discharge Asymmetry of Phase Change Electrodes from Isotropic Solid State Diffusion Models

Lithium based batteries are of current interest for high rate applications in electric vehicles. Some of the electrode materials exhibit phase change characteristics upon lithium intercalation (or de-intercalation). The diffusion phenomenon in these electrode materials coupled with the phase change characteristics can lead to the existence of an asymmetric behavior between charge and discharge, which can be exhibited in the form of a higher cell capacity during charge when compared to discharge (or vice versa) at the same current density. In this work, the isotropic shell-core model is used to describe the diffusion phenomenon coupled with phase change, and approximate solution methods are presented for phase change electrodes. Additionally, the effect of transport properties (diffusivity coefficients) and thermodynamic properties (equilibrium concentrations) of the two phases which can be particle size and temperature dependent, on the charge-discharge asymmetric behavior and battery performance is studied. The diffusion models also suggest that for phase change electrode materials, it is possible that lowering the particle size is desirable not only for the obvious advantages of higher current densities and capacities but also to achieve less asymmetry between discharge and charge. Similar advantages can be obtained when these batteries are operated at higher temperatures. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.049201jes]