Modeling Battery Electrode Properties
Jacqueline Ashmore & David Clatterbuck, TIAX LLC
A brief description of some of physical processes that occur in the battery is as follows: during charging of the battery, a Li atom starts inside a solid particle of the cathode material. It diffuses through the solid to the surface of the particle and disassociates from the particle, creating a Li ion. This ion travels through a liquid electrolyte that occupies the void volume of the electrode. It is then transported through the electrolyte to the other electrode (anode) made of a different material where it again reacts at the surface of a solid particle, enters a solid particle, and finally undergoes solid state diffusion within the anode particle. Below are two typical problems encountered in the design of Li-ion batteries where the cathode consists of a porous network of packed particles of the active material.
Example 1. For a given battery chemistry, the total energy content of the battery increases with the amount of active material and is thus proportional to the packing fraction for a battery of fixed volume. At the same time, the rate of energy delivery (i.e., power) depends on having sufficient mass and electrical transport throughout the electrode. Thus, a battery may be designed to give a high volumetric energy density by increasing the packing fraction, while in another application where high power is needed, a more porous electrode may be used.
Example 2. For some materials, Li transport through the bulk of the individual particles will be the rate limiting step, and thus it is desirable to decrease the particle size to allow for shorter transport distances (diffusion lengths). However, it has been found empirically that very small particles cannot be packed as densely as coarse particles due to surface effects; thus a practical electrode will likely have a particle size which is a compromise between these two effects.
Statement of problem
Some particle characteristics to consider might include:
Some electrode properties of interest include:
Determining all of the electrode properties for all possible combinations of particle characteristics is probably not a manageable task. Rather it may be useful to consider some of the more complex electrode properties for the case of simple particle size distributions (i.e., monodisperse spheres); while for more complex particles, determining the packing fraction may be a sufficiently challenging problem.
Last Changed: Tuesday, 03-Jun-2008 10:11:37 EDT