The Lithium-Ion Battery Interface

The interface (

), found under the branch (

) when adding a physics interface, is used to compute the potential and current distributions in a lithium-ion battery. Multiple intercalating electrode materials can be used, and voltage losses due to solid-electrolyte-interface (SEI) layers are also included.

The physics interface is based on the works of Newman and others. Ohm’s law is used to describe the charge transport in the electrodes. For the electrolyte, concentrated electrolyte theory for a quiescent aprotic (1:1) electrolyte is used to describe charge and mass transport in the electrolyte phase.

Alternatively, a single-ion conductor charge balance, used by default by the (

) entry in the model wizard, may be used for the electrolyte. This option is typically applicable to solid electrolytes.

An extra dimension is included in the porous electrode domains to describe the transport of solid lithium in the solid electrode phase using Fick’s law.

Different combinations of four dependent variables are valid and solved in different domains. The four dependent variables are:

The cl variable is not solved for when using the single-ion conductor charge balance model.

The cs variable is solved for in an extra dimension, using an internal discretization in the particle dimension, not visible in the ordinary model geometry. The cs dependent variable, named liion.cs_xxx (where xxx is the tag of the node, for instance pce1), can be used to set for instance concentration varying diffusion coefficients in the particle. The spatial variable of the particle is named xs_liion_xxx (for instance, xs_liion_pce1).

The concentration variation along the extra dimension, at a given position in the real dimension, can be accessed using the atxd1, atxd2, and atxd3 operators. See also Using Extra Dimensions and Plotting Results in Extra Dimensions in the COMSOL Multiphysics Reference Manual.

The surface, center and average values of cs can also be evaluated in the real dimension by the variable names liion.cs_surface, liion.cs_center, and liion.cs_average, respectively.

When this physics interface is added, these default nodes are also added to the — , , and . Then, from the toolbar, add other nodes that implement, for example, and nonporous , and boundary conditions. You can also right-click to select physics features from the context menu.

	All nodes are described in Shared Nodes for Battery Interfaces.

The is the default physics interface name.

The is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first physics interface in the model) is liion.

	Exclude domains that do not conduct current.

See Out-of-Plane Thickness.

See Cross-Sectional Area.

The option uses concentrated electrolyte theory to solve for the lithium salt concentration and the potential as dependent variables in the electrolyte phase. The option applicable to liquid (or plasticized) organic solvent-based lithium salt electrolytes.

The solves for the electrolyte potential by assuming that all charge in the electrolyte phase is carried by the positive lithium ions only, so that the concentration of lithium ions in the electrolyte can be assumed to be constant. The option is typically applicable to solid phase electrolytes, or electrolytes where the electrolyte conductivity is not changing as a result of the ion transport in the cell. Since the electrolyte concentration is not solved for, the single-ion conductor option reduces the computational load and complexity.

Note that the single-ion conductor option will disable all settings in domain and boundary nodes applicable to the electrolyte concentration dependent variable.

can be added as an additional transport mechanism. By default, the check box is not selected. Select the check box to enable convective transport.

Note that this section is not applicable if the option is selected in the section.

The Physics vs. Materials Reference Electrode Potential setting on the physics interface node can be used to combine material library data for current densities and equilibrium potentials with an arbitrary reference electrode scale in the physics. The setting affects the electrode potentials used for model input into the materials node, as well as all equilibrium potential values output from the materials node.

Note that the setting will only impact how potentials are interpreted in communication between the physics and the Materials node. If the From material option is not in use for equilibrium potentials or electrode kinetics, the setting has no impact.

This section is available when the is selected in the dialog box shown when the button (

) is clicked.

To display these sections, click the button (

) and select from the dialog box. These sections are applicable only if is selected in the section. There are two consistent stabilization methods available and selected by default — and . There is one inconsistent stabilization method, , which is not selected by default.

This physics interface defines the following dependent variables (fields): the electrolyte potential, the electrolyte salt concentration , and the electric potential. The names can be changed but the names of fields and dependent variables must be unique within a model.

The electrolyte salt concentration variable is not solved for when using the Single-ion conductor charge balance model (see previous section).

Certain nodes may add additional dependent variables to the model. For example the intercalated solid lithium concentration in the Porous Electrode and Additional Porous Electrode Material nodes.