Li-Ion Battery Cell Troubleshooting and Guidelines
Lithium/Salt Concentration does not change in solid active materials
Make sure that all solid active materials are electrically connected and have electrical connections to the current collectors.
Convergence problems when initializing with an arbitrary potential distribution
Just like velocity and pressure in an incompressible CFD computation, Lithium/Salt Concentration and Li-Ion Electric Potential are closely related. When an arbitrary potential distribution (for example, the default zero field) is chosen as initial condition, the solution must be converged within the first time-step.
In some cases, this convergence is disrupted during the first inner iterations. The value of the SEI surface overpotential (see [Eqn. (4160)]) is far off the solution value, leading to a spurious computation of the SEI-specific electric current Eqn. (4158). This spurious computation affects the Li-Ion Concentration solver.
To overcome this problem, deactivate the Li-Ion Concentration Solver during the first few inner iterations, by setting the under-relaxation factor to zero.
Convergence problems when initial Lithium/Salt Concentration set to zero or saturation value (maximum)
A uniform zero Lithium/Salt Concentration in electrolyte corresponds to a pure (that is, non-conducting) solvent. Such a setup is physically possible, but breaks the operability of the Li-ion battery cell, and is thus considered an input error.
When the electrode Lithium/Salt Concentration values are exactly , or , the derivative of the SEI-specific electric current with respect to Lithium/Salt Concentration becomes infinite (see Eqn. (4159)). The solver becomes unstable.
Such extreme initial conditions are set when investigating the formation process of a Li-ion battery cell. The implementation in Simcenter STAR-CCM+ cannot handle such cases.
However, simulations in close vicinity of these settings are possible: set initial conditions close to these limits.
Convergence problems when Lithium/Salt Concentration depletes locally in electrolyte, or depletes/saturates in solid electrodes.
When lithium depletes or saturates, the numerical scheme becomes less stable when Eqn. (4158) is evaluated using a 1st-order projection of the Lithium/Salt Concentration towards the SEI. To stabilize the algorithm, use the 0th-order projection.
Convergence problems when setting zero resistance in Butler-Volmer Relationship Parameters
To overcome these issues:
- Start with a non-zero resistance.
- Compute some inner iterations until the residuals for Li-Ion Electric Potential and Lithium/Salt Concentration have decreased substantially.
Li-Ion Cell Report: Cannot find battery cell states where OCV equals voltages at 0% and 100% SOC
This error can occur when a monitor is used for the Li-Ion Cell Report.
Lithium/Salt Concentration is a conserved quantity. However, due to non-converged simulations, Lithium/Salt can be erroneously introduced into the simulation. Due to this accumulation or loss of lithium, the Li-ion battery cell is incapable of assuming states where the OCV corresponds to the user-defined values at 0% and 100% SOC.
This error points at issues where the solution has not properly converged.
Guidelines for Simulating Lithium Depletion and Saturation
High charge or discharge currents can lead to strong gradients of Lithium/Salt Concentration within a lithium-ion battery cell. The enhanced transport, and the concentration differences that are required to sustain the gradients then rapidly deplete or saturate (locally) electrodes or electrolyte.
The Li-Ion Battery Cell model can handle cases of lithium depletion and saturation within solid electrodes, as well as the depletion of salt in electrolyte.
When lithium depletes or saturates, the numerical scheme becomes less stable when Eqn. (4158) is evaluated using a 1st-order projection of the Lithium/Salt Concentration towards the SEI. To stabilize the algorithm, use the 0th-order projection.
To account for the resulting low order of the scheme, use a mesh with sufficiently refined prism layers at the SEI.
Simulation of lithium depletion or saturation requires that an equilibrium state can be found. At that equilibrium state, the driving force for reactions at the SEI vanishes, that is in Eqn. (4160). Make sure that such an equilibrium exists by providing appropriate electrode equilibrium potential profiles. If this requirement is not met, activate Prevent Electrode Depletion.
When the electrolyte depletes, activating Prevent Electrode Depletion corrects the SEI surface overpotential to become when the electrolyte depletes of salt.