Issues with Voltage Drop and Capacity Fade in Custom Li-ion NMC Cell Degradation Simulations

[jcfip]

Hi everyone,
I'm running degradation simulations on a custom-defined LG MJ1 cell in PyBaMM and was hoping to get some help with an issue I've run into. I've been somewhat successful in reproducing the experimental voltage curves, but when I simulate cycling, I'm seeing strange behavior that I can't quite explain.

I'm using the SPMe model with the following sub-models:
• "SEI": "reaction limited"
• "thermal": "lumped"

I'm running 1000 CC charge/discharge cycles at 1C. The issue is that I either:
• Get a normal-looking voltage curve with very little capacity fade (~0.001%), or

• See a more realistic capacity fade (~5%) but with a lower-than-expected initial terminal voltage during discharge.

This behavior seems to depend on parameters like:
• "Initial SEI thickness [m]"
• "SEI reaction exchange current density [A.m-2]"
• "Initial concentration in the positive/negative electrode [mol.m-3]"

In some cases, after a certain number of cycles, I observe a sudden voltage drop where it loses its original discharge structure, transitioning from the normal curve to the lower voltage curve abruptly and skews the capacity results.

Has anyone else encountered this kind of issue or have suggestions on how to debug this behavior?
Any help would be much appreciated!
Thanks!