Discrepancy in Measured Capacity Loss and Degradation Contribution Values

[MirAbbasAli2A]

Hello PyBamm Team -

I have found a discrepancy with the capacity loss evaluation. When I evaluate the measured capacity, the following curve shows ~0.1 Ah capacity loss from the beginning to the end of the test:

However, when I look at LLI contributions, I notice a capacity loss of up to 0.14 Ah:

I know the difference is seemingly small (0.04 Ah) but can this just be explained by an accumulation of rounding error? Or is there something else in play?

FYI, I am using the DFN model with all aging options enabled:

model = pybamm.lithium_ion.DFN(options = {
"thermal":"lumped",
"dimensionality":0,
"cell geometry": "arbitrary",
"SEI": "solvent-diffusion limited",
"SEI porosity change": "true",
"particle mechanics": ("swelling and cracking", "swelling and cracking"),
"SEI on cracks": "true",
"loss of active material": "stress-driven",
"calculate discharge energy": "true",
"lithium plating": "partially reversible",
"lithium plating porosity change": "true"
},
name="TDFN",
)

Thank you for your time.

[valentinsulzer]

Lithium inventory is not identical to capacity. Lithium inventory is the amount of cyclable lithium (Q_Li in this notebook), capacity is the amount of charge you can draw from 4.2V to 2.5V at infinitely low C-rate (zero resistance). They are related but not equal. See the notebook I shared for details.

[MirAbbasAli2A]

Thank you for your prompt reply @tinosulzer . So just to confirm, the capacity loss definitions I provided below are converting the the loss in lithium inventory to a capacity loss and therefore are not identical to the capacity measured?

Q_SEI = sol["Loss of capacity to SEI [A.h]"].entries
Q_SEI_cr = sol["Loss of capacity to SEI on cracks [A.h]"].entries
Q_plating = sol["Loss of capacity to lithium plating [A.h]"].entries
Q_side = sol["Total capacity lost to side reactions [A.h]"].entries

[valentinsulzer]

Correct - although now I see the names are misleading! "capacity" here means "cyclable lithium capacity"

[yazyr]

@MirAbbasAli2A If you don't mind, can you provide me the code that you used to perform the capacity fade? I myself am trying to perform a simulation to see how capacity fades with respect to number of cycles and it would be a great deal of favour if you could share the program. Thanks, in advance.