SEI growth models

[DarioSlaifsteinSk]

Hi!
I'm trying to model the increase in SEI layer thickness $\delta_{SEI}$ with a very simple model:

$\frac{d \delta_{SEI}}{dt}=\frac{i_{SEI} .M_{SEI}}{n_{SEI}.F.\rho_{SEI}}$

The change in volume of the electrolyte $\varepsilon_{e}$ is then:

$\frac{d \varepsilon_{e}}{dt}= -a_s . \frac{d \delta_{SEI}}{dt}$

The model can be found in several papers including:

[1] X. Jin et al., “Physically-based reduced-order capacity loss model for graphite anodes in Li-ion battery cells,” J. Power Sources, vol. 342, pp. 750–761, Feb. 2017, doi: 10.1016/j.jpowsour.2016.12.099.
[2] X. Jin, “Aging-Aware optimal charging strategy for lithium-ion batteries: Considering aging status and electro-thermal-aging dynamics,” Electrochim. Acta, vol. 407, p. 139651, Mar. 2022, doi: 10.1016/j.electacta.2021.139651.
[3] R. D. Perkins, A. V. Randall, X. Zhang, and G. L. Plett, “Controls oriented reduced order modeling of lithium deposition on overcharge,” J. Power Sources, vol. 209, pp. 318–325, Jul. 2012, doi: 10.1016/J.JPOWSOUR.2012.03.003.
[4] G. L. Plett, BATTERY MANAGEMENT SYSTEMS Volume II: Equivalent-Circuit Methods, First edit. Artech House Power Engineering and Power Electronics, 2016. [Online]. Available: https://us.artechhouse.com/Battery-Management-Systems-Volume-II-Equivalent-Circuit-Methods-P2192.aspx

Nevertheless, I cannot seem to find the molar weight $M_{SEI}$ and the density $\rho_{SEI}$ anywhere.
I've already checked in Chen2020.py, Ramadass2004.py, and OKane2022.py as well as their respective papers as per Parameter Sets

This is because none of the references are taking into account the increase of SEI due to variable operation, i.e., that $i_{SEI}=f(i)$ where $i$ is the current passing through the cell.
Is there a workaround for it? Are the molar weight and the density of the SEI layer contemplated in other constants?

Thanks in advance!

The capabilities for modeling battery pack operation are great so far!

[valentinsulzer]

The parameter you want is "Inner SEI partial molar volume [m3.mol-1]" (or outer, they're usually the same in parameter sets), which is M_sei [kg/mol] / rho_sei [kg/m3]. This should be sufficient since M and rho always appear as M/rho. Let us know if you have a counter-example.

[DarioSlaifsteinSk]

thank you!! That was it. I couldn't really know if it came from $M / \rho$ or something else like concentrations and volumes.
thanks!