Inputting specific surface area as a parameter

[isaacbasil]

Hi everyone,

I am modelling a half cell using a DFN model. I want to use the standard approach of solving the spherical diffusion equation for concentration in the particles. However, I don't want the specific surface area to be calculated from the sphere radius (3*eps/radius) - instead I want to input my own value (which could be a measurement from a BET experiment for example).
It seems that I have to use the option “particle shape” : "no particles". But this seems a bit confusing to me, because I don't want "no particles", as I am still solving the diffusion equation in a spherical particle. Anyway, I have tried using this option, and then adding 'Positive electrode surface area to volume ratio [m-1]' to my parameters file, but this returns an error. Am I using the right approach here?

Thanks in advance,

Isaac

[valentinsulzer]

In practice the surface area only appears via the exchange-current density, and changing a messes up conservation of lithium. So you should change your exchange-current density function instead (scale by your calculated area divided by 3eps/R)

[valentinsulzer]

For what I mean by "conservation of lithium" see appendix C of https://iopscience.iop.org/article/10.1149/1945-7111/ac3e48/pdf

[isaacbasil]

Thanks, ok I will modify my i0 function! I had a read of the appendix, but I'm not clear what you mean about it messing up the mass balance. Since changing a would have the same effect as changing i0, and the Li conservation is still respected when i0 is changed, why would changing a directly violate the mass balance?

[valentinsulzer]

The way the equations are formulated in pybamm, mass balance is only respected if a = eps * V / A (eqn 33 of the appendix). Changing i0 or a has the same effect on the overpotential in Butler-Volmer, but different effect on the mass balance

[isaacbasil]

ah ok I see, thanks! (sorry for late reply).