Add the GroupedDFN with flux boundary conditions

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Features
 
+- [#977](https://github.com/pybop-team/PyBOP/pull/977) - Updates the `lithium_ion` models to use flux boundary conditions and adds the `GroupedDFN` model.
 - [#973](https://github.com/pybop-team/PyBOP/pull/973) - Adds example scripts for EIS parameterisation.
 - [#974](https://github.com/pybop-team/PyBOP/pull/974) - Adds voltage components to each grouped model as well as asymmetric and multiphase Butler-Volmer kinetics.
 - [#969](https://github.com/pybop-team/PyBOP/pull/969) - Updates synthetic data and adds example script for thermal parameterisation.

examples/scripts/comparison_examples/grouped_SPMe.py

@@ -18,12 +18,9 @@
 # Use the Chen2020 parameters
 parameter_values = pybamm.ParameterValues("Chen2020")
 
-# Fix the electrolyte diffusivity and conductivity
+# Fix the electrolyte conductivity
 ce0 = parameter_values["Initial concentration in electrolyte [mol.m-3]"]
 T = parameter_values["Ambient temperature [K]"]
-parameter_values["Electrolyte diffusivity [m2.s-1]"] = parameter_values[
-    "Electrolyte diffusivity [m2.s-1]"
-](ce0, T)
 parameter_values["Electrolyte conductivity [S.m-1]"] = parameter_values[
     "Electrolyte conductivity [S.m-1]"
 ](ce0, T)

pybop/models/li_half_cell/sp_diffusion.py

@@ -95,13 +95,14 @@ def __init__(self, name="Single Particle Diffusion Model", **model_kwargs):
         ######################
         # The div and grad operators will be converted to the appropriate matrix
         # multiplication at the discretisation stage
-        self.rhs[sto_p] = pybamm.div(pybamm.grad(sto_p) / self.tau_d(sto_p))
+        N_s_p = -pybamm.grad(sto_p) / self.tau_d(sto_p)
+        self.rhs[sto_p] = -pybamm.div(N_s_p)
 
         # Boundary conditions must be provided for equations with spatial derivatives
         j_p = -I / (3 * Q_th_p)
         self.boundary_conditions[sto_p] = {
-            "left": (Scalar(0), "Neumann"),
-            "right": (-self.tau_d(sto_p_surf) * j_p, "Neumann"),
+            "left": (Scalar(0), ("Flux", N_s_p)),
+            "right": (j_p, ("Flux", N_s_p)),
         }
 
         self.initial_conditions[sto_p] = sto_p_init

pybop/models/lithium_ion/__init__.py

@@ -3,4 +3,5 @@
 #
 from .grouped_spm import GroupedSPM
 from .grouped_spme import GroupedSPMe
+from .grouped_dfn import GroupedDFN
 from .cell_temperature import CellTemperature