Support for half-cell models

[rtimms]

Add a half-cell variant of BPX - this would require writing down the model equations in the standard and having a slightly different schema and validation.

[ejfdickinson]

As a workaround, this can be closely approximated by defining the Li counter electrode (negative electrode, conventionally) as having:

• constant open-circuit potential at 0 V vs Li+/Li
• very low thickness to eliminate conductivity and electrolyte losses
• very high diffusivity to eliminate diffusive losses
• very high surface area per unit volume to mock an 'infinite' Li reservoir
• electrode kinetics either set to very fast, or set as required to achieve an observed counter electrode overpotential

For example (untested):

            "Negative electrode": {
                  "Particle radius [m]": 1e-6,
                  "Thickness [m]": 1e-9,
                  "Diffusivity [m2.s-1]": 1,
                  "OCP [V]": 0,
                  "Entropic change coefficient [V.K-1]": 0,
                  "Conductivity [S.m-1]": 1,
                  "Surface area per unit volume [m-1]": 1e9,
                  "Porosity": 0.5,
                  "Transport efficiency": 1,
                  "Reaction rate constant [mol.m-2.s-1]": 1,
                  "Minimum stoichiometry": 0.5,
                  "Maximum stoichiometry": 0.5,
                  "Maximum concentration [mol.m-3]": 1e6,
                  "Diffusivity activation energy [J.mol-1]": 0,
                  "Reaction rate constant activation energy [J.mol-1]": 0
            },