Is the cell voltage (at a modest charging rate) mostly determined by thermodynamics on the electrode surfaces and resistances of electrolyte and electrode particles, or there is a non-negligible kinetic component to the cell voltage (governed by the Butler-Volmer equation) on the electrode surfaces?

In equivalent-circuit models of cells, voltage (either of the full cell, or of a half-cell) is usually modelled as consisting of three components: open-circuit voltage, diffusion voltage, and hysteresis component.

Open-circuit voltage is determined by the thermodynamics of the electrode process at certain concentrations (stoichiometry) of the electrode (anode or cathode) and the electrolyte. The presence of a solid-electrolyte interface and some lattice effects don't allow to describe it with a single Nernst equation, but it's still thermodynamic in nature.

Diffusion voltage component arises from ion over- or under-concentration at the electrode surface. It's a reflection of the fact that stoichiometry at the surface of an electrode particle is not the average stoichiometry of the whole particle (which is used in the open-circuit voltage relationship).

The remaining hysteresis component is due to the polarisation of the electrode particle surfaces.

So, it looks like equivalent-circuit models disregard any kinetic contributions to the voltage. Does this mean that it's in fact negligible because the kinetics of Lithium-ion (de)intercalation at the electrode surfaces are so facile that a cell can be thought to always be at the thermodynamic equilibrium (when charged or discharged at 1C)? Does this differ appreciably for Graphite anode and typical cathode materials (NMC, LTO, LFP)?

posted almost 3 years ago

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leventov‭

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