I determined diffusion efficiencies in a Li-ion battery cathode at various temperatures and calculated activation energies by using an ARRHENIUS plot.
My results show small activation energies in potential regions with low diffusion efficiencies and high activation energies where diffusion coefficients are high. Can anyone explain that to me?
I would have expected large activation energies to come along with low diffusion efficiencies and small activation energies with high diffusion efficiencies.
I'm not 100% sure what you are measuring (diffusion efficiency), but if you are measuring overall transport properties of a porous electrode, you may see regimes controlled by solid-state diffusion (low activation energy, low diffusion coefficient) and regimes controlled by diffusion in the electrolyte (high activation energy, high diffusion coefficient)
Thank you very much for your answer Keld!
Nevertheless I am sure that I measure properties controlled by solid-state diffusion within the oxide particles at all times since these processes are rate-determining. I calculated diffusion coefficients from GITT curves at various temperatures and obtained activation energies via ARRHENIUS plot.
I wouldn't be so sure - there will always be components from electrolyte resistance and electrolyte diffusion, and the diffusion part will move towards lower frequencies - longer times - when the electrode becomes thicker. The convolution of the different parts is complex for porous electrodes, but you may imagine that in regions where the voltage curve is flat, then a potential step will require a relatively large amounts of Li ions to enter the electrode particles, and transport in the electrolyte becomes more important than in regions where the potential curve is steep.
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