How the electrolyte limits fast discharge in nanostructured batteries and supercapacitors

Solid state and interfacial processes are not necessarily the principal rate limiting process during fast discharge of LiFePO4 composite electrodes with particle size less than 1 μm. A simple model based on salt diffusion to a sharp discharge front explains the observed dependence of discharge rate on electrode thickness, electrolyte concentration, lithium transference number, and dilution of the active material. The effect of changing the electrolyte is dramatic, e.g. discharge to 25% capacity was obtained on a 40 μm thick electrode after only 4 s in an optimised electrolyte, aqueous Li2SO4, showing a rate of 900 C as compared with less than 10 C for a similar cell with an ionic liquid as the electrolyte.