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Relation Between Pore Size Distribution and Discharge Capacity in Carbons Used As Li-O2 Cathodes with an Ionic Liquid-Based Electrolyte
Four porous carbons with similar composition and specific area, but with pores peaking in different points of the mesoporous region (i.e. average pore diameter in the range of 5 - 55 nm) were obtained from a resorcinol-formaldehyde route. These were used as cathodes for Li-O2 batteries using an ionic liquid electrolyte (LiTFSI in PYR14TFSI) at room temperature and 60 ºC. Discharge capacities were related to the experimental pore size distribution determined from N2 adsorption.
The method consisted in testing a linear fit between discharge capacity and the volume occupied by the discharge products. These are assumed distributed in a layer of constant thickness on the pore walls. In addition, the model assumes that pores below a threshold size do not participate to the capacity, because their access becomes clogged much faster than for large pores. Thus, the occupied volume is calculated from the experimental cumulative pore volume and the two parameters, layer thickness and threshold pore size.
The fit quality was mapped as a function of these parameters. No constraints or other fixed parameters were used. Best fits, with a coefficient of determination R2>0.95 were obtained when the layer thickness was of the order of 7 nm, which is consistent with the thickness of an insulating passivation layer. The method also allowed estimating the penetration depth of the discharge reaction in the electrode thickness, which in our electrochemical test conditions resulted of the order of 20 nm at RT and 40 nm at 60 ºC.
The extension of applicability of this method will be discussed.
Acknowledgement
The research leading to these results has received funding from the European Union’s Seventh Framework Programme under EC-GA No. 265971 ‘LABOHR’, and from the Spanish Government under contract MAT2012-39199-C02-01. M.O. acknowledges CSIC for a JAE-DOC research contract.