Inverse Opal Carbons As Li-O2 Cathodes with an Ionic Liquid-Based Electrolyte: Effect of Macropore Size on Capacity, Rate Capability and Reversibility

Inverse opals are beautiful macroporous materials prepared by template synthesis and presenting a well-defined arrangement of pores of tunable monodisperse size. Thus, they are ideal model systems for studying the effect of macropore size in lithium-oxygen electrodes.

In this work, inverse opal carbons were prepared by using poly(styrene-co-methacrylic acid) (PS-MAA) spheres of different size as templates via a resorcinol-formaldehyde sol-gel process. These were used in cathodes for Li-O₂ batteries using different cell configurations and an ionic liquid electrolyte (LiTFSI in PYR14TFSI). The high porosity and open channels with low tortuosity allowed remarkable capacities of over 3000 mAh/g at 0.1 mA/cm².

We could relate the carbon textural properties to the capacity, rate capability and reversibility. We observed better rate capability for discharge with the largest macropores (500 nm diameter), evidencing a mass transport limitation. However, charge profiles denoted a remarkable asymmetry respect to discharge, with low coulomb efficiency. Instead, when macropores were smaller than 100 nm we obtained a remarkable symmetry in time-limited discharge-charge profiles. This proves that confinement of discharge products in small pores effectively prevents the electrode passivation.

 

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.