Rechargeable metal-air (oxygen) batteries are receiving a great deal of interest as possible alternatives to lithium ion batteries, due in particular to their potential to provide higher gravimetric energies.¹ While much attention has been focused on aprotic Li-O₂ batteries, substantial challenges must be addressed before widespread commercial implementation is possible. Recently, a metal-air battery in which lithium is replaced by sodium has received increasing attention. Although Na-O₂ batteries present lower gravimetric energies than Li-O₂ batteries on a cell basis (based on discharge products Na₂O₂/NaO₂: 1605 /1108 Wh/kg, respectively; Li₂O₂, 3505 Wh/kg), much lower charge overpotentials than those in typical Li-O₂ batteries have been reported during discharge and charge (~100 mV vs. ~1000 mV), based on reversible sodium superoxide (NaO₂) formation.^2,3 In addition, Na-O₂ batteries can exhibit energy efficiencies higher than 90 %. Besides cell configuration, the utilization of suitable electrode materials is a point of major concern as they are responsible for achieving efficient deposition of the discharge products. Graphene-based materials have emerged as promising Na-air battery electrodes due to the highly conductive nature of the Csp² structure of the graphene sheets. The preparation of graphene electrodes with tuned porosity could provide an optimal host for the NaO₂ particles (improving capacity) as well as eliminating the diffusional/kinetic limitations associated with the redisolution of discharge products (enlarging efficiency).

Here we report the effect of the pore size of self-standing, binder-free graphene electrodes on the electrochemical performance. Graphene materials with different pore sizes are prepared by the thermal reduction of different graphene oxide structures obtained from the same graphene oxide suspension. The resultant electrodes show very low density, high electrical conductivity and defined pore network, with an adjustable pore size ranging from several nanometers to several tens of micrometers. In addition, electrode fabrication is a very simple process that allows large-scale production at a low cost. Phase-pure NaO₂ (pyrite structure, Fmm) was found as the sole discharge product, as confirmed by Raman and nuclear magnetic resonance spectroscopy. Notably, open porosity does not present the best electrochemical performance – instead there is a compromise between pore size and performance. In this talk, we will unravel the influence of pore size to achieve high-performing electrodes for Na-air batteries.

References:

1 D. G. Kwabi, N. Ortiz-Vitoriano, S. A. Freunberger, Y. Chen, N. Imanishi, P. G. Bruce and Y. Shao-Horn, MRS Bull., 2014, 39, 443–452.

2 I. Landa-Medrano, C. Li, N. Ortiz-Vitoriano, I. Ruiz de Larramendi, J. Carrasco and T. Rojo, J. Phys. Chem. Lett., 2016, 7 (7), 1161-1166.

3 N. Ortiz-Vitoriano, T. P. Batcho, D. G. Kwabi, B. Han, N. Pour, K. P. C. Yao, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. Lett., 2015, 6(13), 2636–2643.