For years, lithium oxygen (Li-O₂) battery has been attracted attention thanks to extremely high energy density which might substitute natural resource [1-3]. However, several issues have obstructed the practical application of Li–O₂ batteries such as poor rate capability, low energy efficiency and short cycle life [4-6]. To overcome those drawbacks, herein we propose a novel approach for preparing multi porous hierarchal (micro-, meso-, and macro-) carbon as an efficient positive electrode for Li-O₂ batteries. Unlike currently existing carbon material, the hierarchical porous carbon spheres have the integrity to accelerate O₂ gas flowing and electrolyte circulation (Li⁺ and O^2-), and to provide enough space for Li₂O₂ accumulation and O₂/Li₂O₂ transformation. As-synthesized cathode delivered higher discharge capacity (3891 mAh g^-1_c) than that of Super P cathode (2427 mAh g^-1_c) in deep charge/discharge condition and guided to a high round trip efficiency (~70%) compared to that of carbon black (Super P) (~63%), and efficiently boosted the overall electrochemical performance of the Li–O₂ batteries.

Furthermore, ruthenium (Ru) nanoparticles were uniformly dispersed inside the hierarchical architecture to increase the cathode surface adsorption energy toward O₂ and decrease the over potential further, thus get benefit from the greater energy efficiency. Ru inclusion in our multi porous hierarchal carbon electrode (Triple hierarchical porous carbon spheres, THPC) effectively modified the morphology of discharge product (Li₂O₂) to a uniform toroidal-shaped, which is renowned for advancing the OER performance. Certainly, appreciation goes to the unique multi-porous structure of THPC cathode which offered a large number of active sites and allowed a uniform distribution Ru catalyst, thus facilitated a high capacity with prolonged cycle life alongside high energy efficiency.

References

[1] K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1.

[2] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nature Mater. 2012, 11, 19.

[3] Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 2012, 337, 563.

[4] B. McCloskey, D. Bethune, R. Shelby, T. Mori, R. Scheffler, A. Speidel, M. Sherwood, A. Luntz, J. Phys. Chem. Lett. 2012, 3, 3043.

[5] B. McCloskey, A. Speidel, R. Scheffler, D. Miller, V. Viswanathan, J. Hummelshøj, J. Nørskov, A. Luntz, J. Phys. Chem. Lett. 2012, 3, 997.

[6] J.-L. Shui, N. K. Karan, M. Balasubramanian, S.-Y. Li, D.-J. Liu, J. Am. Chem. Soc. 2012, 134, 16654.