Hierarchical activated carbon microfiber (ACM) electrodes for rechargeable Li-O₂ batteries

Hierarchical activated carbon microfiber (ACM) and ACM/α-MnO ₂ nanoparticle hybrid electrodes were fabricated for high performance rechargeable Li-O₂ batteries. Various oxygen diffusion channels present in these air-cathodes were not blocked during the oxygen reduction reactions (ORR) in triglyme-LiTFSI (1 M) electrolyte solution. ACM and ACM/α-MnO₂ hybrid electrodes exhibited a maximum specific capacity of 4116 mA h g_c^-1 and 9000 mA h g _c^-1, respectively, in comparison to 2100 mA h g _c^-1 for conventional carbon composite air-electrodes. Energy densities of these electrodes were remarkably higher than those of sulfur cathodes and the most promising lithium insertion electrodes. In addition, ACM and ACM/α-MnO₂ hybrid electrodes exhibited lower charge voltages of 4.3 V and 3.75 V respectively compared to 4.5 V for conventional composite carbon electrodes. Moreover, these binder free electrodes demonstrated improved cycling performances in contrast to the carbon composite electrodes. The superior electrochemical performance of these binder free microfiber electrodes has been attributed to their extremely high surface area, hierarchical microstructure and efficient ORR catalysis by α-MnO ₂ nanoparticles. The results showed herein demonstrate that the air-cathode architecture is a critical factor determining the electrochemical performance of rechargeable Li-O₂ batteries. This study also demonstrates the instability of ether based electrolyte solutions during oxygen reduction reactions, which is a critical problem for Li-O₂ batteries.