Transition Metal Hollow Nanocages as Promising Cathodes for the Long-Term Cyclability of Li⁻O₂ Batteries.

As a step towards efficient and cost-effective electrocatalytic cathodes for Li⁻O₂ batteries, highly porous hausmannite-type Mn₃O₄ hollow nanocages (MOHNs) of a large diameter of ~250 nm and a high surface area of 90.65 m²·g were synthesized and their physicochemical and electrochemical properties were studied in addition to their formation mechanism. A facile approach using carbon spheres as the template and MnCl₂ as the precursor was adopted to suit the purpose. The MOHNs/Ketjenblack cathode-based Li⁻O₂ battery demonstrated an improved cyclability of 50 discharge⁻charge cycles at a specific current of 400 mA·g and a specific capacity of 600 mAh·g. In contrast, the Ketjenblack cathode-based one can sustain only 15 cycles under the same electrolytic system comprised of 1 M LiTFSI/TEGDME. It is surmised that the unique hollow nanocage morphology of MOHNs is responsible for the high electrochemical performance. The hollow nanocages were a result of the aggregation of crystalline nanoparticles of 25⁻35 nm size, and the mesoscopic pores between the nanoparticles gave rise to a loosely mesoporous structure for accommodating the volume change in the MOHNs/Ketjenblack cathode during electrocatalytic reactions. The improved cyclic stability is mainly due to the faster mass transport of the O₂ through the mesoscopic pores. This work is comparable to the state-of-the-art experimentations on cathodes for Li⁻O₂ batteries that focus on the use of non-precious transition materials.