MCNTs@MnO₂ Nanocomposite Cathode Integrated with Soluble O₂-Carrier Co-salen in Electrolyte for High-Performance Li-Air Batteries

Li-air batteries (LABs) are promising because of their high energy density. However, LABs are troubled by large electrochemical polarization during discharge and charge, side reactions from both carbon cathode surface/peroxide product and electrolyte/superoxide intermediate, as well as the requirement for pure O₂. Here we report the solution using multiwall carbon nanotubes (MCNTs)@MnO₂ nanocomposite cathode integrated with N,N′-bis(salicylidene)ethylenediaminocobalt(II) (Co^II-salen) in electrolyte for LABs. The advantage of such a combination is that on one hand, the coating layer of δ-MnO₂ with about 2-3 nm on MCNTs@MnO₂ nanocomposite catalyzes Li₂O₂ decomposition during charge and suppresses side reactions between product Li₂O₂ and MCNT surface. On the other hand, Co^II-salen works as a mobile O₂-carrier and accelerates Li₂O₂ formation through the reaciton of (Co^III-salen)₂-O₂^2- + 2Li⁺ + 2e^- → 2Co^II-salen + Li₂O₂. This reaction route overcomes the pure O₂ limitation and avoids the formation of aggressive superoxide intermediate (O₂^- or LiO₂), which easily attacks organic electrolyte. By using this double-catalyst system of Co-salen/MCNTs@MnO₂, the lifetime of LABs is prolonged to 300 cycles at 500 mA g^-1 (0.15 mA cm^-2) with fixed capacity of 1000 mAh g^-1 (0.30 mAh cm^-2) in dry air (21% O₂). Furthermore, we up-scale the capacity to 500 mAh (5.2 mAh cm^-2) in pouch-type batteries (∼4 g, 325 Wh kg^-1). This study should pave a new way for the design and construction of practical LABs.