MCNTs@MnO<sub>2</sub> Nanocomposite Cathode Integrated with Soluble O<sub>2</sub>‑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 O<sub>2</sub>. Here we report the solution using multiwall carbon nanotubes (MCNTs)@MnO<sub>2</sub> nanocomposite cathode integrated with <i>N</i>,<i>N</i>′-bis­(salicylidene)­ethylenediaminocobalt­(II) (Co<sup>II</sup>-salen) in electrolyte for LABs. The advantage of such a combination is that on one hand, the coating layer of δ-MnO<sub>2</sub> with about 2–3 nm on MCNTs@MnO<sub>2</sub> nanocomposite catalyzes Li<sub>2</sub>O<sub>2</sub> decomposition during charge and suppresses side reactions between product Li<sub>2</sub>O<sub>2</sub> and MCNT surface. On the other hand, Co<sup>II</sup>-salen works as a mobile O<sub>2</sub>-carrier and accelerates Li<sub>2</sub>O<sub>2</sub> formation through the reaciton of (Co<sup>III</sup>-salen)<sub>2</sub>-O<sub>2</sub><sup>2–</sup> + 2Li<sup>+</sup> + 2e<sup>–</sup> → 2Co<sup>II</sup>-salen + Li<sub>2</sub>O<sub>2</sub>. This reaction route overcomes the pure O<sub>2</sub> limitation and avoids the formation of aggressive superoxide intermediate (O<sub>2</sub><sup>–</sup> or LiO<sub>2</sub>), which easily attacks organic electrolyte. By using this double-catalyst system of Co-salen/MCNTs@MnO<sub>2</sub>, the lifetime of LABs is prolonged to 300 cycles at 500 mA g<sup>–1</sup> (0.15 mA cm<sup>–2</sup>) with fixed capacity of 1000 mAh g<sup>–1</sup> (0.30 mAh cm<sup>–2</sup>) in dry air (21% O<sub>2</sub>). Furthermore, we up-scale the capacity to 500 mAh (5.2 mAh cm<sup>–2</sup>) in pouch-type batteries (∼4 g, 325 Wh kg<sup>–1</sup>). This study should pave a new way for the design and construction of practical LABs.