Rechargeable lithium–air (or lithium–oxygen) batteries have generated a great deal of attention due to their ultrahigh theoretical gravimetric energy density outperforming the conventional lithium-ion batteries. Despite its great promise, many obstacles^[1] need to be solved before its practical application, such as low round-trip efficiency, poor catalytic capability and inferior cyclability, mainly caused by their sluggish kinetics. Apparently, to improve this kinetics, a rational design of electrocatalysts toward improving oxygen reduction reaction (ORR, during discharge) and oxygen evolution reaction (OER, during charging) must be devised. Common cathodes used in the previous literature are carbon supported oxygen diffusion electrodes, and the general preparation process is the catalysts mixed with a certain amount of binder into carbon materials, pasting them onto the current collectors. Nevertheless, both carbon materials and organic binders^[2]have been suggested degrading in Li-air batteries. Therefore, there is a critical need to find a new catalyst support to substitute carbon and develop a binder-free cathode preparation strategy for Li-air batteries.

Inspired by the ultrathin nanostructure of graphene carbon materials, two-dimensional (2D) layered molybdenum disulfide (MoS₂) can be exfoliated to give single- or few layer nanosheets with much more exposed edges possesses high electrocatalytic activity.

In this work, we prepared a novel cathode catalyst by growing TiN nanorods on commercially available carbon paper first, followed by growing MoS₂ nansheets on the TiN nanorods to form a core-shell structure materials (see in Fig. 1), and used directly it as a free-standing and binder-free air cathodes materials for the Li-O₂ batteries. The TiN nanorods arrays with excellent mechanical strength and also provide fast transport paths for the electrons and lithium ion. Furthermore, few-layer MoS₂ nanosheets offered high electrode/electrolyte/oxygen interfacial contact areas, promoted rapid charge transfer and offered amount of storage space for the discharged product. The Li-O₂ batteries with the TiN@MoS₂/carbon paper electrode exhibits superior ORR/OER activity with a lower discharge-charge (the first cycle) overpotential of 840 mV than that the cells with the TiN/carbon paper cathode, and a 1100 mV lower overpotential than when a TiO₂/carbon paper electrode was used (see in Fig. 2). 

Figure 1. (a) XRD patterns of the as-prepared various electrode materials; (b, c) SEM images of the as-prepared freestanding TiN@MoS₂/carbon paper electrode; (d, e) TEM images of TiN@MoS₂ catalyst (inset A and B are SAED patterns of TiN@MoS₂catalyst).

Figure 2. Charge-discharge curves of Li/O₂ batteries with the as-prepared various air cathodes at a current density of 100 mAg^-1_catalyst (the specific capacity is calculated based on the mass of MoS₂catalyst in the air electrodes).

Reference

[1] J. Lu, L. Li, J. B. Park, Y. K. Sun, F. Wu and K. Amine, Chem Rev 2014, 114, 5611-5640.

[2] C. V. Amanchukwu, J. R. Harding, Y. Shao-Horn and P. T. Hammond, Chemistry of Materials 2015, 27, 550-561.