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The state-of-the-art design strategies toward highly active catalytic materials and cathode structures for Li–CO2 batteries are reviewed and discussed.

The structure and catalytic activity of the oxygen electrode determine the overall electrochemical performance of lithium–oxygen (Li–O2) batteries. Here, a three-dimensional (3D) porous interconnected network structure combined with ultrathin MoS2 nanosheets with homogeneously dispersed CNTs (MoS2/CNTs) was synthesized via a one-step hydrothermal reaction. The 3D interconnected architecture can efficiently promote the diffusion of O2 and Li ions as well as impregnation of electrolyte and provide more abundant storage space for the accommodation of discharge products, while the incorporation of uniformly dispersed CNTs improves the electronic conductivity and maintains the integrity of the cathode structure. Therefore, the Li–O2 battery based on MoS2/CNTs achieves improved performance with the low overpotentials (discharge/charge overpotentials of approximately 0.29 and 1.05 V), a high discharge specific capacity of 6904 mA h g–1 at a rate of 200 mA g–1, and excellent cycling stability (132 cycles). Experimental studies reveal that the improved electrochemical performance can be ascribed to the synergistic advantages of electronic conductive CNTs and excellent catalytic activity of the MoS2 nanosheets. Moreover, the unique 3D interconnected network structure can effectively facilitate fast charge transfer kinetics and a facile mass transport pathway. These encouraging performances demonstrate the metal sulfide catalyst as a promising catalytic material of oxygen electrodes for Li–O2 batteries.

Catalysts with high performance are urgently needed in order to accelerate the reaction kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium–oxygen (Li–O2) batteries. Herein, utilizing thermodynamically metastable Ti atoms on the Ti3C2Tx MXene nanosheet surface as the nucleation site, oxygen vacancy-rich TiO2 nanoparticles were in situ fabricated on Ti3C2Tx nanosheets (V-TiO2/Ti3C2Tx) and used as the oxygen electrode of Li–O2 batteries. Oxygen vacancy (Vo) can boost the migration rate of electrons and Li+ as well as act as the active sites for catalyzing the ORR and OER. Based on the above merits, V-TiO2/Ti3C2Tx-based Li–O2 battery shows improved performance including the ultralow overpotential of 0.21 V, high specific capacity of 11 487 mA h g–1 at a current density of 100 mA g–1, and excellent round-trip efficiency (93%). This work proposes an effective strategy for researching high-performance oxygen electrodes for Li–O2 batteries via introducing Vo-rich oxides on two-dimensional MXene.

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Lithium-oxygen (LiÀO 2 ) batteries show great potential to become one of the most promising energy-storage and conversion systems owing to their ultrahigh theoretical specific energy (~3505 Wh kg À1 ). However, commercialization of Li-O 2 batteries is constrained by a large charging overpotential caused by the sluggish electrode kinetics and low conductivity of the discharge product, resulting in unsatisfied energy efficiency and poor cyclability. In this paper, aiming to address these issues, we propose unique orderly arranged three-dimensional (3D) flower-like MoS 2 nanospheres combined with carbon nanotubes (f-MoS 2 @CNTs) as an efficient cathode catalyst for LiÀO 2 batteries. Homogeneously dispersed CNTs on the surface of MoS 2 can not only increase the electrical conductivity and thereby lowering the charge overpotential, but also maintain the structural integrity of the cathode, improving the cyclic reversibility. Benefiting from the unique 3D flower-like structure with an interconnected porous network and the excellent catalytic activity of MoS 2 , the LiÀO 2 battery with a f-MoS 2 @CNTs catalyst achieved a lower charge overpotential (1.02 V) and exhibited excellent cyclic reversibility with 141 cycles until the terminal voltage decreased below 2 V at a current density of 500 mA g À1 . The reasonable design of the f-MoS 2 @CNTs-based cathode thus provides a promising solution for practical applications of LiÀO 2 batteries.