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Structural optimization and electrocatalyst utilization in air cathodes have been identified as two major factors affecting the overall performance of a lithium-air battery. Herein, a cobalt@porous carbon composite, Co@C (700-1000) was obtained by facile annealing of a Co/Zn-bimetallic metalorganic framework (MOF). In the Co/Zn-MOF, cobalt cluster was used as a precursor of the catalyst and zinc cluster as a sacrificial template to generate meso-and macropores. The lithium-air battery with the assembled Co@C (700-1000) air cathode revealed a specific capacity as high as 4 mAh cm À2 . Furthermore, the battery exhibited high cycling stability up to 67 cycles (limited capacity of 0.5 mAh cm À2 ). The high cell performance can be in relation to the catalytic activity of uniformly disseminated cobalt nanoparticles in the porous carbon matrix and the rapid diffusion and transport of Li + and O 2 owing to the optimized pore-distribution characteristics. Using a bimetallic MOFderived material, this study sheds fresh light on the design of an air cathode for lithium-air batteries.

The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium–air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium–based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion–exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs.

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A high nickel content of the cathode usually results in a large discharge capacity but causes structural collapse. Ni 2+ ions move to the Li layer when Li + ions are deintercalated during discharge, resulting in irreversible phase transition, cation mixing, dissolution of transition metal ions, and side reactions. A protective barrier is essential for maintaining the layered structures of cathode materials, even after several charge/discharge cycles of Li-ion batteries. Polyaniline (PANi) is an organic coating material with high conductivity and flexibility. PANi-coated cathodes have been widely reported for improving electrochemical performances. However, it is insufficient to prove the correlation between the PANi coating layer and structural stability through further analysis after an electrochemical test. Therefore, we focused on the structural stability and chemical states of the PANi-coated cathode after a cycle test by observing the morphology, lattice patterns, and chemical states of the surface. PANi-coated LiNi 0.9 Co 0.085 Mn 0.015 O 2 (NCM; PANi@NCM) exhibited an initial discharge capacity of 221 mAh g −1 and a capacity retention of 81% after 50 cycles at 45 °C, which corresponded to an improved performance compared to pristine NCM. The cycled PANi@NCM showed an identical morphology to that of the cathode before the test. The R3̅ m layered structure of PANi@NCM was maintained even after 50 cycles, as confirmed by transmission electron microscopy analysis with fast Fourier transform patterns and high-angle annular dark-field images. In addition, PANi@NCM maintains a thinner passivation layer (8 nm) compared with that of pristine NCM (27 nm). According to the X-ray photoelectron spectroscopy results, the surface chemical state of PANi@NCM showed that side reactions between the cathode and the electrolyte were suppressed during the cycle test. Therefore, it is demonstrated that the PANi coating layer prevents cation mixing and side reactions.

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The nickel content of the layered lithium transition metal oxide cathodes is proportional to the discharge capacity of energy storage systems. Ni2+ ion, the oxidation state of Ni, moves to the Li sites when Li+ ion is deintercalated during discharge process due to the similar size of ion (Ni2+: 0.069 nm, Li+: 0.076 nm). The spatial transition of Ni2+ ion causes gradual cation mixing and structure degradation of cathode, resulting in the failure of the battery. Also, thick cathode-electrolyte interphase (CEI) is formed on the cathode surface because of the transition metal dissolution, the degradation of electrolyte, and the residual gas such as H2O and CO2. Coating technology is one of the strategies for the physical protection of the cathode surface and the control of CEI layer thickness. Polyaniline(PANi) is a conductive polymer with stable oxidation state. PANi can enhance the electrochemical property and structural stability as a protective layer. In this study, we synthesized PANi material by the oxidative polymereization and coated NCM with the synthesized PANi by sonication method. The PANi-coated NCM(PANi@NCM) has uniform coating layer with 5 nm thickness. PANi@NCM exhibits an initial discharge capacity of 208 mAhg-1 and a capacity retention of 81% after 50 cycles at 45 oC in Figure 1(a) and (b). After the cycling test, pristine and coated cathodes were disassembled for the comparison of the microstructure. Pristine NCM only shows the structural transition from layered structure to structure in the XRD pattern of Figure 1(c). Cross-sectional SEM image of the pristine NCM after cycling test shows micro and macro cracks are distributed from the bulk to the surface of the cathode as shown in Figure 1(d). On the other side, cracks are barely observed on the PANi@NCM particles in Figure 1(e). The layered structure of PANi@NCM is maintained even after cycling test, which are confirmed by TEM analysis with FFT patterns and HAADF images. Figure (f) and (g) show the FFT pattern of the two cathode surfaces after cycling test. Mixed phase is observed on the pristine cathode surface, while the layered structure is maintained on the PANi@NCM surface. Also, the PANi@NCM maintains a thinner passivation layer compared with that of the pristine NCM (6 nm vs 35 nm). In conclusion, the PANi coating layer prevents the degradation of structure and the formation of thick CEI layer. The structural stability enhances the electrochemical performance. Figure 1

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