Commonly used high-efficiency catalysts include precious metals and alloys, transition metal oxides, perovskites and metal nitrides, carbides, etc. Among them, transition metal oxides have the advantages of low cost, easy synthesis, abundant reserves, and high catalytic activity, and have been widely used as cathode materials for lithium-oxygen batteries. Cerium oxide (CeO2) is a cristobalite structure composed of face-centered cubic unit cells, which has good structural stability and excellent catalytic activity. Its electron arrangement is relatively unique. During the catalysis process, Ce3+ and Ce4+ are easily converted to each other, which can ensure the continuous and rapid occurrence of charge and discharge reactions. In addition, there are oxygen vacancy defects in the structure of CeO2, which can achieve the effect of an oxygen pump in the discharge reaction of a lithium oxygen battery. It is very promising to use it as a lithium oxygen battery catalyst to achieve a substantial increase in its electrochemical performance.
In view of this, a research group innovatively designed an intelligent and reversible lithium-oxygen battery that decomposes the discharge product Li2O2 based on the ultra-assembly framework assembly mechanism, and used the results of density functional theory calculations to explain the intelligent perception of Li2O2 during the discharge process. path.
Figure 1. Lithium-oxygen battery based on ultra-assembled CeO2/C frame material
Point 1: Build a multi-dimensional and multi-scale super-assembled CeO2/C frame material
CeO2 is a material that catalyzes the formation of an overly thick Li2O2 film and has poor conductivity. For this reason, the researchers designed a carbon matrix with an inverse opal structure to accommodate the volume change caused by the discharge of Li2O2. In addition, when the different crystal faces of CeO2 have different catalytic activities and the particle size is above 5 nanometers, it is not conducive to adsorbing oxygen and superoxide. For this reason, a super-assembled CeO2 nanocrystal was precisely designed so that its exposed (100) crystal face with higher catalytic activity exhibited a cubic structure and its size was controlled to about 5 nm to ensure the realization of the catalytic performance of CeO2 material.
The CeO2 nanocube is modified on the carbon wall of the inverse opal structure to form a multi-dimensional and multi-scale super-assembled CeO2/C framework material, exerting the synergistic effect of the two, thereby further improving the electrochemical performance of the lithium oxygen battery.
Figure 2. Schematic diagram of the super-assembled CeO2/C frame material synthesis.
Key point 2: Characterization of the morphology of the super-assembled CeO2/C frame material
The carbon matrix with inverse opal structure was synthesized by biomineralization method, cerium oxide nanocubes were hydrothermally synthesized by liquid-liquid interface method, and super-assembled CeO2/C framework material was obtained by dipping method. The characterization method is relatively conventional, which mainly proves that the CeO2 nanocubes are uniformly loaded on the inverse opal structure carbon matrix, showing good crystallinity and dispersibility. Among them, the CeO2 nanocubes expose the (100) crystal face family with high catalytic activity. It is beneficial to optimize the electrochemical performance of the lithium oxygen battery.
Figure 3. Morphological characterization of the super-assembled CeO2/C frame material.
Point 3: Electrochemical performance test of lithium oxygen battery with ultra-assembled CeO2/C frame material
After the lithium oxygen electrochemical performance test of the super-assembled CeO2/C material, it is found that when the super-assembled CeO2/C frame material is used as the positive electrode of the lithium oxygen battery, the specific capacity is 13000 mAh/g at 100 mA/g, and when the current increases When it is 400 mA/g, the specific capacity is still maintained at 5000 mAh/g, with excellent rate performance. The super-assembled CeO2/C material can cycle 440 cycles when the capacity is limited to 600 mAh/g, and can cycle 130 cycles when the capacity is limited to 1000 mAh/g. Compared with other cerium oxide composite materials, it shows a stable long cycle. life.
Figure 4. Electrochemical performance test of lithium oxygen battery with ultra-assembled CeO 2 /C frame material.
Key point 4: Exploration of the electrocatalytic mechanism of super-assembled CeO2/C framework materials
Characterization methods such as XRD, HETEM, SEM and XPS have characterized the super-assembled CeO2/C positive electrode in different states during the charge and discharge process. It is found that during the discharge process, the CeO2 nanocubes can be used as active sites to catalyze and adsorb lithium peroxide. Thin film, the lithium peroxide thin film is formed in the carbon pores, which shows that the inverse opal structure carbon matrix can accommodate the volume change caused by the lithium peroxide thin film. During the charging process, the disappearance of the lithium peroxide film indicates that the CeO2 nanocube as an active site can promote the reversible decomposition of the lithium peroxide film.
Figure 5. Characterization of the composition and morphology of the super-assembled CeO2/C frame material under different charging and discharging states.
Figure 6. Using DFT simulation to explore the discharge path of the super-assembled CeO2/C positive electrode.
A super-assembled CeO2/C material is used as the positive electrode material of lithium oxygen battery, which fully combines the high catalytic activity of CeO2 and the conductivity and stability of the inverse opal structure carbon matrix to obtain excellent electrochemical performance. This is mainly because There is a large amount of Ce3+ in the CeO2 nanocube, which can promote the electrochemical adsorption and decomposition of the loosely packed Li2O2 film. This subject provides a theoretical basis for explaining the formation of Li2O2 film during the discharge process, and provides a new research idea for the rational design and development of high-performance cerium oxide materials as the anode of smart lithium-oxygen batteries.
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