Exchange unit: Jilin University
Paper doi:10.1038/s 41467-020-15712-z.
In view of the low energy conversion efficiency caused by the slow reaction kinetics of lithium-oxygen battery, researchers usually develop efficient and stable cathode catalysts to reduce the charging polarization voltage of the battery and improve the reaction power. In this work, cobalt atoms were immobilized on nitrogen-doped carbon spherical shell carrier for efficient catalytic reaction of lithium-oxygen battery. It is found that the formation and decomposition of Li2O3 are related to the adsorption energy of LiO2 on monoatomic catalysts. It is clearly pointed out that in the process of discharge, the active sites dispersed by atoms can induce uniform nucleation and epitaxial growth of discharge products, and finally form favorable nano-flower-shaped discharge products. During the charging process, the weak adsorption of the active center of CoN4 on the discharge intermediate LiO2 can induce the charging reaction to change from two electron paths to one electron path. Because the energy level structure and electronic structure of highly dispersed Co-N monoatomic catalyst have changed fundamentally, the charging efficiency and cycle life of the battery have been greatly improved. Compared with the noble metal-based catalyst with the same content, the charging and discharging polarization voltage of 600 mV is reduced, and the long life cycle of 2 18 days is realized.
The theoretical capacity density of lithium-oxygen battery is 0/0 times higher than that of lithium-ion battery/KLOC-,and it is known as a subversive and revolutionary battery technology. However, the battery is still in the initial stage of research and development. Due to the slow electrochemical reaction kinetics of ORR and OER, the actual capacity, rate performance, energy efficiency and cycle life of the battery are still far from the industrial application level. Therefore, it is urgent to develop efficient and stable catalysts to improve the reaction capacity and cycle efficiency of the battery. Atomic-scale nanocrystals have the highest atomic utilization efficiency and unique structural characteristics, and often show different activity, selectivity and stability from traditional nanocatalysts, which provides many possibilities for regulating the electrochemical reaction process. In lithium-oxygen battery, the soluble LiO2 intermediate in electrolyte can regulate the formation and decomposition pathway of discharge product Li2O3. Previous research results show that [1], different formation pathways are related to the adsorption energy of LiO2 _ 2 _ 2 on different crystal planes of the catalyst. Therefore, it may be a new idea to explore the influence of the size effect of monoatomic catalyst on the adsorption energy of LiO2 _ 2 _ 2. This new discovery will provide more choices for designing lithium-oxygen batteries with high energy efficiency and long cycle life.
Monoatomic catalyst is a very important electrocatalyst. Its unique monodisperse structure combines the advantages of homogeneous catalyst and heterogeneous catalyst, and has the largest metal utilization rate, excellent catalytic activity and stability. At the same time, the active center of SACs is relatively simple and easy to control. This unique structure and performance make monoatomic catalyst an ideal material platform for catalytic mechanism research and performance optimization. However, what kind of sparks will be generated when the monoatomic catalyst meets the lithium-air battery? In this paper, nitrogen-doped carbon hollow spheres embedded with Co atoms were designed and synthesized by in-situ polymerization technology, and their charge and discharge processes were analyzed in detail. The results show that the maximum exposure of N-HP-Co and the uniform distribution of active sites of CoN4 atoms on the carbon spherical shell reduce the adsorption capacity of LiO2, effectively change the reaction path of the battery, greatly improve the reaction kinetics of the battery and greatly improve the performance of the battery.
▲ Figure 1 Synthesis process of monoatomic catalyst.
Monoatomic catalysts show high catalytic activity in many catalytic reactions because they improve the uniformity of active centers and the high controllability of coordination environment. Therefore, monatomic Co catalyst was applied to lithium-oxygen battery to explore its influence on the reaction path of Li2O2 formation and decomposition. We used in-situ polymerization method, using silicon dioxide as template and dopamine hydrochloride as carbon source, and pyrolyzed at 900℃ in nitrogen atmosphere.
▲ Figure 2 Characterization of monoatomic catalyst. A, b) SEM image of the sample (A: 1 micron; b:200nm)； C) TEM image of the sample (main image: 200nm；; Illustration:10 nm); D) EDX elemental analysis of the sample (50 nm); E, f) HAADF- stem image of the sample (E: 50 nm; f:2nm)； G) XRD images of samples and comparative materials; H) N 1s XPS spectrum of the sample; I) Nitrogen adsorption curves of samples and comparative materials.
▲ Figure 3 Atomic structure analysis of monoatomic catalyst. A) XANES spectrum of the sample; B) Fourier transformed Co-K edge spectrum of the sample; C, d) EXAFS fits the sample curves in K and R spaces.
Nitrogen-doped carbon spherical shell as carrier is the key step to anchor Co atoms. The successful preparation of monoatomic Co and the existence of highly active sites of CoN4 were confirmed by key characterization techniques such as high-angle annular dark-field spherical aberration electron microscope (HAADF), energy dispersion spectrum (EDX) and X-ray absorption spectrum (XAS).
▲ Figure 4 Study on discharge mechanism of monoatomic catalyst. A) Discharge curves of samples and comparative materials; B) CV curves of samples and comparative materials; C) Multiple properties of samples and comparative materials; D, e, f) SEM images and corresponding XRD spectra (500 nm) of the discharge products of samples and comparative materials; H, i) Discharge mechanism diagrams of samples and comparative materials.
Due to the existence of N-HP-Co capsule, the uniform distribution of exposed CoN4 monatomic active sites on the carbon spherical shell greatly improved the redox reaction kinetics of the electrode, accelerated the generation rate of discharge product Li2O2, and greatly improved the discharge capacity and rate performance of the battery. Compared with the noble metal catalyst with the same content, under the same current density and capacity, N-HP-Co SACs has more reactive sites, which is more conducive to the formation of nano-flake-like Li2O3, and further assembles to form a favorable nano-flower-like Li2O3 through "epitaxial growth". This special discharge mechanism is conducive to breaking the limitation of charge transmission and the essence of electrochemical insulation of discharge products.
▲ Figure 5 Loading characteristics of monoatomic catalyst. A) Ultraviolet-visible spectrograms of samples and comparative materials at different charging stages; B) The charging mechanism diagram of the sample; C-h) Adsorption energy of LiO2 _ 2 _ 2 with different structures on samples and comparative materials.
In order to fully understand the charging mechanism of CoN4 single-site catalyst, density functional theory (DFT) calculation shows that complex coordination environment can significantly change the adsorption capacity of central metal atom CoN4 for LiO2*, thus adjusting the activity and selectivity of the reaction. It can be seen that the weak adsorption energy of CoN4 active center to LiO2, the discharge intermediate, is beneficial to improve the solubility of LiO2 in electrolyte and induce the charging reaction process to change from two-electron path to one-electron path. Thereby being beneficial to improving the charging efficiency of the battery.
▲ Figure 6 Cyclic stability of lithium-air battery. A) Cyclic performance of samples and comparative materials; B-e) SEM images (b, d: 1 micron; c，e:500nm)； F, g) XPS spectra of discharge products of samples and comparative materials in different cycles.
Monoatom-catalyzed lithium-air battery can effectively inhibit the occurrence of side reactions and show excellent cycle stability, which fully verifies the important role of catalyst in accurately regulating discharge products to stabilize the battery system.
▲ Figure 7 Stability of monoatomic catalyst during circulation. A) XPS spectrum of the sample after full cycle; B) EDX spectrum (200 nm) of the sample after multiple cycles; C) XANES spectrum of the sample after repeated cycles; D) Co-K edge spectrum of Fourier transform of the sample after multiple cycles.
After 50 cycles of N-HP-Co, the monoatomic structure of Co still exists. The inherent stability of Co atoms on carbon supports makes them have excellent durability in electrochemical reactions. This remarkable advantage, combined with the advantage of low cost, provides a new strategy for the adjustability of metal monoatomic catalyst in the reaction route of lithium-oxygen battery.
The synthesis of monoatomic catalyst was inspired by the growth process of strawberry. Nitrogen-doped Co monoatomic catalyst was prepared by in-situ polymerization using silica as template. Due to the essential characteristics of monoatomic catalysis, the low coordination environment and the synergistic effect between monoatomic and carbon spherical shell can accurately control the generation and decomposition of discharge products in lithium-oxygen batteries. Compared with noble metal catalysts with the same content, monoatomic catalysts can not only adjust the morphology of discharge products, but also increase the discharge capacity, avoid excessive side reactions and greatly improve the electrocatalytic performance of the battery. The concept, design, preparation and catalytic mechanism of monoatomic catalytic cathode put forward in this study will provide new research ideas and scientific basis for the development of new catalysts in the field of lithium-air batteries, which is clearly leading and pioneering.
refer to
[1] Yao, W. T. et al., adjusting the formation route of Li2O2 through the small plane engineering of MnO2 cathode catalyst. J. Am。 Chemistry. Socialists, 2019,141,12832-12838.
Xu, 198 1, a native of Shanxian County, Shandong Province, is currently a professor and doctoral supervisor at the School of Chemistry of Jilin University, the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, and the Joint Laboratory of Future Science International Cooperation. Deputy Secretary-General of Technical Committee for Standardization of Optical Crystals. Mainly engaged in basic research and technology development in the field of porous new energy materials and devices. His research interests include key materials and devices of lithium (sodium, potassium, zinc) ion batteries, new chemical power sources such as lithium-air (sulfur, carbon dioxide) batteries, and new energy storage and conversion systems assisted by external fields (light, force, magnetism and heat). In recent five years, * * * has published more than 50 SCI academic papers, among which Nat is the first author/correspondent. The third community, Nat. Energy 1, environmental chemistry international edition 2, advanced materials 3, energy environment. Sci. 1 etc. Up to now, the papers have been cited by him for more than 4,000 times, with the highest single citation of 360 times. 12 papers were selected as highly cited papers by ESI, and the research results were reported as highlights by Nature and Science. Authorized invention patents and national defense patents 10. He has won awards or honors such as "Global Highly Cited Scholar" (20 19), top innovative talents in Jilin Province (20 19), Jilin Youth Science and Technology Award (20 18) and discipline leader of Jilin University (20 18).