With the noticeable progress in science and technology and the economic booming, humans grow increasingly reliant on energy. However, non-renewable energies like petroleum and natural gas that account for 85% of the total energy supply are currently running away rapidly. This will not only lead to energy crisis, but will also cause a number of environmental problems, including air pollution and greenhouse effect. Despite some clean energies, wind energy, solar energy and tide energy that are being used, they are vulnerable to influences of regions and climate. As such, people are focusing on storage of energy. Traditional lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries have problems like heavy weight and low energy density, and the fuel cells that are used in high-tech areas like military and aerospace are expensive and costly and may cause potential hazards. For this reason, to cope with the energy crisis and environmental problems, and to develop energy-storing power supply and equipment power for civil/military aerospace field and electric cars, it is urgently needed to develop high-density and large-capacity batteries that are rechargeable.
Lithium oxygen batteries are favored by researchers due to its ultra-high theoretical energy density (~ 3,500 wh kg − 1), and are considered as one of the important candidate systems for the next generation of high-energy chemical power. A lithium oxygen battery is composed of metal lithium anode, electrolyte, diaphragm and porous air electrode (i.e. the positive electrode). For the positive electrode, challenges include slow kinetic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the three-phase interface (oxygen/electrode/liquid electrolyte). In the process of charging and discharging, insulating and insoluble Li2O2 will be produced in the air electrode, which will lead to the blockage of mass transfer and charge transfer channels and positive passivation, and increase of impedance, resulting in very high overpotential (ORR of 0.2 ~ 0.3 V, OER of 1.0 V), resulting in extremely low energy efficiency and poor cycle performance. Therefore, it is urgently needed to develop an efficient ORR and OER cathode catalyst layer and diffusion layer. On the one hand, the positive catalyst can not only affect the discharge/charge potential, but also determine the rechargeability of the battery and the contact interface of Li2O2/cathode. On the other hand, the use of binder will increase the side reaction and produce additional by-products, which will lead to unstable performance. According to Sabatier principle, the catalytic center should have suitable binding energy to adsorb the reaction intermediates, and the adsorption energy should be neither too high nor too low. Therefore, it is very important to design catalysts with appropriate binding energy to efficiently catalyze ORR/OER, rich electrocatalysis centers to improve ORR/OER performance, and adjustable electronic state to improve intrinsic catalytic kinetics and activity, which are very important for the development of high-performance lithium-ion batteries. In view of this, Xu Chaohe's team of Chongqing University has recently synthesized ORR and OER bi-functional single atom electrocatalysis materials with porous structure, stable and high efficiency and with no binder, through space confinement and ion replacement strategy, which significantly reduces the charge/discharge overpotential of lithium-ion batteries and greatly improves the electrochemical performance of the batteries.
Ru sites that were highly evenly distributed in the organic frameworks of foamed metal were observed using the spherical aberration corrected high-angle annular dark field scanning transmission electron microscopy. X-ray absorption fine structure (XAFS) was used to further study the chemical state and coordination environment of Ru sites. The scattering path of Ru-N was found through Fourier transform, and the scattering path of Ru-Ru metal bond was not found. The average coordination number of Ru was obtained as 4 through matching of the extended margin in R space, i.e., Ru-N4 coordination structure. In addition, by controlling the adsorption capacity of metal precursors, Ru SAs-NC with different single atom loads was extended, and the maximum loads of Ru monatoms could be as high as 6.82%.
Fig. 1 Typical preparation roadmap of Ru SAs-NC and Ru NPs-NC
In general, monoatomic catalysts can maximize the number of catalytic active centers and effectively adjust the electronic structure, thus giving the electrocatalyst superior ORR/OER performance. By using the optimized Ru0.3 SAs-NC (the load of monoatomic Ru is 2.48%) as the electrocatalyst, the constructed oxygen lithium battery is able to provide discharge capacity as high as 13,424 mA h g−1 under the condition of 0.02 mA cm−2. The lowest ORR overpotential can be as low as 0.17V given the cutoff specific capacity of 1,000 mA h g−1. The results of in-situ DEMS showed that the e−/O2 value of the discharge process was only 2.14, indicating that the electrocatalytic performance was superior and the reversibility was good. In addition, the high activity of the monoatomic catalyst and the large high load of single atoms are conducive to the formation/decomposition of nano scale flower like Li2O2 in the electrochemical reaction process. DFT simulation further proved that the rate limiting step of ORR process on the catalyst surface was 2e-reaction to generate Li2O2, and the OER process was the decomposition reaction of Li2O2. The results pointed out the direction for the future construction of lithium oxygen battery electrocatalyst, and provided reference for the preparation of monoatomic related materials and better understanding of the structure-activity relationship.
Fig. 2 Theoretical calculation results. Source: J. Am. Chem. Soc.
The above research findings have been published in a top international journal, Journal of American Chemical Society (JACS, IF=14.61) under the title “Ru Single Atoms on N-Doped Carbon by Spatial Confinement and Ionic Substitution Strategies for High-Performance Li–O2 Batteries”. Hu Xiaolin, a doctoral candidate from the School of Aerospace Engineering, is the first author. Researcher Xu Chaohe from the School of Aerospace Engineering is the corresponding author.
