Application And Mechanism Research Of Single Atom Catalysts In Lithium-O₂ And Lithium-CO₂ Batteries

Li-Air batteries(including Li-O2 battery and Li-CO2 battery)have attracted extensive attention and research due to their extremely high theoretical energy density and high output voltage,and have even been regarded as the main candidates for the next generation of energy storage and conversion systems.Due to their unique advantages,Li-O2 battery is considered to be the most ideal power system for longrange electric vehicles,while Li-CO2 battery has a bright application prospect in some high CO2 concentration environments(such as Mars exploration,underwater operations).However,many mutual and unsolvable challenges have seriously restricted their development and application,among which the most intractable problem is their slow and lagging reaction dynamics.Specifically,during the discharge process(ORR and CRR),solid discharge products occupy a large amount of pore space and active site of the cathode catalyst,resulting in low discharge capacity and active site covered inactivation.While in the charging process(OER and CER),insoluble discharge products with wide band gap are often difficult to be completely catalyzed decomposition,and gradually accumulate on the positive electrode surface,which will lead to the increase of overpotential of the battery,and further affect the round-trip efficiency and cycle stability of the battery.Cathode catalysts with high catalytic activity and suitable pore structure are considered to be the optimal solution to this problem.Unfortunately,cathode catalysts reported in LiAir battery often have problems such as low catalytic activity,low utilization rate of active sites and high cost.Hence,take the single-atom catalysts(SACs)with ultrahigh catalytic activity,high utilization rate of active sites and low metal consumption as the entry point,we successfully prepared suitable and efficient SACs for Li-O2 and Li-CO2 battery respectively through reasonable design experiments.In addition,we also systematically studied the mechanism of SACs in the batteries.The main research contents are as follows:1.Ru single atoms were anchored to Co3O4 nanosheet arrays grown on carbon cloth(SA Ru-Co3O4/CC)by a simple,environmentally friendly and low-cost ionexchange-ligand sacrifice strategy,and used as efficient dual-function catalyst for LiCO2 battery.Li-CO2 batteries based on SA Ru-Co3O4/CC cathodes show excellent electrochemical performance.For example,the overpotential is as low as 1.05 V at 100 mA g-1 current density,and the discharge capacity is 30915 mAh g-1;Under 200 mA g-1 current density and 800 mAh g-1 cut-off capacity,the cycle life is as long as 303 cycles(2424 hours).The density functional theory(DFT)calculations show that the single atom Ru species anchored on the surface of Co3O4 nanosheets can change the growth path of the discharge products into the surface-adsorption growth path by enhancing the adsorption of the key reaction intermediate Li2C2O4,thus optimizing the morphology and distribution of the discharge products.In addition,single atom Ru as the active site can also greatly improve the CRR and CER reaction kinetics of Li-CO2 battery.Hence,Li-CO2 batteries based on SA Ru-Co3O4/CC cathodes show excellent electrochemical performance.2.Based on the first work,a universal single-atom catalyst preparation strategy was developed.Eight transition metal atom doped Co3O4 nanosheets(MSA-Co3O4/CC)were prepared and used as cathode catalysts for Li-O2 battery.The experimental results show that,compared with the original Co3O4/CC cathode,some MSACo3O4/CC show better catalytic activity for Li-O2 battery,while others show worse catalytic activity.Considering that MSA-Co3O4/CC doped with different metal atoms has similar physical and chemical properties(such as morphology,size,crystal phase,atomic load,etc.)and the same growth pathway of discharge products,we believed that the differences in their catalytic activities for Li-O2 battery are attributed to their intrinsic catalytic activities,and DFT is used to explain the mechanism in depth.The results show that the energy barrier at the active site and the interaction between the key substances and the active site(electron transfer tendency and adsorption energy)are the main reasons for the difference in catalytic activity of MSA-Co3O4/CC doped with different metal atoms for Li-O2 battery.