Design And Preparation Of Highly Efficient Cathode Catalytic Materials For Li-CO₂ Batteries

With the promotion of global industrialization and continuous population growth,the massive exploitation and consumption of fossil energy have led to energy crisis,environmental pollution,climate change and greenhouse effect.As an efficient,clean and widely used secondary energy,electrochemical energy has become an important support for carbon peaking and carbon neutralization.Besides,the lithium-ion battery energy storage system dominated by the current market is subject to relatively low theoretical energy density,which is difficult to meet the demand of human society for the next generation high-performance energy storage system.Therefore,it is very promising and attractive to explore and develop new energy storage systems with ultra-high energy density.The lithium-carbon dioxide battery system uses the greenhouse gas-carbon dioxide as the reaction gas directly can realize the energy supply and the fixed storage and reduction of carbon dioxide simultaneously,which is conducive to promoting the resource utilization of greenhouse gas and mitigating the greenhouse effect.However,the practical application of lithium-carbon dioxide battery is facing great challenges.Carbon dioxide molecules have stable thermodynamic and kinetic properties.The discharge product lithium carbonate has a wide band gap and is not conductive,resulting in high overvoltage,low energy efficiency and low capacity.Therefore,the design and development of efficient and cheap cathode catalytic materials to enhance the carbon dioxide reaction activity and reduce the product decomposition energy barrier has become the key to improve the performance of lithium-carbon dioxide battery and promote the practical application of the battery.Single-atom catalysts have attracted much attention and been widely used as electrode catalytic materials due to their high atom utilization and clear active sites.Its uniformly dispersed active sites are conducive to the formation of smaller product crystalline particles during the discharge process of lithium-carbon dioxide batteries,thus reducing the resistance of charging reaction and improving energy efficiency.The preparation of single-atom catalysts by doping anchored metal atoms with heteroatoms can regulate the electronic state and charge distribution of metal atoms,thereby improving the catalytic activity.Based on this,this paper is based on the design and synthesis of cheap and efficient single-atom catalysts,and applied them to the research on the performance of cathode catalytic materials for lithium-carbon dioxide batteries.The research contents are as follows:（1）The N atom doped carbon substrate is formed by pyrolysis to anchor metal Cd atoms to prepare Cd monatomic catalytic materials.The results of HAADF-STEM and XAFS indicate that Cd atoms are uniformly dispersed in a single atom form throughout the N-doped carbon network,with a coordination structure of Cd-N₄-C.（2）The effects of calcination temperature,Cd element content,Cd monatomic morphology and Cd nanoparticle morphology on the structure and catalytic activity of the active site of the catalyst were investigated.The results show that the atom level dispersed active site can efficiently realize the electron transfer in the charge discharge process,thus reducing the CO₂ molecular reduction in the discharge process and the decomposition resistance of Li₂CO₃in the charge process.（3）The effects of calcination temperature,Cd content,monatomic morphology and nano particle morphology on the structure of active sites and catalytic activity of the catalyst were investigated.It was found that the use of Cd single-atom catalytic materials as cathode catalyst for Li-CO₂ battery showed excellent performance.The rate performance of Cd SAs/NC＿600 can reach 10 A/g,and remaining stable at a high current density（100 A/g）.The discharge capacity exceeds 160 Ah/g（500 m A/g）;Excellent cycling stability is maintained for 1685 and 669 cycles at 1 A/g and capacities of 0.5 and 1 Ah/g,respectively.At 2 A/g,625 cycles with 500 m Ah/g and 380 cycles with 1 Ah/g were achieved.In terms of capacity and cycle stability,Cd SAs/NC＿600exceeds the recently reported cathode electrode for Li-CO₂ batteries,even the catalysts based on precious metal elements.（4）The mechanism study shows that the discharge and charging process of the battery involves the formation and decomposition of Li₂CO₃ and amorphous carbon.The HAADF-STEM results show that Cd single atoms can still maintain their single atomic form after discharge without aggregating into particle form,and their properties are very stable.The density functional theory calculation results show that the binding energy of the coordination between Cd atoms and four N atoms is lower than the internal fusion energy,and the structure is the most stable.Gibbs free energy diagram shows that the active site of Cd-N₄ has lower energy barrier and higher reaction activity in the key step of discharge and charge reaction.（5）The bimetallic single atom catalytic material is beneficial for directional regulation and modification of the reaction products and mechanism of Li-CO₂ batteries.The use of Cr,Gd/N,S-C bimetallic single atom positive electrode catalytic materials can significantly reduce overvoltage in Li-CO₂batteries and improve the energy efficiency.