Research On The Preparation And Catalysis Mechanism Of Molybdenum-based Carbon Composites Cathode For Lithium Carbon Dioxide Batteries

Li-CO₂ batteries have an extremely high theoretical energy density and are considered a new type of energy storage device for technological iteration.They are one of the most effective technologies to solve energy and environmental crises.However,there are still many challenges in the practical application of Li-CO₂ batteries at present.For example,lithium carbonate in the discharge product is a substance with wide bandgap and stable chemical properties,which has low reaction activity and high decomposition potential,making it difficult to achieve complete decomposition at a voltage below 4.0 V.In addition,the capture capacity of the positive electrode catalyst for CO₂ and the catalytic activity for CO₂ reduction/evolution reactions are not ideal,which limits the application progress of Li CO₂ batteries with high theoretical specific capacity and high cycling performance.Therefore,the development and design of cathode materials with efficient catalytic activity is the key to solving the above problems.This article is based on carbon tube-based materials,with high catalytic activity transition metal compounds molybdenum carbide and molybdenum nitride as catalysts.Different composite materials were prepared through design experimental methods,and the effect of structural composition on the chemical performance of batteries was systematically studied.The main research content is as follows:（1）A nitrogen rich carbon shell composite nanofiber material integrating MoC nanoparticles and MWCNT inner core was prepared through hydrothermal and carbonization reactions NC@MoC/CNTs.The formation of the material before and after charging and discharging was analyzed and studied.Before discharging,there were obvious gaps between NC@MoC/CNTs nanofibers,which were conducive to the transmission of CO₂ and electrolyte through the electrode;After discharge,Li₂CO₃uniformly covers the surface of NC@MoC/CNTs nanofibers;After charging,Li₂CO₃ is basically decomposed.This indicates that NC@MoC/CNTs nanofiber materials have a regulatory effect on the morphology of discharge products and have good catalytic activity for the decomposition of discharge products Li₂CO₃.At a current density of250 mA g^-1 and a limited capacity of 1000 mAh g^-1,high cycle stability was achieved over 150 times,demonstrating good cycle performance.Under the experimental conditions of 250 mA g^-1 current density and 2.25 V cutoff voltage,the specific discharge capacity of the first cycle reached 10965 mAh g^-1.By combining material characterization and theoretical calculation analysis,the adsorption performance of electrode materials for CO₂ and Li₂CO₃ was explored,providing ideas for improving the catalytic activity design of positive electrode materials.（2）An integrated nitrogen doped carbon layer,MoN nanoparticles,Co nanoparticles,carbon nanotubes,and carbon paper were prepared through in-situ growth and nitridation reaction of carbon nanotubes NC@Co/MoN/CNTs/CP self supporting composite materials.The in-situ growth of carbon nanotubes on the surface of carbon paper and carbon fibers avoids the addition of binders,prevents side reactions caused by the decomposition of binders in high potential states of the battery,and improves the exposure rate of catalytic sites;On the other hand,it maintains the pore structure of carbon paper,which is conducive to the rapid transmission of Li⁺and CO₂,as well as the permeation of electrolyte.At a current density of 50 mA g^-1 and a limited capacity of 500 mAh g^-1,over 126 stable cycle lifetimes were achieved;Under the current density of 50 mA g^-1 and the cutoff voltage of 2.25 V,the specific discharge capacity of the first cycle reached 10712 mAh g^-1,which provided an idea for the design and synthesis of Li-CO₂ battery self supporting structure positive pole.（3）A Co/Mo₂C/CNTs composite material integrating carbon nanotubes,Co nanoparticles,and Mo₂C nanoparticles was prepared through high-temperature carbonization reaction.The appropriate pore size distribution of Co/Mo₂C/CNTs composite material promotes the transport of Li⁺and CO₂,as well as the permeation of electrolyte;The high specific surface area of carbon nanotubes provides sufficient space for the deposition of discharge products;In addition,Co nanoparticles and Mo₂C nanoparticles are uniformly loaded on carbon nanotubes,enhancing the catalytic activity of carbon dioxide reduction/precipitation reaction,promoting the uniform deposition of discharge products on the surface of carbon nanotubes,and inhibiting the large agglomeration of discharge products.When the current density is 50 mA g^-1 and the limited capacity is 500 mAh g^-1,the terminating charging voltage in the first 40cycles of charging and discharging is only 3.71 V;Under the current density of 50 mA g^-1 and the cutoff voltage of 2.35 V,the discharge specific capacity of the first cycle reached 9278 mAh g^-1,which provided an idea for the design and synthesis of high-performance cathode.