Template-based Construction Of Highly Active Catalysts And Their Application In Li-O₂ Batteries

The development of next-generation rechargeable batteries with long life and high energy capacity is essential for power storage systems and electric vehicles.The invention of Li-O₂ batteries（LOBs）bring hope to the secondary battery to break through the energy density bottleneck,as the higher theoretical specific energy compared with other rechargeable technologies.However,due to the slow reaction kinetics caused by the insulation and insolubility of the discharge product Li₂O₂ in the organic electrolyte,and the participation of oxygen in the reaction leads to excessive polarization during discharge and charge process,Li-O₂ batteries still face many scientific challenges,such as low energy efficiency,poor rate capability,and short cycle life,etc.Therefore,there is an urgent need to develop effective catalysts to improve reaction kinetics and reduce polarization during charge and discharge process.Metal-organic frameworks（MOFs）with ultra-high specific surface area,excellent electronic conductivity,high porosity,and tunable chemical properties and functionalities are considered as favorable supports for the preparation of highly active catalysts.The graded carbon molecular sieve has a mesoporous and mesoporous structure,which can accelerate the migration of ions,and the hydroxyl groups on the framework can enhance the adsorption capacity of metal precursors,which is an ideal carrier for providing excellent mass transfer.In this paper,MOFs and hierarchical carbon molecular sieves are used as templates to prepare transition metal oxides,nitrides,and alloys with different morphologies as catalysts for Li-O₂ batteries（LOBs）and uses the theories of interface regulation and synergistic effects to modify them in a targeted manner and enhance their bifunctional catalytic activity.The main innovative results of the work are as follows:（1）Preparation of core-shell Co3O4@Ni Co2O4 catalyst with ZIF-8@ZIF-67double MOFs structureBy using the similar structure of ZIF-8 and ZIF-67 with rhombic dodecahedron structure as templates,the core-shell structure of ZIF-8@ZIF-67 was synthesized through aging.Evaporate Zn at high temperature,leaving many vacancies to obtain Co₃O₄ with a relatively high specific surface and high porosity,which accelerates the ion migration rate and provides more space for storing the undecomposed discharge product Li₂O₂.And Ni Co₂O₄ is recombined to release more active sites and improve the reaction kinetics during the charge and discharge process of Li-O₂ batteries.Using this material as the cathode catalyst of Li-O₂ batteries,the battery rate and cycle performance have been improved.When under the current density of 500 m A g^-1 with the limited specific capacity of 1000 m A h g^-1,the battery can be stably cycled 280 cycles.（2）Preparation and application of a self-catalyzed Co3O4/CNTs material byconfinement methodAiming at the problem that the high temperature environment will cause the mixing of metal atoms and the evaporation of carbon atoms,resulting in the reduction of the active sites and the stability of the catalyst,a strategy of using stable m Si O₂ balls at high temperature to wrap ZIF-8@ZIF-67 particles is proposed to reduce the accumulation of metal substances during carbon evaporation and pyrolysis to obtain more active sites.At the same time,the use of m Si O₂ confinement allows Co to autocatalyze the generation of carbon nanotubes（CNTs）,and the prepared Co₃O₄/CNTs material has excellent oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）dual-functional catalytic activity.When used as a Li-O₂ battery catalyst,the battery has a high specific capacity of13663 m A h g^-1,a small overpotential,the overvoltage is 1.02 V,excellent rate performance,under the high current of 1000 m A g^-1,the discharge specific capacity is still 2985.4 m A h g^-1.（3）Autogenous growth of Nitrogen-Doped carbon nanotube composites usingMOFs as templates and analysis of their catalytic performanceN-doped CNT materials with encapsulating metals can not only facilitate electron transfer but also serve as complementary sites for electrochemical reactions.Therefore,fabrication of N-doped CNT materials with ideal porosity and sufficient active sites by a facile and cost-effective strategy is very attractive for improving the catalytic performance of materials.In this chapter,Co²⁺was used as the metal ligand and 1,3,5-trimellitic acid as the metal ligand,the MOF template of Co-BTC was synthesized,then using dicyandiamide as a nitrogen source,Ni/Co₂N@NCNTs was autocatalyzed to generate in an inert gas atmosphere,which was used as a cathode catalyst for Li-O₂batteries.The battery showed a very high discharge specific capacity,under the current density of 100 m A g^-1,the discharge specific capacity of Ni/Co₂N@NCNTs reaches23568.2 m A h g^-1,and the overvoltage of the battery is effectively reduced.Under the current density of 100 m A g^-1,with the limiting specific capacity of 1000 m A h g^-1,the overvoltage is only 0.76 V,meanwhile,the rate performance and cycle performance have been greatly improved,when under the current density of 500 m A g^-1 with the limited specific capacity of 1000 m A h g^-1,the battery can be stably cycled 220 cycles,proving that it has excellent ORR and OER dual-functional catalytic activity.（4）Preparation and application of highly active Li-O2 battery catalyst based on Co3O4/Fe2O3 heterostructureCo-NTA was used as a template to synthesis a rod-shaped Co₃O₄,and then a Co₃O₄@Fe₂O₃heterojunction composite material is generated through a hydrothermal reaction.Compared with pure Co₃O₄,Co₃O₄@Fe₂O₃heterojunction composites have more active sites and oxygen vacancies,Co₃O₄and Fe₂O₃cooperate with each other,and the catalytic activity of the composite s i greatly improved.When it was used as Li-O₂battery catalysts,the capacity of the battery is improved,and the rate performance and cycle performance are greatly improved,and t i can be cycled stably for 280 cycles.Density functional theory（DFT）calculations show that the two materials have strong electronic coupling effects at the grain boundaries and edge positions,and there is obvious electron transfer in the interface region,which can significantly reduce the adsorption energy of the Li O₂intermediate,thereby effectively reducing the overpotential.（5）Highly stable Ru Co alloys confined in graded carbon molecular sieves as catalysts for Li-O2 batteriesAlloy materials have excellent catalytic activity,but pure alloy nanoparticles are prone to agglomeration,which limits the rate capability and cycle performance of batteries.With customizable porosity and high surface area,carbon materials are ideal substrates for carrying active metal substances.Therefore,the combination of highly active metal substances with carbon matrix to construct a coupled catalytic system is an effective strategy to improve catalytic activity.This chapter uses MCM-48 as a hard template for the synthesis of graded carbon molecular sieve CMS with mesoporous and microporous structures.Taking advantage of its structural characteristics,Ru Co alloys were uniformly dispersed on carbon molecular sieves to obtain highly active Ru Co-CMS catalysts,which were used as LOBs catalysts.The battery performance has been greatly improved.At a current density of 100 m A g^-1with a li miting specific capacity of 1000 m A h g^-1,the overpotential is only 1.05 V.Meanwhile,the cycle performance of the battery has been improved,it can cycle stably for 298 cycles.