Core-Shell Porous Carbon Nanocomposites Derived From Covalent Organic Frameworks For Enhanced Lithium Storage

As a new generation of energy storage devices,lithium-ion batteries（LIBs）have been essential energy storage devices for new energy vehicles,communication equipment,and electronic products.The negative electrode is particularly important in LIBs,as it is the key to various properties such as battery voltage,discharge-specific capacity,and cycle life.With the increasing energy demand,commercial graphite cathodes with lower discharge ratio capacities cannot meet the current supply of energy storage.Therefore,the search for battery cathode materials with better performance is becoming a necessary means to build high-performance LIBs.Transition metal oxides（TMOs）and silicon nanoparticles（Si）are excellent negative electrode materials for energy storage due to their high theoretical capacity,environmental friendliness,and low cost.However,their application in the negative electrode of LIBs is severely hampered by their huge volume expansion and huge capacity loss due to electrode collapse during charging and discharging,low Coulomb efficiency,and poor cycling stability.Therefore,the construction of core-shell porous carbon nanocomposites is the key to reducing the volume expansion of electrodes and improving the electrochemical performance of batteries.Covalent organic frameworks（COFs）are widely used in catalysis,sensing,and hydrogen storage due to their porous structure,an abundance of groups,designable structures,and large specific surface area.However,there are few applications in the field of energy storage,mainly due to the poor electrical conductivity of COFs compared with other carbon materials,and the structural characteristics of easy accumulation lead to a poor energy storage effect.The advantages,however,are the designable porous structure and the abundance of groups that provide space for lithium-ion storage,and the designable crystal structure that provides a flexible shell for TMOs and Si,relieving electrode expansion and maintaining electrode stability.Based on the above research background,this thesis focuses on designing different structures of COFs and using them as carbon nanoshell precursors to prepare NCDs@COF_BTH-TT,Si@NC,and Ce-Fe₃O₄@NC composites.The details of the study are as follows:1.Carbon quantum dots（NCDs）-doped NCDs@COF_BTH-TT composites were synthesized by a one-pot hydrothermal method.The successful doping of NCDs changed the morphology of COF_BTH-TT to obtain a hollow rod-like structure.The hollow structure of COF_BTH-TT not only increases the rate of lithium-ion transport but also exposes more active sites,thereby increasing the storage space for lithium ions.NCDs@COF_BTH-TT exhibited excellent lithium storage performance when applied to the anode of LIBs.The NCDs@COF_BTH-TT exhibited an initial discharge-specific capacity of 336.8 m Ah g^-1 at a current density of 0.2 A g^-1 and was stabilized at 207.1m Ah g^-1 after 226 charge-discharge cycles.The work creatively combines zero-dimensional NCDs with two-dimensional COFs,which is of great significance for the construction of lithium-ion batteries with high stability and ultra-long cycle life.2.Flexible porous COFs were used to encapsulate silicon nanoparticles through a dual ligand modification strategy.One-step high-temperature calcination resulted in core-shell porous Si@NC composites.A flexible NC framework with ordered pores not only improves the efficiency of Li⁺transport but also provides limited space for the volume expansion of the silicon nanoparticles.At high hydrothermal temperatures,the silicon nanoparticles are broken down into smaller Si nanoparticles,greatly increasing the Li⁺storage capacity.Si@NC composites prepared with COF_LZU1 as a shell precursor offer great advantages in terms of lithium storage performance.After 100 cycles at a current density of 100 m A g^-1,the discharge-specific capacity was 1534.8 m Ah g^-1.The method can be extended to other core-shell structures prepared based on COFs for a variety of applications.3.The core-shell Ce-Fe₃O₄@NC composites were obtained by simple solvothermal synthesis of Ce-Fe₃O₄ nanoparticles,followed by high-temperature calcination of Ce-Fe₃O₄@COF_LZU1with COF_LZU1 as the precursor and shell.The successful doping of Ce effectively enlarges the lattice of Fe₃O₄,thus providing more space for the embedding of Li⁺,which can effectively improve the battery cycle performance.The nitrogen-doped porous carbon（NC）formed after calcination of COF_LZU1 not only enhances the lithium-ion transport rate but also provides limited space for the volume expansion of Ce-Fe₃O₄,thus slowing down the effect of the expansion of Ce-Fe₃O₄ and maintaining the stability and cycle reversibility of the electrode.Compared to other Fe₃O₄ materials,it has a greater advantage in lithium storage performance.Discharge-specific capacities are up to 662.2 and 923.7 m Ah g^-1after 500 charges and discharges at current densities of 1000 m A g^-1 and 100 m A g^-1.This strategy of combining a core-shell structure derived from Ce-doped nanoparticles with porous COFs has greater promise for application and provides ideas for the preparation of other core-shell negative electrode materials.