Study On Preparation And Properties Of High Performance Nitrogen-rich Organic Electrode Materials

With the development of sustainable energy,lithium-ion batteries（LIBs）are commonly developed in the field of smart grids,electric vehicles,and consumer electronic devices owing to high energy density.However,the electrode materials of traditional lithium-ion batteries are usually based on inorganic compounds,such as transition metal oxides and Li Fe PO₄,which suffer from some problems such as scarce resources,high price,low capacity and hard recyclable.In contrast,organic electrode materials are gaining attention because of their superior structural variability,abundant resources,high redox activity,and environmental friendliness.Hexaazatriphenylene（HAT）and its derivatives（e.g.,hexaazanaphthalene,HATN）are rigid,electron-deficient,and planar structures and they are becoming the focus of research for electrode materials due to high theoretical capacity,π-conjugated structure,and chemical stability.However,HAT-based compounds still have some issues such as high dissolution in electrolyte and low intrinsic conductivity,which limit their cycling stability and rate performance in the reported literature.In view of the above issues,a variety of high-performance HAT and HATN-based polymer electrode materials have been fabricated to enhance their overall electrochemical properties through molecular engineering and micromorphology optimization.In addition,the electrochemical properties,reaction kinetics and mechanism of Li storage are studied.The following are the major investigations:（1）A novel HATN-based polymer（AZO-HATN-AQ）linked by azo and anthraquinone units is designed and fabricated by the polymerization of newly designed trinitro-substituted HATN-based compound（HATNTN）and 2,6-diaminoanthraquinone（DAAQ）under alkaline condition,and used as a cathode material for LIBs.The abundant redox-active sites,extendedπ-conjugated planar conformation,donate-accept（D-A）structure and low energy gap endow the AZO-HATN-AQ electrode with high theoretical capacity,excellent solubility resistance,and fast Li-ion transport.In particular,the fully lithiated AZO-HATN-AQ still keeps the planar structure by theoretical simulations and maintains theπ-πinteraction,which is advantageous to the excellent cycling stability.As a result,AZO-HATN-AQ cathodes show a specific capacity of 240 m Ah g^-1 at 0.05 A g^-1),prominent rate capability(98m Ah g^-1 at 8 A g^-1),and outstanding cycling stability(120 m Ah g^-1 after 2000 cycles at 4 A g^-1 with 85.7%capacity retention)simultaneously.This study demonstrates that rational structure design of the polymer electrodes is an effective approach to achieving excellent comprehensive electrochemical performance.（2）Based on the previous work,in terms of increasing the redox-active sites,HATNTN are self-polymerized through Zn-induced reductive homocoupling chemical polymerization and electrochemical polymerization（HATNTN is used as a cathode material）,and a novel HATN-based azo-linked polymer（AZO-HATN）is constructed.AZO-HATN shows denser redox-active sites than AZO-HATN-AQ,thus achieving a higher specific capacity.Moreover,the extendedπ-conjugation and D-A structure of the AZO-HATN reduce the energy gap and improve the rate performance.Compared with the polymer synthesized by electrochemical polymerization（HATNTN electrode）,the polymer AZO-HATN electrode prepared with Zn-induced reductive homocoupling chemical polymerization delivers a higher specific capacity(262 m Ah g^-1 at 100 m A g^-1),greatly elevated cycling stability(the reversible capacity is 97 m Ah g^-1 after 1500cycles at 500 m A g^-1,approximately 28 times higher than that of HATNTN cathode)and greatly enhanced rate performance(the reversible capacity is 118 m Ah g^-1 at 2000m A g^-1,approximately 90 times higher than that of HATNTN cathode).This study delivers a chemical polymerization approach for constructing extendedπ-conjugated azo polymer electrodes featuring various redox-active sites from nitro compounds.（3）The condition of polymerization significantly affects the degree of polymerization and morphology of the polymers.In this chapter,the degree of polymerization for AZO-HATN is improved by increasing the temperature of homocoupling reaction and the polymer AZO-HATN-150 is prepared,which shows better stability and higher capacity retention under high current density.In addition,the single-walled carbon nanotube（SWCNT）template is used to induce the in-situ growth of nanosized AZO-HATN-150 on the surface of SWCNTs and AZO-HATN-150@SWCNT composites with core-shell nanowire morphology are generated.The porous network structures of AZO-HATN-150@SWCNT nanocomposites enhance the utilization of redox-active sites and capacity significantly,and shorten the pathway of Li-ion diffusion.The high conductivity of SWCNTs and extendedπ-conjugated structure of AZO-HATN-150 enable the rapid electron transfer and Li-ion transport in the AZO-HATN-150@SWCNT,which contribute to excellent rate performance.Furthermore,theπ-πinteraction between SWCNTs and AZO-HATN-150 is beneficial for enhancing the cycling stability.Therefore,AZO-HATN-150@SWCNT cathodes achieve high specific capacity(320.4 m Ah g^-1 at 100 m A g^-1),excellent cycling stability(290 m Ah g^-1 after 800 cycles at 100 m A g^-1 with 91%capacity retention),outstanding rate performance(235 m Ah g^-1 at 2000 m A g^-1)and maintain a high energy density of 491 Wh kg^-1 at a high power density of 9303 W kg^-1.This work demonstrates a combined approach of designing the macromolecular structure and controlling the micromorphology to obtain the high-performance organic polymer electrodes for LIBs.（4）The benzene rings in HATN units are electrochemically inactive.To further improve the specific capacity of electrode materials,hexaazatriphenylene（HAT）units with higher theoretical capacity are used as the core of polymers in this chapter.A novel two-dimensional（2D）π-conjugated phenazine-tetraone linked HAT-based covalent organic framework material（HAPT-COF）is prepared by the condensation reaction of cyclohexanehexaone with 2,3,7,8-tetraaminophenazine-1,4,6,9-tetraone.The abundant redox-active sites and the stable framework endow HAPT-COF with an ultra-high theoretical capacity of 788 m Ah g^-1 and excellent solvent resistance.Additionally,the planarπ-conjugated skeleton promotes the delocalization of electrons in the plane direction,which is beneficial for improving the conductivity of the COF material itself.Through a post-hydrothermal reaction between HAPT-COF and graphene oxide（GO）,we not only achieved the conversion to reduced graphene oxide（rGO）but also construct 2D COF-based nanocomposites（HAPT-COF@rGO）.The effective intercalation and strongerπ-πinteractions between the COF and rGO are established,which lead to improved electrical conductivity,reaction kinetics and structural stability.The theoretical simulation and ex-situ experiments rationalize an overall reversible 18 e^-/18 Li⁺reversible redox process for each repetitive unit.As a result,HAPT-COF@rGO cathodes deliver an ultrahigh reversible capacity(558 m Ah g^-1 at 0.1 C),excellent cycling stability（92%capacity retention after 1000 cycles at 10C）and superior rate performance(318 m Ah g^-1 at 10 C).Meanwhile,HAPT-COF@rGO cathodes maintain a high energy density of 696 Wh kg^-1 at a high power density of 13394 W kg^-1.Notably,these achievements position the HAPT-COF@rGO composite as the top-performing COF-based cathode material reported to date for LIBs.This study offers a pioneering approach to the advancement of high-performance polymer cathode materials.