Designable Synthesis And Lithium Storage Properties Of Nitrogen-containing Organic Cathode Materials

Li-ion batteries occupying the mainstream market in the battery industry usually use inorganic materials such as transition metal oxides and metal phosphates as positive electrodes.Their long-term development will face a series of problems,including:the low actual specific capacity and close to the theoretical value,limited the improvement of energy density;large volume fluctuation during charge and discharge,limited the rate and cycle life;materials mainly come from non-renewable mineral sources,high cost and difficult to recycle;brittle characteristics make it difficult to meet the requirements of flexible electronics.In contrast,organic electrodes are being pursued due to such advantages:composed of light elements,high theoretical specific capacity;easy to control the structure,rational design of electrodes;fast electron transfer reaction;abundant monomer sources,low cost,easy recycling,and flexible processing.At present,organic electrode materials mainly include organic compounds or polymers containing active centers such as C=O,C=N,S,N=N,and free radicals.However,organic electrode materials mainly face problems such as poor intrinsic conductivity,small actual specific capacity（low utilization of active groups）,and dissolution in electrolytes.This dissertation designed and prepared newly organic-inorganic composite electrode materials by compounding nitrogen-containing organic compounds or polymers with conductive carbon（carbon nanotubes,graphene）,and used cyclic voltammetry（CV）,constant current charge-discharge（GCD）and electrochemical impedance（EIS）and other electrochemical techniques,tested and studied the structure-activity relationship and lithium storage mechanism between the material structure and the lithium storage performance of the cathode.The research work includes the following three parts:（1）Used cuprous bromide（Cu Br）/pyridine as the catalytic system,1,3,5-tris（4-aminophenyl）benzene（TAPB）,1,3,5-tris（4-aminophenyl）amine（TAPA）and 2,4,6-tris（4-aminophenyl）-1,3,5-triazine（TAPT）were used as monomers to synthesize the micron-sized spherical conjugated microporous azo polymers by oxidative coupling reaction and labeled as Azo-PB,Azo-PA,and Azo-PT,respectively.Used azo polymers as the organic cathodes active material,it was found that a pair of strong peaks appeared in the range of 1.4～2.0 V for the three electrodes,which was attributed to the reversible redox reaction of the azo group;the Azo-PT framework was rich in the electrochemically active triazine group and the conjugation effect are stronger than that of Azo-PA,resulting in a higher reversible specific capacity and higher rate and cycle stability of the corresponding cathode than Azo-PB and Azo-PA,which provide new ideas to development high performance cathode of LIBs.（2）By mixing carbon nanotubes（CNTs）with 1,3,5-tris（4-aminophenyl）amine（TAPA）and p-phenylenediamine（PDA）monomers,the cuprous-catalyzed in-situ oxidative coupling polycondensation reaction was carried out.The core-shell structure（NHP@CNT）nanocomposites with CNTs as conductive core and hyperbranched azo polymer（NHP）layer thickness of 40-70 nm were obtained.Using NHP@CNT as the electrode active material,it was found that the reversible capacity of the cathode was 122 m Ah g^-1 at 0.05 A g^-1,and with the increased to 1.0 A g^-1,the reversible capacity was 68.1 m Ah g^-1,and the specific capacity can still maintain 85 m Ah g^-1 after 160 cycles;the specific capacity,rate performance and cycling stability of NHP@CNT were higher than pure polymer cathode,which is attributed to the enhanced conductivity,electrochemical reactivity and structural stability of azo polymer by CNTs,at the same time,more fully exposing the active sites of the azo polymer layer.（3）Selected a compound named 1,4,5,8,9,11-hexaazabenzonitrile（HAT-CN）containing a large condensed aromatic ring system with electroactive conjugated hetero-nitrogen groups.Moreover,we developed a universal method,that is,a combination of sonication and hydrothermal process,to scalable synthesis of HAT-CN/graphene composites as novel organic cathodes for high performance LIBs,it was found that two pairs of shoulder peaks appeared in the range of 1.5～2.9 V,corresponding to the multielectron redox reactions of Hexaazatrinaphthalene（HATN）unit in HAT-CN;The reversible capacity is as high as 105.9m Ah g^-1 at 0.05 A g^-1,and the reversible capacity remains 59.7 m Ah g^-1 when increased to 2 A g^-1,and after 60 cycles at 0.05 A g^-1,the specific capacity of HAT-CN/graphene is still as high as about 82.3 m Ah g^-1;HAT-CN/graphene has higher specific capacity,rate capability and cycling stability than pure HAT-CN cathode,due to the graphene inhibition the dissolution of HAT-CN in organic electrolyte,while improve the electrical conductivity,electrochemical reactivity and structural stability compare the pure HAT-CN.