Research Of Organic Small-molecule Cathode Materials In Lithium/Sodium/Potassium-ion Batteries

Compared with the traditional inorganic cathode materials,organic counterparts show great potential to become the new generation of energy storage materials for secondary batteries due to low cost,environmental friendliness,and structural diversity with adjustable performance,thus having aroused extensive research attentions.However,there are still some shortcomings,such as limited active sites,low energy density and poor conductivity,which need to be solved urgently for organic cathode materials.In addition,organic solids primarily comprise molecule agglomerations held together by weak van der Waals forces,and they are easily destroyed by the forces of solvent molecules.As a result,organic small-molecule materials face severe dissolution issues in conventional electrolyte systems,resulting in rapid capacity decay of batteries during charge-discharge cycling.This consequence causes an adverse impact on the practical application of organic cathode materials.Based on the above problems,this dissertation reports several organic small-molecule cathode materials with electrochemical activity,which are designed and synthesized by simple operation methods,such as surface self-carbonization,in-situ electropolymerization and molecular engineering.Consequently,their improved electrode performance in lithium/sodium/potassium-ion batteries and increased feasibility of commercial application can be achieved.The main contents and conclusions are as follows:1.To address the dissolution issue of organic small-molecule cathode materials,herein,an effective surface self-carbonization strategy for organic electrodes is proposed.In addition,a new soluble organic small-molecule compound called[N,N’-bis（2-anthraquinone）]-1,4,5,8-naphthalenetetracarboxdiimide（NTCDI-DAQ）is prepared to verify this concept.Through precise control of the carbonization temperature and time under an inert gas atmosphere,an amorphous carbon layer with a thickness of 30-40 nm is successfully formed on the surface of NTCDI-DAQ particles（NTCDI-DAQ@C）,which eliminates the dissolution of NTCDI-DAQ without affecting its electrochemical properties.Therefore,the electrode performance of NTCDI-DAQ in potassium-ion batteries（PIBs）is significantly improved.In PIBs full cells with the KC₈ anode,NTCDI-DAQ@C can achieve the peak discharge capacity of 236 m Ah g_cathode^-1,the energy density of 255 Wh kg_cathode^-1,and the capacity retention of 40%after 3000 cycles at 1 A g^-1.2.In order to further expand the application range of NTCDI-DAQ@C with unique organic-carbon core-shell structure,its electrochemical behavior continues to be studied in sodium-ion batteries（SIBs）as a cathode.Furthermore,the influence of electrolyte concentration on the performance for the fabricated SIBs is explored.The experimental results indicate that the Na-ion storage performance of the NTCDI-DAQ@C can be better,when the 4 mol/L ether-based electrolyte system with 1,2-dimethoxyethane as the solvent and the sodium hexafluorophosphate as the electrolyte salt is selected.In SIBs full cells with the Na₃Bi anode,NTCDI-DAQ@C delivers the peak discharge capacity of 245 m Ah g_cathode^-1,and the energy density of 245 Wh kg_cathode^-1.Meanwhile,after 8000 cycles at 1A g^-1,NTCDI-DAQ@C still remains the capacity retention of 79%.3.In this work,a bipolar organic small molecule called[N,N’-bis（4-（9H-carbazol-9-yl）phenyl）]-perylene-3,4,9,10-tetracarboxydiimide（PTCDI-DPC）is designed to solve the dissolution problem of organic small-molecule cathode materials and further enhance their electrochemical performance for lithium-ion batteries（LIBs）.It consists of two p-type carbazole groups which can adsorb anions,and one n-type perylene-3,4,9,10-tetracarboxydiimide motif that stores Li cations.Most importantly,the carbazole groups can serve as both redox active sites and electrochemical polymerization reaction positions.Therefore,the in-situ electrochemical polymerization reactions can happen during the high-potential charge process,leading to the formation of conjugated polymers with cross linkage structure,which not only simplifies the pre-synthesized course in traditional methods for polymer cathode fabrication,but also effectively addresses the dissolution issue of organic small molecule materials in electrolytes.Accordingly,the superior rate capability and stable cycling ability of PTCDI-DPC are presented in LIBs.The LIBs full cells assembled with the reduced state（Li C₆）of graphite as the anode and the oxidation state（2PF₆-PTCDI-DPC）of PTCDI-DPC as the cathode can provide the peak discharge capacity of 157 m Ah g_cathode^-1,the high energy density of 385 Wh kg_cathode^-1,and the long cycle life of 2000 cycles.4.Developing cathodes with high working potential（>3 V）plays a paramount role in improving the energy density for rechargeable batteries.Nevertheless,the number of such organic cathodes is extremely rare,and their battery performance can still hardly compete with the commercial inorganic cathodes at present.Herein,a p-type organic small-molecule cathode called[N,N’-bis（triphenylamine）]-dihydrophenazine（PZ2TPA）is specially synthesized.PZ2TPA structurally features the fourπ-conjugated nitrogen atoms as the redox centers,which can be intercalated into four anions from electrolytes while oxidizing to[PZ2TPA]⁴⁺,and thus exhibiting a theoretical capacity of 160 m Ah g^-1.Then,the PZ2TPA cathode is applied to the LIBs/SIBs full cells based on graphite（C）/hard carbon（HC）anode.And the constructed LIBs/SIBs can be successfully activated and operated by the first charge process without any electrode pretreatments.Besides,the LIBs/SIBs can show the two high voltage slopes at 3.79,3.01 V/3.77,3.04 V,the peak discharge capacity of 161/151 m Ah g_PZ2TPA^-1,and stable 1000 cycles without significant capacity attenuation.