Low-Cost Organic Cathode Materials For Rechargeable Lithium Batteries Based On Disulfide And Quinone Groups

Lithium-ion batteries（LIBs）as reliable energy conversion and storage systems have been receiving great attention and booming development has been made with the transformation of energy structure of human society and the increasing demand of energy.However,the commercial cathode materials of LIBs are facing the limitation of theorical capacity,and the production and price are highly restricted by the mineral resources like cobalt and nickel.Hence,it is necessary to develop new type of cathode materials of LIBs as supplements and alternatives of traditional cathode materials.Organic electrode materials（OEMs）involve various tailorable structures and make up of light elements like C,H,O,N,and S,thus,making it possible to achieve high capacity and energy density.Moreover,most OEMs derive from industrial chemicals,dyes,and bio-materials,so,OEMs are less dependent on the mineral resources as well as sustainable,environmentally friendly,and potential low-cost.However,it is difficult to balance the electrochemical performance and the cost of OEMs synthesis in most of previous researches,resulting in the limited value of practical application.Moreover,dissolution of OEMs in electrolyte may results in capacity decay and even low Coulombic efficiency because of the“shuttling effect”.Hence,suppressing the dissolution of active materials is the key to improve the cyclic stability of OEMs.In view of above discussion,this dissertation aims to develop organic cathode materials with high energy density,high cycling stability and low cost.Three cost-efficient organic cathode materials based on disulfide bond and quinone group are studied through electrolyte optimization and materials structure design.The results are detailed as follow.1.Redox mechanism of poly（2,5-dimercapto-1,3,4-thiadiazole）（PDMcT）and the effect of electrolyte on the electrochemical performance.In early studies,ester electrolyte is chosen to do the electrochemical tests of PDMcT.It exhibits low reaction kinetics,poor redox reversibility,and inferior cycling stability resulting from the solubility of discharge products.In this section,we choose ether electrolyte to realize a high energy density(2.64 V×384 m Ah g^-1=1014 Wh kg^-1)and excellent reversibility（ΔV<200 m V）.The redox mechanism is uncovered by ex-situ characterizations and described as two-step reduction process.PDMcT transform to dimers after the first reduction step,then,monomers are generated from the reduction of dimers.This reduction process is totally reversible during the oxidation process.However,the discharge products of PDMcT can react with solvent molecules in ester electrolyte,which is the reason of the reported poor electrochemical performances of PDMcT in ester.We study the effect of ether solvent with different length of ether chains named DME（G1）,DEGDME（G2）,TEGDME（G4）,and PEGDME500（G10）on the electrochemical performance of PDMcT.It’s confirmed that the long-term cycling stability of PDMcT is effectively enhanced in high viscosity electrolyte（1.1 m Li TFSI/G10）.A high specific capacity of 322 m Ah g^–1 and high cycling stability(73%@200^th cycle)can be reached under 100 m A g^–1.This work clarifies the misunderstanding of previous researches on the electrochemical behavior of PDMcT,and is important to its further study.2.Research on the synthesis and electrochemical performance of benzoquinone-piperazine molecule（DBQPz）and polymer（PBQPz）.Electrolyte optimizing can suppress but cannot eliminate the dissolution of active materials.Declining the intrinsic solubleness of OEMs is a fundamental way.This section aims to construct a polymer cathode with stable main chain during redox process.Taking the practical value into consideration,low-cost benzoquinone（BQ）and piperazine（Pz）are chosen as the reactants.DBQPz molecule and PBQPz polymer are generated by changing the molar ratio of BQ and Pz.The structure analysis data indicates that redox-active BQ units and non-conjugated Pz units are linked by C–N bonds.And then,the DBQPz with“2 BQ+Pz”structures and PBQPz with higher degree of polymerization are produced.DBQPz exhibits a high energy density(2.43 V×311 m Ah g^-1=756 Wh kg^-1),but the unsatisfactory cycling stability(42%@100^th cycle)because of the diffluent active materials.PBQPz is almost insoluble in electrolyte,though the energy density is reduced(2.36 V×192 m Ah g^-1=453 Wh kg^-1),the cycling stability is promoted(67%@100^th cycle).It is found that the deficient contact of large PBQPz particle and conductive carbon results in inadequate specific capacity and cycling stability.It may be promoted by further optimizing of electrode production.3.Research on the synthesis and electrochemical performance of benzoquinone-pyrrole based polymer.Pyrrole（Py）and methyl pyrrole（MPy）with conjugated structure is chosen to be the linking units and reacted with BQ.Benzoquinone-pyrrole polymer（PBQPy）and benzoquinone-methyl pyrrole polymer（PBQMPy）are prepared by a solvent-free method.The polymerization mechanism is described as nucleophilic addition reaction to form A units with liner type and subsequent Diels-Alder reaction to form B units with ring type.And the reaction site isα-C not N of Py.Structure analysis data shows that there are phenolic hydroxyl groups in both PBQPy and PBQMPy,which are stable during charge and discharge processes and have no contribution to capacity.PBQPy and PBQMPy share the same electrochemical behaviors,but PBQPy exibits higher energy density(2.32 V×255 m Ah g^-1=592 Wh kg^-1)and cycling stability(81%@1000^th cycle).It may be ascribed to the following three points:1)higher redox potential and specific capacity because of the absence of the methyl groups with electron donating character;2)uniform mixing with active materials and conductive carbon resulting from the small particle size of PBQPy;3)poor dissolvability of PBQPy because of the high hydrogen bonds andπ-πinteractions within polymer chains.Comparing with the reported polymer cathodes based on carbonyl groups,PBQPy shows significant advantages in both synthesis cost and electrochemical performance,which may throw light on the practical application of organic cathode materials.