Preparation And Electrochemical Energy Storage Performance Of Cobalt-carbon Composites Containing Sulfur

Lithium ion battery,as a pioneer in the field of electrochemical energy storage,has made great progress since Professor Goodenough invented lithium cobalt acid and lithium iron phosphate cathode materials,and has occupied a large market share.However,lithium-ion batteries are also limited by their own disadvantages,such as low theoretical specific capacity,low theoretical energy density,and more expensive materials for common cathode materials.In contrast,the sulfur battery has become a new type of secondary battery system promising to break through the above dilemma.Sulfur battery,namely the battery system composed of positive and negative sulfur elemental and lithium/sodium metal,achieves energy storage through the redox reaction between elemental sulfur and metal pole.Lithium/sodium sulfur battery can obtain the theoretical specific capacity of 1675 m A h g^-1.At the same time,the high abundance of sulfur,simple industrial preparation,low cost,will greatly reduce the price per unit capacity.Of course,sulfur batteries still face some urgent problems to be solved.For example,the positive electrode material sulfur and its derivatives lithium polysulfide/sodium are almost insulators with very poor conductivity.Meanwhile,the multi-step conversion process of sulfur is complex and inert,and the reaction kinetics is seriously hindered.In the process of redox,the volume expansion of the positive sulfur electrode is obvious,which easily leads to the destruction and collapse of the positive electrode material.The existence of soluble polysulfide lithium/sodium in the intermediate product will dissolve in the electrolyte,through the diaphragm to the negative electrode,that is,the so-called"shuttle effect",easy to cause the passivation of the negative electrode,but also lead to a large loss of active substances,capacity is greatly reduced.Therefore,the optimal design of sulfur anode has always been the focus of research in the field of sulfur batteries.If a positive sulfur carrier material can be developed which can increase sulfur carrying capacity,reduce sulfur volume expansion,improve electrical conductivity and catalyze the conversion of polysulfide,it will greatly promote the commercialization process of sulfur batteries.As a kind of high-quality sulfur carrier material with simple synthesis,diverse structure and good conductivity,carbon material has first entered the field of sulfur battery researchers’vision.In the last decade or two,a lot of work has been reported on the use of carbon base and carbon base composites as sulfur carriers.At the same time,transition metals and their sulfides,selenides,phosphates and hydroxides have been found to have strong adsorption on soluble polysulfides and can catalyze the conversion of long-chain polysulfides to short-chain polysulfides.On the basis of full investigation of previous literature,we comprehensively considered the needs of increasing sulfur carrying capacity,improving electrical conductivity and accelerating catalytic conversion of polysulfide,and optimized the electrochemical performance of sulfur batteries through two relatively simple synthesis of"tertiary sulfur carrying materials".In addition,during the master’s degree,I fully participated in the project of"Process optimization and performance evaluation of large-scale production of Ferric sodium pyrophosphate battery materials"of the enterprise,and provided some constructive solutions to solve the difficulties in the production and research process of the enterprise by applying some concepts in the design of sulfur-carrying materials.The overall content is as follows:1.A few layers of carbon-doped graphite phase Co/C/C₃N₄ supported by cobalt nanoparticles were constructed,and its feasibility as sulfur carrying materials for sulfur batteries was discussed as follows:A simple multi-step calcination of bulk C₃N₄ was used to peel off a few layers of 2-D graphene like structure.Then it was mixed with carbon source and cobalt source and calcined to form a three-level composite structure of carbon,nitrogen,tetralogy,carbon and cobalt.When the material is used as the cathode material of lithium sulfur battery,the few layers of C₃N₄ in graphite phase not only provide the loading site for the active material cobalt nanoparticles,but also provide the volume expansion space for the elemental sulfur.The widely distributed carbon layer structure acts as a conductive network,aleviating the reaction inertness caused by the insulation of sulfur.As the active center of the reaction,highly dispersed cobalt nanoparticles can catalyze the interconversion between long chain polysulfide and short chain polysulfide while chemically adsorbed polysulfide,thus inhibiting the"shuttle effect".Therefore,compared with C₃N₄ and C₃N₄/Co,the discharge capacity and stability of Co/C/C₃N₄ have been significantly improved.Under the current density of 0.5C,the discharge specific capacity is maintained at 588.5 m A h g^-1 for 300 cycles,and the Coulomb efficiency is close to 100%.However,unfortunately,the electrochemical performance of Co/C/C₃N₄ used in sodium-sulfur batteries is very poor,which may be caused by the fact that C₃N₄ is a semiconductor material with poor electronic conductivity and insufficient ability to transfer sodium ions.2.In Work 1,the introduction of semiconductor materials caused a huge difference in the performance of lithium-sulfur and sodium-sulfur batteries,and the synthesis and preparation was complicated.In this work,nitrogen-doped hollow carbon nanocages with a size of 600 nm were synthesized by one-step calcination process,and there was a protective layer of cobalt-carbon nanotubes with a thickness of 100 nm outside.These cobalt-carbon composite structures with good conductivity improved the transfer ability of lithium ions/sodium ions during the charging and discharging process.Therefore,both lithium-sulfur and sodium-sulfur batteries exhibit good charging-discharge behavior.When it is applied to the sodium-sulfur battery system,the performance of the sodium-sulfur battery is effectively improved because the highly dispersed cobalt nanoparticles in the functional layer can polar absorb sodium polysulfide and act as the catalyst for the conversion of sodium polysulfide to inhibit the"shuttle effect".The discharge capacity of 236.3 m A h g^-1 can be maintained after1000 cycles at 0.5 C current density,and the attenuation rate of each cycle is only0.025%.3.As a professional master’s student,I participated in the production practice of"Large-scale production process optimization and performance evaluation of sodium ferric pyrophosphate".In view of various difficulties encountered in synthesis,route optimization,performance testing and soft package assembly of sodium ferric pyrophosphate,I proposed some constructive solutions based on the design ideas of building sulfur-carrying materials for sulfur batteries.As a polyanionic compound,the poor conductivity of sodium ferric pyrophosphate is one of the important factors restricting its industrialization.We use spray drying and rotary calcination to ensure the uniformity of material particle size and carbon coating,simplify the production process,save time and energy,and meet the requirements of batch preparation.After optimizing the production route,we screened the choice of electrolyte,carbon source,carbon content and sodium supplement,and obtained the optimal performance of half battery and full battery.Finally,kilogram preparation of sodium ferric pyrophosphate and assembly test of soft pack battery were realized,which promoted the industrialization process.