Synthesis And Modification Study Of Cathode For Lithium-ion Batteries And Electrolytes For Lithium-sulfur Batteries With High Energy Density

Lithium-ion batteries as a new type of green and clean energy storage device have been commercialized and applied to various fields.With the rapid development of new energy vehicles,higher requirements are put forward for specific energy of batteries,so it is urgent to develop lithium-ion batteries with high-specific energy.To develop a lithium-ion battery anode material with high specific energy,simple preparation process,low price and high safety is the most direct solution.In addition compared with commercial lithium-ion batteries,lithium-sulfur batteries have a high theoretical energy desity,which are expected to be the next generation of energy storage devices.In recent years,researchers have made a lot of achievements in the research of lithium-sulfur battery,but lithium-sulfur batteries still need to be perfected before they can be commerciallized.The biggest obstacle to the development of lithium sulfur battery is the “shuttle effect” of polysulfide intermediates,sulfur is an insulator and so are discharge byproduct,and the large volume change in the reaction process,whice inevitably leads to the low coulomb efficiency and low cycle life of lithium sulfur battery.Designing a reasonable structure of lithium sulfur battery cathode material can improve the conductivity of lithium sulfur battery and inhibit the “shuttle effect” of polysulfide intermediates,which can improve the electrochemical performance of lithium sulfur battery.Meanwhile,the modification of the electrolyte of lithium sulfur battery can also improve the electrochemical performance of lithium sulfur battery.Therefore,the main research work of this thesis includes the following respects.1.Due to its high work voltage of 4.1 V,high theoretical capacity of 170 mAh g^-1,excellent stability and low price of raw materials,olivine LiMn_1-yFe_yPO₄ is one of the candidates for cathode materials of high-specific energy lithium-ion battery.However,its low electronic conductivity and ion mobility limit its application.Therefore,insertion of 3% vanadium in the LiMn_1-yFe_yPO₄?y = 0.2?with carbon coating was prepared by solid-phase method in this paper.The structure of the material is analyzed.V substitutes for Fe²⁺in the trivalent state V³⁺.This substitution is accompanied with the formation of Fe vacancies while Mnremains in the Mn²⁺valence state,leading to a composition LiMn_0.8Fe_0.2-0.045V_0.03???_0.015PO₄ where ??? is a Fe vacancy.The V-insertion improves the electrochemical properties,due to an increase of the lithium diffusion coefficient by a factor two and an increase of the electric conductivity at any Li-concentration during the cycling process.We expect that this work will provide some data for the improvement of LiMn_1-yFe_yPO₄ by element doping.2.Restraining the “shuttle effect” of polysulfide and improving the conductivity of the positive electrode,which are the most effective way to improve the electrochemical performance of lithium-sulfur batteries.In the current work,we designed a cathode acting simultaneously as a highly efficient polysulfide immobilizer and sulfur host,composed of sulfur embedded in a layer made of hollow porous black TiO₂?b-TiO₂?spheres.These particles are then coated with a polypyrrole?PPy?to form the active cathode materials?b-TiO₂/S@PPy?.The hollow porous structure of the b-TiO₂ layer provides enough space to alleviate the volume expansion of polysulfide.Equally,the PPy coating layer can not only improve the conductivity of the cathodes,but also restrain the shuttle of polysulfide by chemical adsorption.The lithium-sulfur batteries with the b-TiO₂/S@PPy cathode exhibits a high initial discharge capacity(1374 mAh g^-1at 0.1 C),a superior cycling stability(910 mAh g^-1after 200 cycles at 0.1 C),and a good rate performance(725 mAh g^-1at 2 C).3.Introduction of redox mediator?RM?into the lithium-sulfur batteries can improve the capacity and the cycling performance of the lithium sulfur batteries by reducing the energy barrier in the reaction of Li₂S.In this paper,we first report tetrathiafulvalene?TTF?as RM added to the conventional lithium sulfur batteries ether electrolyte,which can accelerate electron conduction and improve the reaction rate of lithium-sulfur batteries.The TTF can promote the redox reaction of Li₂S during charging and discharging.Moreover,the irreversible deposition of Li₂S can return to the reaction,which can improve the utilization rate of Li₂S and slow down the loss of sulfur.The lithium-sulfur batteries cathode materials with a high sulfur content of 69% were made up of the commercial CMK-3 and sulfur.The cell with TTF added electrolyte show a high first discharge capacity(1359 mAh g^-1)and the discharge capacity remains above 850 mAh g^-1after 100 cycles at 0.1 C.4.In the high-donor-number solvents,the reaction intermediate of lithium-sulfur batteries is S₃^·-,which is more stable than that of S₄^2-in low-donor-number solvents.In addition,when lithium-sulfur batteries with a high-donor-number electrolyte,the reaction rate is more faster than low-donor-number electrolyte.However,the lithium anode corrosion is serious,the cells with high-donor-number can't cycle with a long time.We report a high-donor-number electrolyte with the addition of InBr₃.Lithium metal can be effectively protected by forming a dense In layer on the surface of lithium metal,the cells with high-donor-number electrolyte can stable cycle.