Preparation And Properties Of Cathode Materials And Polymer Electrolytes For Lithium-Sulfur Batteries

Nowadays,the traditional lithium-ion battery has been unable to meet the use requirements of people.As one of the best candidates for future energy storage systems,lithium-sulfur batteries have a relatively high energy density(2600Wh·kg^-1).Moreover,sulfur is non-toxic,low-cost,and easy to get on Earth.However,there are still many problems in the commercialization process of lithium-sulfur batteries.The problems include the insulation of sulfur/lithium sulfide,volume expansion during charging and discharging,shuttle effect of polysulfide,dendrite growth of lithium anode,and poor safety of electrolytes.These problems lead to short battery life and rapid capacity decay.To solve the above problems,a three-dimensional porous carbon was designed to improve its conductivity and load active substance.The cerium dioxide embedded porous carbon rod was prepared to inhibit the shuttle effect by chemical adsorption.The solid polymer electrolyte filled with rare earth metal oxide nanorods and the sandwich-structure interface-modified solid polymer electrolyte was synthesized to assemble the lithium-sulfur battery.The safety performance of the battery and compatibility of the electrode-electrolyte interface was improved.Through the performance characterization and theoretical calculation,the action mechanism of various materials in the lithium-sulfur batteries was studied.Meanwhile,the performance effects of cathode materials and polymer electrolytes in the lithium-sulfur batteries were investigated.The main research contents are as follows.Using cellulose as a carbon source,three-dimensional porous conductive carbon materials with large numbers of mesoporous and macroporous materials were prepared by the double-template method.BET test results showed that the three-dimensional porous conductive carbon material had a high specific surface area(694 m²·g^-1),which is conducive to loading sulfur and providing a conductive framework for sulfur.Mesopores were beneficial to confine polysulfide and alleviate the shuttle effect.The macroporous of 50-140 nm could alleviate the volume expansion to prevent the cathode-structure collapse and prolong the battery life during the cycle.Scanning electron microscopy（SEM）test results showed that the material has an interconnected porous structure,which can provide reaction sites for the active material and made the liquid electrolyte contact with active substances to improve the utilization rate of sulfur.The electrochemical performances of the materials were tested in a practical environment.The initial specific discharge capacity was 1072 m Ah·g^-1 at 0.5C.After 100 cycles,the capacity retention rate was 45.22%.Using eco-friendly soybean protein isolate as raw material,cerium dioxide（Ce O₂）nanocrystalline embedded porous carbon rod（PCR）materials were prepared by carbonization,etching and hydrothermal synthesis.X-ray photoelectron spectroscopy（XPS）results showed that both porous carbon rods and Ce O₂ could bond with sulfur to adsorb active substances.The porous carbon rod material reduced the ion/electron transport distance and facilitated the diffusion of lithium-ions in the electrode.And the kinetics of the redox reaction between lithium-ion and sulfur anion was accelerated.The diffusion coefficient of lithium-ion was 6.221×10^-11 cm²·s^-1 by EIS spectrum fitting.The Tafel slope and Li₂S nucleation experiments displayed that the strongly polar Ce O₂ nanocrystals adsorbed polysulfide.Ce O₂ nanocrystals catalyzed the conversion rate of polysulfide and inhibited the shuttle effect.DFT calculation results demonstrated that the binding energy between Ce O₂ nanocrystals and Li₂S₈ was-13.8 e V.At room temperature,the capacity retention rate of the lithium-sulfur battery with Ce O₂/PCR-S cathode was 94.41%after 100 cycles.And the capacity retention rate was 60.61%after 300 cycles.CeO₂,Lanthanum oxide（La₂O₃）and neodymium oxide（Nd₂O₃）nanorods were prepared by hydrothermal synthesis.Using poly（vinylidene fluoride-hexafluoropropylene）（PVDF-HFP）as the polymer matrix,the polymer electrolyte was prepared by casting method,and a solid lithium-sulfur battery was assembled with polymer electrolyte.The interlinked nanorods network formed a channel conducive to the rapid lithium-ion transfer in the polymer electrolyte.XPS test results showed that there were many oxygen vacancies on the surface of nanorods.Oxygen vacancy sites could provide a lewis acid center to accelerate the dissociation of lithium salts.The concentration of free lithium-ion in the polymer electrolyte improved the ionic conductivity and the number of lithium-ion transference.With the addition of nanorods,IR and Raman spectra revealed that the content of TFSI^-in the molecular association state decreased and the content of TFSI^-in the free ionic state increased greatly.Density Functional Theory（DFT）calculated the binding energies of three rare earth metal oxides with TFSI^-.The electrochemical test results showed that when the mass ratio of PVDF-HFP matrix to Nd₂O₃ was 10:0.5,the ionic conductivity was 6.09×10^-4 S·cm^-1,the electrochemical stability window was 4.33 V,and the lithium-ion transference number was 0.48.At room temperature,the initial discharge capacity of Nd₂O₃polymer electrolyte at 0.1C was 940.8 m Ah·g^-1.“Sandwich”structure polymer electrolyte was prepared by interfacial modification with the above polymer electrolyte.Firstly,the ultrathin carbon sheet（UCS）material was synthesized from sodium citrate.UCS/Li TFSI/polyvinylidene fluoride（PVDF）coatings were prepared at the anode-electrolyte interface.The second current collector of UCS/Li TFSI/PVDF（UCS coating）provided more redox-active sites for the sulfur/lithium sulfide on the electrode surface.Meanwhile,UCS coating was used as a barrier layer to prevent the compatibility of polymer matrix and polysulfide.The test results including activation energy of conductivity,lithium-ion transference number,and constant current polarization curve determined that UCS coating was more conducive to lithium-ion transport in the electrode-electrolyte interface than that in the commercial graphite coating.According to the XPS test results of the anode-electrolyte interface after the charge-discharge cycle,the characteristic peak of polysulfide could not be detected after the addition of UCS coating.Combining UCS material with polymer electrolyte precursor slurry prepared the CPE/UCS anode-electrolyte interface layer.CPE/UCS interface layer effectively alleviated the chemical/electrochemical instability of lithium metal and electrolyte.The CPE/UCS interface layer was modified to form a mixed ion/electron conduction interlayer.The charge distribution was balanced to control ion and electron distribution at the anode-electrolyte interface.The static AC impedance of the battery after different cycles and the in-situ AC impedance of a single cycle was measured.The addition of the CPE/UCS interface layer improved the interface compatibility in the anode-electrolyte and a uniform and stable SEI film was generated.At room temperature,the initial specific discharge capacity of a solid lithium-sulfur battery assembled with sandwiches electrolyte was 1050.4 m Ah·g^-1 at 0.2C.After the cycle,the lithium anode planes and the anode-electrolyte interface cross-section were characterized by SEM.The CPE/UCS interface layer effectively alleviated the dendrite growth,and the anode-electrolyte interface became closer and more continuous.The interface layer could avoid the evolution of the interface structure during the cycle and prolong the life of the battery.