Research Of Electrode Material Preparation And Electrochemical Reaction Kinetics Of Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries have attracted much attention of researchers because of their ultrahigh theoretical capacity(1675 mA h g-1)and energy density(2567 W h kg-1).However,there are still some challenges for the commercialization of Li-S batteries,such as the low utilization of sulfur,rapid capacity fading and poor coulombic efficiency.Designing sulfur hosts have always been considered as the most important strategy for improving Li-S batteries,while the reaction kinetics have rarely been concerned.As a matter of fact,the host design relates closely to the reaction kinetics.For example,the low conductivity of sulfur seriously affects the charge transfer.The volumetric expansion/contraction of cathode during discharge/charge leads to the destruction of the conductive network,hindering both the ions transport and electron transfer.The polysulfides diffuse to the lithium anode and form a non-conductive Li2S2/Li2S passivation layer,reducing the negative electrode reaction kinetics.In addition,the lithium anode is prone to generate dendrite during the cycling,resulting in a terrible reaction interface and complicated redox process.Reaction kinetics takes into account both theoretical and practical electrochemical reaction to explore the issues between basic research and practical application.From the point of reaction kinetics in this research,several electrode materials were prepared and the correlation between electrode properties and reaction kinetics in Li-S batteries was studied.The main contents in this research include:1.Altering the essential properties of sulfur material itself.We demonstrate a facile approach to reshape the electronic properties of S and Li2S by simple tellurium(Te)doping and further refine the Li-S chemistry to improve the electrochemical performance of Li-S batteries.DFT calculation indicates that Te doping can effectively facilitate the lithiation/delithiation reactions and lower the lithium ion diffusion energy barrier in Li2S.Additionally,electrochemical studies prove that the reaction kinetics of Li-S chemistry and cycling performance of Li-S batteries have been significantly improved with Te dopants.An exceptional specific capacity of～656 mA h g-1 and a high Coulombic efficiency of～99%have been achieved at 5 A g-1 even after 1000 cycles.More importantly,the capability to manipulate the intrinsic properties of materials and explore the synergistic effects between conventional strategies and element doping provides new avenues for Li-S batteries and beyond.2.Regulating the electron transfer and Li transport of solid sulfides.The epitaxial deposition of solid sulfides prefers a lateral deposition driven by the superior charge transfer of the conductive substrate to produce a continuous insulating 2D film.This situation not only disables the absorbing host to control the dissolved PSs,but also terminates electrochemical redox when the mass transport and electron transfer are completely blocked by the non-conductive layer.Here,we present that the single-atom Co is of significance to control the epitaxial deposition of the solid sulfides to promote a spherical cumulative deposition with 3D morphology.The experimental studies and theoretical calculations reveal that single-atom Co can reconstruct the electronic structure of the solid intermediates owing to the d-p hybridization associated with an electron feedback coupling mechanism,which benefits the non-locality of electrons and octahedron-tetrahedron fields of solid intermediates,promoting both the electron transfer and Li transport of longitudinal deposition to balance the lateral deposition so as to sustain the spherical epitaxial deposition.3.Guiding smooth Li metal deposition by single-atom sites.Lithium dendrite formation results in the porous electrode surface,failure of reaction interface,loss of active substance,which leads to the cell voltage hysteresis,low coulombic efficiency and sluggish kinetics.We demonstrate that the single-atom Zn sites can drive the high dimensional Li deposition kinetics without dendrite.Theoretical calculation predicts the higher surface binding energy and lower migration barrier,providing the possibility toward high dimensional Li deposition.The deposition kinetics reveals the increased new nuclei density,shortened nucleation relaxation time,and lowered nucleation polarized potential.Electron microscopy and in situ optical microscopy identify the high dimensional Li deposition process.Benefiting from the improved kinetics,Li deposition/stripping performance achieves low overpotential about 12 mV and high Coulombic efficiency reaching to 100%.The symmetrical Li-ZnSAs|Li-ZnSAs cell cycles over 800 h with steady voltage oscillations.This work provides a new insight into fundamental understanding of electrochemical deposition of metal anodes by driving high dimensional deposition kinetics using single-atom sites.