Preparation And Performance Of CoS/C Composite In Lithium-sulfur Batteries

Lithium-sulfur batteries have attracted widespread attention as one of the promising alternatives to current commercial lithium-ion batteries.The theoretical capacity of the sulfur cathode is 1675 m Ah g^-1,which is more than 5 times that of Li Co O₂.Although the sulfur cathode has a relatively high theoretical capacity,problems such as poor cycle life,low coulomb efficiency,and low active material utilization rate greatly hinder the further commercial application of lithium-sulfur batteries.The problems faced by sulfur cathodes are related to a variety of material factors,including the large volume effect of sulfur in the lithiation process,the shuttle effect caused by the dissolution of lithium polysulfide in the electrolyte,and the low ion/electronic conductivity of sulfur and lithium sulfide.In order to overcome these problems,extensive research has been conducted,focusing on the structure and composition of the control electrode.Aiming at the problems of lithium-sulfur batteries,this paper designs transition metal sulfide(Co S)and porous carbon composite materials as sulfur cathode support materials,comprehensively combining the strong interaction between transition metal sulfide and lithium polysulfide and the excellent conductivity of carbon materials,improve the conductivity of the sulfur positive electrode and inhibit the shuttle effect,thereby improving the specific capacity and cycle performance of the lithium-sulfur battery.The main research contents and conclusions are as follows:1.With Co-based MOF material ZIF-67 as the precursor and sublimed sulfur as the sulfur source,a two-step method is used to prepare a variety of cobalt sulfide/porous carbon composite materials(Co S/C-700,Co S/C-800,Co S/C-900).The comparison between XRD and SEM shows that Co S/C-800 has better Co S crystallinity,while the rough surface and unique dodecahedron morphology make it have an ideal specific surface area.Co S particles with uniform surface are also for adsorption and catalyzes the conversion of polysulfides to provide more active sites.Electrochemical tests show that the Co S/C/S-800 electrode obtained at a carbonization temperature of800?has the best cycle performance,but the rate performance is insufficient,indicating that Co S/C/S-800 is difficult to withstand the impact of large currents.The CV test shows that the reduction peak potential of Co S/C/S-800 has a significant positive shift compared with the other three materials,indicating that Co S with better crystallinity has better catalytic performance.The EIS test further shows that Co S/C/S-800 has a smaller electrochemical impedance.2.Using ionic liquid[BMIM]BF₄ as nitrogen source and boron source,a N-B co-doped carbon material supported Co S composite material(Co S/NBC)was synthesized through a two-step method.SEM and TEM characterizations show that the Co S/NBC composite has a double-shell structure with uniform particle size.XPS and EDS tests showed that the nitrogen-boron co-doped shell was successfully prepared using ionic liquid as the nitrogen source and boron source.When Co S/NBC is used as a lithium-sulfur battery carrier material,the sulfur cathode exhibits excellent electrochemical performance.At a current density of 167.5 m Ah g^-1,a high specific capacity of 1610.2m Ah g^-1 was demonstrated,and the rate performance was also greatly improved.The improvement in performance is attributed to the double-shell structure which increases the structural stability of the electrode,and the N-B co-doped carbon layer provides high conductivity.It is proved by cyclic voltammetry that the N-B co-doped carbon shell enhances the catalytic effect of the Co S/NBC/S electrode on the conversion of lithium polysulfide.In addition,in order to study the electrochemical reaction mechanism of lithium-sulfur batteries during the discharge process,the relationship between AC impedance spectroscopy and discharge depth was measured and analyzed.Analysis found that the formation and gradual accumulation of soluble lithium polysulfide and insoluble Li₂S and Li₂S₂ greatly affect the overall resistance of lithium-sulfur batteries.3.Combining ZIF-67 with carbon nanotubes(CNT)with high conductivity and calcining at high temperature to obtain a multi-component composite material(Co S/C/CNT)containing cobalt sulfide and having a three-dimensional conductive network.The rich and ordered CNT conductive network can provide a large number of active sites and fast electron and ion transport paths to help Co S particles anchor and convert polysulfides.The synergy of CNT and Co S enhances the electrode's inhibition of"shuttle effect".Using Co S/C/CNT as a carrier material for lithium-sulfur batteries,it exhibits excellent cycle performance and rate performance in electrochemical tests:at a current density of 167.5 m A g^-1,the Co S/C/CNT/S electrode exhibits a high reversible capacity of 1441.4 m Ah g^-1;increasing the current density to 837.5 m A g^-1after 370 cycles still has an initial reversibility of 46.7%capacity;in the rate test,the discharge specific capacity reaches 580.3 m Ah g^-1 at a current density of 3350 m A g^-1.When the current density returns to 167.5 m A g^-1,it still maintains a reversible specific capacity of 927.7 m Ah g^-1.The CV and EIS tests show that CNT not only enhances the catalytic performance of Co S particles on long-chain polysulfides,but also promotes electron/ion transmission on the electrode,thereby greatly improving the utilization of sulfur.The CV of different scanning speeds was further analyzed and the energy storage mechanism of the Co S/C/CNT/S electrode was explored,and it was found that the electrode performance is controlled by both the capacitance behavior and the battery behavior.