Performance Research And Equivalent Circuit Model Construction Of MXene Based High Capacity Lithium-sulfur Battery

Lithium-sulfur battery is known as one of the most promising electrochemical energy storage devices because of its high theoretical specific capacity,low cost and environmental friendliness.However,the shuttle effect of lithium polysulfide and the poor conductivity of elemental sulfur are still the bottlenecks restricting its large-scale commercial development.In view of the above problems,a new type of CNTs/Ti₃C₂T_x-PP modified separator lithium-sulfur battery and a new type of Ti₃C₂T_x-KB/S-PP lithium-sulfur battery with cathode and separator were prepared in this paper.At the same time,the accurate and effective battery model is the guarantee of safe and reliable operation of the battery,and is the key to the battery management system to monitor the actual working conditions of the battery.In this paper,the special reaction mechanism of lithium-sulfur battery is analyzed through theoretical research,which provides reliable identification data for the construction of battery equivalent circuit model,and the equivalent circuit model of lithium-sulfur battery is constructed.The main research contents and conclusions of this paper are as follows:（1）The precursor Ti₃Al C₂ was treated by etching method to prepare a few-layer Ti₃C₂T_x sheet solution.CNTs/Ti₃C₂T_x solution mixed with CNTs was vacuum filtered onto the PP separator to prepare a modified separator and assembled into a lithium-sulfur battery.The effect of modified diaphragm on the electrochemical performance of lithium-sulfur battery was studied by comparing the experimental analysis of different control groups of modified separator battery.It was found that the interconnection of CNTs and Ti₃C₂T_x forms a 3D porous structure,which not only avoids the restacking problem of Ti₃C₂T_x,but also gives full play to the advantages of the polar surface of Ti₃C₂T_x,and combines physical and chemical properties to effectively adsorb lithium polysulfide.According to the analysis results of phase characterization and electrochemical performance,10%CNTs/Ti₃C₂T_x-PP modified separator battery has the best electrochemical performance.It exhibits a high discharge specific capacity of 1116.1m Ah·g^-1 at a current density of 0.1 C.After 100 cycles,it still has a reversible discharge specific capacity of 784.2 m Ah·g^-1,and the capacity attenuation rate per cycle is 0.3%.It maintains a high rate specific capacity of 662.1 m Ah·g^-1 at a high current density of 2 C.After switching the current density back to 0.1 C,it can also recover to a high reversible specific capacity of 932.5 m Ah·g^-1.（2）Ti₃C₂T_x-KB/S cathode material was prepared by melt diffusion method,and then Ti₃C₂T_x-KB/S cathode material was coated on the PP separator to prepare a new type of Ti₃C₂T_x-KB/S-PP electrode,which was used to assemble lithium-sulfur battery.At the same time,KB/S-PP integrated electrode was prepared to compare the advantages of composite materials.The results showed that the composite of KB and two-dimensional Ti₃C₂T_x with good surface area and unique morphological advantages made the specific surface area of Ti₃C₂T_x-KB as high as 783.2 m²·g^-1,and the pore volum as high as 1.42cm·g^-1,which alleviated the volume expansion of the positive electrode and increased the sulfur content.The sulfur content of Ti₃C₂T_x-KB/S was as high as 74.9%.In addition,the conventional Ti₃C₂T_x-KB/S-Al electrode was prepared to compare the advantages of the integrated structure design.The results show that the Ti₃C₂T_x-KB/S-PP integrated electrode has good electrolyte wettability and structural stability.It has an initial discharge specific capacity of 1088.2 m Ah·g^-1 at a current density of 0.1 C.After 100 cycles,the discharge specific capacity still shows 796.8 m Ah·g^-1,which is 73.2%of the initial specific capacity.It maintains a high rate specific capacity of 542.2 m Ah·g^-1 at a high current density of 2 C.When the current density is switched from 2 C to 0.1 C,the Ti₃C₂T_x-KB/S-PP integrated electrode still exhibits a high specific capacity of 975.7m Ah·g^-1.（3）The impedance spectrum characteristics of each SOC point of the selected10%CNTs/-PP modified separator battery were tested,and the first-order and second-order RC equivalent circuit models were established on the basis of them.The experimental data are used to identify the parameters of the model and complete the whole process of modeling.The simulation platform is built by MATLAB/Simulink tool to complete the simulation verification of lithium-sulfur battery under constant current discharge condition.According to the simulation results,compared with the first-order RC equivalent circuit with the maximum simulation error of about 3%,most of the second-order RC equivalent circuits with the simulation error of about 1%are more suitable for fitting the selected lithium-sulfur batteries,so the second-order RC equivalent circuit model is more accurate for the actual discharge conditions of lithium-sulfur battery.The research results can provide technical support for the battery energy storage system of luxury cruise ships,pilot ships,rescue ships,water quality monitoring ships and other types of ships.