Preparation And Electrochemical Performance Of Three-Dimensional Electrode Materials For Lithium-Sulfur Batteries

As a new generation of secondary batteries,lithium-sulfur batteries have the advantages of high theoretical energy density,low cost,environmental friendliness,and safety,making them an ideal choice for energy storage devices in the future.However,lithium-sulfur batteries have disadvantages such as poor conductivity of the positive electrode active material sulfur,volume expansion and crushing of the positive and negative electrodes,low cycle stability of the battery,and the easy growth of dendrites on the lithium metal negative electrode to pierce the diaphragm and cause internal short circuits,which have become practical and commercial A stumbling block on the road to change.In order to improve the charge-discharge cycle performance of lithium-sulfur batteries.In this paper,the active material of sulfur:Super P:PVDF=1:1:1 mass ratio is used as the comparison and basic electrode,and the three-dimensional carbon nanofiber film prepared by molecular design is used as the middle layer of the lithium-sulfur battery to prepare a three-dimensional cathode lithium-sulfur battery.Then,using this positive electrode as a reference,a three-dimensional lithium metal negative electrode is prepared by in-situ growth of copper oxide nanorods and metal lithium sheets on a copper mesh,and a full-cell lithium-sulfur battery is combined with three-dimensional electrodes.In order to further reduce the thickness and mass of the lithium metal electrode,and increase the mass energy density and volume energy density of the full battery,a three-dimensional lithium metal negative electrode was prepared using a metal lithium sheet coated with modified diamond nanoparticles,and a third-dimensional lithium metal negative electrode was prepared in combination with a three-dimensional sulfur positive electrode.Lithium-sulfur battery with three three-dimensional electrodes.The structure and electrochemical performance of the electrode materials were analyzed and characterized by scanning electron microscopy,XRD,XPS and nitrogen adsorption methods.Combining the discharge mechanism of lithium-sulfur batteries to create a capacity ratio study method of II/I,to study the efficiency of the positive electrode material on the treatment of polysulfide compounds.The paper concludes as follows:（1）The three-dimensional self-assembled carbon nanofiber film is placed between the positive electrode and the separator of the lithium-sulfur battery to increase the utilization rate of the active material,inhibit the shuttle effect,and disperse the internal current of the battery,and then obtain the sulfur loading on the surface of the active material material.The amount is 4.54 mg cm^-2,the initial capacity is 947 m Ah g^-1at a rate of 0.1 C,and the electrochemical performance of 908m Ah g-1 can still be maintained after 100 cycles.（2）The theoretical capacity ratio of II/I of the lithium-sulfur battery discharge platform is about 3;the larger the actual capacity ratio of II/I,the higher the efficiency of the positive electrode in converting polysulfide compounds into capacity.The research results show that the capacity of the I discharge platform of the lithium-sulfur battery with an intermediate layer is similar,that is,the conductive structure contributes the same to the utilization rate of the active material sulfur on the pole piece,and the electrode’s processing efficiency for the discharge of polysulfur compounds determines the capacity of the discharge II platform.In the discharge stage II,due to the catalytic effect of nitrogen and oxygen double-doped carbon nanofibers,the utilization rate of Li₂S₄into Li₂S/Li₂S₂increased by 76%.（3）The composite electrode composed of copper oxide nanorods and lithium metal is grown in situ on the copper mesh by using the dispersed lithium ion deposition method.Due to the large specific surface area of??the copper oxide nanorods,the low lithium ion deposition overpotential and the three-dimensional copper mesh The remaining buffer space effectively inhibits the growth of lithium dendrites,alleviates the volume expansion of the lithium metal electrode,and reduces the volume expansion by 44.3%.The initial discharge capacity of the whole battery is1100 m Ah g^-1under the condition of 7.2 mg cm^-2sulfur loading on the active material surface and a rate of 0.1 C.After 150 cycles,the battery capacity retention rate of the composite electrode is 56.8%.（4）Using the rapid ion transfer method,the diamond externally modified magnesium oxide and lithium will generate a lithium oxide magnesium composite artificial prefabricated three-dimensional protective layer at 300℃.Due to the high ion conductivity of the protective layer,the lithium ion can be quickly It is conducted to the inside of the electrode sheet to inhibit the growth of dendrites and alleviate the volume expansion of the lithium metal electrode.The volume expansion of the pure lithium electrode after the cycle is 600%,while the volume expansion of the composite electrode is only 100%after the cycle.In the test of the full battery,the composite electrode battery has an initial capacity of 752 m Ah g^-1at a sulfur load of 3mg cm^-2and a rate of 0.1 C.After 100 cycles,it still maintains 606 m Ah g^-1.This kind of composite electrode not only can well control the volume and quality of the electrode,but also its practical application effect is closer to practicality.（5）Put forward the lithium ion deposition site selection theory,that is,lithium ions deposited on different materials have different deposition potentials,and lithium ions tend to be deposited at sites with lower deposition potentials.Therefore,by modifying the surface of the material,the deposition of lithium ions can be controlled at the desired location,thereby improving the electrochemical performance of the lithium battery.