Preparation Of Transition Metal Sulfur Compounds/Carbon Composites And Their Lithium (Sodium) Storage Properties

Lithium-ion batteries have been commercialized since the 1990s and are used in portable devices,electric vehicles,power grid energy storage and aerospace.However,its sustainable development is limited by the shortage of lithium resources.Sodium-ion batteries have a similar working principle to lithium-ion batteries.Sodium is abundant in the earth’s crust and has a low price.The negative collector fluid of sodium-ion batteries can be used in aluminum foil with a lower price（copper foil is used in the negative collector fluid of lithium ion batteries）,which is expected to replace lithium ion batteries as another large-scale energy storage technology with lower cost and promising.However,the ionic radius of sodium（1.02A）is larger than that of lithium（0.76A）,and the embedding and removal process of sodium ions in the electrode material is more difficult than that of lithium ions.As a result,the diffusion kinetics of sodium ions is slow,the volume expansion of electrode materials becomes larger,and the battery power density is reduced,resulting in the pulverization and shedding of electrode materials.The standard potential of sodium ion（2.7V）is lower than that of lithium ion（3.0V）,narrowing the battery charging voltage range and reducing the energy density of sodium ion batteries.The above factors become the key problems to be overcome in the development of sodium ion batteries.The key technology of battery development is to develop high energy density electrode materials which can stably remove lithium（sodium）ions.Transition metal chalcogenides are considered as potential anode materials for lithium（sodium）ion batteries due to their high theoretical capacity and unique physical and chemical properties.However,the structure rearrangement and volume expansion of these materials during the cycle often lead to the dissolution and comminution of electrode materials,which leads to the rapid capacity attenuation.In this paper,the transition metal chalcogenide/carbon composite materials as the research object,through coating,nanostructure design and interface control strategy to construct and regulate the structure,the transition metal chalcogenide compound anode material with excellent electrochemical performance.The main contents of this paper are as follows:（1）Mo Se₂/C nanofiber composites embedded in nitrogen-doped carbon fibers were prepared by electrospinning and subsequent heat treatment.The obtained Mo Se₂/C nanofibers have uniform morphology with a diameter of about 1μm.Some nanosheets are attached on the surface of the fiber.The nanosheet is composed of a few layers of Mo Se₂,and the internal structure is solid.The amorphous carbon nanofibers are coated with a few layers of Mo Se₂crystal structure.When Mo Se₂/C nanofibers were used as anode materials for lithium-ion batteries,the electrodes showed excellent performance,including high specific capacity of 768 m A h g^-1at 0.1 A g^-1current density and high specific capacity of 286m A h g^-1at 4 A g^-1current density.And good long cycle performance of 2000 cycles at 2 A g^-1current density.The excellent electrochemical performance of the Mo Se₂/C nanofiber electrode can be attributed to the fact that the nitrogen-doped carbon nanofibers are completely coated with fewer layers of Mo Se₂.（2）Ultrafine V₃S₄nanocrystals evenly integrated within three-dimensional（3D）porous N-doped carbon（N-C）by V-C bond are rational designed to address these issues.The interconnected N-C coating provides bi-continuous electron/ion transport pathways.And the 3D porous structure ensures large surface area,as well as robust structural integrity,boosting reaction kinetics.Moreover,the strengthen interface coupling can not only enhance the electron transfer at the interface,but also markedly strengthen structural stability.As expected,the as-prepared electrode showed superior lithium storage capacity,including a high specific capacity of 1021 m A h g^-1at 0.1 A g^-1,superior rate performance of 698 m A h g^-1at2.0 A g^-1and excellent cycling stability and a prolonged cycling stability of 635 m A h g^-1capacity retention over 2000 cycles at 2 A g^-1.Additionally,it also shows good electrochemical performance as an anode material for for SIBs.This work demonstrates the superiority of ultrafine crystal confined in 3D porous carbon network to boost the electrochemical performance in various fields.