Synthesis,Structure And Electrochemical Performances Of Metal Sulfide(Selenide)/Carbon Composites As Anode Materials For Sodium Ion Batteries

With the development of modern society,energy production and storage technology have attracted widespread attention in numerous fields.In particular,lithium-ion batteries（LIBs）have been widely used in portable electronic devices and zero-emission electronic vehicles,however,in view of the high cost and uneven distribution of lithium resources and limited reserves,there is an urgent need to develop new long-life energy storage devices without lithium.Among various potential alternatives to LIBs,sodium ion batteries（SIBs）are considered to be the most promising candidate because of their cost-effectiveness,abundant geographical distribution of sodium and the similar working principle of SIBs to that of lithium ion batteries.At present,hard carbon,metal sulfides,metal oxides,metal selenides,metal alloys,etc.,have been widely investigated as anode materials for SIBs.Among them,metal sulfides and selenides are favored because of their rich redox reactions and high theoretical capacities.Despite some achievements,there are still some severe challenges in terms of sodium ion batteries such as large volume expansion,poor conductivity and unsatisfied cycle performance.In this thesis,three kinds of metal sulfide/carbon and selenide/carbon composite materials have been successfully developed by the strategies of composition optimization and nanostructure construction as anode materials for sodium ion batteries and the crystal structure,micromorphology and electrochemical properties of the electrodes have been systematically investigated.The main research contents and results are as follows:（1）The strategy of using MOF as template has been proposed to prepare in situ nitrogen-doped carbon-encapsulated Cu S nanoparticles（Cu S@N-C）via facile successive carbonization and sulfidation processes,and the relationship between the crystal structure,micromorphology and electrochemical performance of the material was studied.Benefiting from the structure superiority inherited from Cu-MOFs precursors,the synthesized Cu S@N-C composite shows obvious advantages:the porous architecture of the Cu S@N-C can alleviate the large volume stress in the process of repeated sodiation-desodiation;the N-doped carbon contributes to conductivity;and Cu S nanoparticles embedded in carbon matrix can effectively prevent the agglomeration of active nanoparticles.As a result,the Cu S@N-C electrode used as anode for SIBs delivers a prominent rate capability(259.4 mAh g^-1 at 5 A g^-1),remarkable long-life cycling stability(300.2 mAh g^-1 at 5 A g^-1over 1200 cycles)with an ultra-low capacity decay of 0.0035%per cycle.（2）SnS₂-CoS₂@C core-shell nanocubes with abundant mesopores（SCS@C）have been synthesized by the strategies of carbon coating and vulcanization using CoSn（OH）₆as the precursor and resorcinol-formaldehyde as the carbon source,and the relationship between the crystal structure,micromorphology and electrochemical performance of the material was studied.The synthesized SCS@C nanocube shows obvious advantages:the porous nanostructure of SCS@C can shorten the transfer path of Na⁺and facilitate the penetration of electrolyte,while the carbon shell can play a rigid role in resisting the volume stress caused by volume expansion during charging and discharging processes,thus ensuring the structural integrity during long-term cycles.When used as an anode of SIBs,the SCS@C electrode demonstrates superior sodium storage:an outstanding initial coulomb efficiency of 92.8%,remarkable cycling performance of 662 mAh g^-1 at 100 mA g^-1 after 70 cycles with extremely low capacity decay of 0.00013%per cycle,and lasting long cycle durability of 402.5 mAh g^-1 at 5 A g^-1 over 500 cycles.（3）SnSe₂-CoSe₂@C（SCSe@C）core-shell structured nanocubes have been prepared by carbon coating and vulcanization with CoSn（OH）₆ as the precursor and resorcinol-formaldehyde as the carbon source,and the crystal structure,micromorphology and electrochemical performance of the material has been investigated.The synthesized SCSe@C has a large specific surface area,which is conducive to the infiltration of electrolyte,while porous nanostructures can facilitate the rapid transfer of electrons and ions,accelerating the reaction kinetics.In addition,the core-shell structure formed by carbon coating can effectively buffer the volume expansion during cycling process,thus improving the cycling stability of the material.When used as an anode for SIBs,the SCS@C electrode exhibits an outstanding initial coulomb efficiency of 83.2%,remarkable cycling performance of 334.4 mAh g^-1 at 100 mA g^-1after 40 cycles with a capacity decay of 0.4%per cycle,and a specific capacity of 325.9 mAh g^-1 at 5 A g^-1after250 cycles.