Design,Structure And Sodium Storage Properties Of Transition Metal Sulfides/Selenides As Anode Materials For Sodium Ion Batteries

In recent years,the excessive consumption of fossil energy has caused serious energy crisis and environmental pollution.Therefore,it is of great significance to find cost-effective energy storage conversion technologies.Since 1991,lithium-ion batteries（LIBs）have been successfully commercialized.Unfortunately,the uneven distribution and high cost of lithium resources hinder their large-scale application in the future.Sodium and lithium possess similar physical and chemical properties due to the fact that they are located in the same main group.Therefore,recently,sodium ion batteries（SIBs）have been considered as the promising alternatives to LIBs.However,it is more difficult for Na⁺to sodiation/desodiation in anode materials than Li⁺due to the larger radius of Na⁺than Li⁺.As a result,the anode materials for SIBs generally exhibit poor cycling stability and low capacity,hindering the practical applications of SIBs.Among various anode materials for SIBs,transition metal sulfides/selenides have attracted much attention due to their high theoretical capacity and diversified variety.However,most of transition metal sulfides/selenides still suffer from many problems such as large volume expansion,poor conductivity and so on.Herein,three kinds of transition metal sulfides/selenides have been designed by the strategies of morphology design,the composite of carbon and electrolyte optimization,and their structure,microstructure and electrochemical properties of the materials as anode materials for SIBs have been systematically investigated.The main research contents and results are as follows:（1）The NiS₂/NiSe₂ heteroboxes have been constructed and synthesized by thecombined process of hydrothermal reaction and sulfuration/selenization,and their structure,micromorphology and electrochemical performance of the materials as anodes materials for SIBs have been studied in detail.The NiS₂/NiSe₂ heteroboxes possess heterojunction structure and exhibit the morphology of hollow cube.The NiS₂/NiSe₂heteroboxes have many advantages as anode materials for SIBs:1)the lattice distortion and electron redistribution caused by the heterostructure facilitate the transport and storage of electron/Na⁺;2)the abundant pore structure can shorten the transmission path of Na⁺and facilitate electrolyte the infiltration of electrolyte;and 3)the hollow structure can relieve the stress caused by volume expansion generated during the Na⁺insertion/deinsertion and avoid the collapse of material structure.As a result,the NiS₂/NiSe₂ heteroboxes exhibit excellent sodium storage performance:giving a high reversible capacity of 292 m A h g^-1 after 2000 cycles at 10 A g^-1.（2）The NiS₂-Co S₂@C material was synthesized by etching the precursor of ZIF-67with Ni（NO₃）₂ followed by carbonization and sulfuration,and the structure,microstructure and electrochemical properties of the material as an anode for SIBs were studied.The material has a core-shell cubic structure with a cavity inside.As the anode for SIBs,the NiS₂-Co S₂@C exhibits the following advantages:1)the introduction of nickel promotes the formation of bimetallic sulfide,resulting in synergistic effect;2)the cavity can buffer volume expansion and avoid the collapse of material structure;and 3)the carbon shell can provide rigid protection for microstructure of the material.As a result,the NiS₂-Co S₂@C possesses excellent electrochemical properties:after 500cycles at 5 A g^-1,the reversible capacity remains at 385.2 m A h g^-1.（3）The NiSe-Co Se₂@C material was synthesized by etching the precursor of ZIF-67with Ni（NO₃）₂ followed by carbonization and selenization and the structure,microstructure and electrochemical properties of the material as an anode for SIBs were investigated.The NiSe-Co Se₂@C possesses a core-shell micromorphology.The material exhibits the following advantages as the anode for SIBs:1)the core-shell structure can buffer the stress generated in the process of sodiation/desodiation and avoid the collapse of the structure;2)carbon shell plays an important role in improving the stability of structure;and 3)the large specific surface area provides more active sites and promotes the reaction kinetics during charging and discharging process.As a result,the NiSe-Co Se₂@C exhibits an excellent sodium storage performance:it can maintain a reversible capacity of 259.8 m A h g^-1 after 600 cycles at 5 A g^-1.