Study On The Controllable Preparation And Sodium Storage Performance Of Selenium-based Compounds

The extensive use of fossil energy has brought many problems.With the increasing demand for high-efficiency and clean energy,the application of new energy such as solar and wind energy has become more and more extensive,and energy storage technology is facing huge opportunities and challenges.Electrochemical energy storage based on secondary battery technology has gradually revealed its advantages in recent years and has become an emerging energy storage technology.Due to the abundant reserves of sodium resources,non-toxic and environmentally friendly,sodium ion batteriy has become one of the research hotspots in the field of energy storage batteries in recent years.Although the principle of sodium ion batteries is similar to that of lithium ion batteries,the radius of sodium ion is much larger than that of lithium ion.As a result,electrode materials that have been commonly used in lithium ion batteries cannot be directly applied to sodium ion batteries.Therefore,the key to sodium ion battery research is to find the best electrode materials.Among them,selenium-based compounds have become potential candidates for sodium-ion battery anode materials due to their high theoretical specific capacity and abundant reserves.Existing studies have found that the actual capacity of SnSe₂ material was far from the theoretical specific capacity,and there is room for improvement in capacity;Bi₂Se₃ material has a large specific capacity,but it was seldom used in sodium-ion batteries.At the same time,the volume of selenium-based compounds will change greatly during the charging and discharging process,and the structure is easy to collapse,which affects the cycle performance.Therefore,nano-treatment and carbon coating are effective means to solve the problems of low capacity and capacity attenuation.In addition,it is not safe to use metallic sodium as a battery electrode and not meet the requirements of actual use.It is more practical to use sodium vanadium fluorophosphate,which is safe and easy to prepare,as a counter electrode of a selenium-based compound electrode.In this thesis,SnSe₂ and Bi₂Se₃ are used as anode materials for sodium ion batteries through nanometerization and carbon coating to test the sodium storage performance.The performance of the full battery assembled by sodium vanadium fluorophosphate and SnSe₂ was also discussed.The main research work is as follows:(1)SnSe₂ was synthesized by solvothermal method,and the influence of solvothermal reaction temperature and time on the synthesis of SnSe₂ was explored.The study found that the optimal conditions for solvothermal synthesis of SnSe₂ were 180°C for 25 hours.Under this condition,the samples were nanosheets with a thickness of about 97 nm,and the nanosheets were self-assembled from nanoparticles with a size of 5 nm.After assembling the sodium ion battery,the test results shown that at a current density of 1 A g^-1,SnSe₂ has an initial charge capacity of 484 mAh g^-1,a coulombic efficiency of 82%in the first cycle,and361 mAh g^-1 after 200 cycles.(2)Bi₂Se₃ was synthesized by solvothermal method.Firstly,the optimal reaction temperature and time were explored.The experiment found that the optimal conditions for solvothermal synthesis of Bi₂Se₃ were 180°C,40h,and Bi₂Se₃ also had a nanosheet structure.After assembling the half-cell,it was found that the discharge/charge capacity in the first cycle was 562 mAh g^-1 and 504 mAh g^-1,respectively,and the coulombic efficiency reached89.7%.During the first 5 cycles,the capacity quickly decayed and stabilized to about 400mAh g^-1.After 30 cycles,the capacity decayed again,and the final capacity stabilized at about 200 mAh g^-1.The two obvious rapid capacity decays during the charging and discharging process restricted the use of Bi₂Se₃ in sodium ion batteries.In order to suppress the two rapid capacity decays during the charge and discharge process,the Bi₂Se₃ was coated with glucose,and then calcined under argon protection to obtain the Bi₂Se₃@C material.Tests have shown that the carbon coating scheme can partially slow down the two rapid capacity decays,but it still cannot solve this problem fundamentally,and further improvements are needed.(3)The composite material of sodium vanadium fluorophosphate and reduced graphene oxide(NVPF@rGO)was synthesized by microwave-assisted hydrothermal method.It was founded that in the NVPF@rGO sample,the rGO uniformly wrapped the sodium vanadium fluorophosphate particles.When testing the performance of a half-cell with NVPF@rGO as the positive electrode and metallic sodium as the negative electrode,at a current density of1C,the stable cycle capacity was as high as 115.4 mAh g^-1,and the actual capacity exceeds90%of the theoretical specific capacity of NVPF(128mAh g^-1).At a high current density of10C,NVPF@rGO still maintained a reversible capacity of 102.9 mAh g^-1 after 7500 cycles,and the capacity retention rate exceeded 89%.Furthermore,the synthesized NVPF@rGO material was used as the positive electrode material,and SnSe₂ was used as the negative electrode material to assemble the sodium ion battery.After testing,it was found that the first week discharge capacity of the full battery at 0.1A g^-1 was 322.1 mAh g^-1,the first cycle Coulomb efficiency was close to 85%,and the capacity stabilized at 321 mAh g^-1 after 50cycles.The full battery rate performance also performed well,withstood the charge and discharge with a maximum current density of 2A g^-1.The research results show that SnSe₂ is a potential anode material for sodium ion batteries.