| Sodium ion batteries(SIBs)are considered the most ideal alternative to lithium ion batteries due to their low cost and high safety,and have received widespread attention from researchers.However,the large ion radius of sodium ions can easily lead to slow ion diffusion rates and significant volume changes in electrode materials,resulting in poor magnification performance,rapid reversible capacity degradation,and poor cycle life of sodium ion batteries,severely limiting the commercial application of SIBs.At the same time,the sodium storage performance of commercial fossil graphite anode is extremely poor,with a reversible sodium storage capacity of only 30 mAh g-1.Therefore,developing high-performance anode materials suitable for rapid insertion and removal of Na+is one of the key technologies to significantly improve the electrochemical performance of sodium ion batteries.Transition metal selenides are ideal anode materials for SIBs due to their abundant species,high theoretical specific capacity,and good electrochemical activity.However,transition metal selenides have shortcomings such as low electron/ion conductivity and large volume changes during charging and discharging,which lead to poor sodium storage performance and cycle stability of transition metal selenides as anode materials for SIBs,making it difficult to meet practical application requirements.Based on this,this thesis takes transition metal selenide materials(MoSe2,CoSe2)as the research object.Aiming at the problems of low electron/ion conductivity and obvious volume effects in metal selenides,strategies such as heterostructure construction,heteroatom doping,and conductive carbon coating are adopted to effectively solve the problems of low conductivity and poor structural stability in transition metal selenides.At the same time,in-depth analysis of the sodium storage behavior and electrode reaction kinetics of transition metal selenide composite materials is conducted to promote the significant improvement of the sodium storage performance of transition metal selenide negative electrode materials.The main research contents are as follows:(1)In response to the problem of low electron/ion conductivity in transition metal selenides,the author adopted a strategy of heteroatom doping.Co-MoSe2@CN composite material is prepared using the self polymerization characteristics of dopamine hydrochloride under alkaline conditions with a molybdenum source,as well as a simple hydrothermal method to introduce Co atoms and subsequent pyrolysis selenidation.Preparative about 500 nm Co-MoSe2@CN nanospheres and nanosheets covered on them have a larger specific surface area(120.39 m2 g-1),providing more Na+storage sites,making Co-MoSe2@CN anode electrode exhibit excellent electrochemical performance.The specific capacity of the material after 200 cycles at 1 A g-1 is 403 mAh g-1;After 1000 cycles at 10 A g-1,the specific capacity is still 373 mAh g-1,exhibiting good cycle stability.When a full battery is formed with a cathode material of Na3V2(PO4)2O2F,the energy density at a power density of 210 W kg-1 is 103 Wh kg-1.(2)In order to solve the problem of poor magnification performance of transition metal selenides,the author adopted the strategy of constructing a bimetallic selenide heterostructure(CoSe2-MoSe2).CoSe2-MoSe2 nanospheres with core-shell structure of about 800 nm in size were prepared by simple solvothermal reaction and pyrolysis selenidation process.In addition,the specific surface area is about 102.61 m2 g-1.The construction of CoSe2-MoSe2 heterostructures can provide a wealth of lattice distortion and defects,accelerating the transport rate of electrons and ions.At the same time,the yolk shell structure further regulates the mechanical stress,making the electrode material structure stable.As expected,at current densities of 0.2,0.5,1,2,5,and 10 A g-1,the specific capacities of CoSe2-MoSe2 are 553,519,480,452,430,and 413 mAh g-1,exhibiting better magnification performance.Subsequently,the material was combined with a Na3V2(PO4)2O2F cathode material,and the full battery showed an energy density of 133 Wh kg-1 at a power density of 258 W kg-1.(3)Aiming at the problem of poor rate performance caused by low ion conductivity of transition metal selenide materials,,the author adopted a carbon coating strategy to alleviate the problem.Using chitosan as a carbon source,nitrogen doped porous carbon layer supported FeSe2/CoSe2 nanomaterials were prepared by liquid phase ultrasound and pyrolysis selenidation process,and were used as anode materials for SIBs.The results show that by carefully constructing the boundaries of FeSe2/CoSe2-CN electrodes,electrochemical kinetics can be improved and the stress caused by volume expansion can be effectively regulated.The N-doped carbon layer can effectively prevent the volume expansion during sodium insertion and improve the conductivity of the electrode.Due to the above advantages,the capacity of SIBs based on FeSe2/CoSe2-CN anode material remains 350 mAh g-1 after 1000 cycles at 10 A g-1.Research on the sodium storage mechanism indicates that the excellent performance of the composite material is attributed to the synergistic effect of bimetallic selenides,resulting in more active sites and electron redistribution,promoting the rapid adsorption and transfer of Na+.In addition,when assembled into a Na3V2(PO4)2O2F//FeSe2/CoSe2-CN full battery,the energy density can reach 109 Wh kg-1,showing a certain application prospect. |
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