| In recent years,the development and utilization of clean energy has become a research hotspot.However,due to the characteristics of discontinuous energy supply and uneven distribution,the development and utilization of energy storage equipment is the key to further solve the environmental problems.Lithium ion battery is the most widely used secondary battery,and its demand is increasing gradually.However,lithium reserves in the crust are very rare,and 70%of them are distributed in South America and other regions.Sodium and lithium are in the same main group and adjacent in the periodic table,and the two elements have similar physical and chemical properties.In addition,the content of sodium in the earth’s crust is very rich,so in recent years,the development and utilization of electrode materials for sodium ion batteries is a research hotspot in the field of energy storage.The main obstacle to the development of sodium ion battery is the larger atomic radius and the heavier relative atomic mass,in the anode material,some metal sulfides with graphite-like layer structure have very unique advantages.For example,Sn S2,Mo S2,Re S2,etc.,have large interlayer distance（>0.5nm）and weak Van der Waals force exists between adjacent S-M-S layers,which can promote the migration of sodium ions.However,a single metal sulfide has poor conductivity and will produce large volume deformation after repeated redox reaction in the battery.In this paper,the above disadvantages of the material are overcome mainly through the construction of heterostructure and design of microstructure morphology.By synthesizing 2D nanoplates,3D cubes and hollow nanospheres with cavity structures as the basis of the structure,the bimetallic sulfides with three-dimensional layered heterostructure were successfully prepared by combining lamellar Sn S2,Mo S2 and Re S2 on the outside by hydrothermal method.The reasonable structure is designed to improve the electrochemical properties of sodium ion batteries as follows:（1）The electric-field formed by heterostructure promotes the dynamic properties of ions during the charge and discharge of batteries,while bimetallic sulfides make the material have better conductivity.The composite electrode has higher discharge specific capacity and better rate performance at low current density by vertically arranging large layer spacing metal sulfide which is more suitable for the reversible deintercalation of sodium ions on the outside of the structure.（2）From simple 2D nanosheets to 3D nanocube skeletons to hollow nanospheres,the battery cycle stability of composites depends on the precursor that is the basis of the structure.For the two-dimensional sheet metal sulfide heterostructure Ni S@Sn S2,the composite material can effectively delay the structural collapse of the material and the capacity decline of the battery.The three-dimensional nanocube Co S2@Mo S2 has a more stable Co S2 nanocube skeleton inside,so the composite electrode has a higher discharge specific capacity and a longer cycle life.The capacity can still maintain 344.2 m Ah g-1 at the high current density of 1 A g-1 after 300 cycles,capacity retention rate is 82.6%.Ni S2@Re S2 hollow nanosphere has a cavity structure that provides some cushioning space for the volume formation of the material,so the battery has excellent cycle performance,with a capacity retention rate of 82.2%after 200 cycles at a high current density of 1 A g-1.（3）By comparing the proportion of the pseudocapacitance behavior between materials of different sizes in the process of battery discharge,it is proved that smaller material size can effectively improve the pseudocapactiance behavior ratio of the battery.In the 2D bimetal sulfide composite Ni S@Sn S2 electrode,the diameter of the 2D Ni S sheet is about 3-4μm.In the CV test of 1 m V s-1,the small proportion of the pseudo-capacitance is 46%.The side length of the Co S2@Mo S2 nanocube is 400 nm,and the further reduction of the particle size makes more surface redox reactions occur during the charging and discharging of the battery,thus improving the contribution rate of the pseudocapacitance of the battery.Through fitting calculation,it is found that the pseudocapacitance ratio measured by CV test at a scan rate of 1m V s-1 is 83%.When the material has a hollow structure inside,the rapid redox reaction that occurs on the surface of the material accounts for a further increase.Therefore,the pseudocapacitance calculated by fitting in the CV curve of the Ni S2@Re S2 hollow nanosphere at a scan rate of 1 m V s-1 is up to 90%. |
PDF Full Text Request