Design Of Hollow Metal Sulfide Heterostructures And Their Sodium And Potassium Storage Properties

The application of electric energy has already applied into all aspects of people's life.As its important storage equipment,the lithium-ion battery(LIBs)has also ushered in the thirtieth year of commercialization.The increasingly strong market demand has kept the prices of lithium ore and copper foil high.Therefore,manufacturers and scholars have focused on sodium and potassium,which are more abundant in resources.On the other hand,the graphite negative electrode following the intercalation mechanism is not only difficult to meet the technical requirements of large capacity and small volume but also its accumulated technical experience can not be fully applicable to the latter with larger ion diameter,higher reaction activity,and slower reaction kinetics.Therefore,although sodium ion and potassium ion batteries have sufficient development conditions and reference experience,it is still necessary to find and try new negative electrode materials in the research and development process.As a rising star,metal compounds will show huge capacity after conversion reaction and alloy reaction.Some of the metal sulfides not only facilitate the insertion and removal of sodium or potassium ions due to their two-dimensional layered structure(such as Mo S₂,SnS₂,Ni S₂,etc.),but also show a high degree of reaction reversibility due to their high activity.It is worth noting that metal sulfide will face two serious problems when applied to the negative electrode.One is the properties of its semiconductor,and the mediocre conductivity is bound to limit the reaction process.The other is the common problem of volume expansion.The huge volume difference before and after the reaction is not only easy to causes the solid electrolyte interface(SEI)on the material surface to burst,but also the stress change under repeated reaction makes the material structure fragile and causes pulverization or agglomeration.Herein,two or three metal sulfides are introduced to construct heterostructures utilizing ion exchange,ion adsorption,and hydrothermal growth.The overall conductivity and reaction activity is improved by the acceleration and adsorption of the electric field between contact interfaces.At the same time,the hollow overall structure is shaped for the material regarding the sequential template method to improve the cycle stability of the negative electrode material.Firstly,part of Fe S₂ of the intermediate is replaced by Sb₂S₃ in situ by a simple ion-exchange method to form an island heterostructure.Meanwhile,space is created for the interior of the material to solve the expansion problem of the core,supplemented by the external conductive carbon shell to form a stable yolk-shell structure as a whole.The materials under the synergistic action show excellent rate performance,not only showing a high first cycle discharge specific capacity of 702.6 m Ah g^-1 and a high first cycle coulomb efficiency of 85.75%but also can operate continuously for more than 650 cycles without obvious capacity attenuation at a high current density of 1.0 A g^-1.Secondly,the carbon spheres prepared by the soft template method have a uniform size.The cobalt sulfide nanospheres obtained by adsorption,calcination,and vulcanization have a hollow structure.After growing two-dimensional layered Mo S₂ on its surface by the one-step hydrothermal method,a mixed heterostructure is formed.Even if the material does not involve alloy reaction in the process of charge and discharge,by intercalation and conversion reaction,the material can achieve 82.61%high first cycle coulomb efficiency at 0.1 A g^-1.At the same time,the material also shows average reversible capacities of 683,547,415,352,310,and 268m Ah g^-1 at subsequent current densities of 0.1,0.2,0.5,1.0,2.0,and 5.0 A g^-1,respectively.Finally,SnS₂ is pyrolyzed by annealing and reformed into SnS in the inner layer of the carbon shell,which is the same as Ni S with two-dimensional structure composite that forms a hierarchical heterostructure.Before that,Si O₂ coating is introduced as a sacrificial template to create a huge internal space for the material.While giving full play to the advantages of hierarchical heterostructure with large specific surface area,the outer conductive carbon shell and the inner hollow structure also play a good complementary role.Compared with the same period,Ni S_1.03@SnS@Zn S@C not only exhibits excellent rate performance,the capacity platform between each current decreases slowly,but also can run continuously for more than1150 cycles under large current density,and the capacity loss is only 4%.