Electrochemical Performances Of Novel Transition Metal Sulfides For Sodium/Potassium Storage

Transition metal sulfides with large interlayer spacing,high theoretical capacity and controllable microscopic morphology are considered as a promising anode material for sodium/potassium ion batteries.However,the low electronic conductivity of transition metal sulfides is not conducive to electrode reactions.In addition,during the charging and discharging process,the volume expansion is serious that leads to the pulverization and loose contact of the active material,and finally leads to the rapid attenuation of the capacity.To address these above conundrums,this paper designs and prepares nanostructures of transition metal sulfides with different morphologies,and the electrochemical sodium storage and potassium storage mechanisms of electrode materials are discussed in detail.The article mainly includes the following points:?1?Molybdenum-based sulfide compounds are often used in sodium-ion battery electrode materials due to their layered structure and high theoretical capacity,but their cycling stability and rate performance are not good.Molybdenum-based solid spheres are used as templates to prepare hierarchical MoS₂ and MoSe₂ hollow nanospheres via ion exchange mechanism by solvothermal method.The hollow structure provides high surface area,large electrode-electrolyte contact area and a large number of electrochemical active sites for the storage of sodium ions.The permeable thin shell shortens the diffusion distance between sodium ions and electrons and it improves the ionic conductivity of the material.The internal voids effectively alleviate the stress-induced structural changes during long-term electrochemical reactions and improve the stability of the material.The crystal structure,morphology,composition and valence state of the samples are characterized.When used as the anode electrode of sodium ion batteries,MoS₂ and MoSe₂ hollow nanospheres have good performance.The capacities of MoS₂ and MoSe₂ hollow nanospheres only lost 5.51%and 7.79%after 500cycles at a current density of 1 A g^-1,indicating good sodium storage capacity.?2?For further improving MoSe₂ sodium storage performance,the ZIF-8 derived nitrogen-doped porous carbon dodecahedron is used as the precursor by in situ uniform growth MoSe₂ on ZIF-8 derived nitrogen-doped porous carbon dodecahedra,and the obtained nanostructure has a unique core/shell structure,which is conducive to the rapid transfer of electrons and ion diffusion,thus realizing rapid surface control electrode dynamics.The structure can also effectively improve the durability of the electrode and inhibit the change of the electrode structure during charge-discharge process.When used as anode of sodium ion battery,the prepared electrode shows a high initial specific capacity of 464 mA h g^-1 at 0.2 A g^-1,and the specific capacity can be maintained at 437mA h g^-1 after 500 cycles.At a high current density of 2 A g^-1,the electrode can provide a 223 mAh g^-1 specific capacity after 1000 cycles.?3?During the process of sodium storage,the volume expansion of VS₂ is serious which leads to the rapid attenuation of its capacity.An ultra-enlarged interlayer metallic disulfide vanadium nanosheet is prepared by using a simple solvothermal method with a n-octylamine organic solvent as proppant,extending the interlayer spacing of the?001?crystal plane from 5.79?to 10.20??76.16%expansion?.The ultra-enlarged interlayer can effectively reduce the diffusion barrier and provide a wide two-dimensional channel for the rapid diffusion of Na⁺.Furthermore,the micron-scale lateral size of the metallic VS₂ nanosheets and the abundant buffer space between the nanosheets can well maintain the integrity of the electrode material structure.When used as anode material of sodium ion battery,the specific capacity of initial discharge at current density of 0.1 A g^-1 is as high as 570 mAh g^-1.After 100 cycles,the specific capacity remains at 477 mA h g^-1.The capacity keeps 286 mA h g^-1 after cycling 800 cycles at a current density of 0.1 A g^-1.Moreover,as negative electrode of potassium ion battery,its initial discharge specific capacity is 543 mA h g^-1 at current density of 0.1 A g^-1.It still maintains good cycle stability after 200 cycles.