Design,Preparation And Sodium Storage Performance Of Selenium-based Anode Materials

With the development of society,people have realized the seriousness of energy shortage and environmental pollution,and the state has also proposed the strategic goal of carbon neutrality and carbon peaking.As a result,the development of large scale electrochemical energy storage systems was in full swing.Although the commercialization of lithium-ion batteries has greatly changed the production and life of human society,but the limited lithium resources,high costs,and exposure of hidden safety hazards have prompted people to explore the energy storage systems that can replace the lithium-ion batteries.Sodium-ion batteries have a energy storage mechanism similar to the former,and are rich in sodium reserves,low in cost and environmentally friendly,so are highly hoped.Unfortunately,the radius of Na⁺（1.02?）is larger than that of Li⁺（0.76?）,so the traditional lithium-ion battery anode material（graphite）cannot effectively accommodate Na⁺.Therefore,the priority is to find the appropriate high-performance host materials for sodium-ion batteries.The metal selenides is easy-prepared,diverse,and high in theoretical capacity,which has attracted people’s attention.Compared with metal sulfides/oxides,the larger atomic size of selenium facilitates the conversion reaction,and can expand the layer spacing to accommodate more Na⁺.In addition,good electronic conduction and smaller band gaps also make it excellent rate performance.However,the structural collapse,decreased conductivity and the side effects with electrolytes during the cycle are still unavoidable,causing the cycling and rate performance to be unsatisfactory.In order to solve the above problems,nanostructure design,compositing conductive materials and constructing a diversified system are currently commonly used methods.The specific research content and results of this paper are as follows:1.Research on the preparation of Cu₄Mo₆Se₈/C composite materials and its sodium storage performance.The bimetallic（Cu-Mo）MOF obtained by the co-precipitation method were used as the precursor,and using a one-step selenium strategy to design and prepare the Cu₄Mo₆Se₈/C nanocomposite material with a 3D network structure.The 3D network structure composed of ultrathin Cu₄Mo₆Se₈/C nanosheets has a rich gap,which provides a necessary space for the volume expansion during the cycle.The introduction of Cu with different atomic sizes and valence electrons are conducive to reducing Na⁺diffusion and increased conductivity,and the synergy between Cu-Mo can also effectively curb the structural collapse in the cycle.In addition,the Cu-Mo bi-metal centers improve the adsorption capacity and enrich the redox sites,which reduce the dissolution of the polyselenide in the electrolyte and the irreversible decomposition of Na₂Se during the cycle.Finally,carbon can improve the overall conductivity and structural toughness of the material,effectively alleviating the crushing and aggregation of electrode materials,and further improves the electrochemical properties.When used as the anode material of sodium-ion batteries,it shows excellent rate performance(the discharge capacity under 0.1,0.2,0.5,1.0,2.0 and 5.0 A g^-1 is 475,455,467,468,468 and 467 m Ah g^-1,respectively)and cycle performance(474 m Ah g^-1 after 2400 cycles at 2 A g^-1).2.Research on the preparation of N-MXene/WSe₂ composite materials and its sodium storage performance.Melamine formaldehyde resin was used as a self-sacrifice function template and nitrogen source,which used to match the metal ions and restrict the size.Subsequently,through the selenium in the reductive atmosphere to design and prepare the WSe₂ nanoplates evenly distributed on the nitrogen-doped MXene nanosheets.In the prepared materials,N-MXene serves as elastic conductive skeleton to uniformly distribute WSe₂ nanoplates,which can not only effectively alleviate the gathering and volume expansion of WSe₂ nanoplates during the cycle,but also provide a stable conductive network to achieve fast ion/electron transmission.The unique morphological characteristics of WSe₂ are conducive to exposing more edge sites and reducing the resistance of charge transfer,which provide high catalytic activity for electrochemical reactions and improve the overall capacity of the material.The uniformly distributed WSe₂ nanoplates can also be effectively prevented from the restacking of N-MXene nanosheets.In addition,the synergistic effects between N-MXene nanosheets and WSe₂ nanoplates reduce the reaction activation energy,so that the N-MXene/WSe₂ nanocomposite material shows excellent cycle performance when as the anode electrode of the sodium-ion batteries.The discharge capacity is215 m Ah g^-1 after 1300 cycles at 2 A g^-1,and the capacity retention is about 96.4%.