Synthesis And Electrochemical Performance Of Transition Metal Selenide Anodes For Sodium Ion Batteries

Currently,the rising cost of lithium has led to an increase in the cost of using lithium-ion batteries（LIBs）,making it urgent to search for new alternatives.Given the economic volatility of LIBs,sodium ion batteries（SIBs）are potential alternatives to LIBs due to their cost and similar working mechanism to LIBs.However,the larger radius of Na⁺than Li⁺,which means that some materials that perform well in LIBs cannot be used directly in SIBs.Therefore,the development of electrode materials with excellent performance and stability remains a hot topic of research in electrochemistry.Transition metal selenides（TMSs）are being explored as anode materials for SIBs due to their metallic properties,two-dimensional conformation,layered structure,and high theoretical capacity.Metal doping and heterostructure interfaces could improve the kinetics of Na⁺storage reactions in TMSs by inducing lattice expansion or contraction and reducing ion diffusion barriers.In this paper,we focused on the preparation and modification of transition metal selenides for sodium storage applications.The main research work and results are as follows:（1）The N-doped carbon-coated Co（PO₃）₂/Co Se₂ heterostructure was designed via high-temperature selenization by using polydopamine（PDA）-functionalized Co-MOF as a precursor.Heterostructure can achieve high-performance sodium-ion batteries by promoting charge transfer and increasing the contribution of surface capacitance.Heterojunction interfaces can provide a large number of exposed active sites,accelerate Na⁺transport speed.Multiphase synergistic effects accelerates Na⁺diffusion,promotes charge transfer kinetics,and enhances stability.Carbon coating ensures structural integrity of materials during repeated cycling.The electrode designed based on the aforementioned strategy exhibits excellent electrochemical performance.When used as anode of SIBs,Co（PO₃）₂/Co Se₂ could show a high sodium storage capacity of 333.4m Ah g^-1 at 0.1 A g^-1 and maintain a stable capacity of 310.6 m Ah g^-1 after 100 cycles with a capacity retention of about 93.2%.Even after 1000 cycles at 2 A g^-1,it could maintain a reversible capacity of 192.7 m Ah g^-1.（2）N-doped carbon-coated Co-doped MoSe₂ materials were prepared via high-temperature selenization using polydopamine（PDA）functionalized Co Mo-MOF as a precursor.Stable composite materials（Co-MoSe₂（1-5）@PNC）were obtained by optimizing the Co and Mo metal ratios.The N-doped carbon coating can increase the capacity and effectively alleviate volume expansion of active materials during cycling process,improving electrode stability.The flower-shaped structure composed of sheet-like nanosheets increases the specific surface area.Co-doping increases the surface active sites and accelerates Na⁺insertion and extraction rates on the surface.The advantages of these structures enable the synthesized Co-MoSe₂（1-5）@PNC composite material to exhibit high specific capacity,cycling stability and high rate performance.The capacity remains stable at 320.1 m Ah g^-1 after 100 cycles at 0.1 A g^-1,with a capacity retention rate of about 88%,and the specific capacity is 210.5 m Ah g^-1 at a high current density of 5 A g^-1.（3）A Prussian blue analogue（Ni Co-PBA）synthesised by a simple room temperature ageing method was used as a precursor,which was selenised at high temperature with carbon material to obtain carbon modified Co Se₂ and Ni Se₂composites（Ni-Co-Se@C）.The modification of transition metal selenides with carbon materials enhances the reaction kinetics and electrical conductivity of the materials.The Co Se₂ and Ni Se₂ phases in the material form a favourable multiphase synergistic system that enhances the electrochemical performance of the electrodes.The designed material shows better electrochemical performance than the control material,with a capacity retention of 177.1 m Ah g^-1 after 50 cycles at 0.1 A g^-1and good cycle stability.