Preparation Of Nanocomposites Based On Transition Metal Selenides And Their Sodium Storage Performance

As a potential alternative to lithium-ion batteries,sodium-ion batteries have attracted widespread attention in the field of energy storage.Transition metal selenides?TMSs?stand out among many anode materials of sodium ion batteries due to its advantages of abundant varieties and contents,low price,high initial coulombic efficiency and theoretical specific capacity.However,there are many shortcomings when it is used as anode material for SIBs such as poor electrical conductivity,serious particle agglomeration and obvious volume effect,which can lead to poor kinetic behavior and electrochemical stability and limit its further application in the sodium ion batteries.To tackle the above problems,researchers have adopted a series of modification strategies to improve the sodium storage performance of TMSs,including structural design,carbon composite modification,optimization of electrolytes,optimization of cut-off voltage,and control of the composition of materials.Among them,carbon materials modification and nanostructure design are the most researched strategies,which also have the most obvious modification effect.The addition of carbon materials can not only effectively alleviate some problems such as structural damage and rapid capacity decay caused by volume expansion during the charge and discharge process,but also improve the overall conductivity of the material.At the same time,the nanostructure design of TMSs can not only shorten the diffusion distance of Na⁺,improve the kinetic behavior of sodium storage of the material,but also effectively reduce the structural strain of the material during the charge and discharge process,which can further improve its electrochemical performance.However,in the existing reports,there are certain problems in the preparation of mostTMSs materials,such as the complicated process,high cost and even environmental pollution,which cannot meet the requirements of large-scale production and green environmental protection.Based on the above research status,in this paper,aiming at the preparation of TMSs,we sought the direct solid-state reaction method which is simple,low cost,scalable and environmentally friendly.In addition,we chose a suitable carbon source that can form ligands with transition metal ions at room temperature to solve the hurdles of materials such as poor conductivity,serious agglomeration of particles and obvious volume effect.The results show that the introduction of the ligand carbon source enables the nanoscale TMSs nanoparticles to be uniformly embedded in the heteroatom-doped carbon matrix,which not only enhances the conductivity of the material,but also effectively reduces the volume expansion of the material,which significantly improve Na⁺storage performance.Detailed works of this thesis:?1?Using nickel acetate tetrahydrate,o-vanillin and o-phenylenediamine as raw materials,a self-assembled solid-phase reaction was performed at room temperature by direct solid-state reaction to form a complex of bis-Schiff base and Ni?II?.Subsequently,it was mixed with Se powder and then calcined at high temperature,and finally a nanocomposite material with Ni Se particles wrapped in 3D N,Se co-doped carbon matrix was generated?designated as Ni Se???NSe C?.The results show that it can maintain a reversible charge capacity of 291m Ah g^-1 after 100 cycles at 0.1 A g^-1,and the capacity retention is 88%.At the same time,with a large current density of 5 A g^-1,the capacity still remains 197 m Ah g^-1.?2?Using zinc acetate dihydrate,o-vanillin and o-phenylenediamine as raw materials,acomplex of bis-Schiff base and Zn?II?was formed at room temperature.With the progress of carbonization and selenization,ultrafine Zn Se nanoparticles??6.5 nm?were formed in situ in N,Se co-doped carbon matrix?designated as Zn Se???NSe C?.The unique 3D structure makes it show excellent cycle stability which maintains 282 m Ah g^-1 reversible charge capacity after 500 cycles at 0.1 A g^-1 and show excellent rate capability(198 m Ah g^-1 of charging capacity at 5 A g^-1).More importantly,the reversible capacity of 238 m Ah g^-1 was maintained even after 1200 cycles at a current density of 1.0 A g^-1.?3?Manganese acetate tetrahydrate and?-cyclodextrin are used as raw materials.Among them,?-cyclodextrin as a metal ion ligand can self-assemble with Mn???at room temperature to generate?-cyclodextrin and Mn?II?complex.Subsequently,after the carbonization and selenization,Mn Se nanoparticles with a diameter of about 30 nm were formed in situ in the Se doped carbon matrix?designated as Mn Se???Se C?.Unique 2D structure makes it show excellent stability(251 m Ah g^-1 of charging capacity after 500 cycles at 0.1 A g^-1)and good rate performance(134 m Ah g^-1 of charging capacity at 5 A g^-1).