Preparation And Properties Study Of Sodium Storage Materials Based On Molybdenum,Tungsten And Oxygen Group Elements

As an important part of sodium ion batteries?SIBs?,anode material directly affects the electrochemical performance of the battery,and has become one of the focuses in the current research work of sodium ion battery.In order to realize the wide range and large scale application of sodium ion batteries,it is essential to explore and develop high power,low cost,high safety and good cycle stability anode materials.Based on the research status and some existing problems of sodium ion battery electrode materials,the transition metal oxide and selenide electrode materials are brought up in this paper.Different transition metal oxide and selenide electrode materials are successfully synthesized by different preparation techniques and methods,and they were studied for their sodium storage properties.The main research contents are as follows:Firstly,using Na₂WO₄·2H₂O,Co?NO₃?₂·6H₂O and graphene oxide?GO?as raw materials,through hydrothermal method and subsequent high-temperature calcination method,the Cobalt tungstate/reduced graphene oxide?CoWO₄/RGO?nanocomposite particles with the sizes between 40-60 nm were synthesized.Such nanoparticles with smaller particle size can better contact with the electrolyte to provide more sodium ion storage sites,and at the same time effectively increase the conductivity and electrochemical kinetics of overall material due to the addition of reduced graphene oxide.Thus it exhibits excellent electrochemical performance:the first discharge specific capacity is 354 mAh g^-11 at the current density of 0.1 A g^-1,and the value can still maintain at 160 mAh g^-1after 100 cycles.The sodium ion hybrid capacitors based on CoWO₄/RGO nanocomposites as the negative electrode materials while activated carbon?AC?as the positive electrode have also been studied,which possess a specific capacitance of 31 F g^-11 at the current density of 0.1 A g^-1.And it can exhibit an energy density of 87.2 Wh kg^-11 at the power density of 224.6 W kg^-1,showing considerable sodium storage electrochemical performance.Secondly,because the conductivity of metal oxides is worse than that of metal sulfides and selenides so that it exhibits poor electrochemical kinetics and rate performance as the negative electrode material for sodium ion batteries.Therefore,in this article,WSe₂ materials with special nanoflake structure were prepared through a solid phase calcination method using W powder and Se powder as raw materials,which showed pretty sodium ion storage capacity:the initial specific capacity value is433 mAh g^-1at the current density of 0.1 A g^-1.The capacity reduced first and then increased during the subsequent cycles and can maintain at 160 mAh g^-11 after 110cycles.Similarly,the sodium-ion hybrid capacitors using WSe₂ nanoflake materials as the negative electrode materials were also studied in this article,which possesses a specific capacitance of 37.5 F g^-11 at the current density of 0.1 A g^-1.When the current density is increased to 0.5 A g^-1,a specific capacitance of 21.8 F g^-1is still available,demonstrating an excellent rate performance.At the same time,it exhibits an energy density of 105.5 Wh kg^-11 at the power density of 224.4 W kg^-1,showing outstanding sodium ion storage performance.Thirdly,the MoSe₂/RGO nanosheet composites were successfully prepared through hydrothermal method followed by high temperature calcination method using Na₂MoO₄·2H₂O,Se powder and GO as raw materials.The material with the nanosheet structure can provide an advantageous and convenient transmission channel for sodium ions and effectively shorten the ion transport path.At the same time,the MoSe₂/RGO nanomaterial possesses an outstanding electrochemistry kinetics due to the increase of the overall conductivity benefiting from the introduction of RGO,so that exhibiting excellent electrochemical sodium storage properties.It exhibits an initial specific capacity of 472.6 mAh g^-1at the current density of 0.1 A g^-1and is able to maintain at 246.9 mAh g^-1after 100 cycles.Sodium ion hybrid capacitor with MoSe₂/RGO nanosheet composite as negative electrode and AC as positive electrode can provide a specific capacity of 35.8 F g^-11 at the current density of 0.1 A g^-1,and the energy density of 87.7 Wh kg^-11 can be exhibited at the power density of 208.8 W kg^-1,demonstrating an excellent sodium ion storage capacity.