Application Of Tungsten Selenide-based-nitrogen-doped Porous Carbon Composite Superstructure For Sodium/Potassium Storage

With the looming energy crisis,people's increasing awareness of environmental protection has prompted researchers to focus on clean and efficient new renewable energy sources.In recent years,lithium-ion batteries have developed rapidly and successfully achieved mass production.However,due to the limited global lithium resources and cost considerations,there is an urgent need to develop new energy storage devices to meet the growing energy demand.The global reserves of sodium/potassium resources are more abundant and cheaper,and the working mechanism of sodium/potassium ion batteries is akin to lithium ion batteries',as a result,sodium/potassium ion batteries are regarded as an alternative to lithium ion batteries.But,the radius of sodium/potassium ions is relatively large,and there are problems such as slow kinetics and volume expansion during charging and discharging,which will affect the electrochemical performance of the batteries.So sodium/potassium ion batteries are still a long way from the commercialization goal.There is also an urgent need for anode materials suitable for sodium/potassium ion batteries in order to achieve the goal of better performance,safety and stability.This thesis aims at tungsten selenide-based materials,focusing on the structure design of anode materials,showing excellent cycle and rate performances,while in-depth analysis and discuss on the sodium/potassium storage mechanism of the materials.The main contents of the discussion are as follows:1.The tungsten-polydopamine(W-PDA)superstructure was synthesized through simple solution stirring and self-assembly.After high-temperature calcination and etching,WO₂/N-PC with porous carbon was obtained.Finally,tungsten selenide/nitrogen-doped porous carbon(WSe₂/N-PC)superstructure was obtained using selenium powder as the selenium source.From the SEM and TEM images,it can be determined that the porous carbon in the WSe₂/N-PC superstructure remains intact and evenly distributed.The elements W,Se,C and N are also evenly distributed in the superstructure,and the construction of the superstructure can effectively relieve the volume effect and agglomeration of active materials.Therefore,when the WSe₂/N-PC superstructure is served as a negative electrode material for sodium/potassium ion battery,both demonstrate outstanding and stable electrochemical performances.When used as a sodium ion battery,the WSe₂/N-PC superstructure shows excellent cycle performance(at a current density of 0.1 A g^-1,a specific capacity of 390 m Ah g^-1at 200 cycles),long cycle performance(current density of 0.2 A g^-1,the specific capacity of 316 m Ah g^-1at 400 cycles)and excellent rate capability.At the same time,we assembled the WSe₂/N-PC superstructure into a potassium ion battery.Its electrochemical performance is also excellent.When the current density is 0.1 A g^-1,showing a reversible specific capacity of 220 m Ah g^-1at200th cycles.2.The element Mo is introduced on the basis of the WSe₂/N-PC superstructure by a simple solution stirring method at room temperature,adjusting the ratio of Mo and W,and finally a series of Mo_(1-x)W_xSe₂/N-PC superstructures were synthesized.From the SEM and TEM results,it can be determined that its morphology is almost the same as before.Mo and W are uniformly distributed in the entire superstructure,and have the characteristics of both WSe₂and Mo Se₂at the same time,and the electrochemical performances are significantly improved.The XRD results show that the crystal structure of the Mo_(1-x)W_xSe₂/N-PC superstructures are good.As sodium ion battery,the Mo_0.5W_0.5Se₂/N-PC superstructure shows excellent cycle performance:when the current density is 0.1 A g^-1,it shows a reversible specific capacity of 716 m Ah g^-1in the 200th cycles;when the current density is 0.8 A g^-1,the specific capacity for 400cycles is 295 m Ah g^-1and for 1000 cycles is 258 m Ah g^-1;the rate performance is also excellent.