Research On The Improvement Of The Electrochemical Performance Of Transition Metal Selenide By Microstructure Design And Membrane Modification

In the last two centuries,with the development of portable devices such as mobile phones,notebook computers and electric vehicles,the demand of the energy storage device increasing.The new storage device ask higher energy density,higher power density,excellent cycle stability and rate performance.In addition to meeting these requirement,dealing with the environmental pollution problem,energy storage devices should be safe and environmentally friendly.Based on the above requirements and the measurement of price,lithium ion battery has been widely used due to its advantages of good cycle stability,good safety performance and no memory effect.However,with the widespread use of electric vehicles,higher requirements have been placed on the energy density?high specific capacity?,power density?high rate performance?and cycle life of lithium ion batteries.The key factors that limit the performance of lithium-ion batteries are electrode materials,especially anode materials.However,due to the low content and uneven distribution of the lithium source in the earth's crust,the price of lithium resources is getting more expensive,which limits the large-scale development of lithium ion batteries.The sodium ion battery has a similar charge and discharge principle as the lithium ion battery,and the sodium element is abundant,making it one of the most likely new batteries to replace the lithium ion battery.Metal selenide has been widely studied because it has similar physical and chemical properties to metal sulfides and has higher conductivity than metal sulfides.However,metal selenide as a lithium battery anode materials,there are still problems such as volume expansion and shuttle effect,which makes the structure damage and resulting in reversible capacity loss and poor cycle stability.In this paper,the material microstructure design and membrane modification dual strategy will be used to solve these problems and improve their lithium/sodium storage performance.The main point of this article is as follows:?1?We use the polyvinylpyrrolidone and iron nitrate to form a mixture gel,which was blown up by N₂ to form a bubble structure.After calcination and selenization process,3D disordered porous FeSe₂/C electrode materials were generated sucessfully.By testing the structure of the electrode material,it is found that the 3D disordered porous electrode material is composed of mesopores and micropores.The porous structure can alleviate the problem of volume expansion during the process of charge and discharge,and can also provide more active sites.The advantages of this porous structure help FeSe₂ achieve better cyclic stability.After 100 cycles under the current density of 100 mA g^-1,the specific capacity of the material can be maintained at 798.5 mAh g^-1.At the current density of 500mA g^-1,the original 3D disordered porous structure can still be maintained after 200 cycles.?2?We use the SiO₂ spheres as template to synthesis three-dimensional ordered porous ZnSe/C electrode materials by sol-gel method and low temperature selenization method.The carbon content of the composite is optimized by varying the amount of PVP.Firstly,the electrochemical properties of the precursor ZnO/C composite were tested.When the quality of PVP was 0.5 g at 100 mA g^-1,the capacity was still maintained at 925 mAh g^-1after 180 cycles.The optimal carbon content of the precursor was directly decide the electrochemical performance of the selenization production.As a sodium battery anode,the developed electrode delivers a reversible capacity of 380 mAh g^-1 after 60 cycles at a current density of 100 mA g^-1.Its excellent lithium/sodium storage performance is mainly attributed to the three-dimensional ordered porous structure and carbon material.Among them,the three-dimensional ordered porous structure increases the contact area of the electrolyte,provides more active sites,and shortens the diffusion and transmission paths.The carbon material improves the conductivity of the composite,and is beneficial to alleviate the volume expansion.?3?Due to the"shuttle effect"issue in the Li-S battery,during the charged process electronegative polysulfide easily dissolved in the electrolyte effect the cycle stability.Therefore,electronegative additives are often added to electrolyte to prevent the diffusion of polysulfide through the membrane by means of electrostatic repulsion.As we know,put the ferroelectrics in the applied electric field,the internal dipole will be oriented to form a spontaneous internal electric field,and when the applied electric field is removed,the spontaneous electric field can still be maintained.This paper use the BaTiO₃ as the ferroelectric material to mixture with PE membrane.Then applied the poled BTO membrane to the 3D disordered porous FeSe₂/C electrode materials to test the lithium ion battery performance.The poled BTO membrane exhibited an enhanced reversible capacity of 1044 mAh g^-1 at a current density of 500 mA g^-1 over 200 cycles.In contract,the non-poled BTO membrane shows 767 mAh g^-1,at the same time,in the case of commercial PE only remained 704 mAh g^-1.