Application Of Metal Selenides In The Anode Of Lithium/Sodium Ion Batteries

With the continuous development of social economy,energy and environment problems are increasingly prominent and carbon neutrality has become a social consensus.The storage and use of clean energy is one of the important ways to get rid of energy crisis and reduce environmental pollution.As a kind of energy storage device,lithium-ion battery can realize the mutual conversion of electric energy and chemical energy,and thus it plays an indispensable role in smart grid,new energy vehicles,consumer electronics and other aspects.Although the level of lithium-ion battery commercialization has made great progress,the low capacity of graphite anode severely limits its further application.Therefore,it is necessary to seek anode materials with higher energy density and longer cycling life to meet the practical requirements.In view of the limited resources,uneven distribution and rising price of lithium ores,researchers need to develop new battery systems to make up for the shortage of lithium-ion batteries in practical applications.Among them,sodium ion battery as a supplement to lithium-ion battery has become a research hotspot.As the same main group element,sodium-ion battery shares similar working mechanism with lithium-ion battery,and sodium resources are widely distributed around the world,easy to obtain and cheap.Therefore,it is of great significance to develop sodium-ion batteries.The type,morphology and composition of anode materials can directly affect the reversible capacity,cycling stability and cycling life of batteries.Therefore,in the development of battery system,the design of anode material is very important.Metal selenides have been widely reported as anode materials for lithium-ion and sodium-ion batteries due to their advantages of numerous types,abundant phase states,adjustable band gap and high theoretical capacity.However,the conductivity of metal selenides is usually poor,and the cracking of the electrode surface during the electrochemical cycle process cause poor reversible reconfiguration when it is used as a negative electrode and results in poor cycle life and rate performance of the battery.In order to improve the electrochemical cycling performance of metal selenides,the high temperature solution phase synthesis method was selected to achieve the batch preparation of a variety of metal selenides,and which showed excellent electrochemical performance in alkali metal ion batteries.The main content of this paper is divided into the following three parts:In Chapter II,Sn Se₂/Sn Se-r GO composites were prepared efficiently and conveniently at high temperature with oleylamine and tri-n-octylamine as solvents,tin tetrachloride pentahydrate and selenium powder as metal and selenium sources,respectively.The lithium storage properties of Sn Se₂/Sn Se-r GO composites were studied in detail.When the current density is 0.1 A g^-1,the specific capacity of the battery is up to 911.4 m Ah g^-1 after 100 cycles.Sn Se₂/Sn Se-r GO composites also displays excellent cycling performance with the capacity of 374.7 m Ah g^-1 after 1000 cycles at 2.5 A g^-1,which is much better than Sn Se₂ and Sn Se.By means of TEM,XRD,Raman and density functional calculation theory,the contribution of Sn Se₂/Sn Se heterojunction to the excellent performance of batteries has been clearly explained.In Chapter III,we developed a simple and high-throughput wet chemical strategy to synthesize sea urchin-like Fe Se₂/NC composites and used them as anode for sodium-ion batteries.The strong coordination of Fe-N bond limited the aggregation of Fe atoms and volume expansion during the electrochemcial cycle,and the unique biomass morphology of Fe Se₂/NC anode with high electrical conductvity enhanced the dynamics of sodium ion insertion/disinsertion and shortened the ion/electron diffusion length.Fe Se₂/NC electrode material owns a reversible capacity of 387.6 m Ah g^-1 after 56000 cycles at the current density of 20.0 A g^-1.A sodium ion full cell was assembled with Fe Se₂/NC anode and NVP cathode shows 203.5 m Ah g^-1 reversible capacity at 1 A g^-1after 2400 cycles.At the same time,the Fe Se₂-Fe₃Se₄/NC was prepared by the similar method,which did not show significantly capacity decrease after 65000 cycles at the current density of 20 A g^-1.The sodium storage mechanism of Fe Se₂/NC was elucidated by density function theory,in situ and ex situ characterizations.By constructing a unique coordination environment between the active substance and the main carbon material skeleton,which provides a new idea for improving the life of sodium-ion batteries.In Chapter IV,we prepared VSe_2-x/NC composites material and studied their application in the negative electrode of sodium ion battery.VSe_2-x/NC nanoflowers show excellent Na-storage capacity.The VSe_2-x/NC composites exhibit 441.0 m Ah g^-1 reversible capacity after1000 cycles at the current density of 1 A g^-1.A 438.4m Ah g^-1 reversible capacity of VSe_2-x/NC can remain at 5 A g^-1 after 2500 cycles.A sodium-ion full cell assembled with VSe_2-x/NC anode and NVP cathode shows excellent cycling stability at 1 A g^-1during 2400 cycles.In Chapter V,we summarized the batch preparation strategy of metal selenides and their application in lithium/sodium ion batteries and prospected direct synthesis of two-dimension metal selenides with metastable phases.In addition,the application of VSe₂ nanosheets in electrocatalytic sulfur recovery was presented,and the problems and solutions when they were used as negative electrode of potassium ion battery were also discussed.