Preparation And Electrochemical Performance Of Bismuth-based Compound Anode Materials

In recent years,the consumption of energy is increasing to meet the rapid economic development,.The depletion of traditional fossil energy sources such as oil,coal and natural gas,has caused serious environmental pollution,and becomes a worldwide problem that cannot be ignored.Therefore,people urgently need to find clean energy to replace the traditional fossil energy.It is urgent to develop advanced energy storage devices to fully store and convert energy generated from clean energy which is intermittent.Thus,a variety of large-scale energy storage devices have been more attractive as an alternative choice.Lithium ion batteries(LIBs)and sodium ion batteries(SIBs)are the most studied energy storage systems but still possess inherent shortcomings.Key scientific problems including the low electrochemical conductivity,the sluggish electrochemical kinetics and the volume change caused by the insertion/desertion process of ions which can result in the pulverization of the anode materials,can seriously affect the electrochemical performance of the LIBs and SIBs.Hence,it is urgent to develop novel electrode materials not only possess higher energy density,but also can improve the cycle stability and safety of the whole battery.There are two principles for designing and preparing high performance electrode material.One is choosing materials possessing higher theoretical specific capacity,another is constructing appropriate electrode structures to enhance stability.In this dissertation,to improve the electrochemical properties of the metal sulfide Bi2S3 with high theoretical capacity used as the anode material for LIBs and SIBs,doping and compositing with carbon material are adopted to improve the conductivity,self-supporting electrode is constructed to reduce the loss of energy density,and core and shell structure is designed to buffer the volume expansion.The specific research contents are as follows:(1)Bi2S3-CoS@C with core-shell structure was synthesized using ZIF-67 as template.The polyhedral structure of the original ZIF-67 was completely maintained,which was not destroyed after the introduction of Bi atoms source and the later sulfuration treatment.The Bi2S3 and CoS in the composites is small in size,which is conducive to the transportation of ion.When applied as the anode materials for LIBs,Bi2S3-CoS@C electrode exhibited excellent electrochemical properties,and displayed a superior specific capacity of 1390 mAh g-1 at a current density of 100 mA g-1 after 100 cycles.In the long cycling test,the extraordinary reversible capacity of 836.2 mAh g-1 can be obtained at a high current density of 1 A g-1 after 300 cycles.When used as the negative electrode material of SIBs,Bi2S3-CoS@C exhibited a high reversible specific capacity and superior cycling performance,delivering a high capacity of 509.2 mAh g-1 at current density of 100 mA g-1 after 100 cycles.It also displayed an excellent long cycling performance,showing a reversible capacity of 250.9 mAh g-1 after 300 cycles at a high current density of 1 A g-1.The reason why Bi2S3-CoS@C shows excellent electrochemical properties is due to the combination with carbon material,which effectively improved the overall conductivity of the composite and significantly enhanced the electrochemical kinetics.Moreover,carbon layer can be used as the buffer layer to release the mechanical stress caused by the volume expansion of the active material;The small size of Bi2S3 and CoS is beneficial to shorten the ion transport distance.The space in the core-shell structure and between the active materials is beneficial to buffer the volume change caused by the de/insertion of ions and improve the cycling stability of the battery.(2)Taking advantages of nano-sized,heteroatom doping and self-supporting.Se-doped Bi2S3/3D carbon foam(Bi2S3-xSex/CF)is designed and synthesized.The CF obtained by carbonizing commercial melamine foam at high temperature,can directly be acted as the skeleton material of self-support electrode because of the high conductivity and elasticity.Self-supporting Bi2S3-xSex/CF is prepared through hydrothermal-heating three-step treatment.In this composite,the morphology of prepared Bi2S3 is nanoneedles and uniformly coated on the surface of CF.Electrochemcal tests further certify that Se-doping can improve the electrochemical performance of batteries.At a current density of 100 mA g-1 the Bi2S3-xSex/CF delivers a high specific capacity of 441.8 mAh g-1 after 100 cycles and the coulombic efficiency approximates to 100%.When the current density returns back to 50 mA g-1,the special capacity will retrieve to 609.4 mAh g-1.For the electrode of Bi2S3/CF,it only retains a specific capacity of 230 mAh g-1 at 100th.Used as the anode material,Bi2S3-xSex/CF exhibits an outstanding electrochemical performance,which is attributed to the special nanoneedle-like structure of Bi2S3 and the introduction of Se elements.nanoneedle-like structure shorten the ion transfer distance,reduces the mechanical stress.At the same time,compared with S element,Se has the similar chemical properties and a relatively high electronic conductivity.The introduction of Se is benefit to improve the electronic conductivity and forms Se-containing inactive materials during cycling to buffer the mechanical stress,reduce the pulverization and restrain agglomeration.