Design And Synthesis Of VA-based Nanostructured Electrode Materials For Lithium Storage

The excessive consumption of fossil fuels and the resulted environment problems have forced us to look for green and clean energy,such as wind power,solar power,hydrogen power,nuclear power,tidal power,etc.However,the uneven distribution and intermittency of green and clean energy have seriously hindered their practical application.The most effective way is to use energy storage system to store these energy sources for reuse.Lithium ion batteries?LIBs?have gradually become the major energy storage system and be widely used in portable devices and electric vehicles due to its advantages of high open-circuit voltage,long cycle life,large energy density and no memory effect.However,graphite,as the anode materials for commercial LIBs,cannot meet the market demand for large power batteries and energy storage batteries due to its low theoretical capacity(372 mAh g^-1).Therefore,it's urgent for us to explore new anode materials for LIBs.Among these materials,VA group elements?Sb,Bi,etc.?have attracted tremendous attention and show great potential as electrode materials for LIBs due to their relatively low prices,high theoretical specific capacity,high energy density,and suitable lithium deposition potential.However,the VA group element could cause serious volume expansion during charge and discharge process,resulting in poor cycle stability,which also severely limits the application of VA group materials in commercial LIBs.Recently,great effort have been made to improve the electrochemical performance of the VA group element and have achieved many fruitful results.Based on the review on the progress of anode materials for LIBs,we conducted an in-depth study on VA group anode materials.By reasonable structural design of two typical elements of VA group anode materials?Sb and Bi?,we successfully synthesized Sb/N-CM nanorod and BiPO4@void@C/CNT nanorod and analyzed the lithium storage mechanism.With the help of the carbon material with high electronical conductivity to alleviate the volumme expansion,the main problem of the VA group anode material was solved,which also provided theoretical foundation for the practical application of VA group anode materials.The specific research mainly includes the following two parts:?1?The synthesis of Sb@N-CM nanorods and their lithium storage mechanism study.A novel nanorod assembled by Sb nanoparticles encapsulated into N-doped carbon matrix with coating thin carbon layer?Sb@N-CM nanorods?are synthesized using a cation exchange reaction combined with a novel confined route.The Sb@N-CM nanorods not only improve the structural stability,stable SEI layer and high transport of Li⁺,but also suppress SEI layer formation on surface of each Sb nanoparticles due to its advanced structural merits,such as coating N-dopping conductive carbon layer and forming void space by conductive carbon matrix,which can greatly increase electrochemical performances.Besides,In situ transmission electron microscopy?TEM?observations verify that Sb nanoparticles after delithiation are formed networked porous structure to prompt utilization of active materials and the closed connection between Sb nanoparticles and N-doped carbon matrix for the first time,which can deeply improve ion/electron transfer kinetics.As a result,the Sb@N-CM nanorods exhibits a high reversible capacity(673.4 mAh g^-1 at 100 mA g^-1),ultrahigh cycling stability?99.7%capacity retention over 500 cycles?,and excellent rate capability,which to our knowledge shows the best cycling stability and capacity reported to date among all reported Sb based materials.?2?The synthesis of BiPO₄@void@C/CNT nanoparticles and their lithium storage mechanism study.BiPO₄@void@C/CNT nanoparticles was prepared using a template by corrosion and calcination method.The composite has a uniform structure with BiPO₄ conversion into Bi nanoparticles embedded within the Li₃PO₄ matrix by the first discharge process inside C/CNT hollowed structure.The Li₃PO₄ matrix not only can serve as a buffer layer to keep the structural integrity well-maintained during insertion/extraction processes,but also provides an electrolyte-blocking layer that limits the formation of SEI layer on the surface of most of the Bi nanoparticles.Moreover,the CNTs on the surface of BiPO₄@void@C offer an interconnected electron transportation pathway,as well as effective aggregation and separation suppressing among active material was blocked.In addition,the hollow porous structure of provides sufficient free void for the expansion Bi/Li₃PO₄ matrix nanorods.As anodes for LIBs,the BiPO₄@void@C/CNT after rating performance testing delivers a high capacity of³47.0 mAh g^-1 at a high current density of 1000 mA g^-1 and the held until 600 cycles without capacity loss.