Synthesis And Potassium Storage Performanc Of Bismuth-based Composites

In recent years,the disadvantages of lithium-ion batteries have become apparent with additional applications in batteries and energy storage systems,such as their low abundance and uneven distribution in the earth's crust.As an element in the same main group as lithium,potassium has the same physical and chemical properties as lithium and can react with graphite to form potassium-graphite intercalation compounds.However,the ion radius of potassium ions is large and the requirements for the anode material are more demanding,so the development of anode materials with high capacity and good cycle stability plays a very important role.As the anode material of potassium ion battery,bismuth metal not only has a high theoretical specific capacity,but also has a higher reaction potential during the discharge process,which is not easy to precipitate potassium and has higher safety.In this thesis,we mainly focus on the problem that bismuth metal has a large volume change during charging and discharging,thus causing rapid decay of electrochemical performance,to obtain bismuth-based composites with high capacity,long lifetime and high safety by designing bismuth-based compounds,carbon-coated bismuth nanoparticles and composition with other metals from three perspectives.The main research work is as follows:(1)A three-dimensional porous structure of Bi₂O₂CO₃@NC composite material with carbon material cross-linked bismuth subcarbonate nanosheets was designed and synthesized by ball milling and high temperature solid phase method.Upon analysis,the Bi₂O₂CO₃@NC composite material has the following structural advantages:large specific surface area;graded porous structure;robust carbon structure;N-doped carbon can improve the defect concentration of the material.As an anode material for potassium ion batteries,the Bi₂O₂CO₃@NC electrode exhibited relatively excellent electrochemical properties.At a current density of 100 m A g^-1,the reversible specific capacity is 254 m Ah g^-1 after 65 cycles,and the Coulomb Efficiency is 98%.(2)A sponge structure Bi@NC composite with Bi nanoparticles embedded in carbon material was synthesized by a blast drying combined with high temperature solid phase method,and the effect of calcination temperature on the morphological structure and electrochemical properties.Bi@NC composites was investigated.The results show that the Bi@NC-700 composite has the following structural advantages:moderately large bismuth nanoparticles,stable carbon network structure,large specific surface area and graded porous structure,nitrogen atom-doped carbon structure,etc.The Bi@NC-700 composite,benefiting from these structural advantages,exhibits very excellent potassium storage capacity with a reversible specific capacity of 291.1 m Ah g^-1after100 cycles at a current density of 100 m A g^-1 and a capacity retention rate of 88.6%.The reversible specific capacity is 221.3 m Ah g^-1 after 300 cycles at a high current density of 1000 m A g^-1.(3)A graphene-like Bi/Sn@NC composite was prepared by a freeze drying combined with high temperature solid phase method,and the effects of bismuth,tin and the synergistic effect between them on the carbon material were investigated.The Bi/Sn@NC composite has the following structural advantages:Bi/Sn particles are coated with appropriate wall thickness;the synergistic effect of Bi and Sn is beneficial to alleviate the volume expansion of Bi during charging and discharging.As anode material for potassium ion batteries,the Bi/Sn@NC composites exhibit excellent potassium storage capability.It has a reversible specific capacity of 300.7 m Ah g^-1 after100 cycles at a current density of 100 m A g^-1.At a large current density of 500 m A g^-1,the reversible specific capacity is 182.1 m Ah g^-1 after 500 cycles.