Study On Lithium Storage Properties Of Pyrolysised MnO And Ni/Polypyrrole Composite

Lithium-ion battery with a large energy density,working voltage,considerable capacity and many other advantages,has become the research focus in various research fields.In recent years,the main cathode material of lithium-ion batteries are graphite,but its theoretical specific capacity is relatively low(372 m Ah/g)and easy to produce lithium dendrites.How to improve the performance of cathod material is still a problem to be addressed.The transition metal and its oxides have attracted great attention,because of its high theoretical specific capacity,abundant resources and environmental friendliness,etc.However,this kind of material still has many defects,such as poor conductivity and rapid capacity fading resulting from large volume expansion and easy particle aggregation.At present,the methods to improve the material include preparing nano-materials and composites.In this paper,the PPy nanotubes were first synthesized by soft template method.Secondly,the transition metal and its oxides were deposited on the nanotube skeleton by microwave synthesis.Finally,the composites of polypyrrole carbon nanotubes(PPy NTs)with manganese monoxide(p-Mn O@PPy NTs)or metallic nickel(p-Ni@PPy NTs)were obtained by pyrolysis.The micro-structureand compositions of composites were studied by SEM,TEM,XRD,TGA,IR,LAND test system and electrochemical workstation.The main contents and conclusions were as follows:(1)PPy NTs were prepared by soft template method using methyl orange as template,Fe Cl3 as oxidant in 0.1 M pyrrole aqueous solution.The p-PPy NTs active material was obtained by pyrolysis at 700°C for 2h under argon atmosphere.The p-PPy NTs showed amorphous and two-dimensional tubular structure.The specific capacity of p-PPy NTs was less than that of graphite.(2)PPy NTs was mixed with manganese chloride monohydrate.Then diethyenetriamine was added to the mixture.The formation reaction of Mn O wascarried out at 240°C for 10 min.The reaction product was pyrolyzed at 700°C for 2 h under argon atmosphere to obtain p-Mn O@PPy NTs.When the mass ratio of PPy NTs to manganese chloride was 1:20 and the amount of diethylene triamine was 0.2 m L,the p-Mn O@PPy NTs composite exhibited the best lithium storage performance.At the current density of 200 m A/g,the initial discharge capacity reached 1322.3 m Ah/g,and the initial coulomb efficiency reached63.6%.After 200 galvanostatic charging/discharging cycles,the specific capacity still retained 657.4 m Ah/g.At the current density of 500 m A/g,the initial discharge capacity reached 1315.6 m Ah/g,and the initial coulomb efficiency reached 63.7%.After 200 galvanostatic charging/discharging cycles,the specific capacity still retained 587.6 m Ah/g.During charging/discharging course,the PPy NTs not only promote electrolyte penetration,electron transfer and ion diffusion,but also soften impact of the volume change of Mn O,which greatly improved Li+-storage performance of Mn O.(3)Nickel nitrate hexahydrate was mixed with PPy NTs.The formation reactionof Ni metal was carried out at 150°C for 10 min.The reaction product was pyrolyzed at 700°C for 2h under argon atmosphere to obtain p-Ni@PPy NTs.When the mass ratio of nickel nitrate hexahydrate to PPy NTs was 5:1,the p-Ni@PPy NTs composite exhibited the best Li+-storage performance.At the current density of 500 m A/g,the initial discharge capacity reached 1170.2m Ah/g.After 340 galvanostatic charging/discharging cycles,the capacity remained 659.2m Ah/g.At the current density of 2000 m A/g,the initial discharge capacity reached 1115.6 m Ah/g.After 650 galvanostatic charging/discharging cycles,the capacity remained at 614.5 m Ah/g.At the current density of 8000 m A/g,the initial discharge capacity reached 755.1 m Ah/g.After 400 galvanostatic charging/discharging cycles,the specific capacity still retained 433 m Ah/g.Metal Ni on the surface of PPy NTs promoted the electron transfer in active material.On the other hand,the PPy NTs provided the channel for electrolyte penetration and ion diffusion.Therefore,p-Ni@PPy NTs showed excellent rate performance.