The Research On The Energy Storage And Synthesis Of Carbon Composites Based On Phase Separation

Despite the rapid development of the automotive industry,such as electric and hybrid vehicles,the auto companies are facing a huge challenge,which finds a more suitable power system to meet people's needs.Considering that lithium ion batteries?LIBs?have occupied the main position in the portable electronic products in the market,researchers are ready to turn the use of lithium ion batteries from portable electronic devices to power batteries.With several years of research,LIBs have very large market share and have developed well blowout.However,the data on power density and energy density of LIBs still needs further improvement,so that we can meet better the requirements of electric vehicle battery.And the development of any science and technology will be accompanied by contradictions and problems,including the higher demand for energy density and power density and the rapid rise of lithium resources,which will bring more challenges to the development of LIBs.In order to solve the above problems,researchers have worked from different angles in recent years,such as finding suitable materials to replace lithium.And sodium/potassium metal is a good choice,because of its low price,similar chemical properties,rich resources and friendly environment.Therefore,we can build room temperature sodium/potassium ion batteries with a very similar energy storage mechanism to LIBs,which are expected to replace LIBs for the next generation of power source and power supply for large-scale power plants.In view of the problems mentioned above,this paper makes a thorough inquiry into the structure design and synthesis methods of electrode materials.The main contents are as follows:?1?In order to solve volume expansion of MoO₂ in the process of charging/discharging,in chapter 2,we have prepared MoO₂@C core shell nanofibers by a simple electrospinning.In this paper,we discussed the formation mechanism of core shell nanofibers from the choice of different temperatures,explored the different concentrations of AMM,and then analysed optical micrographs of precursor solution.We summed up that the phase separation of AMM and PVA is the key factor to format core shell nanofibers,when we increase the temperature,the core shell structure phenomenon will be more obvious.Finally,we studied the ability of MoO₂@C core shell nanofibers as anodes to store Li⁺.It was found that MoO₂@C core shell nanofibers obtained high reversible capacity of 665 mA h g^-1at a current density of0.5 A g^-1after 600 cycles,and the coulombic efficiency remained at 98%.Even at the current density of 1 A g^-1,the capacity of the electrode can still maintain 537 mA h g^-1after 600 cycles.This work provides a new idea for the preparation of core shell nanofibers,and applies it to energy storage.?2?Based on metal oxide storage mechanism,MoO₃ has a higher theory capacity than MoO₂,so in chapter 3 we successfully prepared the porous h-MoO₃@C nanofibers with a high specific surface area of 400.2 m²g through hot nitric acid oxidating MoO₂@C nanofibers and the carbon layer no obvious damage.As anodes for LIBs compared with MoO₂@C nanofibers,the porous h-MoO₃@C nanofiber electrode shows better performance,which showed a reversible capacity of 302.9 mA h g^-1in the current density of 2 A g^-1after 500 cycles.As anodes for sodium ion batteries?SIBs?,the h-MoO₃@C electrodes also have good rate performances.At a current density of 2 A g^-1at 1000^thcycle the electrode can remaintain a capacity of108.9 mA h g^-1.Even at current density of 5 A g^-1after 1200 cycles the electerode still has a capacity of 91 mA h g^-1.The carbon layer can maintain structural integrity and improve conductivity;at the same time the h-MoO₃ tunnel structure can be used as electron hole separation and Li⁺/Na⁺insertion and extraction provide specific more locations,which makes the composite nanowires as electrode materials show better performance.This work provides a basis for the application of high price transition metal oxide/carbon composites in the field of battery and catalyst.?3?Further studies have found that we can obtain hollow porous carbon nanotubes by completely etching MoO₂ in the MoO₂@C nanowires.So in chapter 4,we first get the core shell structure of MoO₂@C nanofibers through a single electrospinning,and then we can get high specific surface porous carbon nanotubes after hot HNO₃ treatment.As anodes for sodium ion batteries,the porous carbon nanotube electrodes exhibit excellent cycling and rate capacity.Under the current density of 0.05 A g^-1,the porous carbon nanotube electrode can obtain the capacity of503 mA h g^-1.When increased to 1 and 5 A g^-1,the porous carbon nanotube electrodes can have 131.9 mA h g^-1?after 1000 cycles?and 110 mA h g^-1?after 1200 cycles?.This work can provide a simple and broad way to prepare porous carbon nanotubes and their compounds for batteries,catalysis and other applications.?4?Based on the previous works,we try to use other ways to prepare carbon nanomaterials.So in chapter 5,we first prepare red cell-like Mo₂C@C using spray drying,and then use hot dilute nitric acid to etch Mo₂C to get porous red cell-like carbon spheres.At the same time,we also regulate the content of oxygen-containing functional groups on the porous red cell like carbon spheres by heat treatment.It is found that the properties of the electrodes are not only affected by their specific surface area and porosity,but also the larger the amount of oxygen-containing functional groups is,the better the performance is.As the negative electrode of potassium ion battery,under the current density of 0.1 A g^-1,the porous red cell like carbon spheres can have the first discharge specific capacity of 1213 mA h g^-1,and then maintain 366 mA h g-1 after 50 cycles.The specific capacity of 245 mA h g^-1can be maintained after the 300 cycle of the current density of 0.5 A.Even the specific capacity of the current density at 1 and 2 A g^-1is 147 and 86 mA h g^-1,respectively.This work can provide us with an idea that carbon nanomaterials modified by oxygen-containing functionalities may be extended to other fields.