Preparation And Modification Of Porous Carbon Microsphere-Sulfur Cathode Materials

With the increasing demand for energy,traditional lithium-ion batteries limited by their own theoretical energy density(420 Wh kg^-1)have been unable to meet the market requirement.In particular,the development of new energy electric vehicles requires a safe,environmentally friendly and high-energy density secondary energy storage system.Lithium-sulfur batteries have a theoretical energy density of 2600 Wh kg^-1,and are very environment-friendly,which could be a candidate to crack the range anxiety of electric cars.Although Li-S batteries have been proposed since 1962,its application is still not mature,so it cannot be put into commercial mass production and use.Mainly because that Li-S batteries still have many problems,including the insulation of active material and reaction products LiS/Li₂S₂;Severe volume expansion during charging and discharging;the shuttle of intermediate polysulfide and self-discharge phenomenon of the batteries etc.Hydrothermal method was used to prepare carbon microspheres using cheap glucose as the raw material.Three kinds of carbon microspheres with different particle sizes were prepared by controlling different hydrothermal temperature and time.After being activated by KOH,these carbon microspheres were used as the matrix to load with sulfur.And the effects of porous carbon microspheres with different particle sizes,specific surface area and pore volume on the electrochemical performance of lithium-sulfur batteries were investigated.The average particle sizes of hydrothermal carbon microspheres were 674 nm,364 nm and 130 nm respectively.The specific surface areas of porous carbon microspheres correspondingly are1633.95 m² g^-1,431.98 m² g^-1 and 431.5 m² g^-1 respectively after activation.Pore volumes are 2.291 cm³ g^-1,0.655 cm³ g^-11 and 0.608 cm³ g^-1 respectively.The increase of specific surface area and pore volume is helpful to improve the electrochemical performance.At 0.5 C,the coulomb efficiencies of the first cycle are respectively95.42%,93.97%and 92.58%.The initial discharge specific capacity of the battery with the largest specific surface area and pore volume of carbon microspheres in the electrode is 713.6 mAh g^-1,and the capacity retention is 68.7%after 200 cycles.However,compared with Ketjen-black and carbon nanotubes,which are highly conductive carbon,the above porous activated carbon microspheres have relatively poor electrical conductivity.Moreover,when porous carbon microspheres loaded with sulfur,some sulfur does not enter the pores of the carbon microsphere but on the surface,which is not conducive to the adsorption of polysulfide.In order to improve the conductivity of the cathode and the adsorption of polysulfide,multilayer graphene and carbon nanotubes were added to the porous activated carbon microspheres.Multilayer graphene accounts for 10%of the total mass of the cathode material,while carbon nanotubes are used as conductive additives.Multilayer graphene has a sheet structure,which can effectively consolidate sulfur and inhibit the shuttle of polysulfide.While carbon nanotubes can form a three-dimensional network,providing ion electron transport paths for the cathode and greatly improving the electrical conductivity of the cathode.When the content of sulfur is 2.35 mg cm^-2,the initial discharge specific capacity of the battery with multilayer graphene and carbon nanotubes is 605.1 mAh g^-1 at 0.5 C and the discharge specific capacity was 507.1mAh g^-1 after 200 cycles.The capacity retention rate was 83.8%.When the sulfur content is 3.2 mg cm^-2,the battery maintains a capacity of 91.1%after 100 cycles at0.3 C.Compared with batteries without multilayer graphene and carbon nanotubes in the electrode,the capacity retention is significantly improved.These also indicate that the use of multilayer graphene and carbon nanotubes have good modification on porous activated carbon microspheres/sulfur cathode.