Research On The Construction And Electrochemical Performance Of Graphite/Graphene Anode Compound Materials

Lithium-ion batteries have been widely used in many fileds such as portable electronic devices due to their large energy density and long life. Graphite is a anode material for commercial lithium ion batteries in currently. And it has the advantages of abundant resources, low cost and stable performance. Although the role of graphite structure has been supported by the observation that graphites with a higher proportion exhibit a higher reversible capacity and are less vulnerable to exfoliation by co-intercalation of solvents, so it’s concerned by the scientists for a long time. Graphite anode material cannot meet the increasing requirement of electronic devices and electric vehicles with high performance due to the unsatisfactory performance of their specific capacity, cycle stability and rate capability. Therefore, it is significant to further improving the electrochemical performance of graphite anode material.Graphene quantum dots is a new type of carbon materials with the size of 1-2 nm. The introduction of graphite quantum dots can reduce the formation of the solid electrolyte interface film（SEI film） on the surface of graphite and anisotropy of the intercalation of lithium ions. At first, graphene quantum dot was prepared by the pyrolysis of critic acid and cysteine. Then, it was composite with graphite negative electrode material. At last, it was hybridized with graphite and heated at 400℃ in inert atmosphere for 5 h to produce graphene quantum dots-graphite（GQDs-G） composites. We used scanning electron microscope（SEM）, transmission electron microscopy（TEM） and X-ray diffraction（XRD） to analysis the structure and morphology of the materials. These characterization reveal that graphene quantum dots with size of only 1-2 nm were well dispersed on the surface of micron graphite. Although the hybridization can not influence on the crystal structure of graphite, the addition of graphene quantum dots make the surface of graphite particles become more smooth, which significantly enhance the intensity of diffraction peak. Therefore, the introduction of graphene quantum dots significantly improve the conductivity of the composites, and the electrochemical response performance（CV performance） of the composites were increased finally. Electrochemical impedance（EIS） analysis that the resistance of graphene quantum dots-graphite composites were smaller than graphite. This result proved that the introduction of graphene quantum dots can improve the conductivity of graphite, which consistent with the result of CV performance. The reversible capacity of graphene quantum dots-graphite composites reach to 455 m A h g-1. The capacity still remain 368 m A h g-1 after 100 cycles.The addition of N can increase the polarity of graphene, it is beneficial to improve the affinity of electrolyte and electrode materials. At the same time, the conductivity and flexibility of graphene sheets can increase the high-rates performance and structural stability of graphite anode material. Graphite oxide（GO） and urea were dispersed in ultrapure water, then graphite oxide was partly reduced by ascorbic acid. Followed by adding in graphite under vigorous stirring, and then we produced graphene oxide/graphite hydrogel by hydrothermal treatment. The hydrogel was dried and finally annealed in Ar/H2 to obtain N-doped graphene/graphite（N-doped G/C） composites. The result shows that all of graphite particles were dispersed in three-dimensional graphene framework with a rich of open pores. The open pore accelerates the electrolyte transport. Therefore, the graphene framework works as a conductive agent and graphite particle connector and improves the electron transfer. The result indicates that electrical conductivity of the composites reach to 5912 S m-1, which is higher than that of the pristine graphite（4018 S m-1） obviously. The discharge capacity of N-doped G/C composites reach to 781 m A h g-1 at 0.1 C, beyond the theoretical capacity of graphite electrode（372 m A h g-1） of 2 times, which reveals a high-rates performance.Li₄Ti₅O₁₂ is a zero strain material, it has a high discharge voltage platform（1.55 V） and an excellent cycle stability, which can not form SEI film. But it’s great resistance will lead to the low rate performance. With the combination of the advantages of graphene quantum dots, graphene and Li₄Ti₅O₁₂, the performance of graphite can be greatly improved. Firstly, graphene oxide/graphite composites were mixed with titanium tetra-butyl, and then they were dispersed in tert-butanol by ultrasonic. Followed by spotting with ethanol-water mixed solution of lithium acetate dehydrate and graphene quantum dots, to obtain graphene quantum dot-Li₄Ti₅O₁₂ coated graphene oxide/graphite precursor. The precursor were finally annealed in Ar/H2（95:5） to obtain graphene quantum dot-Li₄Ti₅O₁₂/graphene/graphite（GQDs-LTO/G/C） composites. The results reveal that, Li₄Ti₅O₁₂ was coated on the surface of graphite, and graphene quantum dot was dispersed in the outside and inside of Li₄Ti₅O₁₂ crystals. XRD represents that the added of graphene and Li₄Ti₅O₁₂ increase the degree of crystallinity of GQDs-LTO/G/C composites. Raman spectra shows that the added of Li₄Ti₅O₁₂ increasing the degree of disorder of GQDs-LTO/G/C composites, but the existence of graphene quantum dots can reduce this effect. Electrochemical performance reveals that the introduction of Li₄Ti₅O₁₂ and graphene quantum dots decreased the plateau voltage of the composites. Therefore, the rate performance curve shows its good performance at high discharge/charge current, this is the advantages of the introduction of graphene quantum dots. The capacity still remains 313-318 m A h g-1 after 100 cycles, which represents the good cycle performance of composites. In summary, the introduction of graphene quantum dots, graphene and Li₄Ti₅O₁₂ have been providing a good electrochemical performance to lithium ion battery, including high specific capacity, significant rate capability and good cycle performance.