Study On Electronic Effects Of Grafting Groups/Type Of Oxygen-Containing Functional Groups On Lithium Storage Properties Of Graphene

Negative electrode material is an important component of lithium-ion batteries,which directly affects the performance of lithium-ion batteries,such as energy density,cycling performance,rate capability,and low-temperature discharge.As a new type of negative electrode material,graphene has received much attention due to its excellent conductivity,high specific surface area,and outstanding mechanical properties,and is considered a promising negative electrode material.However,in practical applications,graphene cathode materials still suffer from low lithium storage capacity and short cycling life.The functionalization of graphene is an effective way to improve its lithium storage properties.However,it has also been found that lithium storage performance of graphene varied greatly when graphene framework is decorated with different organic groups,such as electron-withdrawing groups,electron-donating groups and oxygen-containing types.This dissertation will focus on the effect of functional groups grafted on the surface of graphene on the lithium storage properties.The research contents are as follows:（1）How the electronic effects of graphene surface groups affect its lithium storage properties.To accurately prepared functionalized graphene materials with a single,well-defined functional group,the"negatively charged graphene"intermediate was firstly prepared by reduction and intercalation of potassium metal towards natural graphite.n-Butyl/4-methoxyphenyl-functionalized graphene（Bu-G^R/4-Me OPh-G^R）with electron-donating functional groups,bromine/trifluoroacetyl-functionalized graphene（Br-G^R/TFAc-G^R）with electron-withdrawing functional groups,and unfunctionalised graphene control sample（G^R）were prepared by the reaction between"negatively charged graphene"intermediate and electrophilic reagents（such as n-butyl bromide,4-methoxobenzene diazotetrafluoroborate,liquid bromine and trifluoroacetic anhydride）.X-ray photoelectron spectroscopy（XPS）and Fourier transform infrared spectroscopy（FT-IR）showed that the functional groups were successfully covalently grafted onto the graphene surface.Elemental analysis（EA）and thermal gravimetric analysis（TGA）indicated that Br-G^R,TFAc-G^R,Bu-G^R,and 4-Me OPh-G^R had similar degrees of functionalization.Raman spectroscopy and Field emission transmission electron microscope tests showed that the prepared functionalized graphene had a relatively complete lattice structure.Finally,the prepared functionalized graphene materials were assembled into lithium-ion batteries for electrochemical performance testing.The results showed that after functionalization of the graphene material,the rate capability and cycling stability of the graphene material were significantly improved.Compared with the unmodified graphene material,the graphene electrode material modified with electron-donating functional groups increased the electronic density of the graphene skeleton,improved the affinity with cations,and was conducive to Li⁺storage,thus having a higher lithium storage capacity.The graphene electrode material modified with electron-withdrawing functional groups reduced the electronic density of the graphene skeleton,which was unfavorable to Li⁺storage,and thus having a lower capacity.At 0.5 C,the specific capacity of Bu-G^R was 512 m Ah g^-1,while that of Br-G^R was only 86 m Ah g^-1.（2）How oxygen-containing groups on the surface of graphene affect its lithium storage properties.It is shown that retaining some oxygen-containing groups on reduced graphene oxide can significantly improve its lithium storage performance.However,there is currently no consensus on how different types of oxygen functional groups affect the lithium storage performance of graphene materials.In this part,three graphene materials with different types of oxygen-containing functional groups—aldehyde-functionalized graphene,hydroxymethyl-functionalized graphene and carboxy-functionalized graphene,namely CHO-G^R,CH₂OH-G^R and COOH-G^R,were prepared using the above reduction functionalization strategy.XPS,FT-IR,EA,and Raman showed that the functional groups were successfully covalently grafted onto the graphene surface and they were similarly functionalized with an intact lattice structure.X-ray diffraction and transmission electron microscope indicated that the introduction of oxygen functional groups does not significantly affect the crystal structure and morphology of graphene.Finally,the prepared functionalized graphene materials were assembled into lithium-ion batteries for electrochemical performance testing.The electrochemical test results showed that the carboxyl functional group of the COOH-G^R electrode material can provide additional lithium storage active sites,which was conducive to lithium ion adsorption and promoted lithium ion embedding,and therefore had the best lithium storage performance.At 0.1 C,the specific capacity of COOH-G^Rwas 400.7 m Ah g^-1,and after 160 charge and discharge cycles,the specific capacity can still be maintained at 336.8 mAh g^-1.