Microwave Preparation Of Novel MXene-rGO Composites And Their Electrochemical Properties As Electrode Materials For Lithium-ion Battery

The increasing requirements for the energy storage devices,such as energy density,power density,cycle life,have put forward with the development of industrialization.Lithium-ion battery as one kind of new energy storage device has attracted abundant attention in the application fields of portable electronics,electric vehicles,and distributed energy utilization due to its advantages of high working voltage,continuous long-term charge-discharge,and so on.In recent years,many studies have shown that two-dimensional materials such as reduced graphene oxide(rGO),MXene possess high specific surface area,electrical conductivity and stable layered structure,which exhibit excellent performance as anode materials for lithium-ion batteries.However,the current existing problems,including the complicated process,the high energy consumption,and the low-yield of rGO as well as the restacking two-dimensional sheets caused by strong van der Waals force,result in the poor electrochemical performance and thus seriously restrict their application in lithium-ion batteries.To solve the above problems,this thesis focused on the rapid reduction of GO to obtain high-quality rGO with low energy consumption.The problem of restacking/agglomeration of 2D rGO and MXene nanosheets can be resolved based on the microwave fast reduction of GO and the characteristics of strong microwave absorption for MXene.Effects of the componental ratio and the microwave power on the microstructure are investigated systematically.The microwave reduction reaction mechanism of MXene-rGO composites are also discussed in detail.The optimization of MXene-rGO composite electrode on electrochemical performance was further studied.The main conclusions are as follows:(1)The(TiO₂/MG)composites composed of MXene and rGO with TiO₂nanocrystals dispersed on the surface of nanosheets were successfully prepared by microwave rapid reduction with the MXene as reducing agent and microwave absorbent.The TiO₂/MG-200W,TiO₂/MG-400W,TiO₂/MG-600W,TiO₂/MG-800W and other composite materials were prepared by adjusting the ratio of MXene/GO and the microwave power.The results showed that the strong reductivity of MXene due to unsaturation of titanium valence state led to the pre-reduction and the reduced oxygen content of GO at room temperature,resulting in the improved microwave absorption capacity.Meanwhile,MXene played the role of catalytic reduction of GO to rGO owing to the excellent microwave absorption ability.That can collect abundant energy and heat to raise the local temperature,resulting in the oxidation reaction and of quickly removed oxygen functional groups on the surface of GO.The reaction mechanism of microwave reduction of GO was studied.In the microwave process,GO can be reduced to rGO at the microwave power of only 200 W(much lower than the 1000 W commonly used in the literature)and the reaction time of 10 s with MXene.In addition,it was found that the low reduction degree of GO results in the existance of massive GO in TiO₂/MG-200W at low microwave power.Violent GO oxidation reaction resulted in abundant defects on the surface of rGO and TiO₂ converted by MXene at high microwave power(over 400 W).Therefore,the TiO₂/MG-400W composite material is the optimized,and it possesses high specific surface area(590.1 m² g^-1),stable pore structure,plentiful channels and excellent conductivity,which facilitates the diffusion and migration of lithium ions and provides and effectively improved the energy storage performance.The optimized TiO₂/MG-400W exhibits a high initial specific capacity of289.2 m Ah g^-1 at the current density of 100 m Ah g^-1 and 328.3 m Ah g^-1 after 100 cycles;the initial specific capacity is 253.6 m Ah g^-1 at 1 A g^-1 and 274.8 m Ah g^-1 after 300cycles.The coulomb efficiency of the electrode material is close to 100%.(2)On the basis of the results above,the MXene and rGO composite films(TiO₂/MGF)were rapidly prepared by vacuum filtration,followed by microwave reduction.In the vacuum filtration process,the MXene-GO films were prepared by self-assemby of MXene nanosheets and GO nanosheets under the force of van der Waals force.The resulting films have a thickness of 7.3?m and excellent mechanical strength.Under reduced microwave powers,MXene working as microwave absorber significantly catalyzes the reduction reaction of GO,thus shortens the reaction time.The microwave power has been demonstrated to have significant effects on the microstructure,mechanical properties and electrochemical energy storage properties of the TiO₂/MGF composite films.For example,the volume and thickness of TiO₂/MGF-400W film is greatly expanded,increasing up to 223.7?m.The thickness reaches about30.6 times higher than that of the MXene-GO film.MXene with excellent electrical conductivity located on the two outer surfaces of the film,which effectively improves the stability of the structure.Therefore,TiO₂/MGF-400W film material was used as the anode material directly for lithium-ion batteries(LIBs).The assembled LIBs exhibit specific energy of 35.3 m Ah g^-1 at 5 A g^-1,and specific energy retention remains well after 3,000 charging-discharging cycles.The TiO₂/MGF-400W film with excellent electrical conductivity,high structural stability can be directly used as anode material.This work provides a new idea for the application of all solid state flexible LIBs.