Building Nanostructural Electrode Materials With MXene For Li-ion Batteries

Comparing with traditional secondary batteries,LIBs have obvious advantages,such as high voltage?³.9 V?,high energy density?²00 Wh/kg?,high power density,wide working temperature range?-40⁷0 ^oC?and steady discharge,therefore LIBs have become the most widely used energy storage technology at present.In order to further meet the demands of the social development,it is still the main trend to develop advanced LIBs with high energy and power density.Electrode materials are the important parts of the LIBs,which determine the electrochemical performance of the devices directly.And the layered materials are very popular in anode materials due to their excellent lithium storage performance.Recently,MXene,a novel 2D layered material,attracts the attention of the researchers owing to its extremely high electronic conductivity and volumetric capacity.Herein,this thesis focuses on the Ti₃C₂T_X,which is the first reported MXene material.Aiming to prevent the dense stacking of Ti₃C₂T_X sheets in the film electrode,the 3D porous structure is constructed in the Ti₃C₂-rGO hybrid film.And benefiting from its synergistic effect of the fast electronic and ionic conductivity,the hybrid film displays excellent electrochemical performance.On the other hand,it is easy for lithium ions to intercalate into the layered Ti₃C₂T_X materials and the intercalation lithium ions can react with Ti₃C₂T_X sheets in situ,forming layered lithium titanate(Li₄Ti₅O₁₂/C,LTO/C)hybrid materials.As the anode of LIBs,it shows good electrochemical performances.The detailed research contents and conclusions are as follows:?1?MXene sheets own the graphene-like morphology and tend to stack densely in the film electrode,which seriously hinders the infiltration of the electrolyte,resulting in poor electrochemical performance.To solve this issue,we introduce graphene oxide?GO?to Ti₃C₂T_X solution and construct a 3D porous structure in the Ti₃C₂-GO hybrid film through facile electrolyte-induced self-assembly method.Then the hybrid film is subjected to hydrazine vapor and annealed at 200 ^oC for 2 h under Ar atmosphere,aiming to reduce the GO to rGO and form 3D porous Ti₃C₂-rGO film.By adjusting the mass ratio of Ti₃C₂T_X to GO in the hybrid film,it displays the best electrochemical performance when the mass ratio(M_Ti3C2/M_GO)is 1:1.And it exhibits high specific capacity of 98.9 mA h g^-1 and good rate capability of 30%capacitance retention at 4 A g^-1.Additionally,the film electrode displays excellent cycling stability without capacity decay after 1000 cycles under high rates(1 A g^-1).Furthermore,the electrochemical analysis also demonstrates that the addition of GO transforms the densely stacked Ti₃C₂T_X sheets to a 3D porous structure,which effectively facilitates the diffusion of electrolyte ions in the film electrode,whereas the Ti₃C₂T_X sheets endow the hybrid film with high electrical conductivity.Because of the synergistic effect of the fast electronic and ionic conductivity,the hybrid film electrode exhibits outstanding electrochemical performance in LIBs.?2?MXene?Ti₃C₂T_X?is prepared by etching the main group element?A-layer?of MAX?Ti₃AlC₂?using HF solution.Due to the wide interlayer spacing of Ti₃C₂T_X,it is easy for alkali metal ions?K⁺,Na⁺,Li⁺?to intercalate into the layered materials.Therefore the intercalation agents can react with Ti₃C₂T_X sheets in situ and the cleavage Ti₃C₂T_X flakes work as the sacrificial template to make the forming hybrid materials with layered structures.In this thesis,Li⁺ions are intercalated into the Ti₃C₂T_X through liquid phase intercalation method.Then the Ti₃C₂T_X@Li⁺aqueous dispersion is sealed in a cylindrical vessel and maintained at 150 ^oC for 12 h,the layered precursor Li_1.81H_0.19Ti₂O₅·nH₂O is prepared.Finally,the layered Li₄Ti₅O₁₂/C hybrid material is fabricated by annealing the precursor at 700 ^oC for 6 h in Ar atmosphere.When the LTO/C is employed as electrode materials for lithium-ion batteries?LIBs?,it exhibits good electrochemical performance.When the current density is increased from 1 C(175 mA g^-1)to 30 C,its specific capacity decreases from 152 mA h g^-1 to 70 mA h g^-1 with good rate capability of 46%capacitance retention.Additionally,the hybrid material also displays excellent cycling stability with high capacity retention?83.6%?after 700 cycles under high rates?10 C?owing to its stable structure.