Preparation And Electrochemical Properties Of The Novel Graphene-Like Ti₃C₂T_x Anode Materials

As a novel two-dimensional metal carbide,pure Ti₃C₂ is likely to be a promising anode material in lithium ion batteries（LIBs）owe to its unique layered structure similar to that of graphite,high conductivity（metallic characteristic）,excellent stability and low barrier（0.07 e V）for Li⁺diffusion.Ti₃C₂ is commonly synthesized by selectively etching the Al element layers with hydrofluoric acid from Ti₃Al C₂.Surface functional groups（T_x）of hydroxyl or fluorine terminate the Ti₃C₂ surface in the exfoliation process.It is usually represented as Ti₃C₂T_x.Unfortunately,the existence of functionalization groups leads to a high diffusion barrier and decreased conductivity.The corresponding theoretical specific capacity of Ti₃C₂,Ti₃C₂F₂ and Ti₃C₂（OH）₂ are 320,130,and 67 m Ah g^-1,respectively.In addition,widespread interlayer stacking is often observed to exist in Ti₃C₂T_x particles,which greatly decreases the bulk electrical conductivity and hinders the accessibility of interlayers to electrolyte ions,limiting its widespread application.Herein,novel Ti₃C₂T_x-based composite materials are designed,and used for the electrode materials of LIBs to study their electrochemical properties,including the following contents:Carbon materials（carbon nanotubes and reduced graphene oxide）bridged,metal oxides（Sn O₂,Mn₃O₄ and Fe₂O₃）coated Ti₃C₂T_x hybrid sandwich-like composites have been designed that fabricated via a simple hydrothermal process.In this 3D carbon materials/metal oxides/Ti₃C₂T_x nanostructure,each component plays unique,indispensable and different roles.Ti₃C₂T_x acts as a conductive framework to enhance the electronic transport properties.Besides,this unique layered structure contributes partially to the enhancement of the total theoretical capacity of the active material.Metal oxides functions as the dominant part of the active material to contribute an extraordinarily high theoretical capacity.Carbon materials can potentially construct a conductivity network and effectively buffer the volume change effect of metal oxides during lithiation/delithiation process.Furthermore,the nanostructure can shorten electron/ion transport distances and construct a highly conductive matrix for the fast transfer of electron and Li⁺.As expected,composites have presented significantly outstanding lithium ion storage capabilities,excellent long-term cyclability and superior rate capability.Spontaneous intercalation of cations from aqueous salt solutions between two-dimensional（2D）Ti₃C₂T_x layers can be be a reality via a simple static precipitation method.Intercalation of cations,including Li⁺and K⁺,can be intercalated,resulting in increases of the c-lattice parameters of Ti₃C₂T_x from 0.971nm to 1.280 and 1.318 nm,respectively.The cations inserted Ti₃C₂T_x shows excellent Li-ion capacity.For an example,the Li⁺@Ti₃C₂T_x anode exhibits high specific capacity of 527.4 m Ah g^-1 at 0.1C,almost 1.4 times as much as that of raw Ti₃C₂T_xanode(369.4 m Ah g^-1).Thus the cations inserted Ti₃C₂T_x anodes can be used as a promising anode material for high performance LIBs.Serious interlayer stacking exists in Ti₃C₂T_x particles,which greatly decreases the electrical conductivity in the bulk and hinders the accessibility of interlayers to electrolyte ions.Herein,conductive carbon nanomaterials（carbon nanotubes,graphene nanowires and carbon nanoballs）grown in the spaces of each stacked Ti₃C₂T_x particle formed conductive bridges between upper and lower Ti₃C₂T_x flakes.Simultaneously,carbon nanomaterials also grew on the outside of each Ti₃C₂T_xparticle,forming a 3D conductive network.In this architecture,carbon nanomaterials could provide effective conductive pathways to effectively facilitate rapid diffusion and transport among Ti₃C₂T_x flakes while still maintaining high electrical conductivity.