Low-Temperature Assembly Of 3D MXene Aerogel For Electrochemical Sodium-Ion Storage

The energy crisis has increased the demand for new energy storage systems as well as new materials,and developing devices like sodium ion batteries and supercapacitors is critical.Transition metal carbide/nitride（MXene）is a type of two-dimensional（2D）materials with excellent electrical conductivity,high specific surface area,and rich surface functional groups.It has a lot of potential in the field of electrochemical energy storage.The intrinsic features of MXene in certain applications may be utilized to a larger degree by constructing 2D MXene into three-dimensional aerogels with robust mechanical strength and porous structure.In this study,on the basis of a reduced graphene oxide（RGO）backbone with appropriate mechanical strength formed during the preparation process,we introduced interfacial modulations to enable 3D assembly of MXene at low temperatures,which overcomes the oxidation issue of MXene during the assembly process.The synthesized 3D MXene/RGOcomposite aerogel materials possess excellent area specific capacity and cycling performance.The details are outlined below.（1）Ti₃C₂T_x/RGO composite hydrogel was synthesized at 35°C by a GO-assisted assembly with L-cysteine as the reducing and cross-linking agent,and Ti₃C₂T_x/RGO composite aerogel was prepared via freeze-drying and heat treatment before being used for electrochemical sodium-ion storage.L-cysteine overcomes the electrostatic repulsion between the Ti₃C₂T_x and GO nanosheet layers,allowing for GO reductive cross-linking at low temperatures.When utilized as a free-standing sodium-ion storage anode material,the Ti₃C₂T_x/RGO composite aerogel preserved both high structural stability of RGO and the electrochemical property of Ti₃C₂T_x,and exhibited a reversible capacity of 172 m Ah g^-1 at0.1 A g^-1.（2）At room temperature,Ti₃C₂T_x/RGO composite aerogels were constructed using cross-linking agents(amino-propyltriethoxysilane,Mn²⁺,Fe²⁺,Zn²⁺,and Co²⁺)as interfacial mediators and ascorbic acid as a reducing agent.When the Ti₃C₂T_x ratio reached87 wt.%,the interfacial mediators significantly reduced the electrostatic repulsion between Ti₃C₂T_x and GO,and the Ti₃C₂T_x/RGO composite aerogel showed high structural stability and porous morphology,with no evident oxidative degradation of Ti₃C₂T_x.On this basis,a sulfur modification process was developed to enhance the surface accessibility and charge storage kinetics for electrochemical sodium-ion storage.The synthesized S-Ti₃C₂T_x/RGO aerogel as a free-standing electrode showed an areal specific capacity of1.26 m Ah cm^-2 with a mass loading of 12.3 mg cm^-2 at a current density of 0.1 A g^-1.When paired with an activated carbon cathode,the sodium-ion hybrid capacitor showed excellent energy and power densities as well as cycling stability.