Construction And Energy Storage Mechanism Of Flexible Titanium-based Double Transition Metal MXene Electrodes

Energy storage technologies（e.g.,high-performance supercapacitors and lithium-ion batteries）are the key support for the large-scale sustainable and stable utilization toward new energy sources such as solar and wind power.And the electrode materials play crucial roles in the electrochemical performances such as capacity,initial coulomb efficiency,and service life.Two-dimensional MXene materials have attracted tremendous attention in the field of energy storage due to their abundant surface functional groups,good hydrophilicity,high electronic conductivity,and tunable layer spacing.Nowadays,the research on MXene electrodes mainly focus on Ti₃C₂ MXene,but it shows unsatisfied specific capacitance as a supercapacitor electrode and reversible specific capacity as a lithium-ion battery anode.To develop MXene electrodes with high capacity and good stability,we take Ti₃C₂ MXene and Ti₂C MXene as the research objects and construct titanium-based double transition metal MXenes.The synergistic effects between the second phase transition metal elements and titanium can adjust their electronic structure and surface terminations of MXenes,consequently improving their stability and energy storage performance.Meanwhile,the improvement mechanisms of stability and electrochemical performance are also investigated with the help of advanced characterization techniques and theoretical calculations.This work is of great significance to promote the application of MXenes in the field of energy storage.The specific research contents are as follows:（1）To increase the specific capacitance of Ti₃C₂ MXene as a supercapacitor electrode and reversible specific capacity as a lithium-ion battery anode,we constructed a double transition metal Ti₂Nb C₂ MXene by introducing the Nb into the M site of Ti₃C₂MXene via alloying strategy.The results show that the introduction of Nb atoms improves the state density of Ti₃C₂ MXene at Fermi level,increases the lithium adsorption capability of Ti₃C₂ MXene,correspondingly improves the electronic conductivity of Ti₃C₂ MXene,enhances the interaction between Ti₃C₂ MXene and electrolyte ions/lithium ions and further increases the electrochemical performances.The Ti₂Nb C₂ MXene exhibits excellent energy storage properties as electrode materials.It delivers a specific capacitance of 281 F g^?1 at 5 m V s^?1 used as a supercapacitor electrode,a reversible specific capacity of 272 m Ah g^?1 at the current of 0.1 A g^?1 used as a lithium-ion battery anode,and a high energy density of 39 Wh kg^?1 at the power density of 115W kg^?1 used as a lithium-ion capacitor anode.（2）To further widen the working potential window of MXene electrodes for improving the energy density of supercapacitors,and develop a implantable,biocompatible and flexible supercapacitor,we constructed a new type of out-of-plane ordered flexible double transiton metal Ti₂Ta C₂ MXene electrode via introducing the Ta species with good stability and bioconpability.The results show that Ti₂Ta C₂ MXene is an out-of-plane ordered structure,where Ti atoms are located in the outermost layer of the transition metal layer and Ta atoms are located in the middle layer.The introduction of Ta species increases the state density of d-orbital,upshifts the d-band center of Ti,enlarges the work function of Ti₃C₂ MXene,which correspondingly improves the electronic conductivity and enhances the chemical affinity and adsorption capability of Ti₃C₂ MXene for electrolyte ions.Most importantly,it improves the chemical stability,bordens the working potential window and increases the energy density of Ti₃C₂ MXene.In a three-electrode system,it delivers a specific capacitance of 312 F g^?1 at a scanning rate of 2 m V s^?1.In symmetric capacitors,its working potential window can reach 1.0 V and its energy density can reach 10 m Wh g^-1,which is much higher than the previously reported energy density of MXene-based symmetric capacitors.In addition,it shows excellent cytocompatibility and biocompatibility in various cells and in vivo applications.（3）The low capacitance hinders the development of MXene electrodes.Herein,we prepared a flexible self-supporting Ti₂C MXene film and investigated its electrochemical performances in different aqueous electrolytes.Moreover,the capacitive behaviors of Ti₂C MXene in various aqueous electrolyte were further revealed by theoretical calculation.The results show that the specific capacitance of flexible Ti₂C MXene electrode in an acidic electrolyte(382 F g^?1)is much higher than that in an alkaline electrolyte(99 F g^?1)and neutral electrolyte(104 F g^?1).In the acidic electrolyte,the coupling of hydronium ions with the oxygen functional groups on the surface of the Ti₂C MXene electrode induces charge redistribution,resulting in a pseudocapacitance behavior and a higher specific capacitance contribution.（4）To further improve the electrochemical cycling stability of Ti₂C MXene,a flexible double transition metal TiNbC MXene electrode was prepared by alloying Nb into the M-site of Ti₂C MXene.The results show that the introduction of Nb species improves the electronic conductivity of Ti₂C MXene,induces the upward shift of the d-band center of Ti,regulates its surface chemical properties,increases its work function,and thus improves its capacitive performances,especially its cycling stability.The as-prepared flexible TiNbC film delivers a specific capacitance up to 381 F g^?1 at a scan rate of 2 m V s^?1 and excellent electrochemical stability without capacitance loss after 10000 charge/discharging cycles.