Ti₃C₂T_x Etched By NaHF₂ And Applied To Sodium Ion Battery

Since its discovery in 2011,MXene,being a new two-dimensional（2D）transition metal carbon/nitride materials,have been extensively studied due to these excellent physicochemical properties.MXenes exhibit electrical conductivity and show significant promising application in energy storage.Sodium-ion battery（SIB）has been regarded as a promising alternative to lithium-ion battery（LIB）due to the abundance of sodium resources,lowcost,high theoretical capacity,high power and power density,SIB is suitable for large-scale energy-storage applications.However,MXene etched by hydrofluoric acid（HF）or lithium fluoride（Li F）and hydrochloric acid（HCl）is prone to collapse and re-stacking during charge-discharge cycling,which affects the ion/electron transport within the sheet layers.Besides,above MXenes cannot provide a large enough layer spacing for Na⁺with a large ionic radius（1.02（?））,which limits the transport of Na⁺between layers and thus reduces the cycling and rate performance of SIBs.Therefore,in this study,a new etchant was used to obtain the Ti₃C₂T_x with Na⁺intercalating between layers to facilitate the Na⁺transport,which was applied to sodium ion battery.Herein,NaHF₂ aqueous solution was used for the selective removal of Al from Ti₃AlC₂,while Na⁺intercalating between layers in one step.The resulting accordion-like Ti₃C₂T_x sheets shortened the Na⁺transport paths in SIB by Na⁺intercalation,and Ti₃C₂T_x electrode provided a stable layer structure for sodium ion battery,promoted Na⁺insertion/de-insertion and maintained good cycling stability in the SIB with a specific capacity of 130 mAh·g^-1.Compared with MXene obtained with other etchants reported in the literature so far,the few-layer Na-Ti₃C₂T_x has higher capacity as the anode electrode of SIB at high current rates.The dispersion and anti-thermal-oxidation stability of Ti₃C₂T_x were analyzed and studied by comparing with MXene prepared by other etchants,and the mechanism for excellent performance was discussed.The main studies and results of this work are as follows:（1）Preparation of Ti₃C₂T_x.The d-spacing of Ti₃C₂T_x etched by NaHF₂ aqueous solution is up to 12.35（?）.The NaHF₂ etchant is mild,and the by-products（Na₃Al F₆）produced by the reaction can be removed by H₂SO₄ treatment,and the product yield can reach about 85%.After a short time of centrifugation and sonication,few-layer（10～20 layers）Ti₃C₂T_x sheets in a large lateral size（ca.3～4μm）can be prepared with high conductivity up to 2.3×10⁵S/m.（2）Dispersion stability and anti-thermal-oxidation stability of Ti₃C₂T_x.The dispersion stability of various colloidal solutions was analyzed by dispersing the exfoliated Ti₃C₂T_x sheets in water,ethanol and other organic solvents（CTAB,SDS,PEG1000,DMSO）,which can further expand the application of Ti₃C₂T_x.The Ti₃C₂T_x powder was calcined at different temperatures,when the temperature was increased to 550℃,the characteristic peak of Ti₃C₂T_x was still remained;it was until the temperature was up to 950℃ that anatase was completely transformed into rutile Ti O₂.The mechanism of the excellent anti-thermal-oxidation stability of Ti₃C₂T_x was also discussed.（3）Application of Ti₃C₂T_x in sodium ion battery.Ti₃C₂T_x sheets（10～20 layers）after centrifugation and sonication were used as the anode electrode of sodium ion battery,and the effect of Na⁺intercalation on the electrochemical performance of Ti₃C₂T_x in SIB was investigated.Ti₃C₂T_x has excellent cycling capacity at various current rates(0.1～2 A·g^-1),and delivers a high reversible capacity of with a charging capacity of 93 mAh·g^-1 at 0.1 A·g^-1.When the current density was restored to 0.5 A·g^-1,the capacity value of the Ti₃C₂T_x electrode(111mAh·g^-1)was higher than its previously tested reversible capacity(69 mAh·g^-1),indicating that the electrode material has excellent reversibility and stability of the electric capacity.Ti₃C₂T_xcan provide a reversible capacity of 70 mAh·g^-1 after 900 cycles at a current density of 1 A·g^-1with an extremely low-capacity decay rate,and the capacity can even be increased to 130mAh·g^-1 after the following 900–1000 cycles with a Coulomb efficiency close to 100%.