Preparation Of Two-dimensional Nb₂CT_x And Electrochemical Performance In Lithium (Zinc) Ion Batteries

In recent years,two-dimensional layered materials have received extensive attention due to their special two-dimensional structure and excellent optoelectronic properties,and there are huge demands in application fields such as hydrogen production,energy storage,sensors,electromagnetic shielding,and biomedicine.In 2011,the Gogotsi research group of Drexel University in the United States reported a new two-dimensional material family-MXene,which quickly attracted people's attention due to its unique optical,electrical,thermal,mechanical and chemical properties,especially It is very competitive in the field of energy storage.Among the MXene family,currently only Ti₃C₂T_X,V₂CT_X,and Mo₂CT_X have been studied in depth and extensively.Nb₂CT_X is known as one of the most potential lithium ion anode materials due to its minimal atomic layer configuration,but its research is limited by etching technology of bulk MAX.At present,the mainstream etching method of Nb₂CT_X is hydrofluoric acid etching.This method not only takes a long etching time(140 hours),but also causes a large number of fluorine-based functional groups,and hydrofluoric acid is toxic and corrosive,and is not environmentally friendly..Studies have shown that fluorine-based functional groups can reduce the electroactive sites of lithium ions in MXene,leading to a decrease in capacity,and the emergence of fluorine-based functional groups should be avoided.Therefore,this article attempts to use a more efficient and environmentally friendly hydrothermal reactive etching method to prepare Nb₂CT_X.The hydrothermal reactive etching not only has a short etching time without the use of hydrofluoric acid,but also has a good etching effect and high yield.This is the future industrialization of MXene Production provides a reference plan.First,in order to verify the feasibility of hydrothermal etching,we compared the samples obtained by HF direct etching,HCl+Li F in-situ HF etching and hydrothermal reaction etching.The SEM and XRD test results show that HF direct etching Etching and HCl+Li F in-situ HF etching methods cannot effectively remove the Al atomic layer at room temperature for 24 hours,while hydrothermal reactive etching can efficiently remove them.And it is concluded that 180?-20h is the best hydrothermal etching condition.Under this condition,the obtained Nb₂CT_X has higher interlayer spacing and crystallinity.According to infrared spectroscopy and Raman spectroscopy,the functional groups on the surface of Nb₂CT_X synthesized under hydrothermal conditions are mainly-OH,=O,-F,which are similar to Ti₃C₂T_X.Next,we conducted a four-probe resistance test and a BET specific surface area test.Under the hydrothermal conditions of180?-20h,the conductivity of Nb₂CT_X is close to 2.5 S/cm,and the specific surface area is close to 100 m²/g.In order to verify that Nb₂CT_X has better electrochemical performance under hydrothermal conditions of 180?-20h,we used Nb₂CT_X prepared under three different hydrothermal conditions for testing and analysis of negative electrodes of lithium-ion batteries.It can be seen that there are no obvious lithiation peaks and delithiation peaks in the CV curve.The energy storage mechanism is the same as that of Ti₃C₂T_X,and it is also the insertion and extraction of lithium ions.EIS test results show that Nb₂CT_X has smaller interface resistance and higher ion diffusion rate under hydrothermal conditions of 180?-20h.This indicates that Nb₂CT_X under hydrothermal conditions of 180°C-20h may be more suitable for lithium-ion battery negative electrodes.Subsequently,the initial capacity of Nb₂CTx under hydrothermal conditions of 180?-20h at a current density of 100m A/g was 248 m Ah/g,while the capacities of the other two materials were 225m Ah/g and 151 m Ah/g,respectively.After cycling,the capacity retention rate is close to 93.5%.In the rate performance test,the capacity of Nb₂CT_X under hydrothermal conditions of 180?-20h is still better than the other two samples.Combining the previous XRD,SEM,FTIR,Raman spectroscopy,BET,four-probe resistivity test and the electrochemical test results in this chapter,it can be concluded that under the optimal hydrothermal synthesis conditions of 180?-20h,the synthesized Nb₂CT_X The material has the advantages of larger specific surface area,higher interlayer spacing,and high electrical conductivity,and therefore obtains better electrochemical performance.In addition,we have also conducted research on the application of Nb₂CT_Xin the protection of zinc negative electrodes of zinc-ion batteries.In order to improve the dendrite growth,corrosion and passivation of the zinc anode,this article uses Nb₂CT_X as a protective layer to prepare the Nb₂C@Zn anode.The Nb₂CT_X ink is evenly sprayed on the surface of the pure zinc,and the second of The three-dimensional layered structure and high specific surface area induce uniform deposition of zinc dendrites.The SEM picture shows that the dendrites on the surface of the modified zinc anode no longer agglomerate,but are deposited uniformly on the surface of the material.Through EIS impedance analysis and self-discharge test,it can be seen that this material has lower interface resistance and higher stability during charging and discharging.Moreover,the Nb₂C protective layer can prevent the zinc from directly contacting the electrolyte,avoiding the occurrence of hydrogen evolution reaction,thereby improving the cycle stability.In a symmetrical battery,at a current density of 0.5m A/cm²,Nb₂C@Zn still maintains normal operation after 360 hours of constant current charge and discharge,while pure zinc fails at 280 hours;between 198-201hours,Nb₂C The hysteresis voltage of@Zn is 40 m V,which is much lower than the 120 m V of pure zinc.Under the high current density of 1 m A/cm² and 5m A/cm²,pure zinc fails in less than 30 hours of charge and discharge test,while Nb₂C@Zn still maintains good stability after 120 hours.The subsequent rate performance test showed the low polarization rate and good stability of Nb ₂C@Zn.In a full battery,at a current density of 100 m A/g,the first cycle capacity of Nb₂C@Zn is 238.6 m Ah/g,while the first cycle capacity of pure zinc is only 85.2m Ah/g.After 200 cycles,the capacity of pure zinc drops to 20 m Ah/g.At this time,the battery has failed,and the Zn negative electrode protected by Nb ₂C maintains its capacity at 87.8 m Ah/g after 200 cycles.