Oxidation Behavior Of Two-Dimensional M₂C?M=Ti Or V? And Its Products For Li-Ion Batteries

MXene has received extensive attention in recent years due to their unique two-dimensional structure and composition diversity,which lead to their applications in the field of energy storage such as secondary-ion batteries and supercapacitors.So far,over 20 different MXene compositions have been synthesized,only Ti₃C₂ has been extensively studied,and the study on other MXenes is still shallow.The theoretical calculations show that V₂C and Ti₂C exhibit higher lithium storage capacity than Ti₃C₂.However,these two kinds of MXenes are chemically unstable and have low experimental capacity.Therefore,we used V₂CT_x and Ti₂CT_x,where T_x represents the surface termination groups,as our research objects to explore the influence of different corrosion methods on electrochemical performance,study the changes of composition and structure of two materials under different surface modification conditions,and establish the relationship between structure and electrochemical performance.In this study,V₂AlC was synthesized by pressureless sintering.Then,V₂AlC is etched by HF acid and HCl+NaF,and the obtained V₂CT_x is annealed and hydrothermal oxidized,and Ti₂CT_x is also hydrothermal oxidized.Characterization and analysis of XRD,SEM and XPS were carried out to study the phase,morphology and valence state of the product.The electrochemical properties of the related products were also tested.The results are as follows:?1?V₂AlC was synthesized by pressureless sintering,and the molar ratio V:Al:C=2:1.1:1.0 was sintered in an argon atmosphere at 1500?for 4 h to obtain V₂AlC with good purity.V₂AlC was etched by HF at 25?and 50?,and V₂CT_x with larger interlayer spacing was obtained by etching at 25?for 7days.The interlayer spacing of the sample was slightly smaller and the surface of the sample was easily oxidized at 50?.High-purity V₂CT_x was obtained after etching at 90?for 7 days with HCl+NaF.The samples obtained by HF acid etching and HCl+NaF etching were cycled 100 times at a current density of 0.1 A g^-1,and the specific discharge capacities were 248 mAh g^-1,266 mAh g^-1,respectively.The reversible capacities can be retained 94 mAh g^-1,58 mAh g^-1after 1000 cycles at a current density of 1 A g^-1,respectively.The samples obtained by HCl+NaF etching have relatively poor cycle performance at large current densities.?2?Annealing of multi-layer V₂CT_x obtained by HF acid etching at room temperature shows that the surface of fluorine can be effectively removed by annealing at 600?.V₂O₃ and Al₂O₃ are formed in the annealed product,and the electrochemical performance of annealed product is poor compared to the sample before treatment.The oxidized V₂CT_x is obtained by hydrothermal oxidation with trace amount of hydrogen peroxide,the results showed that VO₂was formed on the surface of the sample.When V₂CT_x powder is added in an amount of 0.2 g and the concentration of hydrogen peroxide is⁰.17 wt%,the hydrothermal product is used as anode in lithium ion batteries that exhibit better electrochemical performance.The specific capacity of the discharge was 317mAh g^-11 after 100 cycles at a current density of 0.1 A g^-1.?3?The oxidized Ti₂CT_x obtained by hydrothermally treated Ti₂CT_x powder with pure water and alkaline LiOH solution,it showed that Ti₂CT_x was converted to layered anatase TiO₂ in pure water.When the hydrothermal temperature was120?,the obtained sample showed better performance with reversible capacity of 101 mAh g^-11 after 500 cycles at a current density of 1 A g^-1.In the alkaline LiOH solution system,after hydrothermal and calcination,Ti₂CT_x is transformed into layered Li₄Ti₅O₁₂,the sample with the hydrothermal temperature of 150?and calcined at 550?has a good electrochemical performance with the reversible capacity of 126 mAh g^-1after 500 cycles at a current density of 1 A g^-1.