Synthesis And Electrochemical Properties Of SnO₂/Ti₃C₂T_x Composites As Li-ion Batteries Anode

Recently,Ti₃C₂T_x has been regarded as an attractive anode material in lithium-ion battery?LIBs?due to its high volumetric capacity,good conductivity and hydrophilicity,low lithium ion diffusion barrier,etc.However,the design of high-performance LIBs using Ti₃C₂T_x as the anode material is hindered by its low theoretical gravimetric.In addition,the surface functional groups generated in the wet chemical process,can degrade its conductivity and storage capacity,deteriorating the rate capacity.To solve these problems,the following research has been conducted in this dissertation:Firstly,factors that can affect the preparation of few-layered Ti₃C₂T_x was investigated and optimized.To begin with,Ti₃(Al_x Sn_1-x)C₂ MAX phases with different Sn content were prepared by pressureless sintering.Then,A=?Al,Sn?layer were eached with HF and HF/H₂O₂ from MAX phases,respectively.The results showed that layerd structure can be hardly observed for MAX phases with a high Sn content,and accordion-like Ti₃C₂T_x were obtained for MAX phases contained no or few Sn atoms in Ti₃(Al_x Sn_1-x)C₂.The addition of H₂O₂ in HF can accelerate the corrosion rate of Ti₃(Al_x Sn_1-x)C₂ phase,but the product had no layered structure.The accordion-like Ti₃C₂T_x powders were transformed into few-layered and thin electron transparent Ti₃C₂T_x nanosheets after DMSO intercalation and cell fragmentation ultrasound treatment.The few-layered Ti₃C₂T_x can be stably dispersed in water,with Zeta potential of-33.8eV at pH 7.0.Secondly,SnO₂/Ti₃C₂T_x composites were synthesized by hydrothermal method at different temperatures?90,120,and 150??and[Sn²⁺]?0.003,0.009,and 0.027M?.SnO₂ nanoparticles?d=3.5⁶ nm?were uniformly distributed on Ti₃C₂T_x nanosheets when the reaction temperature was 120?.With an increase of[Sn²⁺],the amount of SnO₂ nanoparticles showed a nonlinear increase on Ti₃C₂T_x.In order to remove the surface terminals of Ti₃C₂T_x,heat treatment?500?/Ar/2h?was carried out for Ti₃C₂T_x and SnO₂/Ti₃C₂T_x.After heat treatment,the layered structure of Ti₃C₂T_x and SnO₂/Ti₃C₂T_x materials was intact,while the interlayer water molecules of Ti₃C₂T_x were competely removed,and the amount of surface functional groups?–OH and–F?decreased considerably.However,new phases including TiO₂,SnO and Sn appeared in samples,suggesting that Ti was partially oxidized by surface oxygen functional groups of Ti₃C₂T_x or the anchored SnO₂nanoparticles during heat treatment.Thirdly,the electrochemical properties of Ti₃C₂T_x and SnO₂/Ti₃C₂T_x electrode materials were investigated.Without heat treatment,SnO₂/Ti₃C₂T_x prepared with[Sn²⁺]of 0.027 M showed the best performance:the first discharge/charge specific capacity is 614.1/318.0 mAh/g at the current density of 100 mA/g,402.0/200mAh/g higher than that of Ti₃C₂T_x.At a high current density of 1000 mA/g,the discharge capacity and coulomb efficiency for this sample can maintain as 153.9mAh/g and 99%after 1000 cycles,demonstrating excellent long term cycling stability.The specific capacity,cycle performance and rate performance of electrode materials can be significantly enhanced by heat treatment:in comparison with samples without heat treatment,the discharge capacity of Ti₃C₂T_x,SnO₂/Ti₃C₂T_x?0.003 M?,SnO₂/Ti₃C₂T_x?0.009 M?and SnO₂/Ti₃C₂T_x?0.027 M?was increased by77.1,51.8,65.3 and 171.6 mAh/g after 100 cycles at 100 mA/g,respectively.The rate performance results showed that the percentage of capacity recovery of the electrode materials after heat treatment were larger than 100%.The enhancement of electrochemical property may result from the partial removal of surface functional groups of Ti₃C₂T_x,and the presence of new phases including TiO₂,Sn,and Sn.