Preparation And Electrochemical Properties Of 2D Ti₃C₂-based Layered Materials

As a green environmental energy storage device,lithium-ion batteries（LIBs）have obtained tremendous attentions due to their long cycling performance,high energy density and environmental friendliness.Recently,2D MXenes have attracted the interest of researchers because of their high mechanical strength,excellent electrical conductivity,high specific surface area,and ability to accommodate inserts.However,most of the MXenes derive from HF etching Ti₃AlC₂.Moreover,Ti₃AlC₂ can not be supplied in large quantities and its production cost is high.This paper prepares MXenes using Ti₃SiC₂ as a sufficient and cheap MAX phase.Through selective etching of silicon from Ti₃SiC₂（MAX）using HF and oxidant in this study,the Ti₃C₂ MXenes were obtained,which were firstly evaluated as a promising anode for high durable lithium-ion batteries（LIBs）.The interlayer distance of Ti₃C₂ MXenes can be controllable with the oxidizability of oxidant and etching temperature.In addition,the methods of in-situ growth of TiO₂ and electrostatic attraction ZnCo₂O₄ were used to further improve the performance of the electrode materials,and the structure and electrochemical performance were studied.The main research contents are as follows:（1）Ti₃SiC₂ is used as the low-cost MAX phase raw material to replace Ti₃AlC₂ to prepare 2D layered Ti₃C₂.Because Ti-Si bonds are much stronger than Ti-Al bonds,the use of HF alone for etching cannot successfully produce 2D layered Ti₃C₂.Therefore,using oxidant assisted hydrofluoric acid as an etchant,oxidation of silicon by an oxidant as the first step,and dissolution of silicon oxides by HF as the second step,so as to effectively etch Ti₃C₂from Ti₃SiC₂.When H₂O₂ as the oxidant,the interlayer spacing of Ti₃C₂ can be effectively controlled by adjusting the etching temperature.As the etching temperature increases,the interlayer spacing becomes wider.When the temperature is too high,excessive etching will occur and the layered structure can be destroyed,which is not conducive to ion and electron transport in the electrochemical reaction process.Through a comparative study,it was found that the interlayer distance of Ti₃C₂（H₂O₂-60）is as high as 10.07?,and its layered structure is complete.At a current density of 200 mA g^-1,it is still stable a high discharge specific capacity of 156 mAh g^-1 after 250 cycles,showing the best electrochemical performance.After hydrothermal treatment,TiO₂ nanoparticles were generated in-situ on the surface and interlayer of Ti₃C₂ to prepare Ti₃C₂@TiO₂（H₂O₂-60）composite material,and the interlayer distance was further increased to 10.70?.In addition,the in-situ generated TiO₂nanoparticles provide additional lithium storage capacity,so its electrochemical performance is further improved.After 250 cycles at a current density of 200 mA g^-1,the discharge specific capacity can still be stabilized at 177 mAh g^-1,and at a high current density of 2000 mA g^-1after 1600 cycles,it can still remain stable at 102 mAh g^-1.（2）N-doped 2D Ti₃C₂ with an open lamellar structure has been synthesized using stronger oxidizing concentrated HNO₃ as oxidants at room temperature for 40 hours,and its layer spacing is 12.52?.Nitrogen doping can enhance the electronic conductivity of materials,leading to stronger interfacial binding and hence remarkable improving the cycling stability and rate capability.A high discharge specific capacity of 201 mAh g^-1 can still be kept after 500 cycles at a current density of 200 mA g^-1.The N-doped Ti₃C₂@TiO₂（HNO₃-RT）prepared after hydrothermal treatment,the interlayer distance is further increased to 12.77?,so the specific discharge capacity is still stable at 302 mAh g^-1 after 500 cycles at a current density of 200 mA g^-1.And it can maintain a stable discharge specific capacity of 154 mAh g^-1,after 1500 cycles at a large current density of 2000 mA g^-1,with excellent cycle stability and good rate performance.（3）ZnCo₂O₄ nanoparticles were converted into oxygen-rich vacancy nanoparticles by liquid-phase pulsed laser irradiation,and N-doped Ti₃C₂（HNO₃-RT）was used as an excellent substrate material,and the two were combined in different proportions to produce N-doped Ti₃C₂@ZnCo₂O₄ composite materials.Among them,the ZnCo₂O₄ nanoparticles prepared by liquid-phase pulsed laser irradiation treatment are rich in oxygen vacancies,which can effectively improve their electrical conductivity and electrochemical activity,shorten the ion diffusion length,and reduce the transfer resistance.ZnCo₂O₄ nanoparticles presented positive electromagnetism due to the presence of oxygen vacancy,while N-doped Ti₃C₂ presented negative electromagnetism due to the presence of surface functional groups.They successfully combined by electrostatic attraction,and ZnCo₂O₄ entered the interlayer of Ti₃C₂to complete the composite,and maintained a good layer structure.By controlling the content of ZnCo₂O₄,the N-doped Ti₃C₂@ZnCo₂O₄-40 composite materials with optimal electrochemical performance were prepared,in which the specific discharge capacity of N-doped Ti₃C₂@ZnCo₂O₄-40 electrode is 579 and 454 mAh g^-1 after 100 and 150 cycles at the current density of 200 and 500 mA g^-1,respectively.And,when the current density increases to 2000 mA g^-1,Ti₃C₂@ZnCo₂O₄-40 can still maintain an excellent specific discharge capacity(344 mAh g^-1).Even though,the current density increases to 3000 mA g^-1in the rate test,the discharge specific capacity can also be maintained at 238 mAh g^-1.When the current density returns to 100 mA g^-1,the discharge specific capacity can still be stable at525 mAh g^-1,showing excellent cycle stability and good rate performance.