Preparation Of Ti₃C₂-Based Composites And Study On High Efficiency Lithium Storage Properties

As the most widely used electrochemical energy storage equipment,lithium-ion batterys（LIBs）has attracted wide attention due to its characteristics of environmental protection,high energy density and high cycle stability.In the last decade,two-dimensional transition metal carbon and nitrogen（MXenes）have gained prominence in many fields due to their high mechanical strength,high electrical conductivity,high specific surface area,hydrophilicity and interlayer accommodation of embedders.HF,as the most common MAX phase etch agent,has a good etch effect.Therefore,the in-situ HF etch method with lower risk coefficient is explored.On the basis of using different etched agents,tetramethylammonium hydroxide（TMAOH）and dimethyl sulfoxide（DMSO）were selected as intercalators to peel off multilayer Ti₃C₂.The lithium storage performance of two-dimensional Ti₃C₂ was optimized by adjusting the layer spacing of materials.Secondly,Ti₃C₂ with different layer spacing and high specific capacity electrode materials were combined by different methods,and the structural morphology and electrochemical performance of the composite materials were studied.The main research content can be divided into the following three parts:（1）HCl+LiF was used as etching agent to selectively etch the Al layer between Ti₃AlC₂layers,and the etching effect was poor when the etching time was 24 h,and the etching effect was similar to that of HF after increasing the etching time to 48 h.The layer spacing of Ti₃C₂obtained by HCl+LiF etching and Ti₃C₂ obtained by HF etching is 13.89（?）and 13.05（?）,respectively.After treating Ti₃C₂ with TMAOH,the interlayer spacing increases to 14.80（?）and 14.64（?）,respectively.However,when DMSO was used as intercalator,the interlayer spacing of Ti₃C₂ was reduced to 13.76（?）and 12.52（?）,respectively.The electrochemical properties of different Ti₃C₂ materials have been studied,and the capacity of Ti₃C₂ materials after HCl+LiF etching is higher than that of Ti₃C₂ materials after HF etching.In addition,the capacity of Ti₃C₂ materials after DMSO intercalation is higher than that of Ti₃C₂ materials after TMAOH intercalation.This is because DMSO has better intercalation and stripping effect.In the process of charge and discharge,due to the continuous insertion/de-insertion of lithium ions,the material layer spacing is expanded,and more and more embedments can be accommodated.Ti₃C₂（LiF 48h DMSO）obtained by using HCl+LiF as etch agent for 48h and DMSO as intercalator has the best electrochemical performance.After 300 cycles at the current density of 200 m A g^-1,the discharge capacity can still reach 264.9m Ah g^-1.And it has excellent rate performance.（2）The Ti₃C₂ material obtained by using HCl+LiF as etching agent for 48 h can be further stratified by ultrasonic under nitrogen atmosphere.NiFe-LDH/MXene nanoarray composite was prepared by the combination of ultrasonic treated Ti₃C₂ material with NiFe-LDH.The advantages of NiFe-LDH and Ti₃C₂ were combined through the synergism of NiFe-LDH and Ti₃C₂.The electrochemical test results showed that the discharge capacity of NiFe-LDH/MXene-500 electrode containing 500 mg Ti₃C₂-MXene in the composite can still be stable at 894.8m Ah g^-1 after 300 cycles at the current density of 200 m A g^-1.This was much better than NiFe-LDH(398.5 m Ah g^-1)and Ti₃C₂(220.9 m Ah g^-1).The NiFe-LDH/MXene-500 electrode showed higher rate performance and diffusion coefficient of lithium ion than other mixed materials.In addition,the volume expansion rate of the NiFe-LDH electrode before and after cycling is reduced from 249%to 131%compared with that of the NiFe-LDH/MXene-500 electrode,which indicates that the incorporation of Ti₃C₂-MXene makes the NiFe-LDH/MXene-500 electrodes have very high cycling stability.（3）By improving the stripping process,Ti₃C₂ solution with high concentration and fewer layers was prepared,and the number of layers could be reduced to two layers.The composite of ZnMn₂O₄ nanoparticles with high specific capacity was prepared by a one-step hydrothermal method.Because the surface of Ti₃C₂ shows electronegativity,it can attract cations very well,and the Ti₃C₂/ZnMn₂O₄ composite was successfully prepared.The distribution of ZnMn₂O₄ nanoparticles in the composite can be controlled by changing the amount of Ti₃C₂ solution.The ZMO/Ti₃C₂-5ml electrode showed excellent electrochemical performance when the dosage of few-layer Ti₃C₂ solution was 5m L.The initial discharge capacity of ZMO/Ti₃C₂-5m L electrode is 1018.2 m Ah g^-1 at the current density of 200 m A g^-1.After 120 cycles,the capacity is still stable at 817.9 m Ah g^-1.In the magnification performance test,when the current density increases to 8 A g^-1,the specific discharge capacity can still reach 210.1 m Ah g^-1.After the current density returns to 0.1 A g^-1,the discharge capacity returns to 839.9 m Ah g^-1.The results show that Ti₃C₂/ZnMn₂O₄ composites have excellent cyclic stability and magnification properties.The SEM tests before and after the cycle show that the electrode surfaces become smoother and the surface cracks decrease with the increase of the addition of Ti₃C₂ solution,which further confirms that the addition of Ti₃C₂ can significantly improve the cycle stability of high specific volume materials.