Preparation Of MXene-based Composites And Their Applications In Lithium-ion Batteries And Sodium-ion Batterie

The increasing demand for environmentally friendly,high-energy portable electronic applications and large-scale electrical energy storage devices has enhanced the research on rechargeable lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）,which gradually come into people’s sight.The development of low-cost,high-rate performance,safety and long cycle life of anode and cathode materials are the key factors to the large-scale development and application of LIBs and SIBs.The 2D Ti₃C₂T_x MXene compounds were prepared by selectively etching the Al atom layers of the precursor Ti₃Al C₂ MAX phase in HF.MXene is considered to be promising electrode material due to its excellent conductivity,great mechanical strength,adjustable interlayer spacing and two-dimensional surfaces for ion transport and diffusion.However,the lower theoretical specific capacity and self-stacking properties limit the application of MXene in high-energy LIBs and SIBs.Assembling Ti₃C₂T_xMXene with high specific capacity cathode and anode materials is one of the most common and effective modification strategies.As a cathode material for LIBs,lithium iron phosphate（Li Fe PO₄）has the advantage of abundant raw materials,lower price,higher theoretical specific capacity,and good cycle stability compared with widely commercialized lithium cobalt oxide.However,the application of Li Fe PO₄ is limited by low electrical conductivity and poor Li-ion diffusivity.In addition,molybdenum disulfide（MoS₂）materials have become the research hotspot of anode materials for SIBs due to their adjustable band gap,interlayer distance that facilitates the deintercalation of sodium ions,high-rate performance,and simple preparation.However,the dramatic volume expansion of MoS₂ during Na⁺extraction and insertion severely limit its cycle stability.To solve above problems,we synthesized Ti₃C₂T_x MXene modified Li Fe PO₄ as a cathode material for LIBs by designing the surface structure of electrodes,improved the disadvantages of low electron alongside Li⁺diffusion,and its related electrochemical properties and phase transition of MXene during cycling were investigated.In addition,MoS₂ in situ grew on the surface of Ti₃C₂T_x MXene in order to achieve the effect of inhibiting the agglomeration of MoS₂ and alleviating the volume expansion.Ti O₂ with coexistence of rutile and anatase phases were synthesized utilizing the phase transition generated by MXene.These Ti O₂nanomaterials combined with MoS₂@MXene were applied as anode material for SIBs exhibiting excellent electrochemical performance,which provides ideas for the application of MXene-based composites in LIBs and SIBs.The main researches are as follows:（1）A facile and efficient electrostatic self-assembly method was applied to synthesize MXene-coated Li Fe PO₄@C composites（LFP@C/MXene）at room temperature.Firstly,the shortened[010]direction of LFP nanosheets could improve the lithium-ion diffusion,and the carbon coating could further enhance the conductivity of a single particle.In addition,the highly conductive MXene nanofilms and LFP@C nanosheets could form“dot-to-surface”conductive network and hierarchically porous structure,which not only prevents the aggregation of LFP@C nanosheets,but also increases the contact area with the electrolyte.Moreover,part of the MXene is oxidized into Ti O₂ and carbon during cycling,which provides more active sites for Li-ion intercalation/extraction.As prepared LFP@C/MXene electrodes possess admirable electrochemical performance,including the high-rate performance of 139 m Ah·g^-1 at20 C current density,and long-cycle stability,the capacity maintains 94.8%after 500cycles at 1 C current density.This ingenious design provides a new idea for 2D MXenes as high-performance lithium-ion battery electrode materials.（2）MoS₂@MXene@D-Ti O₂ composites with hierarchical porous morphology were prepared by a one-step hydrothermal method with the assistance of CTAB,in which the MoS₂ lattice spacing was enlarged.In addition,part of MXene was oxidized into dual phase Ti O₂ with rutile and anatase.This unique“plane-to-surface”structure could hinder the MoS₂ nanoflower aggregation and re-stacking,and achieve sufficient electrode/electrolyte interaction.Subsequently,the built-in electric field formed by the heterostructure among the biphasic Ti O₂,MoS₂,and MXene could further promote Na⁺transport.Therefore,the constructed 3D MoS₂@MXene@D-Ti O₂ anode material exhibits an admirably high-rate reversible capacity at room temperature(359.6 m Ah·g^-1 at 5 A g^-1).There is still excellent cycling stability at a low temperature of-30°C(about 200 m Ah·g^-1).Meanwhile,coupling with Na₃V₂（PO₄）₃ cathode,MoS₂@MXene@D-Ti O₂ anode material also possesses superior electrochemical performance in Na⁺full cells.