Preparation And Lithium Storage Performance Of Metal Compound Nanosheets/MXene Composites

Metal compound as anode materials for lithium-ion batteries has the advantages of low cost and environmental friendliness.However,these materials usually suffer from low electrical conductivity during charging and discharging,leading to poor lithium storage performances.To improve the electrochemical performance of the metal compound anode materials,there are two effective strategies,i.e.adjusting their structure or combining with highly conductive materials.Transition metal carbon/nitride（MXene）is a new two-dimensional（2D）material,which has metallic electrical conductivity.In order to obtain high-performance anode materials for lithium-ion batteries,here,2D metal compound nanosheets are prepared by MXene in-situ conversion method or hydrothermal method.Furthermore,through electrostatic self-assembly and mixing-filtration method,the prepared 2D metal compound nanosheets are combined with MXene nanosheets,and high-performance TiO₂/Ti₃C₂T_xMXene,Nb₂O₅/Nb₂CT_xMXene and SnS₂-MXene/rGO composites were obtained.The mainresearch contents are as follows:（1）Using Ti₂CT_xMXene as precursor,porous TiO₂nanoplates with2D layered structure were prepared by in-situ oxidation method.Then,2D-2D TiO₂/Ti₃C₂T_xMXene composites with strong interfacial interaction are fabricated by electrostatic self-assembly between the obtained TiO₂nanoplates and 2D Ti₃C₂T_xMXene with high electrical conductivity.In the 2D-2D TiO₂/Ti₃C₂T_xMXene composites,TiO₂nanoplates evenly distributed on the MXene nanosheets.Together with their porous structure,the electrochemically active sites of TiO₂are exposed fully,which is beneficial for improving the capacity.The introduction of highly conductive MXene can promote the rapid transfer of electrons,significantly enhancing the rate performance of the composite.As a result,the TiO₂/Ti₃C₂T_xcomposites show an initial reversible capacity of 360.8 mAh g^-1at 50 mA g^-1,and a reversible capacity of 84.2 mAh g^-1at 2 A g^-1.Besides,the capacity can still maintain 97.8 mAh g^-1after 10000 cycles at 1 A g^-1.（2）Using Nb₂CT_xMXene as precursor,2D Nb₂O₅nanosheets were prepared by in-situ oxidation method.Then,the 2D Nb₂O₅nanosheets were mixed with Nb₂CT_xMXene nanosheets,and the mixture was vacuum filtrated to fabricate the flexible free-standing Nb₂O₅/Nb₂CT_xcomposite film.The film can be used as anode of lithium-ion batteries directly.The composite film has a layered 2D-2D structure,which can effectively avoid the agglomeration of Nb₂O₅nanosheets,making the active sites fully exposed.It can also buffer the agglomeration of Nb₂O₅nanosheets.Moreover,there are pores between the layers,which provide channels for rapid ion transport.As a result,the flexible free-standing Nb₂O₅/Nb₂CT_xfilm anode shows high capacity,good cycle performance andrate capability.It has a reversible specific capacity of 446 mAh g^-1at 50mA g^-1in the initial cycle and remains a reversible specific capacity of368.8 mAh g^-1after 70 cycles.The film anode also has a specific capacity of 163.4 mAh g^-1at 2 A g^-1.（3）SnS₂nanosheets are prepared by hydrothermal method,and they are combined with MXene and rGO nanosheets by cryogenic hydrothermal method to construct three-dimensional（3D）SnS₂-MXene/rGO aerogel with strong interfacial interaction.The fluffy and porous structure of the 3D aerogel can effectively inhibit the self-stacking of SnS₂nanosheets and effectively buffer the volume expansion of SnS₂nanosheets during lithium storage.In addition,the 3D conductive substrate constructed by MXene/rGO can not only promote fast electron transfer and improve the electrical conductivity of the material,but also facilitate the electrolyte penetration and rapid ion transport.As a result,the 3D SnS₂-MXene/rGO composite exhibits high specific capacity,reaching a reversible specific capacity of 1000.4 mAh g^-1at 0.1 A g^-1and684.4 mAh g^-1at 1 A g^-1.