Preparation Of Two-dimensional MXenes And Metal Selenide Composite And Its Application In Lithium (Sodium) Storage

In the past decades,lithium ion batteries（LIBs）are booming due to a series of merits such as high energy density,green environment protection,no memory effect,relatively long service life and low self-discharge rate.Now LIBs have become an indispensable part of our daily life,and have been widely used in hybrid electric vehicles and fixed power grid storage.However,the capacity of existing graphite-based anode LIBs is difficult to meet the large-scale application needs of people from electric vehicles to smart grid,so it is urgent to develop new high-capacity anode materials for LIBs.Due to the chemical properties similar to lithium and the wide range of sodium resources,sodium ion batteries（SIBs）have become a strong candidate for the next generation of energy storage.Therefore,it is a mutual and imperative mission to develop Li⁺/Na⁺host materials with outstanding performance for LIBs and SIBs.MXenes,a new favorite in the field of two-dimensional materials,shines in the field of energy storage applications because of a series of unique performance combinations.Pure MXenes can maintain a relatively stable cycle as LIBs/SIBs anode material,but the low capacity cannot meet the current energy demand.And metal selenides have high theoretical capacity,but their further application is limited due to their huge volume expansion during charging and discharging.Based on the above background,the purpose of this paper is to prepare MXenes and metal selenide composites and study their electrochemical properties as LIBs or SIBs anode,the main research contents and results are as follows:（1）In order to solve the problem of low specific capacity and easy stacking when layered Ti₃C₂T_x is used as the SIBs anode,Sn Se₂ with high theoretical capacity is selected and synthesized with Ti₃C₂T_x by simple electrostatic adsorption method.Sn Se₂nanosheets not only greatly improve the overall specific capacity,but also successfully suppress the self-stacking problem of Ti₃C₂T_x.At the same time,due to the existence of Ti₃C₂T_x conductive substrate,the agglomeration problem and serious volume expansion problem of Sn Se₂ nanosheets（average diameter of 200～300 nm）are improved.When the Sn Se₂/Ti₃C₂T_x complex was used as the anode of SIBs,the specific capacity was increased 3-4 times compared to that of Ti₃C₂T_x and Sn Se₂.（2）In order to solve the problem that the layered Ti₃C₂T_x layer spacing is limited and the internal active sites are not fully utilized,the lamellar Ti₃C₂T_x with more active sites was selected as the conductive substrate,and the Co Se₂/Ti₃C₂T_x hybrid was prepared by ultrasonic hybridization with Co Se₂ which has high theoretical capacity.With the help of Ti₃C₂T_x flakes,Co Se₂ particles are dispersed evenly and no longer agitate（average particle size is 50-60nm）,and the volume expansion problem is improved significantly,ensuring the integrity of the overall structure.When the Co Se₂/Ti₃C₂T_x hybrid was used as the anode of LIBs,it showed remarkable performance:the specific capacity of Co Se₂/Ti₃C₂T_x hybrid was 1797 m A h g^-1 after 500 cycles at 1 A g^-1,and 896 m A h g^-1 after 1450 cycles at 2 A g^-1.The lithium storage mechanism and reaction kinetics of Co Se₂/Ti₃C₂T_x were investigated by in-situ XRD and GITT test.（3）In order to explore MXene with higher lithium storage capacity and more active sites,layered V₂CT_x with high purity and wide layer spacing was successfully prepared by in-situ hydrofluoric acid generation and hydrothermal etching of V₂Al C,and then combined with high theoretical capacity Fe Se₂,Fe Se₂ nanoparticles were uniformly anchored on the surface of V₂CT_x and embeded into V₂CT_x layers.When it was first applied to LIBs anode,it showed excellent electrochemical performance:the discharge capacity reached 1760 m A h g^-1 after 1130 cycles under 1 A g^-1,and it was found that pseudocapacitance was the main mechanism for lithium storage by exploring its dynamic behavior.To sum up,in this paper,the layered Ti₃C₂T_x,Ti₃C₂T_x flakes and layered V₂CT_x are hybridized with three metal selenides respectively,and the three materials show excellent electrochemical performance when applied to LIBs or SIBs anode,which offers a new option for the development of LIBs and SIBs anode materials,and broadens the application of MXenes in the field of energy storage.