Study On The Performance Of 2D Transition Metal Carbides And Their Composite As Anode Materials For Sodium/Potassium Ion Batteries

Due to the scarcity,high price and uneven distribution of lithium resources in the earth's crust,the further application of LIBs in large-scale energy storage systems is limited.And the sodium-ion and potassium-ion batteries have attracted widespread concern of researchers,due to their low cost,wide operating voltage window and similar charge storage mechanism as lithium ion batteries.However,the ionic radius of Na+and K+are larger than that of Li+,and Na+and K+are difficult to insert and desorb from the electrode materials,so many anode materials suitable for LIBs have weak electrical activity in sodium-ion and potassium-ion batteries.2D transition metal carbide/nitride(MXene)has become the main choice of electrode materials for rechargeable batteries due to its good electrical conductivity and cycling stability However,pure MXene cannot meet the requirements of high energy density of rechargeable batteries.At present,a more effective way for improving its capability of charge-stroage is to delaminate MXene and integrate them with other materials,whereby they could complement each other and produce synergistic effects to significantly improve the electrical performance of MXene-based composites.This thesis mainly focused on MXene(Nb2CTx?Ti3C2Tx MXene@NCFs),as shown below:A simple hydrothermal method was used to synthesize 2D Nb2CTx MXene material,which eliminated the environmental hazard caused by HF volatilization and improved the corrosion efficiency.As the anode material of SIBs,Nb2CTx MXene had a reversible specific capacity about 102 mA h g-1 after 500 charge and discharge cycles at the current density of 1 A g-1 in a half cell for sodium-storage.When acting as the anode material of PIBs,Nb2CTx MXene rendered the reversible specific capacity of 51 mA h g-1 after 200 cycles under a current density of 200 mA g-1.Despite the capacity of potassium-ion battery using Nb2CTx MXene as anode material seems to be unsatisfactory,the electrochemical performance of MXene could be improved by combining with other inorganic materials,due to the unique two-dimensional structure and large specific surface area of MXeneTi3C2Tx MXene@NCFs was synthesized by using high-energy ball milling and electrostatic spinning method.In this composite structure,MXene nanosheets were tightly covered with carbon fibers,so as to inhibit the agglomeration of MXene nanosheets.And the synergistic effect between MXene and NCFs was beneficial to increase the interlayer distance of MXene and form a unique cross-linked structure,which was conductive to shorten the path of potassium ion/electronic transmission,thereby significantly improving the electrochemical performance of the composite.When used as anode material of PIBs,it still had a reversible specific capacity of 224 mA h g-1 after 500 cycles under a current density of 100 mA g-1,which was obviously higher than that of pure Ti3C2Tx MXeneMore importantly,we explored the root cause of the excellent rate performance and stable capacity after a long cycle when MXene was used as the anode material of SIBs.The results showed that remarkable electrochemical performance could be attributed to the emergence of hybrid pseudocapacitance.