Interface Modification Of NCM/CNTs Composite Cathode And Its Application In Lithium Metal Batteries

With the continuous development of new energy electric vehicle industry,the battery energy density is higher and higher.Therefore,the development and application of electrode materials with higher theoretical specific capacities are the top priority of current battery research.Nickel cobalt lithium manganate(NCM)and nickel cobalt lithium aluminate(NCA)have attracted much attention.However,due to the limitation of crystal and surface structures,high nickel ternary materials have met some problems including poor safety and cycling stability.In this thesis,the high nickel ternary cathode material Li Ni_0.6Co_0.2Mn_0.2O₂(NCM622)was modified by surface coating,and NCM/carbon nanotubes(CNTs)composite cathode with excellent ionic/electronic conductivity and stable interface was obtained.The electrochemical performances of high energy density lithium metal batteries based on the NCM/CNTs composite cathode was investigated.The main work and results are as follows:Firstly,NCM622/CNTs composite cathode was prepared by mixing single-wall CNTs slurry with commercial NCM622 powder without adding additional binder.Introduction of CNTs provides three-dimensional conductive network and improve the mechanical stability of the composite cathode.Quasi-solid-state lithium metal batteries based on the NCM622/CNTs composite cathode were constructed by in-situ solidification methode using poly(ethylene glycol)methyl ether acrylate(PEGMEA)and tetraethylene glycol diacrylate(TEGDA)as electrolyte precursors.The results show that the content of CNTs(1 wt%?5 wt%)has an important influence on the specific capacity,rate performance and cycling stability of the batteries.At 0.5 C rate,the quasi-solid-state lithium metal battery with 2 wt%CNTs in the cathode exhibited higher specific capacity(168.9 m Ah g^-1),better rate performance and cycling stability(85.6%capacity retention after 100 cycles).Secondly,to further improve the cycling stability of NCM622/CNTs composite cathode,an elastic PDDA/Li-PAA/OH-CNTs interface layer was coated on the surface of NCM622 particles by electrostatic adsorption and surface polymerization.The elastic interface layer can effectively alleviate₁ the pulverization₁ and fracture₁ of electrode particles₁ during the charge-discharge₁ cycles and provide excellent ionic/electronic conductivity,which is conducive to the diffusion and transport of ions and electrons in the electrode.After activation,the maximum specific capacity of the quasi-solid-state lithium metal battery based on the interface modified NCM622/CNTs composite cathode is up to 162.7 m Ah g^-1(at 0.5 C).Moreover,the batteries present improved cycling stability with capacity retention up to 95%after 100 cycles and 82%after 300 cycles at0.5 C.Finally,to improve the areal capacity of NCM622/CNTs composite cathode for real application,surface modified NCM622/CNTs thick electrodes(the loading mass of NCM622 is 13.3 mg cm^-2,and the thickness is about 160.8?m)were fabricated.Benifiting from the excellent three-dimensional electron/ion conductive network and interface stability of the surface modified NCM622/CNTs composite cathode,the initial specific capacity of the lithium metal battery based on the thick cathode and commercial Li PF₆ liquid electrolyte is 2.16 m Ah cm^-2(162 m Ah g^-1)at 0.2 C,and no capacity decline was observed after 60 cycles,indicating practical application value of the surface modified NCM622/CNTs composite cathode.