Research On The Construction Of High-stability Sodium Metal Anode Based On Carbon Nanotubes

Sodium metal batteries are a strong candidate for replacing lithium metal batteries because of the rich reserves of metal sodium,low price,high theoretical energy density(1165 m Ah g^-1),low electrochemical potential(-2.714 v)and chemical properties similar to metal lithium.However,the development and application of metallic sodium anodes are facing huge challenges.On the one hand,the uneven deposition of sodium ions on the surface of sodium metal can easily produce sodium dendrites,resulting in poor cycle stability and coulombic efficiency;on the other hand,the poor affinity of the substrate material with sodium seriously hinders stable deposition realization of sodium metal.In this paper,to improve the deposition amount and deposition stability of metal sodium,based on the three-dimensional(3D)network structure of multi-walled carbon nanotubes,the acidified multi-walled carbon nanotubes with sodiophilic oxygen-containing functional groups(O-CNTs)and the fluorinated multi-walled carbon nanotubes(F-CNTs)with increased interlayer spacing were prepared by changing the content of oxygen-containing functional groups and changing the fluorination temperature,providing more new ideas for the preparation of high-performance sodium metal anode materials.At the same time,the electrochemical properties of sodium metal batteries were systematically studied.Specific research results are as follows:(1)Through the condensation reflux method,the O-CNTs with hydrophilic sodium and a uniform distribution of 3D network structure has been prepared.At the same time,the problems of hydrophobicity and agglomeration have been resolved without changing itself superior performance.The pore size of O-CNTs also increases,and provides a very superior 3D network structure for metal Na deposition,which can adjust the metal sodium good initial plating and guide subsequent deposition behavior.At the current density of 0.5 m A cm^-2,the average Coulomb efficiency of the active material can be maintained at 99.5%after 1600 cycles,and Coulomb efficiency still can achieve 99%at 1900 cycles.The assembled symmetrical cell can cycle for 820 hours in a stable over-potential range of 15-20 m V.(2)The F-CNTs has been obtained by high temperature treatment,increasing the interlayer distance,allowing metal Na to be deposited more flexibly and freely under a larger current deposition density.In addition,the different fluorination temperatures can influence the properties of F-CNTs.When the active material is used as a negative electrode to deposit metallic Na,it can reach a charge deposition of 6 m Ah cm^-2,and can maintain 99%efficiency for up to 1500 cycles at a current density of 0.5 m A cm^-2.When the current density is increased to 1 m A cm^-2,the long cycle life can be maintained to about 800 cycles(average efficiency is 99%).Compared with O-CNTs,F-CNTs has high stability of 400 cycles at larger current density of 3 m A cm^-2,indicating that F-CNTs has a stronger affinity with Na and good stability when the deposition current density increases.The F-CNTs with a simple preparation process has very excellent development prospects as a negative material for sodium metal batteries.