Study On The Preparation And Electrochemical Properties Of High-performance Anode Materials For Sodium Ion Batteries

Sodium ion battery becomes an ideal next generation large-scale energy storage battery to replace lithium-ion battery due to the abundant reserves of sodium,low price,together with similar physical and chemical properties to lithium.However,the radius of sodium ion is much larger than that of lithium ion,which results in large volume change of sodium ion in the process of embedding into the material lattice.Therefore,we need to develop related materials suitable for sodium ion batteries instead of directly using commercial lithium-ion battery materials.So far,a variety of new high-performance anode materials for sodium ion batteries have been widely studied by researchers.Among them,the hard carbon material has rich pore structure and unique microcrystalline graphite structure,which is considered to be an ideal anode material for sodium ion batteries.However,there are still some problems to be solved,such as low initial coulomb efficiency（ICE）,poor rate performance,and the controversial mechanism of sodium storage.Biomass materials are considered as high-quality carbon sources for hard carbon materials because of their low price,easy processing and wide sources.Therefore,in this thesis,the source of biomass materials,the structural regulation and energy storage mechanism of materials were studied.Using tea residue and waste paper as carbon source,hard carbon material was obtained by high temperature carbonization.The effects of biomass types,preparation conditions and microstructure on the electrochemical performance of sodium ion batteries were systematically investigated by various physical characterization and electrochemical testing methods.The specific research contents are as follows:（1）A series of pyrolyzed waste tea（PWT）hard carbon materials with unique microcrystalline graphite structure were prepared by carbonizing tea slag at different temperatures（1000℃～1600℃）.SEM,HRTEM and XRD were used to characterize the microstructure and graphitization degree of the materials.The results show that the synthesized material has rich pore structure,which can effectively promote the penetration of electrolyte to the inner surface and improve the ion transfer rate.With the increase of pyrolysis temperature,the graphite microcrystalline structure increases gradually,which is conducive to the rapid insertion and removal of sodium ions.In addition,the electrochemical test results indicate that the hard carbon material（PWT-1400）prepared at 1400℃presents the best sodium storage performance,including the highest reversible specific capacity,good rate performance and stable cycle performance.At 0.1 C current density,the first reversible specific capacity of PWT-1400 reaches 282.4 m Ah g^-1,corresponding to 69%ICE;after 200cycles at 0.2 C,the capacity still maintains 173.7 m Ah g^-1.（2）Hard carbon microfiber（HCF）with low specific surface area was prepared by pretreatment and high temperature carbonization（1000℃～1600℃）using printing waste paper as carbon source.The effect of heat treatment temperature on microstructure and electrochemical properties of the material was investigated,and the mechanism of sodium storage was systematically studied.SEM results show that the microstructure of HCF is characterized by the fiber structure with grooves and channels on its surface,which is conducive to the diffusion and migration of sodium ions.TEM,XRD and Raman results show that the degree of graphitization increases with the increase of carbonization temperature.These structural characteristics endow the materials with excellent electrochemical properties.The reversible specific capacity of HCF obtained by carbonization at 1400℃（HCF-1400）is up to 319.6 m Ah g^-1 at 0.1 C current density,and the reversible capacity of 200.6 m Ah g^-1 is still maintained after 100 cycles at 0.2 C,corresponding to 99.3%capacity retention.Ex-situ HRTEM shows that the mechanism of sodium storage is:the plateau specific capacity corresponds to the insertion of sodium ions in the carbon layer,and the slope specific capacity is attributed to the adsorption and filling of sodium ions in the defects and pores.Finally,HCF-1400 was used as the anode and Na₃V₂（PO₄）₃ as the cathode to assemble the full cell,and its comprehensive potential in practical application was explored.It opens up a broad prospect for the application of hard carbon materials in the anode materials of sodium ion batteries.