Study On The Preparation And Electrochemical Properties Of Hard Carbon Anode Materials For Sodium Ion Batteries

In recent years,sodium-ion batteries（SIBs）have been considered as the most prospective supplement to lithium-ion batteries（LIBs）for future application of electric vehicles,energy storage power stations and large-scale energy storage due to the advantages of abundant sodium resources,low cost and high safety performance.Looking for advanced anode materials with superb electrochemical performance is supposed to be a crucial task for the development of sodium-ion batteries technology.Among various anode materials,commercial graphite anodes which have a widespread use in LIBs exhibit poor sodium storage performance due to the instable thermodynamics of Na C₆ intercalation compounds formed in ester-based electrolyte.Thus,it is urgent to explore suitable anode materials for SIBs,which similar to the graphite anodes of LIBs with the advantages of low cost,simple synthesis and excellent electrochemical performance.Among diverse anode materials that have been reported,hard carbon materials are regarded as the most promising anode material for commercial SIBs because of its abundant source,simple synthesis,low cost,low sodium-embedded platform and high sodium storage capacity.However,hard carbon still suffers the drawbacks of low initial coulombic efficiency（ICE）,poor rate performance and terrible cycle stability,which further hinder its commercialization process.In order to address above critical challenges,plenty of efforts have been devoted in this article to improve the electrochemical performance of hard carbon anode materials,consisting of tailoring the microstructure,optimizing electrolyte and coating functional shells.The main contents can be summarized as follows:（1）Employing the phenolic resin and lignin as carbon precursors to build up the intermediate carbon precursor with cross-linked network structure,the structure of the intermediate carbon precursor can be further controlled by changing the mass ratio of the phenolic resin and lignin.In addition,the defect sites,the interlayer distance and specific surface area of the coupling hard carbon can also be controlled through changing the carbonization temperature.With assistance of materials characterization and electrochemical measurements,the results prove that under the mass ratio of 1:1,the hard carbon materials carbonized at 1400°C have the most suitable defects,pores and interlayer distance,thus showing the best electrochemical performance.The reversible capacity of the first cycle is 354.1 m A h g^-1 at the current density of 0.1 C,and the ICE is 87%.After 500 cycles,the reversible capacity is 338 m A h g^-1 with 95.5%capacity retention.It also shows excellent electrochemical performance even at a high current density.It exhibits a reversible capacity of 268 m A h g^-1 after 200 cycles at the current density of 1 C and 205 m A h g^-1 at the current density of 6 C.（2）Based on the above works,two different kinds of electrolytes system are built up in tests.The ether electrolyte is 1 M Na PF₆ in DEGDME and the ester electrolyte is0.8 M Na PF₆ in a mixture of EC and DMC.The influence of electrolyte systems on the coupled hard carbon interface about the structure of SEI and the evolution of component is detected by using SEM,XPS and kinetic measurement,thus further exploring the relationship between SEI layer that derived from different electrolyte systems and the electrochemical performance of coupled hard carbon.The results indicate that he ether-based electrolyte have higher sodium-ion diffusion kinetics.The SEI layer derived from ether-based electrolytes is mainly composed of polyether and sodium/alkoxy compounds,which is conducive to the formation of thin and compact SEI,and the SEI layer can still maintain its original thin and dense state during cycling.The SEI layer derived from ester-based electrolyte is mainly composed of seven kinds of polyesters（C-H,C=O,C-OH）and sodium/alkyl carbonates（RCO₃Na,C-H）,and the SEI layer derived from ester-based electrolytes is too thick and grows continuously,which increases the interfacial impedance.It leads to the deterioration of the kinetics of sodium ion transport at the interface.Therefore,the hard carbon exhibits high ICE（86%）,high specific capacity(0.1 C@350 m A h g^-1),and excellent rate performance(6C@203 m A h g^-1)in the ether-based electrolyte.（3）Coating functional shells on the surface of hard carbon to improve the electrochemical properties and interface stability.A uniform nano-dopamine carbon layer is coated on HC surface by liquid phase polymerization and high temperature carbonization.The physical and chemical properties of dopamine carbon layer are analyzed by SEM,TEM,XRD,Raman and XPS tests,and the following conclusions are obtained by combining the SEM,Raman and XPS tests with different cycles:The dopamine carbon layer is an amorphous,thin,oxygen rich,and nitrogen-containing carbon layer;After modification,the surface defects of hard carbon are significantly reduced;The SEI layer has more inorganic components,which is beneficial to improve the stability of the liquid-solid interface and improve the cycle stability of the battery.At the same time,the stable SEI layer can effectively inhibit the decomposition of electrolyte.The modified hard carbon has a reversible capacity of 299.7 m A h g^-1 for the first cycle at a current density of 0.1 C,and the ICE is as high as 94%.It still has a reversible capacity of 247.1 m A h g^-1 after 800 cycles at a current density of 1 C with87.3%capacity retention.In addition,the reversible capacity is 243 m A h g^-1 at the current density of 6 C.