Research On The Sodium Storage Performance Of Hard Carbon In Ether And Ionic Liquid-based Electrolytes

As the demand for large-scale batteries for electrical energy storage is increasing,sodium-ion batteries have attracted a lot of attention due to the abundance,cost-effectiveness of sodium resources and resemblance with lithium.In the key material selection of sodium ion battery,hard carbon as the most likely commercial anodes has been widely studied in sodium ion battery by optimizing its structure.Electrochemical performance and safety of a sodium-ion battery are affected by the electrolyte,which not only decides the electrochemical window and energy density,but also controls the electrode/electrolyte interfaces.In this paper,from the perspective of organic electrolyte and ionic liquid electrolyte of sodium ion battery,the sodium storage performance of hard carbon in ether electrolyte and ester electrolyte as well as novel ionic liquid combined with organic solvent as mixed electrolyte in hard carbon electrode at high temperature were studied.Phenolic resin based hard carbon was prepared by direct carbonization method as the anode of sodium ion battery,which has short-range ordered amorphous structure and suitable graphitelike microcrystalline layer spacing for sodium storage?0.402 nm?.Kinetic and storage behavior differences of hard carbon electrodes in ether?1M Na PF₆/Diglyme?and ester?Na PF₆/EC:DMC and Na Cl O₄/EC:DEC?electrolytes and characteristics of solid electrolyte interface?SEI?were studied by CV with different scanning rates,XPS,GITT and Ex-situ EIS.In ether-based electrolyte,compact,uniformity,thinner solid electrolyte interphase was formed on the anode,which provides a shorter path for the diffusion of sodium ions.Especially under low potential?<0.1 V?,the energy storage behavior of the ether electrolyte system tends to be capacitive.The rapid diffusion of sodium ions significantly optimizes the reversible electrochemical reaction in the plateau and increases the capacity at plateau.The Ex-situ EIS proved that the impedance at solid electrolyte interface of ether system was much less than that of ester system during the first charge and discharge.Finally,excellent electrochemical performance was obtained:the reversible specific capacity could reach 416 m Ah g^-1?initial coulombic efficiency 90%?at 0.1C current density,and the specific capacity was 355 m Ah g^-1after 300 cycles?capacity retention rate was 98%,0.4 C?.In order to extend the operating temperature range for organic electrolyte,we synthesized a kind of fluorinated ether imidazole ionic liquids,which mixed with ethers and esters solvents.The ionic liquid mixed with ether solvent system shows high ionic conductivity and thermal stability,reflects the low overpotential and polarization in the process of charging and discharging,and presents the best electrochemical performance.Phenolic resin based hard carbon anode can steadily work at 353 K with a specific capacity of 380 m Ah g^-1?0.1 C?and 335 m Ah g^-1?capacity retention rate of 98%?after 100 cycles at 0.4 C.