Design And Properites Of Electrolytes For High-rate Sodium Metal Batteries At Wide Temeratures

Sodium-based batteries have attracted much attentions from researchers due to their low manufacturing cost and have been considered as ideal candidates for large-scale energy storage.With a high theoretical specific capacity(1166 mAhg^-1)and a low standard electrode potential（-2.71 V）,sodium metal anode is regarded as one of the most promising anodes for the next generation of high-energy sodium-based batteries.However,sodium metal batteries（SMBs）still face some problems such as irregular dendrite growth,gas production and unstable solid electrolyte interface（SEI）films before practical application.Although electrolyte modulation and electrode optimization have been adopted in recent years to alleviate these problems,the applications of high-rate SMBs at different temperatures still need to be addressed.This thesis selects conventional electrolyte firstly to investigate the failure mechanism of SMBs.Subsequently,the electrolyte components were optimized to enhance the high-rate performance of SMBs at low temperatures.The main contents are as follows:（1）Research on the failure mechanism of SMBs in conventional electrolytes:The classical ester 1 M NaClO₄-EC/PC（1:1 vol）,1 M NaCl O₄-EC/PC（1:1 vol）+5%vol FEC and ether 1 M NaOTF-DEGDME electrolytes were selected for the study on the failure mechanism of sodium metal electrodes.Electrochemical performance tests were carried out at different temperature and current density,and it was found that similar rules of electrochemical behavior existed in two electrolytes.At the same temperature,increased current density can raise the overpotential and shorten the cycle life;at the same current density,increased temperature can lower the overpotential.However,there are also apparent differences in the electrochemical behavior due to their different dielectric constants,freezing points and solvation structures.Compared with ester electrolytes,Na/Na symmetric batteries with ether electrolyte have a lower overpotential and longer cycle life.The coordination number between Na⁺and anion is relatively high caused by the low dielectric constant of the ether electrolyte.Moreover,the generation of a solid SEI film enhances the compatibility with sodium metal.The ether electrolyte also has a lower freezing point which makes it more suitable for the study of low-temperature high-rate SMBs.（2）Research on low-temperature high-rate SMBs:Adjusting the Na⁺solvation structure through the weak solvation strategy,the desolvation barrier of Na⁺at the electrode surface has been broken,enabling stable cycling of SMBs at high-rate under low temperatures.Electrolytes with different volume ratios（tetrahydrofuran/dimethoxyethane,THF/DME）and salt concentrations（NaOTF）were prepared.Spectral characterization and theoretical calculations verify that the additive THF can produce an anion-dominated solvated structure in the electrolyte.In consequence,the Na/Na symmetric batteries with 0.8 M NaOTF-THF/DME（3:1 vol）could achieve high-rate performance at low temperatures(3 m A cm^-2 at-20°C and 2 mA cm^-2 at-40°C)and a cumulative cycle capacity of 1350 mAhcm^-2 at-20°C and 285.5 m Ah cm^-2 at-40°C respectively.The superlative performance surpasses the previously reported alkali metal symmetric batteries.Subsequently,the anion-derived Na F-rich SEI film generated on the surface of the sodium metal is also analyzed by XPS,and SEM also reveals the suppression of dendrite growth at sodium electrode even under-60°C.Furthermore,the Na-Na Ti₂（PO₄）₃（NTP）full batteries can operate stably in extremely low temperature at-60°C,which is fully proved that this strategy provides effective technical support and new ideas for realizing the high-rate performance of low-temperature alkali metal batteries.