Sodium secondary batteries are promising devices owing to the natural abundance of sodium resources. The use of ionic liquid electrolytes can solve the safety problems associated with sodium secondary batteries, especially for large-scale applications. However, its high viscosity is unsuitable for high rate capability batteries. In an attempt to reduce the viscosity of ionic liquids, electrolyte mixtures of ionic liquid with molecular organic have been studied [1,2]. It was found that small amount of organic solvent reduces significantly the viscosity, meanwhile, the electrochemical properties of mixtures keep similarly to the pure ionic liquid. As some survey reported, combination of butylmethylpyrrolidinium bis(trifluoromethane-sulfony) imide (BMPTFSI) with various Na solutes namely NaTFSI, NaClO₄, NaBF₄ conducted on NaFePO_4, Na_0.44MnO₂ as cathode materials enhanced energy density and specific capacity up to 125 mAh.g^-1 maintained within 20 cycles for sodium batteries [3,4]. Therefore, other ionic liquids also based on anion bis(trifluorome- thanesulfony)imide such as N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfony) imide (Py₁₄TFSI), 1-ethyl- 3-methylimidazolium bis(trifluoromethane-sulfony) imide (EMITFSI) are considered as promising candidate and used as co-solvent with EC:PC (1:1 in vol.) for secured batteries [5–8].

In this study, electrolyte mixtures based on EMITFSI and Py₁₄TFSI with organic solvent have been performed to evaluate compatibility of these ionic liquids with respect to sodium metal anode and some different cathode materials (Na_xMn_1/3Co_1/3Mn_1/3O₂,Na₃F₂V₂(PO₄)₃, Na_0.44MnO₂). Moreover, the thermal stability and the conduction mechanism of electrolyte mixtures will be explored within the diffusion coefficient and sodium transport number. The stabilization of electrode – electrolyte interface could be enhanced by using some different additives: succinic/maleic acid…Interestingly, the coulombic efficiency of sodium metal electrodeposition/dissolution is improved, especially at the elevation of temperature.

References:

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Acknowledgements:

This research is funded by VietNam National Foundation for Science and Technology Development supported (NAFOSTED) under grant number 104.03-2012.46.