Organic Vs. Inorganic Cations of Alkali-Metal Halide Containing Ionic Liquids for Sodium-Ion Battery Applications

The power generation from either fossil fuels or preferably renewable energy sources is on the rise throughout the world.  The pursuit of electrochemical devices for energy storage during peak power grid utilization is viable.  There is great interest in further developing sodium batteries, particularly ZEBRA-type cells with chloroaluminate electrolytes.  This lab has worked on understanding of weak-field, low-melting metal-halide based ionic liquids for use in the sodium-ion conducting batteries at a low-temperature of operation.  This undertaking employed both organic and inorganic cations of low charge density, including the much-studied dialkylimidazolium cation.  The vital issue of decreased ionic conductivity caused by alkali ion-trapping in these electrolytes, long known as a qualitative effect was recently quantified, discussed, and reported by us.  We have since compared the conductivities of binary bromoaluminates in which large inorganic cations PBr₄⁺ formed by a bromide transfer process were used to replace the former dialkylimidazolium cation and here show the differences in the figure below.  Currently, the electrochemical (along with other physical) properties are being characterized in preparations for testing in ZEBRA-type cells.  This study leads to improved understanding of the factors determining electrolyte and electrochemical performance of sodium-containing binary electrolytes over a range of temperatures.  

Figure:  The top set of three conductivity isotherms are for [EMIM]AlCl₄ - NaAlCl₄ solutions, and the bottom set of three are for PBr₄Al₂Br₇ - NaAl₂Br₇ solutions of varying molar fractions.  The completely inorganic electrolyte requires 100^oC (top blue line isotherm) to attain the ionic conductivity of the electrolyte containing the organic cation at 60^oC (mid black line isotherm).  We attribute the difference to incomplete bromide transfer in the formation of the large inorganic cation of the second system.  The “alkali cation-trapping” effect probably remains in effect.