Ionic liquid/boric ester binary electrolytes with unusually high lithium transference number

•Several imidazolium TFSI/FSI derivatives were found to be miscible with boric ester.•One of the binary electrolytes showed extraordinary high lithium transference number of over 0.9.•After fabricating anodic half-cell (Li/electrolyte/Si), specific discharge capacity exceeding 2500 mAhg− 1 was observed.

Binary electrolytes composed of ionic liquids and boric esters were prepared by studying compatibility between various combinations of such systems. The study showed that out of various combinations of ionic liquids/boric esters, only TFSI anion (or FSI anion) based ionic liquids/mesityldimethoxyborane (MDMB) systems were found to be miscible. After equimolar amount of lithium salts was added to ionic liquids, the resulting solution showed high ionic conductivity that was comparable to those for ionic liquids. The lithium transference number (tLi +) of these systems at room temperature was found to be very high. A maximum tLi + of 0.93 was observed for a binary mixture of AMImFSI [1-allyl-3-methylimidazolium bis(fluorosulfonyl)imide]/MDMB. Further, this binary mixture as electrolyte in Li/electrolyte/Si cell showed good reversible lithiation-delithiation with > 2500 mAh/g of delithiation specific capacity.

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