Ionic Liquid/Borane Binary Electrolyte with Remarkably High Lithium Transference Number
Authors: Matsumi Noriyoshi , Matsui Kento , Tamaru Emari , Vedarajan Raman 
Published: February 2020

Design of organoboron electrolytes has been a fruitful method to improve ion conductive properties of various electrolyte matrices [1-7]. Due to boron-anion interaction, either enhanced lithium transference number or improved dissociation degree of lithium salt was observed depending on the strength of boron-anion interaction. In order to enhance lithium transference number of ionic liquid based electrolytes, various organoboron molten salts in which organoboron unit and ionic liquid structure were covalently tethered were reported [6-7]. Such molecular designs were carried out because most ionic liquids were not miscible with typical tricoordinate boron compounds such as trimethoxyborane.

Very recently, we have systematically studied miscibility of various ionic liquids (1) with boron compounds (2 and 3), and it was found that some types of ionic liquids are miscible with mesityldimethoxyborane [Mes(OMe)2] (3). Although immediate phase separation was observed under most combinations of IL/boron compounds, homogeneous solution was readily obtained by mixing ionic liquids (imidazolium TFSI based ILs and some other types) with mesityldimethoxyborane. After mixing, the resulting homogeneous solution showed significantly lower viscosity in comparison with those for ionic liquids themselves. Further, 1:1 volume mixtures of ionic liquids/boron compounds showed high lithium transference number of over 0.5 at ambient temperature. The properties of 1:2 volume mixtures (IL: B) were also studied, and these mixtures showed both higher ionic conductivity and lithium transference number at the same time. For instance, lithium transference number for AMImTFSI /Mes(OM2)2 = 1/2(v/v) reached 0.86, which was extraordinary high as liquid electrolyte (Figure 1). However, further increase in boron content did not result in higher lithium transference number possibly due to unselective motions of ions under very low viscosity of electrolyte. In spite of restricted carrrier ion number under very high lithium transference number, binary electrolytes retained high ionic conductivity of around 10-3 Scm-1 at 51oC. These binary electrolytes also showed relatively wide electrochemical window (from 4 to 5 V). Moreover, successful charge-discharge profiles were obtained after fabricating anordic half cells with moderate coulobmic efficiency.

In conclusion, we developed facile methodology to obtain highly conductive IL/boron binary electrolyte with remarkably high lithium transference number. As far as we know, all the organoboron molten salts reported so far was solid at room temperature. The obtained liquid binary solutions exhibited far better ionic conductivity and lithium transference number at the same time than those for previously reported organoboron molten salts. Furthermore, their moderate potential window (from 4 to 5 V) allowed stable charge-discharge behavior of anodic half cell fabricated using these electrolytes.

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Figure 1