Other types of electrolytes based on highly concentrated aqueous salt solutions (WiSE), gels and polymers

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To minimize the ecological implications associated with battery use, we need to choose chemistries with a minimum footprint on nature. This is why efforts are currently being made on aqueous electrolytes, with the aim of enlarging the electrochemical window of water, which is only 1.23 V, to provide a cheaper, safer and non-toxic alternative to organic electrolytes. This desire was actually realized through the use of salt-concentrated aqueous electrolytes (24 M Li-TFSI), known as WiSE (Water in Salt Electrolyte), which can operate at potentials in excess of 3 V.

This lecture has attempted to put these new electrolytes(e.g. a mixture in which the solute outweighs the solvent in terms of weight and volume) into perspective. A quick review of the literature showed us that this concept is not new, having been introduced by A. Angel in 1993 as "Polymer in salt", with the subsequent observation of beneficial effects in terms of improved cathodic and anodic stability of non-aqueous electrolytes with a high salt content.

Subsequently, the science behind this phenomenon, which lies in the modification of the inner Helmholtz layer both in nature and composition, was described. Thus, during reduction, ^TFSI- anions migrate preferentially to the interface where they decompose, leading to the formation of an insoluble LiF layer, thus postponing water reduction to a lower potential.

On the other hand, the origin of the simultaneous shift of the oxidation wall to higher potentials remains highly conversed to this day. In pursuit of this concept, we have highlighted the possibility of producing aqueous _LiMn2O4/Li cells capable of operating at 4 V, by combining a salt-concentrated aqueous electrolyte (WiSE) with a specific surface treatment via the deposition of an amorphous, highly hydrophobic sheet based on immiscible fluorinated molecules.

Less widely publicized at present, but closer to the applied field, are gel electrolytes and all-solid polymers. The latter are at the origin of the Li-polymer battery, which has to operate at 55°C due to the limited ionic conductivity of the polymer matrix, and powers the "Blue Cars", an integral part of the AUTOLIB project.

Recent improvements have been discussed, via the use of tri-block copolymers to which the ^TFSI- anion is attached, thus increasing the number of transports (_tLi+), resulting in better ionic conductivity while improving mechanical properties.