Ambient temperature plastic crystal fast ion conductors (PLICFICS)

Polymer-salt type solid electrolytes are currently in favor because the electrolyte can change shape under mechanical stress without failure. A disadvantage, however, is that anion transport numbers are not zero. We think the former advantage can be had without the latter disadvantage by using an appropriate anion rotator phase in which the cation, or one of the cations, is mobile. Li₂SO₄ is the prototype material of this type but is limited to high temperature applications. We describe a low temperature material - a double salt of LiBF₄ and methoxyethyldimethyl ethyl ammonium - fluoroborate - which may conduct by the same mechanism as Li₂SO₄ since it exhibits a strong disordering transition at -90°C, and a relatively small entropy of fusion. At 75°C this substance conducts almost as well as the best polyethylene oxide + Li salt combination reported to date. Some variants on the structural theme and their properties and performance will also be described.