Lithium-doped plastic crystal electrolytes exhibiting fast ion conduction for secondary batteries

Rechargeable lithium batteries have long been considered an attractive alte rnative power source for a wide variety of applications. Safety and stabili ty(1) concerns associated with solvent-based electrolytes has necessitated the use of lithium intercalation materials (rather than lithium metal) as a nodes, which decreases the energy storage capacity per unit mass. The use o f solid lithium ion conductors-based on glasses, ceramics or polymers-as th e electrolyte would potentially improve the stability of a lithium-metal an ode while alleviating the safety concerns. Glasses and ceramics conduct via a fast ion mechanism, in which the lithium ions move within an essentially static framework. In contrast, the motion of ions in polymer systems is si milar to that in solvent-based electrolytes-motion is mediated by the dynam ics of the host polymer, thereby restricting the conductivity to relatively low values, Moreover, in the polymer systems, the motion of the lithium io ns provides only a small fraction of the overall conductivity(2), which res ults in severe concentration gradients during cell operation, causing prema ture failure(3). Here we describe a class of materials, prepared by doping lithium ions into a plastic crystalline matrix, that exhibit fast lithium i on motion due to rotational disorder and the existence of vacancies in the lattice. The combination of possible structural variations of the plastic c rystal matrix and conductivities as high as 2 x 10(-4) S cm(-1) at 60 degre es C make these materials very attractive for secondary battery application s.