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

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Rechargeable lithium batteries have long been considered an attractive alternative power source for a wide variety of applications. Safety and stability1 concerns associated with solvent-based electrolytes has necessitated the use of lithium intercalation materials (rather than lithium metal) as anodes, which decreases the energy storage capacity per unit mass. The use of solid lithium ion conductors—based on glasses, ceramics or polymers—as the electrolyte would potentially improve the stability of a lithium-metal anode 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 similar to that in solvent-based electrolytes—motion is mediated by the dynamics of the host polymer, thereby restricting the conductivity to relatively low values. Moreover, in the polymer systems, the motion of the lithium ions provides only a small fraction of the overall conductivity2, which results in severe concentration gradients during cell operation, causing premature failure3. Here we describe a class of materials, prepared by doping lithium ions into a plastic crystalline matrix, that exhibit fast lithium ion motion due to rotational disorder and the existence of vacancies in the lattice. The combination of possible structural variations of the plastic crystal matrix and conductivities as high as 2 × 10-4 S cm-1 at 60 °C make these materials very attractive for secondary battery applications.

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Figure 1: Structure of the alkylmethylpyrrolidinium imide matrix compounds.
Figure 2: Ionic conductivity as a function of temperature for P11 and P12 solid phases doped with Li+.
Figure 3: Dependence of conductivity on Li content in the P12-Li ‘imide’ binary system.
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Correspondence to Douglas R. MacFarlane.

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MacFarlane, D., Huang, J. & Forsyth, M. Lithium-doped plastic crystal electrolytes exhibiting fast ion conduction for secondary batteries. Nature 402, 792–794 (1999) doi:10.1038/45514

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