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Increasing the conductivity of crystalline polymer electrolytes


Polymer electrolytes consist of salts dissolved in polymers (for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows1,2,3,4,5. For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer:salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6:LiXF6 (X = P, As, Sb); however, the conductivities were relatively low6,7. Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6- anions in the crystal structure with isovalent N(SO2CF3)2- ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-xIx ionic conductors8. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

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P.G.B. is indebted to The Royal Society, the EU and the EPSRC for financial support.

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Correspondence to Peter G. Bruce.

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Further reading

Figure 1: Conductivity of PEO6:(LiAsF6)1-x(LiTFSI)x.
Figure 2: X-ray powder diffraction patterns of PEO6:(LiAsF6)1-x(LiTFSI)x.
Figure 3: DSC of PEO6:(LiAsF6)1-x(LiTFSI)x.
Figure 4: Fragment of the crystal structure of PEO6:(LiAsF6)1-x(LiTFSI)x.


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