Lithium ion batteries are a key energy storage technology for electric vehicles and mobile portable devices. The related electrolyte materials are intensely investigated with respect to improvements in ionic conductivity, stability and safety. Solid polymer electrolytes represent a promising alternative, because of their compatibility with large-scale manufacturing processes, mechanical stability and toughness and good electrode adhesion, and improved safety. However, their low lithium ion conductivity, particularly at room temperature, has so far limited their applications. Ion conduction in solid polymer electrolytes is coupled to the segmental motions of the polymer chains, which is particularly slow for crystalline polymers and polymers with low glass transition temperatures. For polyethers such as polyethylene oxide (PEO), Li ion conduction occurs via coordination with the oxygen atoms by intersegmental hopping from one coordination site to another. Since PEO is crystalline below its melting temperature of ca. 60 °C, its segmental mobility at room temperature is very low, limiting its applications.

We systematically investigated the Li ion conductivity of PI-PS-PEO and PB-PEO block copolymers loaded with LiTFSI-salt at high salt loadings from [Li:EO] = 0.1 – 10. We observe that super-stochiomeric addition of LiTFSI leads to the formation of a crystalline PEO/LiTFSI-phase that shows super-ionic conductivity. Concomitantly, the addition of LiTFSI increases the PEO-volume fraction in the block copolymers, thereby inducing a phase transition from hexagonally ordered cylinders to the bicontinuous gyroid phase. The crystalline superionic PEO/LiTFSI phase within the bicontinuous gyroid phase leads to solid electrolyte block copolymer membranes showing exceptionally high Li ion conductivities. We find ion conductivities of > 10 mScm^-1 at 0°C, and values of > 100 mScm^-1 at 80°C, which are in the range of the best inorganic Li ion conductors such as LGPS. The very low activation energies, in the range of 0.2 – 0.3 eV, the sharp increase of the ion conductivity at superstochiometric addition, and the narrow relaxation time distribution as measured by ⁷Li-NMR indicate a super-ionic conduction mechanism in the solid polymer electrolyte. Therefore, these materials open the way for solid polymer electrolytes with sufficiently high ion conductivites for commercial applications.

Stephan Foerster

University of Bayreuth

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