EFFORTS to develop a high-voltage, lightweight rechargeable battery for electrically powered vehicles have focused on batteries based on solid electrolytes, which offer good mechanical strength, easy packaging and high energy densities. An important objective is to identify an electrolyte with the desired combination of mechanical properties, electrical conductivity and stability against powerfully oxidizing and reducing electrodes (lithium is preferred for the anode). Among the most promising materials are rubbery 'salt-in-polymer' electrolytes and highly conducting but brittle superionic glass electrolytes. In the latter category are salts with good lithium-ion conductivity, which are compatible with lithium-anode systems. Here we describe new ionic conductors - 'polymer-in-salt' materials - in which lithium salts are mixed with small quantities of the polymers polypropylene oxide and polyethylene oxide. These materials have glass transitions low enough to remain rubbery at room temperature while preserving good lithium-ion conductivities and high electrochemical stability.
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