Recent Progress with Ethyleneoxy Phosphazenes as Lithium Battery Electrolytes
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1127-T01-05
Recent Progress with Ethyleneoxy Phosphazenes as Lithium Battery Electrolytes
Shih-To Fei and Harry R. Allcock* Department of Chemistry, The Pennsylvania State University University Park, PA 16802, U.S.A.
ABSTRACT We report here a series of high-conductivity liquid- and gel-electrolytes based on a mixture of ethyleneoxy phosphazenes and propylene carbonate, which have potential applications in lithium battery assembly. Phosphazene compounds are known to be good materials for solid polymer lithium electrolytes and, following that idea, we have developed lowvolatility liquid phosphazene electrolytes that maintain 10-4 S/cm conductivity. A high conductivity of 10-3 S/cm can be achieved when polymeric phosphazenes are used as solidifying agents for conventional battery electrolytes. We also report here the fire retardant properties of the materials, which may lead to future use of these materials as fire retardant additives.
INTRODUCTION The phosphorous-nitrogen compounds known as phosphazenes have a wide range of applications due to their synthetic flexibility and organic-inorganic structure. [1] In particular, the electrochemical / radical stability of these compounds naturally lead to their potential as components for energy storage and production devices, such as batteries and fuel cells. One of the most interesting applications for phosphazene compounds is in lithium battery electrolytes. Lithium batteries are electrochemical devices that generate electricity from lithium sources and are noted for high energy density, which makes them especially suitable for applications where lightweight or compact power sources are required, such as portable electronic devices and electrical vehicles. [2] A number of phosphazene – based lithium battery electrolytes have been reported in the past, mainly focused on solid electrolyte systems. [3-5] These types of materials have shown good performance in assembled-cell conditions and are known to be compatible with other battery components such as carbon and metal oxide electrodes [1,6] while maintaining better performance compared to many of the existing solid electrolytes. On the other hand, the potential of phosphazene compounds as gel electrolyte components has not been as thoroughly explored. [7][8] Similarly, liquid electrolyte systems are part of another field where phosphazene research is lacking. Since phosphazenes have high thermal stability, low volatility, and exhibits fire retardant behavior, it is possible to develop component materials that are resistant to leaks, pressure buildup and fire – the most common problems encountered for battery systems. [2,9] Among the possible molecular designs, the species with ethyleneoxy side groups deserve special attention, as this family of materials is known to maintain a liquid state as oligomers and is known to have higher conductivity compared to similarly structured poly (ethylene oxide) (PEO), mostly due to its low crystallinity and main chain flexibility. The materials are also known to
have good compatibili
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