New Electrolytes for Lithium-ion Cells
Commercially available lithium-ion cells use an electrolyte containing a mixture of organic carbonate solvents combined with lithium hexafluorophosphate (LiPF6). These electrolytes have significant disadvantages limiting the performance and safety of lithium-ion batteries. First, the solvents are volatile and flammable, leading to safety issues during production, storage and use of batteries and their behavior under abuse conditions. Second, LiPF6 is not hydrolytically or thermally stable in organic carbonates, leading to degradation of electrolyte, rise in electrode/electrolyte interface impedance, dissolution of active cathode materials and limited battery life. Thirdly, present electrolyte solutions appear to be reactive towards cathode materials at high voltages, which contribute to battery performance deterioration. It also prevents further development of future higher energy cells with 5V cathode materials. Finally, present electrolyte formulations are always a compromise; no one mixture of the solvents has been shown to work well at both low and high temperatures. Performance and safety issues associated with LiPF6 salt in organic carbonates electrolytes are magnified in case of batteries for automotive applications, where the requirements for both performance and safety are more stringent due to the larger batteries used and longer battery calendar and cycle life expected. We propose a novel lithium-ion battery electrolyte composed of a thermally stable lithium imide salt and non-flammable non-carbonate solvents. Use of this advanced electrolyte is coupled by employment of graphite foil as a cathode current collector, which unlike aluminum, does not corrode in lithium imide solutions. Commercial Applications and Other Benefits: Success of this program will lead to new generation of lithium-ion cells. These new cells will be much safer, due to significantly improved stability and non-flammability of electrolyte, and operate longer, both with respect to calendar and cycle life at higher temperatures. Energy density of the cells will increase as the new electrolyte will allow the use of higher voltage electrode materials. The new cells will find use in battery packs for many applications, including HEV, PHEV and EV. Safer, longer lasting batteries will in turn help accelerate the growth of this market and the acceptance of these new vehicles in the marketplace.
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