Promising electrolytes are identified.
Given that lithium-ion (Li-ion) technology is the most viable rechargeable energy storage device for near-term applications, effort has been devoted to improving the safety characteristics of this system. Therefore, extensive effort has been devoted to developing non-flammable electrolytes to reduce the flammability of the cells/battery. A number of promising electrolytes have been developed incorporating flame-retardant additives, and have been shown to have good performance in a number of systems. However, these electrolyte formulations did not perform well when utilizing carbonaceous anodes with the high-voltage materials. Thus, further development was required to improve the compatibility.
A number of Li-ion battery electrolyte formulations containing a flame-retardant additive [i.e., triphenyl phosphate (TPP)] were developed and demonstrated in high-voltage systems. These electrolytes include: (1) formulations that incorporate varying concentrations of the flame-retardant additive (from 5 to 15%), (2) the use of mono-fluoroethylene carbonate (FEC) as a co-solvent, and (3) the use of LiBOB as an electrolyte additive intended to improve the compatibility with high-voltage systems. One of the promising electrolytes identified of the group investigated is l.0M LiPF6 in EC+EMC+TPP (20:70:10 vol %) + 0.15M LiBOB, which was demonstrated to have comparable performance to that of the baseline ternary electrolyte in MPG- 111/Toda (LiNiMnCoO2) coin cells, in terms of reversible capacity and discharge rate capability at room temperature. Thus, improved safety has been provided without loss of performance in the high-voltage, high-energy system.
The use of higher concentrations of the flame-retardant additive is known to reduce the flammability of the electrolyte solution, with 15% concentration resulting in solutions of substantially reduced flammability. Thus, the desired concentration of the flame-retardant additive is the greatest amount tolerable without adversely affecting the performance in terms of reversibility, ability to operate over a wide temperature range, and the discharge rate capability. The use of FEC was used to reduce the inherent flammability of mixtures and improve the compatibility at the interfacial regions, due to desirable surface reactions.
This work was done by Marshall C. Smart and Ratnakumar V. Bugga of Caltech, and G.K. Surya Prakash and Frederick C. Krause of the University of Southern California for NASA’s Jet Propulsion Laboratory.
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