The Development of Electrolytes with Flame Retardant Additives for Multiple Lithium-Ion Chemistries

A number of applications under NASA’s Exploration Technology Development Program would benefit from high specific energy rechargeable batteries that have improved safety characteristics.  To meet these objectives, lithium-ion battery chemistries have emerged as the most promising energy storage devices, due to their high specific energy, long life, and ability to operate over a wide temperature range.  In recent years, advances in anode and cathode materials have created a number of lithium-ion chemistries that that have the potential of exceeded 150 Wh/kg at the cell level.  However, owing to the fact that these chemistries employ the use of organic electrolytes, possessing flammable solvents, there is a desire to improve the safety characteristics of these devises. The flammability of these electrolytes is mainly due to the use of cyclic and linear organic carbonates, which are necessary to provide (i) sufficient conductivity of the media with the use of lithium electrolyte salts, (ii) adequate stability over a wide voltage range, and (iii) the formation of protective surface films at the electrolyte/electrode interfaces.

A number of approaches can be employed to reduce the inherent flammability of the lithium-based electrolyte systems, including (i) the use of phosphorus-based flame retardant additives (FRAs), (ii) the use of ionic liquids, which possess extremely low vapor pressures, and (iii) the use of non-flammable co-solvents, such as fluorinated ethers, esters, and carbonates.  Although the use of ionic liquids and highly fluorinated co-solvents has led to electrolyte formulations that can display dramatically lower flammability, these approaches typically suffer from poor compatibility with the electrode chemistries of interest and lead to a sacrifice in performance.  In contrast, the use of flame retardant additives has proven to be a viable way of reducing the flammability of the electrolytes without compromising the performance of the system. Toward this end, a number of flame retardant additives have been identified in traditional lithium-ion chemistry with success, including triphenyl phosphate (TPP)¹, tris(2,2,2-trifluoroethyl) phosphate², and  dimethyl methyl phosphonate (DMMP).³  After comprehensively investigating the compatibility of a number flame retardant additives in various chemistries, we have identified triphenyl phosphate (TPP) as being the most versatile, being compatible with traditional LiNi_0.8Co_0.2O₂ and LiNi_0.8Co_0.15Al_0.05O₂ –based systems as well as with high voltage, lithium excess, mixed metal oxide-based cathode systems.^4,5

We would like to describe our recent results with TPP-containing electrolytes in both traditional lithium-ion chemistries (i.e., MCMB carbon-LiNi_0.8Co_0.2O₂ and graphite-LiNi_0.8Co_0.15 Al_0.05O₂) as well as in advanced lithium-ion chemistries that possess high capacity anodes and cathodes (i.e., Si/C-LiNi_0.8Co_0.15Al_0.05O₂ and graphite-Li_1.2Ni_0.13Co _0.13Mn_0.54O₂).  Electrolyte formulations have been specifically tailored for each chemistry to achieve the desired benefit to safety without compromising performance.  This was achieved by optimizing the FRA content, the use of fluorinated co-solvents in conjunction with traditional carbonate-based solvents, and the use of film forming electrolyte additives.   The compatibility of the electrolytes with the various chemistries were investigated in experimental coin and three-electrode cells (equipped with lithium reference electrodes), which enable us to perform a number of electrochemical characterization techniques including EIS, DC micropolarization, and Tafel polarization. We will also describe the electrical performance of the most promising electrolyte formulations in prototype lithium-ion cells of varying capacity (0.25Ah to 8.0 Ah) manufactured by various battery vendors (Quallion, LLC, Yardney Technical Products, Inc.  and Saft America, Inc.).

ACKNOWLEDGEMENT

The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and under sponsorship of the NASA-Exploration Technology Development Program (ETDP).

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