The Development of Electrolytes with Flame Retardant Additives for Multiple Lithium-Ion Chemistries
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 LiNi0.8Co0.2O2 and LiNi0.8Co0.15Al0.05O2 –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-LiNi0.8Co0.2O2 and graphite-LiNi0.8Co0.15 Al0.05O2) as well as in advanced lithium-ion chemistries that possess high capacity anodes and cathodes (i.e., Si/C-LiNi0.8Co0.15Al0.05O2 and graphite-Li1.2Ni0.13Co 0.13Mn0.54O2). 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.).
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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