Future NASA missions aimed at exploring Mars and the outer planets require rechargeable batteries that can operate at low temperatures to satisfy the requirements of such applications as landers, rovers, and penetrators. A number of terrestrial applications, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs) also require energy storage devices that can operate over a wide temperature range (i.e., –40 to +70 °C), while still providing high power capability and long life. Currently, the state-of-the-art lithiumion system has been demonstrated to operate over a wide range of temperatures (–30 to +40 °C); however, the rate capability at the lower temperatures is very poor. These limitations at very low temperatures are due to poor electrolyte conductivity, poor lithium intercalation kinetics over the electrode surface layers, and poor ionic diffusion in the electrode bulk.
Two wide-operating-temperature-range electrolytes have been developed based on advances involving lithium hexafluorophosphate-based solutions in carbonate and carbonate + ester solvent blends, which have been further optimized in the context of the technology and targeted applications. The approaches employed include further optimization of electrolytes containing methyl propionate (MP) and ethyl butyrate (EB), which are effective co-solvents, to widen the operating temperature range beyond the baseline systems. Attention was focused on further optimizing esterbased electrolyte formulations that have exhibited the best performance at temperatures ranging from -60 to +60 °C, with an emphasis upon im - proving the rate capability at –20 to –40 °C. This was accomplished by in - creasing electrolyte salt concentration to 1.20M and increasing the ester content to 60 percent by volume to in - crease the ionic conductivity at low temperatures.
Two JPL-developed electrolytes — 1.20M LiPF6 in EC+EMC+MP (20:20:60 v/v %) and 1.20M LiPF6 in EC+EMC+EB (20:20:60 v/v %) — operate effectively over a wide temperature range in MCMB-LiNiCoAlO2 and Li4Ti5O12- LiNiCoAlO2 prototype cells. These electrolytes have enabled high rate performance at low temperature (i.e., up to 2.0C rates at –50 °C and 5.0C rates at –40 °C), and good cycling performance over a wide temperature range (i.e., from –40 to +70 °C). Current efforts are focused upon improving the high temperature resilience of the methyl propionate-based system through the use of electrolyte additives, which are envisioned to improve the nature of the solid electrolyte interphase (SEI) layers.
This work was done by Marshall C. Smart and Ratnakumar V. Bugga of Caltech for NASA’s Jet Propulsion Laboratory.