Ester and Carbonate-Based Low Temperature Electrolytes in High Specific Energy and High Power 18650 Li-Ion Cells for Future NASA Missions

Abstract

NASA continues to have an interest in developing high specific energy and high power rechargeable batteries that can operate well over a wide temperature range. Concepts for applications that could be enabled or enhanced by such technology include: (i) future Mars landers, (ii) future Mars rovers, including a possible Mars Sample Return mission, and (iii) future planetary aerial vehicles, where high specific energy, high power and wide operating temperature range is desired. Future missions to some of the distant icy moons of Jupiter and Saturn are also anticipated to benefit from improved ultra-low temperature rechargeable batteries with high specific energy.1 To meet these needs, the Electrochemical Technologies Group (ETG) at the Jet Propulsion Laboratory (JPL) has developed a number of low temperature Li-ion electrolytes utilizing various approaches. In general, the performance targets of this work is to provide operation over the temperature range of +40oC to -60oC (delivering up to 150 Wh/kg at -40oC at reasonable rates). In addition, continuous operation at low temperatures is desired, so the cells should possess good charge characteristics without undesirable lithium plating.In previous collaborative work with E-One Moli Energy Ltd.1, we have demonstrated excellent specific energy at -40oC (> 150 Wh/kg) at low rates (C/100) in custom 18650-sized Li-ion cells containing JPL-developed electrolytes. The electrolytes investigated included all-carbonate-based low EC-content electrolyte formulations, as well as solutions containing ester co-solvents with various additives.2-6 In an extension of this work, we have investigated the performance of a number of Li-ion electrolytes optimized for low temperature performance in custom high specific energy cells as well high power prototype 18650-size cells manufactured by E-One Moli. The electrolytes evaluated included blends which contain elements of various approaches, including (i) ester co-solvents (such as methyl propionate, methyl butyrate, and propyl butyrate), (ii) the use of electrolyte additives (such as VC and FEC), and (ii) the use of mixed lithium electrolyte salts. In contrast to the previous work that was focused on low rate operation at high temperature, emphasis was placed on characterizing the cells using more aggressive discharge rates over a range of temperatures.To evaluate the high specific energy and high power 18650-size cells, extensive discharge rate characterization was performed over a wide temperature range (down to -80oC). Emphasis was also devoted to establishing the charge acceptance characteristics of the cells at very low temperatures, especially at -40oC. Given that lithium plating when charging at low temperatures is a known degradation mode of Li-ion cells in general, attention was focused upon characterizing the conditions in which its likelihood may be more pronounced and attempting to detect its occurrence indirectly. These results will be compared to baseline commercial off the shelf (COTS) cells. DC current interrupt impedance measurements have also been performed as a function of temperature in an attempt to more fully understand the impact of electrolyte type upon the low temperature performance for the cells.ACKNOWLEDGEMENTThe 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 supported by an internal JPL Research and Technology Development (R&TD) Fund. The information in this document is pre-decisional and is provided for planning and discussion only.