NASA has a strong interest to explore some of the distant icy moons of Jupiter and Saturn, since these bodies are believed to have liquid oceans beneath the icy surface that may harbor life. More specifically, NASA is considering missions to Europa, one of Jupiter’s moons, as well as other icy moons of Jupiter and Saturn, such as Enceladus, Ganymede, Titan and Callisto. To enable surface exploration of  these icy moons using landers and rovers, the power systems will need to be mass and volume efficient and potentially operate at ultra-low temperatures (down to -180^oC) and in high radiation environments (> 5 Mrad). To meet these power requirements, a number of power system technology options are available, including: (i) primary batteries, (ii) solar photovoltaic (PV) power sources coupled with rechargeable batteries, (iii) radioisotope thermoelectric generators (RTG) coupled with rechargeable batteries, and (v) fuel cells.   Of these options, the use of PV solar arrays combined with rechargeable batteries is potentially attractive, since it will enable long duration missions and have low mass.

To address these mission needs, improved rechargeable batteries that can operate at very low temperatures are desired. In response, the Electrochemical Technologies Group (ETG) at the Jet Propulsion Laboratory (JPL) is involved in developing ultra-low temperature rechargeable batteries with high specific energy and long life capability for icy moon surface missions. The performance goal is that these batteries should operate over the temperature range of +40^oC to -60^oC (delivering up to 100 Wh/kg at -40^oC and 75 Wh/kg at -60^oC). In addition, it is desirable that continuous operation is possible at these very low temperatures, so the cells should possess good charge characteristics over a wide temperature range. 

To meet these goals, we are currently evaluating both all carbonate and ester containing low temperature electrolytes that have been incorporated into prototype cells of various chemistries, obtained from a number of vendors, including (i) Eagle Pitcher Technologies-Yardney Division, (ii) Enersys/Quallion, LLC, (iii) E-One Moli Energy Ltd., and (iv) Navitas/A123. These electrolytes have been developed under previous programs, targeting improved low temperature operation of Li-ion cells, for both aerospace and automotive applications. In the case of the Eagle Picher Technologies-Yardney Division Li-ion cells, we are evaluating large capacity prototype cells developed for Mars Science Laboratory (MSL) Curiosity rover and the Mars InSight lander that contain an all-carbonate low temperature electrolyte¹ and a methyl propionate (MP) containing electrolyte², respectively.³ In collaboration with Quallion, we have obtained MCMB-LiNiCoAlO₂-based prototype cells containing both carbonate- and ester-based electrolytes. Previous collaborative efforts with Quallion have demonstrated high power capability over a wide range of temperatures in these cells,  in support of a DoE program focused on automotive applications.⁴ We are also evaluating LiFePO₄-based prototype cells that were obtained from Navitas/A123 with both methyl butyrate (MB) and methyl propionate (MP)-based electrolytes that have been demonstrated to operate well over a wide temperature range (i.e., -60^o to +60^oC).⁵ In collaboration with E-One Moli, we are also engaged in evaluating 18650 Li-ion cells that possess methyl propionate-based electrolytes, which have been observed to provide excellent specific energy down to very low temperatures.⁶

        During the course of this study, we have investigated the impact of the electrolyte type upon the low temperature capability of the different chemistries, with an emphasis upon obtaining high specific energy under relevant conditions.  This involves performing charge and discharge rate characterization over a wide temperature range, as well as cycling continuously at very low temperatures. Furthermore, there is a desire to characterize the likelihood of lithium plating when charging at low temperatures, and the influence that the electrolyte type and additives have upon this behavior.  To augment the prototype cell testing, studies were also performed in experimental three-electrode lithium-ion cells to determine the influence that the electrolyte type has upon the electrode kinetics, consisting of MCMB carbon anode and LiNiCoAlO₂ cathodes (electrodes fabricated by Quallion, LLC). A number of electrochemical techniques were employed to study these cells, including Electrochemical Impedance Spectroscopy, Tafel polarization, and linear micro-polarization. In summary, good reversibility and high specific energy has been obtained at -40^oC with many of the systems, and operating capability has been demonstrated to temperatures as low as -70^oC.

ACKNOWLEDGEMENT