Performance Characteristics of Lithium-Ion Technology Under Extreme Environmental Conditions

Lithium-ion technology has been demonstrated to have high specific energy, high energy density, and relatively long life. In addition, lithium-ion technology is especially attractive since it has the potential to operate over a very wide temperature range, which is particular important for a number of applications. This potential derives from the fact that lithium-ion cells possess organic solvent-based electrolytes, in contrast to aqueous-based electrolytes, which can be tailored to provide high conductivity over a wide range of temperatures. Thus, in recent years, advances in electrolyte formulations have led to dramatic improvements in the capability of lithium-ion technology to operate at extreme temperatures, especially low temperatures. However, it still remains a challenge to demonstrate excellent low temperature capability throughout the life of a cell, especially after being subjected to high temperature cycling or exposure. In order to understand these performance limitations, a number of aerospace quality prototype cells, ranging in capacity from 1 to 45 AKr, have been tested over a wide range of temperatures (-70 to i-75OC). In addition, many cells have been tested under conditions of alternating high and low temperatures to determine the impact that variable temperature cycling has upon cell health and performance. To further elaborate upon possible performance degradation mechanisms present, the results of a number of experimental three-electrode cells will be presented. In addition to enabling us to monitor the individual electrode potentials during cycling, these cells have provided a test vehicle in which electrochemical characterization of electrode can be performed. INTRODUCTION In order to effectively explore the solar system, NASA has identified the need for primary and secondary batteries that can efficiently operate under extreme environmental conditions, including: (1) at ultra-low temperatures, (2) at extremely high temperatures (up to +550°C), (3) in high radiation environments, (4) and under high impact conditions.' For example, current projections of future missions aimed at the exploration of the moons of Jupiter, such as Europa and Titan, will require batteries that will power surface penetrating probes and atmospheric probes in environments that are as cold as -14OOC. Missions planned to explore comets and asteroids are also expected to encounter similarly cold temperatures. In addition, future missions involving the exploration of Mars are expected to encounter environmental conditions as cold at -12OOC. In contrast, future missions planned to explore Venus will require batteries that can operate at very high temperature, in excess of 450°C. Besides harsh temperature 'requirements imposed by many upcoming missions, many applications will require that the batteries possess good tolerance to high intensity g w a radiation, such as that encountered on Europa. Although it may be difficult to difficult to develop technology to effectively operate under these extreme conditions, advances in both primary and secondary batteries are desired to alleviate some of the constraints imposed by the bulky thermal management designs. Due to its favorable characteristics, lithium-ion technology has been recognized as having great potential for meeting some of the low temperature performance requirements projected for some future missions (down to at least -6OOC). In addition to NASA, the 1 American Institute of Aeronautics and Astronautics Air Force and Army both desire secondary batteries that can operate over a wide temperature (-40 to +70°C).2,3,4 Thus, our group has actively been involved in a testing and evaluation program to evaluate the potential and limitations of the technology, as well as, a research and development program to develop improved low temperature lithium-ion batteries. The primary focus of our develop efforts have focused upon developing improved low temperature electrolytes. In this paper, we would like to discuss our recent results obtained in performance characterization of lithium-ion cells under extreme environmental conditions, including: (1) at very low temperatures (< -4OoC), (2) at high temperatures (> +4OoC), (3) and under high intensity radiation. LOW TEMPERATURE DISCHARGE CHARACTERISTICS OF PROTOTYPE CELLS For many applications, the energy storage device is only required to operate at very low temperatures in the discharge mode, with charging occurring at much milder temperatures. When cells are charged at room temperature, prototype cells have been shown to operate effectively down to -30 to -50°C using moderate rates, and even down to -7OOC using very low rates. For example, 7-10 Ah cells developed by Lithion for use in NASA-JPL’s Mars exploration applications display good performance down to 40°C using a C/10 discharge rate to 2.0V, as shown in Fig. 1, with 70% of the room temperature capacity being delivered at -4OOC. The cell chemistry consists of mesocarbon microbeads (MCMB) carbon anodes, LiNi,Col.,Oz cathode materials, and a low temperature electrolyte (1 .O M LiPF6 EC+DMC+ DEC (1 : 1: 1)) developed at JPL.5,6 It must be noted, however, that a significant portion of the capacity derived at these very low temperatures is obtained at voltages below 3.0V. For the purposes of characterization, we have routinely discharged the cells to low voltage (2.02.W) to obtain the most capacity possible. It has been reported that at very low discharge potentials the anode substrate commonly used, e.g., copper foil, can dissolve into the electrolyte solution contributing to cell degradation. However, researchers have demonstrated that this effect is only significant when the anode potential is > +3 V vs. Li+/Li, which should not occur in hll-cells unless the cell voltage is significantly below 1 .OV.’** In addition, this effect is not likely to be as significant at very low temperatures due to the overpotential present at each electrode (ohmic and charge transfer resistances). Fig. 1. Discharge capacity of a 7 Ah lithium-ion cell at various temperatures using a C/10 discharge rate (0.700 A) to a 2.0V cut-off. As shown in Fig. 2, cells discharged under these conditions can deliver > 80 Whkg and > 64 whkg at -30 and 40°C, respectively. Fig. 2. Discharge energy of a 7 Ah lithium-ion cell at various temperatures using a C/10 discharge rate (0.700 A) to a 2.0V cut-off. The performance of these cells drops off rapidly when attempting to discharge the cells at temperatures below 40!Cc, becoming very rate sensitive, and showing negligible capacity at -6OOC with moderate rates ( >C/15). The primary reason for the poor performance below 40°C in this system is due to the low conductivity of the electrolyte and/or the onset of electrolyte ffeezing. This is in part due to the large proportion of ethylene carbonate (33% by volume) present in the electrolyte formulation. Ethylene carbonate is an essential component in the electrolyte due to the fact that it possesses (a) a very high dielectric content providing good salt solvation characteristics and high ionic conductivity, (b) good electrode passivation characteristics leading to ionically conductive and protective surface films, especially on the carbon anode, and (c) good physical properties (e.g., high boiling point) which translate into greater cell stability at hlgher temperatures. However, since ethylene carbonate has a very high 2 American Institute of Aeronautics and Astronautics melting point (+34OC) and high viscosity, it shoyld not be used in large proportion when optimizing electrolyte formulations for low temperatures. Thus, current efforts at JPL9s’0 have focused upon investigating electrolytes with low ethylene carbonate-content to realize improved cell performance at temperatures below 40°C, with the recognition that some high temperature stability (> 55°C) and ambient temperature rate capability may be compromised. Alternatively, propylene carbonate (PC) is often used in place of ethylene carbonate due to the fact that is has a much lower melting point (-55OC) and also possesses a very high dielectric constant, however, it cannot be used with graphitic carbon anodes readily due to an exfoliation process which occurs. In addition, electrolytes with a high proportion of PC display somewhat low conductivity at very low temperatures due to its high viscosity. When prototype cells containing a low temperature electrolyte possessing low ethylene carbonate-content and an optimized blend of linear aliphatic carbonates were evaluated, much better perfonnance was obtained at temperatures below -4OOC. For example, SAFT DD-size cells incorporating 1.0 M LiFP6 EC+DEC+DMC+EMC (I : 1 :I :3 v/v)’’, an electrolyte developed recently at JPL, improved performance is observed as shown in Fig. 3. As illustrated, good performance is displayed down to -5OOC using a C/10 discharge rate when the cells are charged at room temperature, with > 75% of the room temperature capacity being delivered at -5OOC. In addition, the overpotential observed upon discharge observed is much lower than that observed with the previous system described, with > 75% of the capacity being delivered with an operating voltage > 3.0V at 40°C.