Keith Chin

Effect of Electrolyte Type upon the High-Temperature Resilience of Lithium-Ion Cells

The effect of electrolyte type upon the resilience of lithium-ion cells to high-temperature storage has been investigated in experimental mesocarbon microbead carbon-Li x Ni y Co 1 - y O 2 three-electrode cells. Specifically, electrolytes have been studied where the solvent mixtures have been varied, with the intention of determining the impact of ethylene carbonate (EC) content upon performance. In addition to determining the reversible and irreversible capacity losses sustained as a result of high-temperature storage (55 to 70°C), a number of electrochemical measurements (ac impedance, Tafel polarization, and linear polarization) have been performed to determine the impact of the high-temperature exposure upon the electrode kinetics and the nature of the electrode surface films. It was observed that cells containing electrolytes with high EC content (i.e., 70% EC by volume) displayed superior resilience to high-temperature storage, in contrast to cells containing low EC content electrolytes (i.e., 30% EC by volume), which displayed much larger irreversible capacity losses and poorer lithium intercalation/deintercalation kinetics after exposure to high temperatures. Solid-state 7 Li nuclear magnetic resonance measurements were used to determine quantitatively the fraction of Li in the irreversible solid electrolyte interphase (SEI) as compared to Li in the active electrode (both anode and cathode) material. In addition, the electrodes were characterized using a scanning electron microscope equipped with an X-ray energy-dispersive spectrometer to examine the film morphology and composition. The results indicate that the nature of the SEI formed on the anode in low EC content cells correlates with the poor electrochemical performance observed after being subjected to high temperatures.

Performance characteristics of lithium ion cells at low temperatures

The low temperature charge and discharge characteristics of experimental MCMB-Li/sub x/Ni/sub y/Co/sub 1-y/O/sub 2/ cells containing different electrolytes were investigated. The use of low ethylene carbonate (EC)-content electrolyte formulations has resulted in good discharge performance to temperatures as low as -40/spl deg/C. The effect of charge voltage and charge current upon the individual electrode potentials at low temperature was investigated using the three electrode cells (containing lithium reference electrodes). In some cases, lithium plating was observed to occur upon low temperature charge, and found to be facilitated by high charge voltages, high charge currents, and poor anode kinetics. Electrochemical characterization of the cells has helped to establish the conditions under which lithium plating can occur by providing information regarding the polarization effects present at each electrode.

Behavior of Li ion cells in high-intensity radiation environments

Planetary exploration missions to Jupiter and its moons require that the power system components, including batteries, be tolerant to high intensities, about 4 Mrad, of γ-radiation. In view of the several polymeric materials used as separators and binders and the use of organic electrolyte solutions in Li-ion cells, it is difficult to predict their response to such radiation environments. Therefore, a detailed experimental evaluation was undertaken to determine the performance of Li-ion cells after exposure to various levels of cumulative radiation levels up to 25 Mrad, at different levels of intensities. Prototype cells, obtained from two domestic sources for aerospace lithium-ion batteries and consisting of two different chemistries, were used as test articles. Discharge performances, at ambient and low temperatures, as well as, electrical impedance spectroscopy responses were determined after each exposure, and analyses were made for the impedance characteristics and their changes upon irradiation. Postradiation cycling tests were carried out on these cells to assess their cyclability subsequent to radiation exposure. Although control measurements were not made on cells without radiation, these studies reveal that the lithium-ion cells display good tolerance to radiation, with only marginal decline in their capacity, and with no significant change in capacity fade rate during subsequent cycling.

Lithium-ion batteries for aerospace

Under the Mars Surveyor Program (MSP01), lithium-ion batteries were developed by Lithion Inc. (Yardney Technical Products Inc.), each being 28 V, 25 Ah, 8-cells, 9 kg and fully qualified prior to mission cancellation. In addition to the requirement of being able to supply at least 90 cycles on the surface of Mars, the battery was demonstrated to be capable of operation (both charge and discharge) over a large temperature range (-20/spl deg/ to +40/spl deg/C), with tolerance to non-operational excursions to -30/spl deg/ and +50/spl deg/C. After mission cancellation, the batteries delivered to JPL were subjected to generic performance tests to demonstrate the applicability of the technology to meet future NASA aerospace applications. One of the two batteries currently being tested at JPL is undergoing testing according to anticipated performance requirements of future Mars Lander applications. The primary goal of this activity is to determine the performance capability to power surface operation on Mars for a prolonged period (> 3 years) after being subjected to a long cruise period. The second 25 AHr battery is being tested to determine the viability of using lithium-ion technology for future planetary orbiter applications. The test implemented consists of cycling the battery continuously under LEO conditions (30% DOD), while periodically checking the battery impedance and full capacity (100% DOD). Prior to initiating these tests, a number of characterization tests were performed to determine general performance attributes and battery health. In addition to presenting battery data, results obtained with individual cells will also be presented to further describe the capabilities of the technology to meet future applications.

Lithium-ion cell technology demonstration for future NASA applications

NASA requires lightweight rechargeable batteries for future missions to Mars and the outer planets that are capable of operating over a wide range of temperatures, with high specific energy and energy densities. Due to their attractive performance characteristics, lithium-ion (Li-ion) batteries have been identified as the battery chemistry of choice for a number of future applications, including planetary orbiters, rovers and landers. For example, under the Mars Surveyor Program M5P 01 lithium-ion batteries were developed by Lithion (each being 28 V, 25 Ah, 8-cells, and 9 kg) and fully qualified prior to mission cancellation. In addition to the requirement of being able to supply at least 90 cycles on the surface of Mars, the battery demonstrated operational capability (both charge and discharge) over a large temperature range (-20/spl deg/C to +40/spl deg/C), with tolerance to nonoperational excursions to -30/spl deg/C and 50/spl deg/C. Currently, JPL is implementing lithium-ion technology on the 2003 Mars Exploration Rover (MER), which will be coupled with a solar array. This mission has similar performance requirements to that of the 2001 Lander in that high energy density and a wide operating temperature range are necessitated. In addition to planetary rover and lander applications, we are also engaged in determining the viability of using lithium-ion technology for orbiter applications that require exceptionally long life (/spl deg/20,000 cycles at partial depth of discharge). To assess the viability of lithium-ion cells for these applications, a number of performance characterization tests have been performed (at the cell and battery level) on state-of-art prototype lithium-ion cells, including: assessing the cycle life performance (at varying DODs), life characteristics at extreme temperatures (+40/spl deg/C), rate capability as a function of temperature (-30/spl deg/C to 40/spl deg/C), pulse capability, self-discharge and storage characteristics, as well as, mission profile capability. This paper will describe the current and future NASA missions that are considering lithium (Li) ion batteries and will contain results of the cell testing conducted to-date to validate the technology for these missions.

Validation of Lithium-Ion Cell Technology for JPL's 2003 Mars Exploration Rover Mission

n early 2004 JPL successfully landed two Rovers, named Spirit and Opportunity, on the surface of Mars after traveling >300 million miles over a 6-7 month period. In order to operate for extended duration on the surface of Mars, both Rovers are equipped with rechargeable Lithium-ion batteries, which were designed to aid in the launch, correct anomalies during cruise, and support surface operations in conjunction with a triple-junction deployable solar arrays. The requirements of the Lithium-ion battery include the ability to provide power at least 90 sols on the surface of Mars, operate over a wide temperature range (-20 C to +40 C), withstanding long storage periods (e.g., cruise period), operate in an inverted position, and support high currents (e.g., firing pyro events). In order to determine the viability of Lithium-ion technology to meet these stringent requirements, a comprehensive test program was implemented aimed at demonstrating the performance capability of prototype cells fabricated by Lithion, Inc. (Yardney Technical Products, Inc.). The testing performed includes, determining the (a) room temperature cycle life, (b) pulse capability as a function of temperature, (e) self-discharge and storage characteristics mission profile capability, (f) cycle life under mission simulation conditions, (g) impedance characteristics, (h) impact of cell orientation, and (i) performance in 8-cell engineering batteries. As will be discussed, the Lithium-ion prototype cells and batteries were demonstrated to meet, as well as, exceed the requirements defined by the mission.

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 particularly 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 ta ilored 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 prototype cells, ranging in capacity from 1 to 45 AHr, have been tested over a wide range of temperatures ( -70 to +75 o C). In addition, many cells have been tested under conditions of alternating high and low temperature s 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 presente d. These individual cells have provided a test vehicle in which electrochemical characterization of electrodes can be performed, as a function of cycling and storage.

Managing PV power on Mars - MER Rovers

The MER Rovers have recently completed over 5 years of operation! This is a remarkable demonstration of the capabilities of PV power on the Martian surface. The extended mission required the development of an efficient process to predict the power available to the rovers on a day-to-day basis. The performance of the MER solar arrays is quite unlike that of any other Space array and perhaps more akin to Terrestrial PV operation, although even severe by that comparison. The impact of unpredictable factors, such as atmospheric conditions and dust accumulation (and removal) on the panels limits the accurate prediction of array power to short time spans.

The effect of high temperature exposure upon the performance of lithium ion cells

The effect of electrolyte type upon the resilience of lithium-ion cells to high temperature storage has been investigated in experimental MCMB carbon-Li/sub x/Ni/sub y/Co/sub 1-y/O/sub 2/ three-electrode cells. A number of electrolytes have been studied where the solvent mixtures have been varied, with the intention of determining the impact of ethylene carbonate (EC)-content upon performance, as well as, the presence of nonconventional linear carbonates (ie., di-2,2,2-trifluoroethyl carbonate and dipropyl carbonate). In addition to determining the reversible and irreversible capacity losses sustained as a result of high temperature storage (55/spl deg/ to 70/spl deg/C), a number of electrochemical measurements (AC impedance, Tafel polarization and linear polarization) have been performed to determine the impact of the high temperature exposure upon the electrode kinetics and the nature of the electrode surface films. It was observed that cells containing electrolytes with high EC-content (>50% EC by volume) displayed superior resilience to high temperature storage, in contrast to cells containing low EC-content electrolytes (<25% EC by volume) which displayed much larger irreversible capacity losses and poorer lithium intercalation/deintercalation kinetics after exposure to high temperatures.