Ratnakumar V. Bugga

Lithium Plating Behavior in Lithium-Ion Cells

A Li-ion cell does not contain metallic lithium under normal conditions of operation. Under strenuous charge conditions, however, metallic lithium may deposit on the carbon anode in preference to lithium intercalation and may cause problems in terms of performance, reliability and safety of the cell. Factors that affect the anode polarization and also Li intercalation kinetics play a crucial role in determining the propensity for such lithium deposition. Such factors include the nature of electrolyte, anode/cathode capacity ration, which have been studied with specific examples here. Further, various prototype cells from different manufacturers have been examined for their susceptibility towards lithium plating from a set of systematic charge/discharge tests at different charge rates and temperatures.

Reversible Intercalation of Fluoride-Anion Receptor Complexes in Graphite

We have demonstrated a route to reversibly intercalate fluoride-anion receptor complexes in graphite via a nonaqueous electrochemical process. This approach may find application for a rechargeable lithium–fluoride dual-ion intercalating battery with high specific energy. The cell chemistry presented here uses graphite cathodes with LiF dissolved in a nonaqueous solvent through the aid of anion receptors. Cells have been demonstrated with reversible cathode specific capacity of approximately 80 mAh/g at discharge plateaus of upward of 4.8 V, with graphite staging of the intercalant observed via in situ synchrotron X-ray diffraction during charging. Electrochemical impedance spectroscopy and 11B nuclear magnetic resonance studies suggest that co-intercalation of the anion receptor with the fluoride occurs during charging, which likely limits the cathode specific capacity. The anion receptor type dictates the extent of graphite fluorination, and must be further optimized to realize high theoretical fluorination levels. To find these optimal anion receptors, we have designed an ab initio calculations-based scheme aimed at identifying receptors with favorable fluoride binding and release properties.

Lithium Ion Batteries for Space Applications

Interplanetary missions require rechargeable batteries with unique performance characteristics: high specific energy, wide operating temperatures, demonstrated reliability, and safety. Li-ion batteries are fast becoming the most common energy storage solution for these missions, as they are able to meet the more demanding technical specifications without being excessively massive. At JPL, we have undertaken materials development studies on both cathodes and electrolytes with the goal of further enhancing battery specific energy, discharge and charge capability, and functional temperature range. Results of these studies are described below.

Power Management and Distribution for System on a Chip for Space Applications

In this paper a method for achieving integrated power electronics is discussed. Future spacecraft are projected to feature high levels of integration at the system level (i.e., a “systems on a chip” approach) particularly in areas not typically associated with an integrated approach (such as inertial reference systems, RF communications, imaging, sensors, etc.). Taking full advantage of the miniaturization occurring in these other systems will require commensurate reductions in the size of the power electronics. Power electronics are traditionally larger due to the need for high value passive components requiring significant power handling capabilities. Our approach takes advantage of lower projected power requirements and utilizes integrated, on-chip passives and novel high voltage transistors to achieve adaptive distributed on-chip power management and distribution (PMAD). Operating from a single supply, this on-chip PMAD will operate at power levels of up to 1 W, at frequencies of 110 MHz. INTRODUCTION Integrated systems on future nanosatellites will get their supply voltage from a common power bus. These systems will rely on efficient adaptive on-chip power management circuits for generating the internal voltage levels necessary for operation of the sensors, actuators and other subsystems. For space applications, there are several challenges in building an efficient completely integrated power management system, including a) the development of a new generation of miniaturized large value passive components (inductors and capacitors) for DC-DC converter circuits that can be integrated on-chip, b) the development of on-chip power interrupt protection (such as microbatteries), c) the development of high voltage transistors that can coexist with traditional low voltage transistors in the same radiation hardened silicon substrate, and d) the development of a library of mixed-signal/mixedvoltage CMOS cells suitable for the construction of a completely integrated on-chip power management system. This paper summarizes JPL’s effort in overcoming the challenges of building a completely integrated power management system for future avionics microsystems for deep space applications for NASA. PMAD REQUIREMENTS FOR AVIONICS SYSTEM ON A CHIP Figure 1 shows the block diagram of a proposed on-chip adaptive power management system for the next generation of highly miniaturized satellites. Principle components in this on-chip power management system are switching DC-DC converters with large value onchip inductors and capacitors, micro batteries, battery charge/discharge circuits and digital 110 circuits for interface and control. Digital Interface Bus 12C Main Satellite Power Bus

The Kinetics of Sub-Fluorinated Carbon Fluoride Cathodes for Lithium Batteries

Sub-fluorinated carbon materials (CFx, x<1) were prepared by direct fluorination of synthetic graphite (Timcal KS15) and multi-walled carbon nanotubes (MWNT, MER). The fluorination parameters (temperature, fluorine partial pressure, fluorine flow rate and time) were set to achieve a target composition ’x’ in CFx, according to the starting carbonaceous material. The fluorine composition was determined by weight uptake and SEM/XEDX cross analyses.

Performance and Comparison of Lithium-Ion Batteries Under Low-Earth-Orbit Mission Profiles

Abstract The performance of two 28 V, 25 Ah lithium-ion batteries is being evaluated under low-Earth-orbit mission profiles for satellite and orbiter applications. These space flight-qualified batteries were designed and fabricated by Lithion, Inc. (Yardney Technical Products) for the 2001 Mars Surveyor Program Lander, the first major NASA mission that baselined lithium-ion battery technology. Lithium-ion battery chemistry was an enabling technology for the mission because of its ability to provide low temperature operation in a lightweight and compact battery design. The Mars Surveyor Program mission was cancelled before launch, however, the Lander batteries had already been built and flight-qualified. Lithium-ion batteries are being baselined for increasingly more missions, including missions in low-Earth-orbit and geosynchronous orbit. These mission conditions are more challenging for lithium-ion batteries than a short mission on Mars. Many more cycles are required for operation in low-Earth-orbit and a much longer calendar life is required for operation in either low-Earth-orbit or geosynchronous orbit. A ground test program was established that utilized the Lander batteries from the original mission to demonstrate performance and life under various mission conditions. This paper presents results of the low-Earth-orbit (LEO) portion of the testing that is being conducted at NASA Glenn Research Center (GRC) and NASA Jet Propulsion Laboratory (JPL). The batteries discussed are currently undergoing life testing and have each achieved over 12,000 cycles to 40 percent depth-of-discharge. Each battery is cycling at a different temperature, one at 23 °C and the other at 0 °C. In addition to cycling under low-Earth-orbit conditions, the batteries have been characterized at 500 to 1000 cycle intervals throughout the life testing to observe their capacity and DC impedance changes. Because the batteries are not equipped with cell balancing electronics, cell balancing is manually performed on each battery when cell voltage dispersion exceeds the established threshold. The performance of the batteries will be discussed individually and their performance relative to each other at the different test conditions will be compared.

In Situ Polysulfide Detection in Lithium Sulfur Cells

Lithium sulfur batteries promise significant improvements in specific energy compared to Li-ion, but are limited by capacity fade upon cycling. Efforts to improve durability have focused on suppressing the solubility of intermediate polysulfides in the electrolyte. Here we describe an in situ electrochemical polysulfide detection method based on the cyclic volatmmetric response. The voltammetric peaks correlate with increased discharge, consistent with increased polysulfide species in the electrolyte as demonstrated by prior literature measurements using spectroscopic methods. We verified that adding metal sulfide species to the sulfur cathode and ceramic-coatings on the polyolefin separator result in reduced polysulfide concentration, consistent with improved cycle life reported earlier. Further, the use of highly concentrated electrolytes produces no detectable dissolved polysulfide species. Future advances in Li/S technology could utilize this method to determine the polysulfide contents in the electrolyte, and thus quantify the efficacy of the sulfur-sequestering strategies.

Long Term Radiation Effects on Super NiCd Batteries

Due to their unique construction, Super NiCd cells are resistant to degradation by radiation expected from a mission to Europa and other high radiation environments present in interplanetary space. In place of conventional nylon separators the Super NiCd uses a zirconium oxide fabric (Zircar) impregnated with polybenzimidazole (PBI), a fire retardant polymer developed after the fire on Apollo 1. The Super NiCd cells were fabricated at Eagle Picher Power Systems Department (now, Electro Energy, Mobile Products Inc.) and furnished to NASA for the Mars Observer program in 1990. After storage for 10 years the cells were reconditioned then subjected to radiation testing for the Europa Orbiter Program in 2000. The 37 Ah name plate cells yielded capacities greater than 50 Ah both before and after controlled radiation testing. Cells were DPAed after radiation testing with no visible evidence of damage.