(August 2002) Battery Safety/Quality/Testing and Materials
by Donald Georgi
Last month we tried to skim the Power Sources Conference presentations on Lithium, a task which was not easy because of the sheer quantity of information presented. For now, we are grouping ‘other’ battery subjects, so get strapped in and prepare for a wild and bumpy ride over this cornucopia of chemistry. Lithium still plays a big role in the presentations, but this overview also includes more global subjects and chemistries.
Battery Safety Quality Testing
Eagle-Picher, in conjunction with the Naval Warfare Center, China Lake, described the Navy Lithium Battery Safety Program which recognizes thermal battery hazards and growing energy density over the last ten years. Current practice is documented in a technical manual often referred to as S9310. With needs extending to other government agencies, S 9310 could provide integrated testing to characterize and validate system designs by providing better safety, performance and cost effectiveness.
Warheads on torpedoes garner very large safety considerations, but so do Unmanned Undersea Vehicles which have 98 kWh battery packs and are stored before use inside manned submarines. HED Battery Corporation is building a Lithium-thyonyl chloride battery which requires extensive testing for performance under very rugged conditions. Such testing is being done under the direction of Boeing and the Navy .
Safety, performance and environmental capabilities applied to a hand held search and rescue radio were investigated by the Naval Surface Warfare Center, Carderock Division, for three chemistries: Lithium-sulfur dioxide, Lithium-manganese dioxide and Lithium-ion. All three obtained Naval safety approval, but the Lithium-sulfur battery was deemed too costly; the manganese battery is acceptable for operational use in the near term, and the Lithium-ion battery will likely become the battery of choice for training scenarios.
The role of the carbon anode in safety considerations of Lithium-ion cells was investigated by LiTech, LLC. By increasing the anode capacity close to the theoretical capacity of the cathode, the deposition of metallic lithium on the anode during overcharge can be avoided. Better thermal stability of the fully charged lithiated carbon anode (the highly reactive LiC6) could be improved, forming LiCx where x is >6.
Elimination of the hydrazine and turbine for auxiliary power units in the Space Shuttle can make operations safer. To that end, NASA is qualifying 39 kWh Lithium-ion battery powered APUs. Batteries are being qualified with cycle life tests using normal simplified mission profiles and accelerated simplified mission profiles.
What happens when 7 Ah Lithium-ion cells are overcharged? Yardney/Lithion presented test results which showed that 4.7 Volt overcharge causes the cathodes to undergo an exothermic reaction with the electrolyte which can produce enough internal pressure to open the burst disk and cause fire and/or explosion. (Ed. Note: BD commends Yardney/Lithion for presenting the true dangers which exist in Lithium-ion batteries. This is not to condemn Lithium-ion as unsafe, but to highlight conditions which require excellence of design, testing and validation of all systems using this chemistry.)
Since many electronic devices are beginning to include Lithium-ion polymer cells, NASA has investigated the performance and safety of Electrofuel’s PowerPad Lithium-ion polymer batteries. In testing of capacity, rate, high temperature exposure, short circuit, overcharge and overdischarge, no significant venting, excessive heat or thermal runaway was found. Based on the test data, the batteries will be acceptable on Extra Vehicular Activities involving low rate applications.
Sandia National Laboratories is detailing the safety aspects of 18650 Lithium-ion cell’s thermal runaway with accelerating rate calorimetry (See BD# 43, 1-4.) Commercial Sony Lithium cobalt oxide cells with coke anodes were compared to two other experimental 18650 type cells with different ratios of lithium nickel cobalt cathodes, electrolytes and anodes. The Sony cells began onset of self generated heating at 80 0C. Thermal runaway initiates from the decomposition of the anode SEI layer producing “a strong exothermic decomposition reaction...which can result in a rapid disassembly of the cells.” The reader can choose whether “rapid disassembly” means explosion or not. The experimental chemistries, when aged, reduced the thermal runaway response of the cells due to increased SEI passivation of the anode.
(Ed. note: General conversations from the audience after the presentation suggest that the chemical composition of Lithium-ion outgassing could promote explosion of the venting gas if an open flame or spark were present.)
The Safety/Quality/Testing subjects included a presentation “Battery quick testing” by BD’s contributing columnist and Cadex CEO, Isidor Buchmann. His dissertation went beyond lithium and was applicable to other cehmistires including Lead-acid. Here the full consideration of state of charge (SOC) and state of health (SOH) is sought and found in a short test period. To achieve short test times, the components of a Randles model of a battery having both resistance and capacitance must be evaluated. Using a full frequency spectrum, it becomes possible to provide accurate SOC and SOH information rapidly, but at the expense of costly electronics. There is a market for quick testing but possibly at a lower price than is presently available.
Unless one is specific in defining the combination of power sources, the word, hybrid, could be misunderstood because presentations at this Power Sources Conference included many different combinations.
The term hybrid is applied to a supercapacitors when mixing combinations of tantalum as the anode and ruthenium oxide as the cathodes to produce a high Voltage (16 V.) capacitor with a very low RC products of one millisecond. The resultant work, done by Florida State University and Evans Capacitor Company, shows energy density is lower than most supercapacitors but higher than electrolytics This supercapcitor exhibits high power densities.
In the continuing pursuit of lower cost materials and greater capacitance for supercapacitors, work is being done by the University of Reading and Lexcel Technology Ltd. to construct superconductor electrodes of pyrolized polyacrylonitrile. The material is relatively inexpensive and readily available. Their technique results in molecular structures which have high surface area ring-like construction. Test data of supercapacitor samples showed high capacitance with surface areas of up to 990 meters3/g. Pyrolysis temperatures and doping with lithium sulphite were important in achieving high capacitance.
Another implementation of the word, hybrid, is to marry a 15 Watt PEM fuel cell with a 6 Ah Lithium-ion battery in an unregulated system to allow for a cycling load profile to be supported. The U.S. Army Communications an Electronics Command investigated the overall performance over a 21 hour period. The fuel cell alone could only support 3 Wh, the battery alone only 84 Wh while the hybrid provided 462 Wh.
In another hybrid configuration, T/J Technologies reported that DMFCs operating under high pulse current conditions were improved with the addition of high power density ultracapacitors (supercapacitors). A power density of 72 Watts per liter for the DMFC alone was increased to 114 W/liter with a six Farad ultracapacitor.
Another way to implement a hybrid configuration is to marry a primary lithium battery with a supercapacitor in an oceanographic/military application. Tadiran Batteries Ltd. reported on a DD size 40 Amp hour Lithium-thyonyl chloride (Li SOCl2) primary battery which was augmented with a parallel connected AA size hybrid capacitor which when charged to 3.65 Volts, can provide one Amp for 12 minutes or 830 Farads of pseudo capacitance. The battery without the supercapacitor has its performance drop to 70% of rated capacity with only a 100 mA discharge rate.
Expanding the use of the word, hybrid, was a group from the Krakow University of Technology presenting the principles of a five stroke thermodynamic cycle gas turbine engine with motorized flywheel based on the Fijalkowski turbine boosting system. This system has the potential for providing a 95% efficient system with ultra low emissions in small size and low cost configurations for vehicle applications.
BST Systems is investigating domestically available carbon fibers for Lithium-ion anodes to result in lower cost and greater electrochemical performance than mesophase carbon microbeads (MCMB). Testing in half cells of two new materials showed one to have similar specific capacity to MCMB but higher efficiency, and the other to have significantly greater (> 300 mAh/g) specific capacity than MCMB.
When considering total weight in space applications, the battery design which implements part of the battery as one of the structural elements can significantly reduce total weight. Boundless Corp. has a program to develop an integrated structural Lithium-ion battery panel where the rigid carbon composite anode is a part of the structural material. Its fundamental building block is a mesophase-pitch-based carbon fabric composite that has dual functionality of structure and lithium intercalation. Additionally, the carbon fibers have higher thermal conductivity than most metals. In a spiral configuration, cells reached significantly higher temperatures than flat configurations.
Electrolyte characterization for application of Li/MnO2 chemistry to pouch cell construction was the subject of a combined U.S. Army/ Eagle Pitcher investigation. Common salts used in organic solvents were characterized as to their response in sweep Voltammetry. Reduction of the electrolyte on the anode and oxidation on the cathode is a major concern. The excitation energy plays a significant role on the electrolyte degradation.
The thermal behavior of various polymeric battery materials was investigated by the Naval Surface Warfare Center. Modular differential scanning calorimetry (MDSC) was used over differential scanning calorimetry (DSC) because if its ability to properly measure multiple transitions in the same temperature range and has greater sensitivity and resolution. Several complex thermal transitions that could be misinterpreted by DSC were determined, leading to better understanding of the thermal properties of polymeric battery components.
Auburn University is developing ultra thin microfibrous materials for Zinc-air cathodes to improve the pulse performance. Test cells with air cathodes, three times thinner than commercial cathodes, provided greater pulse currents, especially when augmented with a 10 Farad supercapacitor.
Carbon nanotubes and beads from tree resins are being investigated by the Indian Institute of Technology for application to photovoltaic cells, lithium cell anodes and even the killing of cancerous cells. The camphor raw material is a renewable resource.
Also, from India’s Central Electrochemical Research Institute is a starch assisted method of combusting cathode materials for cathodes of Lithium-ion cells. The materials and methods result in excellent cyclability and charge retention. A second presentation from the Institute investigated the synthesis of a series of LiNixMn2-xO4 spinels. Due to the presence of Ni2+ dopant, the initial discharge capacity was found to decrease with respect to the undoped compound. However, lesser capacity fade has been noticed in progressive cycling.
When considering safety for large Lithium-ion batteries, the inherent safety factor of a solid electrolyte is desirable. A group from Argonne National Laboratory, the University of Wisconsin and Quallion LLC are working on a solid polysiloxane polymer electrolyte for just such a purpose. A target for conductivity is in the range of 10-4~10-3 and testing of the electrolyte presented here is in the region of 10-4 at room temperature. Cycling characteristics have not shown capacity fade.