Safety First – The Story of why all Lithium Batteries are not the same Lithium ion batteries have no memory effect and significantly better energy density than other types of batteries - but are they safe? By V. Evan House, PhD; and Fayth Ross, MS, Altairnano Inc. May 1, 2007 http://www.powermanagementdesignline.com/howto/showArticle.jhtml;jsessi onid=3JRMJ1X5LSAOAQSNDLRCKHSCJUNN2JVN?articleID=199202938 We’ve all seen pictures of the infamous exploding laptop, and heard about or been affected by the massive, unprecedented recall of lithium-ion batteries. In August of 2006, Dell recalled 4.1 million notebook lithium-ion batteries, and Apple Computer recalled 1.8 million batteries. A month later, Panasonic recalled 6,000 batteries. As we’ve seen with this substantial recall, current lithium ion packs have one significant drawback: safety. Widely used in consumer electronics, you’ll find lithium-ion (Li-ion) batteries everywhere. From cell phones, to laptops, Li-ion batteries have one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. Li-ion batteries have two to three times the energy density of nickel-cadmium and nickel-metal hydride batteries, and four times the energy density of lead-acid batteries (higher energy density means lighter and more compact batteries). But are they safe? The safety of Li-ion batteries has been in question since before 2001, when the U.S. Department of Transportation’s Research and Special Programs Administration implemented increased safety checks, including puncture and crush tests, to ensure that the batteries could be safely transported. According to the results, 120,000 lithium-ion cells caught fire during the tests – strong evidence that the safety of the battery was in question. Battery Basics All batteries produce energy from electrochemical reactions. Batteries are comprised of several components, but primarily consist of the following: • • • A positive electrode and a negative electrode An ionic electrolyte: a solution that contains and aids the movement of ions (charged particles) back and forth between the two electrodes A porous separator (ensures the two electrodes do not touch but allows ions to travel back and forth between the electrodes) Safety First 1 When a lithium-ion battery is charged, lithium ions travel from the positive electrode to the negative electrode. On discharge, these ions return to the positive electrode releasing energy in the process. What caused the laptop blowup? The cause of the battery blowup originated within the manufacturing of the battery cell itself. Batteries contain several metal parts, which can sometimes result in undesirable metal impurities within the battery. These impurities are typically sharp metal shards from the battery casing or electrode material. In the case of the exploding batteries, the metal impurities rested between an electrode and the separator. Through battery cycling and changes in size in the negative electrode (due to cycling), the metal shard eventually punctured the separator, causing the positive and negative electrode to bridge, resulting in a short circuit. The short circuit produced high heat which ultimately resulted in fire. High heat levels, fire, and/or explosion are all results of thermal runaway: a condition whereby a battery can enter into an uncontrollable reaction. Thermal runaway is a process where the internal temperature of a battery reaches a point (approximately 120°C) that begins an irreversible reaction that is highly exothermic and causes fires. Besides manufacturing defects, additional causes of thermal runaway include physical abuse (such as banging a laptop around) and punctures. The nature of a traditional Li-ion battery is that any type of short circuit will result in thermal runaway. In response to overheating problems, the Japan Electronics and Informational Technology Industries Association (JEITA) and the Battery Association of Japan released new safety guidelines on April 20, 2007. Their guidelines stressed the importance of avoiding high-speed charging using higher than rated voltages. According to JEITA, problems in the Li-ion batteries occur at high charge voltages and when foreign substances are present in battery cells. JEITA was formed one week after Sony Corp. initiated a recall of batteries for its Vaio notebook computers. JEITA is comprised of all PC vendors that used the trouble batteries (excluding Apple Computer Inc.) and focuses on safely using Li-ion batteries in laptops. Li-Ion in Cell Phones Lithium-ion batteries are the battery choice for phone batteries, due principally to their compactness and lightness. Despite the benefits, however, Li-ion batteries in cell phones still have problems. In cell phones, Li-ion batteries can overheat because of a short circuit or improper control. If the temperature rises slowly, the battery can melt. If the temperature rises rapidly, enough pressure can create a small explosion. Consumers have experienced severe burns as a result of these failures. When not controlled, the chemical reaction that produces energy in a Li-ion battery can be quite violent. Sometimes deformed electrode plates are installed in batteries. If batteries are accidentally subjected to strong external impact, which results in a surface dent or similar depression, the deformed plates could pierce the batteries’ internal insulation – resulting in an electrical short during, or right after, charging. Safety First 2 Li-Ion in Electric Vehicles The United States imports nearly 60% of the roughly 20 million barrels per day of total oil consumed, which comes at a cost of nearly $260 billion dollars a year. In 2002, consumption of oil for transportation alone was nearly double the level of domestic production, and in 2005, the Energy Information Administration reported that consumption outstripped domestic production by nearly 4:1. According to the Department of Energy’s National Energy Policy Group, transportation accounted for an estimated 66% of all oil consumed in 2001, or roughly 13 million barrels of oil a day. A shift toward alternative transportation, such as battery or electric vehicles, is essential for reducing oil dependency. An expected 6 million, or about 10%, of all cars sold globally will be hybrid by 2010. Popularity of electric vehicles (EV) is a step in the right direction toward a more efficient fuelbased economy, and major automotive manufacturers have embraced this technology. Today, the Hybrid Electric Vehicle (HEV) market primarily uses nickel metal hydride (NiMH) batteries. NiMH batteries, however, are expensive and self-discharge at a fast rate and are generally agreed upon to not be the ideal solution. NiMH batteries must also be closely controlled as they too can enter into thermal runaway conditions. Traditionally used in consumer electronics and notebooks which require a limited amount of energy, Li-ion use in electric vehicles has been under scrutiny. Li-ion advocates site Li-ion’s high specific energy and low weight as ideal for EV adoption. Skeptics site cost, intolerance of temperature extremes, and safety – safety being the single most significant hurdle in adopting Li-ion batteries for use in electric vehicles and the sole reason that traditional Li-ion batteries are not used in any EV today. Battery electric vehicles typically have their batteries arranged in large battery packs of varying voltage and capacity to supply the required energy to drive the vehicle. The drawback to using current Li-ion batteries is the significant engineering required to both heat and cool, as well as monitor the safety of the pack. Thermal runaway is a significant concern for automotive applications because of the large number of batteries used in a battery pack. Any scale of market adoption in the battery electric vehicle market will not occur until problems with the thermal runaway are resolved. On March 13, 2007, at the Geneva Motor Show in Switzerland, General Motors Corporation Vice-Chairman Bob Lutz confirmed 2010 as the target year for production of the all-electric Chevrolet Volt. While a prototype is expected by the end of 2007, Lutz cautioned that uncertainty remains whether Li-ion batteries can be developed soon enough to power the Volt both economically and safely. Success in the transportation market for EVs, HEVs, and PHEVs is dependant upon a significant improvement in battery technology. For all battery types, the inherent safety and cost of the design is determined by the types of electrodes and electrolytes used, as is the overall energy density capacity, power density capacity, and cycle life of the cell. The SEI Layer in Common Li-Ion Batteries Li-ion batteries generally use metal oxides for the positive electrode, carbon/graphite for the negative electrode, and a lithium salt in an organic solvent for the electrolyte. The energy is released in these batteries through the movement of lithium ions and electron processes at the electrodes. Safety First 3 When a traditional Li-ion battery is first assembled, it typically goes through a processing step called “Formation”. Upon assembly, a traditional Li-ion battery is in an uncharged state and the first step in the formation process is to charge the battery. The first charge results in the formation of an electrolyte decomposition layer on the surface of the negative electrode due to the high reactivity of graphite and lithium ions in the presence of the electrolyte. This layer is called the Solid Electrolyte Interphase (SEI). The SEI layer is a crucial element of traditional Li-ion batteries and acts as a safety feature by maintaining a protective barrier between the reactive negative electrode and the electrolyte. The SEI is just porous enough to allow the passage of lithium ions for low to moderate rate charge and discharges. Safety Problems in Low Temperatures On the downside, the SEI layer limits the discharge rates and seriously renders the battery un-chargeable at cold temperatures and cannot be charged below -5°C. At cold temperatures the pores in the SEI are effectively closed. If charging a cell at severely low temperatures is attempted, lithium metal will plate on the surface of the SEI, resulting in two dangerous conditions, either of which can cause the battery to enter thermal runaway. Safety Problems in High Temperatures If the temperature of the battery rises above 120°C the SEI layer dissolves and the negative electrode can chemically react vigorously with the electrolyte. The SEI breakdown temperature can be relatively easily achieved through aggressive charging or manufacturing defects that result in internal shorting. Either reaction also causes the battery to enter thermal runaway. Safety First: The nLTO-Based Li-Ion Battery One battery manufacturing company, Altairnano, has created a new Li-Ion battery with multiple benefits, including the elimination of safety issues. Altairnano’s technology is based on a nano-size lithium titanate oxide (nLTO) battery electrode material where nLTO is substituted for graphite, the standard negative electrode material employed in common Li-ion rechargeable batteries. Unlike a traditional Li-ion battery, the nLTO-based battery undergoes no formation during the first charge cycle. This is because the nLTO material is not reactive to the electrolyte in the presence of lithium ions. Therefore, no SEI layer is formed. As discussed previously, the SEI in a traditional Li-ion battery is a necessary component for the battery to operate safely. However, this safety can be compromised quite easily as the SEI is meta-stable. Safety in Low Temperatures An nLTO-based battery can be charged to about 90% of its room temperature capacity in 30 minutes at -30°C. Other battery types including lead acid, NiMH, and NiCd show virtually no charge acceptance at such low a temperature. Due to the lack of a SEI, the large surface area, and the ease of lithium atom migration, charge kinetics achieved at normal temperatures are retained at very cold temperatures in an nLTO-based battery. Safety in High Temperatures Within an nLTO-based battery, the nLTO does not react with the electrolyte. Therefore, an nLTO-based battery operates safely at temperatures as high as 75°C. In fact, an nLTO battery will perform better at elevated temperatures in terms of charge and discharge rates than at room temperature, while maintaining its inherent safety. As an nLTO battery operates at a lower voltage than a traditional Li-ion battery (2.3 vs 3.6), there is no danger of encountering the failure modes associated with lithium metal deposition. Safety First 4 Figure 1: High temperature testing results; Left – Traditional Li-Ion battery; Right – nLTO-based battery (note the nLTO battery did not explode) The discharge voltage limit of a traditional Li-ion battery must be tightly controlled as copper used as the negative electrode current collector will dissolve in the electrolyte below about 2V. This can lead to shorting and thermal runaway. A traditional Li-ion battery must be highly controlled in terms of its temperature and voltage states and has a limited window of safety operation with only six regions of temperature and voltage safety stages. An nLTObased battery demonstrates a greater range of safety – anywhere from -40°C to +260°C. Figure 2: Traditional Li-ion battery safe temperature ranges and voltage states are confined to a relatively small window Safety First 5 Figure 3: nLTO-based battery safe temperature ranges and voltage states indicate a greater margin of safety (from -40°C to +260°C) Safety in All Tests All Li-ion batteries should undergo thermal runaway tests including deliberate shorting, via nail penetration tests and thorough super heating the battery to initiate thermal runaway at high temperatures. nLTO-based batteries exhibit pass marks with no smoke or flames in short circuit, forced discharge, over charge, over discharge, nail puncture, crush, over temperature, and drop tests. Furthermore, nLTO-based battery packs do not need significant cooling or temperature monitoring, thereby reducing costs and eliminating safety concerns. About the authors V. Evan House, PhD is the Director of Advanced Materials & Power Systems at Altairnano email: [email protected]; and Fayth Ross, MS is the Marketing Manager at Altairnano - email: [email protected]. Safety First 6
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