Thinking About Lithium Batteries and Safety

We’ve recently seen several high-profile cases with videos of lithium-ion laptop computer batteries burning at near-explosion intensely. (One highly publicized picture of a destroyed truck turned out to have a longer story: laptop batteries had actually ignited ammunition which blew up the gas tank. Dell has recalled millions of Li-ion laptop batteries, followed by Apple. What does this mean for Li-ion battery safety in battery electric (EV) and plug-in hybrid (PHEV) vehicles?

I asked CalCars Tech Lead Ron Gremban to put together an overview of some of the main issues, taking into account what’s public information on the approaches of some companies using lithium batteries for cars, such as (alphabetically) AC Propulsion, EnergyCS, Hybrids-Plus, Hymotion, Phoenix Motorcars and Tesla Motors. Ron has tried to strike a balance between technical discussions and broad descriptions, sticking primarily to existing products and solutions. We’re aware that this is a very complex subject and that technologies are evolving rapidly. We are describing starting points and rasising questions.

First, we need to remind ourselves that the current automotive fuel source is highly flammable and explosive gasoline, stored in a lightweight steel tank. Over time, we’ve learned how to make this reasonably safe — and to live with the occasional explosive consequences, especially in crashes. (In the U.S., over 250,000 vehicles of all types caught fire in 2005. This may amount to 1 in every 1,000 vehicles — far more than one might expect.)

How will Li-ion battery packs compare? The laptop computer fires that prompted the recent recalls are very rare — on the order of one in 60 million cells –but with hundreds or thousands of cells in each vehicle, the likelihood of failure is both greatly increased and more dangerous.

Though we hope all manufacturers will use best design practices, including safety features detailed below, it’s impossible to anticipate all possible modes of failure. Until enough EVs and PHEVs are on the road to gather meaningful statistics, we’ll be guessing about what could happen. Even then, some designs will fare better than others, and we will continue to learn from the school of hard knocks.

NOTE: I am not enough of an expert on battery chemistry for this to be used as a design document.

Li-ion batteries’ major specific issue is their propensity for thermal runaway. Above a certain temperature, usually 80-150 degrees C, a reaction can occur that produces more heat than can be dissipated. And since each cell contains both fuel and oxidizer, the reaction fuels itself. The temperatures and pressures produced are high enough to melt steel barriers and shoot flames over 20 feet. This makes containment difficult. Additionally, any battery can be shorted, producing an explosive energy discharge if not quickly stopped.

Though proper fusing can greatly reduce the likelihood of serious results from inter-cell shorts, little can be done to save a cell with an internal short, such as occurred in the laptop cells that prompted the recalls. "Rick Clancy, a Sony spokesman, said, the problem appears to have been caused by microscopic metal particles in cavities in the battery cells in each battery pack." (San Francisco Chronicle report) Apparently, the particles occasionally caused an internal short after vibration moved them around.

We can divide available prevention methods into cell chemistry, cell design, electronic control and monitoring, and battery pack design and containment.

CELL CHEMISTRY: Lithium is a very volatile metal that will burn if exposed to air at room temperature. Lithium-ion cells are so named because the lithium in them is not pure but combined into ionic compounds — a trade-off that increases safety at the cost of reduced specific energy (energy storage per battery weight). Generally, cobalt, manganese, and/or phosphates are used in these compounds for a Li-ion cell’s cathode. The anode is usually carbon, and the electrolyte is a flammable organic solvent. If the cell’s voltage is allowed to exceed or go below certain values, metallic lithium can "plate out," not only degrading the cell, but making it much more susceptible to future thermal runaway.

Li-ion cobalt batteries, used in laptop computers and cell phones because of their superior specific energy, are the most susceptible to both thermal runaway and the plating-out of metallic lithium. In fact, while reaching a full charge requires a voltage of 4.15V, each cell’s voltage must stay under 4.25V to prevent plating! This requires unusually precise electronics on a cell-by-cell (or set of parallel cells) basis (see below).

Manganese is often used in Li-ion cells for power tools because it produces higher specific power (the rate at which energy can be supplied) despite lower specific energy. This chemistry is less susceptible to thermal runaway and plating. Even less susceptible are phosphate-based cells, but they trade off even more specific energy. Three companies — Valence, Electrovaya, and A123 — are starting to provide these safer phosphate-based cells for electric vehicles. So far, manganese and phosphate cells have been significantly more expensive than their cobalt brethren. This is mainly due to lower manufacturing volume, but has so far limited their use in cost-effective electric vehicles.

CELL DESIGN: Various fail-safe mechanisms can be, but are often not, built into each Li-ion cell:

* An over-temperature and/or over-pressure cutoff device (usually permanently disabling the cell) can save the cell from thermal runaway if the source of the problem is current into or out of the cell.
* A flame-retardant can be added to minimize the effects of a cell fire. Quallion has developed such an additive. It has not yet been incorporated into consumer product batteries, but it could provide a cost-effective additional layer of safety for EV battery packs.

ELECTRONIC CONTROL AND MONITORING: The tight 4.15-4.25V Li-ion charge voltage tolerance (somewhat different for non-cobalt-based cells) means that each cell (or set of cells in parallel) must have its own precise over-voltage protection circuit. Otherwise, the voltage balance among series strings of cells may vary too much due to variations between cells, or to degradation or failure of a single cell. Though under-voltage protection requires less precision, it is usually included in the same circuits. As protection circuits can occasionally fail and allow the very conditions they were designed to avoid, additional layers of protection are required as well.

The next level of defense is to detect overheating and shut things down before thermal runaway. This, as well as monitoring voltages and temperatures that are still within limits, can catch some battery anomalies that might otherwise eventually turn into catastrophic failures.

BATTERY PACK DESIGN AND CONTAINMENT: A well-designed battery pack will normally prevent cells from heating to unstable temperatures. Even such basic thermal design can be overlooked. And though containment of a multi-cell fire may be difficult, optimal design will prevent a single-cell fire from spreading. Tesla Motors says its testing demonstrates that its liquid cooling and careful design does indeed prevent the spread of a single-cell fire.

Battery containment should be designed to vent fumes from malfun
ctioning cells rather than send them into the passenger compartment. Packs should be placed beyond crush zones, if possible. For PHEVs they should also be as far as possible from the fuel tank.

The: US Advanced Battery Consortium (USABC) focuses on specifications for batteries used in vehicles. Its Abuse Test Procedures Manual includes mechanical (shock, drop, penetration, crush, etc.), thermal, and electrical abuse tests. This test regime provides an objective standard with which to judge battery packs for safety. The major automobile manufacturers will no doubt (for very good reasons) insist on their vehicles’ battery packs passing the USABC tests.

Increased public awareness will no doubt put a spotlight on companies building or modifying vehicles into EVs and PHEVs, to explain their approaches to battery safety. Whether or not they submit their packs to formal USABC testing, the USABC safety criteria must be addressed. Tesla Motors Co-Founder Martin Eberhard, for example, responded to questions in a Technology Review Forum on August 4 and then in a Battery White Paper, Tesla delineated the layers of safety features in its battery pack.

Of course, experimenters must also be cognizant of battery safety issues. Though they may not have the resources to validate their designs through destructive testing, they still need to design for both thermal and electrical safety.

Though safety issues must be addressed in vehicle and battery pack designs, we believe well-designed EV and PHEV vehicles with Li-ion battery packs can be as safe as the gasoline vehicles they replace.

Felix is an entrepreneur with a life-long green streak. He enjoys communicating his enthusiasm about what is new, unique, and significant. He is the founder of CalCars.org, The California Cars Initiative, and has been promoting 100+ MPG plug-in hybrids full-time since 2001. He posts his own selection of significant developments for PHEVs at the CalCars News Archive. His first entry at Hybrid Cars, Car Owners Strap into the Drivers Seat, in August 2005, expressed his view that the industrial world is in the midst of a major change — hopefully, it is not too late!


  • rwielk

    Altair Nano claims to have produced a Li-ion battery with very favorable safety characteriistics. They say that by using nano-structured electrodes, they have eliminated the thermal runaway problem. see this link: http://www.b2i.us/profiles/investor/ResLibrary2.asp?BzID=546&GoTopage=&Category=856
    which will probably need to be cut and pasted to be accessed. They will be presenting at the CARB ZEV Symposium. Here is their anouncement: “Advances in the manufacture of safe, fast-charging, long-lasting batteries used to power electric and hybrid electric vehicles will be addressed by Altair Nanotechnologies Inc. (NASDAQ: ALTI) at the California Air Resources Board Zero Emission Vehicle (ZEV) Technology Symposium, held September 25-27, 2006 in Sacramento, California.”

    This should provide ample opportunity to evaluate the claims of the company. I hope Ron Gremban and others will be there to let the CalCars community know whether Altair’s technology may lead to useable Li-ion batteries for vehicles.

  • Guest

    Has anyone looked at the possibility of using a zinc matrix power battery as a replacement for the lithium ion batteries? Better yet, does anyone have any pratical experience in using them? Any sense for the advantages and disadvantages?

    http://www.zmp.com/press/IDF06.html

  • Guest

    I started out by saying we were “sticking primarily to existing products and solutions.” I immediately got emails with links to press releases (including one dating back to 2003) about better solutions.

    Yes, it will be interesting to see what Altair Nano announces — we’ll be there. And people are looking at all sorts of battery options. That’s discussed in many places. But here I would appreciate focusing on existing implementations.

  • rwielk

    According to their news releases, Altair Nano has sold a battery for delivery this September. To quote “In anticipation of Altairnano’s delivery of its first NanoSafe battery pack for use in an electric vehicle in September, this is the final of four planned news releases identifying features of Altairnano NanoSafe batteries that may prove advantageous in the power rechargeable battery market.”

    If not “existing,” this is pretty close.

  • Guest

    Felix, great blog, thanks. Part of the problem occurs when power flows are high, or when recovering energy from regenerative braking. At least one company discussed recently in a Green Car Congress article, has coupled Ultra Caps with the high energy storage form of the Li-Ion battery, so that should lessen the management of the batteries to preclude fires and explosions.

  • Tony Cardella

    FYI Possible solution to exploding batteries.

    3M settles battery suits with CDW and Hitachi Koki
    Tuesday October 30, 11:40 am ET

    3M Co. said Tuesday it settled patent suits over use of its ion battery technology.
    Under terms of the deal, CDW Corp. and Japan’s Hitachi Koki USA certified that they import and sell batteries containing 3M’s patented cathode materials only from manufacturers licensed by 3M.

    ADVERTISEMENT

    In return, Maplewood-based 3M (NYSE: MMM – News) requested that those two companies be dismissed from the U.S. International Trade Commission investigation over lithium ion battery technology and from a patent suit 3M filed in Minnesota district court.

    3M has settled similar suits with Sony Corp., Lenovo Group, Matsushita Industrial Electric Co. Ltd., Panasonic Corp. of North America over its lithium ion technology.

    Lithium ion batteries are used in laptop computers, mobile phones and portable electronic devices, and are an emerging source of power for battery-powered hand tools and hybrid electric vehicles.