Eight Amazing Engineering Stories by Bill Hammack & Patrick Ryan & Nick Ziech

Author:Bill Hammack & Patrick Ryan & Nick Ziech
Language: eng
Format: mobi, epub
Tags: engineering
ISBN: 9780983966135
Publisher: Articulate Noise Books
Published: 2012-04-07T00:00:00+00:00

At the negative electrode -- graphite -- the lithium becomes embedded within the graphite structure as shown in the figure: C6 + xLi+ + xe- → LixC6. At the positive electrode, lithium ions can leave the channels of the cobalt oxide that traps them: LiCoO2 → Li1-xCO2 + xLi+ + xe-. When discharging, the lithium atoms move from the tunnels in graphite to those in the cobalt oxide, and the reverse happens when charging. Because of the motion of the lithium ions, these batteries are often called “swing” or “rocking chair” batteries.

Why Lithium Laptop Batteries Explode

If not designed or used correctly, a lithium battery can explode. This is partly due to its high energy density but also because the materials used in it -- lithium ions, cobalt, and organic solvents -- can lead to a lethal cocktail. While an engineer might like to use other materials, these are the only ones that provide the necessary high voltages. In a lithium-ion battery, all the energy can be released at once by an internal short circuit. Just think of the damage caused if you short out an electrical outlet at home. This short circuit releases energy, causing uncontrolled flows of current, which produces heat that makes the temperature rise inside the battery. A lithium-ion battery can heat itself to over 700 oC in a matter of minutes. (This is called joule heating: heat produced by a current flowing through a conductor.) These high temperatures supply the energy necessary to sustain a host of new reactions.

For instance, the lithium in both the positive and negative electrodes can react with the organic electrolyte to give off ethylene gas. Also, the lithium phosphate salt breaks down in the organic solvent, catalyzed by the cobalt in the positive electrode, to create PF5 gas. All of these reactions are highly exothermic; that is, they give off heat as the reactions occur. This leads to what is known as thermal runaway inside the battery. The reactions begin to generate heat which, in some batteries, produces heating rates as high as 400 oC/minute. This, in turn, increases the rate of the reactions, creating gases until an explosion occurs.

To prevent an explosion, lithium batteries have two key safety mechanisms. First, the separator is designed so that at 125 oC or so its pores close up, thus preventing the flow of ions and stopping the current flow. However, if this separator is pierced, this mechanism won’t work. So lithium-ion batteries have sophisticated electronic controls that keep track of the voltage and shut the battery down if it reaches a dangerous voltage. Although they are manufactured with safety in mind, lithium batteries can also have design flaws that allow them to explode. In 2006, Sony had to recall almost six million laptop batteries used by nearly every major laptop manufacturer because the batteries were found to fail 1 out of 200,000 times. Although this might seem small, in battery manufacturing, safety incidents with correctly made lithium-ion batteries only number one in ten million.

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