Samsung was in the news last year due to problems with its Galaxy Note 7 mobile telephone, which suffered a number of battery failures and fires.
Most mobile telephones use rechargeable lithium-ion batteries, as do a number of other products which have experienced similar problems. These include Hewlett Packard and Sony laptop computers, EE Power Bars, the Boeing 787 Dreamliner aircraft, E-cigarettes and personal hoverboards. Lithium-ion batteries are commonly encountered in the form of “18650” cylindrical cells that are similar in size to AA batteries and commonly used in the battery packs of laptop computers (incidentally, the battery of a Tesla Roadster contains 6831 individual 18650 cells!). Other formats include flat hard case (prismatic) cells such as those used in mobile telephones and pouch cells which do not have a rigid case, often referred to as lithium-ion polymer batteries or LiPo for short.
Left: A battery pack comprising six 18650 cells from a laptop computer.
Lithium-ion batteries, in common with other batteries, contain two electrodes; an anode and a cathode, which are usually made of copper and aluminium. The anode and cathode are formed of thin strips which are rolled or folded together with a separator layer, usually porous plastic, between them. The battery cell is filled with a solution (the electrolyte), which contains lithium ions. The lithium ions migrate between the anode and cathode during use (discharge) of the battery, and in the opposite direction during charging.
Lithium-ion batteries are characterised by very high energy densities, which is one of the reasons they are so popular. The cell chemistry involves some heat production, but the energy density means that thermal runaway, an uncontrolled increase in temperature, can occur if something goes wrong.
Lithium-ion batteries are susceptible to over-charging and over-discharging. Over-discharge can cause the copper electrode to dissolve, leading to other problems during charging that can ultimately lead to thermal runaway. Slight over-charging can cause solid lithium to gather on the electrodes, which can cause dendrites to form, more of which later. More severe over-charging, such as the use of a charger of too high a voltage or current output, can lead to immediate overheating and thermal runaway.
Consequently there is usually charge protection circuitry either within the battery or within the charger. High quality batteries usually have internal protection mechanisms, although of course these can be absent in the case of counterfeit or low quality products. Such mechanisms include safety vents, charge protection circuitry, over-temperature devices and devices that mechanically interrupt charging.
Right: A battery comprising two (silver-coloured) pouch cells and a (green) charge protection circuit board on the left.
However, consider a device in which the charge protection is incorporated into the charger, and not the battery. If a different charger is then used to charge the battery, and that charger does not have charge protection, the battery could be over-charged. This seems particularly to be a risk with e-cigarette batteries, which often seem to be mixed-and-matched with different chargers and in which counterfeit and cheap, low quality products tend to be encountered. Another fault that can arise is short circuiting of the anode and cathode. Foreign bodies, such as shards of metal or welding/solder spatter introduced during manufacture, can puncture the separator and create a short circuit. Dendrites (metallic growths emanating from the electrodes) can also puncture the separator. Tears and punctures of the electrodes during manufacture can also occur. One of the Boeing Dreamliner fires was determined to have been caused by poor manufacturing processes which allowed an internal short circuit to occur, and thermal protection devices that did not account for the most extreme self-heating scenarios.
If a short circuit occurs, localised overheating can develop. Sometimes a short circuit can clear itself, since the separator is usually designed to be “self-healing”. However, on occasion the short circuit does not clear, and thermal runaway can occur.
Left: short circuits that have self-healed.
Thermal runaway raises the pressure inside the battery which can cause it to burst open, ejecting the electrolyte. The electrolyte in many cells includes a flammable organic solvent, which can ignite when it is ejected. This is the “explosion” often reported by witnesses to these events, and it can certainly be dramatic, as witnessed in this video. Such ejections can cause the cell contents or the battery itself to travel some distance, causing a fire away from the original location of the battery. Other types of battery cell, such as lithium-ion polymer (LiPo) cells differ slightly, in that they have a gel, rather than a liquid, electrolyte. Fires involving LiPo batteries therefore tend to be less dramatic, although they can still catch fire. They are often used in mobile telephones and radio controlled toys.
Above left: A fire in a laptop computer caused by a battery fault. Right: a failed laptop battery cell from which the end cap and electrolyte solution were ejected.
Other causes of short circuits include mechanical damage to the battery. These are more likely to occur with pouch cells, which do not have a rigid case, than with laptop batteries for example, which use hard case 18650 cells contained within a rigid plastic case.
Self-heating of lithium-ion batteries can also be initiated by elevated environmental temperatures. Thus batteries in unshaded storage, in ships holds, or otherwise exposed to heat can be susceptible to thermal runaway, particularly if stored in bulk. The tendency to thermal runaway also relates to the charge level, which is why batteries often fail during or after charging. The normal shipment of batteries in a pre-charged state therefore creates a greater risk of fires occurring in transport.
In addition to the usual fire investigation techniques, the investigation of fires involving lithium-ion batteries often requires the use of specialist equipment, such as scanning electron microscopes (SEM) and computerised tomography (CT) scanners to identify the precise cause of the failure.
Right: A CT scan of the batteries in the previous photograph.
Hawkins has investigated many fires involving lithium-ion battery failures and can assist in the investigation of such claims, whether fire or personal injury related. Please contact us if you would like any further information on this matter or if you would like to appoint us to investigate a case.
Prior to joining Hawkins in 2009, Nick spent nine years with the Forensic Science Service in the UK carrying out police forensic work. Now based in our Dubai office, Nick specialises in the investigation of fires and explosions. Nick has investigated several hundred fires and explosions caused by the failure of high and low voltage electrical equipment, faults in electrical appliances and lithium batteries, self-heating, hot work, arson and the ignition of flammable vapours. He has considerable experience of cases involving product recalls, public liability and product liability on behalf of corporate and insurance clients.