Safety of lithium-based batteries has attracted much media and legal attention. Any energy storage device carries a risk, as demonstrated in the 1800s when steam engines exploded and people got hurt. Carrying highly flammable petrol in cars was a hot topic in the early 1900s. All batteries carry a safety risk, and battery makers are obligated to meet safety requirements; less reputable firms are known to make shortcuts and it’s “buyer beware!”
Lithium-ion is safe but with millions of consumers using batteries, failures are bound to happen. In 2006, a one-in-200,000 breakdown triggered a recall of almost six million lithium-ion packs. Sony, the maker of the lithium-ion cells in question, points out that on rare occasion microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell.
Battery manufacturers strive to minimize the entry of metallic particles into the cell during manufacture. The semiconductor industry has spent billions of dollars to find ways in reducing particles that reduce the yield in wafers. Advanced cleanrooms are Class 10 in which 10,000 particles larger than 0.1µm per cubic meter are present (ISO 4 under ISO 14644 and ISO 14698). In spite of this high cleanliness, particle defects still occur in semiconductor wafers. Class 10 reduces the particles count but does not fully eliminate them.
Battery manufacturers may use less stringently controlled cleanrooms than the semiconductor industry. While a non-functioning semiconductor simply ends up in the garbage bin, a compromised Li-ion can make its way into the workforce undetected and deteriorate without knowing. Resulting failures are especially critical with the thinning of the separators to increase the specific energy.
Cells with ultra-thin separators of 24µm or less (24-thousandth of an mm) are more susceptible to impurities than the older designs with lower Ah ratings. Whereas the 1,350mAh cell in the 18650 package could tolerate a nail penetration test, the high-density 3,400mAh can ignite when performing the same test. New safety standards direct how batteries are used, and the UL1642 Underwriters Laboratories (UL) test no longer mandates nail penetration for safety acceptance of lithium-based batteries.
Li-ion using conventional metal oxides is nearing its theoretical limit on specific energy. Rather than optimizing capacity, battery makers are improving manufacturing methods to enhance safety and increase calendar life. The real problem lies when on rare occasions an electrical short develops inside the cell. The external protection peripherals are ineffective to stop a thermal runaway once in progress. The batteries recalled in 2006 had passed the UL safety requirements — yet they failed under normal use with appropriate protection circuits.
Most major Li-ion cell manufacturer X-Ray every single cell as part of automated quality control. Software examines anomalies such as bent tabs or crushed electrode rolls. This is the reason why Li-ion batteries are so safe today, but such careful manufacturing practices may only be offered with recognized brands.
Why Batteries Fail
There are two basic types of battery failures. One occurs at a predictable interval-per-million and is connected with a design flaw involving the electrode, separator, electrolyte or processes. These defects often involve a recall to correct a discovered flaw. The more difficult failures are random events that do not point to a design flaw. It may be a stress event like charging at sub-freezing temperature, vibration, or a fluke incident that is comparable to being hit by a meteor.
Let’s examine the inner workings of the cell more closely. A mild short will only cause elevated self-discharge and the heat buildup is minimal because the discharging power is very low. If enough microscopic metallic particles converge on one spot, a sizeable current begins to flow between the electrodes of the cell, and the spot heats up and weakens. As a small water leak in a faulty hydro dam can develop into a torrent and take a structure down, so too can heat buildup damage the insulation layer in a cell and cause an electrical short. The temperature can quickly reach 500°C, at which point the cell catches fire or it explodes. This thermal runaway that occurs is known as “venting with flame.” “Rapid disassembly” is the preferred term by the battery industry.
Uneven separators can also trigger cell failure. Poor conductivity due to dry areas increases the resistance, which can generate local heat spots that weaken the integrity of the separator. Heat is always an enemy of the battery.
Fire in the hole!
To the right is a clip from one of our tests, where the cell was intentionally overcharged, demonstrating what can occur when no overcharge protection is included.
As the cell reaches full charge, continued application of charging power causes the lithium ions in solution to plate out on the anode in elemental metallic form, rendering the cell highly reactive & unstable.
As the charge is continued, more lithium plates out, and the cell heats up. This heat, along with the reaction of the now metallic lithium with the flammable electrolyte causes further heating. At this point the delithiated cathode is also beginning to react with the electrolyte, releasing various gases. Both processes are extremely exothermic, and the cell ruptures from the combination of heat damage & internal gas pressure.
As the electrode material breakdown continues, driven by thermal runaway & the continuing charge current, the cell vents the resulting breakdown products. Further heating as a result of the combination of chemical/electrochemical processes eventually reaches the autoignition point, and the cell explodes, spewing flames.
Quality lithium-ion batteries are safe if used as intended. However, a high number of heat and fire failures had been reported in consumer products that use non-certified batteries, and the hoverboard is an example.
Incorrect uses of all batteries are excessive vibration, elevated heat and charging Li-ion below freezing. Li-ion batteries cannot be fully discharged and must be stored with a remaining charge. While nickel-based batteries can be stored in a fully discharged state with no apparent side effect, Li-ion must not drop below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical short. If recharged, the cells may become unstable, causing excessive heat or showing other anomalies.
Heat combined with a full charge is said to induce more stress to Li-ion than regular cycling. Keep the battery and a device away from sun exposure and store in a cool place at a partial charge. Exceeding the recommended charge current by ultra-fast changing also harms Li-ion. Nickel-cadmium is the only chemistry that accepts ultra-fast charging with minimal stress.
Li-ion batteries that have been exposed to stresses may function normally but they become more sensitive to mechanical abuse. The liability for a failed battery goes to the manufacturer even if the fault may have been caused by improper use and handling. This worries the battery manufacturers and they go the extra mile to make their products safe. Treat the battery as if it were a living organism by preventing excess stress.
Industrial batteries, such as those used for power tools, are generally more rugged than those in consumer products. Besides solid construction, power tool batteries are maximized for power delivery and less on energy for long runtimes. Power Cells have a lower Ah rating than Energy Cells and are in general more tolerant and safer if abused.
One of the most accident-prone batteries is Li-ion in an 18650 size cell with an unfamiliar brand name. These batteries made available for vaping do not have the same quality and safety as a recognized brand name. Li-ion is safe if made by a reputable manufacturer, but there have been a number fires and injuries with cells that developed defects and caught fire while carrying in clothing and while traveling.