Topic 6: Special Safety Concerns with Lithium-Ion (Li-ion) Batteries
What causes Li-ion to fail?
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 gasoline 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 knowns 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 presence of such particles; however, complex assembly techniques make the elimination of all metallic dust a challenge. 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.
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 sizable 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 damages the insulation layer in a cell, causing an electrical short. The temperature can quickly reach 500ºC (932ºF), 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.