Li-ion Batteries in the Hot Seat (A Primer on the Science of Exploding Smartphones)!
Once again exploding batteries are in the news, and once again, Lithium ion (Li-ion) batteries are in the hot seat (puns intended!). First it was Boeing 787 batteries, then hoverboards, and now it’s the Samsung Galaxy Note 7 smartphone. So why are Li-ion batteries continually finding themselves in hot water (my last bad pun, I promise)?
First, a bit of chemistry 101 as background. All chemical reactions involve the transfer of electrons as chemical bonds are formed. In the formation of a bond, one atom donates an electron to another atom, and that combination is at a lower energy state than the individual pieces. For example, if two hydrogen atoms (which have one electron each) donate those electrons to an oxygen atom, a water molecule is formed, and the excess energy will be released as heat (the most famous example of which is illustrated below in Figure 1):
The bit of magic that batteries perform is that the donation and acceptance of electrons occur in locations physically separated from each other, and the electrons are forced to take a very long pathway (involving an external circuit) to go from the donating atom to the accepting atom. Figure 2 shows a diagram of a Li-ion battery during discharging and charging. In a charged Li-ion battery, the Li atoms are initially located in the ‘anode’, which in its simplest form is graphite (i.e., the same material that makes up pencil leads). During the discharge process shown in Figure 2 (a), the Li atoms inside the graphite anode release electrons, which are collected by the external circuit, and after passing through an electrical load (whether a motor or the circuitry of a cell phone), the liberated electrons end up at the opposite side of the battery (the ‘cathode’). Meanwhile, the resulting Li-ions drive off in search of the lost electrons. The only pathway the Li-ions can take is through the liquid electrolyte, which provides a conductive path for Li-ions (but not electrons) to cross from the anode to the cathode of the battery (along the way passing through a separator film that keeps the anode and cathode from touching). In the cathode, the Li-ions find spaces to park within the structure of the cathode material. The other atoms that comprise the structure of the cathode (in the original Li-ion battery the cobalt atoms present in the form of cobalt oxide) have accepted the electrons arriving from the anode, maintaining electrical charge balance (for every Li-ion arriving through the electrolyte, an electron arrives from the external circuit).
There are only a limited number of places where the Li-ions can fit within the cathode, and once all the parking places are full, the battery must be re-charged. During the re-charging process, shown in Figure 2 (b), an external power supply pulls electrons from the cathode and pushes them into the anode. The Li-ions within the cathode again migrate in search of electrons, and those electrons are obtained in the anode. However, it is during charging that most of the problems occur with Li-ion batteries. During normal charging, the Li-ions migrate to the graphite anode and refill the spaces available between the sheets of carbon atoms that make up graphite. However, if the charging process is too rapid, Li-ions will pick up the available electrons on the outside of the anode, effectively forming a metallic Li deposit. Once a metallic Li deposit forms outside of the anode, it becomes a preferential location for other Li-ions to deposit, and eventually a dendrite of metallic Li will form, as shown in Figure 3. If this dendrite continues to grow, it can eventually punch through the battery separator and cause a short circuit within the cell between the anode and cathode, and with it the potential to cause overheating and in some cases combustion of the battery.
A short circuit is bad for any battery, but other features about Li-ion batteries contribute to make matters worse. Lithium, as the most reactive of the alkali metals, will combust spontaneously if it comes in contact with water, so the electrolyte that fills the battery must be made of a non-aqueous organic liquid which is also highly flammable. If a Li-ion battery ruptures due to overheating, then the exposed electrolyte can also catch fire, and the highly reactive Lithium will contribute to the severity of the fire.
Li-ion batteries have an inherent danger due to their use of highly reactive Lithium and combustible organic electrolytes. However, with proper design (involving battery management circuitry to limit charge and discharge rates), Li-ion batteries can be safely incorporated into electronic devices.
So what got the Galaxy Note 7 into hot water? This matter has been the topic of serious speculation, but unless we were privy to the details of the proprietary battery technology that the Note 7 uses, we can only speculate. Failure mode discussion has centered around battery manufacturing defects and overly aggressive battery charging rates, which is consistent with the idea that dendrite formation could be a contributing factor to the issue. One observation that might be useful for all users of Li-ion batteries is that dendrite formation is inherently a process associated with the charging process, so if at all possible do not leave your devices attached to a charger any longer than necessary. So, sleep tight (with your cell phone disconnected from that charger next to your bed!).