Battery Failure and the 787 Dreamliner
Boeing's 787 Dreamliner is an ambitious feat of engineering that eschews many age-old conventions of airliner construction. For instance, the body of the plane is made of a lightweight composite material instead of the traditional aluminum, it uses technologically advanced and efficient engines, and (most controversially) uses fire-prone Li-ion battery pack in place of hydraulically controlled systems like the brakes and the engine starter.
Is this a modern Icarus story of being too ambitious, or can the grounded 787 get its wings back after flying too close to the sun?
Previous aircraft have had other types of batteries on board, such as the 777 with nickel cadmium batteries, but this battery chemistry wasn't good enough for what Boeing wanted to do with the 787. NiCd batteries have a low energy density, so they end up weighing a lot, which is antithetical to the 787's lightweight designs. What's more, after many charging cycles, NiCd batteries suffer from the memory effect that effectively reduces the energy capacity. Perhaps most importantly, NiCd batteries also can't deliver electricity very quickly, so applications like starting a jet engine aren't practical. Li-ion batteries are more energy-dense, have better power delivery, and don't suffer from the memory effect, so they offer many tangible advantages over NiCd that enable them to be used in new, interesting ways on the 787.
There is one downside for Li-ion batteries, however. They have been known to occasionally burst into flames, seemingly at random. But we live in a physical world where batteries don't just spontaneously combust without a reason; these failures happen because something has gone awry in the battery or its design. To understand what's happening, first we need to talk a little bit about how batteries work. The specific type of Li-ion battery in the 787 is pretty similar to the one in your phone or your laptop. When discharging, lithium ions travel from the the anode to the cathode, releasing electrical energy. The anode is made of graphite, and the cathode material for these batteries is lithium cobalt oxide (LiCoO2).
If the battery is overcharged, a problem can arise with the graphite anode. The lithium atoms sometimes won't stay in the graphite but will start to crystallize as lithium metal instead. The metal grows into a pointy crystal called a dendrite. These long, thin crystals can grow across the battery and cause a short, releasing a bunch of energy all at once. This energy is released as heat in the cell, and the increase in heat can cause cause other unwanted reactions to start happening. These reactions also release heat, reinforcing the earlier reactions, causing a phenomenon known as "thermal runaway" in the battery. This is the point where the flight attendant checks the lavatory for any rebellious smokers, but alas, it's no cigarette, the battery is on fire. The Wall Street Journal has reported that this type of dendrite formation is being heavily investigated by the NTSB as the cause of the fires, although the Bureau cautions in comments to Reuters that "we are still considering several potential causes for the short circuiting" in addition to the dendritic formation.
So thermal runaway and flaming batteries are obviously undesirable. What can be done to prevent this type of failure? The trouble really lies with the dendrite. If Boeing and its battery supplier could prevent the dendrite from forming, there would be no electrical short, no thermal runaway, no fire, and no problem. One way to prevent the dendritic destruction is to make lithium really "want" to be in the anode instead of forming a lithium metal. There are other anode materials that bind lithium atoms more strongly, which will prevent the formation of metal.
Another potential problem is the LiCoO2 cathode. Ji-Guang Zhang, a researcher at the Pacific Northwest National Laboratory, tells the Seattle Times that a safer alternative for the cathode is lithium iron phosphate (LiFePO4). LiFePO4 has a few advantages over it's cobalt-containing counterpart. LiCoO2 is not particularly cheap and cobalt is potentially carcinogenic. In contrast, LiFePO4 could be cheaper (if these types of batteries are built to the same scale as current Li-ion batteries) and are less likely to give you cancer if you are exposed to its innards. And importantly for this discussion, LiFePO4 is more chemically stable than LiCoO2 and is less likely to cause nasty reactions. But there's one major downside: capacity.
LiFePO4 is higher in energy than LiCoO2, and because the energy gleaned from a battery is related to the energy difference of lithium in the anode and the cathode, LiFePO4 batteries are less energy dense. Currently, one of the LiCoO2-containing batteries weighs about 60 pounds, and to store the same amount of energy, a LiFePO4-containing battery would have to weigh even more. Considering the maximum takeoff weight of the 787 is north of 500,000 lbs, a heavier battery wouldn't be a deal breaker. However, it will also take up more space, it might be tricky to make room for it. Also, because of the reduced voltage, the power is also reduced.
Another possibility is that the battery was just designed poorly (Tesla's Elon Musk certainly thinks so). Part of the problem is that the 787's battery puts the individual cells in close contact, making it difficult to dissipate heat and more likely that failure in one cell would be difficult to isolate. The close proximity could cause all of the cells to fail instead of just one.
The root cause of the battery failures could be due to any of these individual problems or some combination of them, and the investigation to get to the bottom of the matter could take years. But Boeing is determined to return its planes to the air and has been hard at work on a fix. Bloomberg reports that Boeing has submitted a proposal for a redesigned battery to the FAA that includes several modifications. The new battery is said to have a glass case to contain fires, increased spacing between battery cells to prevent multiple cells failing, and adding a ventilation system that would prevent a buildup of fumes within the battery. These new batteries are being designed so that they can easily be swapped with the old ones. Notably, Airbus is heading in the opposite direction, scrapping plans for the Li-ion batteries in its next-gen A350 and instead opting for NiCd ones.
Boeing had hoped that they could get the 787 off the ground by March, but United Airlines seems to be slightly more pessimistic, scrubbing the aircraft from its flight schedule until June. Until then, expect to fly the old-fashioned way - NiCd batteries, hydraulics, aluminum, and all.