Warning: Science ahead! Close your eyes and turn away, you've been warned!
Many radio-control enthusiasts experience disappointment with the cycle life of their Lithium-based batteries in electric aircraft. Often this is because they're not entirely sure what's going on inside the battery, and choose a capacity or voltage that's inappropriate for their application. Ultimately, this manifests itself in "swelling" or "ballooning" of a Lithium battery. This editorial attempts to explain what's actually going on when this happens.
Chemically, there can be three causes for the swelling of a LiPo battery, and one exacerbating condition that makes it worse across the board. These occur in hard-shell Lithium Ion batteries, too, but the hard shell can withstand several atmospheres of pressure before expanding.
Note: This is MY understanding of the chemistry involved. I may be off-base, after all, I'm a college dropout. But I did love chemistry class!
EDIT: Lithium manufacturers who's products are implicated in this assertion (read: Hextronik et al, circa 2006-2007, Thunder Power circa 2008) will dispute the assertion of contaminated Lithium. The most common contaminant is water, but there are many others that will cause lithium oxidation in the cell. Basically, any other substance containing oxygen that can be freed by electrolysis or heat will become a contaminant, and any substance that isn't the expected anode, cathode, or separator is a contaminant that will reduce the performance of the cell and cause swelling in other ways. Manufacturers have a fiduciary responsibility to claim that there was no product defect, otherwise they're responsible for a recall. I'll talk about the science and let you draw your own conclusions.
This was the common problem with many cheap Chinese LiPos of around 2005-2008. Most are better now, but it's the #1 cause of premature LiPo failure: water contamination in the plant. Many of China's LiPo factories are on the coast, where the altitude is very low and the humidity is high. You can't run the humidity too low on the assembly floor, because you're working with volatile chemicals that could explode in the presence of a spark, and you can't run it too high because then you end up with a worthless LiPo that swells on first use.
Here's the science. You have three ingredients that are functional in a LiPo battery. The rest is wrapping and wiring attachments.
I'm going to be a little vague in my language here. The chemicals involved vary according to manufacturers, so I don't want to make any assumptions.
Remember your chemistry class? Note the absolute lack of any hydrogen atoms in the reaction. None, zero, zip, nada. If you have water inside your battery -- and virtually all batteries have a little bit -- you've got problems. When the chemical bond of H20 is broken by electrolysis and heat, you end up with free oxygen. You also have free-roaming hydrogen that typically ends up bound to your anode or cathode, whichever side of the reaction it's on and depending on the state of charge of your battery.
Now, this is a pretty unstable situation that's exacerbated by any over-discharge or over-charge condition creating metallic lithium in your cell. The end result is Lithium Hydroxide: 1 atom of lithium, one atom of hydrogen, and one atom of oxygen.
But you still have a free oxygen atom floating around inside the battery casing, that typically combines with one other oxygen atom -- O2, or what we sometimes think of as "air" -- or two other oxygen atoms, to form a characteristic tangy, metallic-smelling substance called "ozone", or O3. Gases expand with heat and contract with cold. Chuck a swollen battery in the freezer and it might come out rock-hard again... until it heats up. It's not frozen, it just got cold enough that the gases inside didn't take up much space at all.
And that free O2 or ozone is just waiting to pounce and oxidize some lithium on the slightest miscalculation on your part. The modest over-discharge during a punch-out, or running the battery a little too low or letting it get a little too hot, or running the voltage up to 4.235v/cell on a cold day when the actual voltage limit per cell is more like 4.1v. All of these create the perfect storm for a puffy battery to quickly turn itself into a ruined battery or an in-flight fire.
Understanding the role of free oxygen in your battery, from water and other causes, is CRUCIAL to understanding why batteries fail, and why sometimes you can get by with flying a puffy battery, and sometimes you can't.
If a Lithium battery is overcharged or charged too quickly, you end up with LOTS of excess free lithium on the anode (metallic lithium plating), and free oxygen on the cathode. A free oxygen atom is small enough to freely traverse the separator without carrying an electric charge, resulting in lithium OXIDE on the anode. Lithium "rust", in reality. Useless to us at this point, just dead weight being carted around inside your battery's wrapper.
But lithium oxide uses fewer oxygen atoms than existed in the ionized state, so you end up with, again, FREE OXYGEN. And people wonder why if you over-charge a LiPo underwater, it still ignites despite the lack of open air...
If it's over-discharged or discharged too quickly, the reverse is true, but you end up with Lithium Oxide on the cathode, but at a lower rate because there's simply less there. Basically, an abused battery quickly develops corrosion on both poles of the battery inside the wrapper. And the more it's abused, the worse it gets as the resistance goes up and it still gets driven hard.
This, by the way, is the most common cause of swelling today for our aircraft when flown with a high-quality pack (not knock-off eBay leftovers from expensive Chinese mistakes of 2004-2009). The reality is, these kinds of cells, regardless of their 'C' rating, are built for use where they last for several hours... not several minutes. While the chemistry if used as designed is good for thousands of cycles, we're driving them so far out of spec that we're lucky to get hundreds of cycles out of them.
In most cases, too, our batteries are under-specced. If slow-charged and slow-discharged, many of these packs would often hold considerably more mAh than we think they do. That's one of the reasons we get the performance we do from them. Higher-C-rated packs also often introduce gelled electrolyte into the separator, and carbon or phosphorous nano-structures on the anode and cathode mixtures rather than the "pound it out thin and hope it's mixed right" approach used with sheets of anodes & cathodes today.
A number of cheap LiPos also use a bad separator formulation. Ultimately, it often boils down to using a dry separator with way too high of an internal resistance to hold up to manufacturer "C"-rating claims. The internal resistance grows over time because a higher and higher percentage of the LiPo is simple Lithium Oxide, and the balloon grows bigger as more oxygen atoms are freed.
I'd also lump "poor anode or cathode chemistry" into this category, too. Ever get a bad battery out of a batch of good ones? Often it's because the mixture of chemicals was inconsistent, and you end up with too much or too little lithium on one side of the battery (well, in certain plates, you get my drift).
A little heat makes everything work better for a Lipo. If you could fly your battery right at 140 Fahrenheit all the time, it would make fantastic power and be operating right in its happy zone. But it generates heat when charging, and when discharging. Hitting 150 results in significant metallic lithium generation, which as we can see from above is a major cause of puffing and cell destruction.
Similarly, the maximum 4.235v/cell limit is only at that mythical 140F. It goes down steadily from there, to about 4.2v/cell at room temperatures, and around 4.0v/cell below 50F, beyond which the over-abundance of electrons will again break chemical bonds and free lithium to bond with oxygen and create lithium oxide... which is just a disaster waiting to bond with humidity in the air if the LiPo ruptures, to create Lithium Hydroxide.
Chemically, there are no LiPos that will not puff under certain circumstances. But tightly-controlled humidity, a superb gel separator, nano-structured anode and cathode, and careful charging and discharging within manufacturer limits should also prevent puffing. Similarly, putting a pack that has been abused into a lower-discharge aircraft, even when puffed, often serves the purpose of stopping the puffing in its tracks because no more metallic lithium is being created in the cell by abuse.
And now you know the answer to today's geeky topic. Why lithium polymer batteries often puff up.