[NSRCA-discussion] Swelling Lipo

[Chris]

Here is the full article with much more detail.
Chris


    What's Really Going On Inside A Dying Lithium Battery

Thu, 07/08/2010

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!


    Cause #1: WATER in the mix.

/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.

    * Cathode: LiCoO2 or LiMn2O4
    * Separator: Conducting polymer electrolyte
    * Anode: Li or carbon-Li intercalation compound

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.


    Cause #2: Formula degradation from over-charge/over-discharge

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.


    Cause #3: Poor separator construction

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).


    Exacerbating factor: HEAT.

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.


    Conclusion

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.


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