What's Really Going On Inside A Dying Lithium Battery

Many radio-control enthusiasts are disappointed by the cycle life of lithium-based batteries in electric aircraft. They frequently choose a capacity or voltage that isn't suitable for their application because they don't fully understand what's happening inside the battery. The "swelling" or "ballooning" of a lithium battery is the ultimate manifestation of this. When this occurs, this editorial aims to clarify what's truly happening.

Three chemical factors can contribute to a LiPo battery's swelling, and one aggravating factor exacerbates the swelling overall. These also happen in lithium-ion batteries with hard shells. However, the hard shell may tolerate many atmospheres of pressure before swelling.

WATER in the mix

Manufacturers of lithium whose goods are linked to this claim (see, for example, Hextronik et al., 2006–2007, and Thunder Power, 2008) will contest the claim that lithium is tainted. Although water is the most frequent pollutant, numerous others can also induce lithium oxidation in the cell. In general, any other oxygen-containing material that can be liberated by heat or electrolysis will be considered a contaminant, as will any material that isn't the anticipated separator, anode, or cathode. These contaminants will impair cell function and result in swelling in different ways. In order to avoid being held accountable for a recall, manufacturers have a fiduciary duty to assert that there was no product problem. After I discuss the science, you are free to make your judgments.

This was a prevalent issue with several low-cost Chinese LiPos between 2005 and 2008. Although most are now better, plant water pollution is the main reason for early LiPo failure. A large number of LiPo manufacturers in China are located along the seaside, where humidity is high, and elevation is very low. On the assembly floor, the moisture cannot be set too low since you are working with volatile chemicals that could blow up in the presence of a spark, nor can it be set too high because that would result in a useless LiPo that swells the first time it is used.

The science is here. A LiPo battery has three working components, and wiring attachments and wrapping make up the remainder.

Cathode: LiMn2O4 or LiCoO2

Separator: Electrolyte of conducting polymers

Anode: Carbon or Li-Li intercalation substance

I will use ambiguous wording here. Since the chemicals used differ depending on the manufacturer, I don't want to assume anything.

Do you recall your chemistry course? Take note of the reaction's complete absence of hydrogen atoms—zip, nada, zero, and none. You're having issues if your battery contains water, which almost all batteries do somewhat. Free oxygen is produced when heat and electrolysis break the chemical link of H20. Additionally, you have free-roaming hydrogen that, depending on the battery's charge level and the side of the reaction it is on, usually becomes linked to your anode or cathode.

Now, any over-discharge or over-charge circumstance that produces metallic lithium in your battery exacerbates this already unstable situation. One atom of lithium, one atom of hydrogen, and one atom of oxygen combine to form lithium hydroxide.

However, there is still a free oxygen atom scattered throughout the battery case, which usually joins with one other oxygen atom (O2), or what we sometimes refer to as "air"—or two other oxygen atoms to create the distinctively acidic, metallic-smelling "ozone" or O3. Heat causes gases to expand, and cold causes them to constrict. If you put a swollen battery in the freezer, it may come out solid once more—that is until it gets hot. It simply became cold enough that the gases inside didn't occupy much space; it was not frozen.

And if you make even the slightest mistake in your calculations, that free O2 or ozone is ready to jump in and oxidize some lithium. Running the battery a bit too low, allowing it to get a bit too hot, or running the voltage up to 4.235v/cell on a chilly day when the actual voltage limit per cell is more like 4.1v are all examples of minor over-discharges that occur during punch-out. These factors combine to make it easy for a puffy battery to degrade or catch fire while in flight rapidly.

Knowing how free oxygen functions in your battery—from water and other sources—is essential to comprehending why batteries malfunction and why you can occasionally get by with a puffy battery and other times you can't.

Formula degradation from over-charge/over-discharge

Excess free lithium on the anode (metallic lithium plating) and free oxygen on the cathode result from overcharging or overcharging a lithium battery. Lithium OXIDE is produced on the anode when a free oxygen atom is sufficiently tiny to move quickly through the separator without carrying an electric charge—the actual "rust" of lithium. Deadweight being carried around inside the wrapping of your battery is useless to us right now.

However, you end up with free oxygen because lithium oxide requires fewer oxygen atoms than existed in the ionized form. People also question why, even in the absence of open air, a LiPo will still ignite if it is overcharged underwater.

The opposite is true if it is discharged too rapidly or excessively, but the amount of lithium oxide on the cathode decreases since there is just less of it. In essence, corrosion rapidly forms on both of the battery's poles inside the wrapper of a mistreated battery. Additionally, it grows worse the more abuse it receives because it still gets driven hard despite increased resistance.

By the way, when our aircraft is flown with a high-quality pack (not knockoff eBay leftovers from expensive Chinese blunders of 2004-2009), this is the most typical cause of swelling today. As a matter of fact, these cells—regardless of their 'C' rating—are designed to endure for several hours, not a few minutes. We're pushing them so far out of specification that we're lucky to get hundreds of cycles out of them, even though the chemistry is good for thousands of cycles if utilized as intended.

Additionally, our batteries are typically underspecified. Many of these packs would frequently hold many more mAh than we believe if they were charged and discharged slowly. They give us the performance we do in part because of this. Unlike the "pound it out thin and hope it's mixed right" method employed with sheets of anodes and cathodes today, higher-C-rated packs also frequently inject gelled electrolyte into the separator and carbon or phosphorous nanostructures on the anode and cathode mixtures.

Poor separator construction

Certain low-cost LiPos also use a poor separator composition. In the end, it frequently comes down to utilizing a dry separator with an internal resistance that is far too high to support the manufacturer's "C" rating claims. Because simple lithium oxide makes up an increasing amount of LiPo, and the balloon expands as more oxygen atoms are released, the internal resistance increases with time.

Additionally, I would include "poor anode or cathode chemistry" in this group. Has a batch of good batteries ever produced a defective one? Inconsistent chemical mixtures frequently result in either too much or too little lithium on one side of the battery (well, in some plates, you understand what I mean).

Exacerbating factor: HEAT.

A little heat improves everything for a lipo. If you could keep your battery at 140 degrees Fahrenheit all the time, it would produce amazing power and be working in its happy zone. However, it produces heat both during charging and draining. As we can see from the above, hitting 150 generates a substantial amount of metallic lithium, which is a key cause of puffing and cell damage.

In a similar vein, the maximum 4.235v/cell limit exists only at the fabled 140F. From there, it gradually drops to about 4.2 volts per cell at room temperature and about 4.0 volts per cell below 50 degrees Fahrenheit. After that, the excess electrons will once more break chemical bonds, releasing lithium to form a bond with oxygen and lithium oxide, which is a recipe for disaster should the LiPo rupture and form lithium hydroxide.

Conclusion

In terms of chemistry, there are no LiPos that won't puff in specific situations. However, puffing should also be prevented by carefully regulated humidity, an excellent gel separator, a cathode and anode with nanostructure, and cautious charging and discharging within manufacturer limitations. In a similar vein, placing an abused pack—even when puffed—into a lower-discharge aircraft frequently has the effect of halting the puffing since abuse stops the cell from producing any more metallic lithium.