# Lithium Overcharge



## rwaudio (May 22, 2008)

gottdi said:


> Got a good one for you all to answer. What is it that causes the Lithium battery to Swell when overcharged. What are the actual changes that cause this problem? I'm not looking for speculation here but actual factual information.
> 
> Pete


Have you watched the video mentioned in the "lithium lecture" thread?
I think you can draw some facts from that.


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## Guest (Dec 10, 2010)

I have and I have but it is still not clear. My understanding is that the structure changes and it must be changing in such a way that it now takes up more volume but no longer accepts lithium.

Pete


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## Jan (Oct 5, 2009)

gottdi said:


> I have and I have but it is still not clear. My understanding is that the structure changes and it must be changing in such a way that it now takes up more volume but no longer accepts lithium.
> 
> Pete


What I understood is that the organic electrolyte changes at a certain temperature, and probably gives of a gas. An organic somewhat explosive gas. With LiCo a bit more explosive than LiFe. The fact the electrolyte changes into something that is not so effective in what it was designed for makes them go bad. And the gas makes them swell before the plug blows. So, never lett them swell. Unless the gas gan be absorbd again and the electrolyt changes back to its original chemistry after cooling down.


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## Guest (Dec 10, 2010)

I know they don't go back from swelling. The one swelled battery I took so I could take it apart is totally dead. One other I took to try to bring back from the dead has taken a charge but the swelling did not subside. All other batteries are fine. I only took the swelled ones for diagnosis. They did not cost me anything.
I don't think its gas. This is where I say, no speculation. I want to know for sure exactly what happens to cause the swelling.


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## DavidDymaxion (Dec 1, 2008)

Not really a chemistry answer for you, but here's a data point. I torture tested some cylindrical cells until one vented (the cylindrical ones don't swell, they vent). It spit out electrolyte so hot it was a steam cloud, plus there was liquid electrolyte on the table. The cell was over 90 Celsius when that happened.

Apparently something gets hot enough to become gas. Sorry don't know if that's just boiling or a chemical change. Anyway, I found a spec for max allowed temperature of 80 Celsius, so now I limit the cells to 70 Celsius during tests, and no more venting events to date.


gottdi said:


> Got a good one for you all to answer. What is it that causes the Lithium battery to Swell when overcharged. What are the actual changes that cause this problem? I'm not looking for speculation here but actual factual information.
> 
> Pete


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## dtbaker (Jan 5, 2008)

the answers are in this video
http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387

very enlightening, but about 1:15 long!


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## DIYguy (Sep 18, 2008)

Hey Pete, did you get the Hi-Power pack that Motorguy got replaced?


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## Guest (Dec 11, 2010)

? Not sure what you are talking about?


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## Guest (Dec 11, 2010)

> the answers are in this video


Watched it already. I see no definitive answer. I am looking for definitive answers.


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## DIYguy (Sep 18, 2008)

gottdi said:


> ? Not sure what you are talking about?


I was just wondering if your Hi_Power cells were from Motorguy. He had his pack replaced by them because some cells were damaged or something. Thought maybe u inherited them..since u mentioned a good deal, or free or something.....


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## Guest (Dec 11, 2010)

Got nothing from Motor Guy. The Freebees were dead or damaged batteries. My good ones were a good deal. Wish I could have gotten some nice 200AH CALBs but for now these will do.These will get me by while I save for better lithiums. 

Pete


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## DIYguy (Sep 18, 2008)

I hear ya. I have CAlb 180's coming. U might want to check with Kevin...maybe u can pick up some more cells from him...


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## dtbaker (Jan 5, 2008)

towards the end, like in the last 15 minutes, he spent about two minutes answering a question on over-heat/overcharge. The overheat boils the electrolyte, becomes unstable, phase change, vents... bad news since the electrolyte is basically a flammable solvent carrying the Li back and forth between anode/cathode.

The other detail that I caught regarding overcharge is that it causes some of the loose Li to permanently plate on (cathode/anode), blocking effective transfer and Li movement, lower capacity, etc.


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## Guest (Dec 11, 2010)

Yes, I remember all that and that is what sparked the question. What exactly happens to cause swelling? I'd like to know on the molecular level what happens.


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## Guest (Dec 11, 2010)

> maybe u can pick up some more cells from him...


No need. I have plenty for two projects for now.


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## dtbaker (Jan 5, 2008)

gottdi said:


> Yes, I remember all that and that is what sparked the question. What exactly happens to cause swelling? I'd like to know on the molecular level what happens.



it sounded to me like the swelling of the plastic case prismatics was a combination of the heat softening the plastic, and the pre-venting vapor buildup as electrolyte starts to vaporize above 90-100 degC. He was talking about the metal can ones basically just rupturing along a seam, and the plastic case ones (TS/CALB) blowing out the designated weak spot in the top of the case.

I would extrapolate that once a cell swells, the bad thing is that it might allow the layers to not be in physical contact, which would disable Li transfer, which kills capacity. Sort of like a short across plates in a floodie. The conclusion for me is that if you are not pulling continuous high C amps enough to over-heat the electrolyte you are in good shape, and probably don't need to ever really worry about the endplate compression even.


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## Guest (Dec 11, 2010)

Again, I don't want speculation or extrapolations. I want exactly what it is and it looks like I need to hunt elsewhere for the answer. 

Pete


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## dtbaker (Jan 5, 2008)

gottdi said:


> Again, I don't want speculation or extrapolations. I want exactly what it is and it looks like I need to hunt elsewhere for the answer.
> 
> Pete


try contacting that guy from the video... I'd say he could probably explain at an atomic level.


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## Guest (Dec 11, 2010)

I will be contacting them. I was hoping someone here was privy to that information. Hope they have the answer for me. 

Pete


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## Tesseract (Sep 27, 2008)

gottdi said:


> Watched it already. I see no definitive answer. I am looking for definitive answers.


Watch it again, then?

Ray says (at two different points) that gas is formed whenever cell voltage goes above 4.3V or below 0V (ie - reversed). More precisely, it is the dissociation of the polar, aprotic electrolyte (e.g. - ethylene carbonate, propylene carbonate, etc.) that causes the gas to form. In other words, electrolysis of the solvent, just like what happens when you "boil" a lead-acid battery.


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## icec0o1 (Sep 3, 2009)

Just like you can split water into oxygen/hydrogen at 0.6v, you can split the electrolyte of lithium batteries at 4.3 volts into gasses.


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## vpoppv (Jul 27, 2009)

I can see why there's so much debate, because it seemed clear to me from the video that when overcharged, lithium iron phosphate separates into lithium and iron phosphate. The oxygen is then free to vent; this also explained why lithium iron phosphate is safer than lithium cobalt because the oxygen bond in iron phosphate is stronger than that of cobalt and oxygen (FePo4 vs. CoO2). I didn't see that the electrolyte had anything to do with gassing when overcharged?? Ok, I admit I only watched the first 10 minutes of the videoThen again, it's been 20 years since I took any kind of chemistry class, and I can't even remember what I had for breakfast this morning.....


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## dtbaker (Jan 5, 2008)

the last 20 or so minutes are the Q and A that are interesting regarding charging, but it is worth the first hour to get a solid understanding of the internals.


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## Guest (Dec 12, 2010)

I understand the issues with gassing out but want to know if the crystalline structure actually changes shape that causes expansion. Like when water freezes, the structure changes and takes up more space. Is this also what happens? I know the structure changes as well as the issue with gassing but how does the shape change. Well the video is really an overview anyway and not and in depth review. 

Just for the record, I fully understand what was said in the video and know what was said. 

Pete


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## Tesseract (Sep 27, 2008)

gottdi said:


> I understand the issues with gassing out but want to know if the crystalline structure actually changes shape that causes expansion....


No, he said specifically that the intercalation process (migration of Li+ from the LiFePO4 crystal) does not result in a change in the crystal's structure. I mean, this is specifically why the LFP chemistry is so good for EVs compared to, say, LiCoO2, where both over-charging and -discharging result in permanent damage or destruction.

Over-discharge and too high a rate of discharge causes the graphite to get plugged up with either LiCO2 or lithium metal, respectively (both resulting in a gradual to severe loss of capacity by taking active Li+ out of the system). 

The expansion is caused by the gas produced when the electrolyte breaks down (we've destroyed enough cells to know that gas is formed, as Ray implies, both from over-charging and over-discharging, especially if the cell is reversed). Additionally, these cells are constructed much like AGM batteries in that they do not have free-floating electrolyte - it is all absorbed into the graphite and LFP that coat the anode and cathode respectively. So, the loss of even a small amount of electrolyte from gassing is exceptionally bad for the cell (just like it is with AGM).

Note that the electrolyte is not turning to gas from boiling - the combination of carbonate esters used as the solvent for the LiPF6 salt (the actual electrolyte) boil at anywhere from 110-245C. The LiPF6 ion is actually unstable in solution above 60C, hence why Prof. Whitacre recommends keeping cell temperature below that for long life.

One additional point of distinction Prof. Whitacre made is that it is not practical to attempt to _manage_ individual cells but it is certainly practical, and even desirable, to _monitor_ individual cells. In other words, attempting to bypass charging (much less discharging!) current around each cell is impractical (and I agree based on the little tinkering I've done with such) but that knowing whether an individual cell has gone out of bounds in voltage or temperature then shutting off the entire pack is a good strategy for anything but the most cost-constrained (and liability-impervious) applications. He also *seems* to make a distinction between an actual individual cell and a bank of cells in parallel that will act as a single cell electrically when he refers to the impracticality of Tesla monitoring all 7500 (?) of the cells in its Roadster (I thought there were 6831, but maybe that's changed?); monitoring each parallel bank of cells that are then stacked in series to make a "string" is presumably entirely practical, though he does not actually state this.

Prof. Whitacre also recommends that all of the cells for a new pack be connected in parallel then _either_ charged, or discharged, to the respective knee voltage to ensure they are all at the same SoC. In other words, _either a top or bottom balance is fine_, as long a you use a cell-level monitoring system to disconnect or otherwise limit the pack as soon as the first cell goes out of bounds in temp/voltage.


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## JRP3 (Mar 7, 2008)

I think he actually said he preferred bottom balancing for safety reasons. As for swelled cells being ruined, Jack has compressed some swelled cells back into shape and found they still function, though probably not as well. I imagine it depends on how much they swell and how much venting takes place.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> I think he actually said he preferred bottom balancing for safety reasons. As for swelled cells being ruined, Jack has compressed some swelled cells back into shape and found they still function, though probably not as well. I imagine it depends on how much they swell and how much venting takes place.


I didn't get much impression of him preferring one way or the other, but either way it is certainly a lot easier to charge a parallel bank of cells up to, say, 3.8V, than it is to discharge them down to, say, 2.9V.

If the problem with overcharging is electrolytic decomposition of the solvents (ethylene carbonate plus others) then no amount of squeezing is going to reform them. Note that electrolysis of the solvents can occur from overcharging (taking a cell above 4.3V) and overdischarging (specifically from reversing a cell).

This is just basic electrochemistry. The mechanism by which LFP cells operate is certainly different from, say, a lead-acid or Ni-Cd cell - lithium metal ions have to physically move from one electrode to the other during charging and discharging - but other than that quirk the behavior is eminently predictable _as long as you know exactly what components were used to make the cell!_ Unfortunately, LFP cell chemistry is constantly evolving so it is difficult to give one set of universal recommendations on how to charge, discharge and otherwise care for them. 

That said, the salient points I took away from the video ares to stay below 4.3V during charging, above 1V during discharging, under 60C at all times and make a Ragone plot to determine the tradeoff in current draw vs. capacity.


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## dtbaker (Jan 5, 2008)

JRP3 said:


> I think he actually said he preferred bottom balancing for safety reasons.


I recall he said EITHER end would work as long as you get off the long flat plateau in the middle. I know in my case I am more comfortable with the idea of top-balance because that is where I need to be sure the cells are balanced as well as possible and hit the CA-CV voltage setpoint close to the same time while charging.


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## Guest (Dec 12, 2010)

> intercalation process (migration of Li+ from the LiFePO4 crystal) does not result in a change in the crystal's structure.


During normal charge and discharge cycles. Changes do occur in over and under charge. I will go back to class and listen again.


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## Elithion (Oct 6, 2009)

gottdi said:


> What is it that causes the Lithium battery to Swell when overcharged.


I am not a chemist. Having said that, here is my understanding:

There are two mechanisms that result in cell expansion:


During charging, the active material in the anode expands slightly as it receives Lithium ions (through intercalation); this process is reversed during discharge. This expansion is proportional to the SOC.
When the cell is nearly full, any further charging will result in part of the electrolyte to be converted to a gas; that gas is converted back to a liquid electrolyte when the cell SOC drops below 100 %. How much gas is produced depends on the chemistry of the cell, and varies considerably from manufacturer to manufacturer. The presence of this gas increases the pressure inside the cell, which may result in cell expansion (which can be quite large). Overcharging will result in this conversion of electrolyte into gas to continue, often resulting in permanent deformation of the cell, and in release of the gas, either in a controlled manner, or in an explosion.
Note that that second mechanism of expansion only occurs when the _STATE _of the cell is around 100 % SOC. This point is very important. The expansion is _NOT _due to heat, and it is _NOT _due to the act of charging. The expansion is the same regardless of cell temperature (cooling won't prevent the expansion) and cell current (at a given SOC level, the expansion is the same, regardless of whether the cell is charging, discharging or sitting on a shelf).

Cylindrical cells will inherently contain that expansion. Pouch cells won't do so, so they must be contained. Prismatic cells will contain expansion to a small extent, so they too need to be contained. How much containment is required is up for debate, and I discuss it in the addenda to section 6.1.1.3.2 of the Li-Ion book which is freely available on line. The jest of it is that you need to allow for some expansion (for mechanism 1) , but not too much (for mechanism 2), and the answer depends on the particular model of cell.

Davide


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## JRP3 (Mar 7, 2008)

dtbaker said:


> I recall he said EITHER end would work as long as you get off the long flat plateau in the middle.


He did, but as I said he also mentioned that for safety he prefers bottom balancing.


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## Jan (Oct 5, 2009)

JRP3 said:


> He did, but as I said he also mentioned that for safety he prefers bottom balancing.


I was surprised you could discharge them to zero volts, without distroying them. Below zero is distructive. Not a very likely to happen situation....


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## Elithion (Oct 6, 2009)

Jan said:


> I was surprised you could discharge them to zero volts, without [destroying] them.


Depends on the cell.

Cobalt cells: no you absolutely must not let the OCV (Open Circuit Voltage) go below a certain voltage; they will be immediately damaged.

LiFePO4 cells: depends on the brand:


K2 cells: time spent with the OCV below 2 V will reduce the lifetime, though not immediately destroy them
A123 cells: 0 V is OK, even slightly beneficial.
Others: I don't know, but I'd err on the side of keeping above 2 V
Regardless, negative voltages are absolutely not to be allowed.

Davide


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## JRP3 (Mar 7, 2008)

Jan said:


> Below zero is distructive. Not a very likely to happen situation....


It could happen if you deeply discharge a top balanced pack without cell level protection. The smallest capacity cell goes to zero as the other cells keep putting current through it.


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## JohnM (Jan 12, 2010)

gottdi said:


> I will be contacting them. I was hoping someone here was privy to that information. Hope they have the answer for me.
> 
> Pete



Hi Pete,

I hope you share that information here once you contacted M.r Whitacre. At any rate, what I remember from the lecture is that the graphite structure is like a honeycomb and it is flexible and he even said it is used as a lubricant because of this property. Once the battery is charged, the graphite layer expands to accommodate the Lithium ions( or the electrons, I forgot which one). Once discharges, the graphite layers go back into its original structure. So, it could be that if the battery is overcharged, these graphites bulges as well and thus making the battery swell.

JohnM


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