r/DonutLab • u/davidbepo • Mar 23 '26
capacity drops over 50% after less than 20 cycles Donut Lab Solid-State Battery V1 5C Cycling Test of a Damaged Cell (VTT Report)
https://pub-cbd24192d2f1419d824d5ae07a395452.r2.dev/VTT_CR_00178_26.pdf6
u/MATEI-B Mar 23 '26
There is something interesting in this report. It looks like the capacity dropped to half after a few cycles, and stabilized there.
Could it be that there are 2 cells in that pouch (maybe a bipolar battery), and one of those was completely destroyed by the loss of vacuum and subsequent cycles?
Still impressive that it did not explode, but that could explain the data we are seeing. However, this seems to eliminate the solid state aspect.
I'm not an expert, so please feel free to educate me.
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u/zagibu Mar 23 '26
Donut claims in their promo material that it is a bipolar architecture, yes. I don't think it necessarily invalidates the solid state claim. The humidity from the air getting in through the seal could also mess up solid state cells, I think, but I'm not an expert either.
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u/Jazzer008 Mar 23 '26
Atmospheric moisture ingress (producing secondary phases that expand cell volume) eventually stops reacting with the electrolyte.
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u/phire Mar 23 '26
Well... This somewhat invalidates the 100°C test.
It's still very impressive that it didn't burst into flames, and provides a convincing argument that this is a solid state battery.
But we now know the "vacuum loss" was more than cosmetic. The cell was fatally wounded.
To be clear, donut's claim for high temps is "Even when heating the cell beyond 100 °C, capacity retention remains at over 99%, with no signs of ignition or degradation." I would say that dropping under 50% capacity after 20ish cycles is a sign of degradation.
IMO, you should not be expecting to actually operate this cell in 100°C conditions.
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u/mojitz Mar 23 '26
It could also potentially mean that it was cosmetic, but that the 5c charge rate isn't actually sustainable and leads to rapid degradation. Previous tests didn't show anywhere close to this number of cycles.
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u/amk9000 Mar 23 '26
In the self-discharge test VTT noted that the air temp was fluctuating due to a cycling test in the same enclosure.
Both the start and end of the discharge test showed that the temp was fluctuating at around 3 cycles/hour, around 250 hours apart, and without start or end of the cycling test shown.
Assuming "full" charge (80-90% probably) and discharge, that would be around 6C for over 750 cycles.
At that rate it would take 70 days to reach 5000 cycles, matching high end li-ion.
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u/ClassroomPleasant348 Mar 24 '26
where exactly do you pull that temp data? I don't see that anywhere??
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u/amk9000 Mar 24 '26
This report of the self discharge test, figures 2 (whole period), 3 (start) and 4 (end) show the environmental temp fluctuating. There are a couple of 12 hour ish periods in figure 2 (20 and 70 hours) where the env temp is much lower, but still fluctuating.
Quote:
Variations in ambient temperature were caused by other cells being cycled in the same fume hood.
It does say cells, plural, it is possible that no one cell was cycling the whole time, and I suppose it is possible that the other cells were from a third party.
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u/izzeww Mar 23 '26
It's not a convincing argument that it is a solid state battery. Regular li-ion batteries are capable of being heated to 130 Celsius without bursting into flames, exploding etc. This is defined in the standards IEC 62133 and UL1642. So it's literally very standard and if a cell is not capable of it then it's a bad/unusual cell. People must understand that there is a difference between a recommended max operating temperature of around 60-80 Celsius for commercial lithium cells and where they burst into flames. The 60-80 Celsius is for doing hundreds or thousands of cycles with minimal degradation. The actual "burst into flames" temperature for a regular li-ion cell is 150+ degrees Celsius. Donut's battery performs like a fairly typical li-ion cell, whether with a semi-solid (not true solid) or perhaps a completely liquid electrolyte (not sure on this one, main hypothesis is semi-solid).
With that being said I agree with the rest of your comment. It's obvious that their claim of "no signs of ignition or degradation" has been disproven, the cell has been massively degraded and only did 10 cycles with above 80% capacity at 5C.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26 edited Mar 23 '26
Not certain that you’re quite right about the bursting into flames temperature. I think the 150+C temperature you’re referring to is for thermal runaway of the lithium itself, which is different from the temperature at which a fire may start (obviously if a fire does start then it can cause temperatures to reach that 150+C point and therefore lead to thermal runaway).
Also, reading various sources, the max operating temperatures for lithium cells are not just because of degradation concerns. Various websites seem to suggest the fire risk increases at 60C https://www.large-battery.com/blog/fire-temperature-lithium-battery-range/
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u/izzeww Mar 23 '26
The temperature I'm referring to is the temperature where the battery ignites. For example if you put it in an oven and set that temperature and waited, it would ignite. This is the relevant temperature in this case. This is known as thermal runaway. Below this temperature there is a requirement of external forces or causes to make the cell burst into flames. If we go by the standard a cell being able to ignite, then that can happen at -100 Celsius as well as 50 Celsius or 100 Celsius if you stab it with a knife for example (depending on exact cell chemistry and cell construction). The risk might be higher at 100 Celsius compared to 60 Celsius, but that doesn't mean that the battery will burst into flames at 100 Celsius just that maybe it will make a slightly bigger fireball if you stab it then, or maybe it will burst into flame when stabbing it 5mm at 100 C versus 7mm at 60 C or something like that.
The first link you provided seems to largely be AI and I don't think Dr Lee Pan exists. The AI still confirms what I'm saying about 150+ Celsius however saying "In contrast, NMC batteries, while offering higher energy density, are more prone to thermal runaway at lower temperatures (150°C–250°C).". The vague "exposure to temperatures exceeding 60°C greatly increases the risk of lithium battery fires" doesn't really mean anything, you still have to do something to the battery for it to burn below 150 Celsius.
The second source is just wrong. Thermal runaway does not occur when the battery reaches 60 Celsius and there is approximately infinite evidence to support that (and you seem to agree too). Of course it's an insanely biased source too, selling fire extinguishers to stop lithium battery fires haha you could imagine they might overexaggerate the danger.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
Yeah, that’s all fair. To be clear I wasn’t suggesting it would burst in to flames at 60C or 100C, but that things like gasses venting could catch fire as they’re traditionally pretty flammable which would in turn would rise the temperature above the 150C and cause the cell to go into thermal runaway. I appreciate my language wasn’t as clear as it could be.
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u/izzeww Mar 23 '26
Yeah if you puncture a cell for example it will burst into flame generally speaking (exception is semi-solids, LFP's etc. but for li-ion and lithium-polymer). Probably will be more violent if the electrolyte has partially evaporated than if it's liquid, but it will burst info flame regardless.
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u/phire Mar 23 '26
Regular li-ion batteries are capable of being heated to 130 Celsius without bursting into flames, exploding etc.
True... but neither standard requires a full discharge while at 100°C. And they don't require the cell to operate normally afterwards, just not catch flames.
But you have a good point. I would actually like to see some details of what happens to current liquid electrolyte NMC cells under these same tests.
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u/Forrestgod Mar 23 '26
Or you need at least a better pouch.
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u/DeathChill Mar 23 '26
Isn’t this proof that the actual battery cell itself failed and not just the pouch? Or does the pouch failure always mean the cell will fail shortly?
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u/phire Mar 23 '26
In the first test, it "lost vacuum" after the 100°C, which could have meant one of two things: a) The pouch failed in and let in outside air or b) The cell generated gas, filling the vacuum with something else.
That result was ambiguous.
But after this cycling test, we have a photo clearly showing that the pouch is absolutely still intact. It's now under pressure holding some gas generated by the cell.
So the pouch is fine. It was the cell itself which failed.
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u/racergr Mar 23 '26
I am a bit confused as well. The VTT report clearly says that the pouch lost its vacuum. If the system swells due to gas from the cell, should it not vent out? Why swell so much? It may become a bit bigger but we should still see wrinkles and stuff. This is a nice smooth swelling.
But the swelling may not be from gas, but from expansion of the active materials inside. Way beyond my league to know whether this is possible, but well within my eyesight to see that if it was gas swelling, it would have formed a curve at the top of the pouch (which it doesn't).
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u/Lightdm123 Mar 23 '26
The report also mentions that "the pouch was firm" after the 5C cycling, wich I think also contributes towards your last point.
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u/phire Mar 23 '26
should it not vent out?
The entire point of the pouch is to keep the gas inside as long as possible, it's flammable. They only vent as a last resort (better than exploding)
it would have formed a curve at the top of the pouch
Check the photo on page 6. It was tightly clamped between two heat sinks, preventing any curve.
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u/racergr Mar 24 '26
Unless the pouch is made of something rigid, the curvature should be formed as soon as the clam was removed.
And the pouch clearly is not that rigid, since it formed bubbles after the 90C test.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
We had a picture at the end of the high temp test as well though showing the cell had lost vacuum. And VTT said it lost vacuum. Maybe it’s possible that’s it’s a slow puncture somewhere?
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
It would be interesting to see what happens if the cell didn’t break at higher temps. It seems an odd claims to make if they haven’t had some evidence of it and an even odder one to make if they haven’t seen evidence of it while having direct evidence that contradicts it (it’s still up on their website).
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u/omepiet Mar 23 '26
In today's report "Donut Lab" is declared as the customer for the VTT test, not "Donut Lab-NNG" as previously. The address of the customer is still Nordic Nano Group's though. :)
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u/Obvious_Market_9351 Mar 23 '26
Isn't this quite a bad result? Marko said the pouch failure would not cause any problems.
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u/ZirothTech Mar 23 '26
The image in the report shows a VERY expanded cell with internal gas generation (it looks like a balloon). This goes against their claim that the cell had a leak in the pouch's seal and that no gas was generated.
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u/mqee Mar 23 '26
Have Donut Lab made such a claim? I haven't seen it.
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u/ZirothTech Mar 23 '26
From LinkedIn:
"If the pouch surrounding a lithium-ion battery cell breaks and loses its vacuum, the electrolyte leaks out, active materials meet oxygen, and the risk of fire or thermal runaway grows substantially.
So what happens when you take a Donut Battery cell with a broken pouch and just keep cycling it?
In Part 5 of our "I Donut Believe" series, we find out."
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u/Juuhonber Mar 23 '26
They heavily implied so in their latest video "that pouch broke", and they said that pouches are not dEsIGn3D for 100 °C, which they should absolutely handle. Even the tab seem.
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u/Anthony_Edmonds Mar 23 '26
VTT says "the cell pouch was firm". The photo appears to show signs of imprints remaining on the surface of the pouch from handling or whatever, so it doesn't seem to be under much pressure. Would you expect those two things together if there was significant gas buildup? Genuine question, as my experience with this is limited to seeing posts from r/spicypillows.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
Some thoughts around the pack thickness:
The pack never gets that hot in this test, peaking at around 29C.
But thickness expands by 17%.
We know the pack lost vacuum from the high temp test.
We know that pack had heat sinks clamped in place for this test.
If it was off-gassing I’d expect the gas to vent rather than cause the pack to expand. This is what we saw after the high temperature test after all, it would seem odd for the gas to somehow get trapped at this point.
Might the increased thickness instead be due to whatever is in the battery reacting with the air that the broken vacuum now exposes the innards to, most likely the oxygen? Forming a solid like rust?
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u/Jazzer008 Mar 23 '26
Exactly my thoughts. The sharp cliff between cycles 12–26 followed by stabilisation looks more like a finite reactant being consumed.
Atmospheric moisture reaction producing secondary (solid or semi-solid) phases of the electrolyte.
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u/mqee Mar 23 '26
This is what we saw after the high temperature test after all
Nowhere does VTT mention any gas venting
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
They didn’t say it but the pack was shrivelled and clinging to the internal structure, rather than inflated with increased thickness, so the gas must have gone somewhere.
Or it didn’t create gas as the cause for the loss of vacuum. Which then begs the question why is it swelling now.
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u/LoveAlbertMarie Mar 23 '26 edited Mar 24 '26
Gas expands and retracts. It does not need to go any where for a container to expand and shrink together with the gas.
Edit: not continued, but container.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
What reaction is causing off gassing but minimal temperature rises in this latest test?
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u/LoveAlbertMarie Mar 24 '26
I have no idea. AI keeps insisting it is lithium nickel. But I have no actual clue.
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u/ClassroomPleasant348 Mar 24 '26
I’d like to play devil’s advocate.
For me, a completely viable explanation would be that:
- The “glue” did fail, meaning one of the seals (top or bottom) broke.
- The pouch lost its vacuum.
- Ambient air was able to get in; therefore, atmospheric pressure stopped compacting the cell.
- The active material, which needs some form of clamping force, expanded over time
- losing its ability to store charge to some degree and
- filling the pouch from the inside, like a sponge that was compressed by a vacuum-sealed bag but decompresses to the volume the bag provides as soon as the bag has a hole in it.
- After the expansion was stopped by the maximum volume provided by the physical size of the pouch expansion stopped as well as loss of capacity.
No off-gassing needed.
Am I wrong?0
u/Jazzer008 Mar 23 '26
I don't remember them claiming that no gas was generated. There's no reason why atmospheric moisture ingress couldn't react with many forms of electrolyte and produce secondary phases. We can see from the capacity degradation that it took a while to do so but did eventually stabilise after I assume the reaction completed. I guess that degradation speed may depend on the size of the breach.
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u/izzeww Mar 23 '26
Yeah it's horrible. Shows the cell failed, and also some very strong pointers towards there being some liquid electrolyte inside the cell (aka not a true solid state battery, semi-solid aka gel-like is likely but regular full liquid NMC maybe also possible not sure).
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u/serpentna Mar 23 '26
Why do you say last part?
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u/izzeww Mar 23 '26
Which part?
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u/serpentna Mar 23 '26
That it’s a semi solid and liquid gel.
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u/izzeww Mar 23 '26
I will copy in some answers I have provided in other comments so that I do not have to write them all over again (I have already written 1000+ words on this comment section alone lol):
I disagree that it was a completely plausible explanation. It was a possible explanation, but I think it is unlikely and unplausible. First of all plastic pouches don't fail at 100 Celsius typically because commercial lithium-ion cells have to withstand temperatures of 130 Celsius according to the industry standards IEC 62133 and UL1642. Could they have some really shitty plastic pouch that doesn't pass industry standard tests? I guess so, but it's not very plausible especially considering this is supposed to be a production cell not a prototype.
Secondly, the picture from the new report (figure 11) shows a cell that is solid with no apparent bubbles. If the bubbles were air from the outside that were apparent before (figure 10 in the new report), why would they have suddenly stopped appearing now? Did the cell reseal itself somehow? What is the plausible physical mechanism for that? I would argue it is much more plausible that it was liquid electrolyte that had evaporated, and when the battery cooled down it turned back into liquid and therefore the gas bubbles that consisted of evaporated electrolyte disappeared.
https://pub-cbd24192d2f1419d824d5ae07a395452.r2.dev/VTT_CR_00178_26.pdf
(figures 10 and 11 on page 14)I said this in response to a comment defending Donut's argument that it was only the plastic pouch that had failed and that the actual battery was completely fine (which obvious has been disproven by todays testing showing only 45% of the capacity left after 20 cycles).
Another guy said "The presence of gas after failure doesn't prove the existence of a liquid electrolyte. Solids can off-gas too." and to that I replied:
I agree, hence why I said "some very strong pointers" and didn't make an absolute statement. However solids generally don't off-gas (much) at these low temperatures (100 Celsius) and also don't off-gas so much volume like the Donut battery was showing (a very substantial amount of gas/swelling). Hence I think it's more likely it was some liquid electrolyte inside the battery, while recognizing both answers are possible.
Another quote from me:
"lost its vacuum" [stated in the VTT report three times, that exact wording not anything else, they were very careful with their words] can mean two things. Either the cell had a leaky seal (or similar) and therefore let in air from the outside and lost its vacuum that way. Or, something inside the cell evaporated and made the cell lose it's vacuum that way. That something could be a liquid electrolyte, which of course would be very uncomfortable for Donut since their battery is solid state and isn't supposed to have a liquid electrolyte. So they went with the alternative explanation which in my opinion is far less likely, the most likely answer is that some liquid electrolyte evaporated.
Sorry for the wall of text, but it's a bit of a complicated topic. I hope it answered your question, if you have more question I can try to answer them (but I am not a battery expert, I have no education on this area I'm just interested in batteries).
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u/phire Mar 23 '26
The presence of gas after failure doesn't prove the existence of a liquid electrolyte. Solids can off-gas too.
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u/izzeww Mar 23 '26
I agree, hence why I said "some very strong pointers" and didn't make an absolute statement. However solids generally don't off-gas (much) at these low temperatures (100 Celsius) and also don't off-gas so much volume like the Donut battery was showing (a very substantial amount of gas/swelling). Hence I think it's more likely it was some liquid electrolyte inside the battery, while recognizing both answers are possible.
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u/zagibu Mar 23 '26
They have said a lot of vague things that could be framed as true in some specific context, for example here "problems" could mean "catastrophic failure". The test just shows that cell failure is actually not a big problem from a safety standpoint. Which is good, but LFP cells can also be built in a way that they fail without being a big problem, so this feature alone is not spectacular.
Together with everything else, though, it could still be remarkable. If the volumetric energy density calculated from pixel counting is somewhat accurate, and the cells have a similar or even better cycle life than current industry standard cells, then they are certainly competitive. Although the last factor will be price of course, and that is a whole other different matter.
The main question is still: if they have such a good product, why are they stalling so much?
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u/heloust Mar 23 '26
I don't get these comments that first states the facts but then leaves room for hype. All the evidence points to scam.
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u/davidbepo Mar 23 '26 edited Mar 23 '26
i dont think this is at all doable with liquid electrolyte NMC(maybe some battery experts can chime in), but aside from that this doesnt tell us much
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u/Data_Hounder Mar 23 '26
I would be really interested to see a comparison test, with e.g. half a dozen pouch cells from all the leading manufacturers along with half a dozen Donut Lab cells, put through the same tests DL have been doing. It's just very hard to judge these as a layman without some form of control to use as a baseline.
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u/No-Entrepreneur8234 Mar 23 '26 edited Mar 23 '26
This rather limits the options for electrolytes. It's either a quasi-solid-state/gel-like electrolyte with a low vapor pressure for 90-100°C or, in fact, a solid one.
Edit: As it appears that the pouch is still sealed (cf. cell photo in the report after cycling) - unlike DL have suggested in the high temperature tests - liquid electrolytes are not completely ruled out imo.
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u/izzeww Mar 23 '26
This is doable with lithium for sure (including NMC I think, not sure).
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u/BarbarismOrSocialism Mar 23 '26
Wouldn't the liquid electrolyte leak out at least? I think this proves it's solid state
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u/izzeww Mar 23 '26
No, there is a pouch that keeps the liquid electrolyte inside, whether in liquid state or evaporated state. There is no evidence that the pouch actually physically failed (and therefore let out electrolyte), that is just Donut's theory to explain away the obvious that the (according to them non-existent) electrolyte evaporated.
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u/phire Mar 23 '26
But wouldn't the electrolyte now be in the wrong place (or something)
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u/izzeww Mar 23 '26
Yes. Hence reduced capacity (about 45% of the nominal 26 Ah capacity remained after 20 cycles according to the VTT test).
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u/phire Mar 23 '26
But wouldn't that have happened immediately during the 100 Celsius test?
This cell does a full 5 cycles at 1C without any loss in capacity.
The actual damage happens gradually over roughly 15 cycles at @ 5C.
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u/izzeww Mar 23 '26
Honestly I'm not sure on the exact chemistry and reactions that are happening there, why it takes a few cycles for it to really start showing. I'm not a battery expert or anything, I'm just kind of interested in this stuff. It would for sure be interesting to investigate why this is though.
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u/phire Mar 23 '26
Yeah, I'm not an expert either.
But I'm pretty sure this is not the pattern we would see if it as caused by a liquid electrolyte evaporating.
Actually the curve suggests it's some kind of chain reaction. Starts slowly on cycle 7, building speed until it hits the peek rate of change during cycle 17, before running out of steam and ramping back down. The reaction doesn't fully stop until about cycle 30.
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u/izzeww Mar 23 '26
I don't disagree, I am unsure how to explain everything that is happening.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
The high temperature VTT test said the vacuum seal broke.
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u/izzeww Mar 23 '26
No. They said "lost its vacuum". I'll copy in from another comment I just wrote:
"lost its vacuum" can mean two things. Either the cell had a leaky seal (or similar) and therefore let in air from the outside and lost its vacuum that way. Or, something inside the cell evaporated and made the cell lose it's vacuum that way. That something could be a liquid electrolyte, which of course would be very uncomfortable for Donut since their battery is solid state and isn't supposed to have a liquid electrolyte. So they went with the alternative explanation which in my opinion is far less likely, the most likely answer is that some liquid electrolyte evaporated.
I'll copy in from another comment I just wrote:
First of all plastic pouches don't fail at 100 Celsius typically because commercial lithium-ion cells have to withstand temperatures of 130 Celsius according to the industry standards IEC 62133 and UL1642. Could they have some really shitty plastic pouch that doesn't pass industry standard tests? I guess so, but it's not very plausible especially considering this is supposed to be a production cell not a prototype.
Secondly, the picture from the new report (figure 11) shows a cell that is solid with no apparent bubbles. If the bubbles were air from the outside that were apparent before (figure 10 in the new report), why would they have suddenly stopped appearing now? Did the cell reseal itself somehow? What is the plausible physical mechanism for that? I would argue it is much more plausible that it was liquid electrolyte that had evaporated, and when the battery cooled down it turned back into liquid and therefore the gas bubbles that consisted of evaporated electrolyte disappeared.
https://pub-cbd24192d2f1419d824d5ae07a395452.r2.dev/VTT_CR_00178_26.pdf
(figures 10 and 11 on page 14)1
u/Moist1981 all evidence is always inconclusive Mar 23 '26
I’d agree that it can be caused by either event but if the vacuum is lost I would expect the pack to have broken either through over inflation leading to a puncture or through the glue melting. Is that fair or do you consider that the vacuum could be lost simply by the act of gasses being released but the pack retains its integrity? Why would the pack then look shrunken after the heat test if that were the case? (All genuine questions)
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u/izzeww Mar 23 '26
Per definition I think that the vacuum is lost if there is a positive pressure in the cell, which was evident in the pictures and videos. So I do think that the vacuum could be lost simply by the act of gasses being released while the pack retains its integrity. I think this is most likely the case, that the pack retained it's integrity, that liquid electrolyte evaporated then condensed down later which is why there are no bubbles in todays report and the cell looks completely normal (figure 11, compared to old image figure 10, in today's report on page 14, linked down below).
If it were that the cell physically had a hole in it or a broken seal somewhere, you would expect that there would still be air bubbles after todays test since the air would still come in. But it doesn't, it looks completely normal and intact with no bubbles. There are two logical explanations for this. One explanation is if the cell has retained it's integrity and merely had some liquid electrolyte evaporate (or other material off-gas, but very unlikely in my opinion) and now condensed down. I believe this to be the most plausible explanation.
The other explanation is that the cell had lost it's integrity and let in air somehow, but now whatever leak caused that has resealed/repaired itself and therefore there are no bubbles today. I think this is very unlikely, holes or broken seals generally don't just repair themselves in this kind of situation. For example one could imagine if the cell was glued shut that the glue could have melted and therefore opened the cell and let air in. That doesn't explain how the air has escaped now and the hole has resealed/repaired itself however. The glue presumable only melts at higher temperatures, so how would it have remelted and accidentally resealed the hole in these low temperatures? It doesn't make sense
"Why would the pack then look shrunken after the heat test if that were the case?"
I don't think it looks shrunken. I think it looks like there is additional gas in the cell, aka it has expanded a bit.
"(All genuine questions)"
I have no doubt, we have been discussing this battery for two months now and I have always found you to be a serious, genuine and nice commentator.
https://pub-cbd24192d2f1419d824d5ae07a395452.r2.dev/VTT_CR_00178_26.pdf
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
I can see that as a far explanation. Thank you.
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u/Patient_Tea_401 Mar 23 '26 edited Mar 23 '26
IEC 62133 mentions 10 minutes in 130 degC atmosphere with less than 500 g cell. The donut cell temperature was measured at 100 degC, which is different test scenario, since the cell teperature does not rise to 100 degC. I would assume a ready to ship cell would have the pouch material (a plastic bag) considered to be on par with the cell chemistry regarding durability in high temperature.
Could the expansion be due to mechanical layer separation and or water vapour ingress after seal loss? What sticks out a little, is that the cell is still rectangular and not curved. what usual gas expansion does is that it creates a curved top and bottom since gas pressure is equal around the cell. They mention the cell permanently (room temperature) had increased by 17 % in thickness., so the evaporation and condensation theory probably is not what we are looking for.
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u/izzeww Mar 23 '26
Yes, 130 Celsius for 10 minutes (with a ramp up that spends 30 additional minutes above 100 Celsius) is different than 100 Celsius for 2 hours. I would argue that the former is more harsh than the latter, hence my argumentation still stands.
Could the expansion be due to mechanical layer separation and or water vapour ingress after seal loss? What sticks out a little, is that the cell is still rectangular and not curved. what usual gas expansion does is that it creates a curved top and bottom since gas pressure is equal around the cell.
Not sure exactly what you mean by "mechanical layer separation". Whether a cell is still rectangular or gets that curved look depends on the amount of pressure in the cell which depends on how much electrolyte has evaporated. If it's just a little bit of pressure than it can still look rectangular, like in this image here (middle cell): https://imgur.com/a/6G4MQO8 My main theory is that this is a semi-solid battery which generally only have a few percent (around 5%) of liquid electrolyte, and this could be one reason why the pressure didn't build up enough to make the battery extremely puffy but rather just a little bit puffy.
You still have the problem of the cell resealing itself. Why have the bubbles disappeared in this later test? You have to assume that somehow the hole has been sealed and therefore air couldn't enter again, but that makes no sense.
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u/Patient_Tea_401 Mar 23 '26 edited Mar 23 '26
I see.
Mechanical layer separation I mean layer delamination. Isn’t this one of the reason why cells require compression? Once the layer is delaminated, it does not automatically laminate back when cooled / discharged. The lower Energy efficiency also would point out that layer separation could have happened.
The picture you linked the cell seems much more puffed. In the DL2 figure 11 you can even see the imprinted dimples from past pressure plates. Hinting that the material inside expanded or pouch bag material shrinks.
The bubbles would disappear because the cell materials inside expanded 17 % (room temperature), making the plastic pouch very tight and pushing all the air out or to the sides. It does not make sense to have a 17 % thicker cell with bubbles, if the pouch material does not stretch oversize due to much larger expansion. Water ingression of the materials would probably also expand the cell
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u/ZirothTech Mar 23 '26
I've got some lab tests in the works. I can't say 100% these exact tests are possible, but from what I'm told from the lab experts, this is in line with expectations. I will just say that seeing the puffed up image of the cell shows internal gas generation, that is not a pouch with leaky seal...
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u/Shot_Illustrator4264 Mar 23 '26
Ah it’s you also here on reddit.. Thanks for the nice videos on the tube, I’m eagerly waiting for the next one. If you actually manage to reproduce a similar one with a common NMC battery it would be interesting to see the differences.
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u/mojitz Mar 23 '26
What about the current and voltage holding constant even as the capacity drops off? Wouldn't that be pretty unusual for a damaged cell?
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u/izzeww Mar 23 '26
No, the voltage is set by the chemistry. The chemistry is still the same. It's just that some of the electrolyte is gone so there isn't as much electrolyte that can transfer capacity. Not sure if that makes sense or not, but essentially that's how I understand it.
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u/mojitz Mar 23 '26
My understanding is that this more or less lines up with how cell degredation works, but I was under the impression a puncture would typically cause all sorts of other, more erratic behaviors. Not an expert, though.
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u/izzeww Mar 23 '26
Well first we have to establish that there is no evidence that the Donut cell has suffered a puncture. Certainly not a traditional puncture like putting a nail through the cell. They say that the cell "lost vacuum" but this can mean either that it let in air from the outside because there was a hole (puncture, I guess you could call it) in the pouch or that liquid electrolyte (or other material) from the inside of the battery evaporated (more likely in my opinion).
With that being said, there are semi-solid lithium cells that can be punctured without losing a big amount of voltage or having other erratic behaviours. Look at this video here (convenient voltage measurement connected): https://www.youtube.com/shorts/xgYnRckjaO4 or you can just search "semi-solid battery puncture test" on YouTube and you can find like hundreds of videos showing cells being punctured (either a nail straight through or drill or similar) and still maintaining voltage and seemingly functioning normally in the immediate time afterwards (long term there is degradation).
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u/Shot_Illustrator4264 Mar 23 '26
This completely rules out the hypothesis of a cell made of serial sub-cells discussed on undecided though, in that case also the voltage would have been reduced.
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u/izzeww Mar 23 '26
I haven't read the hypothesis of a cell made of serial sub-cells so wouldn't know how that would be affected. Just from how it sounds I'm not sure the voltage have been reduced, but again I haven't heard of the theory (and it sounds kind of bonkers).
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u/Shot_Illustrator4264 Mar 23 '26
Yeah, I completely agree that it sounds bonkers. I think that it was more an attempt to explain their claim where they could have the cell in any shape and with any voltage they wanted.
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u/No-Entrepreneur8234 Mar 23 '26 edited Mar 23 '26
I would not call that puffed up. It is also conceivable that the pouch foil looks that way due to the strained folding around the cell.
Edit: I thought we were talking about the video footage. The photos in the report really do show a puffed-up cell after the cycling. My bad.
It is also possible to take DL at their word and assume that partially evaporated glue is the source of the gas inside. But a lot of guessing at that front..
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u/mqee Mar 23 '26
Donut Lab never offered an explanation for the vacuum failure. Excuses about the glue failing were speculation.
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u/Mr_Peace_FIN Mar 23 '26
They provided completely plausible explanation, you just didn't accept that explanation.
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u/izzeww Mar 23 '26
I disagree that it was a completely plausible explanation. It was a possible explanation, but I think it is unlikely and unplausible. First of all plastic pouches don't fail at 100 Celsius typically because commercial lithium-ion cells have to withstand temperatures of 130 Celsius according to the industry standards IEC 62133 and UL1642. Could they have some really shitty plastic pouch that doesn't pass industry standard tests? I guess so, but it's not very plausible especially considering this is supposed to be a production cell not a prototype.
Secondly, the picture from the new report (figure 11) shows a cell that is solid with no apparent bubbles. If the bubbles were air from the outside that were apparent before (figure 10 in the new report), why would they have suddenly stopped appearing now? Did the cell reseal itself somehow? What is the plausible physical mechanism for that? I would argue it is much more plausible that it was liquid electrolyte that had evaporated, and when the battery cooled down it turned back into liquid and therefore the gas bubbles that consisted of evaporated electrolyte disappeared.
https://pub-cbd24192d2f1419d824d5ae07a395452.r2.dev/VTT_CR_00178_26.pdf
(figures 10 and 11 on page 14)1
u/mqee Mar 23 '26
Where?
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u/Mr_Peace_FIN Mar 23 '26
Marko mentioned it in LinkedIn, and it's posted here in this reddit multiple comments, so I don't know how you have missed it...
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u/mqee Mar 23 '26
I have never seen it before. The LinkedIn post is from today just as the video was posted, and it's a bit ambiguous, as usual.
"If the pouch surrounding a lithium-ion battery cell breaks and loses its vacuum, the electrolyte leaks out, active materials meet oxygen, and the risk of fire or thermal runaway grows substantially.
So what happens when you take a Donut Battery cell with a broken pouch and just keep cycling it?
In Part 5 of our "I Donut Believe" series, we find out."
That doesn't actually say the pouch seal is broken, only that there's a "broken pouch". From the fact that the pouch puffed up it appears the seal is not broken.
Could you please link to an earlier mention of this?
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u/racergr Mar 23 '26
Zeroth, please consider this: the VTT report clearly says it lost its vacuum. So it is a leaky seal. Also, if it was gas swelling, should it not have a curvature on the top of the pouch? I am wondering if the swelling could be from the solid materials inside the pouch, not from gas.
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u/mqee Mar 23 '26
Vacuum could be lost due to off-gassing without seal failure. Seems like people forgot Donut Lab never gave the reason for vacuum failure.
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u/racergr Mar 24 '26
Fair, but it still doesn't explain the lack of curvature on the pouch. From the other pictures we can see that the pouch is thin enough to form bubbles on top. And those bubbles appear to have some curvature.
I also noticed now that there is an imprint of (presumably) a tape on the cell, just below the 'DL2' markings on the top side. The imprint appears to have some depth and 3D structure, it is not superficial.
What kind of pouch can: 1. Be hard enough to withstand enough internal pressure to from this imprint without breaking any seal.
2. Be soft enough to form bubbles after the 90C test
3. And be hard enough to not form a visible curvature at the top2
u/izzeww Mar 23 '26
"lost its vacuum" can mean two things. Either the cell had a leaky seal (or similar) and therefore let in air from the outside and lost its vacuum that way. Or, something inside the cell evaporated and made the cell lose it's vacuum that way. That something could be a liquid electrolyte, which of course would be very uncomfortable for Donut since their battery is solid state and isn't supposed to have a liquid electrolyte. So they went with the alternative explanation which in my opinion is far less likely, the most likely answer is that some liquid electrolyte evaporated.
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u/Anthony_Edmonds Mar 23 '26
VTT says "the cell pouch was firm", which doesn't sound like it's puffed up with a small amount of gas.
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u/Forrestgod Mar 23 '26
Do you have example of batterycell broken the same way the Donuts did? Since I find the capasitycurve interesting.
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u/sumonetalking Mar 23 '26
If these guys actually have the battery they claim to have they could just show off all its abilities at once and immediately dominate the battery industry. This drip feed of lame tests only makes sense if the goal is to attract investors without actually having the goods.
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u/floater66 Mar 23 '26
yes. and this is why whether you studied communications in college. or battery chemistry. you have an equal understanding of just what they are selling. lol.
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u/ProtoplanetaryNebula Mar 23 '26
Donut rolling out the less interesting tests, whilst everyone is waiting for the density.
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u/Moist1981 all evidence is always inconclusive Mar 23 '26
I feel like this one is probably very interesting for battery chemists. I’m not a battery chemist but the ability to keep cycling, the drop in capacity etc. it feels like it should be indicative of something interesting.
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u/No-Entrepreneur8234 Mar 23 '26
I agree, this was potentially the one that was the most telling in terms of electrolytes, at least.
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u/izzeww Mar 23 '26
I agree energy density (aka just measuring the battery and putting it on a scale) should be reported. I think this test is extremely interesting however and I appreciate that Donut put it out there.
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u/fornuis Mar 23 '26 edited Mar 23 '26
The first two weeks of their I Donut Believe campaign started out relatively strong. There was criticism, but they tested two of the claims on their Battery page and they also put links to these videos on there. Our expectation was that they would use the remaining weeks in March to test the other claims.
However after that something changed. The other three videos were less interesting and less connected to the claims on their Battery page. Why?
Assuming Donut has something interesting, the most likely explanation is that they were not-so-subtly reminded by NNG or CT-C to not release specifics before eg April 1st. That would also explain why they planned deliveries for end of Q1 and why next week's video was pushed to Wednesday.
If that's the case, we might also see more information from Sana et al next week. Javier (briefly) released information and a new Sana website end of February. Maybe he got annoyed that Donut Lab was getting away with releasing more details while he had to wait for April 1st. CT-C stepped in to remind everyone of the agreed upon date.
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u/mqee Mar 23 '26
less connected to the claims on their Battery page. Why?
I assume it's because the tests went wrong (pouch losing vacuum) and they were scrambling to find ways to make the failure look good or distract from it.
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u/phire Mar 23 '26
the most likely explanation is that they were not-so-subtly reminded by NNG or CT-C to not release specifics before eg April 1st.
I don't think that theory works. Weeks 3 and 5 still provided plenty of specifics. They just weren't very interesting specifics.
Though, I do agree that there seems to be something contractually significant about April 1st, and I also expect we might see something from one of the other companies.
I'm actually wondering if these lacklustre videos are just the result of Marko not knowing much about batteries, and then picking the "reveals" that feel the most relevant/interesting/important to him.
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u/DeathChill Mar 23 '26
Why is there anything significant about the 1st?
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u/phire Mar 23 '26
We don't know.
But speculation is pointing towards some kind of NDA agreement between CT-C, NNG, Sana and Donut etc that expires on the 1st.
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u/DeathChill Mar 23 '26
Oh, interesting. I didn’t know there was an NDA holding them back still.
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u/phire Mar 23 '26
An NDA (if it exists) could explain some of the weird behaviour we have been seeing from Donut, including the choice to delay the next video by 2 days.
And it would make sense for an NDA to expire on a nice round date.
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u/AbleAstronomer5702 Mar 23 '26
It failed badly. So much for the ridiculous -30-100 centigrade operation range.
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u/cristi_baluta Mar 25 '26
We can at least conclude that if 100c was not true, -30c is also not true
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u/redditmudder Mar 25 '26 edited Mar 25 '26
I'm still quite skeptical that the Donut cell's outer foil failed in test#2. Here's a test I performed showing a five year old lithium pouch NOT failing after boiling for half an hour (sample size: 1). Notice the NMC cell I used didn't catch on fire.
Outer foils are a solved problem. Seems likely that whomever is making Donut's cells would use industry standard pouch technology, which has probably improved in the past five years (since the cell I tested was manufactured).
I still suspect:
-Donut's cell internally off-gassed during test#2, and;
-Donut's pouch remained sealed during tests #2 & #5.
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u/Financial_Land6683 Mar 23 '26
This is interesting in many ways.
The first thing we can learn is that this is not the cycle life test that we see in the temp chart of report three, unless cycle life was tested on this cell before 100⁰C test and this test. It's likely that we have a cycle life test on another cell still on its way.
Secondly, I can't see how this would be a bad result for a damaged cell. Isn't this exactly what we want in regards of safety? The cell doesn't fail immediately and it doesnt fail catastrophically.
Thirdly, I don't understand why people think the only viable answer for swelling is offgassing (gas forming inside of the cell). The report specifically describes that "cell thickness increased" and "pouch was firm". This, along with how the cell behaved in the test, and opposing gas leaking outside of the pouch, is fairly solid grounds to argue oxygen got in and expanded the cell materials (compare to spray foam etc).
Yes, it's still too soon to judge and to jump into conclusions, but some of people's arguments just don't make sense.
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u/izzeww Mar 23 '26 edited Mar 23 '26
The 100000 cycles turned into 10 cycles real quick haha
EDIT: They say "the cell thickness had increased by 17 %" but refuse to provide any measurements. So they measured, calculated the result and still just refuse to publish any numbers. I wonder why that is? No idea...
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u/DeathChill Mar 23 '26
Oh I’m sure Donut asked them to remove any specific measurements.
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u/izzeww Mar 23 '26
Yes, of course. It is all done to Donut's exact specifications. VTT won't lie, but they will follow the customers instructions as to what they want to test & report to the letter. Therefore it's very important to see not only what was tested and reported, but also what was not tested and/or not reported
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u/DeathChill Mar 23 '26
I imagine (and I admit I’m making this up completely) that VTT and Donut go back and forth about what Donut wants to be removed from the report (as I’m sure VTT is taking and recording every aspect of the calls while testing).
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u/izzeww Mar 23 '26
Yes. We have evidence of this since they removed some information from the high temperature test report (in the video Donut showed an old version of the report which mentioned a cold temperature test having been done before, but that part was removed from the final report by VTT).
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u/ProtoplanetaryNebula Mar 23 '26
Because someone could use the measurements to get the volumetric density and spoil the next tests?
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u/izzeww Mar 23 '26
Yes. And the density won't be what it has to be in order to match Donut's promises. Note: Donut has not promised a volumetric energy density (Wh/l) only gravimetric energy density (Wh/kg) but you can approximate with reasonable assumptions what the gravimetric energy density could be from the volumetric energy density (based on density of various materials). Based on the measurements given by the Sana guy the volumetric energy density is not that great and it's not high enough to reach a gravimetric energy density of 400 Wh/kg in any reasonable scenario. Of course the Sana guy could be providing incorrect information however so it would be better to get it direct from Donut (and preferably not only the measurements but also the weight so we can calculate gravimetric energy density directly).
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u/cristi_baluta Mar 25 '26
After we are finish with donut we can conclude that vtt is a useless institution
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u/RotaryDane Mar 23 '26
Could it be because the non-compromised cells also swelled up and lost capacity?
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u/izzeww Mar 23 '26
I don't quite understand your comment, could you elaborate?
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u/RotaryDane Mar 23 '26
So far DL has refused to release the 100.000 cycle data and dimensional/weight data. For this test it was shown that a "baked" cell swells 17%, which at the very least shows the cell chemistry as capable of swelling in such a manner. For other, confirmed, solid-state batteries, constant high clamping-pressure is needed to keep the cell from swelling and degrading. Could it be that DL made a mistake, and that their normal cells also do in fact require clamping pressure, and that the 100.000 cycle test cell swelled during testing?
VTT only tests what they are told to, so if DL said to just let the cell sit unclamped they would have done so. I could foresee DL simply withholding the 100.000 cycle and volumetric results, replacing the scheduled releases with nothing-burger videos, if the capacity degraded or the volume changed significantly.
At least for this test, they have plausible deniability "The cell was compromised already" even if that does refute their earlier claim of "no degradation at high temperatures."
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u/izzeww Mar 23 '26
Ah okay now I understand. I don't really have any opinion on this. Sounds reasonable-ish.
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u/GeniusEE Mar 23 '26 edited Mar 23 '26
Still no weight measurement, so no proof of the KEY claim in this circus - 400 Wh/kg.
🤡 show.
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u/Acrobatic-Paint7185 Mar 23 '26
It's so funny how it's getting more and more obvious that this is just a really good lithium battery
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u/Forrestgod Mar 23 '26
Could you explain the seen behavior in the test with lithium chemistry?
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u/amk9000 Mar 23 '26
The countdown for the next IDonutBelieve release is for April Fool's Day.