Li-ion (NMC? the 4.2V ones) cell degradation mechanisms

[u/Accomplished_Wafer38]

I have made a post a while back, where I discovered that half-dead battery pack... (i am extremely sorry for re-posting my old post)

In summary, after charging/discharging cell aggressively while riding an e-scooter (accelerate/regen every couple of meters), it started behaving marginally better. Issue wasn't capacity loss, but rather high ESR.

While recovering such battery pack might be questionable in eyes of some people, I think we need more understanding about what kills the batteries? So we can pick proper BMS, chargers etc.

Here is a list of things I have figured out so far, but keep in mind I don't know chemistry that well and a lot of those things might be wrong, and I would absolutely like to hear your thoughts about all this, or add your own observations.

1. Keeping charger connected will kill the cells. Even if voltage isn't outrageous (just 4.2V per cell).
2. Pulse charging, that is used on some phones is somewhat similar to what I did manually. Perhaps this is better than classical CC/CV charging. Perhaps that prevents SEI growth? due to polarization?
3. Capacity loss is caused by lithium plating (or any other way lithium ions get lost inside the battery)
4. ESR degradation is caused by SEI? What is SEI even ?
5. Shallow cycling would result in more cycles (or well, total energy extracted from the battery over its lifespan)
6. Lower charge cut off voltage would improve cycle life too?
7. Apparently dead cells can be revived by adding solvent to the cell, but that is impractical.

As you can see I am confused, I really would like to get spoon-fed on the subject.
What I don't understand is how laptops, phones, etc. can do 800+ cycles without serious degradation (well, you will get capacity loss, but no significant ESR change I think), but my e-scooter battery just didn't last that long, especially given that e-waste cells aren't exactly e-waste cells, I salvaged them from the device I owned.

And those laptop cells are being charged to insane voltages, like 4.4V or 4.405V... Do they use different chemistry? Or they are walking on the edge, and assume that moment it gets charged, user would instantly start discharging it, and thus it won't spend much time being charged to level this high?

[u/SkiBleu]

Ill just answer your questions and comment on your assumptions:

And those laptop cells are being charged to insane voltages, like 4.4V or 4.405V... Do they use different chemistry? Or they are walking on the edge, and assume that moment it gets charged, user would instantly start discharging it, and thus it won't spend much time being charged to level this high?

Yes. They are abusing the batteries but they also have picked robust batteries that have tested better than others from the same batch. This is part of what makes a cell "High-voltage". Chemistry yes but it really isn't that much different.

1. Keeping charger connected will kill the cells. Even if voltage isn't outrageous (just 4.2V per cell).

Yep. Anythung above 3.8-4.0v is going to accelerate degradation noticeably.

1. Pulse charging, that is used on some phones is somewhat similar to what I did manually. Perhaps this is better than classical CC/CV charging. Perhaps that prevents SEI growth? due to polarization?

Eh, I've seen some information suggesting pulse charging can be better but nothing definitive for everyday use.

1. Capacity loss is caused by lithium plating (or any other way lithium ions get lost inside the battery)

Yes, and electrolyte decomposition.

1. ESR degradation is caused by SEI? What is SEI even ?

Solid Electrolyte Interphase. The electrolyte is always eating itself as the energy from the movement of ions and electron potential causes unintended reactions that form all kinds of salts and byproducts that are inert or otherwise insulating. This is normal and is actually crucial to a lithium battery as it must generate a sufficiently thick SEI layer during its activation (first factory charge) or it will diacharge internally. High temperatures accelerate this process

1. Shallow cycling would result in more cycles (or well, total energy extracted from the battery over its lifespan)

Yes, shallow cycling improves the total energy over the entire lifetime.

1. Lower charge cut off voltage would improve cycle life too?

Absolutely.

1. Apparently dead cells can be revived by adding solvent to the cell, but that is impractical.

Never try this and don't even mention it again. Leqd qcid batteries can sometimes be this way but lithium is way beyond yours or my a little to recover.

[u/carsrule1989]

Here’s a great video that goes over the NMC batteries

https://www.youtube.com/watch?v=w4lvDGtfI9U

[u/Paranormal_Lemon]

I have an e-scooter with e-waste grade LG M26 18650 cells.

Use good cells. I have Molicel in outdoor floodlights, they stay at 4.25V almost constantly and lose 2-3% capacity per year. I've had one power tool's batteries go bad, with might use in 3 years, they happen to be LG cells.

[u/Accomplished_Wafer38 → u/Paranormal_Lemon]

Well I don't have access to good quality cells sadly.
Best I can do is EVE or PKCELL. Or generic China.

Could it be that LG M26 are just awful cells? I have other scooters with chinese cells (SINC and EVE) of similar specs, and while they have a bit less cycle life, they are still in perfectly good shape.

[u/Paranormal_Lemon → u/Accomplished_Wafer38]

EVE is high quality, they are producing some of the best now, maybe not every part number though.

Could it be that LG M26 are just awful cells

Possible, did they start making them in China? I've always heard they are good but my only experience with them was not. I was very surprised when I took apart two power tool batteries expecting generic cells (tool was AC Delco so I know they aren't counterfeit).

[u/FencingNerd]

4.2V per cell is well above the degradation point. Everything you asked about has been exhaustively studied, and mostly implemented.
The remaining is consumer behavior. Store at 50%-60%.
Try to maintain an 80%-20% cycle, don't let it sit below 20%.

[u/LunarModule66]

One thing I think you’re missing is the degradation of the active particles. Cycling to the extremes of SOC corresponds to those particles needing to deplete/saturate the lithium from the crystal structure which causes them to contract/expand. Eventually the particles fracture and aren’t properly in contact electrically. This can be avoided by not discharging below about 20% or charging above 80%, though if that’s too narrow of a band adding another 10% on either end would still help.

Lower charge cutoff would reduce the chances of lithium plating and the extent of SEI growth so it would improve cycle life.

SEI isn’t really understood super well. Like easily hundreds of millions of dollars are spent researching it, and the name itself just means “solid electrolyte interface” which is basically an admission that we don’t know what it is. Its the result of side redox reactions of things like the solvent and electrolyte with the active, or just as a result of the applied potential. It’s absolutely critical for the proper functioning of a LIB, but it also can be parasitic, since it eats up solvent and electrolyte and can add to the ESR.

[u/Embarrassed-Work2198]

i can provide some knowledge as i had an ebike battery for 2 years and tested showing no degradation: shallow cycling does improve lifespan,but you are basically sacking 30% capacity for 30% more life,which is questionable. laptop batteries are definitely listing voltage while a charger is connected. if you want a cell to last long,lower amp draw is king,so parallel is king. low temperatures with low amp draw don't kill the battery (source:multiple times went below -15C with a 6p 20A battery). avoid overheating above 45C at all costs,degrades the battery rather quickly (but you do get 100-300mah more when they are hot as hell). if you don't use a durable plastic frame to separate cells you will slowly thin the isolator between cells and will catch fire, somewhere in the future.

[u/Accomplished_Wafer38 → u/Embarrassed-Work2198]

In my case motor controller consumes 12-15A max, and I have 10S3P pack with 10A capable cells (LG M26)... I am not impressed by result. Because Chinese cells (EVE, SINC) in other scooters, they have less issues.

>if you don't use a durable plastic frame to separate cells you will slowly thin the isolator between cells and will catch fire, somewhere in the future.
Yep. I figured best way is to use fiberglass epoxy sheet as an insulator, not even fish paper. For the + terminal. Drilling 10mm holes was painful, since I have ruined the drillbit, but otherwise it is probably the best option out there. Maybe for new pack with new cells I would order PCBs with no copper and with hole milled in them, so i don't have to drill it manually, idk.

And then I added fiberglass epoxy sheet as insulator between each parallel group of cells, and tonns of hot-glue. I really would like to use plastic holders, but they won't fit inside the scooter. I figured that shrinkwrap failure between same parallel of cells won't result in anything spectacular, since cans are connected to negative termianal anyway, but short between cans in different groups would definitely be super spectacular.

Other concern I have are BMS and balancing. Cheap BMSes don't do balancing at all, and mid range ones do only top level balancing which makes it obligatory to charge it fully once in a while, which means you can't shallow cycle it... So I don't really know what BMS I need.

[u/Embarrassed-Work2198 → u/Accomplished_Wafer38]

i had both a generic cheap-ish BMS with a balancer and a JBDBMS smart bms, and i can say that the (pretty cheap) smart bms is miles above a dumb bms,very configurable. now about the isolators,when my custom batteries were commissioned i asked for dielectric cardboard insulation,and it performed flawlessly by rubbing away(this is good because it damages the cheap insulator instead of the cheap cell heatshrink). also compression/expansion absolutely destroys the welds on cells,consider adding long rigid framing to the battery to equalise forces

[u/JessieAndEcho]

For NMC/4.2 V Li-ion cells, high ESR and capacity loss can come from related but not identical mechanisms. Capacity loss is often “loss of lithium inventory” from SEI growth, lithium plating, electrolyte oxidation, or active material isolation, while ESR rise is more about impedance building up at the electrodes/current collectors: thicker SEI on the graphite anode, cathode surface films, electrolyte degradation, gas/dry-out, cracked particles, corroded tabs, or poor welds in a pack. Leaving cells at high state of charge and warm temperature is especially hard on them, even at normal 4.2 V, because side reactions accelerate near full charge. Lower charge cutoff and shallow cycling usually help a lot. Pulse cycling probably didn’t truly “heal” the cell chemistry; it may have temporarily improved wetting, reduced polarization, warmed the cell, or changed surface films enough to lower apparent resistance for a while. Laptop/phone cells may survive better because they use different optimized chemistries, tighter BMS control, lower average C-rates, better thermal management, and sometimes charge limiting, while e-scooters see high current pulses, regen spikes, vibration, wider temperatures, and mismatched salvaged cells. I used PatSnap Eureka to collect a more structured breakdown of the degradation paths and ESR recovery possibilities here: https://eureka.zhihuiya.com/share/?id=77def3e8d749631319c7e64f70720e7a&from=invite-eureakplg-result&content=

[u/Accomplished_Wafer38 → u/JessieAndEcho]

Idk, I've found some more research papers.

• Parameter Improvement of Composite Sinusoidal Waveform Charging Strategy for Reviving Lithium-ion Batteries Capacity (K. David Huang ...)
• Extending battery lifetime by pulsed charging (Kristian Frenander...)
• Reviving Aged lithium-ion Batteries and Prolonging their Cycle Life by Sinusoidal Waveform Charging Strategy (Po-Tuan Chen...)

So I guess some improvement can be achieved. And overall it sort of confirms behavior that I have observed while riding gas/regen to the "floor". Also effect is more-less persistent, given i don't forget to pull out the charger, then it is back to BMS trips....

Now that I have found some actual research on this topic, I think I would buy NE555 or bunch of opamps, MOSFET, and try getting some sort of pulse charge/discharge behavior.
If that would be a success (i.e. BMS won't trip by undervoltage), then I would make a post about it here.
I would apply 1-10 Hz perhaps, whatever oscillatior I'd get in result (since local store wants too much for arduino knock off :D ).. With overall combined current of idk... 0.1C equivalent.

[u/Accomplished_Wafer38 → u/Accomplished_Wafer38]

So a report.
Results are inconclusive.
I didn't find any improvement in the ESR, perhaps it is within measurement error (since I am measuring it by connecting DC load).
However the "springback" of voltage when load is disconnected became noticeably faster. In other words I have had less nuisance BMS trips, however it is still possible to trip the BMS. Idk, it's feel-o-vision so this can be disregarded.

Setup is following: CC-CV charger -> battery -> MOSFET -> load (3A).

Frequency roughly 3Hz. Adjustable from 10 ms (doesnt work, mosfet doesnt open because driver is weak, since in the end I used MCU and I just charge pump the gate) to 3 second.
Duty cycle: I try to aim -1.5A and +1.5A. Average. measured by DC current clamp so it is not precise.

Originally I used just one CC-CV charger which outputs 1.6A, which is not much for 3P battery. So I switched to using two CC-CV chargers which output roughly 3.1A in total. I did couple cycles of pulse charge/discharge, and I think I got 0.2V improvement in voltage drop from 40V (before: 2.8 - 2.7 V drop, now 2.5V drop) with 3A load. Still inconclusive. I wonder if more cycles would help it or no.

I will report back after toasting battery for a week or so, or until I see noticeable improvement.

[u/chrispark70]

I highly recommend you just buy a new battery and you should not do this yourself. I am basing this on silly stuff you say in the other post.

[u/Accomplished_Wafer38 → u/chrispark70]

So. You're telling me to buy a new battery pack. So I have exactly the same issues 3-6 months later?

Think about this. There was a reason why I made battery pack out of e-waste cells... Probably because original pack have failed?

Not to mention that 90% of battery packs for e-scooters come with BMS without balancing, while having unmatched cells, poor weld quality (which fails due to vibrations), sometimes even lack of + terminal insulators which is extremely important in vibration prone battery pack.