Cells that lightly swell during charging

HighTechLab

Small Business Owner
Joined
Sep 23, 2019
Messages
435
Does anyone have an exact reason why brand new perfectly flat cells slightly swell on their first charge? No guesses please, and please cite your source or whitepaper.


I have a theory but I am really trying to get to the bottom of this. I'll omit the theory for now and stick to the facts of this particular test.

I just took 4 brand new perfectly flat CATL grade A+ cells from a sequential production batch of a few hundred cells that I just received. These cells literally were perfectly flat in the beginning. I began charging them up without any fixture or compression. I left ~1/8" gap between cells for this test. I charged at a relatively low current, around 40a in 12v series. As I got closer to 3.4~3.5v the cells were NOT flat anymore, and between two cells they were now nearly touching. I hooked up to the electronic load/precision shunt and as we have been doing this c/5 discharge test they have slowly but surely been returning to their original flat state.

From my experience, it is best practice to keep cells flat from the very beginning by using the 300kgf fixture (or as close to this as possible). Then the cells never have a chance to move/grow. Many suggestions lately have been that the compression is pointless and only prolong life cycles.

This is very different from the other sequential production batch of CALB L173F230 230ah cells, which also arrived PERFECTLY flat, but even at 100% SOC & floating for 4 hours at 3.65v, never showed the slightest growth.

Are there any studies that cover the growth/expansion of even perfectly new grade A cells anyone has shared lately? Are there any battery chemists out there in the crowd being buried by the guesses and uninformed knowledge floating around the internet?
 

HighTechLab

Small Business Owner
Joined
Sep 23, 2019
Messages
435
This is the closest explanation that I can find, but I am not sure where this information was sourced from. @RCinFLA :
I have a suspicion many of the new cells since about Jan 2021 are made with pre-lithiated anodes. It also roughly corresponds to the timing of change over to welded stud posts although they are functionally unrelated items.

Pre-lithiated anodes reduces the manufacturing cost by eliminating the manufacturing charge forming process and probably explains most of the 280AH to 300 AH capacity jump in same size/weight cell as pre-lithiated anode do not suffer the cathode lithium loss normal to regular manufacturing charge forming process.

The benefits to the manufacturers are too hard to resistance - lower manufacturing cost and higher cell AH selling price.

One side effect is some slight bloating on first few charging cycles. It is likely acceptable and should de-bloat over time.
 

toms

Solar Addict
Joined
Jul 24, 2020
Messages
748
I don’t know where that information came from either. The only pre lithiated anodes i’ve seen are recycled ones. I would expect much lower lifespan from this method.

I don’t know of any published papers that accurately describe the SEI formation process (which causes bloating in early cycles). It is a closely guarded process by the manufacturer i use.

Your best bet for information is to directly work with a chemist from the manufacturer.
 

kuranaga

Solar Enthusiast
Joined
Sep 24, 2021
Messages
137
maybe some cells just have extra space in the casing? So the guts still expand but not the case?
 
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