I recently opened several SIB coin cells after cycling was complete. The electrolyte turned yellow (in some cases brown) from the initial colorless state. The electrolyte also changed from a watery liquid into gel-like (or rock-like in some cases) textures. After consulting with a colleague from another research group, it seems that the problem is fairly common in SIBs.
I used 1M NaClO4 in PC as the electrolyte and Whatman glassfiber as the separator; I cycled hard carbon based electrodes against sodium metal.
I would really appreciate if you can provide good literature references regarding this issue or provide best practice guideline to mitigate such issue.
I'm spitballing here, but your electrolyte coloration could be due to aromatic decomposition products of carbon, such as mellitic or humic acids. I've seen this in aqueous electrolytes, but I'm not sure if these break-down products are water specific. Once again, this is just a guess. Good luck.
Hi Jialiang,
I am wondering the amount of electrolyte was taken in a coin-type cell that you can observe its colour or nature after cycling. Secondly, have you seen any change in the electrochemical processes (CV or cycling)?
Hi Dr. Mardilovich,
Are you referring to the reactions between carbon and electrolyte or between the aromatic impurities on carbon and electrolyte? The hard carbon sample I used was prepared at low temperature 700C, so it is more likely for the latter senario to take place.
Hi Dr. Sarkar,
I usually add electrolyte till the entire glassfiber separator is wet (so maybe 5-6 drops). The cycling of cells seem fine (delivering reasonable capacities during rate testing); that's why I was surprised to see the electrolyte degradation. One thing I forgot to mention is that the cells were resting in charged state (no sodium in carbon) for a month before I opened them last week, so the degradation could take place during the resting period but not during cycling. I am going to cycle a few cells and open them at different stages of cycling and resting.
I was referring to the product of the reaction of carbon and electrolyte. I'm not familiar with your system, so I can't say for sure, but if I remember correctly PC and graphite don't play together well in lithium batteries (for example, some discussion here: http://pubs.acs.org/doi/abs/10.1021/jp066762j?journalCode=jpccck). Not sure if you can have a similar problem with Na and hard carbon.
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