I have made many LiCoO2 vs Li coin cells for charging and studying structual changes during the Li removal, but my voltage profiles do not follow the conventional behaviour in the literature. I often find before the two-phase region at 3.9V my cells undergo a long process at ~3.5V, which has to come to an end before charging begins (FIG: first charging sweep of four cells prepared using the same cathode material, charged to different extents, all exhibiting a significant anomolous process at low voltage).
(LiCoO2|LP30|Li cells. Cathode prepared with LiCoO2, PVDF, and carbon black in 8:1:1 mass ratio, charging at C/25.)
Making 20 cells from the same cathode batch, no two cells exhibited the same gravimetric capacity for this process, so it isnt tied to the mass loading of LiCoO2, and two of the cells didn't possess it at all. When I tried using powder cathodes with no polymer binder, just carbon and LCO the cells showed no sign of this process, and when I make the cells using solid ptfe balls as binder the process is much less pronounced but often still present.
Has anyone ever seen this kind of thing before? I've been told that Li2CO3 forming on the surface of the LCO particles can cause this, as can carbonyl groups on the surface of the carbon black, but I have ruled these out with the powder cell experiment. I'm starting to think it is something not related to the materials themselves but their geometry. Can a thick carbon layer give rise to this somehow?
Thanks a lot for your help.
\J
what is the waiting time? maybe you can wait more time for charging
Hi Süleyman,
This was my supervisor's first question too.
I leave my cells overnight (approx 10 hours) before placing them in the cycler, and the beginning of my cycling programme has a 6 hour rest under voltage monitoring. I believe that is a significantly excessive resting period to rule out electrolyte permeation.
Thanks
- Badges
- Science topic