I have developed LiNi0.5Mn1.5O4 (LNMO) material and tested it in half-cell (LNMO vs. Li/Li+) and works perfectly as shown in the picture attached.
However, when I used the same material to make a full cell (LNMO vs. graphite), it showed strange behaviour as can be seen in the voltage vs. time plot in the attachment. What can be the possible reasons? The data shown is for the formation cycle, where the current equivalent to C/10 was applied for both charge and discharge.
Note: I made some similar materials, and for them, the full-cells worked perfectly.
Full-cell parameters:
Graphite = 3 mAh cm-2
LNMO = 2.5 mAh cm-2
N/P around 1.2
Glass fibre as a separator
1M LiPF6 EC:DMC 1:1 wt.% as electrolyte
Half-cell parameters:
LNMO = 1.2 mAh cm-2
Glass fibre as a separator
1M LiPF6 EC:DMC 1:1 wt.% as electrolyte
As you mentioned, you have the successful experience with some similar materials. And the LNMO half-cell also worked well. Your charge profiles are quite strange, so please confirm whether the cell was properly assembled or not. In addition, please also check the graphite half-cell.
An-Giang Nguyen thanks for your comment. The cells were apparently assembled well. The graphite half-cells were fine, but I had some issues with a couple of cells from graphite half-cells as well. But in the full-cells, it is really difficult to find where the issue is coming from.
Hi Umair, are you maybe slightly short-circuiting somehow the full cells? Because in the first 20 hours (which I assume is OCV) the cell voltage decreases quite much, while it should remain more or less constant, hence I am wondering if you have some problems during the assembly phase that can cause this. Are you using coin cells?
Or maybe, the CV step you are doing at the end of the charge (which btw. seems a bit noisy) is causing an excessive decomposition of the electrolyte. How long does it last? In the end, the LP30 you are using is not really stable at such high voltages.
Let me know if these suggestions are helpful or not! :)
Alessandro Innocenti thanks for sharing your opinion and nice thoughts.
(1) right after assembling the cell, a pulse current was applied to reach the voltage of 1.5V, after which the cell was left at OCV for 20 h. The purpose of the initial pulse is to form a protection film on Cu current collector. Overall, the OCV is quite stable.
(2) You are right there is noise in the CV step, and ofcourse it can be due to electrolyte decomposition, but since other cells are running fine, I am not sure what is the real problem with these cells. Would be happy to further hear your thoughts.
1) Ok, I understand :)
2) All the cells with LNMO/graphite that you are running show the behavior seen in the pictures? Have the other cells you say are running fine a CV step at 4.8 V too, and are they running in the same voltage range?
Alessandro Innocenti Not all the cells have this problem, some materials run very nicely in full-cells. This makes me think that there might be some issue with the graphite, but at the same time, the graphite electrodes were coated on the piot line, and should be of good quality.
Dear Umair Nisar ,
also, could you try, please, some diagnostic EIS measurements[1,2] for the two (cell) classes:
a) Normal/common,
and
b) Uncommon/'bad',
cells ?
1. Frequency range: 10 mHz < Freq. < 0.1 or 1 MHz.
2. The cells are polarized at VDC=OCV.
Ioannis Samaras Unfortunately, the bad cells were discarded. But once I make the new cells, and find this issue again, I can definitely perform EIS on them. Many thanks.
Hi Umair Nisar,
The problem is the Voltage range that you applied on the full cell (2.8-4.8 V) is too high for commercial electrolytes without additives. We can see on the full cell charge curve that shows evidence of electrolyte decomposition ( not reaching the value of 4.8 V and having noise). The full cell matches the cathode and anode so the range of full cell should be lower than half cell 1.0 V ( in the case LNMO-Graphite). You can try the full cell from 2.5 to 2.7 V. On the other hand, you need to double-check the half cell of the anode to ensure it works well with that electrolyte.
Tuyen Truong thanks for your comment.
(1) That is true that commercial electrolyte is not very stable at higher voltages, however, in our project we have to use commercial electrolyte to evaluate our cathode materials. In the later stage, we can ofcourse go for the optimized electrolyte.
(2) Full-cell between 2.5-2.7V, or is there a typing mistake?
(3) I am trying to retest my graphite half-cells. However, many full-cells worked perfectly fine with these graphite anodes.
Tuyen Truong that's fine. Actually, this material LNMO has a redox potential at 4.7V, so if we limit our upper cut-off voltage to 4.7V, the battery wouldn't charge.
Umair Nisar,
It depends on the half-cell profile of the anode and cathode work with your electrolyte. Based on your result attached, you should try with the applied voltage from 2.5 to 4.7 V. For my electrolyte with more additives; it is also good with a cut-off of 4.8 V. Furthermore, you could think about the charge and discharge testing setup (holding the voltage, change the C-rate around 4.5-4.7 V, for example). It is my idea, hope it is helpful to you.