Mostly in lithium/sodium ion batteries we observe a smooth voltage profile in galvanostatic measurements. However, in my recent experiments which includes sodiation of Sn metal, I am observing some uneven dips in my voltage profile only when I use a thick metalic tin electrode. (In case of thin electrodes the voltage profile is smooth. Also the current densities are same for both thin and thick electrodes) Researchers speculate that it might be due to different phase formations of NaSn casuing this abnormal behavior. it might be true but seems to me a very superficial explaination. I would like to know whether there is thermodynamic, kinetic or both reasons behind it causing the voltage to show this strange behavior. I also obeserve this kind of profile in other thick alloying electrodes as well. Kindly find the attached voltage profile in question. First sodiation process of Sn electrode 60 um thick with 1M Na-perchlorate in PC using a coin cell.
With thicker electrodes, you can have any number of things happening. One is that the strain that accompanies the sodiation produces larger forces in thicker films, which may lead to partial delamination or cracking. Another is that a thicker Sn film might have sufficient thickness to produce several layers of grains, with lower layer being kinetically hindered.
I'm not sure whether what you are seeing can be connected to "staging" of various Na-Sn phases, since that looks a bit different.
Dear Pavel, Thank your for your answer. I am observing prominant plateaus which can be related to new phase formations as shown in your figure. With in one plateau i am observing noise like signal which are more prominant in thick films as compare to thin films. It might be related to the production of severeal grains or cracking. Does different grain size changes the thermodymanics or kinetics or both?
I'm not sure with sodiation of Sn, but I've seen papers that looked at lithiation of some cathode materials, and showed heterogeneous lithiation, i.e. grain by grain difference, or even varying concentration within a single grain. (Actually, just searching lithiation and FIB threw this out: http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.6b00155?journalCode=cmatex It's Li-Si, but some of the dynamics could be the same.) I imagine that factors such as dislocations, grain size/surface energy, and stress/strain can have an effect on the thermodynamics or sodiation. How does the noise change with different charging rates?
Oh. That is strange. Could it be at the lower end of instrument capability? I had output stability issues before, trying to run a 10 mA charge/discharge on a 3A rated battery tester.
Dear Umair Gulzar,
could you, please, replace time with x (x: NaxSn) in your plot ?
Dear Pavel Correct me if am wrong. To me high rate means less time for sodium to diffuse, So less reactions, less destructions of electrode, smooth curve. So why its strange. The currents are quite far from the limits. So thats not a problem
Ionnis I will share it once i make it thanks
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