Hi,

Dear Evan, as you have mentioned, there is only one difference between cells 1,3 and cells 2,4 and it is the application of plasticizer in your polymer electrolyte. I think it means the viscosity of your polymer will decrease and it should cause lower resistance for Li ions in the cell to move around. However as you have mentioned you are using Succinonitrile as a plasticizer which has two CN functional groups. I think these two CN groups can have coordination bound with some of Li-ions that supposed to be free for moving around, which means when your cells are reaching to the mass control stage (end of discharge) Li ions movement would be more restricted a little bit compare to the start of discharge.

So, in my opinion it might be because of internal resistance of your cells and using plasticizer. as you can see those two cells in their charge and discharge profiles have shown higher difference between the voltage of charge and discharges in their plateau compare to cell 3 (you can add some perturbation in charge profile as a result of this issue as well). so when LFP is going to reach the mass control reaction stage, cells will face a little bit perturbation in discharge profile because of higher resistance for Li movement inside the battery. as you can see there is a higher negative slope in the discharge of those cells compare to the cell number 3, which can be correlated to the voltage drop resulted from higher internal resistance (impedance) of cells, and as far as you have prepared all sample with the same material (LFP), this issue might be the outcome of the internal resistance because of added plasticizer. another possible option might be the repeatability of cell preparation that you can test it by increasing the number of cells evaluation.

Hi Hossein Ghafarian-Zahmatkesh , you have given an interesting answer I haven't explored yet. I will review in detail about the phenomena happening at the mass control stage. Also, as you rightfully pointed out about the repeatability of the data, for other cells with lower discharge capacity, this was not observed.

Evan Kurian

Hi, if we consider these differences are not completely from cell repeatability and your cells prepared almost the same, I think there is another important point that you should consider. as you can see in the charging graph of cells without plasticizer, there is a delay to reach to the kinetic control stage of the reaction (I mean to reach to potential of 3.5 V in which the electrochemical reaction happened) compare to those cells with plasticizer showing a higher positive slope in the beginning of charge. it can be correlated to the presence of plasticizer, at the beginning the viscosity reducing effect of additive, can improve the kinetic of electrochemical reaction but at the end when the reaction needs much more free Li ions, it can play a role as a barrier for freely movement of li ions. in addition there is a slightly difference between cell 1 and cell 3 at the beginning of charge, I suggested it might be probably because of using different polymers in those two cells that one of them has shown higher viscosity compare to the other one.

I am not well versed with the kinetic control and mass control concepts. Can you provide any directions to learn more about them. I also have doubt whether the wider voltage window (2 to 4 V) used for the test have any significance (may limit the mass control stage) in relation with to your suggestions. Thank you Hossein Ghafarian-Zahmatkesh for the answer.

Hi Evan Kurian

Usually we have mass control and kinetic control region in each electrochemical reaction. in the kinetic control stage, rate of reaction is controlled by the kinetic of electrochemical reaction (because at the start of reaction the migration of Li ions is so fast that the reaction is controlled by its kinetic or charge transfer resistance). In Li-ion batteries and in this case (LFP) it is correlated to the electron transfer needed for electrochemical reaction and changing the state of Fe. in this stage the electrochemical reaction which is related to the redox couple of Fe will happen. But in mass control region the rate of reaction is controlled by the rate of mass delivery to the reaction (in this case Li ion migration). if you are interested in having more detail I think you can find some information in electrochemistry books.

following image is from Alen J Bard book (ELECTROCHEMICAL METHODS Fundamentals and Applications).

check this image