I made some Li ion cells include NMC622 as working electrode and Li metal as counter electrode. After the first charge-discharge cycle, they analysed with EIS. First, Nyquist plots were composed of one semicircles at the high to middle frequency region and a slope in the low frequency region. When we repeat it, Nyquist plot included two semicircles at the high to middle frequency region and a slope in the low frequency region.
yes , this is very common factor because the metal conductivity is totally different from the battery active material . The first one is metal therefore low impedance and another one is ceramic material with carbon and some additive all are having higher impedance for this purpose you are getting such effect. by proper heat treatment you will get the reduced impedance i.e. the conductivity improves this may merge. it is always depend upon the way of preparation of electrode and contact.
Thank you for your reply. You mentioned that heat treatment is important. َAt what step is heat treatment is important?
Dear Nasrin Azad ,
just after a very strong (NMC622 Li-half) cell disturbance, a charge-discharge cycle, the VOCV value(s) of a Li-WE has a very rich (and multifactor) time-line, a VOCV(t) transient towards the thermodynamic equilibrium phase, for, at least, 1h, relaxing to a steady state. It is obvious, that ALL Nyquist plots, during this transient phase, are very different, following the specific (NMC622 Li-half) cell thermodynamic relaxation path(s).
So, what is/are
1) the last Voltage value, V0, after the 1st (charge-)discharge (semi-)cycle ?
2) the OCV value, V1(=VDC1), just before the 1st Nyquist plot ?
3) the OCV value, V2(=VDC2), just before the 2nd Nyquist plot ?
Dear Ioannis Samaras
Thanks. The last voltage after first cycle that battery tester showed, was about 3.5V. But when I put the cell on EIS analyzer, the voltage lost to about 2.9V and then gradually increased to about 3.4-3.5V. After that, I started the EIS test. After first EIS , voltage was about 3.5 V that was voltage before the second Nyquist plot.
After EIS test, CV test was taken. When again put it on the battery tester, voltage was 2.8V and gradually inceased to 3.5V.
Dear Nasrin Azad ,
please, 'goto'
https://www.researchgate.net/post/What_are_the_reasons_for_voltage_loss_in_Li_ion_coin_cells_after_EIS_and_CV_tests
What can be the reason for low voltage of Li ion cell made from NMC622 material immediately after crimping ? I made before some cells from this material and the voltage was about 2.3 V after crimping with the same conditions.
Dear Nasrin, Partially I can understand the problem, think about your moisture content of your electrolyte, which may passivate the lithium metal and dissolute the active material. Similarly, the electrolyte volume also important. Moreover, in half-cell the mass is imbalanced, hence it is not reliable and advised to pair with graphite or something else.
The gradual voltage increment might be an instrumental error, cross-check with a multimeter.
EIS, a non-destructive method to probe the interface and all the cell components have been involved, even poor wetting or coating and pore-filling, increasing the Rs, and Rct.
(includes the ans for ur previous Question on Aug 1)
Dear Vadivel
I already fabricated some cells consisted of NMC622 working electrode and Li metal as reference with the similar conditions and same electrolye.They had good electrochemical performance. But now, with coating the cathode active material, fabricated cells have a bad performance.While most research shows that coating of active material improves the performance.
Dear Nasrin Azad ,
how do you make your coating and
what is the mass ratio, mR (= mcoating/mNMC622) ?
dear Nasrin,
you are right, the coating will improve the performance in terms of cycle life (due to the suppression of parasitic reaction, hence significantly reduce the active material dissolution and lithium loss), however, the coating material determines the interfacial property. If the coated material supposed to have poor electronic conductivity or lattice mismatching etc leads to poor performance like voltage decay and capacity decay.
Just presume that the coated material reacts with HF (hydrolysis of LiPF6), it turns to metal-fluoride and has poor electronic conductivity leading to poor performance.
As you said, at the earlier, the material is working good, which fails now. is both the material are the same (pristine) or different?
If it is same, then the problem might be ur coating and other fabrication else it may come from the surface modifier.
Dear Vadivel Selvamani
The selected potential rang for charge- discharge was 4.3-2.5V. Can it be the reason for low voltage of discharge?
Dear Nasrin,
The potential window is safer for Ni-rich cathodes.
Moreover, the upper cut-off voltage for NMC is 4.4 V vs Li+/Li. thereafter no significant electrochemical contribution has been noted, instead, lattice oxygen release leads to dramatic capacity loss.
The electrochemical lithiation potential for Ni-rich cathode is ~ 1.8 V, hence it is safe to operate above the potential.
I have experience with electrochemical lithiation of Ni-rich cathode. After the lithiation sighniifcatnly higher charging and subsequent discharge capacity have been noted with the immediate cycle, however, the capacity is fading is very much on later cycles.