Higher power output requires faster charge transfer reactions in the charge/discharge process. Lower activation energy directly correlates to faster Li ion diffusion. Morphology of active materials, binders, separators and electrolytes play an important role in determining the activation energy of a battery system. In [1] the authors showed through electrochemical impedance spectroscopy the impact of binders on the activation energy for Li Ni_1/3 Mn_1/3 Co_1/3 O₂ (NMC)  and for Li(Ni_0.4Co_0.6)O₂ (NCO) in [2]. For this study we demonstrate a similar electrochemical method to determine the activation energy for ionic diffusion in electrode materials via Electrochemical Impedance Spectroscopy and quantify its dependence on separator type and electrolytes.

Experiment:

Here in this study we have investigated three different variations of NMC cathode material namely NMC 442 from a commercial cell, NMC 622 and NMC 622 with surface modifications from Umicore. The core idea is to check the activation energy of these active materials with commercially available separators and electrolytes and compare them with advanced ceramic separators and a new type of electrolyte. In this way, we can demonstrate the influence of each element on the activation energy and to identify good combinations to achieve high power output cells.

Figure 1 shows the discharge capacity test results for all the 3 NMC cathodes at C/30 rate from 4.2- 3 V potential window. The NMC 622 type cathodes were tested with the advanced separators and the new electrolyte and the NMC 442 from the commercial cell was tested with Cellgard 2325 PP-PE-PP separator and LP30 (LiPF₆ in EC: DMC: 1:1) electrolyte. For NMC 442, the electrodes were recovered from the commercial cells whereas for the NMC 622 electrodes, they were provided by the battery manufacturer.

Figure 2 a) shows the incremental capacity curve for NMC 622 surface modified electrode with the advanced ceramic separator and new electrolyte. This test was performed at C/30 rate to determine the peak potential at which the electrochemical impedance spectroscopy needs to be performed. Figure 2 b) shows the results of the EIS at three different temperatures and one can see a clear variation in the behaviour with change in temperature.

Using the Arrhenius equation the activation energy will be calculated by plotting the quantity of interest or thermally activated parameter as a function of reciprocal temperature. Here the quantity of interest will be the charge transfer resistance (R_ct), which will be calculated after fitting the impedance spectroscopy results at various temperatures for every condition and plotting it against 1/T. From the information obtained from this experiments can lead to a better understanding of the role of the separators and electrolytes in diffusion processes occurring in different NMC electrodes.

References:

 [1] J. Xu, S.-L. Chou, Q. Gu, H.-K. Liu, and S.-X. Dou, “The effect of different binders on electrochemical properties of LiNi_1/3Mn_1/3Co_1/3O₂ cathode material in lithium ion batteries,” J. Power Sources, vol. 225, pp. 172–178, 2013.

[2] M. Ecker, T. K. D. Tran, P. Dechent, S. Kabitz, A. Warnecke, and D. U. Sauer, “Parameterization of a Physico-Chemical Model of a Lithium-Ion Battery: I. Determination of Parameters,” J. Electrochem. Soc., vol. 162, no. 9, pp. A1836–A1848, Jun. 2015.

Acknowledgements:

The research leading to these results has received funding from the [European Union's] [European Atomic Energy Community's] Seventh Framework Programme ([FP7/2007-2013] [FP7/2007-2011]) under grant agreement n° 608936 (see Article II.30. of the Grant Agreement). Further, we acknowledge the support to our research team from ”Flanders Make”.