Effect of Electrode Mixing Conditions on the Performance of Lithium-Ion Batteries Analyzed by Fast Fourier Transform Electrochemical Impedance Spectroscopy

High power and energy density lithium-ion batteries (LIBs) with high charge/discharge rate are required for electric vehicles. Although it is inevitable to clarify the appropriate electrode manufacturing methods to enhance the performance of LIBs, there have been many uncertain points. In the present study, we investigate the influence of positive electrode mixing conditions on the performance of LIBs at 30 °C. Test cells were assembled with positive electrodes of LiCoO₂ mixed with acetylene black as conductive filler and PVDF binder, electrolyte solution of ethylene carbonate (EC)/dimethylcarbonate (DMC) (1:1 vol.) containing 1 M LiPF₆, and negative electrodes of graphite mixed with PVDF binder.

We employed fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS) to analyze the internal resistances under discharge to take advantage of its short measurement time since measurable decrease in the state of charge (SOC) cannot be avoided during EIS measurements with the conventional frequency response analyzer (FRA). Cross-sectional scanning electron microscope (SEM) observations of the positive electrodes were also carried out.

Pre-mixing of LiCoO₂ particles and acetylene black under an appropriate mill rotation speed is effective for the performance enhancement. In both cases without the pre-mixing and with pre-mixing under too high mill rotation speed, the discharge capacities decrease.

The impedance spectrum by FFT-EIS exhibits two semi-circles at 1000 mA/g. In the case without the pre-mixing, the high frequency arc becomes significantly large, while both high and low frequency arcs become significantly large in the case with pre-mixing by too high mill rotation speed.

The appropriate pre-mixing possibly thus decreases the resistance of electron transfer, i.e. contact resistance, between the LiCoO₂ particles and acetylene black by good dispersion increasing the contact between them. This view corresponds to the assignment that the high frequency arc is attributed to the surface insulating layer of LiCoO₂ particles[1,2] contacting with acetylene black. The SEM observations support this view.

On the other hand, the pre-mixing by too high mill rotation speed possibly increases the contact among LiCoO₂ particles, decreasing the contact between the LiCoO₂ particles and acetylene black. This also decreases the contact between the LiCoO₂ particles and the electrolyte. This view agrees with the assignment that the high frequency arc is ascribed to the surface insulating layer of LiCoO₂ particles described above while the low frequency arc represents the Li ion charge transfer at the interface between the LiCoO₂ particles and the electrolyte[1,2]. The SEM observations support this view also in this case.

References

[1] E. Barsoukov, in Impedance Spectroscopy, Theory, Experiment, and Applications, 2^nd ed., E. Barsoukov and J. R. Macdonald, Editors, John Wiley & Sons, Hoboken, NJ (2005).

[2] Advances in Lithium-Ion Batteries, W. A. V. Schalkwijk, and B. Scrosati, Editors., Kluwer Academic/Plenum Publishers, New York (2002)