Novel Measurement Method for Distinction between Electronic and Ionic Conductivity in Composite Electrodes

       Composite electrodes containing active materials, carbon and binder are widely used in Li-ion batteries. It is well known that their morphology influences the electrochemical performance of batteries. For the improvement of performance in practically used Li-ion batteries, it is necessary to tune the parameters of composite electrodes such as porosity, compounding ratio and thickness. The morphology of composite electrodes varies the effective electronic and ionic conductivity in composite electrodes. However the tuning of composite electrodes has been mainly based on the intuition and experiences, since it is difficult to distinguish the electronic conductivity and the ionic conductivity in composite electrodes. For composite electrodes, the traditional 4-probe method cannot be applied because charge-discharge currents are flew during applying voltage. Therefore, a measurement method for distinction between electronic and ionic conductivities in composite electrodes is required. In this study, we have developed a method for simultaneous measurement of electronic and ionic conductivities in composite electrodes. This method is applied to the various porosities composite electrodes.

       Fig. 1 shows the measurement setup and the arrangement of the electrodes and samples. Two Al foils were bonded with polypropylene. 75 wt% carbon-coated LiFePO₄ powder, 10 wt% acetylene black and 15 wt% PVDF were mixed in 1-methyl-2-pyrrolidinone anhydrous (NMP, Sigma-Aldrich) solvent. The slurries were coated onto the aluminum foils. Drying was done at 70 °C to remove solvent and additional drying was performed at 80 °C in a vacuum oven to vaporize the residual solvent. These LiFePO₄-based composite electrodes were pressed at 0 kgf, 300 kgf, 600 kgf, 900 kgf, 1200 kgf pressures to control their porosity. As shown in Fig.1, the electrochemical cell contains 6 probes connecting with the composite electrode. For the electronic and the ionic conduction electrodes, Al foils and Li foils were used, respectively. Two potentiostats and bias voltage were connected to the cell. After open circuit voltage is measured, the two potentiostats were operated with this voltage as the set point. And then, a bias voltage was applied between the two working electrodes. The ionic current was measured at A1 or A3 current meter, and the electronic current was measured at A2 or A4 current meter. The measurement principle is explained in the literature [1].

       Fig. 2 shows the effective electronic and ionic conductivities for various porosity composite electrodes. The electronic conductivity is at least 10 times higher than the ionic conductivity. This indicates the dominant contribution to the electrochemical performance is the ionic conductivity. For the high porosity electrodes, the effective ionic conductivity is almost constant. On the other hand, when the porosities of the electrodes are less than 47%, the ionic conductivity decreased. This indicates that the compressed composite electrode structure narrowed the ionic conduction path, resulting the decrease of the ionic conductivity. From the result of the rate capability test in the LiFePO₄ composite electrodes, the decrease of porosity causes the decrease of discharge capacity at 10 C rate. Therefore, the low ionic conductivity from low porosity lowers the rate capability of LiFePO₄composite electrodes.

References

 [1] Siroma, Z.; Hagiwara, J.; Yasuda, K.; Inaba, M.; Tasaka, A. J. Electroanal. Chem. 2010, 648, 92-97.