Effect of Electrode Pressing Process on the Electrode Characteristics of LiCoO2 Cathode.
Authors: Kobayashi Takahiro , Nara Hiroki , Mukoyama Daikichi , Koiwa Ichiro , Momma Toshiyuki , Osaka Tetsuya 
Published: February 2020

Introduction

　Lithium-ion battery (LIB) has been used in a power source of mobile devices. Currently, LIB is demanded to be used for electric vehicles as a power source. Therefore, LIB with properties of high capacity, high power and long life have been requested. However, it is difficult to produce a LIB to meet all these requirements. In recent years, we have been selecting the condition to realize cathode sheets to perform for power usage or high energy storage to match the requirements for using the existing active material. In this study, we focused on the process of pressing cathode sheets. LiCoO2 cathodes were prepared by coating followed by pressing at varied pressures, as cause a change in packing density of active material and conducting carbon. Effect of the pressure on charge and discharge characteristics was investigated with various current loading rate. Furthermore, we tried to assess the effect of pressing cathode sheets using electrochemical impedance spectroscopy (EIS).

Experiment

　LiCoO2 cathode was prepared in our laboratory by coating method. The thickness of the cathode before pressing is defined as thickness of 100 %. Then cathode sheet was pressed into the thickness upto 70 % with varying the pressure. Laminated cell was prepared with inserted reference electrode using a pressed LiCoO2 cathode. The anode sheets were prepared with graphite as the active material. The anode was pressed until the thickness of 90 %. 1.0 mol dm-3 LiPF6 in EC: DEC = 1:1 was used as electrolyte solution. Li-Al alloy reference electrode was used as the reference electrode. The inserted Al wire (f 25 mm ×4 mm) was alloying with Li by applying current of 5 mA for 60 minutes between Al wire and cathode. Initial charge-discharge of the laminate cells was carried out for 2 cycles at 7 mA. Rate performance was studied by 0.1 C, 0.5 C, 1 C, 5 C and 10 C for 2 cycles each. EIS was carried out at the SOC (state of charge) of 50 %, with frequency range 100 kHz - 10 mHz and amplitude 10 mV. The ac responses of the cathode and the anode were measured simultaneously by multi-potentiostat.

Results and Discussion

　Discharge overvoltage was observed for the cathodes with thickness of 100 %, 90 %, 75 % in the charge and discharge curve. Discharge overvoltage that was observed in the charge and discharge curve at 0.5 C was 130 mV between the cathode thickness of 90 % and 75 %. In addition, overvoltage of between 100 % and 75 % was 370 mV. Discharge overvoltage that has been observed at 1.0 C was 220 mV between the film thickness of 90 % and 75 %. In addition, overvoltage of between the thickness of 100 % and 75 % was 520 mV. This result suggests that overvoltage decreases with an increasing of the pressing pressure. The results of the EIS are shown in Fig. 3. The radius of the semicircle in high-frequency region appeared in Nyquist plot becomes smaller in the order of 100 %, 90 %, and 75 % in thickness. From this result, the electric contact of cathode active material particles is considered likely to increase with increasing the pressing pressure. Also, the radius of the lower frequency side semicircle from Nyquist plot is found to becomes smaller in the order of 100 %, 90 %, and 75 % in thickness. This result suggests that charge transfer resistance decreases with increasing the pressure. A change in the charge transfer resistance as compared with the resistance value of the high-frequency side is small. These results suggest that resistance value of the high frequency sid has large influence on the charge and discharge characteristics.

Conclusion

　Rate performance of the cathode sheet for LIB was investigated with varying the pressure for the press prior to the cell configuration after the cathode coating. The cathode sheets were pressed to be the thickness to 100 % (no press), 90 %, and 75 %. The overvoltage during the discharge of the cell decreased by the electrode pressing process. The EIS study suggests that both the electric resistance inside the cathode layer and the charge transfer of the cathode active material are reduced by the pressing of cathode sheet in the thickness range of 100 % and 75 %.

Acknowledgements

　This work was partially aided by the MEXT-supported Program for the Strategic Research Foundation at Private Universities.

Figure 1