2019 Volume 60 Issue 12 Pages 2576-2579
Herein, a stable LiCoO2/acetylene black (AB) acetone suspension was prepared using LiCoO2 and AB powders with iodine dissolved in acetone as solvent. Using the LiCoO2/AB acetone suspension, LiCoO2/AB films without binders were deposited on stainless steel substrates via direct current (DC) and pulsed electrophoretic deposition (EPD) methods. The thickness of the resulting films deposited via both methods was greater than 50 µm with 5.0% AB content. The capacity of the DC-EPD film was 100 mA·h/g in the first cycle; this capacity decreased with increasing number of cycles, and subsequently began to peel off from the substrate. The capacity of the pulsed-DC EPD deposited films was 115 mA·h/g in the first cycle; this capacity slightly decreased with an increase in the number of cycles, and film degradation was not observed after the charge–discharge cycles.
Lithium-ion batteries (LIBs) are widely used in portable devices and hybrid and/or electric vehicles. Therefore, LIBs must have high capacity, fast recharge time, and high cycle stability. LiCoO2 (or LiCoxNi1−xO2) is mainly used as a positive electrode material in LIBs. Various methods have been investigated for the fabrication of the positive electrode of LIBs, such as CVD,1,2) sputtering,3,4) spin coating,5,6) inkjet printing,7) and electrophoretic deposition.8–10) Kanemura et al. have reported the fabrication of PTFE containing LiCoO2 electrodes via direct current (DC) electrophoretic deposition (DC-EPD);9) in addition, Ui et al. investigated the fabrication of binder-free LiCoO2 electrodes via DC-EPD.10) Both studies reported that the deposited cathode film possessed good electrochemical properties.9,10) We fabricated LiCoO2 films via DC-EPD and conducted charge–discharge measurements on the resulting films. After the charge–discharge cycles, some degradation was observed in the films. The use of alternating current or pulsed-DC bias induces very little decomposition of water.11) The formation of gas bubbles in the deposited films was negligible;11) therefore, dense and intact LiCoO2 electrode films were deposited via pulsed-DC EPD. In this study, we used acetone as a solvent to prepare a LiCoO2 suspension solution. A previous report states that gas bubbles caused by electrolysis of water are not generated when acetone is used as solvent.12) Therefore, the LiCoO2 film deposited via pulsed-DC EPD seems to be effective in preventing the electrolysis of the suspension when acetone is used as solvent.
Herein, after fabricating binder-free LiCoO2 films via DC and pulsed-DC EPD, we performed charge–discharge measurement for the resulting LiCoO2 films and evaluated the macro- and microstructure of the films both before and after the charge–discharge cycles.
LiCoO2 powder (Toshima manufacturing Co., Ltd., Japan; particle size: 100–500 nm) and acetylene (ethyne) carbon black (hereinafter, AB) powder (STEM Chemicals Inc., USA; average particle size: 42 nm) were used as starting materials. We prepared the suspension under the conditions described in a previous study.10) Iodine (99.8%; Toshima Co., Ltd., Japan) was dissolved in 80 ml of acetone (99.5%; Wako Pure Chemical Industries, Ltd., Japan), and the solution was stirred at 100 rpm for 1 h. Post stirring, 0.30 g/L LiCoO2 powder and 0.10 g/L of AB powder were added into the resulting iodine solution; then, the powders were dispersed via supersonic agitation for 5 min and ball milled for 24 h. We confirmed that the LiCoO2 and AB powders in the resulting suspension did not precipitate for 24 h; hence, the suspension was used for deposition. Planar stainless steel (SS) plates (2 × 5 cm) were used as substrates that served as electrodes, placed 2.5 cm apart, in the EPD cell. A pulsed-DC electrical bias (square wave) was applied to the SS electrodes (substrates) at 1 kHz with an alternating bias voltage of 0 or −20 V using a universal source (HP-3245A, Agilent Technologies, USA) for 10 min; for the DC-EPD, a DC source (E3640A, Agilent Technologies, USA) of −20 V was applied for 5 min. Then, the substrates were removed from the suspension, and the films were dried at 60°C. To evaluate the quantity of LiCoO2 in the deposited film, the deposited powders were scratched off the substrate’s surface and fired at 800°C in air to remove AB particles. The weight before and after firing the specimen was measured, and the LiCoO2 content in the film was estimated.
The structure of the resulting films was characterized using an X-ray diffractometer (XRD, Rigaku Miniflex, Rigaku, Japan) with CuKα radiation (30 kV, 15 mA). The microstructure and thickness of the deposited films were obtained using a scanning electron microscope (SEM, JCM-6000 Plus, JEOL Ltd., Japan).
The charge–discharge measurement of the electrode films was performed using a potentiostat/galvanostat (HABF5001, Hokuto Denko Corp., Japan) at a rate of 0.1 C (0.08 mA·cm−2) at room temperature and a three-electrode electrochemical cell with the following components: Pt wire as counter; Ag wire as a reference electrode; the deposited LiCoO2 film substrate with an active area of approximately 2 × 2 cm2 as the working electrode; and 1.0 M LiClO4 in polyethylene carbonate (Kishida Kagaku Corp. Japan) as the electrolyte.
A suspension of LiCoO2/AB powder was prepared as described in the experimental procedure. The suspension was observed to be stable for 2 days, without any precipitation. The quantity of iodine added to the suspension was either smaller or higher in the reports where LiCoO2 powder precipitated from the suspension. Ui et al. prepared the LiCoO2/AB powder suspension using the same solvent, acetone,10) but the concentration of iodine was different from that used in the present work. The particle size of the LiCoO2 powder used herein was smaller than that used in Ui et al.’s study;10) thus, the optimum iodine concentration in the suspension used in this study was assumed to be different than that used previously.10)
LiCoO2 films were deposited on SS substrates via DC-EPD and pulsed-DC EPD using the suspension. Hereinafter, we refer to the film deposited via DC-EPD and pulsed-DC EPD as the DC film and pulsed-DC film, respectively. Figures 1 and 2 show the XRD patterns and photographs, respectively, of the DC and pulsed-DC films. Except for the peaks arising from the SS substrate, all peaks were assigned to LiCoO2. According to the photographs, homogeneous films were deposited on the SS substrate via DC and pulsed-DC EPD. Figure 3 shows the surface and cross-sectional SEM images of the films deposited via DC-EPD and pulsed-DC EPD. From the surface and cross-sectional morphologies of the films, cracks and pores were not observed in the films obtained through both methods. The thicknesses of the DC and pulsed-DC films were 62.1 and 57.8 µm, respectively; these film thicknesses were sufficient to conduct the charge–discharge measurement. The LiCoO2 and carbon contents in the deposited films were estimated, and the weight loss of the powder after firing was found to be 5.0%. Therefore, the carbon content in the film was assumed to be 5.0%. We also evaluated the capacity of the electrode using the mass of the LiCoO2 content in the film.
X-ray diffraction patterns for the direct current (DC) and pulsed-DC films.
Photographs of the DC and pulsed-DC films.
Scanning electron microscope (SEM) images of surface and cross-sectional DC and pulsed-DC films.
Figure 4 shows the charge–discharge curves of the DC and pulsed-DC films; the measurement was conducted at a charge–discharge rate of 0.1 C (0.08 mA/g) at 25°C in a potential range of 0–1.1 V. Wide potential plateaus around 0.8–0.9 V (vs Ag+/Ag) were clearly observed in the discharge region of the films deposited via both methods. The capacity of the DC film was 100 mA·h/g in the first cycle; this capacity considerably decreased after the second cycle. The capacity of the pulsed-DC film was 115 mA·h/g in the first cycle; this capacity slightly decreased after the second cycle. Both films served as the positive electrode, but the capacity of the DC film significantly degraded after the charge–discharge process. To evaluate the difference in degradation between the films deposited via the DC and pulsed-DC methods, we conducted an SEM analysis on the films after the charge–discharge cycles.
Charge–discharge cycle of the DC and pulsed-DC films.
Figures 5 and 6 show the SEM images and photographs, respectively, of the deposited films after three charge–discharge cycles. As shown in Fig. 6, a part of the DC film’s surface peeled off from the substrate, but this was not observed in the pulsed-DC film. Some cracks, pores, and peeling were observed on the surface and cross-section SEM images of the DC films, but no significant change was observed on the surface and cross-section of the pulsed-DC films both before and after the charge–discharge cycle. The degradation of the film due to the charge–discharge cycle could be suppressed by fabricating the LiCoO2 positive electrode via pulsed-DC EPD. Hence, pulsed-DC EPD is a more suitable technique for fabricating LiCoO2 electrode films.
SEM images of the surface and cross-sectional DC and pulsed-DC films after the charge–discharge cycles.
Photographs of the DC and pulsed-DC films after the charge–discharge cycles.
LiCoO2/AB binder-free films were fabricated via DC and pulsed-DC EPD using a LiCoO2/AB suspension with optimal iodine addition. Dense films were formed at the initial stage of the deposition using both methods, and both films exhibited a reversible charge–discharge property. After the charge–discharge cycle, some part of the DC film’s surface peeled off, but no peeling or degradation was observed on the pulsed-DC film. Therefore, we conclude that pulsed-DC EPD is more suitable for fabricating a LiCoO2 positive electrode for LIBs.
