Electrochemical Analysis Using Microelectrode for Cathode Materials Deposited on a Solid Electrolyte Pellet By Using Aerosol Deposition Method

Abstract

All solid-state batteries are expected as one of next generation batteries, because of its high safety. However, it is difficult to make a good interface between cathode layer and solid electrolyte. Aerosol deposition (AD) method which has been utilized for a room temperature impact consolidation process, is one of good method to form cathode layer on a solid electrolyte [1]. We have studied all state batteries composed of oxide-based solid electrolyte Li6.25Al0.25La3Zr2O12(Al-LLZ), Li3BO3(LBO)-LiCoO2(LCO) composite cathode deposited by AD method and Lithium metal. By the way, we have studied a single particle measurement using microelectrode to discuss a charge transfer resistance between one active material particle and liquid electrolyte and diffusion of lithium ion in the active material particle [2]. As one of advantage for the single particle measurement, lithium ion diffusion in electrolyte can be eliminated from a rate-determining steps because of rapid spherical diffusion in the electrolyte. In this study, this method was employed to evaluate charge transfer resistance of the cathode layer / solid electrolyte interface and diffusion of lithium ion in the cathode layer in LCO-LBO/Al-LLZ/Li cell with a micro area. The LCO-LBO composite cathode was prepared by heat treatment at 800 °C for 2 hours under air atmosphere after mixing LCO and LBO with a planetary ball mill. LCO and LBO are synthesized by a citric acid complex method using acetate salt and citric acid, and a solid state method using lithium carbonate and boron oxide, respectively. Al-LLZ pelet was prepared by sintering of planetary ball milled powder at 1140 °C for 20 hours. LCO-LBO powders were deposited on Al-LLZ covered by a polystyrene foil having holes around 200-300 µm in diameter. Then, cathode layers having 200-300 µm in diameter were deposited on the Al-LLZ pellet. After the deposition of the cathode layers, LCO-LBO/Al-LLZ was heated at 750 °C for 1 hour. Then, all solid state cell was prepared using lithium metal anode. Au wire as current collector was contacted to the one cathode layer, and then electrochemical measurements were carried out in a glove box filled by Ar. (Fig. 1) Fig. 2 shows charge-discharge curves of a cathode layer having 200 µm in diameter at 25 °C. Charge-discharge was repeated for three times. Current value was 10 nA (0.27 C). The cell showed an operating voltage over 3.8 V vs. Li/Li+ and good capacity retention during three cycles even at 25 °C. From these results, it can be said that the all solid-state battery with a micro area does work so well. The discharge rate capability was also evaluated at 20, 30, 40 nA (0.54, 0.81, 1.08 C, in Fig. 3). All charge processes were conducted at 10 nA. The discharge capacity at 40 nA was 78 % of the discharge capacity at 10 nA. The charge transfer resistance and diffusion of lithium ion in the cathode layer can be discussed from these results. [1] S. Iwasaki et al.,J. Power Sources. 2014, 272, 1086-1090. [2] K. Kanamura et al., Electrochemistry, 2016, 84(10), 1–6. Figure 1