Recently, all-solid-state thin-film rechargeable lithium batteries (TFBs) have expected to be used in various applications such as wireless sensor, smart device and other small devices for an auxiliary power supply. The TFBs are fabricated by forming each layers (i.e. cathode, electrolyte, anode and current collectors) mainly using physical vapor deposition process1,2). As a solid electrolyte, lithium phosphorus oxynitride (LiPON) thin film has been used commonly due to same advantages as no grain-boundary, isotropic property, less electronic conductivity and electrochemical stability. Ionic conductivity of LiPON thin film has been reported ~3.3 x 10-6 S/cm at 25 deg.C3), but it is lower compared with other solid electrolyte4). It is necessary to reduce the internal resistance of TFBs for applying to the device as described above. We improved this issue by reducing the thickness of LiPON film and reported in the previous meeting5). Internal resistance was successfully reduced. However, we had another issue that interfacial resistance between a cathode and an electrolyte was increased by planarization of a cathode surface. In this study, we investigated the mechanism of increasing of interfacial resistance and improved it with new surface modification material (LiCo0.9Al0.1O2).
2. Experimental
Lithium cobalt oxide (LCO) film as a cathode was prepared by RF and DC hybrid magnetron sputtering method on platinum film as a current collector. Ar was used as sputtering gas, and process pressure was kept at 1.6 Pa. New surface modification material, LiCo0.9Al0.1O2 (LCAO) film was prepared by RF magnetron sputtering method on LCO film. Sputtering gas and process pressure were same as LCO deposition. After deposition, LCO and LCAO were annealed at 600deg.C under atmospheric pressure by lamp heating system. Solid electrolyte (LiPON) film was prepared by RF magnetron reactive sputtering method with lithium phosphate (LPO; Li3PO4) target and N2 gas. Lithium film as an anode was prepared by vacuum evaporation method. Acrylic monomer was coated on the anode surface, covered with barrier lid, and cured by ultraviolet light for formation as an encapsulation. The fabricated TFB cells were investigated by electrical and electrochemical properties at room temperature.
3. Results and discussion
Figure 1 shows the scanning electron microscopy (SEM) images of LCO films with and without LCAO film after annealing process. As result , it was confirmed that top of LCO columnar grain became sharp by coating with LCAO film. We think this result shows decomposition reaction was suppressed with LCAO film. Figure 2 shows the nyquist plot of TFB and each resistance with and without LCAO film. It was also confirmed that the resistance of 1st semicircle was not changed by LCAO film thickness and the resistance of 2nd semicircle is was decreased by it. These results show LCAO film improved interfacial resistance between a cathode and an electrolyte.
Reference
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3) Xiaohua Yu, J. B. Bates, G. E. Jellison, Jr., and F. X. Hart, J. Electrochem. Soc., 144, 524 (1997).
4) N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y.Kato, S. Hama, K. Kawamoto and A. Mitsui, Nature Materials, 10, 682 (2011).
5) A. Suzuki, S. Sasaki, I. Kimura and T. Jimbo, Abstract#449, 227th ECS Meeting (2015).
