Formation of Positive Composite Electrode By Aerosol Deposition Method for All-Solid-State Rechargeable Lithium Batteries

Abstract

Introduction All-solid-state rechargeable lithium batteries using non-flammable solid electrolytes are promising as next-generation batteries with high safety. A fabrication of all-solid-state batteries requires a high temperature treatment to form an electrochemical interface between electrode and solid electrolyte materials. However, the heat treatment limits the combination of electrode and solid electrolyte materials because of a formation of impurities through chemical reactions at high temperatures. Aerosol Deposition (AD) method(1) which can form a ceramic film on a substrate without heat treatment has been investigated as one of effective methods to solve interfacial problems between electrode and solid electrolyte materials in all-solid-state rechargeable lithium batteries. The powder with an optimal particle size is needed to form a thick deposition layer by AD method. In this study, we synthesized the composite particles composed of LiCoO2 and Li3BO3 (2), and applied to prepare a thick LiCoO2 layer on Al-doped Li7La3Zr2O12 (LLZ) by AD method. Experimental LiCoO2 was synthesized by a sol-gel method using CH3COOLi and Co(CH3COO)2･4H2O. The precursor gel was calcinated at 400 °C for 5 hours and then calcinated at 750 °C for 10 hours to obtain LiCoO2. Li3BO3 precursor was synthesized from Li2CO3 and B2O3 by a solid-state technique using a planetary-ball mill. The LiCoO2/Li3BO3 composite particles with two different mixing ratios of 9:1 and 7:3 in weight were prepared as aerosol powder by calcining the mixture of LiCoO2 and pulverized Li3BO3 precursor at 800 °C for 2 hours. LLZ powder was synthesized by solid-state reaction using LiOH･H2O, La(OH)3, ZrO2, and g-Al2O3, and then pelletized and sintered at 900 °C for 3 hours and 1200 °C for 12 hours. A performance of cell was investigated by a constant current charge/discharge test with Li metal anode. The synthesized LiCoO2 powder and LiCoO2/Li3BO3 composite powder were deposited on LLZ pellets, respectively, by spraying 0.7 mg of each powder under the pressure difference between 70 Pa and 0.6 MPa. Results and discussion Figure 1 shows the cross-sectional SEM and EDS images of LiCoO2 cathode layer and LiCoO2/Li3BO3 composite cathode layer on LLZ-pellets. The thicker cathode was formed using LiCoO2/Li3BO3 composite particles compared with LiCoO2 particles. In addition, the composite particles with the higher Li3BO3 ratio provided the thicker cathode. Those results suggest that Li3BO3 works like binder. Figure 2 shows the 1st charge-discharge curves of the all-solid-state batteries with LiCoO2/Li3BO3 composite and LiCoO2 cathodes at 60 °C, respectively. The initial discharge capacity and utilization of LiCoO2 were estimated to be 12.8 mA h g-1 and 9.3 % (LiCoO2), 12.3 mA h g-1 and 9 % (LiCoO2/Li3BO3 composite 9:1), 55.5 mA h g-1 and 40.1 % (LiCoO2/Li3BO3 composite 7:3), respectively. This result suggests that Li3BO3 works as not only binder but also Li+ conducting pathways to improve the utilization of LiCoO2. Reference [1] J. Akedo, J. Am. Ceram. Soc., 89, 1834-1839 (2006) [2] S. Ohta, et al., J. Power Sources, 238, 53-56 (2013) Acknowledgement This study was supported by Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST). Figure 1