Cathode Composite Films for Oxide-Based All-Solid-State Rechargeable Lithium Batteries Prepared By Aerosol Deposition at Room Temperature

Abstract

Introduction Oxide-based all-solid-state rechargeable batteries (Ox-SSBs) have been expected as next generation energy storage devices with both high energy density and safety. Especially, thin-film type Ox-SSBs will be the most successful battery and are commercially available. In case of those thin-film Ox-SSBs, crystalline cathode electrodes are generally used, which have been prepared by RF magnetron sputtering, PLD, et al with heating process. On the other hand, aerosol deposition (AD) is a novel technique for preparing dense ceramics films at room temperature by jetting dry powders to the substrates. In addition, the deposition rate of the AD is much higher (5-50 μm min-1) than those in conventional deposition techniques such as RF magnetron sputtering (0.01-0.05 μm min-1)[1]. Hence, we focus on the AD to deposit thin films of composite cathode electrodes for thin-film type SSBs. In this work, we prepared composite electrode films composed of LiNi1/3Co1/3Mn1/3O2 (NMC) and Li1.4Ti2Si0.4P2.6O12-AlPO4(LATP) by the AD. These composite films were combined with LiPON and Si anode film, and electrochemical properties of the thin-film SSB of Si/LiPON/NMC-LATP were investigated. Experimental Composite powders of NMC (Nihon Kagaku Inc.) and LATP (Ohara Inc.) were prepared using a dry-powder mixer (NOB-MINI, Hosokawa Micron Co.). The mixing ratio of NMC to LATP was 100 : 5 in weight. Obtained composite powders were deposited onto a SUS substrate or a Si wafer by AD to fabricate a composite electrode. The resultant thin film was coated with a lithium phosphorus oxynitride (LiPON) glass electrolyte thin film by radio frequency (RF) magnetron sputtering. Subsequently, the Si anode thin film was deposited on the top of a LiPON film by RF magnetron sputtering. Cross-sections of a fabricated SSB (Si/LiPON/NMC-LATP) were observed by FE-SEM. Impedance measurements were performed in frequency ranges within 100 mHz to 200 kHz. Charge-discharge measurements were carried out for fabricated SSBs using 50 μA cm-2 (0.5C) for charging and 50-2000 μA cm-2for discharging. All electrochemical measurements were carried out at 100 °C in Ar-filled-glove boxes. Results and Discussion Figure 1 shows the cross-sectional FE-SEM image of a SSB (Si/LiPON/NMC-LATP) on a Si wafer substrate. A dense NMC-LATP composite film was successfully deposited on SUS by “one-scan” AD (ca. 6 second) as shown in Fig. 1. The thicknesses of NMC-LATP, LiPON, and Si anode layers were approximately 2.6 μm, 4.0 μm, and 300 nm, respectively. Figure 2 shows charge-discharge curves for a SSB on a SUS substrate. The discharge capacity for the first cycle at 50 μA cm-2 was 147 mAh g-1 where the cutt-off voltage was 3.0 V. This value corresponds to 98% of the capacity of a NMC-LATP composite film measured in liquid electrolyte cells. Moreover, 40 mAh g-1 was still obtained as the discharge capacity at 2.0 mA cm-2 (20C). Highly Li+-conductive-glass-ceramic LATP is supposed to promote the Li+ conduction and inhibit crack propagations associated with volume changes of NMC particles. These effects are likely to be the reason that this SSB provides higher rate capability than that in previous work[2]. We will further examine the effect of film thickness on the rate capabilities and stabilities. Reference 1. J. Akedo, S. Nakano, J. Park, S. Baba, and K. Ashida, Synthesiology 1, 130 (2008). 2. S. Iwasaki, T. Hamanaka, T. Yamakawa, W. C. West, K. Yamamoto, M. Motoyama, and Y. Iriyama, J. Power Sources, 272, 1086 (2014). Figure 1