A Study of Lithium Silicon Oxynitride (LiSiON) Deposited By RF Sputtering for All-Solid-State Thin Film Batteries

At present, we suffer from various environmental issues such as air pollution and rapid weather change. Air pollution is usually caused by the use of fossil fuels. To overcome the environmental issues, LIBs are reasonable candidates. In these days, lithium ion batteries (LIB) are very promising power suppliers for electronic devices, electrical vehicles (EV), and energy storage system (ESS) because of their high power densities.[1] Although the LIB has advantages compared to other power sources, LIBs with a liquid electrolyte have safety issues such as explosion and fire contributed by thermal or chemical instability. All-solid-state batteries are the solution to the problem of LIBs with liquid electrolytes. All-solid-state batteries have many advantages such as high energy densities, high stability, and applying high voltages compared to conventional LIBs.[2] As the portable and miniaturization of electronic devices and the development of wearable devices are to be in the spotlight, the development of a power supply for drive them is indispensable. The type of batteries that can meet this demand is an all-solid-state thin-film battery. Thickness of thin-film batteries is about 10 μm, which makes it suitable for power source of miniaturized electronic devices such as smart cards, RFID tags and medical devices. All-solid-state thin-film batteries also have better thermal stability than conventional Li-ion batteries. Higher capacity of thin-film batteries can be realized by applying high voltages. The key to the performance of all-solid-state thin-film batteries is a solid electrolyte. In order to deposit thin-film electrolytes, there are various available techniques such as a sputter, plasma laser deposition (PLD), e-beam evaporator, and so on. Among these techniques, the sputtering technique has an advantage compared to other deposition techniques. The sputtering techniques could deposit oxide and nitride materials. In addition, sputtering techniques are simple process and can be deposition with thin films uniformly. Comparing PLD and molecular beam epitaxy (MBE), the sputtering techniques are lower cost and low temperature for deposition. Therefore, sputters are suitable for commercialization. Generally, solid electrolytes are classified under oxide system and sulfide system. While sulfide electrolytes have high ionic conductivity and instability, oxide electrolytes have low ionic conductivity and high stability. A lot of oxide based solid state electrolytes were researched. The oxide-based solid electrolytes include LiPON, Li7La3Zr2O12 (LLZO), Li1+xAlxTi2-x(PO4)3(LATP), and lithium boron oxynitride. Among them, LiPON thin-film electrolytes are representative and commonly used as a thin-film electrolyte. However, since the ionic conductivity of LiPON is relatively low, it is necessary to improve the low ionic conductivity. In this study, a new lithium silicon oxynitride (LiSiON) thin-film electrolyte was deposited by RF sputtering technique. Surface morphologies and cross-sectional views of the thin-film electrolyte were characterized by field emission scanning electron microscope (FESEM). The thin film showed smooth surface without any cracks and pinholes. It can be thought that the smooth surface could decrease interfacial resistance between electrolyte and electrodes. In addition, surface morphologies were also characterized by atomic force microscopy (AFM). The sputtering rates were calculated by thickness of thin films on cross sectional views. Structural properties of the thin films were characterized by x-ray diffraction (XRD). The thin film showed amorphous properties compared to the target material which is a crystalline material. In addition, structural properties of the thin film were also characterized by transmission electron microscope (TEM). The thin film showed also amorphous properties with partially crystalline in LiSiON structures deposited by RF sputtering. Ionic conductivity of LiSiON was measured by electrochemical impedance spectroscopy (EIS). Cu thin films used as blocking electrodes with 150nm thickness were deposited by a direct current (DC) magnetron sputtering using a target with 2-inch diameter. The DC power and working pressure were set on 30W and 7 mTorr in Ar atmosphere, respectively. LiSiON thin film was deposited by an RF magnetron sputter at 200W power using Li4SiO4 target with 4-inch diameter in Ar/N2 (2:8) atmosphere. Ionic conductivity of the LiSiON thin film showed 2.47 ´ 10-6 (S/cm) which is similar to other lithium oxide thin films. For this reason, LiSiON thin-film electrolytes are research-worthy materials for use in all-solid-state thin-film batteries. Reference [1] M. Armand, J. Tarascon, Nature, 451, 652 (2008). [2] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, et al., J. Power Sources, 208, 210 (2012).