Composition-controlled Atomic Layer Deposition of Lithium Phosphorus Oxynitride for Lithium Metal Microbatteries

Abstract

Lithium-ion batteries have been widely studied and commercialized more successfully than other secondary batteries due to their excellent characteristics such as high cell voltage, small self-discharge rate, and long life. Along with the explosive demand increase, the higher energy density of lithium ion batteries and a high level of stability are requested. An ideal method for increasing the energy density of a lithium ion battery is to use lithium metal instead of graphite used as a negative electrode material. However, when lithium metal is used as the negative electrode material, dendrites of lithium metal grow at the interface between the lithium metal and the liquid electrolyte during charging/discharging of the battery, which may cause an explosion or fire due to a short circuiting between the negative electrode and the positive electrode. If a solid electrolyte is used instead of the liquid electrolyte to prevent this, dendrite formation can be suppressed, which can greatly improve the stability and energy density of the lithium ion battery. Most of the solid electrolytes that exhibit high ionic conductivity are sulfide-based solid electrolytes. In addition to high ionic conductivity, it has the advantage of excellent mechanical flexibility and easy formation of a contact with an electrode. However, sulfide materials are very reactive and has poor stability, so it is difficult to develop manufacturing facilities for mass production. On the other hand, oxide-based solid electrolytes are very stable compared to sulfides but have relatively lower ionic conductivity. Numerous studies have been conducted to compensate for this, and oxide-based solid electrolytes having high ionic conductivity such as LISICON and NASICON-type solid-state electrolytes are widely being studied. In addition to the ionic conductivity, requirements that must be satisfied for the solid electrolyte to be used in a solid-state battery with lithium metal anode include stable contact with lithium metal, a wide stability window, and ease of processing. Among the solid-state electrolytes, LiPON exhibits reasonable ionic conductivity, has stable contact with lithium metal anode, and has a wide stability window. Furthermore, LiPON can be easily fabricated as a thin film. The LiPON deposited by sputtering showed the ionic conductivity of 10-6 S/cm, but the LiPON deposited by ALD showed a relatively lower ionic conductivity. It was caused by the relatively low concentration of N in the anions of LiPON. To improve this, a super-cycle ALD technique was used to control the anion concentration of the LiPON thin film and to change the ionic conductivity. In this thesis, it was possible to control the ionic conductivity of LiPON thin films. Eventually, LiPON with high ionic conductivity required for three-dimensional thin-film solid-state batteries was successfully deposited by super-cycle ALD and can be applied to next-generation all-solid-state batteries.