Fabrication of LiCoO2 cathode and Li2O-B2O3-P2O5 electrolyte for thin film battery by Aerosol Flame Deposition

Abstract

Rechargeable thin film battery has various applications to electronic devices such as micro-electro-mechanical system, smart card, medical appliances, and so on. To realize the thin film battery, solid electrolyte with electrochemical stability and lithium ion mobility is indispensable and the necessity of solid electrolyte expedited the related researches during past tens of years. In this study, an oxide glass electrolyte was deposited on the pre-deposited LiCoO2 cathode layer to fabricate a single cell of thin film battery. We employed the completely new method to synthesize the oxide glass electrolyte and cathode film, called "Aerosol Flame Deposition (AFD)" method. The similar method was employed to produce the preform of optical fiber or optical waveguide devices based on oxide glasses applied to optical communication networks [1]. In the deposition process, single or multi-component amorphous oxide nano particles (soot) are synthesized by supplying metal halide sources such as chloride in the form of gas into oxy-hydrogen flame. The deposited film composed of synthesized nano particles is subsequently consolidated into solid film by high temperature heat treatment. The whole processes are carried out under normal air atmosphere with only optional control for moisture. The aerosol flame deposition was applied to fabricate glass electrolyte film of Li2O-B2O3-P2O5 and cathode of LiCoO2. The aqueous precursor solution was first atomized with an ultrasonic vibrator (1.7MHz). LiCoO2 were formed on silicon wafer or quartz glass wafer by oxy-hydrogen flame. To fabricate glass electrolyte, the gases of POCl3 and BCl3 produced by Ar bubbling under controlled temperature were delivered to an oxy-hydrogen torch and hydrolyzed to form porous soot film on Si substrates. Li source were incorporated in the soot by feeding the aerosol of solution into the torch. Glass soot was first deposited on substrate placed in rotating stage, which was kept at 150°C to eliminate H2O produced during hydrolysis. C2H3LiO2, POCl3 and BCl3 were hydrolyzed in an oxy-hydrogen flame and converted into oxide or hydroxide by the hydrolysis reaction and subsequent Brownian coagulation. The deposited film composed of synthesized particles is subsequently consolidated into solid film by high temperature heat treatment. XRD diffraction of a precursor and an annealed LiCoO2 film on wafer are shown in Fig.1. The as-deposited soot of LiCoO2 showed relatively low crystallinity, while it was improved with high temperature heat treatment . The thickness of as-deposited LiCoO2 film was about 6 ?m and after consolidation process by solid state sintering it was reduced to approximately 1 ?m. The formed particles of glass soot had spherical shape and the size of approximately 50~100nm. XRD analysis revealed that the amorphous phase and crystalline phases were mixed in glass soot and the crystalline phases were identified to B(OH)3 and B2O3. The deposited Li2O(Li2CO3)-P2O5-B2O3 glass soot on wafer was completely densified for 10h at 490°C. Densified glass film exhibited a broad diffraction peak at 2θ=21˚, which is attributed to the amorphous glass state. The crystalline B(OH)3 and B2O3 found in glass soot completely disappeared by heat treatment. The cross section of thin film electrolyte is shown in Fig. 2 and the thickness of film was about 1 ?m.