Study on the Thermionic-Enhanced Sputter Deposited Lithium Iron Phosphate as a Positive Electrode for Thin Film Battery

• Main Authors: Hung-en Tu, 凃弘恩
• Other Authors: Ju-Liang He
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/29958349249860774561

碩士 === 逢甲大學 === 材料科學所 === 95 === Thin film batteries with slim feature, all solid state construction, and flexibility on flexible substrates, high power density and conformability to any shape/size, are promptly researched and developed into a circumstance of ready-to-use recently. Meanwhile, the applications being aggressively proposed include battery-backed computer memories, smart labels/tags, bio-chips, advanced drug delivery, wireless sensor platforms and other MEMs. Positive electrode materials used in the thin film battery have attracted much attention, among which lithium iron phosphate (LiFePO4) is a promising candidate. However, the annealing temperature must be 500-600oC to obtain triphylite structure and lower electrical conductivity is about 10-9 S/cm. In this study, thermionic emission technique was applied in sputter deposition of LiFePO4 positive electrode thin film for achieving better battery performance without post annealing. The carbon-mixed LiFePO4 film was deposited using dual-target (LiFePO4/Graphite) sputter deposition system with plasma enhancement. It may be obtained that lower electrical resistivity and high discharge capacity of the LiFePO4 thin film. The influences of the intensified plasma with various process parameters, include thermionic emission, deposition temperature and carbon-mixed, to the crystal structure, microstructure and electrochemical properties. Experimental results show that the deposition temperature, at which the LiFePO4 triphylite was obtained, was obviously decreased from 600oC to 300oC on condition that the plasma density was increased from 1.36×1010 to 5.69×1010 cm-3, when utilizing thermionic enhancement. It was attributed to higher plasma density enhances energetic ion bombardment. Therefore it provides additional energetic species to arrive at the growing film and to achieve forming crystalline structure at a low deposition temperature. Using the second graphite target as carbon source, the carbon content of the thin film is about 7.36 wt% that decreases the resistivity of the deposited film with four order of magnitude than the film without carbon incorporation. The better crystallinity beneficial from the increased plasma density and the decreased resistivity brought about by the carbon incorporation results in the highest discharge capacity about 41.22 μAh/cm2μm for the film deposited at 500oC. This study presents that through the use of a thermionically enhanced plasma to aid in the deposition of LiFePO4 thin film, it obtains the deposition temperature to be reduced and the electrochemical properties of carbon-mixed thin film be improved.