Fabrication And Electrochemical Performance For All-Solid-State Thin Film Lithium Battery

Innovation over the past decades in low and ultra-low power devices such as battery-backed complementary metal oxide semiconductor (CMOS), microelectromechanical systems (MEMS) and micro-sensors have been booming intense studies on thin film micro-batteries. The unique features of all-solid-state thin film lithium battery (TFLB) , high power densities, low self-discharge rate and long cycle life and easy to be fabricated in specific shape and size, make them be one of the most promising micro-power sources.An all-solid-state thin film lithium battery is composed of a cathode film, a solid electrolyte film and an anode film. The properties of any of these three components and their interfacial quality will substantially affect the electrochemical performance of the battery. Lithium phosphorus oxynitride (LiPON) thin film has been regarded as the most promising inorganic solid-state electrolyte due to its high ionic conductivity , low electric conductivity and excellent electrochemical stability. RF magnetron sputtering has been widely used to fabricate LiPON film, but its major drawbacks such as low deposition rate and small deposition area limit its application. Searching for an efficient method to fabricate LiPON, novel electrode materials and a way to improve the interfacial properties of films becomes the goal of this thesis.The results in this thesis are summarized as follows:1. LiPON electrolyte thin films have been successfully fabricated by nitrogen plasma assisted deposition of e-beam reactively evaporated Li₃PO₄ for the first time. This method can fabricate large area films (ca. 1200cm²) at a high deposition rate (ca. 0.4μm /h). A dense, smooth and uniform LiPON thin film without pinholes and cracks was obtained with the nitrogen plasma power of 350 W and the e-beam gun power of 200W. The AC impedance spectra of Al/Lipon/Al structure exhibited a typical behavior of lithium ion conductor. At 300K, the LiPON film presented an ionic conductivity of 6.0×10^-7 S/cm, an electric conductivity of less than 1.0×10^-10 S/cm, and an electrochemical stable window of 5.0 V, respectively. The dependence of ionic conductivity of LiPON thin film on the temperature obeyed an Arrhenius equation and the activation energy was estimated to be 0.57 eV. LiPON thin films were examined by XRD, SEM, EDX, FTIR and XPS. The deposited LiPON thin films had an amorphous structure which might benefit lithium ion mobility. The insertion of nitrogen into Li₃PO₄ may cause the substitution of N for O in the P-O-P structure and the formation of the triply coordinated P-N<_P^P and doubly coordinated P-N=P, resulting in the increase of ionic conductivity and the decrease of the activation energy.2. By using the LiPON thin film deposited by nitrogen plasma assisted deposition of e-beam reactively evaporated Li₃PO₄, a new all-solid-state thin film lithium battery of Li/LiPON/Ag_0.5V₂O₅ has been successfully prepared. The cathode thin film of Ago 5V₂O₅ was fabricated using pulsed laser deposition (PLD), and the metallic lithium anode thin film was prepared via vacuum thermal evaporation. For Li/LiPON/Ag_0.5V₂O₅ battery, the open circuit voltage was 3.0V and the first discharge capacity was 62μAh/cm²-μm at a current density of 14μA/cm². The capacity fading was about 0.2% per cycle after 10 cycles, and this battery can be cycled above 550 cycles. In addition, two kinds of all-solid-state thin film lithium battery, Li/LiPON/LiCoO₂ and Li/LiPON/LiNi_0.8Co_0.2O₂ , have been fabricated by using RF magnetron sputtering. Electrochemical behavior of these batteries with amorphous, nano-crystalline and crystalline LiCoO₂ and LiNi_0.8Co_0.2O₂ cathodes, were investigated, respectively. The battery with a 700℃ annealed LiCoO₂ cathode film can be cycled above 450 cycles and possessed the discharge capacity of 110mAh/g.3 . A "lithium-free" thin-film rechargeable battery with a very simple configuration of SS/LiPON/Ag, including a stainless steel (SS) foil used as a substrate and anode current collector, a LiPON electrolyte film, and a metallic silver film, has been successfully fabricated for the first time using magnetron sputtering techniques. In this battery , there is "no" anode and cathode at all for the as-prepared battery, and only the lithium ion electrolyte LiPON supplies lithium for creating both the anode and cathode. The battery was activated by electrochemical plating a metallic lithium anode and electrochemical creating a reactive layer cathode, respectively, after the initial charge potential up to 3.7V. Electrochemical cycling between 0.5 and 4.2V demonstrated a reversible discharge capacity around 12μAh/cm² at 20μA/cm² over 450 cycles. The thin-film battery of SS/LiPON/Ag is "lithium-free" and no lithium intercalation compound as a cathode material, which simplifies the fabrication and package processes of thin-film battery and reduces the cost. In order to understand the electrochemical reaction processes during cycling the cell, electrochemical measurements such as CV curve, EIS and discharge/charge curve, and physical characterizations such as SEM, XPS and In-situ Raman techniques, have been performed. Results showed that plated metallic lithium and Ag(I) compound in the glassy LiPON were formed at the interfaces of SS/LiPON and LiPON/Ag, respectively, after the initial charge potential up to 3.7 V. Based on these results, a possible electrochemical reaction mechanism was proposed.4. To improve the interfacial quality and electrochemical performance of all-solid-state thin film lithium batteries (TFLBs), a sequential thin film deposition equipment has been successfully developed for the first time. This equipment consists of a dry glove box for packing TFLB and four vacuum chambers, one chamber of them for deposition lithium film anode by flash thermal evaporation, and the other three vacuum chambers for deposition of current collectors, cathode and solid electrolyte by magnetron sputtering, respectively. This equipment has been used to sequentially deposited Au current collector, TiO₂ cathode, LiPON (lithium phosphorus oxynitride) solid electrolyte and metallic Li anode thin films, and to prepare a TFLB with a structural configuration of Li/LiPON/TiO₂/Au via an in-situ process without breaking vacuum. Results showed that the TFLB exhibited lower interface resistance and better electrochemical performance than that fabricated via an ex-situ process.In addition, a combined power system, consisting of an all-solid-state thin film lithium battery and an array of solar energy micro-cell, has also been successfully developed in our laboratory. An electronic watch could be driven by using this combined power system.