Fabrication and Characterization of Porous Thin Film Cathodes Prepared By RF Sputtering for Li-Ion Microbatteries

Abstract

Li-ion batteries play an important role in powering the electronic devices and in storing energy due to its high energy and power density, which are expected to be a solution for the future energy storage requirements. Due to the lack of suitable on-board power sources, the advances in the miniaturization of microelectronics is growing which then open up the opportunities to explore the both cathode and anode materials [1]. Olivine-type LiFePO4 becomes a popular cathode owing to its thermally stable, low cost, more abundant, and less toxic [2]. The strong Fe-O covalent bonds in LiFePO4 cathodes has such a special characteristic compared to layered LiCoO2-type materials. This covalent bond greatly improve the stability of O in the lattice, thus increasing the safety of the materials [2]. In the field of thin-film microbatteries, a study of the thin film cathodes is essential for the fundamental research and application since neither binder nor conductive carbon additive are utilized. It means a good contact between the active material and the current collector could be greatly established [3]. By using the thin film cathodes, the intrinsic drawbacks such as a low electronic conductivity and low Li+ diffusion mobility can also be suppressed because the thickness of the cathode material has been reduced. It would also help facilitating the transport and diffusion of electrons and ions through the thin electrodes compared to their bulk counterparts [4]. Similar to olivine-type LiFePO4, NASICON type-Li3Fe2(PO4)3 showed their own advantages such as low toxicity, low cost, a good ionic conductor and naturally abundant [5]. In addition, the synthesis of Li3Fe2(PO4)3 is easier because the Li3Fe2(PO4)3 cathode can be prepared directly in air without avoiding the oxidation of Fe2+. In the present work, radio frequency (RF) sputtering is utilized to fabricate the thin film cathodes. The results show that the annealing temperatures at 500 °C after deposition is found to be an optimal condition to obtain a porous olivine type-LiFePO4 film (Figure 1a). By further increasing the annealing temperature up to 700 °C , a complete transformation from porous olivine type-LiFePO4 to porous NASICON-type Li3Fe2(PO4)3 can be observed (Figure 1b); this film provides better cycling stability. Porous NASICON-type Li3Fe2(PO4)3 film attains a good capacity of ~103 mAh g-1 (~32 µAh cm-2 µm-1) at C/10 rate with an excellent cycling performance over 110 cycles. In this work, the effect of various annealing temperatures on morphology and electrochemical performances of the thin film cathodes will be discussed.