J. Phys. Chem. C 2009, 113, 11373–11380
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Structure, Morphology, and Electrochemical Investigation of LiMn2O4 Thin Film Cathodes Deposited by Radio Frequency Sputtering for Lithium Microbatteries Bing-Joe Hwang,*,†,‡ Chien-Yu Wang,† Ming-Yao Cheng,† and Raman Santhanam§ Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, #43 Keelung Road, Section 4, Taipei, 106 Taiwan, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, and Nanoexa Corporation, 863 Mitten Road, Burlingame, California 94010 ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: April 27, 2009
Spinel lithium manganese oxide (LiMn2O4) cathode thin films were successfully deposited on different substrates such as Si, Pt/Al, and indium tin oxide (ITO)/Pt/Al by radio frequency (RF) sputtering. These thin films were investigated as positive electrodes for lithium microbatteries. The thickness of the deposited films was estimated by surface-profilometer at different deposition time intervals from 30 min to 15 h, and the thickness was found to be 20-600 nm. The structure and surface morphology deposited thin films of LiMn2O4 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The phase transformation from amorphous to crystalline phase was found at around 700 °C for the electrode films deposited on Si substrate. The LiMn2O4 deposited with ITO interlayer on Pt/Al substrate was found to be well crystallized relatively at lower temperature (e300 °C). No significant changes were observed on the surface morphology between the as-deposited and the annealed film. Electrochemical performance was measured for LiMn2O4 films deposited with or without ITO interlayer on Pt/Al substrate. The discharge capacity around 60.9 and 67.5 mAh g-1 are obtained, respectively, for the 500 °C-annealed LiMn2O4/Pt/Al film and as-deposited LiMn2O4/ITO/Pt/Al film. Because the crystallization temperature of LiMn2O4/ITO/Pt/Al film was found to be relatively lower (e300 °C), this work could possibly be extended to design and fabricate flexible lithium microbatteries using polymer substrates. Introduction Lithium-ion batteries are widely used in portable electronic devices such as cellular phones, notebook computers, and video cameras and recently they have received much attention as a power source for hybrid electric vehicles and electric vehicles as a result of their high energy density, low cost, high rate capability, and safety. Greater advances in the miniaturization of electronic devices are the major driving force behind the development of all solid state thin film lithium rechargeable batteries.1,2 Thin film lithium batteries have particular applications as suitable power sources for emerging microelectronics such as smart cards, remote sensors, implantable medical devices, antitheft protection, miniature transmitters, personal data assistant systems, microelectromechanical system (MEMS) devices, and so on.3-7 In recent past, an electronic market has seen an exponential growth and the physical size of the electronic device shrinks to micro level which requires high reliable and long shelf life power source. Therefore, the pressure was mounted on the developers of secondary batteries to produce reliable and long shelf life microbattery by reducing its cell geometry to make the device smart.4,8 The development of microbattery with high energy density is a challenging task for researchers and battery manufactures, because the energy density of microbattery depends upon the surface area of the deposited electrode film. However, it is possible to design and fabricate a microarray electrode system which can make in series or * To whom correspondence should be addressed. Tel.: 886-2-27376624. Fax: 886-2-27376644. E-mail:
[email protected]. † National Taiwan University of Science and Technology. ‡ National Synchrotron Radiation Research Center. § Nanoexa Corporation.
parallel depend on the requirement of voltage and current, respectively, to meet the need of these applications.9-12 On the other hand, the advancement of miniaturized microsensor actuators and microelectromechanical systems (MEMS), the integrated circuits (ICs), can be designed to incorporate the power source as an integral part of the ICs rather than a separate component to provide an uninterrupted electrical power to the individual components and device.13 Thin film electrodes have several advantages over composite electrodes for understanding interfacial reactions and they are free from impurities like binders and conductive carbons. The composite electrodes with polymeric binders and conductive carbons may not suitable to study the characteristic electrochemical properties of these metal oxides. Therefore, thin film cathodes have received considerable interest to study the intrinsic electrochemical properties of lithium transition metal oxides.14-16 The typical thin film battery systems were developed by Oak Ridge National Laboratory with lithium metal anode, glassy lithium phosphorus oxynitride (“LiPON”) electrolyte and a transition metal oxide such as LiCoO2. The electrochemical performance is sensitive to the method of preparation, structure, and composition of the electrode materials.17-20 It is well-known that LiMn2O4 is one of the promising positive materials for lithium ion batteries due to its high energy density, low cost and low toxicity. It has an isotropic structure, which provides a 3D network for lithium ion diffusion, and hence, this material is suitable for fast lithium insertion and deinsertion reactions.21-27 It has also been shown that LiMn2O4 and its derivatives exhibited excellent cycleability, safety, and rate capability when combined with lithium titanium oxide, Li[Li1/3Ti5/3]O4.28,29 In this study, we fabricated the LiMn2O4 thin films by RF sputtering technique, which can be used as a cathode electrode
10.1021/jp810881d CCC: $40.75 2009 American Chemical Society Published on Web 06/09/2009
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J. Phys. Chem. C, Vol. 113, No. 26, 2009
material for flexible lithium microbatteries. The LiMn2O4 films deposited by various methods such as laser ablation, electron beam, sol-gel, spin coating, and other synthesis process were found to be delivered reasonable capacity but their structural properties like crystallinity and grain size slightly changed with preparative and annealing conditions.30-40 Hwang et al. prepared LiMn2O4 films by RF sputtering and electrode films annealed at 750 °C showed the best cycle performance because of its good crystallinity.27 In this work, to produce good crystalline electrode films relatively at lower temperatures (
