Improvement of Power Characteristics of All-Solid-State Thin-Film Rechargeable Lithium Batteries
Recently, all-solid-state thin-film rechargeable lithium batteries (TFBs) has expected in various applications such as wireless sensor, smart device and other small devices for an auxiliary power supply. The TFBs are fabricated by forming each layers (i.e. cathode, electrolyte, anode and current collector) using physical vapor deposition process1,2). As a solid electrolyte, lithium phosphorus oxynitride (LiPON) thin film has been used commonly due to no grain-boundary, isotropic property, less electronic conductivity and electrochemical stability. Ionic conductivity of LiPON thin film has been reported ~3.3 x 10-6 S/cm at 25 deg.C3), but it is lower compared with other solid electrolyte4). It is necessary to reduce the internal resistance of TFBs for applying to the device as described above. In order to improve this, it has to be thinner LiPON film or change the electrolyte material having higher ionic conductivity. However, there is a limited to the thickness of electrolyte film without short- circuit because the surface of cathode film is rougher with thicker cathode film5). In this study, we investigated the planarization of cathode film for thinner electrolyte film.
Lithium cobalt oxide (LCO) film was prepared by RF and DC hybrid power magnetron sputtering method on platinum film as current collector. Ar was used as sputtering gas, and process pressure was kept at 1.6 Pa. Furthermore, LCO was deposited with applying each bias power to the substrate for investigating the planarization. After deposition, LCO was annealed at 600 deg.C under atmospheric pressure by lamp heating system. Solid electrolyte (LiPON) film was prepared by RF magnetron reactive sputtering method with only N2gas. Lithium film as an anode was prepared by vacuum evaporation method. Acrylic monomer as a sealant was coating on the anode surface, covered with barrier lid, and cured by ultraviolet light for encapsulation. The fabricated TFB cells were investigated by electrical and electrochemical properties at room temperature.
3. Results and discussion
Figure 1 shows the scanning electron microscopy (SEM) image of 3-μm-thick LCO film under different bias power condition. As the result of this, it was confirmed that surface of LCO becomes smoother with higher bias power and surface morphology changes from pyramid-like to dome-like by re-sputtering. TFB cell was fabricated with 2-μm-thick LiPON and 2-μm-thick lithium and discharge capacity was also checked. The charge method is CCCVmode (current density: 0.21 mA/cm2, cut-off: >4.2 V and <0.021 mA/cm2) and the discharge method is CC mode (current density: 0.21 mA/cm2, cut-off: <3.0 V). Figure 2 shows the discharge curve of TFB cells. It was confirmed that discharge capacity of TFB cell is higher with increasing bias power. It is thought that film density of LCO becomes high due to ion peening effect. The improvement of thinner LiPON film and the other are on evaluating.
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