Recently, all-solid-state thin-film rechargeable lithium batteries (TFBs) have expected to be used 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 collectors) mainly using physical vapor deposition process^1,2). As a solid electrolyte, lithium phosphorus oxynitride (LiPON) thin film has been used commonly due to same advantages as 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. We tried to improve internal resistance used by Li3PO4-Li4SiO4 (LPO-LSO) electrolyte.

Experimental

LPO-LSO film was prepared by RF sputtering with 4 inch LPO-LSO target (Toshima manufacturing Co.). The LPO-LSO films were prepared by 0.1 Pa, 0.25 Pa and 0.6 Pa. LiPON film was prepared by RF magnetron reactive sputtering method with lithium phosphate (LPO; Li3PO4) target and N2 gas by 0.25 Pa. The substrate was glass wafer. The cathode film (LiCoO2) was prepared by RF and DC spattering method. The anode film (Metal Li) was prepared by vacuum evaporation method. Acrylic monomer was coated on the anode surface, covered with barrier lid, and cured by ultraviolet light for formation as an encapsulation. The LPO-LSO films quality were measured by FT-IR and SEM. The fabricated TFB cells were investigated by electrical and electrochemical properties at room temperature.

Result and discussion

Fig.1 shows the nyquist plot of Half-cell (Li/LPO-LSO/Pt, Li/LiPON/Pt) prepared by each pressure. The bulk resistance of LPO-LSO film prepared by 0.1 Pa and 0.25 Pa are lower than the bulk resistance of LiPON film. And the pressure condition of lowest bulk resistance was 0.25 Pa.

Fig.2 shows the nyquist plot of TFB that electrolyte were LPO-LSO and LPO-LSO/LiPON . Two semi-circle were observed TFB that electrolyte is only LPO-LSO. We thought 1^st semi-circle was the bulk resistance of LPO-LSO and 2^nd semi-circle was the interfacial resistance between LPO-LSO and LiCoO2 because 2^nd semi-circle was not observed in half-cell of LPO-LSO. We could improve the interfacial resistance by insertion of LiPON film (100nm) between LPO-LSO and LiCoO2. These results show LPO-LSO/LiPON electrolyte improved internal resistance of TFB.

References

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