Lithium borophosphate thin film electrolyte as an alternative to LiPON for solder-reflow processed lithium-ion microbatteries
Résumé
Microbatteries are energy sources well adapted to power various microsystems. To be considered as a microelectronic component, the microbatteries must be compatible with the solder-reflow treatment. During this industrial process, the temperature reaches 260 °C during few seconds. Having noticed an increase of the activation energy of the most used solid electrolyte lithium phosphate thin film (LiPON) after this type of thermal treatment, from 0.57 eV to 0.80 eV, we have investigated the correlations between composition, local structure (by XPS) and the electrical properties of lithium borophosphate solid electrolyte thin films. The lithium borophosphate thin films were prepared by radio-frequency sputtering using five different targets having the following compositions x LiBO2: (1 − x) Li3PO4 (x = 5, 10, 15, 20 and 25) in a pure argon discharge gas. In the absence of a thermal treatment, the addition of a small amount of boron (B/P ratio equal to 0.1) leads to an increase of the ionic conductivity (1.1 × 10− 6 S*cm− 1) and a decrease of the activation energy (0.52 eV) compared to a pure lithium phosphate. However, for B/P ratio higher than 0.12, the ionic conductivity decreases. The study of the local structure by X-ray photoelectron spectroscopy (XPS) shows that the borate groups have less reacted with the phosphate groups leading to a phase separation reducing the connectivity between phosphate and borophosphate networks and so the ionic conductivity. The solder-reflow type thermal treatment leads to an increase of the ionic conductivity. The best ionic conductivity, 1.9 × 10− 6 S*cm− 1, is obtained for a B/P ratio equal to 0.24 with an activation energy of 0.57 eV instead of 0.80 eV for the LiPON thin film. For a use of Li-ion microbattery at low temperature after the solder-reflow type thermal treatment, the lithium borophosphate solid electrolyte would be a potential candidate to replace LiPON with several industrial advantages.