Electrical insulation properties of RF-sputtered LiPON layers towards electrochemical stability of lithium batteries

Electrochemical stability, moderate ionic conductivity and low electronic conductivity make the lithium phosphorous oxynitride (LiPON) electrolyte suitable for micro and nanoscale lithium batteries. The electrical and electrochemical properties of thin-film electrolytes can seriously compromise full battery performance. Here, radio-frequency (RF)-sputtered LiPON thin films were fabricated in nitrogen plasma under different working pressure conditions. With a slight decrease in the deposition pressure from 6 to 1 × 10−3 mbar, the 600 nm thick LiPON film reveals an electric resistivity increase from 108 to 1010 Ω · cm, respectively. UV– micro-Raman spectroscopy confirms the nitrogen incorporation on the Li3PO4 material, while scanning electron microscopy, Rutherford backscattering spectrometry and nuclear reaction analysis show a well-defined compact structure with a composition of Li2.2PO2.2N0.6 for the higher electrical-resistivity film. An ionic conductivity close to 3 × 10−7 S cm−1 at room temperature (22 °C) was measured by AC impedance spectroscopy. Thermal properties were investigated through the differential scanning calorimetry technique. LiPON films reveal high optical transmission (>75%) in the UV–vis range, which could be interesting for transparent electronic devices.This work is supported by FCT with project reference UID/ EEA/04436/2013, by FEDER funds through the COMPETE 2020–Programa Operacional Competitividade e Internacionalização (POCI) with project reference POCI-01-0145- FEDER-006941, by the UID/FIS/50010/2013 project and by the strategic project PEST–C/QUI/UI0686/2013. EMFV is grateful for financial support through FCT grant SFRH/ BPD/95905/2013. NPB gratefully acknowledges FCT support through the UID/Multi/04349/2013 project. MRC acknowledges the funding provided by FEDER through the COMPETE 2020 Programme and National Funds through FCT–Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013 and RECI-II/ FIS-NAN/0183/2012 (FCOMP-01-0124-FEDER-027494).