The dependence of the impedances of lithium phosphorous oxynitride (LiPON) thin film and solid electrolyte interphase (SEI) formed by decomposition of some electrolytes on the electrode potential was investigated by electrochemical impedance spectroscopy. A LiPON thin film was prepared on a Ni electrode by radio frequency magnetron sputtering of Li3PO4 under nitrogen atmosphere. The resistance of the LiPON thin film decreased with lowering the electrode potential in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing 1M LiTFSA. The similar potential dependence of the impedance of the SEI formed in 1M LiTFSA/BMPTFSA was observed, suggesting that the Li+ carrier density in the LiPON thin film and SEI increased with lowering the electrode potential probably due to the doping of Li+ from the electrolyte into the thin Li+ conductors in order to compensate the negative charge on the electrode. On the other hand, the potential dependence of the SEI formed in LiTFSA-tetraglyme (G4) solvate ionic liquid was insignificant because of the high concentration of Li+ in the SEI and electrolyte. The resistance of the SEI formed in 1M LiClO4/EC (ethylene carbonate) + DMC (dimethyl carbonate) (1 : 1 vol%) did not depend on the electrode potential, suggesting the thin and highly Li+ conductive SEI is formed in the organic electrolyte.
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