Potential Dependence of the Impedance of Solid Electrolyte Interphase in Some Electrolytes

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 1 M LiTFSA. The similar potential dependence of the impedance of the SEI formed in 1 M 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 1 M 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.

Automated summary

The impedance of LiPON thin film and SEI is dependent on the electrode potential, with a decrease in resistance observed with lower electrode potential in certain electrolytes. This is attributed to the doping of Li+ from the electrolyte into thin Li+ conductors to increase Li+ carrier density.