Deposition and Dissolution of Lithium through Lithium Phosphorus Oxynitride Thin Film in Some Ionic Liquids

Aprotic ionic liquids have been studied as alternative electrolytes for rechargeable lithium batteries because of non-volatility, which is expected to improve safety of the batteries. However, most of the organic cations giving room-temperature ionic liquids are instable against metallic lithium. In case of carbonate-based organic electrolytes, a passivation film, so called solid-electrolyte interphase (SEI) film, is known to form as a result of reductive decomposition of organic solvents. The SEI film is considered as a lithium ion conductor, though its nature has not been clarified yet. On the other hand, the reductive decomposition of the organic cations of ionic liquids hardly leads to formation of good SEI film. In the present study, electrochemical deposition and dissolution of lithium were investigated in some aprotic ionic liquids through a typical inorganic lithium ion conductor, lithium phosphorus oxynitraide (LiPON), thin film. Reversible deposition and dissolution of lithium were possible on a nickel electrode covered with LiPON thin film in bis(trifluoromethylsulfonyl)amide (TFSA^–) based ionic liquids consisting of 1-butyl-1-methylpyrrolidinium (BMP⁺) and 1-ethyl-3-methyl imidazolium (EMI⁺). No cathodic current attributable to reductive decomposition of these cations was observed on the LiPON-coated electrodes, indicating that LiPON thin film acted as an artificial SEI film and that lithium ions were transported between the ionic liquids and LiPON. The deposition of lithium between the nickel electrode and LiPON thin film was confirmed by electrochemical quartz crystal microbalance (EQCM) measurements and X-ray photoelectron spectroscopy.