Deposition and Dissolution of Lithium through a Lithium Carbonate Thin Film in an Ionic Liquid Electrolyte

Abstract

Li-ion batteries are used widely for portable energy storages because of large capacity. Generally, the carbonate-based organic solvents are used as the electrolytes. Most of these organic solvents decompose during the first charge, resulting in formation of the solid electrolyte interphase (SEI) film, which plays an important role for Li-ion batteries on the active materials. Although there are many reports about the SEI film, its nature and function have not been clarified yet. We have already reported that lithium phosphorous oxynitride (LiPON) film acts as an artificial SEI film in 1-butyl-1-methylpyrrolidinium (BMP+) and 1-ethyl-3-methylimidazolium (EMI+) bis(trifluoromethylsulfonyl)amide (TFSA–) ionic liquids containing LiTFSA, and LiTFSA-glyme solvate ionic liquid. [1,2] Ionic liquids are expected to be the alternative electrolytes for Li-ion batteries, since ionic liquids have low flammability and low volatility, which are expected to improve safety of the batteries. However, the SEI film formed by the decomposition of ionic liquids is not preferable for deposition and dissolution of lithium. In this study, deposition and dissolution of lithium through a Li2CO3 thin film prepared by RF magnetron sputtering were investigated in BMPTFSA containing LiTFSA. The Li2CO3 thin film was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractrometer, and X-ray photoelectron spectrometer (XPS). All the electrochemical measurements were performed using an airtight three-electrode cell. The Ni electrode with or without the Li2CO3 thin film was used as a working electrode. Li foil was used as a reference and counter electrode. After electrochemical measurements, the electrode was washed with dimethyl carbonate (DMC) and characterized by SEM and XPS. SEM observation and XPS measurement showed that the Ni substrate was covered with the Li2CO3 thin film completely. The Li2CO3 thin film on the substrate was X-ray amorphous. However, the crystalline domains were observed sparsely by TEM observation. The deposition and dissolution of lithium were possible through the Li2CO3 thin film in BMPTFSA containing 1 M LiTFSA. Although the cathodic current attributed to the decomposition of the electrolytes was observed on the Ni electrode, it was suppressed on the Ni electrode with the Li2CO3 thin film. These results indicate that the Li2CO3 thin film has the possibility to act as an artificial SEI film. The depth analysis by XPS revealed that lithium deposited between the Li2CO3 thin film and Ni electrode. From AC impedance measurement, the ionic conductivity of the Li2CO3 thin film was estimated to be 1.3 × 10−8 S cm−1.