Properties of surface film on lithium anode with LiNO3 as lithium salt in electrolyte solution for lithium–sulfur batteries

The properties and the deposition process of surface film on lithium metal with LiNO3 as lithium salt in electrolyte solution are investigated using X-ray photoelectron spectroscopy (XPS), scanning probe microscopy (SPM), scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS). XPS spectra and depth profile of the surface film show that LiNO3 represents a significant role in the formation of the film, which is attributed to the reaction between lithium metal and the electrolyte solution. With the strong oxidation of LiNO3, the surface film consists of both inorganic species such as LiNxOy and organic species such as ROLi and ROCO2Li. The height and phase images obtained by a SPM system show that the surface film formed with LiNO3 is a homogenous surface film. Combined with EIS spectra and SPM images, the appropriate rate of the reaction between lithium metal and electrolyte solution with LiNO3 is a significant factor to from a homogeneous surface film. The SEM images show that the surface film on lithium metal immersed in electrolyte solution with LiNO3 for several hours becomes smooth and compact. The compact and homogenous surface film formed with LiNO3 brings about the enhanced stability of lithium anode and the improved cycle life of lithium–sulfur batteries.