In Situ AFM Observation of Surface Morphology of Highly Oriented Pyrolytic Graphite in Propylene Carbonate-Based Electrolyte Solutions Containing Lithium and Bivalent Cations

Abstract

Propylene carbonate (PC) is one of the promising solvents of lithium-ion batteries for use in cold area. Lithium salt concentrated PC and both lithium salt and bivalent salt dissolved PC enabled lithium ion to intercalate at graphite negative electrodes. The key to use PC is a solid electrolyte interphase (SEI). In this study, the SEI formation process on a highly oriented pyrolytic graphite was investigated by in-situ atomic force microscopy (AFM) in 4 mol dm−3 lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 M(TFSA)2 (M = Ca or Mg)/PC. Based on the in-situ AFM observation, the SEI formation processes in 4 mol dm−3 LiTFSA/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Ca(TFSA)2/PC were the co-intercalation type and that in 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Mg(TFSA)2/PC was the surface decomposition type. Relation between the SEI formation process and the charge–discharge properties was also discussed.