Influence of electrolyte additives on the cathode electrolyte interphase (CEI) formation on LiNi1/3Mn1/3Co1/3O2 in half cells with Li metal counter electrode

• Influence of film forming additives on the CEI composition, thickness and growth.
• Correlation between PEO formation in the CEI layer and electrolyte aging.
• Protective effects of VC and FEC during aging are figured in Li metal half cells.

Traditional solid electrolyte interphase (SEI) forming additives of vinylene carbonate (VC), fluoroethylene carbonate (FEC) and ethylene sulfite (ES) are studied with respect to their impact on the formation and growth of the cathode electrolyte interphase (CEI) layer. T-half cells are assembled and undergo three different electrochemical investigation plans: after formation (0.1C, 5 cycles) and long term cycling (0.1C, 5 constant current cycles + 1C, 100/150 constant current/voltage cycles), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and gas chromatography-mass spectrometry (GC-MS) are combined to investigate morphology, CEI composition, CEI thickness and aging products for cells with different electrolyte systems. The obtained results reveal a significant influence of these additives on the CEI composition and CEI growth. With the help of SEM, it is found that large areas of electrolyte decomposition products are formed at the aged electrode surfaces (=after cycling), with the exception when 2 vol% of FEC is added into the reference electrolyte. From XPS measurements, CEI thicknesses are calculated. The reference electrolyte with 2 vol% of FEC shows the thinnest layer after long time aging (0.8 ± 0.2 nm). For the addition of 2 vol% of VC, an incremental growth of the CEI thickness occurs from the 100th to 150th cycle (from 1.0 ± 0.1 nm to 2.9 ± 0.4 nm). By correlating the CEI thickness values with the electrochemical performance, it can be observed that for lithium metal based half cells, the existence of a thinner CEI layer corresponds to a better cycling behavior, with 2 vol% of FEC showing the highest discharge capacity of 114.4 ± 0.2 mAh/g after 150 cycles at 1C. GC-MS shows that both VC and FEC help to prevent fast electrolyte aging.