In-Situ Observation of Solid Electrolyte Interphase on LiCoO2 during Cycling to 4.5 V By AFM

LiCoO₂ is the most common cathode material for commercial lithium-ion batteries due to its high tap density, excellent cycling performance and high rate capability. Though LiCoO₂ has a theoretical capacity of 274 mAh/g, its reversible capacity is limited to around 140 mAh/g when cycled between 3.0 V and 4.2 V vs Li metal. Increasing the upper cutoff voltage can extract more lithium ions beyond Li_0.5CoO₂, which can increase the reversible capacity of LiCoO­₂. However, the electrolyte will be oxidized at such high voltage, which will consume the electrolyte, and form solid electrolyte interphase (SEI) on the surface. The thickness, stability will influence the electrochemical performance of LiCoO₂cathode at high voltage.

In this work, we used atomic force microscope (AFM) to study the properties and evolution of SEI films on LiCoO₂ cathode during electrochemical cycling. Different structure and evolution processes of SEI on different crystal faces //(001) and „¨(001) were observed in-situ during the electrochemical reaction. We also study the protecting effect of a thin layer of Al₂O₃ film on the surface of LiCoO₂micro-crystal prepared by atomic layer deposition (ALD) technique.

Figure. (a) LiCoO₂ micro-crystal prepared by melting-salt method. (b) LiCoO₂ micro-crystal embedded in Al substrate. Topography images of LiCoO₂ micro-crystal //(001) face obtained in-situ by AFM at original (c) and charged to 4.5 V (d). Topography images of LiCoO₂ micro-crystal „¨(001) face obtained in-situ by AFM at original (e) and charged to 4.5 V (f).