Time-of-flight secondary ion mass spectrometry study of lithium intercalation process in LiCoO2 thin film

Abstract

A detailed Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) analysis of the lithium de-/intercalation in thin films of the insertion cathode material lithium cobalt oxide is presented. The LiCoO 2 (LCO) thin films are deposited by radiofrequency magnetron sputtering at 600°C, having a (003) preferred orientation after deposition. The thin electrode films are cycled with liquid electrolyte against lithium metal, showing 80-86% extractable capacity. After disassembling the cells, the depth resolved elemental distribution in the LCO is investigated by ToF-SIMS and Glow Discharge Optical Emission Spectroscopy (GDOES). Both techniques show a stepwise lithium distribution in charged state, leading to a lithium depleted layer close to the surface. In combination with the electrochemical results, the qualitative comparison of the different lithium depth profiles yields a reversible lithium extraction in the depleted area below the stability limit for bulk materials of LCO. For bulk LCO a phase change normally occurs when the lithium concentration in Li x CoO 2 is lower than x=0.5. As a possible cause for the inhibition of the phase change, the preferred orientation and thus pinning of the crystal structure of the film by the substrate is proposed.

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