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

•Magnetron sputtering of crystalline LiCoO2 on CrN thin film current collector.•Systematic study of the Li de-/intercalation in LiCoO2 thin films by TOF-SIMS.•Stepwise Li profiles in charged state indicate a lithium extraction below Li0.5CoO2.•Thin film pinning is identified as possible cause for delithiation under Li0.5CoO2.

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 LiCoO2 (LCO) thin films are deposited by radio frequency magnetron sputtering at 600 °C, having a (003) preferred orientation after the deposition. The thin electrode films are cycled with liquid electrolyte against lithium metal, showing 80–86% extractable capacities. After disassembling the cells, the depth resolved elemental distribution in the LCO is investigated by ToF-SIMS and glow discharge optical emission spectroscopy. 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 LixCoO2 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.