In situ Raman spectroscopy of LixCoO2 cathode in Li/Li3PO4/LiCoO2 all-solid-state thin-film lithium battery

A thin-film battery was fabricated in the structure Li/Li3PO4/LiCoO2 using pulsed laser deposition technique. The structural change in LixCoO2 during battery operation was investigated using in situ Raman spectroscopy. Raman measurements were performed from the front side of the cell beside the lithium anode through the solid electrolyte and from the backside of the cell below the lithium anode through a transparent electrode. The Raman peaks of A(1g) and E-g modes were observed to shift to lower and higher wavenumbers at around 4 V due to Li extraction and insertion, respectively, according to the structural change between H1 and H2 phases of the LixCoO2. The Raman shift for the LixCoO2 showed hysteresis during the charging-discharging process when observed from the front side of the cell, although no hysteresis is seen in those measured from the backside. The observed hysteresis is explained as a delay in structural change due to the chemical diffusion of lithium in the LixCoO2 cathode.