Structural damage in LiCoO2 films during chemical delithiation

The development of suitable Li leaching methods to recover Li in all-solid Lithium-ion batteries (LIBs) is essential for the sustainable mass production of these novel devices. As part of this effort, we investigate here the capacity of the oxalic acid (C2H2O4) to be used as a highly selective leaching agent in LiCoO2 thin films. We analyze the balance between Li extraction and the formation of structural defects, which provides us with a measure of the loss of selectivity of the reaction, in the process of chemical delithiation of LiCoO2 once exposed to oxalic acid. Structural changes attributed to selective Li extraction and damage formation (no selective etching) are investigated using synchrotron radiation x-ray diffraction, scanning electron microscopy, micro Raman spectroscopy, photoelectron spectroscopy and conductive atomic force microscopy. We identify three different delithiation regimes, related to surface processes, bulk delithiation and damage generation. We find that only a fraction of the grains is affected by the delithiation process, which may create local inhomogeneities; while homogeneous damages (increase in the film roughness and porosity) and heterogeneous (expelled material and pits) are detected.