Analysis of the self-discharge process in LiCoPO₄ electrodes: bulks

Highlights: • Electrochemically de-lithiated LiCoPO4 electrodes spontaneously self-discharge in open circuit conditions. • Ex-situ synchrotron XRD and FTIR of self-discharged de-lithiated LiCoPO₄ electrodes highlight the spontaneous reincorporation of lithium in the CoPO₄ lattice to give back LiCoPO₄. • Upon the spontaneous self-discharge in lithium cells, charged LiCoPO₄ electrodes oxidize the organic carbonates in the electrolyte leading to the release of gaseous CO₂. - Abstract: LiCoPO₄ (LCP) is a promising high voltage cathode for next-generation high energy Li-ion batteries. However, once electrochemically de-lithiated, it suffers from a spontaneous self-discharge process in open circuit conditions. Here we present our systematic study about the phase composition and structural changes of LCP electrodes upon charging and self-discharging processes by ex situ synchrotron X-ray diffraction and Fast-Fourier Transform Infrared Spectroscopy (FT-IR) analyses. Various initial charging states and different current rates have been studied. Our investigation highlights large structural alterations of the de-lithiated CoPO₄ phase upon self-discharge. However, after 160 hours with open circuit the pristine LCP structure is recovered. The electrochemically de-lithiated phase reincorporates lithium by a spontaneous reduction reaction releasing gaseous CO₂ and likely degrading the electrolyte molecules.