Visualization of the Delithiation Mechanisms in High-Voltage Battery Material LiCoPO4

LiCoPO4 is a high-voltage Li-ion battery material seen as a potential candidate for electric vehicles due to its high energy density. However, LiCoPO4 cathodes suffer from severe degradation on cycling. To date, most LiCoPO4 studies have involved bulk characterization techniques that do not allow the phases formed to be spatially resolved; thus, information on which phases contribute to the severity of degradation, and reasons why, is lost. Here, the delithiation mechanisms of LiCoPO4 are visualized by mapping changes in the valence state of Co across the electrode using ex situ electron energy loss spectroscopy (EELS). To understand the effect of Co–O hybridization on LiCoPO4 cyclability, changes in the O K-edge across the electrode during the first cycle and later cycles were also mapped. Co valence state EELS mapping showed that lithium-poor phases initially form on the outer edge of particles, corroborating a shrinking-core delithiation mechanism, which was previously proposed from in situ X-ray diffraction (XRD). At higher potentials, the presence of Li-poor CoPO4 correlates with Co–O bond hybridization; thus, the instability of CoPO4 leads to attack from the electrolyte and degradation at the electrode/electrolyte interface. The instability of the delithiated phase results in Li reincorporation at the surface at high potentials, shown by Co valence state EELS by Co(II)-rich regions forming on the surface of particles at high potentials. By the 10th cycle, CoPO4 no longer forms and capacity loss is caused by Li retention in the LiCoPO4 lattice. The Co valence state EELS study reveals that strategies to improve the cyclability of LiCoPO4 should focus on improving the stability of CoPO4 or on methods to shield CoPO4 from electrolyte degradation.