context: Although STEM-HAADF imaging has become the most widely used imaging mode in STEM, the strong atomic number dependency of STEM-HAADF contrast also means that the imaging contrast of light atoms is easily submerged by the adjacent heavy atoms. In contrast, STEM-ABF phase-contrast imaging based on wave interference provides better visualization of light elements with the presence of heavy elements, such as oxygen in the SrTiO3 crystal, lithium in LiCoO2 and even hydrogen in metal hydrides. An elegant example was reported by Gu et al., in which aberration-corrected STEM-ABF imaging was applied to directly visualize lithium columns in LiFePO4 and identify an intriguing lithium staging structure in the partially delithiated LixFePO4 (x ≈ 0.5) crystal. Fig. 6(a–c) present the atomic-resolution STEM-ABF images of the LiFePO4 cathode at different charging states. Compared with the pristine and fully-charged structures of LiFePO4 in Fig. 6(a and b), half-delithiated LiFePO4 in Fig. 6(c) demonstrated that part of the lithium remains in the lattice (yellow circles) at every other row of Li-extracted sites (orange circles). This ordered structure contradicted the previous model of LiFePO4/FePO4 two-phase separation, but was analogous to the stage-II phase in some layer compounds, and contributed to minimizing the Li–Li repulsive interaction to stabilize this intermediate phase. This result indicated that a continuous phase transition occurred in partially delithiated LixFePO4 through a metastable phase and helped to understand the high-rate capability of LiFePO4 cathode.
answer:  LiFePO4