Atomic Resolution Imaging of Structure Evolution in Li-Rich and Disordered Rock-Salt Cathode Materials

Atomic resolution transmission electron microscope (TEM) can allow direct imaging of defects in battery materials, but for imaging of cathode materials there are significant challenges. These challenges include the beam sensitivity of the materials, especially in their charged state, and the low image contrast of low atomic number elements such as Li and O. We have shown that the simultaneous application of annular dark-field (ADF) imaging and ptychography in the scanning TEM (STEM) can overcome these challenges. The ADF image allows for identification of the TM sublattice. Ptychography, being a phase-sensitive imaging mode, allows observation of Li and O. It also makes efficient use of the scattered electrons, allowing lower doses and therefore reduced beam damage, and also the correction of residual aberrations improving contrast and precision in images of cathode materials. By dismantling cells at various points in the charge-discharge cycle and using anaerobic transfer to the microscope, the evolution of the degree of crystallinity and the nature of defects has been studies using the simultaneous ADF and ptychography approach.<br/><br/>As an example of this approach to studying cathode nanostructure, in previous work we have previously shown that an O sublattice distortion occurs in Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (LRNMC) on charge that persists on discharge which can be associated with the voltage hysteresis seen in these materials [1].<br/><br/>We have gone on to show that honeycomb ordering within TM layers consists of domains of different rotations. The projection through the inclined boundaries between such domains can be erroneously interpreted as being a rhombohedral disordered phase. We find the domains shrink on charge, but that they can partially regrow in size on discharge.<br/><br/>At the end of the TM-oxidation region and before the high voltage O oxidation plateau, we observe Li occupying alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li-Li dumbbell configurations. We also observe the in- and out-of-plane TM migration as well as a partial phase transition from O3 to O1 stacking. In the O1 stacking phase, tetrahedral Li is absent, consistent with our DFT calculations indicating the O1 phase is not thermodynamically stable to accommodate tetrahedral Li.<br/><br/>Finally, we show how direct real-space imaging can reveal details of the structural correlations associated with short-range order in a range of disordered rock-salt cathode materials.<br/><br/>[1] Song, W. X.<i> et al.</i> <i>Joule</i> <b>6</b>, 1049, (2022).