Atomic Structural Features of Stacking Faults and Domain Connections in the Li- and Mn-Rich Cathode.

Li- and Mn-rich layered oxides (LMRs), a class of earth-abundant materials for rechargeable Li-ion battery cathodes, crystallize into layered structures of two different symmetries: 2/ represented by LiMnO and 3̅ represented by LiMnNiO. Fundamental questions about how the 2/ and 3̅ domains spatially correlate within the same oxide grain and how the 2/ stacking faults arrange themselves when this happens still remain. Here, by using integrated differential phase contrast imaging in scanning transmission electron microscopy (STEM-iDPC), we probe the structural and compositional details of a prototypical, cobalt-free LMR material, 0.3LiMnO·0.7LiMnNiO (LiMnNiO). The connection between the 2/ and 3̅ domains is found to be abrupt, facilitated by the small lattice mismatch between the two structures. Stacking faults in the 2/ domains feature atomic plane shifting that accommodates stacking sequence changes, which explains why the stacking faults form in a random manner. Furthermore, a local disordering mechanism was identified to correlate with the 2/ stacking faults. Chemically, it is found that Ni coexists with Mn at the transition metal sites within the nominal LiMnO domain. This study demonstrates that STEM-iDPC is a very useful tool for capturing all the elements in a single image, revealing atomic details on domain connections and stacking faults in the LMRs.