Abstract text (incl. figure legends and references)
Li-rich Li1.2Ni0.13Mn0.54Co0.13O2 (Li-rich NMC) can deliver a high capacity over 250 mAh g-1 as a Li-ion battery cathode material compared with conventional layered metal oxides (<200 mAh g-1). Such high capacity results from the redox reaction of the lattice O2- ions and transition metals (TMs). Despite the boost in the capacities, Li-rich NMC suffers from voltage hysteresis and degradation in the voltage and capacity over cycling [1]. Synthesis efforts aim to overcome the issues but its structure is rather inhomogeneous and the role in affecting the materials performance is unclear [2]. The as-prepared material contains a number of defects, such as stacking faults, and has raised debates on whether the material is a coherent mixture of C2/m Li2MnO3 and R-3m LiTMO2 phase or a solid solution with the monoclinic phase [2].
In this work, we performed simultaneous annular dark field (ADF) imaging and electron ptychography on Li-rich NMC. Figure 1a shows the ADF image containing three types of monoclinic domains projected along [100], [110] and [1-10] zone axis, respectively. The domain variants are due to the faulted stacking of the TM layers in c direction. The TM layers composing the three types of domains have an in-plane rotation angle of 120 degree with respect to each other and the resulting stacking faults are referred to as rotation type. In the TM layers, Li+/Ni2+ and Co3+/Mn4+ cation ordering in a honeycomb pattern leads to the dumbbell contrast in ADF that arising from the Co/Mn atom columns. The dumbbell contrast is not uniformly seen as shown in Figure 1b, where cation disordering contrast is seen in the transition regions between the [110] and [1-10] domains. Such transition regions possibly result from the boundaries of the two types of domains or the in-plane cation disordering. Figure 1c displays several types of TM layers with cation ordering (O), mixing ordering/disordering (O/D) and disordering (D) contrast in ADF imaging. In addition to the rotation-type stacking faults, Figure 1d shows another type resulting from out-of-plane TMs layer shift in c direction, namely shift type. Both types of stacking faults rely on the imaging of TMs whilst lack the understanding of oxygen stacking. To probe the oxygen lattice, we carried out focused-probe electron ptychography to reconstruct the phase image and use the ADF image to point out TMs. Figures 2 show the ADF and ptychographic image and their composite image, respectively. Although TM layers possess a number of stacking faults, the oxygen layers are seen in an O3-type stacking. The imaging on the TMs and O layer stacking shows different structure defects in Li-rich NMC that can affect the electrochemical performance [3].
Figure 1 Stacking faults of Li-rich NMC. (a) ADF image of rotation-type stacking fault. White region is magnified in (b). The red, green and orange tetragons indicate monoclinic domains projected along [100], [110] and [1-10] zone axis, respectively. (c) ADF image of various types of TMs layers. (d) ADF image of shift-type stacking fault. Scale bar is 1 nm.
Figure 2 Imaging of faulted stacking. (a-c) Rotation type. (d-f) Shift type. (a, d) ADF image. (b, e) Ptychographic image. (c, f) Composite image. Superposed is the crystal model. Purple sphere is TM, green is Li and red is O. Scale bar is 0.5 nm.
Reference
[1] House, R.A., et. al., (2020) Nat. Energy 5, 777-785.
[2] Shukla, A.K., et. Al., (2018) Energ. Environ. Sci. 11, 830-840.