Stacking Disorders in Honeycomb Layered NaKNi2TeO6 and Mixed-Cation Transport
Authors: Masese Titus , Kanyolo Godwill , Miyazaki Yoshinobu , Saito Tomohiro 
Published: December 2024

The utilisation of multiple alkali atoms to develop mixed alkali layered oxides has been gaining considerable interest in recent years for its potential to yield materials with versatile crystal structures and remarkable electrochemical functionalities.
[1,2]
By leveraging the properties derived from the variant alkali atoms, fascinating functionalities such as enhanced ionic transport, unique crystal chemistry and appealing electrochemistry can be achieved by this emergent class of materials. However, the adoption of this approach to aid in the augmentation of honeycomb layered oxide materials
[3]
is vastly unexplored with extremely scarce literature on the subject matter.
In this study, we investigate our novel honeycomb layered composition NaKNi2TeO6

[1]
obtained from Na2Ni2TeO6 and K2Ni2TeO6 precursor materials. The atomic-resolution imaging was accomplished using an aberration-corrected scanning transmission electron microscopy (STEM). Figure 1a shows a high-angle annular dark field (HAADF)-STEM image of NaKNi2TeO6 taken along the [100] zone axis. The image reveals that Na and K are separated into different layers in an alternating sequence. From an enlarged view of the atomic-scale HAADF-STEM mapping (Figure 1b), aperiodic shifts between adjacent Ni/Te slabs are contingent on whether the interlayer spaces are occupied by Na or K atoms. No shifts are observed when Ni/Te slabs are separated by a K layer. It is also evident that the interlayer distance depends on the alkali atom species sandwiched between adjacent Ni/Te layers. The Ni/Te layers with Na atoms are separated by 0.55 nm whereas the interlayer distance for the layers with K is 0.62 nm. These interlayer distances attained closely resemble those of the precursor compounds Na2Ni2TeO6 and K2Ni2TeO6.
[4-6]

We will reveal the intricate local atomic structures present in honeycomb layered NaKNi2TeO6, through a series of atomic-resolution scanning transmission electron microscopy (STEM) in multiple zone axes. Further, we will validate the electrochemical potential of this material to enable mixed-alkali ion transport. It is hoped that this study will pave way for further realisation of new honeycomb layered oxide compositions; thus, expanding the database of known honeycomb layered oxide frameworks. We also anticipate that the new findings will not only serve as a stepping-stone for unlocking new functionalities but also inspire the development of high-performance mixed alkali-ion energy systems. Extension of this work to new materials for Ca and Mg batteries will also be highlighted.

References :

[1] T. Masese, Y. Miyazaki, G. M. Kanyolo, T. Saito et al., Nat. Commun., 12, 1-16 (2021)

[2] R. Berthelot et al., Inorg. Chem., 60, 14310-14317 (2021); E. M. Mpanga et al., Chem. Mater., 36, 892-900 (2024)

[3] G. M. Kanyolo et al., Chem. Soc. Rev., 50, 3990-4030 (2021)

[4] T. Masese, Y. Miyazaki et al., Materialia, 15, 101003 (2021)

[5] T. Masese, Y. Miyazaki et al., ACS Appl. Nano Mater., 4, 279-287 (2021)

[6] T. Masese et al., Nat. Commun., 9, 1-12 (2018).

Figure 1. (a) HAADF-STEM image illustrating the unique stacking sequence in NaKNi2TeO6.
[1]
In the layers where K atoms occupy the interlayer space, Te / Ni slabs are not shifted with respect to each other (marked by a green line). However, for layers where Na atoms reside, shifts of the Te / Ni slabs are observed. The yellow and blue lines show shifts in different directions. Note the aperiodicity in the stacking sequence. (b) Enlarged view of the domain highlighted in (a), showing that the interlayer distance is dependent on the alkali atom species (Na or K) sandwiched between the Te / Ni layers.

Figure 1