Stacking Disorders in MixedAlkali Honeycomb Layered Oxide NaKNi2TeO6 and Feasibility for Mixed-Cation Transport

Abstract

<b>We demonstrate the feasibility of using a combination of alkali atoms (Na and K) to develop a robust mixed-alkali honeycomb layered oxide NaKNi<sub>2</sub>TeO<sub>6</sub>. Through a series of atomic-resolution transmission electron microscopy in multiple zone axes, we reveal for the first time the local atomic structural disorders characterised by aperiodic stackings and incoherency in the alternating arrangement of Na and K atoms. Our findings indicate great structural versatility that renders NaKNi<sub>2</sub>TeO<sub>6</sub> an ideal platform for investigating other fascinating properties such as mixed ionic transport and intriguing electromagnetic and quantum phenomena amongst honeycomb layered oxides. Finally, we unveil the possibility of inducing mixed Na- and K-ion transport electrochemistry of NaKNi<sub>2</sub>TeO<sub>6</sub> at high voltages (~ 4V), thus epitomising it as a competent cathode candidate for the emerging dendrite-free batteries based on NaK liquid metal alloy as anodes. The results not only betoken a new avenue for developing functional materials with fascinating crystal versatility, but also prefigure a new age of ‘dendrite-free’ energy storage system designs that rely on mixed-cation electrochemistry.</b>