Na⁺ Redistribution by Electrochemical Na⁺/K⁺ Exchange in Layered Na_<i>x</i>Ni₂SbO₆

This work investigates the electrochemical Na&lt;sup&gt;+&lt;/sup&gt;/K&lt;sup&gt;+&lt;/sup&gt; ion exchange mechanism occurring in layered Na&lt;sub&gt;3&lt;/sub&gt;Ni&lt;sub&gt;2&lt;/sub&gt;SbO&lt;sub&gt;6&lt;/sub&gt;. Structural characterizations using X-ray diffraction and transmission electron microscopy uncover a remarkable and rich phase evolution as K is inserted in partially desodiated Na&lt;sub&gt;x&lt;/sub&gt;Ni&lt;sub&gt;2&lt;/sub&gt;SbO&lt;sub&gt;6&lt;/sub&gt;. Rather than simple addition of K to the structure, we see significant Na rearrangement, with discrete Na phases traversing the full range of Na compositions, even though the overall Na content in the sample is not changed. At any given time as much as three distinct phases can be present in the sample, consistent with thermodynamic equilibrium rules. Using DFT computations we demonstrate that this remarkable phase behavior during ion exchange is due to the repulsion between Na&lt;sup&gt;+&lt;/sup&gt; and K&lt;sup&gt;+&lt;/sup&gt; which creates distinct phases of them. Our analysis should be applicable to other ion-exchange systems where the exchanged ions are not well miscible in the host structure.