Electrochemical Performance of Layered Honeycomb Na₂(Ni_2–<i>x</i>Co_<i>x</i>)TeO₆ (<i>x</i> = 0, 0.10, 0.25) Oxides as Sodium-Ion Battery Cathodes

To develop economically and environmentally viable sodium-based solid-state batteries, we investigated Na₂(Ni_2-<i>x</i>Co_<i>x</i>)TeO₆ (<i>x</i> = 0, 0.10, 0.25). After synthesizing phase-pure compositions, we confirmed <i>P</i>6₃/<i>mcm</i> space group and P2 coordination through high-resolution synchrotron diffraction data, where Na⁺ occupies three different crystallographic positions in the unit cell: Na1, Na2, and Na3. With the incorporation of cobalt into the nickel lattice, an increase in the cell volume is seen. Bond parameters show that the average Ni-Ni distance increases as a result, but the local structure and coordination do not show marked differences. Our density functional theory calculations revealed that sodium at the Na1 site is energetically more favorable and that Co doping increased the lattice constants, supported by our X-ray diffraction data. Electrochemical measurements performed on half-cells versus sodium metal using CR2032-type coin cells revealed exceptionally high specific capacity matching the theoretical value and retained around 120 mAhg^-1 at the smallest but optimum concentration of cobalt. The kinetics of storage mechanisms in these compositions reflect pseudo-capacitive behavior. Our results indicate that substitution pathways in the layered oxide family of Na₂Ni₂TeO₆ offer the potential for the development of Na-based cathodes with enhanced cycling stability and ionic conductivity.