  Natural abundance, impressive chemical characteristics and economic
feasibility have rekindled the appeal for rechargeable sodium (Na) batteries as
a practical solution for the growing energy demand, environmental
sustainability and energy independence. However, the scarcity of viable
positive electrode materials remains a huge impediment to the actualization of
this technology. In this paper, we explore honeycomb layered oxides adopting
the composition Na$_2$Ni$_{2-x}$Co$_x$TeO$_6$ ($x = 0, 0.25$ and $0.50$) as
feasible positive electrode (cathode) materials for rechargeable sodium
batteries at both room- and elevated temperatures using ionic liquids. Through
standard galvanostatic assessments and analyses we demonstrate that
substitution of nickel with cobalt in Na$_2$Ni$_2$TeO$_6$ leads to an increase
in the discharge voltage to nearly $4$ V (versus Na$^+$ / Na) for the
Na$_2$Ni$_{2-x}$Co$_x$TeO$_6$ family of honeycomb layered oxide materials,
which surpasses the attained average voltages for most layered oxide positive
electrode materials that facilitate Na-ion desertion. We also verify the
increased kinetics within the Na$_2$Ni$_{2-x}$Co$_x$TeO$_6$ honeycomb layered
oxides during operations at elevated temperatures which lead to an increase in
reversible capacity of the rechargeable Na battery. This study underpins the
doping of congener transition metal atoms to the honeycomb structure of
Na$_2$Ni$_2$TeO$_6$ in addition to elevated-temperature operation as a
judicious route to enhance the electrochemical performance of analogous layered
oxides.
