Ca-doped NaxCoO2 for improved cyclability in sodium ion batteries

•NaxCayCoO2, synthesized by a solid-state method, is examined as a cathode in SIBs.•Ca2+ substituted for two Na+ enhances structural stability during cycling.•Ca-doping contracts the cell dimensions and suppresses abrupt phase transitions.•Na0.6Ca0.07CoO2 shows the best cyclability (from 105 to 101 mAh g−1 for 60 cycles).•Na0.6Ca0.07CoO2 also shows a better rate performance than undoped Na0.73CoO2.

NaxCayCoO2 (0.45 ≤ x ≤ 0.64, 0.02 ≤ y ≤ 0.10) is synthesized via a solid-state method and examined as a cathode material to improve the cyclability of sodium ion batteries (SIBs). The substitution of one Ca2+ for two Na+ does not alter the P2-phase of undoped Na0.73CoO2, accompanying the continuous contraction of the unit cell volume with Ca-doping. The existence of Ca2+ in the prismatic sites between CoO6 slabs also affects the phase transition behaviors during charge–discharge (C–D). The suppression of Na+-vacancy ordering, ascribed to the immobility of Ca2+ in NaxCayCoO2, leads to continuous changes of the cell voltages with no abrupt voltage drops during C–D. The reduced cell dimension and the mitigation of distinctiveness between biphasic and solid-solution domains during C-D improve the cyclability of NaxCayCoO2 in SIBs. As an example, the negligible capacity fading in Na0.60Ca0.07CoO2 during 60 cycles (0.07 mAh g−1 cycle−1) is contrasted with a substantial decrease in the reversible capacity of Na0.73CoO2 (0.56 mAh g−1 cycle−1). Furthermore, a slower Na+ diffusion of NaxCayCoO2 in Na+-vacancy ordering regions, is sufficiently offset by a more rapid diffusion in all the other regions, which results in an improved rate performance. The platform presented here (multi-valent cation substitution for Na+) could be utilized in other layered cathode materials to improve the electrochemical performance of SIBs.