Ca-doped NaxCoO2 for improved cyclability in sodium ion batteries

Abstract Na x Ca y CoO 2 (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 Ca 2+ for two Na + does not alter the P2-phase of undoped Na 0.73 CoO 2 , accompanying the continuous contraction of the unit cell volume with Ca-doping. The existence of Ca 2+ in the prismatic sites between CoO 6 slabs also affects the phase transition behaviors during charge–discharge (C–D). The suppression of Na + -vacancy ordering, ascribed to the immobility of Ca 2+ in Na x Ca y CoO 2 , 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 Na x Ca y CoO 2 in SIBs. As an example, the negligible capacity fading in Na 0.60 Ca 0.07 CoO 2 during 60 cycles (0.07 mAh g −1  cycle −1 ) is contrasted with a substantial decrease in the reversible capacity of Na 0.73 CoO 2 (0.56 mAh g −1  cycle −1 ). Furthermore, a slower Na + diffusion of Na x Ca y CoO 2 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.