In quest of a stable structure throughout redox reactions, an approach of B-site ordering (0D arrangement) of cations in double perovskites is adopted. Here, we report B-site cation ordering in double perovskite Sr 2 CoMoO 6-δ (DP-SCM) that tends to a favorable rock salt structure (0D arrangement). The synergy of Co/Mo having good redox ability further facilitates high oxygen mobility. A high content of oxygen vacancy examined using XPS and EPR facilitates a high oxygen anion diffusion rate (2.03 × 10 −11 cm 2 s −1 ). Moreover, fast kinetics (ΔE P ≈ 0.013 V@ 1 mV s −1 ) of charge storage prohibits any phase transformation reflecting the excellent cycle life (125% retention up to 5000 cycles). Such fast kinetics is majorly furnished from anion intercalation with little involvement from double layer mechanism (C dl ≈ 42.1 F g −1 ). DP-SCM achieves a resultant capacitance of 747 F g −1 @ 1 A g −1 with a rate capability of 56% up to 10 A g −1 . Motivated by outstanding performance of DP-SCM electrodes, a symmetric cell is assembled with a 1.4 V operating potential that delivers a high energy density of 64 Wh kg −1 @855 W kg −1 . This work on double perovskites suggests that the advance understanding of cation ordering and charge storage mechanism can provide a new direction to fabricate highly capacitive electrode materials.
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