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
Sr2CoMoO6‑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 cm2 s–1). Moreover, fast kinetics (ΔEP ≈ 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 (Cdl ≈ 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.