Zero-Dimensional Ordered Sr₂CoMoO_6-δ Double Perovskite as High-Rate Anion Intercalation Pseudocapacitance

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 Sr₂CoMoO_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 cm² 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.