Heterointerface synergistic Na+ storage fundamental mechanism for CoSeO3 playing as anode for sodium ion batteries/capacitors

• CoSeO 3 is chosen as the anode to understand its Na + storage mechanism in SIBs/SICs. • CoO/SeO 2 heterointerfaces are formed by in-situ conversion of CoSeO 3 . • The coupled heterointerfaces accelerate the electron and Na + diffusion kinetics. The transition metal selenite CoSeO 3 single crystal nanoparticles with primary particle size distribution ranging from 80 to 200 nm is systematically investigated as anode material for sodium ion batteries (SIBs)/capacitors (SICs). It achieves stable Na + storage capacity of 280 mAh g −1 at a high current density of 10 A g −1 in CoSeO 3 ||Na SIBs. Furthermore, the corresponding CoSeO 3 ||Activated carbon (AC) SICs also presents a high energy density of 51 Wh kg −1 at a power density of 2 kW kg −1 , along with 72% energy retention after 3000 cycles at 1 A g −1 . Combining advanced microscopy (HRTEM, SEM), density functional theory (DFT) calculations and surface science (XPS), it is demonstrated that the CoSeO 3 in-situ transform into binary oxides CoO/SeO 2 and form heterointerfaces during the initial discharge/charge cycle. The coupled heterointerfaces between CoO and SeO 2 could construct strong internal electric field, accelerating the electron/Na + diffusion kinetics during the subsequent charge–discharge process and boosting Na + pseudocapacitance storage. The finding of heterointerfaces synergistic sodium-ion storage fundamental mechanism of transition metal selenite might provide inspiration for the development of new anode material for sodium-based storage devices.