Highly reversible lithium storage in cobalt 2,5-dioxido-1,4-benzenedicarboxylate metal-organic frameworks boosted by pseudocapacitance

Exploiting novel metal-organic frameworks (MOFs) as electrode materials with superior rate capabilities and understanding their electrochemical behaviour in detail are crucial for boosting the application of MOFs in the field of energy storage. Herein, we prepared Co₂(DOBDC) (DOBDC = 2,5-dioxido-1,4-benzenedicarboxylate) via a hydrothermal method and explored its electrochemical performance as an anode material for lithium-ion batteries. The as-prepared Co₂(DOBDC) MOF exhibits a reversible capacity of 526.1 mA h g⁻¹ after 200 charge/discharge cycles at a current density of 500 mA g⁻¹ and also demonstrates an impressive rate capability, with a high capacity of 408.2 mA h g⁻¹ at a high current density of 2 A g⁻¹. Furthermore, synchrotron-based soft X-ray absorption spectroscopy (sXAS) and electron paramagnetic resonance (EPR) spectroscopy have been applied to investigate the spin state of cobalt in the electrodes at different states of charge. Our results suggest that localized electrons in high-spin (S = 3/2) Co²⁺ in pristine Co₂(DOBDC) are gradually delocalized after discharging. It was also found that the high rate capability of Co₂(DOBDC) is mainly ascribed to an ultrafast ion intercalation pseudocapacitance process, which results from its unique microporous architecture and adequate specific surface that offers sufficient electrode/electrolyte contact and benefits fast Li⁺ ion diffusion.