High-capacity cobalt-based coordination polymer nanorods and their redox chemistry triggered by delocalization of electron spins

The design and synthesis of nanoscale metal organic frameworks (MOFs) or coordination polymers (CPs) is of great significance in deepening their application for rechargeable batteries and supercapacitors. Here we present the designed fabrication of cobalt-nitrilotriacetic acid (CoHNta) CP nanoarchitectures via judiciously formulating reacting solvent and choosing cobalt precursor. The CoHNta with a nanorod structure (r-CoHNta) exhibits the most impressive lithium storage performance, delivering a high reversible capacity of 875 mA h g⁻¹ at 500 mA g⁻¹ after 300 cycles. Even at 2.4 A g⁻¹, it still maintains a reversible capacity of ~460 mA h g⁻¹. The performance superiority of r-CoHNta over other nanostructures is attributed to its mesoporous nanorod architecture with rapid ion transport, good flexibility, large electrode-electrolyte contact area, and robust structure stability upon prolonged cycling. More importantly, the lithiation/delithiation behavior of r-CoHNta is investigated via synchrotron-based soft X-ray spectroscopy and electron paramagnetic resonance techniques. The findings suggest that localized high-spin Co²⁺ in pristine r-CoHNta would gradually convert to delocalized high-spin Co²⁺ upon discharging, accompanying by the rehybridization of O-2p and Co-3d orbitals.