Thienyl Metalloporphyrin Cathodes for Stable and High-Energy-Density Magnesium-Ion Batteries.

Magnesium-ion batteries (MIBs) suffer from low capacity, sluggish kinetics, and poor cyclability of organic cathode materials. Here, we show that modulating the substituents on the porphyrin ring by replacing phenyl with thienyl groups can substantially enhance magnesium-storage performance through the introduction of additional sulfur-based redox-active sites. A comparative study of two nickel-based metalloporphyrin complexes, [5,15-bis(ethynyl)-10,20-diphenylporphyrinato]nickel(II) (NiDEPP) and [5,15-bis(ethynyl)-10,20-dithienylporphyrinato]nickel(II) (NiDETP), reveals that the thiophene-functionalized NiDETP delivers a high specific capacity of 227 mAh g-1 at 0.3 A g-1, along with an energy density of 253.5 Wh kg-1 and a power density of 7168.25 W kg-1, while maintaining 80.9% capacity retention after 1000 cycles at 1.0 A g-1. In situ and ex situ spectroscopies confirm that the nitrogen atoms on the porphyrin ring and the sulfur atoms on the thiophene groups serve as synergistic redox-active sites for reversible Mg2+ storage. This work establishes heteroatom functionalization as a generalizable molecular design strategy for constructing multiple active sites in organic cathodes, offering a viable pathway to overcome performance bottlenecks in multivalent metal batteries.