Ultra-long-life zinc-iodine batteries enabled by efficient conversion of iodine on high-spin Fe single-atom sites embedded into 3D porous graphene

The practical application of aqueous zinc-iodine batteries as promising low-cost and large-scale energy storage devices is still hindered by the polyiodide shuttle effect and sluggish iodine redox kinetics. Herein, we designed highly dispersed Fe atom coordinated by planar N dopants with axial Cl ligand on three-dimensional porous graphene nanosheets (3DGNFe-Cl) as the cathode host to promote iodine conversion for efficient zinc-iodine batteries, which was developed through a simple impregnation of ammonia-treated 3D porous graphene with FeCl3 followed by calcination. The 3D porous graphene nanosheets provide abundant sites to accommodate iodine and ensure iodine with high conductivity, while the atomically dispersed, high-spin FeN4 moieties induced by the axial Cl ligand serve as efficient catalytic centers. These centers enhance the direct conversion of the I−/I2 redox reaction with less formation of polyiodide via an additional σ-bonding interaction between the Fe 3d and I 5p orbitals, effectively solving the critical issue of the polyiodide shuttle. As a result, the zinc-iodine battery with 3DGNFe-Cl containing I2 loading of 50% as cathode delivers a high specific capacity of 0.47 mAh cm−2 at 1 mA cm−2 and an exceptional long-term cycling stability with retaining 100% of its capacity after 85 000 cycles at 10 mA cm−2, exceeding all previously reported results. Moreover, a corresponding single pouch cell achieves an areal capacity of 3 mAh cm−2. This work demonstrates an effective strategy of developing high-performance zinc-iodine batteries through the modulation of the spin-state of Fe atom sites to improve iodine conversion.