Highly Stable Metal-Organic Framework with Redox-Active Naphthalene Diimide Core as Cathode Material for Aqueous Zinc-Ion Batteries.

Recently, metal-organic frameworks (MOFs) as the cathode materials for aqueous zinc-ion batteries (ZIBs) received growing attention. Herein, a novel MOF, Ni-Ndi-trz (Ndi-trz=2,7-di(4H-1,2,4-triazol-4-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) was synthesized through a solvothermal method. Its rational design using a naphthalene diimide (Ndi) core allowed the formation of a four-fold interpenetrated pcu (primitive cubic) topology. The as-synthesized Ni-Ndi-trz is highly stable over a wide pH range (0-12) for 30 days, which is critical to ensure the decent cyclability of zinc-ion batteries (ZIBs). When used as the cathode material of ZIBs, it shows a high initial specific capacity of 90.7 mAh g and excellent cycling stability. Remarkably, three-electrode system tests, ex situ FTIR, UV/Vis and XPS spectra revealed that the Ndi core of Ni-Ndi-trz undergoes a reversible interconversion between the keto and enol forms when interacting with Zn ions. This work may shed light on the feasibility of designing novel MOFs and exploring their mechanisms for zinc ion batteries.