Fe(III) dihydroxybenzoquinone-based metal organic framework for sodium battery cathodes: Properties, charge-discharge kinetics and redox reaction mechanisms

Sodium batteries are promising energy storage devices that could promote the transition to clean energy. For this purpose, it is desirable to develop safe cathode materials that can be easily produced from cheap and abundant elements. One of the attractive options is Fe2(dhbq)3, a metal-organic framework (MOF) that is synthesized from 2,5-dihydroxybenzoquinone (H2dhbq) and Fe(III) salts. This work provides a comprehensive analysis of Fe2(dhbq)3, including its structure, properties, charge-discharge kinetics, and redox reaction mechanism in sodium batteries. Using a set of experimental and theoretical methods, we show that sodiation of Fe2(dhbq)3 in the potential range of 1.1–3.8 V vs. Na+/Na is accompanied by a reversible two-electron reduction of dhbq ligands, while iron is only marginally involved and formally remains in +3 state. The material delivers a high specific capacity of up to ∼180 mAh g−1. Diffusion of Na+ ions in NaxFe2(dhbq)3 at low-to-moderate sodiation degrees (≥2.2 V vs. Na+/Na) is so fast that it is kinetically indistinguishable from supercapacitance. Additionally, we show that Fe2(dhbq)3 can be synthesized by simply mixing solutions of H2dhbq and salts of Fe(III) at room temperature, which makes its production especially simple and scalable.