High-Capacity and High-Rate Ni-Fe Batteries Based on Mesostructured Quaternary Carbon/Fe/Feo/Fe <sub>3</sub>O <sub>4</sub> Hybrid Material

Abstract

There is now an increasing demand for green, long-lasting and sustainable battery technology, due to the rapid development of photovoltaic and other renewable energy power systems. The Ni-Fe battery is a promising alternative to lithium ion batteries due to its long life, high reliability and eco-friendly characteristics. However, passivation and self-discharge of the iron anode are currently two of the main issues with Ni-Fe batteries, which lead to low specific energy (20-30 Wh·kg-1) and poor cycle efficiency (~65%). Here, we demonstrate that controlling the valence state of the iron and coupling with carbon can solve these problems. We develop a novel mesostructured quaternary carbon/Fe/FeO/Fe3O4 hybrid, containing iron oxides wrapped with a few atomic layers of carbon synthesized by a one-step solid state reaction. Experimental evidence reveals that the optimized system with three valence states of iron facilitates the redox kinetics, while the carbon layers can effectively enhance the charge transfer and suppress self-discharge. The hybrid anode exhibits high specific capacity of 604 and 528 mAh · g-1 at 1 and 10 A·g-1, respectively and high cyclic stability. A Ni-Fe button battery is fabricated using the carbon/Fe/FeO/Fe3O4 hybrid, delivering specific device energy of 127 and 110 Wh·kg-1 at a power density of 0.58 and 5.07 kW·kg-1, respectively. ~98.5 % Coulombic efficiency and ~90 % capacity retention have been achieved after 2500 cycles at 300 mA ·g-1. These features suggest that the Ni-Fe battery holds promise to become an important alternative in stationary energy storage.