A High-Efficiency Iodide-Ferrocene Multiple Redox Suspension for Li-Based Flow Batteries

Semisolid flow batteries (SSFBs) have the characteristics of high energy and high flexibility, exhibiting great application potential in renewable energy storage. However, the liquid-phase electrolyte with the largest volume ratio fails to provide energy in the majority of semisolid suspensions, hence limiting further improvements. This study employs ferrocene (Fc) as the solid-phase active material and incorporates highly soluble lithium iodide (LiI) as the liquid-phase active material, resulting in a synergistic multiple redox semisolid–liquid (MRSSL) suspension. This MRSSL suspension is utilized in a Li-based flow battery, significantly enhancing energy density while also achieving high voltage and energy efficiency. An appropriate amount of LiI can significantly improve the reversibility of solid-phase Fc, hence extending the cycle life of the Li-based flow battery. The volumetric capacity of 79.1 Ah L–1 and an energy density of 250 Wh L–1 are demonstrated by the 1.2 M LiI + 2.65 M (31 vol %) Fc MRSSL suspension, which is ∼19 times superior to that of a single-phase Fc saturated catholyte, and the capacity retention reaches 84.1% after 100 cycles. Additionally, the closer redox potentials of LiI and Fc result in voltage and energy efficiencies of 97% and 95%, respectively, which benefit stack design and battery management. Finally, the continuous-flow mode test demonstrates a 50 h long operation, with Coulombic efficiency and energy efficiency above 90% and 82%, respectively, which validates the applicability of the LiI + Fc MRSSL suspension. This work proposes a promising approach to enhance the energy density and efficiency of SSFBs by optimizing the liquid-phase electrolyte.