Understanding the degradation process in zinc–iodine hybrid flow batteries

Abstract

Zinc–iodine hybrid flow batteries (ZIHFBs) represent promising stationary energy storage with a theoretically high volumetric capacity (>250 Ah L⁻¹). However, their broader commercialization is prevented, mainly by their short lifetime, particularly when charging to a higher areal capacity of the negative half-cell (>130 mAh cm⁻²). In our study, we investigated the origins of the performance degradation of a lab-scale ZIHFB single-cell due to excessive charging. This is manifested by a local peak on the charging voltage profile (voltaic bulge), resulting in decreased coulombic efficiency for the subsequent battery cycling. Systematic variation of the selected experimental conditions (including charging SoC limit and electrolyte composition) and battery construction (use of non-conductive felt in individual half-cells and hydraulic shunt of electrolyte tanks), combined with post-mortem characterization of internal components (pressurized membrane tightness test and microtomographic evaluation of Zn distribution within the felt electrodes), revealed the origin of the performance degradation. It originates from non-homogeneous Zn deposition leading to the formation of a compact zinc layer near the electrode-membrane interface, which restricts ion supply to the rest of the 3D negative electrode. As a consequence, Zn dendrite growth towards the positive electrode is promoted, leading to membrane perforation and malfunction. With the optimized operating conditions and battery construction, we achieved stable and efficient mid-term cycling with a coulombic efficiency of ≥95% and energy efficiency of >83% at 100 mA cm⁻², and a low-capacity fade of 0.02% per cycle. The enhanced insight into the degradation mechanism will be further used to design effective mitigation strategies to enhance the areal capacity and durability of ZIHFBs and related zinc-based chemistry.