Understanding the degradation process in zinc-iodine hybrid flow batteries

Abstract

Zinc-iodine hybrid flow battery (ZIHFB) represents a promising stationary energy storage with a theoretically high volumetric capacity (>250 Ah L^-1 ), however its broader commercialization is obstructed mainly by its reduced lifetime, particularly when charging to higher areal capacity of the negative half-cell (>130 mAh cm⁻²). This is typically manifested by a steep local increase of battery overvoltage during the charging and its subsequent drop (referred to as voltaic bulge) resulting in the decreased coulombic efficiency of the battery operation. In our study we investigated the origins of the performance degradation of a lab-scale ZIHFB single-cell by a systematic variation of the selected experimental conditions (incl. charging SOC limit, electrolyte composition) and battery set-up (use of non-conductive felt in individual half-cells, hydraulic shunt of electrolyte tanks). Careful analysis of these experiments, together with post mortem characterization of the inner cell components (pressurized membrane tightness test, microtomographic evaluation of Zn distribution within the felt electrodes), revealed that the performance degradation originates from a non-homogeneous Zn deposition leading to a formation of a compact zinc layer near the electrode-membrane interface, which restricts ionic supply to the rest of 3D negative electrode. As a consequence, a Zn dendrites growth is promoted, leading to membrane perforation and its malfunction. With the optimized operating conditions and battery construction we achieved stable and efficient mid-term cycling with CE ≥ 95% and EE > 83% at 100 mA cm⁻², and a low-capacity fade of 0.02% per cycle. The deepened 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 chemistries.