Electrolyte additive design to address side reactions at cathode and anode in flowless aqueous Zn-Br2 batteries

Abstract

Aqueous Zn-Br2 batteries hold immense promise for large-scale energy storage systems due to their inherent safety and high energy density. However, achieving high battery performance necessitates addressing critical side reactions, including dendrite formation and hydrogen evolution at the anode, as well as bromine species diffusion and evaporation of volatile bromine at the cathode. Importantly, these reactions impact overall battery performance, underscoring the need for comprehensive mitigation strategies. This study proposes an electrolyte additive design strategy to address side reactions at both the anode and cathode. This study introduces a multi-type electrolyte additive strategy combining cerium chloride (CeCl3) and 1-ethylpyridinium bromide (1-EPBr). Trivalent Ce3+ forms an electrostatic shielding layer to prevent Zn2+ from concentrating at zinc metal protrusions, while the high electron-donating nature of Cl− mitigates H2O decomposition on the zinc metal surface by reducing the interaction between Zn2+ and H2O. These combined cationic and anionic effects significantly enhance the reversibility of the zinc metal reaction. Additionally, 1-EPBr, a known bromine complexing agent (BCA), effectively mitigates bromine species diffusion and the evaporation of bromine at the cathode. This enables the flowless aqueous Zn-Br2 battery to reliably cycle with exceptionally high capacity (>400 mAh after 5,000 cycles) even in a large-scale battery configuration of 15 × 15 cm2. This work presents a novel approach by utilizing both cationic and anionic additives to stabilize the zinc metal anode. These findings advance the understanding of stabilization mechanisms in aqueous battery systems. Building upon this foundation, this study proposes a single-type electrolyte additive to simultaneously address critical challenges related to both anode and cathode reactions in aqueous Zn-Br2 batteries. This work introduces poly(N-(2-hydroxyethyl)-4-vinylpyridinium bromide), a multifunctional poly(ionic liquid) (PIL). The pyridinium moieties within the PIL, featuring electron-deficient aromatic rings, establish π-anion interactions with polybromide anions (Br3 −, Br5 −, Br7 −, etc.), effectively mitigating bromide diffusion and evaporation. Additionally, the PIL electrostatically adsorbs onto the Zn anode, forming an electro-shielding layer that is expected to inhibit Zn dendrite formation. The hydroxyl (−OH) functional groups within the PIL further enhance its functionality by participating in Zn2+ solvation structures, thereby reducing the HER at the Zn anode through modulation of solvation dynamics. This marks the first application of a PIL as an electrolyte additive in aqueous Zn-Br2 batteries. This study highlights the potential of PIL-based additives for extending to other aqueous battery systems, paving the way for next-generation, high-performance energy storage technologies.