Aqueous zinc batteries are highly attractive for large-scale storage applications owing to their inherent safety, low-cost, and durability. Yet, their advancement is hindered by a dearth of positive host materials (cathode) due to sluggish diffusion of Zn 2+ inside solid inorganic frameworks. Here, we report a novel organic host, tetrachloro-1,4-benzoquinone (also called: p-Chloranil), which due to its inherently soft crystal structure can provide reversible and efficient Zn 2+ storage. It delivers a high capacity of ≥200 mAh g-1 with a very small voltage polarization of 50 mV in a flat plateau around 1.1 V, which equate to an attractive specific energy of > 200 Wh kg-1 at an unparalleled energy efficiency (~95%). As unraveled by density functional theory (DFT) calculations, the molecular columns in p-Chloranil undergo a twisted rotation to accommodate Zn 2+ , thus restricting the volume change (-2.7%) during cycling. In-depth characterizations using operando X-ray

Aqueous reversible zinc batteries using near-neutral aqueous electrolytes like ZnSO4/H2O are the cheaper, safer and more durable alternatives to typical nonaqueous Li-ion batteries, and seem more suited for renewable and large-scale energy storage. Yet their advancement is hindered by a limited choice of positive host materials due to sluggish solid-state diffusion of divalent zinc ions inside most inorganic structures. Much work has focused on layered and tunneled transition metal oxides with large interstitial sites or open interlayer galleries for high capacity and high rate Zn2+ storage. While these oxides deliver attractive electrochemical performance, formation/disappearance of an insulating side product - Zn4SO4(OH)6·nH2O – during battery discharge/charge has been noted for many of them. The insulating byproduct may not only be detrimental to power capability and extended cycling, its origin has been linked to parasitic side reactions involving metal dissolution and consequent increase of the electrolyte pH. An unequivocal understanding of its reversible appearance is missing, however. This presentation will delve into this topic, conveying a comprehensive mechanism that leads to the hydroxysulfate (Zn4SO4(OH)6·nH2O) formation/decomposition during zinc battery cycling, and demonstrate strategies to inhibit it. In addition, this presentation will cover our work on novel organic hosts, which owing to their malleable crystal structure provide facile and reversible Zn2+ storage. Operando X-ray diffraction, electron microscopy, and impedance analysis have been applied together with theoretical calculations to elucidate the Zn2+ storage mechanism. The deeper understanding gained has been channeled to tune the cathode design towards stable electrochemical cycling