Electrochemically reversible redox couples that embrace more electron transfer at a higher potential are the eternal target for energy storage batteries. Here, we report a four-electron aqueous zinc-iodine battery by activating the highly reversible I2/I+ couple (1.83 V vs. Zn/Zn2+) in addition to the typical I−/I2 couple (1.29 V). This is achieved by intensive solvation of the aqueous electrolyte to yield ICl inter-halogens and to suspend its hydrolysis. Experimental characterization and modelling reveal that limited water activity and sufficient free chloride ions in the electrolyte are crucial for the four-electron process. The merits of the electrolyte also afford to stabilize Zn anode, leading to a reliable Zn-I2 aqueous battery of 6000 cycles. Owing to high operational voltage and capacity, energy density up to 750 Wh kg−1 based on iodine mass was achieved (15–20 wt% iodine in electrode). It pushes the Zn-I2 battery to a superior level among these available aqueous batteries.

Summary The highly reversible zinc-bromine redox couple has been successfully applied in the zinc-bromine flow batteries; however, non-electroactive pump/pipe/reservoir parts and ion-selective membranes are essential to suppress the bromine diffusion. This work demonstrates a zinc-bromine static (non-flow) battery without these auxiliary parts and utilizing glass fiber separator, which overcomes the high self-discharge rate and low energy efficiency while the advantages of the zinc-bromine chemistry are well preserved. It is achieved by a multifunctional additive, tetrapropylammonium bromide (TPABr), which not only mitigates the bromine cross-diffusion by regulating the fluidic bromine to a condensed solid phase but also provides a favorable interface for zinc electrodeposition toward non-dendritic growth. The proposed zinc-bromine static battery demonstrates a high specific energy of 142 Wh kg −1 with a high energy efficiency up to 94%. By optimizing the porous electrode architecture, the battery shows an ultra-stable cycling life for over 11,000 cycles with controlled self-discharge rate.

The reversible storage of Zn2+ ions in Prussian blue analogues with typical aqueous solution was challenged by fast degradation and poor coulombic efficiency, while the mechanism is yet to be uncovered. This study correlates the performance of the nickel hexacyanoferrate to the dynamics of H2O in the electrolyte and the associated phase stability of the electrode. It demonstrates severe Ni dissolution in conventional diluted aqueous electrolyte (1 m ZnSO4 or 1 m Zn(TFSI)2), leading to structure collapse with the formation of an electrochemical inert phase. This is regarded as the descriptor for the fast decay of nickel hexacyanoferrate in diluted aqueous electrolyte. However, a well‐preserved open framework for zinc storage was obtained in concentrated aqueous electrolyte (1 m Zn(TFSI)2 + 21 m LiTFSI)—the H2O activity is highly suppressed by extensive coordination—thus, reversible capacity of 60.2 mAh g−1 over 1600 cycles could be delivered.

The sulfur chemistry is transited from the conventional dissolution–precipitation category to solid–solid biphasic conversion by tuning the coordinating structure of the carbonate based electrolyte, paving the way for achieving Li–S batteries with lower E/S ratios.