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.

polysulfide by the host materials are conventional approaches to tackle the dissolution problem, [3,4] nonetheless, these could only partially alleviate the shuttle effect due to low fraction of host material in the electrode. Using "flooded" electrolyte could dilute the polysulfide solution thus somehow improves the redox kinetics due to enhanced mass/charge transfer, although it inevitably decreases the actual energy density of the battery. [1] Alternatively, recently established proactive electrocatalysis strategy could not only effectively improve the redox kinetics but also provide a "root" solution to solve the polysulfide shuttling. [5,6] The catalyzed sulfur redox reaction showed faster conversion kinetics, especially for the conversion of soluble LiPSs into the insoluble final products, thus preventing the continuous accumulation of LiPSs in electrolyte that is responsible for the shuttle effect. [7,8] An increasing number of electrocatalysts have been applied to Li-S batteries, [5,[9][10][11][12] however, systematic study on the kinetic and underlying basis to evaluate the exact role of catalyst addressing the PS shuttling issues is still overlooked. The classic electrocatalysis process is mostly referred to as heterogeneous catalytic reaction [13] -catalyzing a specific reaction instead of a series of reactions-as exemplified by the wellestablished electrocatalysis in fuel cells and water splitting such as the oxygen reduction reaction, oxygen evolution reaction (OER), and hydrogen evolution reaction. [14,15] The electrolysis of sulfur reduction reaction is much more complicated due to the consecutive multistep reduction reaction of sulfur, which brought huge challenge for the design of catalyst. [16] The diversified intermediate products of sulfur species would dynamically disproportionate or centralize in the electrolyte solution. [17] Moreover, one specific sulfur intermediate acts as both the reactant and product for the adjacent consecutive reductions, indicating that the sulfur species in the electrocatalysis reactions are highly conjugated and coupled. In addition to the multistep conversion reactions, the sulfur conversion reaction also involves multiphase transition because the long-chain polysulfides (Li 2 S x , 4 ≤ x ≤ 8) are soluble whereas the shortchain polysulfides (Li 2 S x , 1 ≤ x ≤ 2) is insoluble in the electrolyte. [18] Only a few catalysis works have paid attention to such complicated conversion reactions but mostly focused on the The diversity and coupling of sulfur redox intermediates and its associated solid-liquid-solid multiphase conversion mechanism pose great challenges in designing a proper electrocatalysts for Li-S batteries. In this report, it is proposed that an ideal catalyst should possess two catalytic centers which catalyze liquid-liquid conversion and liquid-solid conversion in tandem within one structure, with the use of 2D MOF nanosheets with different metal centers for validation. It is uncovered that the Ni-MOF is more effective in catalyzing the reduction of lo...

Inexpensive and energy-dense Zn metal anodes is key to the promise of aqueous Zn-ion batteries, which are heralded as an exciting battery chemistry for renewable and stationary storage. Yet, Zn deposition instability under demanding cycling conditions leads to rapid dendritic cell failure, and the hydrogen evolution reaction aggravates the issue. Electrolyte additives are a scalable solution to address the problem, but a high volume fraction is typically required for a noticeable effect. Here, a benign alcohol molecule propylene glycol is presented as an electrolyte additive that enables remarkably stable Zn anode cycling of over 1000 h at a practical 2 mA-2 mA h cm −2 at a low volume concentration when the reference cell shorts only after 30 h. The dramatic performance improvement at the low additive concentration is attributed to the effective morphology regulation and inhibition of hydrogen evolution, as revealed by spectroscopic and microscopic investigations. Ab initio molecular dynamics simulations reveal unprecedented atomistic insights behind the concentration-dependent effectivity of propylene glycol as an electrolyte additive. Excellent full cell cycling with two different positive host materials, even with high loading, highlights the potential for practical development.