Electrolyte Strategies For Highly Reversible Iodine Conversion Reactions In Zinc-based Batteries

As global energy demand continues to grow and concerns about environmental sustainability intensify,there is an urgent need to develop high-performance,low-cost,and environmentally friendly electrochemical energy storage systems.Zinc-iodine batteries have emerged as a research focus in the field of large-scale energy storage due to their abundant resources,stable high working voltage,low cost,and relatively high energy density.Building upon the research foundation of zinc-iodine flow batteries,static zinc-iodine batteries with iodine-containing materials as the primary cathode have achieved rapid development in the past two years.However,there is still limited research on the behavior of iodine-containing electrolytes in the energy storage process.This study focuses on zinc-iodine batteries with iodine-containing electrolytes and adsorptive cathodes,addressing issues such as low coulombic efficiency,short cycle life,and poor zinc anodic surface quality encountered during the cycling.By analyzing the inherent solvated structure of the electrolyte and utilizing in-situ and ex-situ characterization methods,the fundamental causes of these problems are identified.Additionally,the study employs electrolyte strategies to mitigate these issues and further explores the application and expansion of iodine redox reactions in zinc-based batteries.The specific researches with conclusions are summarized as follows:（1）The failure reasons of a static zinc-iodine battery with aqueous electrolyte of 2 M Zn（CF₃SO₃）₂+0.5 M KI and porous activated carbon were investigated.This device failed owing to a voltage barrier at a low charge/discharge rate,and exhibited low coulombic efficiency that could not be improved by adjusting the operating voltage range.Solvated structure analysis revealed that the high activity of free water molecules in H₂O-riched solvent is the main cause of aforementioned phenomena:on one hand,a large number of water molecules form the Zn²⁺solvated structure,participate in the interfacial charge transfer of zinc stripping/plating,destroy the acid-based equilibrium,and induce side reactions such as gas precipitation and hydroxide deposition,thus deteriorating the surface morphology of the anodic zinc.On the other hand,weak solvent interaction of I^-in a H₂O-rich environment leads to a multi-step I^-/I₂conversion.Since the high solubility of I3^-as intermediate product,it weakens the adsorption of polyiodides by porous activated carbon,thus reducing the reversibility of interfacial reaction on cathode.（2）Eutectic electrolyte based on N-methylacetamide（C₃H₇NO）was constructed to achieve highly reversible iodine conversion on cathode.In the eutectic solution,Zn²⁺shows a unique double-layer solvated structure,including a compact anhydrous inner layer and a dilute-H₂O outer layer formed by partial substitution of C₃H₇NO,resulting in a gentle and mild anodic interface reaction.More importantly,the Gibbs free energy of I₂/I₃^-conversion increases,effectively inhibiting the formation of I₃^-as intermediate product during the I^-oxidation.When paired with porous activated carbon as adsorptive cathode,the zinc-iodine battery maintains a reversible areal capacity of 2.19 m A h cm^-2 and a capacity retention rate of 98.7%after 5000 cycles,with coulombic efficiency consistently close to 100%.In-situ XRD patterns reveal the buffering effect of porous activated carbon during I₂ adsorption/desorption,while a reversible and direct I^-/I₂ conversion is confirmed by in-situ UV-vis adsorption spectra based on a cuvette cell.（3）Quasi-solid-state electrolyte based on calcium sulfate dihydrate（Ca SO₄·2H₂O）was constructed to enhance the reversibility and stability of zinc-iodine batteries.In quasi-solid-state electrolyte,a large number of SO₄^2-replace H₂O to coordinate with Zn²⁺,while Ca²⁺enhances solvation effect of I^-.Also,the conductivity of Zn²⁺is promoted in quasi-solid-state electrolyte due to its dense and orderly ion network,and limited activity of free water molecules endows it with anti-corrosion and suppressed side reaction activity.Based on these advantages,quasi-solid-state electrolyte exhibits a stable zinc stripping/platting for up to 1000 h.When paired with a porous activated carbon cathode,zinc-iodine battery demonstrates excellent anti-polarization capability and cycling stability within a current density range of 1.0 to 5.0 m A cm^-2.（4）Electrolyte based on dual-energy storage ion solute(Zn²⁺,I^-)and ethylene glycol partially substituted solvent was developed,which owned a dual-energy storage mechanism of Zn²⁺（de）intercalation in low-voltage region and I^-/I₂ conversion in high-voltage region in a zinc-based battery with a bifunctional cathode containing NH₄V₄O₁₀ and porous activated carbon.Ethylene glycol molecules form mono-/bidentate coordination with Zn²⁺,weakening the activity of free water molecules.Also,the 90%substitution volume ensures the electrolyte still has ideal wettability and fiuidity.Benifitting from the activation of dual-mechanism,zinc-based battery exhibits a reversible capacity of 0.43 m A h at 1 A g^-1,which can be distinguished as specific capacity of 343 m A h g^-1 for（de）intercalation of Zn²⁺as well as areal capacity of 0.16 m A h cm^-2 for I^-/I₂ conversion.At current density as low as 0.2 A g^-1,the battery shows a single-cycle capacity decay of only 0.032%within 500 cycles,highlighting a distinct enhancement in both structural stability of NH₄V₄O₁₀ and reversibility of I^-/I₂ conversion at low charge-discharge rates.Compared to vanadium-based cathodes with Zn²⁺（de）intercalation as their sole energy storage mechanism,dual-mechanism provides approximately a 40%increase in discharge medium voltage for zinc-based batteries.Additionally,the anti-freezing characteristic of ethylene glycol enables the battery to maintain stable operation at temperature as low as-20°C.