Faced with worsening environmental problems and fossil energy depletion, renewable energy has become a focus of the global energy transition, whereas its intermittency and variability require large-scale energy storage systems. Commercial lithium-ion batteries are limited by safety hazards and high costs, making novel, safe, and low-cost energy storage systems critical. Thus, building-integrated structural energy storage systems have emerged, in which the separator, a core component, must perform multiple functions. Cement-based materials, because of their abundant raw-material sources and excellent stability, are ideal for separators and have been applied in supercapacitors and zinc-based batteries. Aqueous zinc-iodine batteries, featuring intrinsic safety, low cost, and good redox reversibility, are suitable for structural energy storage systems. The liquid active material system (iodide dissolved in electrolyte) avoids the drawbacks of solid iodine cathodes, and its combination with porous cement separators can meet building operation requirements (low maintenance, long-term stability). As a core factor regulating battery performance, electrolyte concentration determines active substance content and ion conduction. Thus, clarifying its effect on cement-based separators and zinc-iodine batteries and optimizing electrolyte formulation are essential. This study integrates foamed cement paste separators (FCPS) into zinc-iodine batteries. By regulating the cement paste pore structure through foaming and adjusting the zinc iodide concentration, this study aims to optimize ion conduction and mechanical and electrochemical properties. The study explores the influence of zinc iodide concentration on the physicochemical properties of FCPS and battery performance, and determines the optimal electrolyte formula, aiming to provide theoretical and experimental support for high-performance cement-based zinc-iodine batteries and building-integrated structural energy storage systems.

FCPS specimens were prepared by a foaming technique with a water-cement ratio of 0.5. After incorporating 0.1% aluminum powder by cement mass, the mixture was poured into molds for curing. For battery assembly, active carbon (AC), acetylene black, and polyvinylidene difluoride (PVDF) were mixed at a mass ratio of 8:1:1, blended with N-methylpyrrolidone (NMP) to fabricate AC cathodes with a loading of 2.48 mg·cm^-2. The electrolyte consisted of 2 mol/L zinc sulfate (ZnSO₄) and 0 to 0.20 mol/L zinc iodide (ZnI₂). FCPS were vacuum-soaked in the electrolyte for 7 d, and the zinc-iodine batteries were assembled with a cathode-separator-anode configuration. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase and microstructure of FCPS. Compressive strength before and after electrolyte immersion was measured by a UTM5105 tester, and pore structure was analyzed by mercury intrusion porosimetry (MIP). Electrochemical behaviors including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic chargedischarge (GCD) were tested to evaluate battery performance, and ionic conductivity, areal capacity, areal energy density, and areal power density were calculated.

The pristine FCPS presented a complex phase composition, consisting of unhydrated cement clinker minerals and various hydration products. After immersion in the electrolyte, the diffraction peak intensity of calcium hydroxide decreased, whereas characteristic peaks of zinc sulfate hydroxide hydrate (Zn₄(OH)₆SO₄⋅4H₂O, ZSH) and gypsum appeared, indicating chemical reactions between cement hydration products and the electrolyte. Pore-structure analysis revealed that the original FCPS had a porosity of 44%. After electrolyte soaking, the porosity decreased with obvious pore refinement, reduced pore connectivity and increased tortuosity. A porosity of 33% was obtained at 0.15 mol·L^-1 ZnI₂, yet the FCPS still maintained stable ion transport performance. The ionic conductivity of FCPS first increased and then decreased slightly as the ZnI₂ concentration increased, reaching a maximum of 10.20 mS·cm^-1 at 0.15 mol·L^-1, which was predominantly governed by the intrinsic conductivity of the electrolyte. The compressive strength of FCPS was substantially enhanced after electrolyte treatment and reached its maximum (7.54 MPa) at 0.15 mol·L^-1 ZnI₂, closely associated with pore filling and porosity evolution.

Electrochemical tests of FCPS-based zinc-iodine batteries indicated that increasing the ZnI₂ concentration reduced the solution resistance, with the lowest value obtained at 0.15 mol·L^-1. CV results confirmed the highly reversible I₂/I^- redox reaction, and the peak current increased with higher ZnI₂ concentration. GCD and rate-performance results showed that a moderate ZnI₂ concentration extended the charge-discharge plateau and improved areal capacity. Under the optimal electrolyte formula (2 mol·L^-1 ZnSO₄ and 0.15 mol·L^-1 ZnI₂), the battery delivered an areal energy density of 2.00 mW·h·cm^-2 at a power density of 0.25 mW·cm^-2, and retained 82% of its initial capacity after 1000 cycles, outperforming most previously reported cement-based batteries. Regarding the energy-storage mechanism, the multi-scale pore structure of FCPS provided sufficient electrolyte storage space and ion transport pathways. The in situ formation of ZSH and gypsum balanced mechanical stability and electrochemical performance. The optimized ZnI₂ concentration ensured high ionic conductivity and sufficient active iodine sources. The synergistic effect of these three factors enabled the integration of structural load-bearing and electrochemical energy storage for building applications.

In this study, a safe and environmentally friendly zinc-iodine battery is integrated with foamed cement paste to develop a novel energy storage system that integrates structural load-bearing capacity, electrochemical functionality, and large-scale application potential. A porous cement paste separator (FCPS) is fabricated via a foaming technique, enabling it to serve dual functions: acting as a battery separator that facilitates ion conduction and electrolyte retention while providing sufficient load-bearing capacity to maintain structural integrity. A zinc sulfate solution is used as the base electrolyte, into which different concentrations of zinc iodide are introduced to regulate the content of iodine active species and the ionic transport environment. The results demonstrate that the synergy between the porous structure and the optimal zinc iodide concentration is critical to enhancing both the ionic conductivity of the separator and the electrochemical performance of the battery. The foaming technique endows FCPS with abundant pore structures, which construct continuously connected ion transport channels and provide sufficient storage space for the electrolyte. Meanwhile, the optimal zinc iodide concentration (0.15 mol·L^-1) improves the ionic conductivity of the electrolyte and, as an active iodine source, increases the number of iodide ions participating in the I₂/I^- redox reaction. This synergy enables the foamed cement paste separator to achieve a porosity of 33%, an ionic conductivity of 10.20 mS·cm^-1, and a compressive strength of 7.54 MPa. Based on this optimal zinc iodide concentration, the assembled zinc-iodine battery delivers a high areal capacity of 1.60 mA·h·cm^-2 and an energy density of 2.00 mW·h·cm^-2 at a power density of 0.25 mW·cm^-2, with a capacity retention of 82% after 1000 cycles. This study provides a new strategy for large-scale energy storage and demonstrates the promising application of zinc-iodine batteries in building structural energy storage.