Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions

Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions Guanjie He, Ruwei Chen, Haobo Dong, Yuhang Dai, Jingyu Wang, Jie Chen, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6372434/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Four-electron aqueous metal-iodine batteries embrace high theoretical capacities, abundant raw materials, and superior safety, making them highly promising for next-generation large-scale energy storage applications with high energy and power densities. However, harnessing this four-electron redox chemistry has traditionally relied on high-concentration, corrosive halogen-containing electrolytes (up to 46 moles) to stabilize hypervalent iodine cations, posing considerable economic and environmental challenges to access their full potential. Here, we proposed a universal electrolyte design, the KA-PA-Nuc standard, which employs kosmotropic anions (KA), polar anions (PA), and nucleophilic species (Nuc) to achieve reversible four-electron aqueous metal-iodine batteries. PA facilitates a water-deficient, anion-enriched interface, while KA disrupts hydrogen bonding between Nuc and their hydration shells, which in turn form stable halogen bonds with hypervalent iodine cations. For the first time, this electrolyte design grounded in anionic chemistry achieves reversible four-electron iodine redox reactions in halogen-free electrolytes at an exceptionally low concentration (2.4 moles). The KA-PA-Nuc standard was validated across diverse Nuc in aqueous Zn-I₂ and Al-I₂ batteries, demonstrating its broad applicability and effectiveness for aqueous metal–I2 batteries. By eliminating the reliance on high-concentration, corrosive halogen-containing electrolytes, this work establishes a new paradigm and provides a new avenue for low-cost and sustainable batteries. Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Chemistry/Energy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The increasing demand from renewable energy sources, industry grids, and electronic devices call for reliable and sustainable electrochemical energy storage systems beyond lithium-ion batteries. 1 Alternative battery chemistries integrating multi-electron redox couples of earth-abundant elements with low-cost aqueous electrolytes show great promise for future large-scale energy storage applications. 2 Emerging aqueous metal batteries coupled with iodine cathodes are regarded as one of the most promising candidates, leveraging the cost-effectiveness and earth-abundance of metal anodes (e.g., Zn and Al) and iodine, along with multielectron conversion chemistry considering the diverse chemical valences of iodine. 3 Unlike cathodes with intercalation chemistry (e.g., V-based, Mn-based, and Prussian Blue analogues), which are constrained by the hosting capacity of intercalated charge carriers, conversion-type iodine cathodes achieve electron transfer by reversibly altering their chemical states, offering a notable capacity advantage over their intercalation counterparts, especially when multi-electron transfer is involved. 4 The electrochemical performance of aqueous metal-iodine (I 2 ) batteries is determined by the number of electron transfer or valence transition in iodine conversion chemistry. Substantial research has focused on the I - /I 0 redox couple with a two-electron transfer process, providing a theoretical capacity of 211 mAh g -1 and a redox potential of 0.54 V vs. standard hydrogen electrode (SHE). 5 In contrast, the four-electron reaction involving successive I - /I 0 /I + redox couples offers a duplicate theoretical capacity of 422 mAh g -1 and a higher redox potential of 1.07 V vs. SHE, ensuring higher energy density and outperforming most prevailing intercalation-type cathodes. 6 However, the I 0 /I + conversion process generally suffers from poor reversibility, which is considerably inferior to that of the I - /I 0 conversion process due to the thermodynamic instability of I + cations. 3,7 Consequently, despite its potential, the development of reversible I - /I 0 /I + redox couples remains in its infancy and continues to be a long-standing challenge. The crux of iodine conversion chemistry lies in interfacial reactions, where the electrolyte’s composition and interface profoundly influence the reaction pathways. The I + cation, being highly electrophilic due to its electron deficiency, is prone to hydrolysis in aqueous electrolytes due to nucleophilic attacks by electron-rich OH groups of water molecules, resulting in poor reversibility of the I 0 /I + conversion process. 8,9 In prior attempts, four-electron aqueous Zn-I 2 batteries were activated by stabilizing the hydrolysis of electrophilic I + via the formation of halogen-bond complexes (e.g., ICl 2 or IBr 2 ) in intensive chloride- or bromide-based electrolytes. 10,11 Analogously, four-electron aqueous Al-I 2 batteries have been demonstrated using concentrated AlCl 3 electrolytes. 12 In these systems, the I + cation functions as a halogen-bond donor, forming noncovalent halogen bonds with two nucleophilic species (Cl - or Br - ) by accepting electrons in the lobes of its empty p-orbital. 2,13 However, in water-enriched interfaces, polar water molecules preferentially form hydrogen bonds with nucleophilic species, competing with and destabilizing halogen-bond complexes (Fig. 1a). To mitigate this, in all prior studies, high-concentration, halogen-containing electrolytes were generally employed to reduce water activities via intensive ion coordination and promote the population of free halogen ions (Fig. 1b). 10,14 Unfortunately, the use of such high-concentration electrolytes significantly increases costs, offsetting the primary economic advantage of aqueous systems. Moreover, halogen-containing electrolytes are highly corrosive to standard battery components, severely limiting their broader applicability (Fig. 1d and Supplementary Fig. 1). Therefore, the development of halogen-free electrolytes with low concentrations that can support reversible I - /I 0 /I + redox chemistry is highly desirable but seems impossible due to the inherent inverse correlation between the water activity and the electrolyte concentration. Addressing this challenge will unlock the full potential of aqueous metal–I 2 batteries, making them viable candidates for large-scale, cost-effective, and sustainable energy storage solutions. Giving that the competition between halogen bonds and hydrogen bonds at water-enriched interfaces mainly contributes to the poor reversibility of the I 0 /I + conversion process, creating a water deficient electrolyte interface at low concentrations, while simultaneously manipulating hydrogen bonds between water molecules and nucleophilic species, has become the crux for overcoming this challenge. Salt anions not only affect the bulk water structure but also play a pivotal role in modulating micro-environment of the electrolyte-electrode interface. 15,16 Meanwhile, anion-specific-anion effect, namely the distinct interactions among anions, water molecules, and solutes at the molecular level, hold great potentials in manipulating hydrogen bonds between water molecules and nucleophilic species. 17,18 Herein, we propose a universal guideline leveraging anionic chemistry to achieve reversible I - /I 0 /I + reaction in halogen-free, low concentration aqueous electrolytes. Systematic experimental characterizations and theoretical calculations reveal that strong-polar anions (PA) facilitate an anion-enriched, water-deficient electrolyte interface, while kosmotropic anions (KA) disrupt hydrogen bonds between nucleophilic species (Nuc) and their hydration shells, thereby stabilizing I + cation involved halogen bonds (Fig 1c). This novel KA-PA-Nuc electrolyte standard was validated across four nucleophilic species (urea, acetamide, pyridine, and nicotinamide) in aqueous Zn-I 2 and Al-I 2 batteries, demonstrating its general applicability and effectiveness. For the first time, this electrolyte design enables outstanding and reversible I⁻/I⁰/I⁺ redox reactions in low-cost and halogen-free electrolytes at very low concentrations, outperforming most existing aqueous metal-I 2 batteries (Fig. 1e-f and Supplementary Table 1-2). By eliminating the dependence of hypervalent iodine conversion on high-concentration, expensive and corrosive halogen-containing electrolytes, this work will pave the way for cost-effective, scalable, and sustainable aqueous energy storage solutions. Versatile electrolyte standard for four-electron iodine conversion reactions The hydrolysis of I + is the primary factor limiting the reversibility of the four-electron iodine conversion reaction in aqueous electrolytes. Previous studies relied on high-concentration, corrosive halogen-containing electrolytes to suppress I + hydrolysis by reducing water activity and releasing free halogen ions to form stable halogen complex (ICl 2 or IBr 2 ), thereby mitigating the competition between hydrogen and halogen bonds. 19,20 In contrast, this work introduces a completely different insight to overcome this long-standing challenge. Specifically, strong polar anions feature preferential adsorption at the electrolyte-electrode interface under an electric field, establishing a water-deficient, anion-enriched micro-environment. Concurrently, kosmotropic anions polarize the hydration shells of nucleophilic species, breaking hydrogen bonds between these species and surrounding water molecules. This dual effect, grounded in anionic chemistry, releases more free nucleophilic species, which in turn form stable halogen bonds with I + cations (INuc 2 ) within the water-deficient micro-environment (Fig. 2a and Supplementary Fig. 2). As a result, a new electrolyte standard, termed KA-PA-Nuc, is proposed, offering a more sustainable way for four-electron iodine conversion reactions. As a demonstration, the effectiveness of KA-PA-Nuc electrolyte standard was validated in aqueous Zn-I 2 batteries across four different nucleophilic species (Supplementary Fig. 3). Theoretically, the I + cation acts as a strong halogen bond donor, capable of interacting with two nucleophilic molecules containing electronegative centers, such as nitrogen or oxygen (halogen bond acceptor), to form linear [Nuc-I-Nuc] + halogen bonds. These halogen bonds are the foundation of four-electron iodine conversion reactions. 21 Fig. 2b shows the CV profiles with urea (U) as the nucleophilic species. In the KA-U electrolyte, only a single redox peak corresponding to the I - /I 0 conversion reaction is observed. The PA-U electrolyte displays an additional irreversible redox peak associated with the I 0 /I + conversion reaction. Notably, the KA-PA-U electrolyte shows two distinct and reversible redox peaks, highlighting the synergistic effects of KA and PA. From the GCD profiles, the KA-U electrolyte only exhibits one pair of plateaus (Fig. 2e). In the PA-U electrolyte, a second charge plateau accompanied by a long tail emerges, attributed to the further oxidation of iodine, but the corresponding discharge plateau is negligible, indicating the irreversibility of the I 0 /I + conversion reaction. 10 Conversely, the KA-PA-U electrolyte presents two pairs of reversible plateaus, confirming the successful realization of reversible four-electron iodine conversion reactions. These GCD results are consistent with the CV data, proving the indispensable interplay among KA, PA, and U. By leveraging the reversible I 0 /I + conversion reaction, the KA-PA-U electrolyte achieves significantly improved stability and capacity (Supplementary Fig. 4). Moreover, the KA-PA-Nuc electrolyte standard was further demonstrated using other nucleophiles, including acetamide (AE), nicotinamide (NA), and pyridine (PD). In each case, reversible four-electron iodine conversion reactions can also be achieved under the synergistic contributions of PA and KA, indicating the broad applicability of the KA-PA-Nuc standard (Fig. 2c-d, Fig. 2f-g, and Supplementary Fig. 5-7). The role of polar anions To identify the indispensable role of polar anions in the KA-PA-Nuc standard, SO 4 2- was maintained as the baseline kosmotropic anion, while additional anions with varying polarities were introduced separately. The electrostatic potentials of three selected anions, including CF 3 SO 3 - , CF 3 COO - , and CH 3 COO - , were calculated by density functional theory (DFT) to visualize their polarities (Supplementary Fig. 8). Notably, these anions possess similar structures, effectively excluding the effects of spatial configuration and valence state. Compared to the electron withdrawing trifluoromethyl group in CF 3 SO 3 - and CF 3 COO - , the electron donating methyl group in CH 3 COO - increases the electron density on the opposite side of the anion, resulting in the polarity sequence CH 3 COO - ˃ CF 3 COO - ˃ CF 3 SO 3 - . According to the classical ion-shuttling model, ions shuttle through the electrolyte, accumulate at the electrode interface, and ultimately participate in electrochemical reactions under an applied electric field. 22,23 Among these processes, ions involved electrolyte-electrode interface critically determines the micro-environment of the electrochemical reaction. Molecular dynamics (MD) simulations were employed to visualize the behavior of anions with varying polarities at the electrolyte-electrode interface (Fig. 3d). As shown in Fig. 3e-f, introducing the strong polar CH 3 COO - into the baseline electrolyte significantly reduces the density of water molecules while increasing the density of anions at the interface. In contrast, the water molecule densities remain high with the addition of CF 3 COO - and CF 3 SO 3 - . These observations indicate that strong polar anions preferentially adsorb on the electrode surface, creating an anion-enriched, water-deficient electrolyte-electrode interface. As an experimental validation, in situ attenuated total reflection infrared (ATR-IR) spectroscopy was conducted to examine mass distribution at the electrolyte-electrode interface. The distinct peak located at 1640 cm -1 corresponds to the δ-HOH vibration of water molecules, where other peaks are associated with corresponding anions (Supplementary Fig.9). 24-27 For electrolytes containing CF 3 SO 3 - and CF 3 COO - , ATR-IR spectra exhibit prominent δ-HOH peaks, indicating substantial adsorption of water molecules at the interface (Fig.3g-h). Conversely, in the electrolyte with CH 3 COO - , a pronounced reduction in the δ-HOH peak intensity is observed alongside a significant increase in intensities of anion-related peaks, suggesting dramatically decreased water molecules and increased anion contents at the interface (Fig.3i). Furthermore, alternating current voltammetry was employed to ascertain the adsorption behavior at the electrolyte-electrode interface (Supplementary Fig.10). The calculated differential capacitances follow the trend: CH 3 COO - < CF 3 COO - < CF 3 SO 3 - , where anions with stronger polarity exhibit lower capacitance, reflecting the preferential adsorption of strong polar anions at the interface. 28 These results align with the MD simulations, confirming the capability of strong polar anions to establish an anion-enriched, water-deficient electrolyte-electrode interface. After clarification the effect of anion polarities on the electrolyte-electrode interface, the interplay between electrolyte-electrode interface and electrochemical performance was further correlated. In the electrolyte containing CF 3 SO 3 - , no distinct redox peak and charge/discharge plateaus corresponding to the I 0 /I + conversion reaction are observed in CV and GCD profiles, indicating that the I 0 /I + conversion reaction cannot be activated in this electrolyte environment (Fig. 3a and Supplementary Fig. 11a). Although the I 0 /I + conversion reaction can be activated in the presence of CF 3 COO - with a moderate polarity, the redox peak is highly irreversible, and the charge/discharge plateaus gradually diminish after several cycles, attributed to the water-enriched interface (Fig. 3a-b and Supplementary Fig. 11b). By contrast, the electrolyte containing strong polar CH 3 COO - achieves a reversible I 0 /I + conversion reaction with significantly improved stability and capacity, highlighting the pivotal role of the anion-enriched, water-deficient interface created by the strong polar anion in enabling reversible four-electron iodine conversion reactions (Fig. 3c and Supplementary Fig. 11c). The role of kosmotropic anions Building upon the understanding of polar anions, the critical role of kosmotropic anions in the KA-PA-Nuc standard was further explored. Within this standard, dissolved nucleophilic molecules with electronegative nitrogen or oxygen centers act as halogen bond acceptors by forming halogen bonds with generated I + cations during electrochemical reactions. However, these electronegative centers also extensively interact with water molecules via hydrogen bonding regarding high water content in low concentration electrolytes (Supplementary Fig. 12). Such Nuc-water interactions pose significant obstacles to achieving the reversible four-electron iodine conversion reaction, explaining its irreversibility in PA-Nuc electrolytes even with anion-enriched, water-deficient interfaces (Fig. 2). It has been observed that different anions have distinguishable impacts on the solubility of protein in aqueous solutions through molecular interactions among anions, water molecules, and polymer chains, which is known as the anion-specific effect (Supplementary Fig. 13). 29,30 Such effect has been studied in hydrogel fabrication. 31,32 However, its potential for regulating molecular interactions for aqueous electrolytes remains unexplored. Drawing inspiration from the anion-specific effect, five commonly used anions were selected (excluding halide anions) and systematically analyzed to elucidate their role in stabilizing electrochemical performances. The effect of these anions on electrochemical performance was systematically evaluated. Although the introduction of these anions into the same polar anion baseline didn’t affect the initial specific capacity, their cycling stabilities varied significantly (Supplementary Fig. 14). For example, the GCD profiles in the electrolyte containing ClO 4 - deteriorated rapidly, indicating inferior stability (Fig. 4c). Slight improvements in cycling stability were observed in electrolytes containing CF 3 SO 3 - or extra CH 3 COO - (Fig. 4b and Supplementary Fig. 15). Remarkably, electrolyte containing SO 4 2- demonstrated near-identical GCD profiles over repeated cycles, suggesting excellent stability (Fig. 4a). The stability of electrochemical performance follows the order: SO 4 2- ˃ CH 3 COO - ˃ CF 3 SO 3 - ˃ ClO 4 - , which correlates well with the anion-specific effect. Notably, NO 3 - deviates from this sequence, which may attribute to its instability under the applied voltage (Supplementary Fig. 16a-b). 33 To gain insight into the underlying mechanism behind the electrochemical performance, nuclear magnetic resonance spectroscopy (NMR) and MD simulations were performed to probe molecular interactions among anions, water molecules, and Nuc molecules. Interactions between anions and water molecules were characterized using 2 H NMR spectrum (Fig. 4d). The 2 H peaks shifted gradually to higher chemical shifts (lower fields) in the sequence SO 4 2- ˃ CH 3 COO - ˃ NO 3 - ˃ CF 3 SO 3 - ˃ ClO 4 - , indicating enhanced de-shielding effect and stronger hydrogen bonds. This trend reflects the anion-specific effect, where chaotropic anions (right side) weakly coordinate with water molecules, while kosmotropic anions strongly coordinate with water molecules. 34,35 This distinction in water-anion coordination will significantly affect water-Nuc interactions. For 15 N spectra, peaks for Nuc molecules shifted to higher chemical shifts following the same order (SO 4 2- ˃ CH 3 COO - ˃ NO 3 - ˃ CF 3 SO 3 - ˃ ClO 4 - ), indicating enhanced de-shielding effect (Supplementary Fig. 19). This de-shielding effect is closely related to hydrogen bonds between N atoms and water molecules. Kosmotropic anions like SO 4 2- has the strong coordination with water molecules that can polarize the hydration water molecules, which destabilizes hydrogen bonds between Nuc molecules and their hydration water, resulting in electron density reduction around N atoms. 30 Conversely, chaotropic anions like ClO 4 - directly interfere with the hydrophobic backbone of Nuc molecules, which adds extra charge and facilitates hydrogen bonds between Nuc molecules and their hydration water, thereby increasing in electron density around N atoms. 18 These varying interactions were further evidenced by the differing solubilities of Nuc molecules in the presence of different anions (Supplementary Fig. 20). MD simulations also confirmed that kosmotropic anions significantly reduced water-Nuc interactions, highlighting their critical role in modulating molecular interactions (Supplementary Fig. 21). In addition, chemical shifts of 1 H and 13 C peaks showed an inverse order (SO 4 2- < CH 3 COO - < NO 3 - < CF 3 SO 3 - < ClO 4 - ), validating interactions between chaotropic anions and the Nuc backbone (Fig. 4e-f, Supplementary Fig. 17, and Supplementary Fig. 18). Corresponding 2D 1 H- 15 N heteronuclear multiple bond correlation (HMBC) spectra also exhibited similar trends in chemical shifts for both 1 H and 15 N peaks, further proving above findings (Fig. 4g-i and Supplementary Fig. 16c). As a result, these results highlight the critical role of kosmotropic anions, particularly SO 4 2- , in disrupting water-Nuc interactions and stabilizing four-electron iodine conversion reactions. Understanding the interfacial electrochemical behavior The conversion chemistry of iodine is driven by interfacial reactions, visualizing these interfacial processes is crucial for a comprehensive understanding of interfacial electrochemical behavior. To gain deeper insight, particularly into the synergistic contributions of KA and PA to the interfacial electrochemical behavior, in situ electrochemical impedance spectroscopy (EIS), the distribution of relaxation times (DRT), and in situ Raman spectroscopy were conducted, followed by a detailed analysis. DRT is an effective technique to correlate relaxation times with characteristic electrochemical processes by deconvoluting EIS data. 36 Typically, relaxation times (τ) in the range of 10 -6 -10 -4 s correspond to the intrinsic impedance (τ 1 ). Leveraging low concentration electrolytes with high ionic conductivities, all systems exhibit minimal intrinsic impedances (Fig. 5a-c). Relaxation times within the range of 10 0 -10 2 s reflect ion (mass) diffusion impedance (τ 5 ). 37,38 During a full charge-discharge cycle, the intensity of τ 5 peaks gradually increases at the low-voltage region, suggesting a rise in ion diffusion impedance (Supplementary Fig. 22). As the voltage progresses toward the high-voltage region, the intensity of τ 5 peaks gradually decreases, indicating reduced ion diffusion impedance. Theoretically, a successful four-electron iodine conversion reaction involves a transition from ionic iodine species to solid iodine and then to ionic iodine species at the electrolyte-cathode interface. These transitions are well captured by the observed shifts in ion impedance. Notably, throughout this process, the KA (SO 4 2- )-PA (CH 3 COO - )-Nuc electrolyte demonstrates the lowest impedance and the best reversibility, underscoring the synergistic contributions of KA and PA in stabilizing the interfacial electrochemical behavior of the four-electron iodine conversion reaction. In the mid-frequency region (10 -4 -10 0 s), interfacial charge transfer impedance (τ 2 -τ 4 ), including the cathode-electrolyte interface and the electron transfer process of redox species, exhibits notable variations. DRT plots in the electrolyte without PA present two distinct peaks (τ 3 and τ 4 ) at this region, while electrolytes containing PA exhibit three prominent peaks (τ 2 , τ 3 , and τ 4 ). The additional τ 2 peak is attributed to the cathode-electrolyte interface, induced by the preferential adsorption of PA, further demonstrating the critical role of PA in facilitating an anions-rich interface (Fig. 5a-c). 39 The τ 3 and τ 4 peaks in electrolytes containing PA are greatly affected by voltage and become more pronounced at higher voltages, indicating a successful multi-electron charge transfer reaction. 40 In contrast, the τ 3 and τ 4 peaks in the KA (SO 4 2- )-CF 3 SO 3 - -Nuc electrolyte gradually merge into a single peak at high voltages (Fig. 5c). Notably, compared with the CF 3 SO 3 - -PA (CH 3 COO - )-Nuc electrolyte, the KA (SO 4 2- )-PA (CH 3 COO - )-Nuc electrolyte exhibits lower intensity peaks, suggesting a more complete conversion reaction, leading to enhanced stability and reversibility. Furthermore, in situ Raman spectroscopy was performed to track the evolution of iodine species. An obvious peak representing NA-I-NA complex appears upon charging to 1.5 V, while this peak gradually diminishes during subsequent discharging process, confirming the reversible conversion between I 0 and I + species in the electrolyte with both KA and PA (Fig. 5d-f). These findings highlight the synergistic contributions of KA and PA in keeping stable and reversible interfacial electrochemical behaviors. Electrochemical performances of four-electron aqueous metal-iodine batteries Following the elucidation of the synergistic mechanism of the KA-PA-Nuc standard, its broad applicability in four-electron aqueous metal-iodine batteries was investigated. Aqueous Zn-I 2 batteries were firstly assembled using zinc anodes. As shown in Supplementary Fig. 23a, CV profiles in the KA-PA-Nuc electrolyte reveal two distinct redox peaks with well-maintained shapes across increasing scan rates (0.1 to 1 mV s -1 ), indicating reversible four-electron iodine conversion reactions and excellent conversion kinetics. 9 To demonstrate the superiority of the KA-PA-Nuc electrolyte, the widely used high-concentration ZnCl 2 electrolyte was selected as a representative halogen-containing counterpart for comparison. The Zn-I 2 batteries in the KA-PA-Nuc electrolyte exhibit superior rate performances, showing higher reversible capacities of 487, 459, 422, 362, 324, and 299 mAh g -1 at current densities of 0.5, 1, 2, 4, 6, and 8 A g -1 , respectively (Supplementary Fig. 23b). In contrast, batteries in the 30m ZnCl 2 electrolyte show lower capacities with significant capacity decline, especially at low current densities, indicating irreversible reactions despite the high electrolyte concentration. Furthermore, corresponding GCD profiles also display improved reversibility and stability at various current densities compared to the ZnCl 2 system (Supplementary Fig. 23c and Supplementary Fig. 24). Long-term cycling tests further highlight the performance benefits of the KA-PA-Nuc electrolyte. Zn-I 2 batteries in the KA-PA-Nuc electrolyte maintained a high capacity of 394 mAh g -1 with a capacity retention of 93% after 1,000 cycles at a current density of 1 A g -1 (Fig. 6a). In comparison, batteries in the 30m ZnCl 2 electrolyte shows rapid capacity degradation, retaining only 42% of their initial capacity. The reliability of four-electron aqueous Zn-I 2 batteries with the KA-PA-Nuc electrolyte was also examined at a higher current density of 2 A g -1 , achieving an outstanding capacity retention of 94% over 2,000 cycles, corresponding to an extremely low capacity decay rate of 0.003% per cycle (Supplementary Fig. 25). Notably, this work represents the first successful demonstration of reversible four-electron iodine conversion reactions with remarkable electrochemical performance in halogen-free, low-concentration electrolytes, outperforming most existing aqueous metal-I 2 batteries that employing high-concentration, halogen-containing electrolytes (Supplementary Table 1-2). To further validate the application of the KA-PA-Nuc standard, pouch cells were assembled (Supplementary Fig. 26). The pouch cell delivers an initial capacity of 184.9 mAh, retaining a large capacity of 114.9 mAh after 500 cycles (Fig. 6c). To extend the application of the KA-PA-Nuc standard to other aqueous metal-iodine systems, aqueous Al-I 2 batteries were assembled using Al-Zn alloys as anodes. These batteries present two pairs of reversible plateaus associated with four-electron iodine conversion reactions in the KA-PA-Nuc electrolyte (Supplementary Fig. 27). Long-term cycling tests demonstrate a reversible capacity of 346 mAh g -1 with a capacity retention of 82% after 1000 cycles, significantly outperforming the only reported four-electron aqueous Al-I 2 battery that utilized a high-concentration, halogen-containing electrolyte (Fig. 6b). 12 These results validate the broad applicability of the KA-PA-Nuc standard in advancing four-electron aqueous metal-iodine batteries. To assess the practical potential of four-electron aqueous metal-iodine batteries utilizing the KA-PA-Nuc standard, a systematic techno-economic analysis was conducted for the first time (see Supplementary Information for details). 41 Figure 6d-e show the cost structure of four-electron aqueous Zn-I 2 batteries (4e ZIBs) using different electrolytes. Compared to 4e ZIB employing the 30 m ZnCl 2 electrolyte (425 USD KWh -1 ), the overall cell cost of 4e ZIB in the KA-PA-Nuc electrolyte (67 USD KWh -1 ) is reduced by 6.34 times, primarily due to the elimination of costly high-concentration halogen electrolytes and expensive titanium current collectors. Figure 6f compares the cell costs of various battery systems, where the overall cell cost of 4e ZIB in the KA-PA-Nuc electrolyte is significantly lower than that of commonly used Li-ion batteries (LIBs) chemistries and state-of-the-art K-ion battery (KIB) chemistry. 42 Zinc is an ideal anode material due to its high specific capacity (819 mAh g -1 ) and low cost (2.36 USD kg -1 ). Although the negative-to-positive capacity ratio of 4e ZIB (2.00) is higher than that of LIB (1.02), zinc remains a more economical anode option (Supplementary Table 3). Meanwhile, the cathode cost for 4e ZIB is also significantly lower than that of LIB, benefiting from iodine’s lower unit cost (0.80 USD kg -1 ) and high theoretical capacity (422 mAh g -1 ) enabled by four-electron conversion chemistry. Compared to LIB, where the cathode and anode account for 74.96% of the overall cell cost, the corresponding cost contribution in 4e ZIB is only 20.1% (Supplementary Table 4). It is worth noting that 4e ZIB in the KA-PA-Nuc electrolyte still incurs a higher current collector cost due to the use of stainless steel, which remains relatively expensive (Fig 6d). Therefore, 4e ZIB in the KA-PA-Nuc electrolyte still has huge potential in further cost reduction by adopting more cost-effective current collectors, highlighting its immense economic benefit and commercial value for cost-effective and sustainable energy storage solutions. Discussion In summary, we have developed a new and universal electrolyte paradigm leveraging anionic chemistry, termed KA-PA-Nuc, for aqueous metal-iodine batteries. Comprehensive experimental and computational analyses reveal that PA facilitates a water-deficient, anion-enriched interface, while KA disrupts hydrogen bonding between nucleophilic species and their hydration shells, thereby guaranteeing stable halogen bonds of I + cations. As a result, this electrolyte paradigm spearheaded achieves reversible multi-electron iodine conversion chemistry in halogen-free electrolytes at an exceptionally low concentration (2.4 moles), eliminating the reliance on high-concentration, corrosive halogen-containing electrolytes. A systematic techno-economic analysis further reveals that the overall cell cost of aqueous metal-iodine batteries using the KA-PA-Nuc electrolyte is reduced by 6.34 times (from 425 USD KWh -1 to 67 USD KWh -1 ), making it more cost-effective than conventional Li-ion batteries. This work not only enhances the practicality of aqueous metal-iodine batteries but also provides a versatile electrolyte principle, which will arouse new inspiration and pave the way for cost-effective and sustainable energy storage solutions. Declarations Acknowledgements This research was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/V027433/3), UK Research and Innovation (UKRI) under the UK government‘s Horizon Europe funding (101077226; EP/Y008707/1), EPSRC Centre for Doctoral Training in Molecular Modelling and Materials Science (EP/L015862/1). We would like to acknowledge the BatPaC for batteries cost calculation. Author contributions R.W.C. and G.J.H. conceived this work. R.W.C. designed and conducted the experiments. R.W.C. wrote the manuscript. G.J.H. supervised the research and edited the manuscript. H.B.D. performed the theoretical calculations and corresponding analysis. Y.H.D., J.Y.W., J.C., J.Y.L., J.R.F., Y.P.Z., and W.Z. helped with measurements and data analysis. W.Z., H.Y., and Z.C. helped with materials synthesis and electrodes fabrication. Competing interests. The authors declare no competing interests. Additional information Data supporting the findings of this study are available from the Supplementary Information. References Liang, Y., Yao, Y. Designing modern aqueous batteries. Nature Reviews Materials 8 , 109-122 (2022). Xu, C. et al. Practical high-energy aqueous zinc-bromine static batteries enabled by synergistic exclusion-complexation chemistry. Joule 8 , 461-481 (2024). Liu, T. et al. Aqueous electrolyte with weak hydrogen bonds for four-electron zinc-iodine battery operates in a wide temperature range. Adv Mater 36 , e2405473 (2024). Li, X., Xu, W., Zhi, C. Halogen-powered static conversion chemistry. Nat Rev Chem 8 , 359-375 (2024). Hao, J. et al. Advanced cathodes for aqueous Zn batteries beyond Zn 2+ intercalation. Chem Soc Rev 53 , 4312-4332 (2024). Liu, T. et al. Practical four-electron zinc-iodine aqueous batteries enabled by orbital hybridization induced adsorption-catalysis. Sci Bull 69 , 1674-1685 (2024). Zhang, S. J. et al. 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Development of rechargeable high-energy hybrid zinc-iodine aqueous batteries exploiting reversible chlorine-based redox reaction. Nat Commun 14 , 1856 (2023). Du, H. L. et al. Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency. Nature 609 , 722-727 (2022). Wagner, A., Sahm, C. D., Reisner, E. Towards molecular understanding of local chemical environment effects in electro- and photocatalytic CO 2 reduction. Nature Catalysis 3 , 775-786 (2020). Hua, M. et al. Strong tough hydrogels via the synergy of freeze-casting and salting out. Nature 590 , 594-599 (2021). Wu, S. et al. Poly(vinyl alcohol) hydrogels with broad‐range tunable mechanical properties via the hofmeister effect. Advanced Materials 33 (2021). Guan, D. et al. π-d conjugated coordination mediated catalysis for four-electron-transfer fast-charging aqueous zinc-iodine batteries. Matter (2024). Zhang, F., Wang, Q. & Tang, Y. Extended iodine chemistry: Toward high-energy-density aqueous zinc-ion batteries. Matter 4 , 2637-2639 (2021). Ward, J. S., Fiorini, G., Frontera, A. & Rissanen, K. Asymmetric [N-I-N] + halonium complexes. Chem Commun (Camb) 56 , 8428-8431 (2020). Dai, Y. et al. Zn 2+ -mediated catalysis for fast-charging aqueous Zn-ion batteries. Nature Catalysis 7 , 776-784 (2024). Yang, H. et al. Reunderstanding aqueous Zn electrochemistry from interfacial specific adsorption of solvation structures. Energy & Environmental Science 16 , 2910-2923 (2023). Zhu, Y., Hao, J., Huang, Y., Jiao, Y. A new insight of anti‐solvent electrolytes for aqueous zinc‐ion batteries by molecular modeling. Small Structures 4 (2023). Liu, X. et al. Understand the effect of the confinedtrifluoromethane sulfonate (OTf − ) anions by the adjacent MXene nanosheets on oriented design of Zn ion storage. Carbon 219 (2024). Wu, M. et al. Highly reversible and stable Zn metal anodes realized using a trifluoroacetamide electrolyte additive. Energy & Environmental Science 17 , 619-629 (2024). Wang, Y., Li, Y., Zhou, Z., Zu, X., Deng, Y. Evolution of the zinc compound nanostructures in zinc acetate single-source solution. Journal of Nanoparticle Research 13 , 5193-5202 (2011). Luo, J. et al. Regulating the inner Helmholtz plane with a high donor additive for efficient anode reversibility in aqueous Zn-ion batteries. Angew Chem Int Ed Engl 62 , e202302302 (2023). Gregory, K. P., Wanless, E. J., Webber, G. B., Craig, V. S. J., Page, A. J. The electrostatic origins of specific ion effects: quantifying the Hofmeister series for anions. Chem Sci 12 , 15007-15015 (2021). Jaspers, M., Rowan, A. E., Kouwer, P. H. J. Tuning hydrogel mechanics using the Hofmeister effect. Advanced Functional Materials 25 , 6503-6510 (2015). Zou, H., Meng, X., Zhao, X., Qiu, J. Hofmeister effect-enhanced hydration chemistry of hydrogel for high-efficiency solar-driven interfacial desalination. Adv Mater 35 , e2207262 (2023). Ren, J. et al. Super-tough, non-swelling zwitterionic hydrogel sensor based on the Hofmeister effect for potential motion monitoring of marine animals. Adv Mater 36 , e2412162 (2024). Zhou, L. et al. Two-dimensional Cu plates with steady fluid fields for high-rate nitrate electroreduction to ammonia and efficient Zn-nitrate Batteries. Angew Chem Int Ed Engl 63 , e202401924 (2024). Huang, S., Hou, L., Li, T., Jiao, Y., Wu, P. Antifreezing hydrogel electrolyte with ternary hydrogen bonding for high-performance zinc-ion batteries. Adv Mater 34 , e2110140 (2022). Kou, R., Zhang, J., Wang, T., Liu, G. Interactions between polyelectrolyte brushes and hofmeister ions: Chaotropes versus Kosmotropes. Langmuir 31 , 10461-10468 (2015). Chen, J., Quattrocchi, E., Ciucci, F., Chen, Y. Charging processes in lithium-oxygen batteries unraveled through the lens of the distribution of relaxation times. Chem 9 , 2267-2281 (2023). Huang, J. D., Meisel, C., Sullivan, N. P., Zakutayev, A. & O’Hayre, R. Rapid mapping of electrochemical processes in energy-conversion devices. Joule 8 , 2049-2072 (2024). Hu, T. et al. Development of inverse-opal-structured charge-deficient Co 9 S 8 @nitrogen-doped-carbon to catalytically enable high energy and high power for the two-electron transfer I + /I - electrode. Adv Mater 36 , e2312246 (2024). Sun, S. et al. Eliminating interfacial O-involving degradation in Li-rich Mn-based cathodes for all-solid-state lithium batteries. Science advances 8 , eadd5189 (2022). Wang, H. et al. Accelerating sulfur redox kinetics by electronic modulation and drifting effects of pre-lithiation electrocatalysts. Adv Mater 36 , e2307741 (2024). Knehr, K. W., Kubal, J. J., Nelson, P. A., Ahmed, S. Battery performance and cost modeling for electric-drive vehicles (a manual for BatPaC v5. 0). (Argonne National Lab.(ANL), Argonne, IL (United States), 2022). Dhir, S., Wheeler, S., Capone, I., Pasta, M. Outlook on K-ion batteries. Chem 6 , 2442-2460 (2020). Methods Preparation of electrolytes All chemicals were purchased from Sigma-Aldrich and used as received. The KA-PA-Nuc electrolytes were prepared by dissolving KA, PA, and Nuc into deionized water at room temperature. Specifically, KA was 1.5 M ZnSO 4 and PA was 0.5 M Zn(CH 3 COO) 2 for aqueous Zn-I 2 batteries. To demonstrated broad applicability of the KA-PA-Nuc standard, various Nuc were employed, including 2 M urea, 2 M acetamide, 0.2 M pyridine, or 0.4 M nicotinamide were employed as Nuc. To explore the function of polar anions, a baseline electrolyte containing 1.5 M ZnSO 4 and 0.4 M nicotinamide was established. Subsequently, Zn(CH 3 COO) 2 , Zn(CF 3 SO 3 ) 2 , or Zn(CF 3 SO 3 ) 2 (at a concentration of 0.5 M) were added to the baseline for comparative analysis. To evaluate the role of kosmotropic anions, a baseline electrolyte containing 0.5 M Zn(CH 3 COO) 2 and 0.4 M nicotinamide was prepared. Various salts, including ZnSO 4 , Zn(NO 3 ) 2 , Zn(CF 3 SO 3 ) 2 , or Zn(ClO 4 ) 2 (at a concentration of 1.5 M), were then dissolved into the baseline. For the application of the KA-PA-Nuc standard in aqueous Al-I 2 batteries, the electrolyte was prepared by dissolving 1.5 M Al 2 (SO 4 ) 3 , 0.5 Mg(CH 3 COO) 2 , and 0.5 M nicotinamide into deionized water. Preparation of electrodes The evaporation-adsorption method was used to prepare iodine-activated carbon (AC) material. In brief, 0.5 g iodine was ground with 0.5 g AC for 10 min. The resulting mixture was sealed in a Teflon reactor and heated at 120 ℃ for 6 h. For the preparation of iodine cathode, the synthesized I 2 -AC material, polytetrafluoroethylene and Super P were mixed in deionized water with a mass ratio of 8:1:1. Then the slurry was pressed onto a titanium mesh, followed by drying in an oven at 60 ℃ for 3 h. The areal iodine loading was controlled by adjusting the thickness of slurry film on the titanium mesh. Commercial Zn foils were used as received, without further treatment. To assess the potential of the KA-PA-Nuc electrolyte standard in multi-electron aqueous Al-I 2 batteries, AC electrodes were prepared as both reference and counter electrodes. Specifically, AC, polytetrafluoroethylene and Super P were mixed in deionized water with a mass ratio of 8:1:1. The resulting slurry was then pressed onto a titanium mesh and dried in an oven at 60 ℃ for 6 h. By adjusting the electrode thickness, the capacity of AC electrode was controlled to be much higher than that of the cathode, ensuring its capability as a stable reference and counter electrode for evaluating the electrochemical performance of the KA-PA-Nuc electrolyte. To further validate the applicability of the KA-PA-Nuc electrolyte standard in multi-electron aqueous Al-I 2 batteries, Al-Zn alloys were fabricated as anodes following a previous work. 43 Briefly, symmetric cells were assembled using Zn foils as electrodes and 2M Al(CF 3 SO 3 ) 3 as the electrolyte. During charging, Al 3+ in the electrolyte deposited on and reacted with the metal Zn substrate, forming the Zn-supported Al-Zn alloys. Materials characterizations In situ ATR-IR measurements: In situ attenuated total reflection infrared (ATR-IR) spectra were recorded by a Thermo Scientific Nicolet iS50 spectrometer equipped with a liquid nitrogen-cooled MCT-A detector. Gold-coated silicon plates were employed as working electrodes, with Zn foils and glass fiber used as reference electrodes and separators, respectively. During testing, the assembled batteries were connected to a Gamry Interface 1000 potentiostat to run a galvanostatic charge-discharge (GCD) program. All spectra were collected at a resolution of 4 cm -1 , with 50 scans per measurement. In situ Raman measurements: Raman spectra were recorded using a Bruker Senterra II Raman spectrometer with a 532 nm excitation wavelength. The iodine-AC cathodes, Zn foils, and glass fiber served as the working electrodes, reference electrodes, and separators, respectively. Raman spectra were collected during GCD cycling using an external Gamry Interface 1000 potentiostat. Fourier-transform infrared (FTIR) spectra of various electrolytes were conducted on an ATR-FTIR (BRUKER, platinum-ATR). Nuclear magnetic resonance (NMR) analyses, including 1 H, 13 C, and 1 H- 15 N HMBC spectra, were carried out on a Bruker NMR instrument (Avance Neo 500). 15N spectra were collected using a Bruker NMR instrument (Avance Neo 700). Deuterium oxide was used as the solvent for all NMR measurements. Electrochemical measurements Coin cell configuration: Electrochemical studies were performed in CR2025 coin cells. For aqueous Zn-I 2 batteries, the synthesized iodine-AC electrodes served as cathodes, Zn foils were used as anodes, and Whatman GF/D glass fiber was utilized as separators. For aqueous Al-I 2 batteries, as fabricated Al-Zn alloys were used as anodes. Pouch cell configuration: For pouch cells, large-area iodine-AC electrodes with a slurry film thickness of 30 µm were employed as cathodes, Zn foils and Whatman GF/D glass fiber were utilized as anodes and separators, respectively. The electrolyte consisted of 1.5 M ZnSO 4 , 0.5 M Zn(CH 3 COO) 2 , and 0.4 M nicotinamide. Titanium foils were connected to electrodes as tabs. After lamination, the assembled batteries were encapsulated in an aluminum-plastic films, and subjected to controlled pressure using a customized model during electrochemical measurements. GCD tests were conducted at room temperature using a Neware battery test system. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) measurements were performed using a Biologic VMP-3 potentiostat. Distribution relaxation times (DRT) were calculated from EIS data using MatlabR2019 with the DRT-tool developed by Professor Francesco Ciucci's research group. Theoretical calculations All the molecular dynamics (MD) simulations were performed using Forcite package. Here, we keep the number of water molecules constant and then adjust the ion species to obtain electrolytes of different ions. For each MD simulation, COMPASSIII force field and NVT ensemble were chosen, 44-46 and the temperature was controlled using a Nosé thermostat with a target temperature of 298 K. A time step of 0.01 ps was chosen, and the total number of steps was set as 5000000. For the calculation of the electrostatic potential, preoptimizations and the stationary point calculations were carried out using M06-2X/6-311+G(2df,2p) 47 for all the selected configurations using the Gaussian 16 software. The density difference was calculated with a periodic slab model using the Vienna ab initio simulation program (VASP). 48-50 The generalized gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. 51 The projector-augmented wave (PAW) method was utilized to describe the electron-ion interactions, and the cut-off energy for the plane-wave basis set was 450 eV. 48 Brillouin zone integration was accomplished using 6×6×6 Monkorst-Pack k-point mesh. Data availability The data that support the findings of this study have been included in the main text and the Supplementary Information. They are available from the corresponding author upon reasonable request. References Yan, C. et al. Architecting a stable high-energy aqueous Al-ion battery. J Am Chem Soc 142 , 15295-15304 (2020). Sun, H., Ren, P., Fried, J. R. The COMPASS force field: parameterization and validation for phosphazenes. Computational and Theoretical Polymer Science 8 , 229-246 (1998). Akkermans, R. L. C., Spenley, N. A., Robertson, S. H. COMPASS III: automated fitting workflows and extension to ionic liquids. Molecular Simulation 47 , 540-551 (2020). Sun, H. et al. COMPASS II: extended coverage for polymer and drug-like molecule databases. Journal of Molecular Modeling 22 , 47 (2016). Zhao, Y., Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06 functionals and 12 other functionals. Theoretical Chemistry Accounts 119 , 525-525 (2008). Kresse, G., Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Physical Review B 59 , 1758-1775 (1999). Kresse, G., Hafner, J. Ab initio molecular dynamics for liquid metals. Physical Review B: Condensed Matter and Materials Physics 47 , 558-561 (1993). Kresse, G., Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B: Condensed Matter and Materials Physics 54 , 11169-11186 (1996). Perdew, J. P., Burke, K., Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys Rev Lett 77 , 3865-3868 (1996). Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6372434","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":461076176,"identity":"acd6b658-1a45-44a0-a977-e8a4a31b06bb","order_by":0,"name":"Guanjie 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electrolytes.\u003c/strong\u003e a, Mechanism of the competition between halogen bond and hydrogen bond. b, Schematic illustration of the working mechanism in high-concentration, halogen-containing electrolytes. c, Schematic illustration of the working mechanism in proposed low-concentration, halogen-free electrolytes, showcasing the role of anionic chemistry. d, Polarization curves in different electrolytes. e, Comparison of the cost and the concentration of reported electrolytes in aqueous metal-I\u003csub\u003e2\u003c/sub\u003e batteries. f, Comparison of the electrolyte cost and electrochemical performances of reported aqueous metal-I\u003csub\u003e2\u003c/sub\u003e batteries.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/8b664b6828692eaa0efcce98.png"},{"id":83536511,"identity":"94559710-0c7e-499a-98dc-ea43d56658cb","added_by":"auto","created_at":"2025-05-28 06:49:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":196638,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eVersatility of proposed KA-PA-Nuc electrolyte standard.\u003c/strong\u003e a, Molecular interplay governed by anionic chemistry and halogen bonding. b, Cyclic voltammetry (CV) profiles of iodine conversion reaction using urea as Nuc at 0.6 mV s\u003csup\u003e-1\u003c/sup\u003e. c, CV profiles using acetamide as Nuc at 0.6 mV s\u003csup\u003e-1\u003c/sup\u003e. d, CV profiles using nicotinamide as Nuc at 0.6 mV s\u003csup\u003e-1\u003c/sup\u003e. e-g, Corresponding galvanostatic charge-discharge (GCD) profiles for b-d at 1 A g\u003csup\u003e-1\u003c/sup\u003e, respectively.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/2096c8c9090d4eb7e8ad159f.png"},{"id":83536510,"identity":"83c9acac-41eb-45a4-86db-487bf03d2e03","added_by":"auto","created_at":"2025-05-28 06:49:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":291280,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCritical role of polar anions.\u003c/strong\u003e a, CV profiles in electrolytes with different anions. b, GCD profiles in electrolytes with different anions. c, Cycling stability in electrolytes with different anions. d, Schematic snapshot of the interfacial structure in the electrolyte with CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e. e, Corresponding interfacial cumulated density profiles of H\u003csub\u003e2\u003c/sub\u003eO in electrolytes with different anions. f, Corresponding interfacial cumulated density profiles of anions in electrolytes with different anions. g-i, \u003cem\u003eIn situ\u003c/em\u003e ATR-IR spectra in the electrolytes with CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/5ad4fd3d175be03dc578db38.png"},{"id":83536512,"identity":"3d04c344-e780-45f1-afde-2b1d1cfc1523","added_by":"auto","created_at":"2025-05-28 06:49:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":194088,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCritical role of kosmotropic anions.\u003c/strong\u003e a, GCD profiles in the electrolyte containing SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e at different cycles. b, GCD profiles in the electrolyte containing CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e at different cycles. c, GCD profiles in the electrolyte containing ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e at different cycles. d, \u003csup\u003e2\u003c/sup\u003eH NMR spectra of electrolytes containing different anions. e, \u003csup\u003e1\u003c/sup\u003eH NMR spectra of electrolytes containing different anions. f, \u003csup\u003e13\u003c/sup\u003eC NMR spectra of electrolytes containing different anions. g, 2D \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e15\u003c/sup\u003eN HMBC NMR spectrum of the electrolyte containing SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e. h, 2D \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e15\u003c/sup\u003eN HMBC NMR spectrum of the electrolyte containing CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. i, 2D \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e15\u003c/sup\u003eN HMBC NMR spectrum of the electrolyte containing ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/ebe9d1443d8e6ac4cb18566e.png"},{"id":83536513,"identity":"4e6ce75b-eb33-43f3-ae6b-a0719b1cfe39","added_by":"auto","created_at":"2025-05-28 06:49:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":246312,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCharacterizations of interfacial electrochemical behaviors.\u003c/strong\u003e a, DRT profiles in the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte at different states. b, DRT profiles in the CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte without KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e) at different states. c, DRT profiles in the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-Nuc electrolyte without PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e) at different states. d, GCD profiles of in situ Raman test. e, In situ Raman spectroscopy in the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte. i, Corresponding contour plots of e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/c8f0e89ea513f9879905c70c.png"},{"id":83537393,"identity":"ce894ea3-5741-4bc5-90ba-592897f9c679","added_by":"auto","created_at":"2025-05-28 06:57:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":193613,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical performances of aqueous Zn-I\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and Al-I\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e batteries.\u003c/strong\u003e a, Cycling performance of Zn-I\u003csub\u003e2\u003c/sub\u003e batteries in 30m ZnCl\u003csub\u003e2\u003c/sub\u003e and KA-PA-Nuc electrolytes at 1 A g\u003csup\u003e-1\u003c/sup\u003e. b, Cycling performance of Al-I\u003csub\u003e2\u003c/sub\u003e batteries in the KA-PA-Nuc electrolyte at 1 A g\u003csup\u003e-1\u003c/sup\u003e. c, Cycling performance of Zn-I\u003csub\u003e2\u003c/sub\u003e pouch cell, inset shows corresponding digital image. d, Cost structure of 4e ZIB cell in the KA-PA-Nuc electrolyte. e, Cost structure of 4e ZIB cell in the 30 m ZnCl\u003csub\u003e2\u003c/sub\u003e electrolyte. f, Comparison of cell cost of different batteries.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/6d234012e08af62ace31d77e.png"},{"id":83537529,"identity":"5eba15de-07bf-43c8-9d71-ba2e11e0c60b","added_by":"auto","created_at":"2025-05-28 07:05:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2510214,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/ec1d73ce-d8f0-4fe7-9d89-91e905256625.pdf"},{"id":83536515,"identity":"f4325c0c-ea24-4d10-9844-d39a69c5e2be","added_by":"auto","created_at":"2025-05-28 06:49:08","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4471426,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6372434/v1/8d4f0e956a75ec2ba830f350.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe increasing demand from renewable energy sources, industry grids, and electronic devices call for reliable and sustainable electrochemical energy storage systems beyond lithium-ion batteries.\u003csup\u003e1\u003c/sup\u003e Alternative battery chemistries integrating multi-electron redox couples of earth-abundant elements with low-cost aqueous electrolytes show great promise for future large-scale energy storage applications.\u003csup\u003e2\u003c/sup\u003e Emerging aqueous metal batteries coupled with iodine cathodes are regarded as one of the most promising candidates, leveraging the cost-effectiveness and earth-abundance of metal anodes (e.g., Zn and Al) and iodine, along with multielectron conversion chemistry considering the diverse chemical valences of iodine.\u003csup\u003e3\u003c/sup\u003e Unlike cathodes with intercalation chemistry (e.g., V-based, Mn-based, and Prussian Blue analogues), which are constrained by the hosting capacity of intercalated charge carriers, conversion-type iodine cathodes achieve electron transfer by reversibly altering their chemical states, offering a notable capacity advantage over their intercalation counterparts, especially when multi-electron transfer is involved.\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe electrochemical performance of aqueous metal-iodine (I\u003csub\u003e2\u003c/sub\u003e) batteries is determined by the number of electron transfer or valence transition in iodine conversion chemistry. Substantial research has focused on the I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e redox couple with a two-electron transfer process, providing a theoretical capacity of 211 mAh g\u003csup\u003e-1\u003c/sup\u003e and a redox potential of 0.54 V vs. standard hydrogen electrode (SHE).\u003csup\u003e5\u003c/sup\u003e In contrast, the four-electron reaction involving successive I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e redox couples offers a duplicate theoretical capacity of 422 mAh g\u003csup\u003e-1\u003c/sup\u003e and a higher redox potential of 1.07 V vs. SHE, ensuring higher energy density and outperforming most prevailing intercalation-type cathodes.\u003csup\u003e6\u003c/sup\u003e However, the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion process generally suffers from poor reversibility, which is considerably inferior to that of the I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e conversion process due to the thermodynamic instability of I\u003csup\u003e+\u003c/sup\u003e cations.\u003csup\u003e3,7\u003c/sup\u003e Consequently, despite its potential, the development of reversible I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e redox couples remains in its infancy and continues to be a long-standing challenge.\u003c/p\u003e\n\u003cp\u003eThe crux of iodine conversion chemistry lies in interfacial reactions, where the electrolyte\u0026rsquo;s composition and interface profoundly influence the reaction pathways. The I\u003csup\u003e+\u003c/sup\u003e cation, being highly electrophilic due to its electron deficiency, is prone to hydrolysis in aqueous electrolytes due to nucleophilic attacks by electron-rich OH groups of water molecules, resulting in poor reversibility of the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion process.\u003csup\u003e8,9\u003c/sup\u003e In prior attempts, four-electron aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries were activated by stabilizing the hydrolysis of electrophilic I\u003csup\u003e+\u003c/sup\u003e via the formation of halogen-bond complexes (e.g., ICl\u003csub\u003e2\u003c/sub\u003e or IBr\u003csub\u003e2\u003c/sub\u003e) in intensive chloride- or bromide-based electrolytes.\u003csup\u003e10,11\u003c/sup\u003e Analogously, four-electron aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries have been demonstrated using concentrated AlCl\u003csub\u003e3\u003c/sub\u003e electrolytes.\u003csup\u003e12\u003c/sup\u003e In these systems, the I\u003csup\u003e+\u003c/sup\u003e cation functions as a halogen-bond donor, forming noncovalent halogen bonds with two nucleophilic species (Cl\u003csup\u003e-\u003c/sup\u003e or Br\u003csup\u003e-\u003c/sup\u003e) by accepting electrons in the lobes of its empty p-orbital.\u003csup\u003e2,13\u003c/sup\u003e However, in water-enriched interfaces, polar water molecules preferentially form hydrogen bonds with nucleophilic species, competing with and destabilizing halogen-bond complexes (Fig. 1a). To mitigate this, in all prior studies, high-concentration, halogen-containing electrolytes were generally employed to reduce water activities via intensive ion coordination and promote the population of free halogen ions (Fig. 1b).\u003csup\u003e10,14\u003c/sup\u003e Unfortunately, the use of such high-concentration electrolytes significantly increases costs, offsetting the primary economic advantage of aqueous systems. Moreover, halogen-containing electrolytes are highly corrosive to standard battery components, severely limiting their broader applicability (Fig. 1d and Supplementary Fig. 1). Therefore, the development of halogen-free electrolytes with low concentrations that can support reversible I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e redox chemistry is highly desirable but seems impossible due to the inherent inverse correlation between the water activity and the electrolyte concentration. Addressing this challenge will unlock the full potential of aqueous metal\u0026ndash;I\u003csub\u003e2\u003c/sub\u003e batteries, making them viable candidates for large-scale, cost-effective, and sustainable energy storage solutions.\u003c/p\u003e\n\u003cp\u003eGiving that the competition between halogen bonds and hydrogen bonds at water-enriched interfaces mainly contributes to the poor reversibility of the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion process, creating a water deficient electrolyte interface at low concentrations, while simultaneously manipulating hydrogen bonds between water molecules and nucleophilic species, has become the crux for overcoming this challenge. Salt anions not only affect the bulk water structure but also play a pivotal role in modulating micro-environment of the electrolyte-electrode interface.\u003csup\u003e15,16\u003c/sup\u003e Meanwhile, anion-specific-anion effect, namely the distinct interactions among anions, water molecules, and solutes at the molecular level, hold great potentials in manipulating hydrogen bonds between water molecules and nucleophilic species.\u003csup\u003e17,18\u003c/sup\u003e Herein, we propose a universal guideline leveraging anionic chemistry to achieve reversible I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e reaction in halogen-free, low concentration aqueous electrolytes. Systematic experimental characterizations and theoretical calculations reveal that strong-polar anions (PA) facilitate an anion-enriched, water-deficient electrolyte interface, while kosmotropic anions (KA) disrupt hydrogen bonds between nucleophilic species (Nuc) and their hydration shells, thereby stabilizing I\u003csup\u003e+\u003c/sup\u003e cation involved halogen bonds (Fig 1c). This novel KA-PA-Nuc electrolyte standard was validated across four nucleophilic species (urea, acetamide, pyridine, and nicotinamide) in aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e and Al-I\u003csub\u003e2\u003c/sub\u003e batteries, demonstrating its general applicability and effectiveness. For the first time, this electrolyte design enables outstanding and reversible I⁻/I⁰/I⁺ redox reactions in low-cost and halogen-free electrolytes at very low concentrations, outperforming most existing aqueous metal-I\u003csub\u003e2\u003c/sub\u003e batteries (Fig. 1e-f and Supplementary Table 1-2). By eliminating the dependence of hypervalent iodine conversion on high-concentration, expensive and corrosive halogen-containing electrolytes, this work will pave the way for cost-effective, scalable, and sustainable aqueous energy storage solutions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVersatile electrolyte standard for four-electron iodine conversion reactions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe hydrolysis of I\u003csup\u003e+\u003c/sup\u003e is the primary factor limiting the reversibility of the four-electron iodine conversion reaction in aqueous electrolytes. Previous studies relied on high-concentration, corrosive halogen-containing electrolytes to suppress I\u003csup\u003e+\u003c/sup\u003e hydrolysis by reducing water activity and releasing free halogen ions to form stable halogen complex (ICl\u003csub\u003e2\u003c/sub\u003e or IBr\u003csub\u003e2\u003c/sub\u003e), thereby mitigating the competition between hydrogen and halogen bonds.\u003csup\u003e19,20\u003c/sup\u003e In contrast, this work introduces a completely different insight to overcome this long-standing challenge. Specifically, strong polar anions feature preferential adsorption at the electrolyte-electrode interface under an electric field, establishing a water-deficient, anion-enriched micro-environment. Concurrently, kosmotropic anions polarize the hydration shells of nucleophilic species, breaking hydrogen bonds between these species and surrounding water molecules. This dual effect, grounded in anionic chemistry, releases more free nucleophilic species, which in turn form stable halogen bonds with I\u003csup\u003e+\u003c/sup\u003e cations (INuc\u003csub\u003e2\u003c/sub\u003e) within the water-deficient micro-environment (Fig. 2a and Supplementary Fig. 2). As a result, a new electrolyte standard, termed KA-PA-Nuc, is proposed, offering a more sustainable way for four-electron iodine conversion reactions.\u003c/p\u003e\n\u003cp\u003eAs a demonstration, the effectiveness of KA-PA-Nuc electrolyte standard was validated in aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries across four different nucleophilic species (Supplementary Fig. 3). Theoretically, the I\u003csup\u003e+\u003c/sup\u003e cation acts as a strong halogen bond donor, capable of interacting with two nucleophilic molecules containing electronegative centers, such as nitrogen or oxygen (halogen bond acceptor), to form linear [Nuc-I-Nuc]\u003csup\u003e+\u003c/sup\u003e halogen bonds. These halogen bonds are the foundation of four-electron iodine conversion reactions.\u003csup\u003e21\u003c/sup\u003e Fig. 2b shows the CV profiles with urea (U) as the nucleophilic species. In the KA-U electrolyte, only a single redox peak corresponding to the I\u003csup\u003e-\u003c/sup\u003e/I\u003csup\u003e0\u003c/sup\u003e conversion reaction is observed. The PA-U electrolyte displays an additional irreversible redox peak associated with the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction. Notably, the KA-PA-U electrolyte shows two distinct and reversible redox peaks, highlighting the synergistic effects of KA and PA. From the GCD profiles, the KA-U electrolyte only exhibits one pair of plateaus (Fig. 2e). In the PA-U electrolyte, a second charge plateau accompanied by a long tail emerges, attributed to the further oxidation of iodine, but the corresponding discharge plateau is negligible, indicating the irreversibility of the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction.\u003csup\u003e10\u003c/sup\u003e Conversely, the KA-PA-U electrolyte presents two pairs of reversible plateaus, confirming the successful realization of reversible four-electron iodine conversion reactions. These GCD results are consistent with the CV data, proving the indispensable interplay among KA, PA, and U. By leveraging the reversible I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction, the KA-PA-U electrolyte achieves significantly improved stability and capacity (Supplementary Fig. 4). Moreover, the KA-PA-Nuc electrolyte standard was further demonstrated using other nucleophiles, including acetamide (AE), nicotinamide (NA), and pyridine (PD). In each case, reversible four-electron iodine conversion reactions can also be achieved under the synergistic contributions of PA and KA, indicating the broad applicability of the KA-PA-Nuc standard (Fig. 2c-d, Fig. 2f-g, and Supplementary Fig. 5-7).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThe role of polar anions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo identify the indispensable role of polar anions in the KA-PA-Nuc standard, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e was maintained as the baseline kosmotropic anion, while additional anions with varying polarities were introduced separately. The electrostatic potentials of three selected anions, including CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, and CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, were calculated by density functional theory (DFT) to visualize their polarities (Supplementary Fig. 8). Notably, these anions possess similar structures, effectively excluding the effects of spatial configuration and valence state. Compared to the electron withdrawing trifluoromethyl group in CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, the electron donating methyl group in CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e increases the electron density on the opposite side of the anion, resulting in the polarity sequence CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e ˃ CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e ˃ CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAccording to the classical ion-shuttling model, ions shuttle through the electrolyte, accumulate at the electrode interface, and ultimately participate in electrochemical reactions under an applied electric field.\u003csup\u003e22,23\u003c/sup\u003e Among these processes, ions involved electrolyte-electrode interface critically determines the micro-environment of the electrochemical reaction. Molecular dynamics (MD) simulations were employed to visualize the behavior of anions with varying polarities at the electrolyte-electrode interface (Fig. 3d). As shown in Fig. 3e-f, introducing the strong polar CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e into the baseline electrolyte significantly reduces the density of water molecules while increasing the density of anions at the interface. In contrast, the water molecule densities remain high with the addition of CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e and CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. These observations indicate that strong polar anions preferentially adsorb on the electrode surface, creating an anion-enriched, water-deficient electrolyte-electrode interface.\u003c/p\u003e\n\u003cp\u003eAs an experimental validation, in situ attenuated total reflection infrared (ATR-IR) spectroscopy was conducted to examine mass distribution at the electrolyte-electrode interface. The distinct peak located at 1640 cm\u003csup\u003e-1\u003c/sup\u003e corresponds to the \u0026delta;-HOH vibration of water molecules, where other peaks are associated with corresponding anions (Supplementary Fig.9).\u003csup\u003e24-27\u003c/sup\u003e For electrolytes containing CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, ATR-IR spectra exhibit prominent \u0026delta;-HOH peaks, indicating substantial adsorption of water molecules at the interface (Fig.3g-h). Conversely, in the electrolyte with CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e, a pronounced reduction in the \u0026delta;-HOH peak intensity is observed alongside a significant increase in intensities of anion-related peaks, suggesting dramatically decreased water molecules and increased anion contents at the interface (Fig.3i). Furthermore, alternating current voltammetry was employed to ascertain the adsorption behavior at the electrolyte-electrode interface (Supplementary Fig.10). The calculated differential capacitances follow the trend: CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e \u0026lt; CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e \u0026lt; CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, where anions with stronger polarity exhibit lower capacitance, reflecting the preferential adsorption of strong polar anions at the interface.\u003csup\u003e28\u003c/sup\u003e These results align with the MD simulations, confirming the capability of strong polar anions to establish an anion-enriched, water-deficient electrolyte-electrode interface.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAfter clarification the effect of anion polarities on the electrolyte-electrode interface, the interplay between electrolyte-electrode interface and electrochemical performance was further correlated. In the electrolyte containing CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, no distinct redox peak and charge/discharge plateaus corresponding to the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction are observed in CV and GCD profiles, indicating that the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction cannot be activated in this electrolyte environment (Fig. 3a and Supplementary Fig. 11a). Although the I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction can be activated in the presence of CF\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e with a moderate polarity, the redox peak is highly irreversible, and the charge/discharge plateaus gradually diminish after several cycles, attributed to the water-enriched interface (Fig. 3a-b and Supplementary Fig. 11b). By contrast, the electrolyte containing strong polar CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e achieves a reversible I\u003csup\u003e0\u003c/sup\u003e/I\u003csup\u003e+\u003c/sup\u003e conversion reaction with significantly improved stability and capacity, highlighting the pivotal role of the anion-enriched, water-deficient interface created by the strong polar anion in enabling reversible four-electron iodine conversion reactions (Fig. 3c and Supplementary Fig. 11c).\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThe role of kosmotropic anions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBuilding upon the understanding of polar anions, the critical role of kosmotropic anions in the KA-PA-Nuc standard was further explored. Within this standard, dissolved nucleophilic molecules with electronegative nitrogen or oxygen centers act as halogen bond acceptors by forming halogen bonds with generated I\u003csup\u003e+\u003c/sup\u003e cations during electrochemical reactions. However, these electronegative centers also extensively interact with water molecules \u003cem\u003evia\u003c/em\u003e hydrogen bonding regarding high water content in low concentration electrolytes (Supplementary Fig. 12). Such Nuc-water interactions pose significant obstacles to achieving the reversible four-electron iodine conversion reaction, explaining its irreversibility in PA-Nuc electrolytes even with anion-enriched, water-deficient interfaces (Fig. 2). It has been observed that different anions have distinguishable impacts on the solubility of protein in aqueous solutions through molecular interactions among anions, water molecules, and polymer chains, which is known as the anion-specific effect (Supplementary Fig. 13).\u003csup\u003e29,30\u003c/sup\u003e Such effect has been studied in hydrogel fabrication.\u003csup\u003e31,32\u003c/sup\u003e However, its potential for regulating molecular interactions for aqueous electrolytes remains unexplored. Drawing inspiration from the anion-specific effect, five commonly used anions were selected (excluding halide anions) and systematically analyzed to elucidate their role in stabilizing electrochemical performances.\u003c/p\u003e\n\u003cp\u003eThe effect of these anions on electrochemical performance was systematically evaluated. Although the introduction of these anions into the same polar anion baseline didn\u0026rsquo;t affect the initial specific capacity, their cycling stabilities varied significantly (Supplementary Fig. 14). For example, the GCD profiles in the electrolyte containing ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e deteriorated rapidly, indicating inferior stability (Fig. 4c). Slight improvements in cycling stability were observed in electrolytes containing CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e or extra CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e (Fig. 4b and Supplementary Fig. 15). Remarkably, electrolyte containing SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e demonstrated near-identical GCD profiles over repeated cycles, suggesting excellent stability (Fig. 4a). The stability of electrochemical performance follows the order: SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e ˃ CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e ˃ CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ˃ ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, which correlates well with the anion-specific effect. Notably, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e deviates from this sequence, which may attribute to its instability under the applied voltage (Supplementary Fig. 16a-b).\u003csup\u003e33\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eTo gain insight into the underlying mechanism behind the electrochemical performance, nuclear magnetic resonance spectroscopy (NMR) and MD simulations were performed to probe molecular interactions among anions, water molecules, and Nuc molecules. Interactions between anions and water molecules were characterized using \u003csup\u003e2\u003c/sup\u003eH NMR spectrum (Fig. 4d). The \u003csup\u003e2\u003c/sup\u003eH peaks shifted gradually to higher chemical shifts (lower fields) in the sequence SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e ˃ CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e ˃ NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ˃ CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ˃ ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, indicating enhanced de-shielding effect and stronger hydrogen bonds. This trend reflects the anion-specific effect, where chaotropic anions (right side) weakly coordinate with water molecules, while kosmotropic anions strongly coordinate with water molecules.\u003csup\u003e34,35\u003c/sup\u003e This distinction in water-anion coordination will significantly affect water-Nuc interactions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor \u003csup\u003e15\u003c/sup\u003eN spectra, peaks for Nuc molecules shifted to higher chemical shifts following the same order (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e ˃ CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e ˃ NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ˃ CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ˃ ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e), indicating enhanced de-shielding effect (Supplementary Fig. 19). This de-shielding effect is closely related to hydrogen bonds between N atoms and water molecules. Kosmotropic anions like SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e has the strong coordination with water molecules that can polarize the hydration water molecules, which destabilizes hydrogen bonds between Nuc molecules and their hydration water, resulting in electron density reduction around N atoms.\u003csup\u003e30\u003c/sup\u003e Conversely, chaotropic anions like ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e directly interfere with the hydrophobic backbone of Nuc molecules, which adds extra charge and facilitates hydrogen bonds between Nuc molecules and their hydration water, thereby increasing in electron density around N atoms.\u003csup\u003e18\u003c/sup\u003e These varying interactions were further evidenced by the differing solubilities of Nuc molecules in the presence of different anions (Supplementary Fig. 20). MD simulations also confirmed that kosmotropic anions significantly reduced water-Nuc interactions, highlighting their critical role in modulating molecular interactions (Supplementary Fig. 21). In addition, chemical shifts of \u003csup\u003e1\u003c/sup\u003eH and \u003csup\u003e13\u003c/sup\u003eC peaks showed an inverse order (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e \u0026lt; CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e \u0026lt; NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e \u0026lt; CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e \u0026lt; ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e), validating interactions between chaotropic anions and the Nuc backbone (Fig. 4e-f, Supplementary Fig. 17, and Supplementary Fig. 18). Corresponding 2D \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e15\u003c/sup\u003eN heteronuclear multiple bond correlation (HMBC) spectra also exhibited similar trends in chemical shifts for both \u003csup\u003e1\u003c/sup\u003eH and \u003csup\u003e15\u003c/sup\u003eN peaks, further proving above findings (Fig. 4g-i and Supplementary Fig. 16c). As a result, these results highlight the critical role of kosmotropic anions, particularly SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, in disrupting water-Nuc interactions and stabilizing four-electron iodine conversion reactions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eUnderstanding the interfacial electrochemical behavior\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe conversion chemistry of iodine is driven by interfacial reactions, visualizing these interfacial processes is crucial for a comprehensive understanding of interfacial electrochemical behavior. To gain deeper insight, particularly into the synergistic contributions of KA and PA to the interfacial electrochemical behavior, in situ electrochemical impedance spectroscopy (EIS), the distribution of relaxation times (DRT), and in situ Raman spectroscopy were conducted, followed by a detailed analysis. DRT is an effective technique to correlate relaxation times with characteristic electrochemical processes by deconvoluting EIS data.\u003csup\u003e36\u003c/sup\u003e Typically, relaxation times (\u0026tau;) in the range of 10\u003csup\u003e-6\u003c/sup\u003e-10\u003csup\u003e-4\u003c/sup\u003e s correspond to the intrinsic impedance (\u0026tau;\u003csub\u003e1\u003c/sub\u003e). Leveraging low concentration electrolytes with high ionic conductivities, all systems exhibit minimal intrinsic impedances (Fig. 5a-c). Relaxation times within the range of 10\u003csup\u003e0\u003c/sup\u003e-10\u003csup\u003e2\u003c/sup\u003e s reflect ion (mass) diffusion impedance (\u0026tau;\u003csub\u003e5\u003c/sub\u003e).\u003csup\u003e37,38\u003c/sup\u003e During a full charge-discharge cycle, the intensity of \u0026tau;\u003csub\u003e5\u003c/sub\u003e peaks gradually increases at the low-voltage region, suggesting a rise in ion diffusion impedance (Supplementary Fig. 22). As the voltage progresses toward the high-voltage region, the intensity of \u0026tau;\u003csub\u003e5\u003c/sub\u003e peaks gradually decreases, indicating reduced ion diffusion impedance. Theoretically, a successful four-electron iodine conversion reaction involves a transition from ionic iodine species to solid iodine and then to ionic iodine species at the electrolyte-cathode interface. These transitions are well captured by the observed shifts in ion impedance. Notably, throughout this process, the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte demonstrates the lowest impedance and the best reversibility, underscoring the synergistic contributions of KA and PA in stabilizing the interfacial electrochemical behavior of the four-electron iodine conversion reaction.\u003c/p\u003e\n\u003cp\u003eIn the mid-frequency region (10\u003csup\u003e-4\u003c/sup\u003e-10\u003csup\u003e0\u003c/sup\u003e s), interfacial charge transfer impedance (\u0026tau;\u003csub\u003e2\u003c/sub\u003e-\u0026tau;\u003csub\u003e4\u003c/sub\u003e), including the cathode-electrolyte interface and the electron transfer process of redox species, exhibits notable variations. DRT plots in the electrolyte without PA present two distinct peaks (\u0026tau;\u003csub\u003e3\u003c/sub\u003e and \u0026tau;\u003csub\u003e4\u003c/sub\u003e) at this region, while electrolytes containing PA exhibit three prominent peaks (\u0026tau;\u003csub\u003e2\u003c/sub\u003e, \u0026tau;\u003csub\u003e3\u003c/sub\u003e, and \u0026tau;\u003csub\u003e4\u003c/sub\u003e). The additional \u0026tau;\u003csub\u003e2\u003c/sub\u003e peak is attributed to the cathode-electrolyte interface, induced by the preferential adsorption of PA, further demonstrating the critical role of PA in facilitating an anions-rich interface (Fig. 5a-c).\u003csup\u003e39\u003c/sup\u003e The \u0026tau;\u003csub\u003e3\u003c/sub\u003e and \u0026tau;\u003csub\u003e4\u003c/sub\u003e peaks in electrolytes containing PA are greatly affected by voltage and become more pronounced at higher voltages, indicating a successful multi-electron charge transfer reaction.\u003csup\u003e40\u003c/sup\u003e In contrast, the \u0026tau;\u003csub\u003e3\u003c/sub\u003e and \u0026tau;\u003csub\u003e4\u003c/sub\u003e peaks in the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-Nuc electrolyte gradually merge into a single peak at high voltages (Fig. 5c). Notably, compared with the CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte, the KA (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e)-PA (CH\u003csub\u003e3\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e)-Nuc electrolyte exhibits lower intensity peaks, suggesting a more complete conversion reaction, leading to enhanced stability and reversibility. Furthermore, in situ Raman spectroscopy was performed to track the evolution of iodine species. An obvious peak representing NA-I-NA complex appears upon charging to 1.5 V, while this peak gradually diminishes during subsequent discharging process, confirming the reversible conversion between I\u003csup\u003e0\u003c/sup\u003e and I\u003csup\u003e+\u003c/sup\u003e species in the electrolyte with both KA and PA (Fig. 5d-f). These findings highlight the synergistic contributions of KA and PA in keeping stable and reversible interfacial electrochemical behaviors.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical performances of four-electron aqueous metal-iodine batteries\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFollowing the elucidation of the synergistic mechanism of the KA-PA-Nuc standard, its broad applicability in four-electron aqueous metal-iodine batteries was investigated. Aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries were firstly assembled using zinc anodes. As shown in Supplementary Fig. 23a, CV profiles in the KA-PA-Nuc electrolyte reveal two distinct redox peaks with well-maintained shapes across increasing scan rates (0.1 to 1 mV s\u003csup\u003e-1\u003c/sup\u003e), indicating reversible four-electron iodine conversion reactions and excellent conversion kinetics.\u003csup\u003e9\u003c/sup\u003e To demonstrate the superiority of the KA-PA-Nuc electrolyte, the widely used high-concentration ZnCl\u003csub\u003e2\u003c/sub\u003e electrolyte was selected as a representative halogen-containing counterpart for comparison. The Zn-I\u003csub\u003e2\u003c/sub\u003e batteries in the KA-PA-Nuc electrolyte exhibit superior rate performances, showing higher reversible capacities of 487, 459, 422, 362, 324, and 299 mAh g\u003csup\u003e-1\u003c/sup\u003e at current densities of 0.5, 1, 2, 4, 6, and 8 A g\u003csup\u003e-1\u003c/sup\u003e, respectively (Supplementary Fig. 23b). In contrast, batteries in the 30m ZnCl\u003csub\u003e2\u003c/sub\u003e electrolyte show lower capacities with significant capacity decline, especially at low current densities, indicating irreversible reactions despite the high electrolyte concentration. Furthermore, corresponding GCD profiles also display improved reversibility and stability at various current densities compared to the ZnCl\u003csub\u003e2\u003c/sub\u003e system (Supplementary Fig. 23c and Supplementary Fig. 24).\u003c/p\u003e\n\u003cp\u003eLong-term cycling tests further highlight the performance benefits of the KA-PA-Nuc electrolyte. Zn-I\u003csub\u003e2\u003c/sub\u003e batteries in the KA-PA-Nuc electrolyte maintained a high capacity of 394 mAh g\u003csup\u003e-1\u003c/sup\u003e with a capacity retention of 93% after 1,000 cycles at a current density of 1 A g\u003csup\u003e-1\u003c/sup\u003e (Fig. 6a). In comparison, batteries in the 30m ZnCl\u003csub\u003e2\u003c/sub\u003e electrolyte shows rapid capacity degradation, retaining only 42% of their initial capacity. The reliability of four-electron aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries with the KA-PA-Nuc electrolyte was also examined at a higher current density of 2 A g\u003csup\u003e-1\u003c/sup\u003e, achieving an outstanding capacity retention of 94% over 2,000 cycles, corresponding to an extremely low capacity decay rate of 0.003% per cycle (Supplementary Fig. 25). Notably, this work represents the first successful demonstration of reversible four-electron iodine conversion reactions with remarkable electrochemical performance in halogen-free, low-concentration electrolytes, outperforming most existing aqueous metal-I\u003csub\u003e2\u003c/sub\u003e batteries that employing high-concentration, halogen-containing electrolytes (Supplementary Table 1-2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo further validate the application of the KA-PA-Nuc standard, pouch cells were assembled (Supplementary Fig. 26). The pouch cell delivers an initial capacity of 184.9 mAh, retaining a large capacity of 114.9 mAh after 500 cycles (Fig. 6c). To extend the application of the KA-PA-Nuc standard to other aqueous metal-iodine systems, aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries were assembled using Al-Zn alloys as anodes. These batteries present two pairs of reversible plateaus associated with four-electron iodine conversion reactions in the KA-PA-Nuc electrolyte (Supplementary Fig. 27). Long-term cycling tests demonstrate a reversible capacity of 346 mAh g\u003csup\u003e-1\u003c/sup\u003e with a capacity retention of 82% after 1000 cycles, significantly outperforming the only reported four-electron aqueous Al-I\u003csub\u003e2\u003c/sub\u003e battery that utilized a high-concentration, halogen-containing electrolyte (Fig. 6b).\u003csup\u003e12\u003c/sup\u003e These results validate the broad applicability of the KA-PA-Nuc standard in advancing four-electron aqueous metal-iodine batteries.\u003c/p\u003e\n\u003cp\u003eTo assess the practical potential of four-electron aqueous metal-iodine batteries utilizing the KA-PA-Nuc standard, a systematic techno-economic analysis was conducted for the first time (see Supplementary Information for details).\u003csup\u003e41\u003c/sup\u003e Figure 6d-e show the cost structure of four-electron aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries (4e ZIBs) using different electrolytes. Compared to 4e ZIB employing the 30 m ZnCl\u003csub\u003e2\u003c/sub\u003e electrolyte (425 USD KWh\u003csup\u003e-1\u003c/sup\u003e), the overall cell cost of 4e ZIB in the KA-PA-Nuc electrolyte (67 USD KWh\u003csup\u003e-1\u003c/sup\u003e) is reduced by 6.34 times, primarily due to the elimination of costly high-concentration halogen electrolytes and expensive titanium current collectors. Figure 6f compares the cell costs of various battery systems, where the overall cell cost of 4e ZIB in the KA-PA-Nuc electrolyte is significantly lower than that of commonly used Li-ion batteries (LIBs) chemistries and state-of-the-art K-ion battery (KIB) chemistry.\u003csup\u003e42\u003c/sup\u003e Zinc is an ideal anode material due to its high specific capacity (819 mAh g\u003csup\u003e-1\u003c/sup\u003e) and low cost (2.36 USD kg\u003csup\u003e-1\u003c/sup\u003e). Although the negative-to-positive capacity ratio of 4e ZIB (2.00) is higher than that of LIB (1.02), zinc remains a more economical anode option (Supplementary Table 3). Meanwhile, the cathode cost for 4e ZIB is also significantly lower than that of LIB, benefiting from iodine\u0026rsquo;s lower unit cost (0.80 USD kg\u003csup\u003e-1\u003c/sup\u003e) and high theoretical capacity (422 mAh g\u003csup\u003e-1\u003c/sup\u003e) enabled by four-electron conversion chemistry. Compared to LIB, where the cathode and anode account for 74.96% of the overall cell cost, the corresponding cost contribution in 4e ZIB is only 20.1% (Supplementary Table 4). It is worth noting that 4e ZIB in the KA-PA-Nuc electrolyte still incurs a higher current collector cost due to the use of stainless steel, which remains relatively expensive (Fig 6d). Therefore, 4e ZIB in the KA-PA-Nuc electrolyte still has huge potential in further cost reduction by adopting more cost-effective current collectors, highlighting its immense economic benefit and commercial value for cost-effective and sustainable energy storage solutions.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn summary, we have developed a new and universal electrolyte paradigm leveraging anionic chemistry, termed KA-PA-Nuc, for aqueous metal-iodine batteries. Comprehensive experimental and computational analyses reveal that PA facilitates a water-deficient, anion-enriched interface, while KA disrupts hydrogen bonding between nucleophilic species and their hydration shells, thereby guaranteeing stable halogen bonds of I\u003csup\u003e+\u003c/sup\u003e cations. As a result, this electrolyte paradigm spearheaded achieves reversible multi-electron iodine conversion chemistry in halogen-free electrolytes at an exceptionally low concentration (2.4 moles), eliminating the reliance on high-concentration, corrosive halogen-containing electrolytes. A systematic techno-economic analysis further reveals that the overall cell cost of aqueous metal-iodine batteries using the KA-PA-Nuc electrolyte is reduced by 6.34 times (from 425 USD KWh\u003csup\u003e-1\u003c/sup\u003e to 67 USD KWh\u003csup\u003e-1\u003c/sup\u003e), making it more cost-effective than conventional Li-ion batteries. This work not only enhances the practicality of aqueous metal-iodine batteries but also provides a versatile electrolyte principle, which will arouse new inspiration and pave the way for cost-effective and sustainable energy storage solutions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/V027433/3), UK Research and Innovation (UKRI) under the UK government\u0026lsquo;s Horizon Europe funding (101077226; EP/Y008707/1), EPSRC Centre for Doctoral Training in Molecular Modelling and Materials Science (EP/L015862/1). We would like to acknowledge the BatPaC for batteries cost calculation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eR.W.C. and G.J.H. conceived this work. R.W.C. designed and conducted the experiments. R.W.C. wrote the manuscript. G.J.H. supervised the research and edited the manuscript. H.B.D. performed the theoretical calculations and corresponding analysis. Y.H.D., J.Y.W., J.C., J.Y.L., J.R.F., Y.P.Z., and W.Z. helped with measurements and data analysis. W.Z., H.Y., and Z.C. helped with materials synthesis and electrodes fabrication.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests.\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData supporting the findings of this study are available from the Supplementary Information.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLiang, Y., Yao, Y. 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W., Kubal, J. J., Nelson, P. A., Ahmed, S. Battery performance and cost modeling for electric-drive vehicles (a manual for BatPaC v5. 0). (Argonne National Lab.(ANL), Argonne, IL (United States), 2022).\u003c/li\u003e\n\u003cli\u003eDhir, S., Wheeler, S., Capone, I., Pasta, M. Outlook on K-ion batteries. \u003cem\u003eChem\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 2442-2460 (2020).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003ePreparation of electrolytes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll chemicals were purchased from Sigma-Aldrich and used as received. The KA-PA-Nuc electrolytes were prepared by dissolving KA, PA, and Nuc into deionized water at room temperature. Specifically, KA was 1.5 M ZnSO\u003csub\u003e4\u003c/sub\u003e and PA was 0.5 M Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e for aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries. To demonstrated broad applicability of the KA-PA-Nuc standard, various Nuc were employed, including 2 M urea, 2 M acetamide, 0.2 M pyridine, or 0.4 M nicotinamide were employed as Nuc. To explore the function of polar anions, a baseline electrolyte containing 1.5 M ZnSO\u003csub\u003e4\u003c/sub\u003e and 0.4 M nicotinamide was established. Subsequently, Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e, Zn(CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, or Zn(CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (at a concentration of 0.5 M) were added to the baseline for comparative analysis. To evaluate the role of kosmotropic anions, a baseline electrolyte containing 0.5 M Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e and 0.4 M nicotinamide was prepared. Various salts, including ZnSO\u003csub\u003e4\u003c/sub\u003e, Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, Zn(CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, or Zn(ClO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (at a concentration of 1.5 M), were then dissolved into the baseline. For the application of the KA-PA-Nuc standard in aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries, the electrolyte was prepared by dissolving 1.5 M Al\u003csub\u003e2\u003c/sub\u003e(SO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e, 0.5 Mg(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e, and 0.5 M nicotinamide into deionized water.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of electrodes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe evaporation-adsorption method was used to prepare iodine-activated carbon (AC) material. In brief, 0.5 g iodine was ground with 0.5 g AC for 10 min. The resulting mixture was sealed in a Teflon reactor and heated at 120 ℃ for 6 h. For the preparation of iodine cathode, the synthesized I\u003csub\u003e2\u003c/sub\u003e-AC material, polytetrafluoroethylene and Super P were mixed in deionized water with a mass ratio of 8:1:1. Then the slurry was pressed onto a titanium mesh, followed by drying in an oven at 60 ℃ for 3 h. The areal iodine loading was controlled by adjusting the thickness of slurry film on the titanium mesh. Commercial Zn foils were used as received, without further treatment.\u003c/p\u003e\n\u003cp\u003eTo assess the potential of the KA-PA-Nuc electrolyte standard in multi-electron aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries, AC electrodes were prepared as both reference and counter electrodes. Specifically, AC, polytetrafluoroethylene and Super P were mixed in deionized water with a mass ratio of 8:1:1. The resulting slurry was then pressed onto a titanium mesh and dried in an oven at 60 ℃ for 6 h. By adjusting the electrode thickness, the capacity of AC electrode was controlled to be much higher than that of the cathode, ensuring its capability as a stable reference and counter electrode for evaluating the electrochemical performance of the KA-PA-Nuc electrolyte. To further validate the applicability of the KA-PA-Nuc electrolyte standard in multi-electron aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries, Al-Zn alloys were fabricated as anodes following a previous work.\u003csup\u003e43\u003c/sup\u003e Briefly, symmetric cells were assembled using Zn foils as electrodes and 2M Al(CF\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e as the electrolyte. During charging, Al\u003csup\u003e3+\u003c/sup\u003e in the electrolyte deposited on and reacted with the metal Zn substrate, forming the Zn-supported Al-Zn alloys. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaterials characterizations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIn situ ATR-IR measurements:\u003c/strong\u003e In situ attenuated total reflection infrared (ATR-IR) spectra were recorded by a Thermo Scientific Nicolet iS50 spectrometer equipped with a liquid nitrogen-cooled MCT-A detector. Gold-coated silicon plates were employed as working electrodes, with Zn foils and glass fiber used as reference electrodes and separators, respectively. During testing, the assembled batteries were connected to a Gamry Interface 1000 potentiostat to run a galvanostatic charge-discharge (GCD) program. All spectra were collected at a resolution of 4 cm\u003csup\u003e-1\u003c/sup\u003e, with 50 scans per measurement. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIn situ Raman measurements: \u003c/strong\u003eRaman spectra were recorded using a Bruker Senterra II Raman spectrometer with a 532 nm excitation wavelength. The iodine-AC cathodes, Zn foils, and glass fiber served as the working electrodes, reference electrodes, and separators, respectively. Raman spectra were collected during GCD cycling using an external Gamry Interface 1000 potentiostat.\u003c/p\u003e\n\u003cp\u003eFourier-transform infrared (FTIR) spectra of various electrolytes were conducted on an ATR-FTIR (BRUKER, platinum-ATR).\u003csup\u003e \u003c/sup\u003eNuclear magnetic resonance (NMR) analyses, including \u003csup\u003e1\u003c/sup\u003eH, \u003csup\u003e13\u003c/sup\u003eC, and \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e15\u003c/sup\u003eN HMBC spectra, were carried out on a Bruker NMR instrument (Avance Neo 500). 15N spectra were collected using a Bruker NMR instrument (Avance Neo 700). Deuterium oxide was used as the solvent for all NMR measurements. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCoin cell configuration:\u003c/strong\u003e Electrochemical studies were performed in CR2025 coin cells. For aqueous Zn-I\u003csub\u003e2\u003c/sub\u003e batteries, the synthesized iodine-AC electrodes served as cathodes, Zn foils were used as anodes, and Whatman GF/D glass fiber was utilized as separators. For aqueous Al-I\u003csub\u003e2\u003c/sub\u003e batteries, as fabricated Al-Zn alloys were used as anodes. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePouch cell configuration:\u003c/strong\u003e For pouch cells, large-area iodine-AC electrodes with a slurry film thickness of 30 \u0026micro;m were employed as cathodes, Zn foils and Whatman GF/D glass fiber were utilized as anodes and separators, respectively. The electrolyte consisted of 1.5 M ZnSO\u003csub\u003e4\u003c/sub\u003e, 0.5 M Zn(CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003e, and 0.4 M nicotinamide. Titanium foils were connected to electrodes as tabs. After lamination, the assembled batteries were encapsulated in an aluminum-plastic films, and subjected to controlled pressure using a customized model during electrochemical measurements. \u003c/p\u003e\n\u003cp\u003eGCD tests were conducted at room temperature using a Neware battery test system. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) measurements were performed using a Biologic VMP-3 potentiostat. Distribution relaxation times (DRT) were calculated from EIS data using MatlabR2019 with the DRT-tool developed by Professor Francesco Ciucci\u0026apos;s research group.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the molecular dynamics (MD) simulations were performed using Forcite package. Here, we keep the number of water molecules constant and then adjust the ion species to obtain electrolytes of different ions. For each MD simulation, COMPASSIII force field and NVT ensemble were chosen,\u003csup\u003e44-46\u003c/sup\u003e and the temperature was controlled using a Nos\u0026eacute; thermostat with a target temperature of 298 K. A time step of 0.01 ps was chosen, and the total number of steps was set as 5000000. For the calculation of the electrostatic potential, preoptimizations and the stationary point calculations were carried out using M06-2X/6-311+G(2df,2p)\u003csup\u003e47\u003c/sup\u003e for all the selected configurations using the Gaussian 16 software. The density difference was calculated with a periodic slab model using the Vienna ab initio simulation program (VASP).\u003csup\u003e48-50\u003c/sup\u003e The generalized gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.\u003csup\u003e51\u003c/sup\u003e The projector-augmented wave (PAW) method was utilized to describe the electron-ion interactions, and the cut-off energy for the plane-wave basis set was 450 eV.\u003csup\u003e48\u003c/sup\u003e Brillouin zone integration was accomplished using 6\u0026times;6\u0026times;6 Monkorst-Pack k-point mesh. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study have been included in the main text and the Supplementary Information. They are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReferences\u003c/strong\u003e\u003c/p\u003e\n\u003col start=\"43\"\u003e\n\u003cli\u003eYan, C.\u003cem\u003e et al.\u003c/em\u003e Architecting a stable high-energy aqueous Al-ion battery. \u003cem\u003eJ Am Chem Soc\u003c/em\u003e \u003cstrong\u003e142\u003c/strong\u003e, 15295-15304 (2020).\u003c/li\u003e\n\u003cli\u003eSun, H., Ren, P., Fried, J. R. The COMPASS force field: parameterization and validation for phosphazenes. \u003cem\u003eComputational and Theoretical Polymer Science\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 229-246 (1998).\u003c/li\u003e\n\u003cli\u003eAkkermans, R. L. C., Spenley, N. A., Robertson, S. H. COMPASS III: automated fitting workflows and extension to ionic liquids. \u003cem\u003eMolecular Simulation\u003c/em\u003e \u003cstrong\u003e47\u003c/strong\u003e, 540-551 (2020).\u003c/li\u003e\n\u003cli\u003eSun, H.\u003cem\u003e et al.\u003c/em\u003e COMPASS II: extended coverage for polymer and drug-like molecule databases. \u003cem\u003eJournal of Molecular Modeling\u003c/em\u003e \u003cstrong\u003e22\u003c/strong\u003e, 47 (2016).\u003c/li\u003e\n\u003cli\u003eZhao, Y., Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06 functionals and 12 other functionals. \u003cem\u003eTheoretical Chemistry Accounts\u003c/em\u003e \u003cstrong\u003e119\u003c/strong\u003e, 525-525 (2008).\u003c/li\u003e\n\u003cli\u003eKresse, G., Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. \u003cem\u003ePhysical Review B\u003c/em\u003e \u003cstrong\u003e59\u003c/strong\u003e, 1758-1775 (1999).\u003c/li\u003e\n\u003cli\u003eKresse, G., Hafner, J. Ab initio molecular dynamics for liquid metals. \u003cem\u003ePhysical Review B: Condensed Matter and Materials Physics\u003c/em\u003e \u003cstrong\u003e47\u003c/strong\u003e, 558-561 (1993).\u003c/li\u003e\n\u003cli\u003eKresse, G., Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. \u003cem\u003ePhysical Review B: Condensed Matter and Materials Physics\u003c/em\u003e \u003cstrong\u003e54\u003c/strong\u003e, 11169-11186 (1996).\u003c/li\u003e\n\u003cli\u003ePerdew, J. P., Burke, K., Ernzerhof, M. Generalized Gradient Approximation Made Simple. \u003cem\u003ePhys Rev Lett\u003c/em\u003e \u003cstrong\u003e77\u003c/strong\u003e, 3865-3868 (1996).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6372434/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6372434/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Four-electron aqueous metal-iodine batteries embrace high theoretical capacities, abundant raw materials, and superior safety, making them highly promising for next-generation large-scale energy storage applications with high energy and power densities. However, harnessing this four-electron redox chemistry has traditionally relied on high-concentration, corrosive halogen-containing electrolytes (up to 46 moles) to stabilize hypervalent iodine cations, posing considerable economic and environmental challenges to access their full potential. Here, we proposed a universal electrolyte design, the KA-PA-Nuc standard, which employs kosmotropic anions (KA), polar anions (PA), and nucleophilic species (Nuc) to achieve reversible four-electron aqueous metal-iodine batteries. PA facilitates a water-deficient, anion-enriched interface, while KA disrupts hydrogen bonding between Nuc and their hydration shells, which in turn form stable halogen bonds with hypervalent iodine cations. For the first time, this electrolyte design grounded in anionic chemistry achieves reversible four-electron iodine redox reactions in halogen-free electrolytes at an exceptionally low concentration (2.4 moles). The KA-PA-Nuc standard was validated across diverse Nuc in aqueous Zn-I₂ and Al-I₂ batteries, demonstrating its broad applicability and effectiveness for aqueous metal–I2 batteries. By eliminating the reliance on high-concentration, corrosive halogen-containing electrolytes, this work establishes a new paradigm and provides a new avenue for low-cost and sustainable batteries.","manuscriptTitle":"Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-28 06:49:03","doi":"10.21203/rs.3.rs-6372434/v1","editorialEvents":[],"status":"published","journal":{"display":false,"email":"[email protected]","identity":"nature","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nature","sideBox":"Learn more about [Nature](http://www.nature.com/nature/)","snPcode":"","submissionUrl":"","title":"Nature","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8a1659c6-7fef-46a4-9f96-4df78320d9b4","owner":[],"postedDate":"May 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":48977825,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":48977826,"name":"Physical sciences/Chemistry/Energy"}],"tags":[],"updatedAt":"2026-03-27T13:44:48+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-28 06:49:03","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6372434","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6372434","identity":"rs-6372434","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}