Carbon-halogen bond substitution enables high-utilization four-electron iodine redox in noncorrosive dilute electrolytes

Domain: chemistry
Published: 2026-02-21
Editorial outcome: minor_revision

Abstract
Aqueous Zn||I2 batteries, involving I-/I0/I+ redox, are promising yet usually facing low I2 utilization dominated by I0/I+ redox, especially under high loadings. Unlocking alternative pathway to I0/I+ redox, preferably in noncorrosive dilute electrolytes, is a crucial solution. Here, we report a pathway towards more thermodynamically favorable I0/I+ redox, via a unique carbon-halogen bond substitution. This pathway is realized with a low-concentrated (0.7 M), noncorrosive organohalide additive (2-bromoacetamide, BrAce), triggering a reversible Br-C···I(0) and C-I(+)-Br bond substitution. Compared with conventional interhalogen bonding (I-Br) pathway, this pathway synchronously lowers the barrier for I⁰/I⁺ redox and strengthens the anti-hydrolysis of I+ species, by elaborately regulating axial δ hole activity of interhalogen bond (I(δ+)-Br). Notably, this pathway enables sustainable operation of four-electron Zn||I2 batteries with high I2 loading (8.6~24.0 mg cm-2), featuring improved performances: (1) high I2 utilizations (55%~80%) at high rates (5.8~46.4 mA cm-2), (2) long lifespan (>400 cycles) with practical areal capacity (~3.85 mAh cm-2) and 99.5% retention even at 47.5 mA cm-2. This pathway opens an exciting research direction to unlock unusual halogen chemistry for scalable, high-energy, sustainable aqueous batteries.

Introduction
Aqueous Zn||I2 batteries, involving cascaded I-/I0/I+ redox, are promising high-energy systems for various potential application scenarios1,2. Despite the good compatibility of I-/I0 redox with conventional electrolytes, I0/I+ redox works mostly in highly corrosive, halogen anion (X-=Br-, Cl-)-concentrated electrolytes3–6. The corrosive cell environment shortens the lifespan of the Zn negative electrode due to pitting effects7,8. In addition, the use of highly concentrated electrolytes compromises the cost merits of batteries9,10. More crucially, the I+ species hydrolysis can accelerate the shuttle effect, leading to low I2 utilization (below 30%) at high loadings (≥10 mg cm-2)11. Hence, the key to promoting the sustainable application of full cells lies in simultaneously triggering I0/I+ redox with high anti-hydrolysis and prolonging Zn negative electrode lifespan, preferably in mild and dilute electrolytes12–15.

Implementing I0/I+ redox involves the formation and stabilization of I+ species, which are usually generated in the form of interhalogen compounds (e.g., IBr, ICl)3,16–19. In halogen anion (X-=Br-, Cl-)-dilute electrolytes, the generated interhalogen compounds are prone to hydrolysis, due to the presence of active axial δ hole of interhalogen bonds20,21. For the long-term implementation of I0/I+ redox, a strategy is appropriately weakening the activity of the axial δ hole on the I side (δ+…I-X). In halogen anion (X-=Br-, Cl-)-concentrated electrolytes, the generated interhalogen compounds (IX) can further form chain molecule structures (e.g., [···I-X···I-X···]n) or anion clusters (e.g., IX2-) via axial δ hole interaction4,22–25, which represents the typical interhalogen bonding pathway (Fig. 1a) during I0/I+ conversion. Therefore, the stabilization of IXs usually needs X--concentrated chemical environments6,26. The key to implementing I0/I+ redox in mild and dilute electrolytes lies in breaking this shackle, by constructing paradigms to replace the chain IX molecule structures or anion clusters.

Although inorganic compounds, halogen can stably bond with non-metallic elements (e.g., C, S, N, P)27,28, no studies have addressed axial δ hole regulation of I-X by non-metallic element-X bonding in organic molecules. Recent studies showed that linking the electron-drawing groups to the I side of IXs, can enhance the halogen bonding constant (K) by 2-3 orders of magnitude29. The introduction of non-interhalogen bonding could be a latent strategy to regulate the axial δ hole of I-X towards more thermodynamically favorable I0/I+ redox.

Herein, we decouple I0/I+ redox kinetics from harsh electrolyte environments with a noncorrosive, environment-benign organohalide additive (BrAce, 0.7 M). This additive enables a pathway with a carbon-halogen bond substitution (Br-C···I(0) ↔ C-I(+)-Br) (Fig. 1b) that alters the electronic landscape of iodine intermediates. Compared with the conventional interhalogen bonding pathway, our approach strategically modulates the axial δ⁺ hole activity of I-X bonds, simultaneously reducing the barrier for I0/I+ conversion and suppressing hydrolysis of I+ species. This dual regulation enables sustainable operation of four-electron Zn||I2 batteries with high I2 loading: (1) high I2 utilizations (55%~80%) at high rates (5.8~46.4 mA cm-2), (2) long lifespan (>400 cycles) with areal-specific capacity (~3.85 mAh cm-2) and 99.5% retention at a high current density (47.5 mA cm-2). This pathway eliminates reliance on corrosive halide salts, prolonging Zn negative electrode lifespan while reducing electrolyte toxicity and cost. Beyond iodine, this carbon-halogen substitution concept offers a universal design principle for stabilizing multivalent halogen redox towards scalable, high-energy, and sustainable aqueous batteries.

Results
Among interhalogen compounds (XY), the bigger the electronegativity differences (ΔEN) between X and Y, the more stable the XY is, but the more difficult it is to form bonds30. Based on ΔEN values for IBr (0.3), ICl (0.5), and IF (1.5), I+ species should be more challenging to be triggered by additives with C-Cl and C-F bonds31. We focus on the I0/I+ redox triggered by organic additives with C-Br bonds. Balancing cost and ionic conductivity, a sulfate electrolyte with 3 M ZnSO4 formulation is employed as baseline (denoted ZSO)32. Water-soluble organic additives, with a formation of R-CH2-Br (R = electron-donating or -withdrawing groups) (Fig. 2a), are respectively introduced into ZSO to examine their efficacy. For comparison, ZnBr2 salts are introduced into ZSO as well.

The regulation efficacy of additives is scrutinized by cyclic voltammetry (CV) tests of Zn||I2 batteries (I2 loading: 3 mg cm-2; cut-off voltage: 0.3 − 1.75 V; scan rate: 1 mV s-1) (Fig. 2b). Acetamide (Ace) without C-Br bonds and 2-bromomethyltetrahydrofuran (BrTHF) with an electron-donating group cannot activate I0/I+ redox (the pair of peaks at higher potential), whereas BrAce, bromoacetone (BrPK), 3-bromopropionamide (3-BrPace), and bromoacetonitrile (BrAN) with electron-withdrawing groups show distinct I0/I+ redox behavior. The corresponding Galvanostatic charge-discharge (GCD) curves (Fig. 2c) at 1 A g-1 further confirm this trend, i.e., no plateaus of I0/I+ redox for Ace and BrTHF (Supplementary Figs. S1 and S2). Still, they appear with BrAce, BrPK, 3-BrPace, and BrAN (Supplementary Figs. S3–S5). These results verify that the activation of I0/I+ redox is highly associated with the C-Br bond and the electronic properties of R. When C-Br bonds and electron-withdrawing R coexist, the electron-drawing ability (Hammett parameter: σ > 0; Supplementary Table S1) of R influences I2 utilization (ratio of theoretical capacity to actual capacity of I-/I0/I+ redox) (Fig. 2d). The σ value can be used to quantify the electron-withdrawing strength of the organic additives, i.e., a more positive σ value indicates greater electron-withdrawing ability33. Compared to BrAce with amide group (σ = 0.28), BrAN (nitrile, σ = 0.66) and BrPK (ketone, σ = 0.51) additives provide inferior I2 utilization. Theoretically, optimal electron-withdrawing groups can effectively bind to I+ species, preventing them from acting as independent active sites. This consequently weakens the nucleophilic attack of H2O molecules and thus enhances the cycling stability of the battery. However, an excessive electron-withdrawing effect reduces the nucleophilicity of the C-Br bond, hindering the formation of I-Br. In contrast, electron-donating groups compromise the stability of charged products and accelerate capacity fading. The amide group in BrAce shows a slightly higher σ value than that of Br (0.28 vs. 0.23). The slightly higher σ value of electron-withdrawing groups than that of bromine of C-Br bond could be more beneficial to achieve the trade-off for iodine redox reversibility and I2 utilization. Among six additives, BrAce with amide group (σ = 0.28) allows for the highest I2 utilization (83.1%), confirming the crucial role of the appropriate σ value in triggering the I0/I+ redox. Hence, the σ value could be an effective descriptor for regulation of I0/I+ redox triggered by C-Br bonds. Besides, with chloroacetamide (ClAce) and 2,2-difluoroacetamide (DFAce) additives, I0/I+ redox can be triggered only at higher voltage windows (~1.85 V for ClAce, 1.95 V for DFAce), and Zn||I2 batteries typically show inferior cycling stability (Supplementary Figs. S6 and S7). This result conforms to the higher ΔEN values for ICl (0.5), and IF (1.5).

BrAce is selected for subsequent electrochemical investigation. To accurately recognize the effect of C-Br bonds, ZnBr2 salts with equimolar Br- are added into ZSO for comparison. The introduction of BrAce enhances the overpotential of hydrogen and oxygen evolution reactions, as revealed by linear sweep voltammetry (LSV) curves (Supplementary Fig. S8). Compared to other high-concentration salt electrolytes, the BrAce electrolyte exhibits lower viscosity and higher ionic conductivity, which is beneficial to obtain four-electron Zn||I2 batteries with fast redox kinetics (Supplementary Fig. S9). When the Zn||I2 battery is further charged to 1.85 V at 1 A g-1, a higher charge plateau appears at 1.78 V (Supplementary Fig. S10), corresponding to the Br-/Br0 conversion; during discharging, the corresponding discharge plateau appears, along with a decrease in the Coulombic efficiency (CE). When scanned at 1 mV s-1 up to 2.0 V, CV curves of Zn||I2 batteries also show the Br0/Br+ redox peaks (Supplementary Fig. S11)34. It indicates that excessively high charging voltage causes electrolyte decomposition, leading to the irreversible fracture of C-Br bonds of additives. Furthermore, Zn battery using iodine-free positive electrode is evaluated in the BrAce electrolyte (specific current: 1 A g-1; carbon loading: 2.5 mg cm-2; voltage window: 0.3 - 1.75 V). The GCD curves (Supplementary Fig. S12) display a typical capacitive behavior, without identified bromine redox conversion. It suggests that the cut-off voltage of 0.3 - 1.75 V can eliminate bromine redox interference in four-electron iodine conversion. To ensure the efficacy of BrAce, an upper voltage limit of 1.75 V is chosen for the following battery tests.

The solubility limit of BrAce in ZSO is approximately 1.0 M. Among various BrAce concentrations, 0.7 M BrAce endows the Zn||I2 battery with the highest capacity and the longest cycling stability at 1 A g-1 (Supplementary Fig. S13). The near-saturated BrAce can reduce the reversibility of iodine conversion to some extent. Therefore, 0.7 M BrAce and 0.35 M ZnBr2 were introduced into ZSO as electrolytes, respectively. At high rates of 1-60 A g-1, discharge curves of Zn||I2 batteries show the advantage of BrAce over that by ZnBr2, in triggering I-/I0/I+ redox with higher capacity and well-kept plateaus (Fig. 2e). When the specific current increases from 1 to 50 A g-1, Zn||I2 batteries in BrAce electrolytes display much smaller polarization than that in ZnBr2 electrolytes (0.08 V vs. 0.20 V). The trend is more obvious when discharge curves at different rates are compared individually (Supplementary Fig. S14). The Zn||I2 batteries with BrAce also show higher I2 utilization than that with ZnBr2 (Fig. 2f). Even at 50 A g-1, also >50% I2 utilization remains obtained with BrAce, while the counterpart with ZnBr2 is only ~25%. Color changes in the cycled BrAce electrolytes (always colorless) and ZnBr2 electrolytes (from colorless to shallow yellow) manifest no Br3- and a large number of Br3- formation (inset in Fig. 2f), respectively35. This high I2 utilization render the Zn||I2 batteries with merits of specific energy/power (~450 Wh kg-1Iodine, ~18 kW kg-1Iodine) over other conversion positive electrode chemistries (Fig. 2g, Supplementary Table S2), including I-/I0, Br-/Br0, and multi-electron chalcogen redox36–43.

To unveil the BrAce-regulated pathway to I0/I+ redox, in situ Raman spectroscopy is initially conducted to track iodine speciation dynamics on the positive electrode side during GCD cycling (Fig. 3a, b). The Zn||I2 batteries are firstly discharged to 0.3 V (D-0.3 V) and then scrutinized by Raman spectroscopy. The conventional interhalogen compounds (i.e., IBr) form when Zn||I2 batteries operated in Br--rich electrolytes in previous studies5. Raman spectra in Fig. 3a show the presence of the IBr signal (from charged to 1.6 V (C-1.6 V) to discharged to 1.6 V (D-1.6 V), at 428 cm-1), suggesting the conventional pathway to I0/I+ redox via interhalogen bonding (I-Br) in 0.35 M ZnBr2 electrolytes17,34. Meanwhile, when charged to 1.4 V (C-1.4 V), I3- species appear (at 110 cm-1), which usually results in shuttle effect44. Additionally, during IBr formation, strong signals assigned to Br3- species appear at 160 cm-1, indicating the coexistence of IBr formation and hydrolysis19,35,45. The hydrolysis dissociates Br- to initiate Br0/Br- redox, which would accelerate negative electrode corrosion and lower conversion kinetics, leading to battery decay. In contrast, I-Br signals in 0.7 M BrAce electrolytes are well-kept from C-1.6 V to D-1.6 V (Fig. 3b), demonstrating reversible I0/I+ conversion. Notably, no polyiodide (I3-) or polybromide (Br3-) signals are detected, suggesting that the BrAce-regulated pathway effectively suppresses hydrolysis and prompts cascaded I-/I0/I+ redox. Additionally, in 0.7 M BrAce electrolytes, I-Br signals appear a redshift (Supplementary Fig. S15), compared to the counterparts in 0.35 M ZnBr2 electrolytes. This results from the electron-withdrawing effect of amide group. Notably, the in situ Raman spectra exhibit weakened peak intensity for C-I-Br compared to I-Br (Fig. 3a, b). This is attributed to that the amide group in BrAce reduces the electron density of adjacent atoms or functional groups, thereby lowering the polarizability of charged intermediates46. These results confirm that the I-Br bond in the BrAce system is not isolated like conventional one, but connected by electron-withdrawing organic group.

The intermediates at different charge/discharge potentials are further characterized by X-ray photoelectron spectroscopy (XPS). In 0.7 M BrAce electrolytes, both the Br 3d and I 3d XPS spectra of intermediates at high potentials (C-1.75 V, D-1.6 V) appear the signals associated with I+ species at 74.0 and 631.5 eV (Supplementary Fig. S16), respectively. Both the binding energies are different from those for the counterparts (70.3 eV and 631.8 eV) in 0.35 M ZnBr2 electrolytes (Supplementary Figs. S17 and S18). Specifically, I+ species signals in Br 3d of intermediates in 0.7 M BrAce electrolytes show a 3.7-eV higher binding energy than those in 0.35 M ZnBr2 electrolytes. Such a large gap manifests that, in addition to bonding between Br and I, the electron density around the Br also reduces, which could be caused by inductive effect of electron-withdrawing amide group. This is consistent with the result revealed by in situ Raman spectra in Fig. 3a-b, i.e., the C-I-Br pathway lowers the polarizability of charged product. In the I 3d XPS spectra, the characteristic peak of I+ species is closer to the C-I bond (631.4 eV), indicating similar chemical properties of the I+ species to alkyl iodide substances.

To further discern the C-Br bond evolution of BrAce, the positive electrode during GCD processes is characterized by in situ attenuated total reflectance-infrared spectroscopy (ATR-IR) and ex situ 1H/13C nuclear magnetic resonance (NMR)47. ATR-IR spectra in Fig. 3c reveal that, during GCD processes, the signals associated with -C-Br (at 600.6 cm-1), -CH2-Br (at 977.9 and 1411.4 cm-1, respectively), and -C=O (at 1650.9 cm-1) remain, suggesting that BrAce is not electrochemically decomposed48,49. However, when charged to 1.75 V (C-1.75 V) and discharged to 1.6 V (D-1.6 V), a new peak appears at 1055.5 cm-1, and meanwhile, the peak at 1411.4 cm-1 disappears, which could be associated with the isomerization of -CH2-Br caused by the formation of -CH2-IBr. Furthermore, 1H NMR and 13C NMR spectra at high potentials (from C-1.2 V to C-1.75 V and to D-1.6 V) in Fig. 3d, e appear split peaks of H (A: ~1.18 ppm; B: ~3.65 ppm) and C (A: ~18.23 ppm; B: ~50.09 ppm) of -CH2-Br, which is different from that of pristine BrAce (Supplementary Fig. S19a). For pristine BrAce, the H in -CH2-Br is under chemical equivalence, thereby showing no splitting. The splitting of H signals in -CH2-Br implies that the chemical equivalence is broken by isomerization of -CH2-Br. To simulate operational environment of BrAce in aqueous batteries, BrAce is dissolved in deionized water for 1H NMR characterization. The amide proton signals (2.45 ppm and 3.3 ppm) almost disappear (Supplementary Fig. S19b-d) due to the interference of hydrogen-bonding interaction between water and amide group. In contrast, the hydrophobic -CH2Br protons remain distinct at 3.8 ppm, demonstrating their relative insensitivity to aqueous solvation. The positive electrode samples during GCD cycling are subsequently analyzed. When charging beyond 1.2 V, a new resonance at 1.18 ppm appears (Fig. 3d). These peaks experience analogous potential-dependent shifts to the -CH2Br signal at 3.65 ppm (-CH2Br proton) (Fig. 3d). This emergent signal is assigned to one proton of the -CH2Br group, where asymmetric electronic environments resulting from interaction (chemical adsorption or bonding) with iodine species decouple the methylene protons, yielding distinct chemical shifts. Such decoupling implies covalent or coordinative bonding between BrAce and iodine intermediates during charge transfer. The splitting phenomena of H and C also accord with the ATR-IR results.

The peak upshift and downshift of 1H NMR peaks are usually caused by adjacent electron-donating and -withdrawing groups, respectively50. For 13C NMR spectra, the effects of adjacent electron-donating and -withdrawing groups on peak upshift and downshift are consistent with those in 1H NMR spectra51. As depicted in Fig. 3d-e, the electrochemical evolution during charging from 1.2 V to 1.4 V (stage ①②) initiates the I-/I0 conversion, where iodine species (I0) function as Lewis acids that initially coordinate with the bromine atom of the -CH2-Br moiety. Given the high polarizability of I0, this Br···I coordination could facilitate electron withdrawal from the Br-C bond through n→σ* anti-bonding orbital interactions. Consequently, the methylene group experiences deshielding, accounting for the downfield shifts in both 1H and 13C NMR spectra. Subsequently, during the voltage elevated to 1.5 V (stage ②③), the -CH2-Br···I0 undergoes isomerization to form I0···CH2···Br structure. In this rearranged state, I0 transitions into an electron donor, cooperatively enhancing electron density at the methylene carbon alongside bromine. This electronic redistribution elevates shielding effects, thereby inducing upfield shifts of the methylene proton and carbon resonances in NMR spectra. Electrostatic potential simulations further corroborate this dynamic behavior (Supplementary Fig. S20, Supplementary Data 1–3). When the CH2Br···I0 intermediate forms, iodine accumulates electron density while depleting electron density around the methylene group, rationalizing the initial downfield NMR shifts. Following isomerization to the I0···(CH2)···Br intermediate, I0 donates electron density to the methylene carbon, increasing its electron cloud density and inducing the upfield shift. These computational results align with Fig. 3d, e.

We further characterized the charged intermediates (C-1.75 V) in 0.7 M BrAce electrolytes by liquid chromatography-mass spectrometry (LC-MS). In the LC-MS spectra, a strong signal appears at m/z 264.8 (Supplementary Fig. S21), corresponding to the weight of NH2-C(O)-CH2-(IBr), confirming the presence of C-(I-Br) moiety. To identify the bonding mode of C-(I-Br), we investigated the vibration change of C-Br bond in the BrAce system. Based on the Raman spectra of different electrolytes (Supplementary Fig. S22), the characteristic vibration signals of SO42- and C-Br bond were identified (at 600 cm-1 and 700 cm-1, respectively). As shown in Supplementary Fig. S23, within the in situ Raman testing range of 500–800 cm-1, only the SO42- signals were detected during charge/discharge in the ZnBr2 system. In contrast, within the range of C-1.6 V to D-1.6 V, the BrAce system exhibited reversible split double peaks at around 630 cm-1, which is assigned to stretching vibration of C-(I-Br) with I-Br coupling effect. Meanwhile, the characteristic peak of the C-Br bond disappeared with the formation of C-(I-Br) and then gradually recovered after the disappearance of C-(I-Br) (from D-1.6 V to D-0.3 V). This confirms a reversible transformation between the C-Br bond and C-(I-Br). Based on the charge distribution of covalent bonds, the bonding of I+ enhances the covalent bond, leading to an increase in its force constant and a subsequent rise in vibration frequency (manifested as blue shift). This confirms that the C is connected with I+ in the C-(I-Br) moiety, i.e., its presence in the form of C-I+-Br. If bonding occurs in the form of C-Br-I+, the characteristic peak of the C-Br bond should undergo a certain degree of blue shift rather than disappearance after I+ bonding to the end of the C-Br bond. This phenomenon suggests that the C-Br bond breaks and re-bonds in the form of C-I+-Br.

To investigate the chemical properties of C-I-Br, we further compared the 1H NMR of C-I-Br samples (C-1.75 V) with commercial IAce and BrAce. As shown in Supplementary Fig. S24, the characteristic signal peak of the α-hydrogen attached to IBr in C-I-Br appeared between the characteristic peaks of IAce and BrAce. Notably, this signal peak is closer to that of IAce and farther from that of BrAce. It confirms similar chemical properties of the C-I-Br sample to IAce, and provides evidence that the α-carbon is bonded with I rather than Br. During the discharge process, R-CH2–I–Br is reduced to R-CH2Br and I2 (I+/I0 conversion), followed by the conversion of I2 to I- at the I0/I- stage. The reduced involvement of C-Br bonds in the discharge process accounts for the spectral differences between the charged states (C-1.4 V, C-1.2 V) and discharged states (D-1.4 V, D-1.2 V). The retention of the -CH2–I–Br signal at the high-voltage plateau (C-1.75 V/D-1.6 V) and its complete recovery to the original -CH2Br signal upon discharge (D-1.4 V/D-1.2 V) confirm the electrochemical reversibility of BrAce.

We further conducted iodine color reactions to verify the reversible transformation between the C-Br bond and C-I-Br, chemical differences between C-I-Br and I-Br, and similarity of chemical properties of C-I-Br to C-I bond in the IAce. Usually, non-polar iodides change color in deuterated chloroform (CDCl3), which essentially arises from the interaction between the low polarity of CDCl3 and iodides via van der Waals forces, and this weak interaction barely alters the electronic structure of iodides52. As shown in Supplementary Fig. S25a, BrAce and IAce appeared colorless and magenta in CDCl3, respectively; while I- (from ZnI2), I2, and I-Br appeared colorless, purple-black, and yellow-brown in CDCl3, respectively. When the products of the iodine positive electrode at different potentials (with the same iodine content) were dissolved in CDCl3, it was found that Br-C···I0 (C-1.3 V) shows the same color as BrAce, while the colors of I···C···Br (C-1.5 V) and C-I-Br (C-1.75 V, D-1.6 V) gradually approached that of IAce (Supplementary Fig. S25b). Finally, we assembled and tested a Zn||I2 battery using IAce (0.4 M, ~5 mg cm-2) as the iodine source, ZnBr2 (0.35 M) as the initiator, and carbon cloth as the positive electrode current collector. With the cutoff voltage of 0.3 − 1.75 V, distinct I-/I0/I+ redox couple characteristic peaks appeared (Supplementary Fig. S26). In addition, the battery showed no obvious capacity decay during 500 cycles, indicating a stable C-I-Br structure formed in the system as well. This result further verifies the reliability of the above conclusions.

Based on the discussion above, we summarize the carbon-halogen bond substitution pathway to I-/I0/I+ redox as follows (Fig. 3f, Supplementary Data 1-4):

(I-=I0): 2I- ↔ 2e- + R-CH2-Br(BrAce) → 2R-CH2-Br···I(0) (R, NH2-(C=O)-) (1)

(isomerization transition): 2R-CH2-Br···I(0) → 2R-(I0···(CH2)···Br) (2)

(I0=I+): 2R-(I0···(CH2)···Br) ↔ 2e- → 2R-CH2-I(+)-Br (3)

(I+=I0): 2R-CH2-I(+)-Br + 2e- → 2R-CH2-Br + I2 (4)

(I0=I-): I2 + 2e- → 2I- (5)

To investigate the redox kinetics and anti-hydrolysis of the carbon-halogen bond substitution pathway, energy barriers are further calculated by density functional theory (DFT). On the basis of the C-Br bond-regulated pathway summarized in Fig. 3f, structural models of species involved in I-/I0/I+ redox are firstly established and optimized. Subsequently, Gibbs free energy changes (ΔG) across all electrochemical steps are calculated (Fig. 4a, Supplementary Table S3). For comparison, ΔG values for I-/I0/I+ redox regulated by ZnBr2 are also calculated. In contrast to the ZnBr2-regulated pathway (blue), C-Br bond-regulated pathway (red) shows lowered barriers for I-/I0 redox (ΔG1(red) vs. ΔG2(blue)) and I0/I+ redox (ΔG3)13,43. This result verifies that C-Br bond substitution is far more thermodynamically stable than conventional interhalogen bonding regulation. This is strongly supported by the enhanced rate capability by BrAce (Fig. 2e).

At scan rates of 1–20 mV s-1, Zn||I2 batteries with BrAce electrolytes exhibit well-defined redox pairs (I-/I0 and I0/I+) (Supplementary Fig. S27). Even at 20 mV s-1, minimal peak separation (<120 mV) remains, confirming low polarization and fast kinetics enabled by BrAce; while Zn||I2 batteries with ZnBr2 electrolyte show widened redox peak separations, indicating severe polarization and sluggish kinetics53. Pseudocapacitive contributions in the I-/I0/I+ redox related to redox kinetics are further evaluated by power-law relationship (i = aνb, i: peak current; v: scan rate; a and b: constant) (Supplementary Fig. S28)53,54. The higher b-values feature the kinetics merits of both I-/I0 and I0/I+ redox in BrAce over ZnBr2 electrolytes. Deconvolution of pseudocapacitive (kν) and diffusion (kν1/2) contributions at difference scan rates (1-20 mV s-1) is further conducted (Supplementary Fig. S29)55. Zn||I2 batteries in BrAce electrolytes exhibit higher pseudocapacitive contributions (43%~80%) at various scan rates than those in ZnBr2 electrolytes (27%~69%). Typically, CV curves at 1 mV s-1 present the pseudocapacitive contribution distinction (47% vs. 30%) of I-/I0/I+ redox in BrAce and ZnBr2 electrolytes (Supplementary Fig. S30). These further validate enhanced I-/I0/I+ redox kinetics by BrAce relative to ZnBr2.

In addition, based on electrochemical impedance spectra (EIS) at various temperatures (Supplementary Figs. S31–S33) and the Arrhenius equation (Supplementary Fig. S34), we calculate the activation energy (Ea) for I-/I0 and I0/I+ redox, respectively (Fig. 4b)5. Similar to the results calculated by DFT, both I-/I0 and I0/I+ redox by BrAce show reduced Ea values compared with that by ZnBr2. Specifically, in the case of BrAce, 11.48 kJ mol-1 (charge) and 8.83 kJ mol-1 (discharge) for the I-/I0 conversion, which is 76%–78% lower than that of ZnBr2 system (56.73 and 33.52 kJ mol-1, respectively); while for I0/I+ redox conversion, Ea values are 11.36 kJ mol-1 (charge) and 10.40 kJ mol-1 (discharge) by BrAce, which is 39%–42% lower than that by ZnBr2 (19.68 and 17.1 kJ mol-1, respectively). Such results verify the kinetics advantage of the carbon-halogen bond substitution pathway.

Anti-hydrolysis of I+ species regulated by carbon-halogen bond substitution and interhalogen bonding is further investigated by DFT. To judge the influence of SO42- concentration on the stability of R-CH2-IBr, we tested Zn||I2 batteries using 1 M ZnSO4 + 0.7 M BrAce and 2 M ZnSO4 + 0.7 M BrAce as the electrolytes, respectively. These batteries both exhibit no significant capacity decay over 1000 cycles at 2 A g-1, with specific capacities of 340~350 mAh g-1 following an initial activation (Supplementary Fig. S35). Both iodine utilization and cycling stability are close to the 3 M ZnSO4 + 0.7 M BrAce. These results confirm that I0/I+ redox process is primarily governed by the modulation effect of carbon-halogen bond substitution rather than SO42- concentration. Therefore, during the DFT calculations of hydrolysis energy barrier for the I0/I+ redox reaction, the influence of ZnSO4 is rationally neglected. For IBr hydrolysis (Fig. 4c, top), water molecules tend to first attack the active axial δ hole, and then oxygen (electronegative) and hydrogen (electropositive) of H2O combine with I(+) and Br(-) of IBr respectively. Finally, IBr is split into HIO and HBr (Supplementary Data 5–8). This pathway has been mentioned in previous studies13. While for R-CH2-IBr hydrolysis (Fig. 4c, bottom), it is hypothesized with a similar pathway to the former, i.e., oxygen and hydrogen of H2O firstly combine with I(+) and Br(-) of IBr, respectively. The final species are IAce with -CH2-I, OH-, and HBr (Supplementary Data 8–11). After structural model optimization, the IBr and R-CH2-IBr hydrolysis barriers are calculated (by subtracting the free energy of the initial reactants from the free energy of the hydrolysis products of R-CH2-IBr or IBr) (Fig. 4d), where R-CH2-IBr shows a ~1.50-fold higher barrier than that of IBr (1.01 and 1.51 eV, respectively). This implies that R-CH2-IBr has enhanced anti-hydrolysis, which will enable enhanced I2 utilization and prolonged battery lifespan. Self-discharge tests further substantiate this point (Supplementary Fig. S36). After resting for 12 h at a fully charged state, Zn||I2 batteries using BrAce electrolytes exhibit a lower self-discharge rate, maintaining a CE of 77.44%. Conversely, batteries using ZnBr2 electrolytes undergo a CE decay to 42.6%, accompanied by the complete disappearance of the discharge plateau of I0/I+ redox. This observation indicates the high susceptibility of IBr to hydrolysis. In contrast, the majority of the discharge plateau of I0/I+ redox is preserved in BrAce electrolytes, verifying the stronger anti-hydrolysis capability of R-CH2-IBr.

In situ ultraviolet-visible (UV-Vis) spectral characterization is further conducted with a cuvette cell (Supplementary Fig. S37). As revealed by in situ UV-vis spectra, I3- (ca. 280 and 350 nm) and Br3- (ca. 295 nm) appear in ZnBr2 electrolytes, during discharging of batteries (Fig. 4e, Supplementary Fig. S38)35,44,45. In contrast, no I3- or Br3- signals can be detected in BrAce electrolytes (Fig. 4f, Supplementary Fig. S39). These results also correspond to the color change of electrolytes cycled (inset in Fig. 2f). The absence of I3- or Br3- in BrAce electrolytes could result from enhanced anti-hydrolysis of R-CH2-IBr, which is beneficial for shuttle-free I-/I0/I+ redox.

When certain methyl groups were introduced on the amide side additives (e.g., 2-bromo-N-methylacetamide, 2-bromo-N, N-dimethylacetamide) in place of BrAce, although I0/I+ redox could be triggered, the capacity and I2 utilization of the Zn||I2 batteries at 2 A g-1 are lower (~310 and 260 mAh g-1, 73.8% and 61.6%, respectively) (Supplementary Fig. S40). In addition, when using N-bromoacetamide, an additive with N-Br bonds, the capacity retention of the cell is only 37.5% after 500 cycles at 2 A g-1 (Supplementary Fig. S41), which further confirms the distinct efficacy of the C-Br bond regulation and the advantages of the BrAce.

Zn||I2 batteries involving I-/I0/I+ redox are usually evaluated with relatively low I2 loading (Supplementary Fig. S42). We examine the long-term cycling performance of Zn||I2 batteries with conventional I2 loading (~3.0 mg cm-2). During 2000 cycles at a small specific current of 2 A g-1, Zn||I2 batteries in BrAce electrolytes show stable cycling performance, while those in ZnBr2 electrolytes experience rapid capacity decay (Supplementary Figs. S43 and S44). At specific currents (10, 20, and 40 A g-1), Zn||I2 batteries in BrAce electrolytes show higher capacities and higher I2 utilization than those in ZnBr2 electrolytes (Supplementary Figs. S45 and S46). Specifically, Zn||I2 batteries in BrAce electrolytes can be operated over 13 000 cycles with 82.9% retention and 66.0% I2 utilization at 10 A g-1, over 8000 cycles with 95.2% retention and 61.3% I2 utilization at 20 A g-1, over 16 000 cycles with 95.6% retention and 55.2% I2 utilization at 40 A g-1, respectively. In contrast, I2 utilizations of Zn||I2 batteries in ZnBr2 electrolytes almost halve, only 35.8%, 35.2%, and 28.2% at 10, 20, and 40 A g-1, respectively. Moreover, for Zn||I2 batteries in BrAce electrolytes, dual plateaus corresponding to I-/I0 and I0/I+ redox are well-kept at different cycles. Compared with other Zn||I2 batteries (involving I-/I0 and I-/I0/I+ redox) reported elsewhere, the Zn||I2 batteries in BrAce electrolytes exhibit advantages in merits, in terms of rate capability and long-term cycling performance (Supplementary Fig. S47).

The effect of BrAce on Zn negative electrodes is scrutinized. In ZnBr2 electrolytes, Zn negative electrode of Zn||I2 batteries (I2 loading: ~3 mg cm-2) cycled experiences severe pitting corrosion (Supplementary Fig. S48); while that in BrAce electrolytes remains intact. Compared with ZSO and ZnBr2 electrolytes, BrAce electrolytes effectively lower corrosion, as substantiated by Tafel curves at 1 mV s-1 (Supplementary Fig. S49). CV curves of Zn||Cu cells, long-term Zn plating/stripping performance of Zn||Cu and Zn||Zn cells suggest enhanced kinetics and prolonged cycle life by BrAce (Supplementary Figs. S50 and S51). Zn negative electrodes cycled in BrAce electrolytes show smooth surfaces, as revealed by SEM and LSCM images (Supplementary Figs. S52 and S53). Also, surficial passivation of the Zn negative electrode is alleviated by BrAce as revealed by XRD patterns (Supplementary Fig. S54). These confirm that the efficacy of BrAce can be extended to Zn negative electrode modulation.

Due to severe shuttle and/or hydrolysis of I+ species, high-loading implementation of I0/I+ redox usually faces low utilization and short cycle life. To explore the loading limit, we first evaluated the rate capabilities of Zn||I2 batteries with high I2 loadings (8.6, 14.5, and 24.0 mg cm-2). SEM image reveals electrode thicknesses of 374 and 493 μm, respectively, corresponding to I2 loadings of 8.6 and 14.5 mg cm-2 (Supplementary Fig. S55). In BrAce electrolytes, Zn||I2 batteries with different I2 loadings show favorable rate capabilities (Fig. 5a, Supplementary Fig. 56a). Specifically, for the I2 loading of 14.5 mg cm-2, specific capacities of 4.6, 4.3, 4.1, 3.8, and 3.5 mAh cm-2 are obtained at 0.4, 0.8, 1.2, 2.0, and 3.2 A g-1 (areal current densities of 5.8, 11.6, 17.4, 29.0, and 46.4 mA cm-2), respectively. The corresponding GCD curves exhibit distinct plateaus for I0/I+ redox even at high rates (≥ A g-1) (Fig. 5b). In contrast, in ZnBr2 electrolytes, despite similar rate capability for the I2 loading of 8.6 mg cm-2 (Supplementary Fig. 57), severe capacity decay appears with higher I2 loadings (14.5 and 24.0 mg cm-2), especially at high rates (≥ A g-1) (Fig. 5c, Supplementary Fig. 56b). In the corresponding GCD curves, plateaus for I0/I+ redox almost disappear and severe polarization appear at high rates (≥ A g-1) (Fig. 5d, Supplementary Fig. 58), which is caused by sluggish I-/I0/I+ redox kinetics. Moreover, Zn||I2 batteries with high I2 loadings display enhanced I2 utilization in BrAce electrolytes relative to ZnBr2 electrolytes (Fig. 5e). Specifically, at the I2 loading of 24 mg cm-2, the I2 utilization of the Zn||I2 battery is 65.2% vs. 34.2% at 1.2 A g-1, and 59.2% vs. 2.5% at 2 A g-1. These results verify the kinetics advantage of carbon-halogen bond substitution in compatibility with high I2 loadings.

We further evaluated the long-term cyclability of Zn||I2 batteries with an I2 loading of 14.9 mg cm-2. Over 400 cycles at 3.2 A g-1/47.5 mA cm-2, an area-specific capacity of 3.85 mAh cm-2 is obtained with BrAce electrolytes, corresponding to a 99.8% retention and 61.5% I2 utilization (Fig. 5f). The corresponding differential charge/discharge profiles also confirm well-kept I-/I0/I+ redox reversibility (Supplementary Fig. S59). In contrast, Zn||I2 batteries in ZnBr2 electrolyte exhibit rapid capacity fade (∼80% loss during only 7 cycles), followed by short-circuiting, highlighting the efficacy of BrAce. Compared with other Zn||I2 batteries (involving I-/I0 and I-/I0/I+ redox) reported elsewhere, the Zn||I2 batteries in BrAce electrolytes exhibit advantages in merits, in terms of rate capability and long-term cycling performance (Supplementary Fig. S47).

To scrutinize the practicality of BrAce electrolytes, Zn||I2 batteries under practical conditions are further evaluated. At a low negative/positive electrode capacity (N/P) ratio, Zn||I2 batteries in BrAce electrolytes (I2 loading: 9.8 mg cm-2; N/P ratio: 2.4) show stability over 200 cycles at 1.2 A g-1/11.8 mA cm-2 with an area specific capacity of 3.3 mAh cm-2, corresponding to 90.4% retention and 98.5% CE (Fig. 6a). In contrast, Zn||I2 batteries in ZnBr2 electrolytes appear rapid capacity decay after initial 25 cycles. The corresponding GCD curves at different cycles (1st, 50th, 100th, 150th, and 200th) show well-kept dual plateaus in BrAce electrolytes (Fig. 6b, bottom); while in ZnBr2 electrolytes, the discharge curves appear evident fluctuation since the 50th cycle (Fig. 6b, top), which could be caused by accelerated pitting corrosion of thin Zn negative electrode (~20 μm).

Pouch cells with different high I2 loadings are further assembled in BrAce electrolytes. Figure 6d shows the digital photograph of a pouch cell with a theoretical capacity of 58.4 mAh, size of 4 × cm2, and electrolyte/positive electrode (cathode) (E/C) ratio of 22.2 μL mAh-1. After 210 cycles at 2.75 mA cm-2, this pouch cell delivers an actual capacity of 54.5 mAh, corresponding to 63% I2 utilization and 87.3% retention (Fig. 6c). The corresponding GCD curves at different cycles show well-kept dual plateaus (Fig. 6e). In addition, long-term cycling performance of pouch cell with a theoretical capacity of 35 mAh is evaluated. This pouch cell also shows stability over 312 cycles at 3.125 mA cm-2, corresponding to 66.6% I2 utilization and 86% retention (Supplementary Fig. S60). These results substantiate the practicality of pouch cells with BrAce electrolytes. Based on the commercial cost of components, the cost of BrAce electrolyte is as low as 0.1 US$ mL-1, which is lower than those of the electrolytes (e.g., highly concentrated salts, hybrid electrolytes) for I-/I0/I+ conversion (Fig. 6f, Supplementary Table S5)3–8,13,21,62. The positive electrode fabrication employs ambient-temperature wet processing with iodine powder and activated carbon (AC), circumventing expensive catalysts or carbon materials (e.g., carbon nanotubes). This approach delivers economic advantages through simplified processing and scalable production pathways. Compared to organic electrolytes, the aqueous ZnSO4–BrAce system exhibits higher ionic conductivity, lower viscosity, non-flammability, reduced toxicity, and minimal ecological footprint. The cost advantage and environmental impact advantage of BrAce electrolyte will pave the way for its application.

Methods
Chemicals
All reagents and starting materials were used as received without any further purification. Zn foil (99.9%, 20 and 100 μm) and Cu foil (99.9%, 12 μm) were purchased from Shenzhen Kejing Star Technology. Zinc bromide (ZnBr2, 99.9%), zinc sulfate heptahydrate (ZnSO4·7H2O, 99%), 2-(Bromomethyl)Tetrahydrofuran (BrTHF, 95%), and acetamide (Ace, 99%) were purchased from Aladdin. 2-bromoacetamide (BrAce, 97%), 3-bromopropionamide (3-BrPace, 97%), 2-chloroacetamide (2-ClAce, 97%), 2,2-difluoroacetamide (DFAce, 98%), iodoacetamide (IAce, 98%), 2-bromo-N-methylacetamide (97%), 2-bromo-N,N-dimethylacetamide (97%), N-bromoacetamide (97%), Bromoacetonitrile (BrAN, 97%), Bromopropanone (BrPK, 95%), and IBr (98%) were purchased from Macklin. Deuterated chloroform (CDCl3) and deuterated water (D2O) were purchased from Sigma Aldrich.

Electrolytes preparation
3 M (mol L-1) ZnSO4 solution (ZSO) was prepared by adding 3 M ZnSO4 to 1 mL of deionized water and stirring for 2 h at 25±1°C. Then, 0.35 M ZnBr2 and 0.7 M BrAce were added to the as-prepared ZSO solutions, respectively, and stirred for 2~4 h at 25±1°C to obtain ZnBr2 electrolytes and BrAce electrolytes. Similarly, other additives (e.g., Ace, BrAN, and BrTHF) were also configured into electrolytes at 0.7 M concentration for comparative investigations. All preparations were conducted in an ambient atmosphere.

Battery assembly
The I@AC composite material was made by sufficiently grinding the activated carbon (AC, 99.99%, Canrd) with iodine powders (99.99%, Aladdin) at a mass ratio of 1:1. The I@AC, Super P (99.9%, Canrd), and sodium carboxymethyl cellulose binder (CMC aqueous solution, 99.99%, Anaiji) at a mass ratio of 8:1:1 were mixed by grinding to a slurry and drop-cast onto the carbon cloths current collector (Suzhou Sineros Technology), and then dried (8 h in an oven at 60 oC) to obtain the I@AC electrode (single-side coated). The dried electrode was then punched into disks with a diameter of 12 mm using a precision disc cutting machine (Shenzhen Kejing Star Technology, MSK-T10). The average areal I2 loading of the I@AC composite was ∼2.5−3.0 mg cm−2 in the electrode. For the iodine-free positive electrode, the proportion of iodine was completely replaced by AC, while all other conditions remained unchanged. Metal foils (Zn, Cu) were also punched into 12 mm-diameter circular discs using the same precision disc cutting machine. Unless the temperature and environment are explicitly specified, all preparations were conducted in an ambient atmosphere at 25±1°C.

All cells were assembled in the configuration of CR2032-type coin cells. The cell structure was composed of two working electrodes and a glass fiber separator (Whatman GF/D, 16 mm in diameter, 675 μm in thickness); the remaining components included a stainless-steel spacer (1.0 mm thick) and a spring-loaded plunger (stainless steel, with a constant force of 1.5 kN). The Zn||I2 batteries were assembled using the as-obtained ZSO solution with different additives as the electrolytes, the I@AC electrodes as the positive electrodes, the metallic Zn foils (100 μm) as the negative electrodes, glass fibers as the separators. For the high depth of discharge (DOD) tests, the Zn foils with a thickness of 20 μm were used as the negative electrodes. The average areal I2 loading of the I@AC composite was ∼8.6−24.0 mg cm−2 in the electrode.

For the fabrication of single-layer pouch cells, the same slurry mentioned above was coated onto 4 cm × 4 cm carbon cloths, with the mass loading of I2 controlled at 13 mg·cm-2. The assembled electrodes (Zn foil: 100 μm, 4 cm × 4 cm) were placed into an aluminum-laminated bag and vacuum-sealed, with one electrolyte injection port reserved for electrolyte filling. Approximately 1.3 mL of electrolyte was injected through this port, followed by a second vacuum-sealing step. The cells were then rested for 6 h to ensure sufficient wetting of all components by the electrolyte. During the battery cycling test, an external pressure of 1.2 MPa was applied to the pouch cells using a fixture. The pouch cells were tested at a cutoff voltage of 0.6–1.8 V and a current density of 2.75 mA·cm-2.

Material characterizations
The ionic conductivities of electrolytes were measured by a conductivity meter (DDS-308A). XRD analysis was used to determine the phase structures of the negative electrode samples with a Rigaku instrument (CuKα, λ = 1.5418 Å). The measurements were conducted at a scan rate of 5–10°·min-1 and a step size of 0.02°. Infrared spectra of samples were recorded on an FT-IR spectrometer (Nicolet iS50R, Thermo Scientific). Raman spectra (HORIBA, LabRAM HR800) of the powder samples were measured at 25±1°C with laser excitation at 532 nm. The scanning electron microscope (SEM, Hitachi S-3400 N, beam voltage: 5–15 kV) with X-ray energy dispersive spectroscopy (EDS) was used to observe the morphology and elemental composition of the samples. Roughness of the surface of the resultant sample using laser scanning confocal microscopy (LSCM). The 1H and 13C NMR spectra were recorded by a Bruker AVANCE III 500 MHz Superconducting Fourier using CDCl3 as the solvent and trimethylsilane (TMS) as the standard internal substance. For ex situ 1H and 13C NMR spectra tests, the positive electrodes with different states of charge (SOC) were soaked in 0.6 mL CDCl3 for 30 min and filtered to obtain a clear solution, then transferred to the NMR tube for analysis. The UV-vis spectra were collected by a UV-visible near-infrared spectrophotometer (Shimadzu). The chemical states of the electrodes after tests were determined by XPS (Thermo Fisher-Escalab 250Xi), and binding energies were calibrated using carbon (284.8 eV for C 1s). The mass spectrum was collected by Thermo Fisher Scientific Q Exactive Focus to qualitative determination of the formation of hypervalent iodine products.

Preparation and characterization of the ex situ samples
For ex situ XPS, NMR, and mass spectrometry measurements, the cells were disassembled in an ambient atmosphere to investigate the surface chemical properties of the electrodes after cycling. After 3 cycles at a specific current of 0.5 A g⁻¹, the cycled iodine electrodes (at different states of charge) were harvested and rinsed three times with deionized water to remove residual electrolyte salts. Subsequently, the samples were dried at 60°C for 1 h. The dried electrode samples were then transferred to the characterization instruments using a dedicated airtight transfer vessel. All characterization tests were conducted at 25±1°C. For the characterization of the negative electrode, the cycling was performed for 50 cycles under the same specific current, with all other procedures remaining unchanged.

Preparation and characterization of the in situ samples
For in situ Raman, ATR-IR, and UV-Vis spectroscopy measurements, the iodine electrodes were first discharged to 0.3 V at a specific current of 0.5 A g-1 or 0.2 A g-1, followed by monitoring the variations of the electrodes over 1 full cycle (at different states of charge). All characterization tests were conducted at 25±1°C in an air atmosphere.

Theoretical simulation calculation
Free energy calculation by DFT. Density functional theory (DFT) chemical description for the geometry optimization, the electronic structure calculations, including energies, and frequencies of all the stationary points (reactants, transition states, products) were carried out by the Gaussian09 program package with M062X exchange-correlation functional and 6-311+G** basis set. Single energy calculations were performed by the aug-cc-pVTZ basis set.

Equilibrium potential calculation. The change in the Gibbs free energy (ΔG) can be calculated by using the electron-transfer numbers (n) and the difference in the electrochemical potential (ΔU).

ΔG = -neΔU

where n is the electron transfer number, e is the elemental charge, and ΔU is the electrochemical potential difference.

Calculation of hydrolysis energy barriers. Hydrolysis energy barriers (HEB) for I-Br bonds or C-I-Br bonds were calculated using the M062X/def2-TZVP level. The calculation formula is as follows:

A + H2O → B* + C + D

HEB = GB + GC + GD - GA - G(H2O)

where G is the Gibbs free energy value of I-Br or C-I-Br, and GB, GC, and GD correspond to the Gibbs free energy values of the hydrolysis products of I-Br or C-I-Br in water attack, respectively.

Electrochemical measurements
For the assembly of Zn||Zn symmetric cells, two identical Zn foils were sandwiched between a piece of glass fiber separator and then assembled into a coin cell under an ambient atmosphere. The Zn||Cu half-cells were fabricated similarly to the symmetric cells, with a cutoff voltage of 0.5 V. In the three-electrode system, Zn foil, Pt plate, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. These electrodes were utilized to perform LSV, Tafel and CV measurements on a Gamry electrochemical workstation. For LSV measurements: the corresponding scan rate was 1 mV·s-1, and the cutoff voltage was -0.6–2.5 V. For CV measurements: the corresponding scan rates were in the range of 0.5–20 mV·s-1, and the cutoff voltage was 0.3–1.75 V. GCD tests were carried out on a Neware battery test system (model CT-4008-10V50mA-164, from Shenzhen, China) within an electrochemical window of 0.3 − 1.75 V. EIS measurements were recorded by a Gamry electrochemical workstation, with a frequency range spanning from 0.01 Hz to 100 kHz. The measurements were performed in potentiostatic mode, with a sinusoidal amplitude of 10 mV, and 10 data points collected per frequency decade. Before each measurement, the cell was stabilized at the open-circuit potential (OCP) for at least 1 h until the voltage change was less than 1 mV per minute.

All batteries in this study were assembled and tested in an ambient atmosphere at 25±1°C. The capacity retention in the cycling test is calculated as the ratio of the discharge capacity of the last cycle to the discharge capacity of the first cycle. The calculation formula for self-discharge is the ratio of the discharge capacity after resting to the charge capacity of the charging cycle prior to resting. All data were obtained from at least three replicate measurements.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I appreciate the authors’ efforts to elucidate the proposed C–I bond formation. However,\nthe collected 1H and 13C NMR spectra at various states of charge and other\nsupplemented experiments do not provide convincing evidence for the claimed C–I(+)–\nBr covalent bond, which is the central mechanism of this study. If such a covalent bond\nwere indeed formed, the C–I(+)–Br moiety should behave analogously to an alkyl iodine\nspecies in organic chemistry. In that case, it would be expected to exhibit characteristic\nspectroscopic signatures and chemical reactivity distinct from those of simple IBr. The\nabsence of these features casts doubt on the proposed structural assignment.\nFurthermore, if the “electron-withdrawing amide group of BrAce diminishes electron density in adjacent atoms,” then the\nadjacent I+ center should be less polarized. A less polarized I+ would be expected to exhibit a lower valence state\ncompared to IBr, which contradicts both the XPS data and the 13C NMR results. In addition, such a less polarized I+ in the\nC–I(+)–Br structure should yield a lower discharge voltage than conventional IBr, which is not observed.\nThe self-discharge tests also raise concerns: the I+/I2 plateau is nearly absent in the charge–discharge curves, suggesting\nthat BrAce electrolytes fail to stabilize I+ effectively. Moreover, under high iodine loading, the capacity contribution from the\nI+/I0 redox process is minimal, indicating that the I0 → I+ oxidation step may be kinetically hindered.\nThe comparison of voltage polarization reported in Figure 2e is not fair.\nGiven these issues, the mechanistic interpretation of iodine transformations remains insufficiently justified. I regret that I\ncannot provide a positive assessment of this work again.", "reviewer_2": "The authors have addressed my comments successfully.", "reviewer_3": "Comment on NCOMMS-25-35778A: (Carbon-Halogen Bond Substitution Enables High-Utilization Cascaded Iodine Redox\nin Noncorrosive Dilute Electrolytes)\nThe authors have made appropriate revisions according to the suggestions. Therefore, I suggest acceptance for publication.\n1. Line 114/532: change “Hametter” to “Hammett”.\n2. Lines 230/235: ATR-IR units should be cm⁻¹, not nm; please check for consistency.\n3. Replace “electron-drawing” with “electron-withdrawing” in the text and Fig. 1 as they\nare different in meaning.\n4. Supplementary figures: change “figure 50” to “Supplementary Fig. S50” and apply\nthe same style to later figures.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Response Letter\nDear Reviewers,\nThank you very much for your message pertaining to our submission. We have studied\nthe comments and suggestions. In our point-by-point responses, the Reviewers’\ncomments are presented in black, our responses are provided in Blue. The modifications\nimplemented during the revision are indicated in both the revised Manuscript and the\nSupporting Information, highlighted in Yellow. The following is a summary of our\nrevisions and responses to referees.\nReviewer #2:\nThe authors have addressed my comments successfully.\nReply:\nWe greatly appreciate your professional evaluation of our research.\nReviewer #3:\nComment on NCOMMS-25-35778A: (Carbon-Halogen Bond Substitution Enables\nHigh-Utilization Cascaded Iodine Redox in Noncorrosive Dilute Electrolytes) The\nauthors have made appropriate revisions according to the suggestions. Therefore, I\nsuggest acceptance for publication.\nReply:\nThank you for your positive and professional evaluation of our work.\n1. Line 114/532: change “Hametter” to “Hammett”.\n2. Lines 230/235: ATR-IR units should be cm⁻¹, not nm; please check for consistency.\n3. Replace “electron-drawing” with “electron-withdrawing” in the text and Fig. 1 as\nthey are different in meaning.\n4. Supplementary figures: change “figure 50” to “Supplementary Fig. S50” and apply\nthe same style to later figures.\nReply:\nThank you for your suggestions. Issues in the manuscript, including typos, unit\ninconsistencies, improper word usage, and incorrect abbreviations of image formats,\nhave been corrected. Please review.\nReviewer #4:\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReply:\nThanks for your valuable comments on our manuscript in conjunction with the\nother reviewers.\nReviewer #1:\nI appreciate the authors’ efforts to elucidate the proposed C–I bond formation. However,\nthe collected 1H and 13C NMR spectra at various states of charge and other\nsupplemented experiments do not provide convincing evidence for the claimed C–I(+)–\nBr covalent bond, which is the central mechanism of this study. If such a covalent bond\nwere indeed formed, the C–I(+)–Br moiety should behave analogously to an alkyl iodine\nspecies in organic chemistry. In that case, it would be expected to exhibit characteristic\nspectroscopic signatures and chemical reactivity distinct from those of simple IBr. The\nabsence of these features casts doubt on the proposed structural assignment.\nReply:\nThank you for your thoughtful questions.\nWe need to politely reiterate that, given the varying sensitivity of different\ncharacterization techniques to the four-electron iodine conversion regulated by BrAce,\nit is quite challenging to rely on any single method to capture the full picture of the\nreaction. Instead, comprehensive characterization techniques are essential to obtain a\ncomplete evidence chain.\nIn our investigation, we have employed an integrated array of analytical methods,\nincluding Raman spectroscopy, NMR, FTIR, LC-MS, XPS, and UV-Vis, to confirm the\nexistence of the C–I(+)–Br bond and its marked chemical distinction from conventional\nI–Br species.\nFirstly, we monitored the dynamic changes of the iodine cathode in different\nelectrolytes (BrAce and ZnBr ) via in-situ Raman spectroscopy (Fig. 3a, b).\n2\n(1) During the charge/discharge process, reversible signals of the I-Br bond at 420-\n440 cm-1 were clearly identified in both the BrAce and ZnBr systems. This verified\n2\nthat the Br atom of the C-Br bond in the BrAce molecule can bond with I+ activated\nelectrochemically.\nMeanwhile, in the ZnBr system, distinct characteristic peaks of I - and Br - were\n2 3 3\nalso detected, which is attributed to the slow kinetics and hydrolysis of traditional\ninterhalogen compounds. In contrast, in the BrAce system, only a reversible I+ species\nsignal peak was observed at 420-440 cm-1 (within the range of C-1.6 V to D-1.6 V),\nwith no by-product signals detected. This indicates that the I+ species formed in BrAce\nexhibits significant chemical differences from I-Br.\n(2) Compared with traditional I-Br, the I+ species in the BrAce system shows an\nobvious redshift, which is consistent with the electron-withdrawing inductive effect\n(Supplementary Fig. S15). In addition, the intensity of the I+ species in the BrAce\nsystem is much weaker than that in the ZnBr system. Generally, the intensity of Raman\n2\nscattering fundamentally depends on the magnitude of polarizability change during\nmolecular vibration. In this work, the electron-withdrawing amide group in BrAce\nreduces the electron density of adjacent atoms or functional groups, thereby decreasing\nthe dynamic polarizability in the vibration mode and ultimately leading to weakened\nRaman scattering intensity of C-(I-Br). These results confirmed that the I-Br bond in\nthe BrAce system is not isolated like conventional one, but connected by electron-\nwithdrawing organic group.\n(3) Subsequently, we clearly confirmed the existence of C-(I-Br) moiety by LC-\nMS. As shown in Supplementary Fig. S21, a strong signal peak appeared at a m/z of\n264.8 when C-1.75 V. This peak is consistent with the molecular weight of the\ntheoretically predicted product NH -C(O)-CH -I-Br, thus verifying the existence of the\n2 2\nC-(I-Br) structural unit.\nSecondly, we further analyzed the bonding mode of C-(I-Br):\n(1) Based on Raman spectra of different electrolytes (Fig. R1), we identified the\ncharacteristic vibrations of SO 2- and C-Br bond (600 cm-1 and 700 cm-1, respectively).\nintensity of the SO 2- characteristic peak changed in the ZnBr system, which may be\n4 2\ncaused by the variation of the electrolyte environment during charge/discharge. In sharp\ncontrast, within the range of C-1.6 V to D-1.6 V, the BrAce system exhibited reversible\nsplit double peaks at around 630 cm-1, which is assigned to the stretching vibration of\nC-(I-Br) with the I-Br coupling effect.\n(2) Meanwhile, the characteristic peak of the C-Br bond disappeared with the\nformation of C-(I-Br) and gradually recovered after the disappearance of C-(I-Br) (from\nD-1.6 V to D-0.3 V). This indicates a reversible transformation between the C-Br bond\nand C-(I-Br). Based on the charge distribution of covalent bonds, the bonding of I+\nenhances the covalent bond, leading to an increase in its force constant and a subsequent\nrise in vibration frequency (manifested as blueshift). This confirmed that the C is\nconnected with I+ in the C-(I-Br) moiety, i.e., its presence in the form of C-I(+)-Br.\nIf bonding occurs in the form of C-Br-I(+), the characteristic peak of the C-Br bond should\nundergo a certain degree of blueshift rather than disappearance after I+ bonding to the\nend of the C-Br bond. This phenomenon suggests that the C-Br bond breaks and re-\nbonds in the form of C-I(+)-Br.\nThirdly, we further investigated the chemical properties of C-I(+)-Br:\n(1) The 1H NMR spectra of the C-I(+)-Br sample was compared with those of IAce\n(NH -C(O)-CH -I) and BrAce. As shown in Supplementary Fig. S24, the characteristic\n2 2\nsignal peak of the α-hydrogen attached to IBr in C-I(+)-Br appeared between the\ncharacteristic peaks of IAce and BrAce. Notably, this signal peak is closer to that of\nIAce and farther from that of BrAce. This result confirms that the C-I(+)-Br sample\nexhibits chemical properties similar to IAce and provides evidence that the alpha-\ncarbon is bonded with I rather than Br.\n(2) Iodine color reaction clearly confirmed the reversible transformation between\nthe C-Br bond and C-I(+)-Br, significant chemical differences between C-I(+)-Br and I-\nBr, and the similarity of chemical properties of C-I(+)-Br to C-I bond in the IAce.\nThe iodine color reaction is commonly used to determine the presence and characteristics\nof iodides (Nature 1947, 160, 87). Non-polar iodides change color in deuterated\nchloroform (CDCl ), which essentially arises from the interaction between the low\n3\npolarity of CDCl and iodides via van der Waals forces, and this weak interaction barely\n3\nalters the electronic structure of iodides. As shown in Fig. R3a, BrAce and IAce appear\ncolorless and magenta in CDCl , respectively, while I- (from ZnI ), I , and I-Br appear\n3 2 2\ncolorless, purple-black, and yellow-brown in CDCl , respectively. When the products\n3\nof the iodine cathode at different potentials (with the same iodine content) were\ndissolved in CDCl , it was found that C-Br…I0 (C-1.3 V) had the same color as BrAce,\n3\nwhile the colors of I…C…Br (C-1.5 V) and C-I-Br (C-1.75 V, D-1.6 V) gradually\napproached that of IAce (Fig. R3b).\nFinally, we assembled a Zn||I battery using IAce (0.4 M, ~5 mg cm-2) as the\niodine source, ZnBr (0.35 M) as the initiator, and carbon cloth as the cathode current\n2\ncollector. The results showed that within the voltage window of 0.3–1.75 V, distinct I-\n/I0/I+ redox couple characteristic peaks appeared (Fig. R4). In addition, the battery\nshowed no obvious capacity decay during 500 cycles, indicating that a stable C-I(+)-Br\nstructure was also formed in the system. This result further verifies the reliability of the\nabove conclusions.\nThis relevant data and explanation have been added to the revised Manuscript\n(Pages 7-8, 10-11) and Supporting Information (Pages 11-13). Please see the\nhighlighted parts.\nFig. R1 Raman spectra of ZSO, ZSO+ZnBr , ZSO+Ace, and ZSO+BrAce electrolytes.\n2\na b c\nZnBr BrAce\ne\ng\nr\na\nh C-I-Br\nc\ne s\nm id\nTi\ne\ng\nr\na\nh\nc\n0.9 1.8 500 600 700 800 500 600 700 800\nVoltage (V) Raman shift (cm-1) Raman shift (cm-1)\nFig. R2 (a) Typical charge/discharge curves and corresponding in-situ Raman spectra\nof cathodes in (b) 0.35 M ZnBr and (c) 0.7 M BrAce electrolytes.\n2\na\nBrAce IAce ZnI I I-Br\n2 2\nb\nC-1.3 V C-1.5 VC-1.75 V D-1.6V D-1.2V D-0.3V\nFig. R3 (a) Photographs for BrAce, IAce, and different iodides dissolved in CDCl .\n3\n(b)\nPhotographs for the products of the iodine cathode at different potentials dissolved in\nCDCl .\n3\na b c\n6 3\n1.8 100\nV ( V o ) lt a g e\n0\n1\n.\n.\n6\n2 -1\nd\n( m ) Q A / h d V V\n-3\n0 3 -2\nC\n) apacity ( m Ah c m 1 2\n2\n4 6 8\n0\n0 0 0 C E ( %)\nIAce+ZnBr 10 mA cm-2 10 mA cm-2 10 mA cm-2\n2 -6 0 0\n0.0 0.5 1.0 1.5 0.6 1.2 1.8 0 100 200 300 400 500\nCapacity (mAh cm-2) Voltage (V) Cycle number\nFig. R4 (a) GCD curves, (b) corresponding differential charge-discharge curves, and (c)\ncorresponding long-term cycling performance of Zn∥I batteries with IAce+ZnBr\n2 2\nelectrolyte.\n2. Furthermore, if the “electron-withdrawing amide group of BrAce diminishes electron\ndensity in adjacent atoms,” then the adjacent I+ center should be less polarized. A less\npolarized I+ would be expected to exhibit a lower valence state compared to IBr, which\ncontradicts both the XPS data and the 13C NMR results. In addition, such a less\npolarized I+ in the C–I(+)–Br structure should yield a lower discharge voltage than\nconventional IBr, which is not observed.\nReply:\nThank you for your thoughtful questions.\nThe influence of microscopic electron cloud density on the macroscopic valence\nstate of a compound depends on the bonding type of compounds (Angew. Chem. Int.\nEd. 2020, 59, 984-1001). Both C-X bonds and I-X compounds exist as covalent bonds,\nwhere atoms form chemical bonds through shared electron pairs. The electron pairs are\n\"shared\" between atoms, with no obvious electron transfer. Although the electron-\nwithdrawing effect can alter the distribution of local electron cloud density, it cannot\nbreak the electron-sharing characteristic of covalent bonds. Therefore, it does not cause\natoms to actually gain or lose electrons, and thus does not directly change the valence\nstate.\nUsually, I+ species exist in the form of interhalogen compounds (e.g., IBr, ICl).\nIn concentrated electrolytes containing halogen anions (X- = Br-, Cl-), the I-X can further\nform chain-like molecular structures (e.g., [···I-X···I-X···]n) or anion clusters (e.g., IX -,\n2 2 3\n2025, https://doi.org/10.1002/adma.202415979). It can be seen that when the I+ center\nis bonded to multiple electron-withdrawing groups (Cl-, Br-), there is no significant shift\nin the XPS spectra of the I+ species. In our work, in the I 3d XPS spectra\n(Supplementary Fig. S18), the characteristic peak of C-I-Br is slightly lower than that\nof I-Br (631.5 eV vs. 631.8 eV). Notably, it is closer to the C-I bond position (631.4\neV). This result is consistent with the literature above. This also provides evidence that\nC-I-Br exhibits similar properties to alkyl iodide substances.\nThe discharge voltage and polarization of a battery also need to consider multiple\nfactors, such as product stability and anode corrosion. When the charging products\nundergo hydrolysis, the battery polarization effect can be intensified, leading to a\ndecrease in discharge voltage (Nat. Energy 2024, 9, 714-724). This is because\nhydrolysis results in the formation of various by-products, which in turn causes the\nmismatch between ion transport and the demand of electrode reactions. As a\nconsequence, ohmic polarization, electrochemical polarization, and concentration\npolarization are all significantly enhanced, making the actual discharge voltage deviate\nfrom the equilibrium potential. The relevant data and explanation are as follows:\n1) Combining with in-situ Raman spectra, UV-Vis spectra, and hydrolysis energy\nbarrier calculations, it can be concluded that severe hydrolysis and the shuttling effect\nof I - and Br - will significantly disrupt the I-Br reaction pathway. In contrast, C-I-Br\n3 3\nexhibits a higher hydrolysis energy barrier and better stability.\n2) As shown in Supplementary Fig. S47, compared with BrAce, ZnBr exhibits\n2\nstronger corrosivity toward the Zn anode, which hinders ion transport at the anode\ninterface. Therefore, the C-I-Br exhibits higher capacity than the I-Br, with a more\ncomplete high-plateau reaction, higher discharge voltage, and lower polarization.\nMeanwhile, the multi-electron conversion systems of I-/I0/I+ and Br-/Br0 reported in\nrelevant literature (Adv. Mater. 2024, 36, 2401924; Adv. Mater. 2025,\nhttps://doi.org/10.1002/adma.202415979) also indicates that after activation of the Br-\n/Br0 redox couple, the discharge plateau of I-/I0/I+ decreases to a certain extent due to\nthe aggravation of side reactions (Fig. R6).\nThis relevant data and explanation have been added to the revised Manuscript\n(Page 8). Please see the highlighted parts.\nFigure Redacted\nFig. R5 Multi-electron conversion systems reported in relevant literature: (a) I-/I-\nCl/ICl - (Nat. Commun. 2023, 14, 1856); (b) I-/IBr -/IBr - (Adv. Mater. 2025,\n2 2 3\nhttps://doi.org/10.1002/adma.202415979).\nFigure Redacted\nFig. R6 Multi-electron conversion systems reported in relevant literature: (a) I-/I0/I+(A\ndv. Mater. 2024, 36, 2401924); (b) Br-/Br0 (Adv. Mater. 2025,\nhttps://d oi.org/10.1002/adma.202415979).\n3. The self-discharge tests also raise concerns: the I+/I plateau is nearly absent in the\n2\ncharge–discharge curves, suggesting that BrAce electrolytes fail to stabilize I+ effectively.\nMoreover, under high iodine loading, the capacity contribution from the\nI+/I0 redox process is minimal, indicating that the I0 → I+ oxidation step may be\nkinetically hindered.\nReply:\nThank you for your insightful suggestions.\nIt has been acknowledged that in terms of self-discharge performance, 4e- Zn||I\n2\nbatteries do not show obvious advantages compared with traditional 2e- Zn||I batteries.\ncompounds are prone to hydrolysis, due to the presence of active axial δ-hole of\ninterhalogen bonds. The hydrolysis of I+ significantly impairs its self-discharge\nresistance. Nevertheless, the BrAce system still exhibits remarkable advantages over\nthe traditional ZnBr system: after 12 h of storage, its capacity retention rate is much\n2\nhigher than that of ZnBr (77.44% vs. 42.6%).\nthe state-of-the-art relevant literature on 4e- Zn||I batteries via other modulation\n2\nstrategies (e.g., high-concentration salt electrolyte, Nat. Commun. 2021, 12, 170;\nsupport adsorption-catalysis, Sci. Bull. 2024, 69, 1674-1685; Organic halide additive,\nJ. Am. Chem. Soc. 2025, 147, 19, 16350-16361; eutectic electrolytes, Adv. Energy Mater.\n2025, https://doi.org/10.1002/aenm.202501460, and N-I coordination, Energy Environ.\nSci. 2023, 16, 4502-4510). As shown in the Fig. R7 and Table R1, it can be seen that\ncompared with these strategies, the self-discharge performance regulated by BrAce\nremains at a relatively high level. Therefore, regulating 4e- iodine conversion via\ncarbon-halogen bonds is a feasible strategy. In the future, a combination of multiple\nregulation approaches may be adopted to better address the severe self-discharge issue\ncaused by I+ hydrolysis.\nNotably, compared with existing strategies, carbon-halogen bond regulation\nexhibits distinct kinetic advantages. As shown in Fig. R8, existing strategies can barely\nachieve charge-discharge tests under high rates. When the current density exceeds 5 A\ng-1, the capacity contributed by the I0/I+ conversion decreases sharply; even though\nsome literature has conducted tests at current densities above 20 A g-1, they can hardly\nmaintain high-plateau conversion (1–2 mg cm-2). In contrast, the BrAce system enables\nultrahigh-rate operation (1–60 A g-1) with a load of 4 mg cm-2 while sustaining stable\ndual-plateau conversion (Fig. R10). The excellent rate capability indicates that it\npossesses fast reaction kinetics. Meanwhile, under high-loading conditions, the existing\nsystems only show a very low proportion of capacity contributed by high-plateau\nconversion when the areal capacity reaches 2 mA h cm-2 (Fig. R9); in contrast, the\nBrAce system can still maintain stable dual-plateau conversion at a high areal capacity\nof 3.85 mA h cm-2 and a large current density of 47.5 mA cm-2.\nAll these results demonstrate that the carbon-halogen bond regulation strategy\nreduces the reaction barrier of the I0/I+ conversion and enhances its conversion kinetics.\nFigure Redacted\nFig. R7 Different strategies for regulating the self-discharge performance of 4e-iodine\nconversion. (a) High-concentration salt electrolyte (Nat. Commun. 2021, 12, 170), (b)\nsupport adsorption-catalysis (Sci. Bull. 2024, 69, 1674-1685), (c) organic halide\nadditive (J. Am. Chem. Soc. 2025, 147, 19, 16350-16361), (d) eutectic electrolyte (Adv.\nEnergy Mater. 2025, https://doi.org/10.1002/aenm.202501460), and (e) N-I\ncoordination (Energy Environ. Sci. 2023, 16, 4502-4510).\nFigure Redacted\nFig. R8 Different strategies for regulating the GCD curves of 4e-iodine conversion. (a\n) High-concentration salt electrolyte (Angew. Chem. Int. Ed. 202\n5, https://doi.org/10.1002/anie.202515633), (b) Eutectic electrolyte (Angew. Chem. I\nnt. Ed. 2025, https://doi.org/10.1002/anie.202514375), (c) Organic halide additive (Ang\new. Chem. Int. Ed. 2025, https://doi.org/10.1002/anie.202513747), (d) Support adsorpt\nion-catalysis (Adv. Mater. 2024, 36, 2312246), and (e) N-I coordination (J. Am. Chem.\nSoc.2025, 147, 30, 26889–26897).\nFigure Redacted\nFig. R9 GCD curves of 4e- iodine conversion under (a) high loading (Angew. Chem. I\nnt. Ed. 202 4, 63, e202 404784) and (b) pou ch cell (Adv. Mater. 2025,\nhttps://do i.org/10.1002/adma.2025141 17) conditions in relevant literature.\nTable R1. Different self-discharge performance of 4e- iodine conversion in literatures.\nDifferent strategies Rest time CE\nHigh-concentration salt 12 h 80.9%\nelectrolyte\nSupport adsorption- 6 h 70.5%\ncatalysis\nOrganic halide additive 6 h 82.4%\nEutectic electrolyte 12 h 63.67%\nN-I coordination 3 h 93.5%\nThis work 12 h 77.44%\n4. The comparison of voltage polarization reported in Figure 2e is not fair.\nReply:\nThanks for the thoughtful question.\nWhen we compared the discharge curves at different rates individually (Fig. R10),\nwe can see that at current densities ranging from 1 to 60 A g-1, the discharge plateaus\ncorresponding to the I-/I0 and I0/I+ redox couples in the BrAce-containing system are\nwell-maintained. Additionally, the discharge capacity is much higher than that of the\nZnBr system, and the voltage polarization is significantly lower than that of the ZnBr\n2 2\nsystem.\nThis relevant data and explanation have been added to the revised Manuscript\n(Page 7) and Supporting Information (Page 8). Please see the highlighted parts.\nFig. R10 Discharge curves of Zn||I batteries at ultrahigh rates of 1-60 A g-1 with BrAce\n2\nand ZnBr electrolytes.\ninsufficiently justified. I regret that I cannot provide a positive assessment of this work\nagain.\nReply:\nWe greatly appreciate your concerns about our research and your professional\ncomments. We have provided the above response to address your concerns."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I don't have other comments. The manuscript can be published."}, "author_response": ""}]

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