Dual‐Affinity Interphase Engineering Enables Stable Aqueous Zn–S Batteries

ABSTRACT

Aqueous Zn–S batteries have garnered significant attention for grid‐scale storage but suffer from rapid capacity fade and sluggish reaction kinetics. Although existing strategies can improve redox reversibility, they fail to fundamentally address capacity attenuation arising from oxidation‐driven ZnS decomposition loss. In this study, a nano‐copper‐based cathode/electrolyte interphase (Cu CEI) featuring a unique sulfur/ZnS dual‐affinity is rationally designed to accelerate both S─S and Zn─S bond dynamics, effectively preventing ZnS accumulation and suppressing its decomposition via preferential Cu‐ZnS binding. Specifically, the strong binding affinity of the Cu CEI stabilizes ZnS by reducing its direct contact with interfacial water. Meanwhile, the strong interaction between Cu nanoparticles and S₈ activates ring‐opening and facilitates S─S bond cleavage, elevating the discharge voltage to 0.75 V. Cu‐mediated weakening of Zn─S bonds in ZnS synergistically lowers the apparent activation energy from 69.4 to 29.5 kJ mol⁻¹, establishing a robust interfacial redox pathway with a low voltage hysteresis of 0.23 V. Consequently, the Cu CEI enables Zn–S system with excellent cycling stability over 1000 cycles at 5 A g⁻¹ and a high areal capacity of ∼6.5 mAh cm⁻² over 200 h in a pouch cell, underscoring the practical feasibility of this dual‐affinity interphase design for high‐performance Zn–S batteries.

A nano‐copper‐based cathode/electrolyte interphase (Cu CEI) with unique sulfur/ZnS dual‐affinity is engineered to concurrently regulate S─S and Zn─S bond dynamics and minimize ZnS exposure to interfacial water, thereby suppressing ZnS decomposition and enabling a high‐performance aqueous Zn–S battery with low voltage polarization, high output voltage, and long‐term cycling stability.

1. Introduction

As a low‐cost and environmentally benign conversion‐type cathode [1, 2, 3], sulfur (S) undergoes a one‐step solid‐to‐solid (STS) conversion mechanism between S and ZnS in aqueous Zn–S battery systems [4]. This reaction pathway inherently avoids the shuttling of soluble sulfur intermediates, which drives rapid capacity decay in Li‐S batteries using organic electrolytes, while simultaneously offering an ultrahigh theoretical capacity of 1675 mAh g⁻¹ [5, 6, 7, 8, 9, 10, 11, 12]. However, during repeated cycling, the intrinsically high bond dissociation energies of S─S bonds in S₈ and Zn─S bonds in ZnS impose substantial energetic penalties for their continuous cleavage and reformation. Meanwhile, the rapid capacity fade also presents a major challenge to the cycling stability of Zn–S batteries. In addition, the pronounced volumetric mismatch between S and ZnS induces irreversible structural deformation of sulfur particles [13, 14, 15, 16, 17, 18], further aggravating polarization and accelerating capacity fading [19, 20, 21]. Therefore, simultaneously accelerating conversion kinetics and suppressing capacity decay is essential for developing high‐performance aqueous Zn–S batteries.

Current research efforts primarily aim to overcome the sluggish ZnS↔S conversion by employing catalyst or redox‐mediator engineering [22, 23, 24, 25, 26], electrolyte optimization [22, 27, 28, 29], and electrostatic mediation strategies [4, 30, 31]. Among these, iodide‐based redox mediation (I⁻) is the most widely adopted. This strategy requires charging to a relatively high potential (>1.2 V vs. Zn/Zn²⁺) to oxidize I⁻ to I⁰ [32], which subsequently promotes ZnS oxidation (Figure 1a). Nevertheless, this mediation process contributes little to sulfur reduction during discharge, thereby requiring a high charge cut‐off voltage and resulting in a low average output voltage and compromised energy efficiency. Alternatively, the development of cosolvents (e.g., propylene glycol methyl ether; N,N‐diethylformamide) is demonstrated to facilitate the reduction of S₈ to ZnS during discharge [29, 33]. However, achieving simultaneous enhancement of both charge and discharge kinetics within a limited operating voltage window remains highly challenging, originating from the insufficient ability of existing strategies to directly regulate the strong S─S and Zn─S bonds at the atomic scale. In particular, the large activation barrier associated with the initial S₈ ring‐opening imposes unavoidable kinetic and mass‐transport constraints. The stable crown‐shaped S₈ molecules exhibit extremely low electronic conductivity (∼5 × 10⁻²⁸ S cm⁻¹) due to poor electron delocalization [29, 31], while the high S─S bond dissociation energy (∼169.5 kJ mol⁻¹) [34] further hinders both electron transfer and S₈ ring‐opening during discharge, collectively limiting the practical voltage output of Zn–S batteries. Beyond kinetic limitations, the chemical instability of ZnS in aqueous environments represents another often‐overlooked challenge. ZnS is susceptible to H₂O/O₂‐induced corrosion at the cathode/electrolyte interface [35, 36], as well as electrochemically driven oxidation during charging, causing its conversion into soluble sulfate species (Equation 1). This irreversible process results in pronounced active‐material loss and rapid capacity decay, particularly severe at high areal loadings and under moderate‐rate conditions (e.g., 1 A g⁻¹), where prolonged exposure of large quantities of ZnS to the electrolyte accelerates cumulative SO₄ ²⁻ leaching (Figure 1a). Therefore, developing a tandem strategy that (i) directly modulates S─S and Zn─S bond dynamics to accelerate both charge and discharge kinetics, thereby mitigating ZnS accumulation, and (ii) stabilizes ZnS against aqueous corrosion to fundamentally suppress its decomposition, is essential for realizing high‐areal‐capacity aqueous Zn–S batteries with low voltage polarization and long‐term cycling stability at practical low cycling rates.

$$\begin{array}{cc}\hfill 2\mathrm{ZnS}\left(\mathrm{s}\right)& +4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)-10{\mathrm{e}}^{-}\to \mathrm{S}\left(\mathrm{s}\right)+2{\mathrm{Zn}}^{2+}\left(\mathrm{aq}\right)\hfill \\ & +{{\mathrm{SO}}_4}^{2-}\left(\mathrm{aq}\right)+8{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\hfill \end{array}$$ (1)

FIGURE 1.

Proposed mechanism of Cu CEI in Zn–S batteries. Schematic illustrations of the redox process for Zn–S battery with (a) I⁻ and (b) Cu CEI, and the Cu CEI‑mediated suppression of SO₄ ²⁻ formation. Electrostatic potential (ESP) distributions for (c) S₈ and (d) Cu‐S₈. (e) Laplacian bond order (LBO) statistics for S₈ and Cu‐S₈. (f) Photograph of the as‐prepared Cu CEI.

In this work, a homogeneous artificial nano‐copper‐based cathode/electrolyte interphase (Cu CEI) layer with sulfur/ZnS dual‐affinity was constructed to enable fast and reversible conversion kinetics (Figure 1b,f). The substantially stronger binding energy of Cu‐ZnS (−3.25 eV), compared with that of H₂O‐ZnS (−0.46 eV) indicates preferential ZnS interaction with the Cu CEI rather than with active water molecules, effectively suppressing both water‐induced ZnS decomposition and the loss of forming soluble SO₄ ²⁻ species. Moreover, the significant adsorption affinity of Cu CEI for both S₈ and ZnS not only promotes S₈ ring‐opening and reduction, but also triggers a domino‐like ZnS oxidation, as detailed in the following section. Accordingly, the Cu CEI lowers the activation energy of the Zn–S system from 69.4 to 29.5 kJ mol⁻¹. Benefiting from the accelerated redox kinetics, the Zn–S battery exhibits a low voltage hysteresis of only 0.23 V and a high average discharge voltage of 0.75 V at 0.5 A g⁻¹. The Zn–S coin‐type full cell with the Cu CEI demonstrates a markedly enhanced cycling stability, maintaining performance over 1000 cycles at 5 A g⁻¹. Furthermore, a corresponding Cu CEI‐based Zn–S pouch cell delivers a high areal capacity of ∼6.5 mAh cm⁻² for more than 200 h at 0.1 A g⁻¹, highlighting its strong potential for practical applications.

2. Results and Discussion

2.1. Failure Mechanisms Analysis in Aqueous Zn–S Batteries

To evaluate the electrochemical characteristics and capacity‐decay mechanisms of aqueous Zn–S batteries, coin cells comprising a Zn anode and a S@C composite cathode (Figures S1–S3) were assembled as benchmark systems. As shown in Figure 2a, the S@C electrode delivers an initial discharge capacity of approximately 1500 mAh g⁻¹ at 0.5 A g⁻¹, corresponding to ∼90% of the theoretical capacity based on the S⁰/S²⁻ redox couple (1675 mAh g⁻¹). In sharp contrast, in the absence of a ZnI₂ mediator, the accumulation of insulating ZnS renders the sulfur electrode electrochemically inert (Figure S4). These results confirm that the presence of I⁻ species in the electrolyte is beneficial for promoting the electrochemical oxidation of ZnS (Figures S5 and S6) [23]. Despite this mediation effect, the S cathode still suffers from severe capacity fading upon stepwise increases in current density from 0.5 to 5 A g⁻¹ (Figures 2b and S7). More critically, when the current density is reverted to 0.5 A g⁻¹, the capacity recovers to merely ∼500 mAh g⁻¹, far below its initial value. This pronounced irreversible capacity loss highlights the intrinsically poor rate capability of Zn–S batteries and is consistent with observations reported in prior studies [19, 22, 23, 29, 31, 37, 38].

FIGURE 2.

Failure analysis of the aqueous Zn–S batteries. (a) Charge/discharge curves of Zn||S@C at 0.5 A g⁻¹. (b) Rate performances of Zn||S@C. (c) Charge density differences of H₂O‐ZnS (311). Yellow and blue–green regions represent charge accumulation and depletion, respectively. IC results of the Zn–S batteries with (d) GF and (e) Cu@GF, and (f) the corresponding quantitative analysis with GF. The IC analysis was performed on the cycled sulfur cathode, separator and Zn anode using an electrolyte of 1 m Zn(CH₃COO)₂ + 0.05 m ZnI₂. Figures 2d and 2e show stacked Y‑axis plots with the same intensity range. The absolute values do not represent actual signal strengths; only relative peak intensities are compared. (g) EIS results of the sulfur electrode before and after 5 cycles. In situ XRD results of the Zn–S batteries with (h) GF and (i) Cu@GF. (j) Schematic illustration of active sulfur leakage from the carbon host during cycling. (k) Self‐discharge test results of Zn||S@C at 0.5 A g⁻¹.

Density functional theory (DFT) calculations provide mechanistic insight into this degradation behavior, revealing that oxygen atoms from active water molecules can directly attack Zn─S bonds in ZnS (Figure 2c; Figures S8–S10), thereby oxidizing active sulfur into soluble sulfate species (SO₄ ²⁻). In contrast, the weak interaction between H₂O and S₈ suggests that direct water‐induced activation of S₈ is unlikely to be the dominant origin of active sulfur dissolution (Figure S11). Experimentally, ion chromatography (IC) analysis of the Zn||S@C cell employing a glass‐fiber (GF) separator in zinc acetate electrolyte shows a relatively enhanced SO₄ ²⁻ signal (∼6.75–7.67 µS) during discharge (Figure 2d), which is already higher than the baseline value of 4.77 µS obtained from the pristine sulfur cathode (Figure S12). This observation indicates spontaneous ZnS decomposition driven by the combined effects of water and dissolved oxygen. Notably, during charging, the SO₄ ²⁻ signal increases continuously from 12.34 µS at 1.2 V to 25.41 µS at 1.6 V, demonstrating that ZnS decomposition is strongly coupled with electrochemical oxidation. In contrast, after introducing a Cu CEI consisting of a nano‐copper layer uniformly coated on the surface of glass fiber (Cu@GF, Figures S13–S18), the SO₄ ²⁻ intensity is effectively maintained at a low level of 4.64–5.58 µS during discharge (Figures 2e and S19). Upon charging, it increases only moderately from 7.09 µS at 0.7 V to 9.51 µS at 1.1 V, clearly indicating that the Cu CEI significantly stabilizes ZnS and suppresses sulfur/electrolyte interfacial side reactions. Quantitative analysis further reveals that, in the GF‐based cell, a sulfur molar loss rate of approximately 29.4% (the generation of ∼6.89 mg L⁻¹ SO₄ ²⁻ per milligram of sulfur) occurs after full charging (Figure 2f; Figures S20 and S21). This sulfur loss directly accounts for the irreversible capacity decay of the Zn–S battery and originates from the gradual migration of sulfur species out of the carbon host during prolonged cycling. Consistent with this interpretation, electrochemical impedance spectroscopy (EIS) reveals an increase in the electronic resistance of the cycled sulfur electrode (Figure 2g), possibly reflecting the electronic conductivity degradation caused by the leakage of insulating sulfur away from the conductive carbon framework. Further evidence is provided by in situ XRD (x‐ray diffraction): during cycling, the diffraction signal of sulfur increases markedly, particularly near 25°, where a growing sulfur peak progressively overwhelms the broad carbon background (Figure 2h). This behavior indicates continuous sulfur recrystallization, attributable to sulfur migration and surface reorganization on the carbon host (Figure 2j). Thermodynamically, this process is driven by the strong tendency of S²⁻ to migrate toward the electrode surface and precipitate as ZnS upon reaction with Zn²⁺ (Equation 2), which weakens sulfur‐carbon interactions and exacerbates corrosion‐induced active‐material loss in aqueous environments.

$${\mathrm{S}}^{2-}\left(\mathrm{aq}\right)+\mathrm{Z}{\mathrm{n}}^{2+}\left(\mathrm{aq}\right)\to \mathrm{ZnS}\left(\mathrm{s}\right)\mathrm{\Delta }G=-139.87\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}<0$$ (2)

$$\mathrm{S}\left(\mathrm{s}\right)+2\mathrm{Cu}\left(\mathrm{s}\right)\to \mathrm{C}{\mathrm{u}}_2\mathrm{S}\left(\mathrm{s}\right)\mathrm{\Delta }G=-86.19\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}<0$$ (3)

When the Cu@GF separator is employed, the attenuation of the carbon background near 25° proceeds much more slowly, and sulfur‐related diffraction peaks in the 30°–40° range emerge only at later stages (Figure 2i). This phenomenon confirms that the Cu CEI strongly adsorbs sulfur species and modulates the polarity of crystalline S₈ (Figure 1c,d), likely driven by the spontaneous reaction between Cu and S (Equation 3), thereby suppressing sulfur leaching. Correspondingly, the reduced laplacian bond order (LBO) of most S─S bonds in the Cu‐S₈ system reflects Cu‐induced electronic modulation and suggests weakened S─S bonding, as well as a decrease in S─S bond dissociation energy (Figure 1e) [39], thereby facilitating S₈ ring opening and electrochemical reduction. This interpretation is supported by the original LBO study, which demonstrated that LBO is correlated with bond polarity, bond dissociation energy, and bond vibrational frequency, supporting its use as a quantitative descriptor of covalent bond strength. Although sulfur migration remains inevitable during long‐term cycling, the strong atom‐level interaction between sulfur species and the conductive Cu CEI effectively reactivates otherwise insulating S₈ (Figure 2j), enabling its continued participation in subsequent redox reactions. As a result, the Zn||S@C cell employing the Cu@GF separator exhibits an outstanding capacity retention of 98.82% after a 24 h self‐discharge test, substantially outperforming the GF‐based counterpart (Figure 2k).

2.2. Charge Storage Mechanism of Zn–S Batteries

To further elucidate the reaction mechanism, the role of the Cu CEI in regulating the Zn–S reaction pathway was investigated by tracking the structural evolution of S and Cu at different electrochemical stages. As depicted in Figure 3b, the sharp characteristic diffraction peaks of crystalline S₈ observed in the pristine S@C electrode (stage A) are largely masked by the carbon cloth substrate (Figure S1a), with only weak and inconspicuous reflections near ∼26°. During the initial discharge of the Zn||S@C cell employing a bare GF separator (Figure 3a, stage A to D), broad diffraction peaks emerge at ∼28.6°, ∼33.1°, ∼47.5°, and ∼56.3°, which can be unambiguously assigned to the formation of crystalline ZnS (JCPDS no. 05–0566). This observation confirms that the dominant thermodynamic pathway involves the rapid reaction between S²⁻ and Zn²⁺ to form ZnS (Equation 2). Upon charging from stage D to G, the ZnS diffraction peaks gradually weaken and eventually disappear, while the characteristic reflections of S₈ reappear and intensify, indicating the effective oxidation of ZnS and regeneration of S₈ facilitated by the high concentration of I⁻ in the electrolyte. However, three‐dimensional (3D) ion mapping reveals that, after cycling, sulfur species (S⁺ signals) become heterogeneously distributed and lose intimate contact with the conductive carbon framework (Figures 3f and S22). This behavior is accompanied by substantial accumulation of sulfate species (SO₄ ²⁻, SO⁺ signals), demonstrating that although the I⁻ mediator enhances ZnS oxidation kinetics, it fails to suppress active sulfur migration and water‐induced ZnS decomposition.

FIGURE 3.

Redox mechanism of Zn||S@C battery. Initial charge/discharge profiles of Zn||S@C batteries with (a) GF and (c) Cu@GF at 0.5 A g⁻¹, and (b,d) the corresponding ex situ XRD results of S@C cathode and Cu CEI. (e) Ex situ Raman results of S@C electrode with Cu@GF during the initial cycle. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) analysis of the S@C electrodes after 5 cycles at 0.5 A g⁻¹ in an electrolyte containing 1 m Zn(CH₃COO)₂ and 0.05 m ZnI₂, with (f) GF and (g) Cu@GF separators. Charge density differences of (h) Cu‐S₈ and Cu‐ZnS (311) and (j) I⁻‐S₈ and I⁻‐ZnS (311). Yellow and blue–green regions represent charge accumulation and depletion, respectively. Partial density of state (PDOS) results of the Zn 3d orbital in ZnS (311) after interacting with (i) Cu and (k) I⁻. (l) Mechanistic diagram of the enhancement of Cu CEI in promoting sulfur redox kinetics.

In contrast, when the Cu CEI is introduced, distinct mechanistic features emerge. During discharging (Figure 3c), the appearance of new diffraction peaks at ∼25.5°, ∼29.3°, ∼42.2°, ∼49.9°, and ∼52.4° (Figure 3d, stage B), confirms the formation of CuI (JCPDS no. 06–0246). This result implies that I⁻ rapidly interacts with the Cu mediator following S₈ activation (Figure S23), a conclusion further corroborated by the suppressed I⁻ signal intensity observed in ion chromatography (IC) at the early discharge stage (Figure 2e). As discharge proceeds from stage B to D, the CuI diffraction signals diminish, while the corresponding I⁻ peak intensity in the ion chromatogram increases, reflecting the reduction of CuI to metallic Cu (JCPDS no. 04–0836) under continuous electron flux (Equation 4) [40, 41]. Meanwhile, the accompanying growth of ZnS diffraction peaks confirms sustained sulfur reduction facilitated by the Cu/CuI evolution. Additionally, the charge density difference plot in Figure 3h displays a high Cu‐S₈ binding energy of −2.47 eV and the significant charge accumulation between Cu and S₈, suggesting the electron transfer from Cu to S₈ that facilitates S─S bond dissociation. Bond elongation generally reflects a reduction in bond dissociation energy, thereby promoting bond cleavage and sulfur reduction. Consistently, the calculated S─S bond length in S₈ elongates from 2.06 to 2.10 Å upon interaction with Cu. Similarly, adsorption of I⁻ on the S₈ surface also induces S─S bond elongation (Figure 3j).

During charging (stages D to G), the gradual intensification of CuI diffraction peaks coincides with the ZnS → S₈ conversion (Figure 3d), indicating a reversible ZnS oxidation pathway mediated by the transformation of Cu to CuI. Notably, extremely high lattice energy of ZnS (3400 kJ mol⁻¹) presents a substantial kinetic barrier for Zn–S covalent bond cleavage, thereby severely limiting its intrinsic conversion rate [42]. The strong adsorption affinities of both Cu and I⁻ toward ZnS (Figure 3h,j), together with the increased I⁻ signal observed in IC at the early charging stage, imply preferential binding of Cu and I⁻ to ZnS. This interaction induces Zn─S bond elongations of 3.8% and 6.4%, respectively. More importantly, electronic structure analysis reveals that adsorption of Cu and I⁻ shifts the Zn 3d‐band center of the ZnS (311) surface from −6.19 eV to −6.09 eV and −6.04 eV, respectively (Figures 3i,k and S24), moving it closer to the Fermi level. This shift indicates surface electronic reconstruction and an increase in electron density around Zn atoms, which enhances Zn─S antibonding interactions and weakens the Zn─S covalent bond. The characteristic S‐S vibrational modes of S₈ disappear upon full discharge (D‐0.1 V) in ex situ Raman spectroscopy (Figure 3e), concomitant with the emergence of Zn–S vibrational signals corresponding to ZnS. Upon full charging (C‐1.1 V), these spectral features are completely reversed. Furthermore, 3D ion mapping further confirms that the cycled active sulfur species remain homogeneously distributed between the conductive carbon network and the Cu CEI (Figures 3g and S25), preserving effective electrical contact. Simultaneously, the near absence of surface SO₄ ²⁻ firmly confirms the dual functionality of the Cu CEI in accelerating sulfur redox kinetics and stabilizing ZnS against aqueous corrosion.

$$\begin{array}{cc}\hfill 2\text{CuI}(\mathrm{s})& +{\text{Zn}}^{2+}(\text{aq})+2{\mathrm{e}}^{-}\to 2\text{Cu}(\mathrm{s})+2{\mathrm{I}}^{-}(\text{aq})\hfill \\ & +{\text{Zn}}^{2+}(\text{aq}),\mathrm{\Delta }G=35.73\text{kJ}{\text{mol}}^{-1}>0\hfill \end{array}$$ (4)

$$\begin{array}{cc}\hfill \mathrm{ZnS}\left(\mathrm{s}\right)& +\mathrm{Cu}\left(\mathrm{s}\right)+{\mathrm{I}}^{-}\left(\mathrm{aq}\right)-3{\mathrm{e}}^{-}\to \mathrm{S}\left(\mathrm{s}\right)+\mathrm{CuI}\left(\mathrm{s}\right)\hfill \\ & +{\mathrm{Zn}}^{2+}\left(\mathrm{aq}\right),\mathrm{\Delta }\mathrm{G}=36.23\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}>0\hfill \end{array}$$ (5)

Based on these findings, the Cu CEI‐enhanced Zn–S redox mechanism is schematically illustrated in Figure 3l. During discharging, the strong Cu‐S₈ interaction drives spontaneous electron transfer from Cu to S₈, facilitating S─S bond dissociation and promoting S₈ ring opening and reduction to S²⁻, which subsequently reacts with Zn²⁺ to form ZnS. In parallel, the transient Cu oxidation is buffered by iodide via reversible CuI formation, thereby ensuring sustained catalytic activity of the Cu CEI. During charging, both Cu and I⁻ strongly adsorb onto ZnS, significantly weakening Zn─S bonds and lowering the lattice energy of ZnS. This interaction induces partial structural disorder, triggering a domino‐like oxidation of ZnS. Owing to the high affinity between Cu and S, a portion of ZnS may undergo transient conversion into copper sulfide species (e.g., Cu₂S) at the interface. Upon completion of charging, both ZnS and Cu₂S are fully oxidized to elemental S (Equation 5). Notably, the rapid regeneration of CuI at the end of charge indicates that the Cu‐based CEI adaptively maintains a stable, highly reversible sulfur redox cycle through dynamic Cu/CuI interconversion (Figures S26 and S27).

2.3. Electrochemical Performances of Zn||S@C Batteries

Remarkably, the Zn||S@C equipped with the Cu CEI exhibits markedly enhanced rate capability. As shown in Figure 4a, high capacity retentions of 97.59%, 93.42%, 76.47%, 66.81%, and 56.34% are achieved at current densities of 0.8, 1, 2, 3, and 5 A g⁻¹, respectively, relative to the capacity at 0.5 A g⁻¹. The corresponding charge‐discharge profiles (Figure 4b) further reveal that the introduction of the Cu CEI significantly elevates the average discharge voltage of the sulfur cathode from below 0.4 to 0.75 V (Figure S28), while simultaneously reducing the voltage hysteresis from 0.84 to 0.23 V (Figure 4c). To the best of our knowledge, these values rank among the first echelon of previously reported aqueous Zn–S batteries (Figure 4e,f; Table S1 and S2) [4, 20, 21, 23, 27, 29, 30, 31, 37, 38, 43, 44, 45, 46, 47, 48]. As presented in Figure 4d, the Zn||S@C cell with Cu CEI shows negligible capacity fade over 50 cycles at a moderate current density of 1 A g⁻¹, whereas its counterpart employing a bare GF separator undergoes rapid capacity fading. Additionally, the high conductive carbon content renders the sulfur cathode hydrophobic (electrolyte contact angle ∼144.5°, Figure S29) [49], leading to an uneven I⁻ distribution and hindering ZnS oxidation. Fortunately, the hydrophilic Cu CEI (electrolyte contact angle ∼25.3°) could not only ensure adequate electrolyte wetting at the interface between sulfur and Cu CEI, but also benefit from the efficient cooperation of I⁻ and Cu in improving the conversion reversibility of sulfur electrode (Figures S30 and S31). As a direct consequence of these synergistic interfacial and kinetic advantages, the Zn||S@C cell incorporating the Cu CEI demonstrates outstanding long‐term durability, maintaining stable operation for 1000 cycles at a high current density of 5 A g⁻¹ (Figure 4g).

FIGURE 4.

Electrochemical performances of Zn||S@C batteries with Cu@GF. (a) Rate performance of Zn||S@C and (b) the corresponding charge/discharge profiles. (c) Charge/discharge profiles of Zn||S@C batteries with GF and Cu@GF at 0.5 A g⁻¹. Cycling performances of Zn||S@C batteries (d) at 1 and (g) 5 A g⁻¹. Comparisons of (e) voltage hysteresis and (f) average output voltage between the Zn||S@C with Cu@GF and the other reported Zn–S batteries.

Moreover, distribution of relaxation times (DRT) analysis derived from the in situ EIS (Figures S32 and S33) reveals that the introduction of Cu CEI significantly reduces the interfacial charge‐transfer resistance (R _ct) of the sulfur cathode (Figure 5a,b). The relatively low R _ct during discharging reflects the accelerated S─S bond dissociation and ZnS formation, which can be attributed to the enhanced sulfur reduction kinetics and improved electrolyte wettability enabled by the Cu CEI with S affinity nature. In addition, the Cu CEI with ZnS affinity also promotes the oxidation of ZnS during charging, thereby facilitating Zn²⁺ transport within the sulfur electrode. This effect is evidenced by the reduced Warburg diffusion resistance (W_diff) during the charging process (Figure 5b). Based on the modified Arrhenius relationship of 1/R _ct =  Aexp(− E _a/RT), the introduction of Cu CEI can significantly decrease the reaction activation energy of the Zn–S system from 69.4 to 29.5 kJ mol⁻¹ (Figures 5d and S34). As a result, even at a high sulfur loading mass of ∼3 mg cm⁻² (Figure 5c), the Zn||S@C cell with Cu@GF can deliver a high areal capacity of ∼2.6 mAh cm⁻² for 50 cycles at 0.5 A g⁻¹, outperforming many recently reported advanced Zn–S batteries (Figure 5g and Table S3) [4, 20, 23, 29, 31, 37, 43, 48]. The well‐maintained shape of the differential capacity curves (Figure S35) also indicates that the robust Cu CEI effectively mitigates the key challenges under the high‐loading conditions, such as sulfur leakage, incomplete ZnS oxidation, and active water attack, thereby advancing the development of practical Zn–S batteries. To further evaluate practical applicability, a Zn||S@C pouch cell (4 × 5 cm²) incorporating the Cu CEI was assembled. The as‐fabricated pouch cell delivers an impressive areal capacity of ∼6.5 mAh cm⁻² at an ultra‐low rate of 0.1 A g⁻¹ and maintains stable operation for more than 200 h (Figure 5e,f), demonstrating excellent durability under practical operating conditions. Compared with recently reported MnO₂, vanadium oxide, and MXene‐based cathodes for aqueous zinc‐based batteries, the Cu CEI‐enabled Zn–S battery exhibits superior areal capacity at comparable active material loadings, particularly in a pouch cell configuration, underscoring its promise for high‐energy‐density energy storage (Table S4) [50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63]. Moreover, a series‐connected pouch‐cell module can power an LED panel (Figure 5h), providing a direct demonstration of the practical feasibility of the Cu CEI‐enabled Zn–S battery system.

FIGURE 5.

Kinetic analysis and high‐loading performance of Cu CEI‐enabled Zn||S@C batteries. DRT results of Zn||S@C with (a) GF and (b) Cu@GF. (c) Cycling stability of Zn||S@C with Cu@GF at 0.5 A g⁻¹. (d) Arrhenius plots obtained from EIS spectra of Zn||S@C with GF/Cu@GF. (e) The voltage–time curves of Zn||S@C pouch cell with Cu@GF and (f) the corresponding cycling performance. (g) Comparisons of the electrochemical performances between Zn||S@C with Cu@GF and the other reported Zn–S batteries. (h) Photograph of the Zn||S@C pouch cell powering an LED panel.

3. Conclusion

In summary, the designed Cu CEI preferentially interacts with ZnS over water molecules, effectively suppressing ZnS decomposition into SO₄ ²⁻ and improving the cycling stability of Zn–S batteries, particularly at relatively low rates (<2 A g⁻¹). In addition, the strong intrinsic interactions of the Cu CEI with S₈ and ZnS significantly weaken the S─S and Zn─S bonds. In synergy with I⁻, the activation energy of the Zn–S battery is substantially reduced from 69.4 to 29.5 kJ mol⁻¹. As a result, for the Zn–S battery with the sulfur/ZnS dual‐affinity Cu CEI, the charge‐discharge polarization is reduced from 0.84 to 0.23 V, and the average output voltage increases from below 0.4 to 0.75 V. More importantly, with a high loading mass of ∼3 mg cm⁻², the Zn–S cell delivers stable cycling performance for 50 cycles at 0.5 A g⁻¹, with an areal capacity of ∼2.6 mAh cm⁻². Furthermore, the Zn–S pouch cell also maintains ∼6.5 mAh cm⁻² for over 200 h at a lower rate of 0.1 A g⁻¹, demonstrating that Cu CEI endows Zn–S batteries with excellent long‐term stability under practical conditions.

Author Contributions

Zeheng Lv: conceptualization, investigation, validation, writing – original draft. Peiyao Wang: methodology, formal analysis. Sirui Lin: methodology, validation, investigation. Xinran Li: formal analysis and data curation. Ruibo Sun: visualization and validation. Kaiwen Li: investigation and visualization, Hong Lin: investigation, formal analysis, software. Fanxiang Meng: software, data curation, visualization. Minghao Zhang: validation and formal analysis. Yang Yang: writing – review editing and project administration, Hao Luo: resources, writing – review and editing. Jinbao Zhao: writing – review editing, funding acquisition, and supervision. Dongliang Chao: supervision, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73206‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.