Direct Visualization and Regulation of Interfacial Ion Concentration Layer at Zinc Metal Interfaces via an Ion‐Buffering Artificial Solid Electrolyte Interphase

ABSTRACT

Direct experimental visualization of the electrical double layer (EDL) and its associated layer formation at metal–electrolyte interfaces remains challenging yet is essential for understanding interfacial instability in zinc metal anodes. Herein, we engineer a PEO/PIM‐1 hybrid polymer (PIP) as an artificial solid–electrolyte interphase (SEI) that replaces field‐driven interfacial ion accumulation with confined ion buffering, thereby suppressing EDL amplification and chaotic interfacial dynamics. Liquid‐phase atomic force microscopy (AFM) measurements demonstrate the elimination of voltage‐dependent force amplification upon PIP modification, evidencing effective decoupling of electric‐field‐induced ion crowding at the Zn surface. In situ Raman spectroscopy, electrochemical analysis, and molecular dynamics simulations further reveal reduced interfacial water activity, regulated ion distribution, and homogeneous Zn stripping and deposition. PIP significantly enhances anode stability, enabling Zn|Zn symmetric cells to cycle for 2700 h and delivering a Coulombic efficiency of 98.6% in Zn|Ti cells. When coupled with I₂ cathodes, PIP@Zn supports long‐life 4e⁻ Zn|I₂ batteries, achieving >6000 cycles at 4 A g⁻¹ and >27 000 cycles at 8 A g⁻¹. For the practical application, 4e⁻ Zn|I₂ pouch cell delivers 500 mAh capacity and 500 cycles. This work offers a mechanistic framework for probing EDL dynamics and a generalizable SEI strategy for stabilizing Zn metal.

A PEO/PIM‐1 artificial SEI regulates the electrical double layer and its associated interfacial ion concentration layer at zinc metal interfaces by redistributing Zn²⁺ within the membrane. This interfacial‐gradient decoupling suppresses field‐induced ion crowding, concentration polarization, dendrite growth, and parasitic reactions, enabling stable Zn metal anodes and long‐life aqueous Zn–I₂ batteries.

1. Introduction

The development of safe and low‐cost energy storage technologies is a key pathway toward a sustainable future. Among them, aqueous zinc‐ion batteries (AZIBs) have attracted widespread attention owing to the high theoretical capacity of zinc (820 mAh g⁻¹), excellent safety, and low cost [1, 2]. Considerable efforts have been devoted to electrolyte engineering to mitigate dendrite growth and suppress interfacial side reactions. However, under an applied electric field, zinc metal anodes inevitably develop pronounced interfacial ion concentration gradients, forming a highly heterogeneous electrical double layer (EDL) at the electrode/electrolyte interface. Excessive ion accumulation within the EDL amplifies local electric‐field inhomogeneity and the associated tip effect at surface asperities, driving highly localized and kinetically unbalanced Zn²⁺ flux and interfacial reactions. Such field‐amplified kinetic heterogeneity ultimately leads to spatially non‐uniform Zn stripping and plating, thereby exacerbating dendritic growth and interfacial instability [3]. While numerous studies have focused on the solvation structure of Zn²⁺ in bulk electrolytes, the EDL constitutes the immediate reaction zone that directly governs metal–electrolyte interactions under operating conditions. Therefore, directly probing ion distribution and interfacial dynamics within the EDL is of greater practical relevance for understanding and regulating Zn anode stability [4].

Current understanding of the EDL relies predominantly on theoretical models (e.g., COMSOL and molecular dynamics simulations), which inevitably oversimplify complex electrochemical realities, including dynamic ion flux, interfacial reactions, and transient pH fluctuations [5]. Moreover, conventional AFM characterization of Zn anodes has been largely limited to ex situ morphological analysis after cycling, offering limited access to real‐time interfacial ion dynamics. Herein, we repurpose liquid‐phase AFM from a passive surface‐imaging tool into a sensitive in situ force probe for monitoring the local ionic environment near the Zn surface [6]. In the classical Gouy–Chapman–Stern–Grahame model, the metal–aqueous electrolyte interface is generally described by an inner Helmholtz layer, an outer Helmholtz layer, and a diffuse layer extending toward the bulk electrolyte. However, the aqueous Zn‐ion electrolyte used in this work presents a complex interfacial environment, mainly due to the strong hydration/coordination capability of Zn²⁺. The stable Zn²⁺ solvation shell, together with ion–solvent interactions, surface polarization, and interfacial ion coupling, can substantially regulate the local charge distribution and broaden the ionic response near the Zn surface. Therefore, the AFM‐detected interaction region should be understood as the combined contribution of the conventional EDL and the diffuse‐double‐layer‐related ionic response, rather than the thickness of a single static EDL structure. Therefore, the ∼150 nm interfacial region detected by liquid‐phase AFM is not defined here as a conventional equilibrium electric double layer, but rather as an “interfacial ion concentration layer” arising from the combined contribution of specific adsorption, non‐specific ion accumulation, and partially diffusive ion enrichment near the Zn surface [7].

Within this ion concentration layer, electric‐field‐driven ion enrichment gives rise to measurable mechanical responses in AFM force–distance spectra. In general, a higher local ion concentration at the interface produces a stronger mechanical response, allowing liquid‐phase AFM to qualitatively and semi‐quantitatively probe the evolution of interfacial ion accumulation. Importantly, we establish a correlation between the measured force signals and local Zn²⁺ enrichment, where stronger interaction forces indicate higher interfacial ion density. Such localized ion crowding is not merely a structural feature of the interface; it can accelerate parasitic reactions, induce local pH perturbations that promote corrosion, impede ion transport, and ultimately compromise deposition uniformity and electrochemical stability [8]. Leveraging this capability, we directly track the dynamic evolution of interfacial ion enrichment during Zn plating and stripping in real time using liquid‐phase AFM [9].

Herein, to mitigate excessive EDL‐associated interfacial ion accumulation and concentration‐gradient polarization, we construct a Poly(Ethylene Oxide) (PEO)/ Polymers of intrinsic microporosity (PIM)‐1 hybrid polymer interphase, denoted as PIP, on the Zn metal surface. Rather than functioning merely as a passive physical barrier, the PIP layer serves as an ion‐buffering artificial SEI. The ether oxygen sites in PEO and the polar nitrogen‐containing groups in PIM‐1 provide abundant weak coordination/adsorption sites for Zn²⁺, while the intrinsic disordered microporous structure of PIM‐1 offers spatially confined ion‐accommodation channels. Under an applied electric field, this hybrid interphase enables transient Zn²⁺ storage and redistribution within the polymer matrix, thereby converting direct ion crowding at the Zn metal surface into distributed in‐membrane ion buffering. This “in‐membrane ion storage–interfacial gradient decoupling” mechanism suppresses excessive EDL‐related ion enrichment, mitigates concentration polarization, and promotes more homogeneous Zn stripping/plating behavior.

Meanwhile, the PIP layer reduces interfacial water accumulation and suppresses corrosion reactions and parasitic byproduct formation. During cycling, PIP@Zn further evolves into a robust organic–inorganic hybrid SEI, in which the organic‐rich outer layer and inorganic‐rich inner layer jointly enhance interfacial mechanical stability and alleviate stress/cracking induced by uneven deposition and corrosion. As a result, PIP@Zn enables highly reversible Zn electrochemistry over a wide current‐density range of 1–25 mA cm⁻². Notably, Zn|Zn symmetric cells achieve stable cycling for 2700 h at 1 mA cm⁻²/1 mAh cm⁻² and 2100 h at 2 mA cm⁻²/5 mAh cm⁻². Zn|Ti cells deliver a high Coulombic efficiency of 98.6% for over 1700 cycles. Furthermore, 4e⁻ Zn|I₂ batteries with PIP‐protected Zn anodes exhibit 85.8% capacity retention over 6000 cycles at 4 A g⁻¹ and 83.4% retention after 27 000 cycles at 8 A g⁻¹. For a practical demonstration, a 4e⁻ Zn|I₂ pouch cell delivers a capacity over 500 mAh and stable operation over 500 cycles.

2. Results and Discussion

2.1. Interfacial Ion Concentration Layer on the Zn Metal Anode Surface

To ensure high ionic conductivity, the ions must migrate rapidly within the liquid electrolyte and are therefore readily influenced by the electric field, resulting in ionic concentration gradients. This localized, relatively high‐concentration environment hinders ionic transport and impairs deposition behavior. To mitigate the excessive interfacial ionic concentration gradients induced by the EDL, designing an artificial SEI is the most straightforward and practical strategy to address this issue. EDX‐SEM was applied to demonstrate that the PIP layer is coated on the Zn anode surface with a tightly conformal contact, and the thickness of the PIP layer is ∼8 µm (Figure S3).

ToF‐SIMS was used to probe the Zn‐ion distribution within the PIP interphase, with CN⁻ and Zn²⁺ signals representing the organic layer and Zn‐containing species, respectively. As the sputtering depth increased, the Zn²⁺ signal gradually intensified while the CN⁻ signal weakened, indicating that Zn‐containing species are mainly distributed near the Zn metal surface beneath the PIP layer (Figures S2 and S4). Depth‐profiling XPS further shows a gradual shift of the Zn 2p peak from higher to lower binding energy, confirming the transition from ionic Zn species to metallic Zn and agreeing well with the ToF‐SIMS results (Figure S5).

Polymers featuring a disordered pore‐channel architecture are particularly suitable as artificial SEI materials because they enable spatially confined ion accommodation rather than purely directional ion transport. In contrast, ordered pore structures primarily facilitate rapid ion migration with limited ion‐retention capability. Disorder‐engineered pores introduce controlled kinetic hindrance that transiently trap ions within the polymeric channels, thereby buffering interfacial ion flux and mitigating diffusion‐driven concentration polarization. Accordingly, PIM‐1, which intrinsically possesses a highly disordered microporous framework, was selected as the primary component of the artificial SEI, with PEO serving as a Zn ion transport channel [10]. To clarify the roles of different interphase components, pristine PIM‐1 and PEO were first evaluated at 2 mA cm⁻² and 2 mAh cm⁻². Although PIM‐1 provides ion‐storage capability, its discontinuous and disordered porous channels lead to localized Zn deposition at incompletely covered regions, resulting in poor cycling stability (Figures S6 and S7). PEO exhibits better ion‐conduction ability but suffers from limited interfacial stability due to its high hydrophilicity. By combining PIM‐1 with PEO, their ion‐storage and ion‐conduction advantages are effectively integrated, enabling more uniform Zn deposition and markedly improved long‐term cycling stability.

We first applied cyclic voltammetry (CV) to evaluate the non‐Faradaic interfacial capacitive contribution of bare Zn and PIP@Zn. The higher interfacial capacitance‐derived capacity contribution of PIP@Zn is likely attributed to the dense polar functional groups in the PIP interphase, which can temporarily accommodate solvated Zn²⁺ ions via surface complexation/ion adsorption, forming a buffered charge‐storage layer within the PIP matrix rather than direct ion accumulation at the Zn metal surface (351.49 µF cm⁻² vs. 111.46 µF cm⁻²) (Figure S8). To enable a more rigorous analysis of the interfacial capacitive behavior, Bar Cu and PIP@Cu were used as the working electrodes, with Ag/AgCl serving as the reference electrode and Zn as the counter electrode. Electrochemical measurements were conducted at a fixed frequency of 10 Hz, a regime typically associated with EDL capacitance characterization. The applied potential was swept from the open‐circuit potential (≈ 40 mV) toward the cathodic direction down to −0.9 V, close to the onset of Zn deposition, thereby isolating the capacitive response governed by interfacial ion adsorption and redistribution prior to Faradaic metal plating [11]. For Bare Zn, the interfacial capacitance initially decreases, likely due to anion desorption under cathodic polarization and the accompanying reorganization of the electric double layer. With further negative polarization, the accumulation of Zn²⁺ at the interface leads to an increase in capacitance; however, as this process is dominated by physical adsorption, the capacitance rise remains gradual. In the PIP@Zn, the increased surface electron density induces polarization of the PIP molecular chains, while the porous structure and interchain gaps provide abundant anchoring sites for Zn²⁺, leading to a rapid increase in interfacial capacitance. When the potential reaches approximately −0.6 V, local ion saturation and enhanced electrostatic repulsion hinder further Zn²⁺ accumulation, resulting in a decrease in capacitance. At more negative potentials, partial desolvation of Zn²⁺ occurs, increasing the local Zn²⁺ concentration at the interface and giving rise to a secondary increase in capacitance (Figure 1b) [12, 13]. This process effectively shifts the dominant electric‐double‐layer charge‐storage region away from the immediate Zn metal surface and into the PIP interphase, where excess interfacial charge can be buffered. As a result, severe ion accumulation and localized electric‐double‐layer effects at the Zn/electrolyte interface are suppressed, thereby mitigating concentration‐gradient‐induced polarization and reducing Zn‐ion aggregation.

FIGURE 1.

(a) Schematics of the metal–aqueous solution interface featuring the newly observed soft layer, sandwiched between the diffuse layer and the diffusion layer. (b) Electrochemical double‐layer capacitance evaluation based on cyclic voltammetry. AFM force curves obtained at the (c) plating process and (d) stripping process of the Bare Zn anode surface and the (e) plating process and (f) stripping process of the PIP@Zn anode surface. (g) Tip effect schematics of relatively low Zn ion concentration in the Zn anode surface. (h) Schematic of the PIP‐based SEI membrane functioning as a confined Zn²⁺‐ion storage layer.

In situ electrochemical impedance spectroscopy (in situ EIS) further supports this picture: Bare Zn shows lower post‐cycling EIS impedance, consistent with accelerated interfacial kinetics induced by uncontrolled Zn²⁺ enrichment, dendritic protrusion, and an amplified tip effect, which collectively decrease charge‐transfer resistance but signify a parasitic, inhomogeneous interfacial state. In contrast, PIP@Zn exhibits persistently higher impedance yet markedly enhanced cycling stability, and the absence of “tip‐effect” signatures, or mossy dendritic tips, indicates that high‐concentration Zn²⁺ clustering and the associated tip amplification are effectively suppressed (Figure S9) [14].

To directly visualize the evolution of local ion concentration at the Zn–electrolyte interface, we employed liquid‐phase AFM to probe the molecular interfacial characteristics under an applied electric field. Liquid‐phase AFM was used to measure the interfacial interaction forces on Bare Zn and PIP@Zn during Zn plating and stripping. It should be noted that the AFM‐detected interaction region does not correspond to the physical thickness of the pure electric double layer. Instead, the recorded force signals reflect the overall mechanical response of the interfacial ionic environment, including contributions from the Helmholtz region and the diffuse‐layer‐related ion redistribution. Therefore, the measured forces primarily represent the external‐field‐induced interfacial ion enrichment, concentration gradients, rearrangement of solvated Zn²⁺, and long‐range ionic interactions near the Zn surface [7]. To protect the Al‐coated AFM probe from corrosion/dissolution, the electrolyte concentration was limited to 0.1 m Zn(OTf)₂ for all measurements (Figure 1a).

First, we compared the force–distance responses of Bare Zn and PIP@Zn in the absence of an applied electric field. No obvious difference in the interfacial ion‐concentration‐related force response was observed between the two samples under open‐circuit conditions [15]. This result indicates that the subsequent changes in the force–distance curves under polarization mainly originate from the electric‐field‐driven redistribution/enrichment of ions near the Zn interface, rather than from the intrinsic mechanical response of the PIP layer itself. (Figures S10 and S11) [16, 17].

When zinc deposition was performed on bare Zn with an external voltage of 100 mV, the interaction force between AFM and the electrolyte components in the interfacial ion concentration layer (119.1 nm) was 407.3 pN, which indicates a relatively higher ion concentration compared with the bulk electrolyte [18]. In contrast, for PIP@Zn, no significant change in interaction force was observed with applied voltage (100 mV) in the plating process. This form of Zn²⁺ storage within the PIP membrane structure replaces the conventional mode of forming a concentration‐gradient polarization layer at the electrode/electrolyte interface that arises from the external electric field (100 mV) (Figure 1c,e) [9].

The stripping process likewise exerts a significant influence on the cycling stability of the Zn anode. Accordingly, we further examined the EDL formation and interfacial ion rearrangement during the stripping stage to elucidate how the formation of EDL contributes to Zn anode stability. The Bare Zn surface still exhibits the formation of a strong EDL; however, the interaction force measured during stripping is notably lower than that observed in the plating process. This difference may arise from the shift in the dominant ions governing EDL formation: during plating, Zn²⁺ primarily contributes to the interfacial structure, whereas during stripping, the EDL is mainly regulated by the larger, more uniformly charged OTf⁻ anions. The presence of these bulkier anions leads to weaker localized interactions with the probe, thereby reducing the measured interaction force. Similarly, the abundant disordered pores within the PIP layer can effectively accommodate the anions involved in the EDL formed under the applied electric field. As a result, no pronounced ion concentration gradient develops at the PIP@Zn interface. This buffering effect enables more uniform Zn stripping kinetics, rather than the chaotic and accelerated Zn²⁺ migration and stripping that typically occur under a highly concentrated anion‐driven environment (Figure 1d,f).

The PIP structure enables the coordination‐based adsorption and spatially confined storage of Zn²⁺, thereby establishing an ion‐buffering storage mechanism at the electrode/electrolyte interface. This mechanism partially substitutes the conventional diffusion‐limited concentration‐gradient polarization layer that forms on the electrode surface due to potential‐induced under high anodic potentials. Owing to the abundant functional groups within the PIP, Zn²⁺ ions can be uniformly captured and distributed within the membrane matrix. Consequently, the interfacial ion transport and migration processes no longer rely on potential‐driven ion accumulation through immediate‐adjacent concentration gradient buildup at the Zn anode surface, which effectively suppresses the formation of asymmetric ion enrichment/depletion and the associated concentration‐gradient polarization layer. This “in‐membrane ion storage + interfacial gradient decoupling” mechanism mitigates the potential‐induced interfacial concentration imbalance and further promotes the evolution of Zn deposition toward more stable and homogeneous plating behavior.

2.2. Interface Ion Distribution in the Zn Anode Surface

To further substantiate the transition in the EDL formation mechanism, in situ Fourier‐transform infrared spectroscopy (in situ FTIR) was employed to probe and elucidate the interfacial chemical environment and ion–molecule interactions at the electrode/electrolyte interface. In situ FTIR measurements reveal that during Zn deposition, the accumulation of interfacial water on the PIP‐coated electrode is significantly lower than that on the bare electrode and gradually decreases with time. More intriguingly, during the stripping process, the interfacial water signals on both electrodes continuously decline, while a pronounced water‐band inversion emerges on the PIP surface at the late stage of stripping (current density: 0.5 mA cm⁻²), indicating substantial reconstruction of the interfacial solvation environment [12].

During Zn deposition, the bare Zn electrode lacks an effective interfacial regulation layer. Under an applied electric field, Zn²⁺ rapidly accumulates near the metal surface, as verified by AFM measurements. This interfacial enrichment mainly originates from the redistribution and accumulation of solvated ions in the diffuse layer during EDL formation. The increased local Zn²⁺ concentration directly induces fast and uncontrolled Zn deposition. Meanwhile, water molecules released during Zn²⁺ reduction can also rapidly accumulate at the interface, further aggravating the heterogeneity of the interfacial environment and amplifying the tip effect, ultimately leading to uneven Zn growth.

In contrast, the PIP‐modified electrode exhibits distinctly different interfacial kinetics. The porous PIP framework redistributes Zn²⁺ near the interface, weakens ion enrichment in the EDL and its diffuse layer, suppresses the direct formation of a strong EDL on the Zn surface, and lowers the local Zn²⁺ concentration at the metal/electrolyte interface. Therefore, Zn²⁺ does not undergo abrupt accumulation on the Zn surface, thereby avoiding fast and uncontrolled Zn deposition. Meanwhile, partial desolvation of Zn²⁺ occurs within the PIP layer, leading to a more gradual release of coordinated water molecules and a reduced hydration degree of Zn²⁺ reaching the Zn surface. In addition, weak hydrogen‐bond interactions between PIP and water can temporarily immobilize the released water molecules, suppressing their rapid accumulation at the interface and reducing the formation of high‐water‐activity regions. Therefore, the PIP layer effectively buffers the interfacial Zn²⁺ concentration gradient induced by EDL evolution, reduces ion enrichment in the diffuse layer, and suppresses the formation of a strong EDL. As a result, it mitigates uncontrolled fast Zn deposition and rapid interfacial water release, ultimately reducing tip‐effect‐induced dendritic growth.

During stripping, Zn⁰ is oxidized to Zn²⁺, which undergoes interfacial re‐solvation and actively captures surrounding water molecules, leading to a continuous decrease in the IR water signal. In the bare system, the regenerated Zn²⁺ rapidly diffuses into the bulk electrolyte, allowing efficient replenishment of interfacial water. In contrast, Zn²⁺ diffusion in the PIP‐coated system is kinetically restricted, resulting in sustained local water consumption and the formation of a water‐deficient interfacial microenvironment, which thermodynamically suppresses parasitic corrosion reactions. This process ultimately gives rise to the observed water‐band inversion at the late stripping stage (Figure 2a,b) [19].

FIGURE 2.

In situ FTIR for the plating and stripping process of Zn for (a) Bare @Cu; and (b) PIP@Cu; (c) In situ Raman for the plating process for Bare Zn (top) and PIP@Zn (bottom); (d) In situ Raman Mapping of the interfacial H₂O in for Bare Zn and PIP@Zn; (e) Tafel curve for the Bare Zn and PIP@Zn; Molecular dynamics simulation snapshot for (f) Bare Zn; and for (g) PIP@Zn; (h) MSD and the diffusion coefficient of Zn in H₂O, PEO and PIM.

Due to the new ion storage mechanism for the Zn anode surface, in situ Raman is applied to detect the ion distribution in the Zn anode surface. The presence of characteristic C─H and C─O bonds confirms the successful formation of a PIP layer at the Zn anode interface. The area integration of the O─H stretching vibration peak of the H₂O molecules in different bare Zn and PIP@Zn was performed, respectively. The concentration change of Zn²⁺ during continuous deposition was further analyzed by analyzing the signal change of OTf⁻ at 1035 cm⁻¹ (Figure 2c). The OTf⁻ signal intensity of bare Zn at the interface is higher than that of PIP@Zn [20]. The stronger OTf⁻ signal confirms the Zn²⁺ enrichment associated with EDL formation, which can amplify the tip effect and induce rapid Zn deposition. This rapid deposition further evolves into disordered Zn growth and eventually leads to dendrite formation. This observation is mutually consistent with the AFM results. This disorder rapid Zn ion deposition process also reflected in EIS (Figure S12). Bare Zn exhibits lower interface resistance due to the uncontrolled Zn deposition process. And PIP@Zn shows higher interface resistance with the controlled Zn deposition process. In addition, the Zn²⁺ diffusion behavior on the surface of bare Zn and PIP@Zn anodes was analyzed by chronoamperometry (CA). The current of the PIP@Zn symmetric cell with an overpotential of −150 mV quickly stabilized after 8 s. The achievement of a stable current indicates that uniform Zn deposition is achieved by forming uniformly dispersed nuclei in three dimensions rather than active, dense nuclei on bare Zn (Figure S13) [21].

The area integration of the O─H stretching vibration peak of the H₂O molecules in different Bare Zn and PIP@Zn was performed, respectively (Figure 2d). It can be seen from the in situ Raman spectra and corresponding mapping diagram that Bare Zn is enriched with a large amount of interfacial water in Bare Zn, which is thermodynamically unstable, leading to metal zinc corrosion and the formation of by‐products. In PIP@Zn, the signal of interfacial water enrichment is almost negligible. Tafel test shows that PIP@Zn exhibits a lower corrosion current density of 0.63 mA cm⁻² and a higher corrosion potential of −21 mV, compared with the Bare Zn, indicating that the PIP@Zn interface layer may isolate moisture from contact with Zn, and the interface stability is enhanced (Figure 2e) [22]. To further evaluate the stability of the Zn metal with the protection of PIP, an immersion experiment was carried out to evaluate. For the different immersion times (72, 168, 240 h), with the protection of PIP, PIP@Zn anode significantly suppresses the corrosion of the electrolyte (Figures S14–S16) [23].

To elucidate the Zn²⁺ transport behavior and the formation mechanism of the interfacial ion concentration layer, molecular dynamics (MD) simulations were performed to model ion migration in H₂O, PEO, and PIM media. To quantitatively assess the ion transport kinetics, the mean square displacement (MSD) of Zn²⁺ ions was calculated from the equilibrium trajectories, enabling a direct comparison of ion mobility and diffusion characteristics across different environments (Figure 2f,g). Compared with the different media, Zn²⁺ exhibits the highest ion transport capability in H₂O, which promotes the rapid development of an EDL‐related interfacial ion‐enriched region. As time progresses, Zn²⁺ ions rapidly accumulate at the Zn metal surface, leading to a progressively enhanced local ionic response [24]. In contrast, in PEO/PIM, Zn²⁺ ions show no obvious interfacial enrichment, and their local concentration near the Zn surface remains nearly unchanged relative to the pre‐bias state. This indicates that PEO/PIM‐1 effectively suppresses EDL‐driven Zn²⁺ accumulation, in good agreement with the AFM force–distance curve results (Figure 2h).

2.3. Zn Deposition Behavior in the Zn Anode

In addition, compared with Bare Zn, PIP@Zn exhibits a lower Zn²⁺ desolvation activation energy of 24.23 kJ mol⁻¹ (compared to 36.3 kJ mol⁻¹ of bare Zn) (Figure 3a–c). In addition, due to the lower ion polarization in the Zn anode interface, a higher Zn²⁺ migration number of 0.63 for PIP@Zn (compared to 0.3 of bare Zn) (Figures S17 and S18). The lower desolvation energy can accelerate the Zn deposition kinetics. PIP@Zn accelerates the desolvation process of hydrated Zn²⁺ and further reduces the side reactions related to water, which is consistent with the results of the linear polarization curves (Figure 2e).

FIGURE 3.

(a) Temperature‐dependent EIS spectra of (a) Bare Zn and (b) PIP@Zn electrodes; (c) Arrhenius plots of interfacial resistance for Bare Zn and PIP@Zn; In situ optical observation for the Zn plating behavior for (d) Bare Zn; and PIP@Zn; SEM for the Zn cycle with 1 mA cm⁻² 1mAh cm⁻² for (e) Bare Zn and PIP@Zn; and (f) SEM for the Cu deposited with 1 mA cm⁻² 10 mAh cm⁻²; Surface morphology and height profiles of Bare Zn and PIP@Zn electrodes (g) Bare Zn; and (h) PIP@Zn.

The structural evolution of bare Zn and PIP@Zn anodes after 30 min of electroplating was studied using in situ optical microscopy. At 10 min, a significantly non‐uniform Zn coating can be observed on the bare Zn surface, and the tip effect is generated and persists throughout the subsequent process. In contrast, on PIP@Zn, the Zn surface coating is flat and dense after 30 min of electroplating, without obvious dendrites or bubbles (Figure 3d). Scanning electron microscopy (SEM) and X‐ray diffraction (XRD) also confirmed the nucleation and growth evolution of zinc and the difference in deposition morphology. The surface morphology evolution after cycling was further examined by scanning electron microscopy (SEM) of Zn|Cu asymmetric cells (current density of 1 mA cm⁻²/1 mAh cm⁻²) supplemented by elemental mapping analysis. The bare Zn surface showed an obvious dense stacking structure and adhered rod‐shaped glass fibers. In contrast, the above‐mentioned byproducts and impurities were absent on PIP@Zn, and a uniformly dispersed Zn and Cu distribution was shown (Figure 3e) [25]. To more intuitively observe the effect of PIP@Zn during Zn deposition, the changes in the deposition morphology of Zn on the copper substrate were observed after 30 cycles of charge and discharge at current densities of 1 mA cm⁻² and 10 mAh cm⁻². As shown in Figure 3f, Zn nucleation on the exposed Bare Cu surface is random and gradually develops into large protrusions. Such non‐uniform deposition is closely associated with the excessive Zn²⁺ accumulation caused by EDL formation, which amplifies the local “tip effect” and further promotes dendrite growth during subsequent Zn plating. In contrast, no obvious non‐uniform deposition is observed on the PIP@Cu surface, indicating that the suppressed EDL effectively mitigates local Zn²⁺ enrichment and thus prevents tip‐effect‐induced uneven Zn deposition. To systematically study the differences in Zn ion deposition behavior on bare Zn and PIP@Zn anodes, the nucleation behavior of Zn²⁺ was studied by cyclic voltammetry (CV) at a scan rate of 10 mV s⁻¹ on a Zn|Ti cell. Zn nucleation on the PIP@Zn anode exhibits a nucleation overpotential of about 31 mV higher than that of bare Zn. According to Equation S1, the higher the nucleation overpotential, the smaller the size of the deposited Zn particles. The increase in the reaction barrier for PIP@Zn deposition can promote the growth of more uniform and finer Zn particles, inhibit the formation of Zn dendrites, and ultimately achieve a more uniform and smoother Zn deposition morphology (Figure S19).

Confocal laser scanning microscopy (CLSM) was further used to observe the overall morphology of the Zn anode after 30 cycles of electroplating/stripping (current density of 1 mA cm⁻² and an areal capacity of 1 mAh cm⁻²) (Figure 3g,h). A smooth surface was observed on PIP@Zn, which was reflected in the fluctuation amplitude controlled within ±2 µm, and the uniformity was significantly improved compared with that of the bare Zn surface. The reduction in surface roughness inhibits the spike deposition and local over‐deposition behavior of zinc, showing a smoother and denser deposition morphology [26]. In order to confirm that, AFM is applied to observation for Zn anode surface during cycling. The results are consistent with the CLSM, PIP@Zn significantly suppresses the growth of the Zn dendrites, making the Zn anode surface uniform (Figures S20 and S21). To further confirm the dendrite‐suppression effect of the PIP interphase, SEM was performed after Zn deposition on different substrates. After cycling at 1 mA cm^{− 2} and 10 mAh cm^{− 2} for 30 cycles, Zn deposited on bare Cu shows uneven morphology with obvious protrusions, indicating nonuniform deposition. In contrast, the PIP‐modified substrate exhibits compact and uniform Zn deposition without apparent dendritic structures, further confirming the regulated Zn plating behavior induced by PIP (Figure S22).

2.4. Interfacial Stability and Electrochemical Evaluation of Zn Metal Anodes

By comparing the XRD spectra of deposited Zn after 90 cycles in the Zn|Zn symmetric cell at a current density of 1 mA cm⁻² and 1 mAh cm⁻², the byproducts and crystal orientation of the Zn surface are studied. The byproduct Zn_xOTf_y(OH)₂x−y∙nH₂O (xZn + yOTf⁻ + (2x−y)OH⁻ + nH₂O = Zn_xOTf_y(OH)_2x−y∙nH₂O) appears on bare Zn, but no corresponding byproduct appears on the PIP@Zn surface. This is attributed to the water shielding effect of the PIP@Zn interface layer, which is consistent with the results of the ex situ Raman mapping (Figure 4a) [27].

FIGURE 4.

(a) XRD for the Zn anode after 90 cycles with 1 mA cm⁻² and 1 mAh cm⁻²; FIB‐SEM with Zn anode cycled after 30 cycles with 1 mA cm⁻² and 1 mAh cm⁻² for (b) Bare Zn; and (c) PIP@Zn; (d) XPS for the Zn anode after 30 cycles with 1 mA cm⁻² and 1 mAh cm⁻² for (d) Bare Zn; and (e) PIP@Zn; ToF‐SIMS for (f) Bare Zn; and (g) PIP@Zn.

X‐ray photoelectron spectroscopy (XPS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), and focused Ion Beam—Scanning Electron Microscopy (FIB‐SEM) were used to gain insight into the SEI chemistry and the correlation between PIP and surface components [28]. The FIB‐SEM results more intuitively demonstrate the positive role of the inorganic‐rich inner layer and organic‐rich outer layer of PIP@Zn in maintaining the cycle stability of the negative electrode interface [29]. After 30 cycles, the inorganic‐rich SEI layer on the bare Zn is unevenly distributed, and the excessive local Zn²⁺ concentration triggers uneven deposition/corrosion, resulting in severe corrosion reactions and the influence of uneven deposition stress, which ultimately leads to cracking of the zinc negative electrode [30]. In contrast, PIP@Zn is rich in flexible organic PIP and inorganic SEI film, which effectively alleviates the undesirable electrode pulverization and stress cracking and achieves a more uniform and controllable deposition behavior (Figure 4b,c). In the C 1s region, the CF₃ species originate from the incomplete reduction products of OTf⁻ anions or trace Zn(OTf)₂ residues on the Zn surface. Notably, Zn₂(OH)₂CO₃ passivates were detected on the Bare Zn, which can be attributed to the significant increase in local pH caused by water decomposition (Figure 4d,e). In addition, in the Zn 2p region, the PIP@Zn surface signal is weak, which may be due to the coverage of the artificial organic layer. As the sputtering time increases to 8 min, the Zn 2p signal is significantly enhanced, and the metallic Zn at the bottom is gradually exposed, further indicating that the organic layer is mainly located on the surface (Figures S23 and S24) [31]. Therefore, through the constructed PIP@Zn, a composite SEI with an inorganic‐rich inner layer and an organic‐rich outer layer is generated on the surface of the Zn metal anode, which helps to improve the stability of the anode. The inorganic‐rich SEI layer of Bare Zn and PIP@Zn with an inorganic‐rich inner layer and an organic‐rich outer layer were studied using ToF‐SIMS. Compared with bare Zn, the Zn signal intensity of PIP@Zn is more obvious during the etching process, indicating that the Zn matrix is continuous in the depth direction and there are fewer byproducts on the surface (Figure 4f,g and Figure S25). In addition, the ZnS and ZnF₂ signals are mainly distributed in the surface layer, indicating the formation of an inorganic‐rich SEI gradient layer, which also confirms the significant advantages of the PIP@Zn inner inorganic layer in terms of thinness, density, and uniform distribution [32].

2.5. Electrochemical Performance of Long‐Term Zn|I₂ Battery

With the protective PIP layer, the Zn anode is effectively safeguarded, thereby exhibiting markedly enhanced stability during cycling. The PIP@Zn|Ti cell operated stably for 1776 cycles with an average Coulombic efficiency (CE) of 98.6%. In contrast, the bare Zn asymmetric cell had a relatively short operating life of less than 500 cycles. The corresponding capacity‐voltage curves of the PIP@Zn|Ti asymmetric cell have higher voltage polarization and stable voltage hysteresis, which also demonstrates the reversible and stable Zn deposition/stripping in PIP@Zn (Figure S27). Zn|Zn symmetric cells were also assembled to evaluate the charge–discharge reversibility and cycling stability of bare Zn and PIP@Zn. The PIP@Zn symmetric cell achieved a cycle life of 2700 h at 2 mA cm⁻² and 2 mAh cm⁻² (Figure 5a). With insufficient PIM‐1 content, PIP‐15% shows weaker suppression of the local tip effect, resulting in a shorter cycling life of about 1600 h. In contrast, excessive PIM‐1 in PIP‐5% restricts ion‐transport kinetics and promotes nonuniform Zn deposition, leading to reduced stability of about 2200 h. Therefore, PIP‐10% provides an optimal balance between regulating interfacial Zn deposition and maintaining efficient Zn²⁺ transport (Figure S28). In particular, at an area capacity density of 5 mA h cm⁻², the PIP@Zn symmetric cell still exhibits stability with a cycle life of more than 2100 h, which is 17.4 times that of the bare Zn||Zn symmetric cell (125 h) (Figure 5b). In addition, the rate performance of the Zn anode was evaluated. Compared with Bare Zn, PIP@Zn sustained stable Zn stripping/plating at an ultrahigh current density of 25 mA cm⁻² under highly demanding conditions (Figure S29). To further assess the practical potential of the PIP@Zn anode, electrochemical tests under more demanding conditions were conducted. The PIP@Zn symmetric cell maintained stable cycling for over 600 h at 10 mA cm^{− 2} and 20 mAh cm^{− 2}, corresponding to a high Zn DOD of 68.3% (Figure S30).

FIGURE 5.

(a) The cycling stability of the bare Zn and PIP@Zn symmetric cells at 2 mA cm⁻² and 2 mAh cm⁻². (b) Long‐term cycling performance of the bare Zn and PIP@Zn symmetric cells at 5 mA cm⁻² and 5 mAh cm⁻². (c) Long‐term performance and (d) charge and discharge curves of the bare Zn and PIP@Zn at current of 0.8 A g⁻¹. Long‐term performance of the bare Zn and PIP@Zn at current of (e) 4 A g⁻¹ and (f) 8 A g⁻¹. (g) Cycling performance of the 4 e⁻ Zn|I₂ pouch cell at 2 C and 2.2 mAh cm⁻².

After confirming the stability of the Zn metal anode, we further validated the overall cell stability using full Zn|I₂ batteries. In contrast to Mn‐ and V‐based cathodes, I₂ cathodes are prone to the I₃ ⁻ shuttle phenomenon, which causes pronounced parasitic reactions and corrosion at the Zn anode. Consequently, Zn|I₂ batteries serve as a more rigorous model system for probing anode stability and for evaluating the effectiveness of interfacial coatings in suppressing anode degradation. In addition, we applied 4 e⁻ Zn|I₂ battery is applied in here for high capacity. At 0.8 A g⁻¹ (2 C), with the protection PIP of Zn anode keeps stable in the cycle process, it delivers high stability for 0.8 A g⁻¹, 98.4% capacity retention after 535 cycles (Figure 5c,d). Due to the high stability of PIP@Zn, we achieved high‐rate performance in a Zn|I₂ battery at 8 A g⁻¹, achieving 27 899 cycles with 83.4% capacity retention (Figure 5f). This long‐term durability can be attributed to PIP regulating Zn²⁺ transport and suppressing uncontrolled interfacial reactions. By mitigating EDL‐driven Zn²⁺ accumulation and reducing water‐induced side reactions, the PIP layer promotes uniform Zn deposition/stripping and prevents dendrite growth, corrosion, and interfacial passivation. Moreover, the coin‐type full cell delivered stable cycling for 150 cycles at 1 C with a Zn DOD of 41.3%, confirming the effectiveness of the PIP interphase under high Zn utilization and more practically relevant conditions (Figure S31). In addition, with an areal capacity of 2.2 mAh cm⁻². 4 e⁻ Zn|I₂ pouch cell delivers high cycles over 500 cycles (Figure 5g). Under high cathode loadings, the pouch‐cell tests further confirm the effective suppression of the shuttle effect on the Zn anode enabled by the PIP layer, demonstrating its strong potential for practical implementation in battery devices.

3. Conclusion

In summary, we establish a direct experimental framework for probing the electrical double layer (EDL) and its associated layers' dynamics at zinc metal interfaces by integrating liquid‐phase AFM with in situ spectroscopic and electrochemical analysis. The pronounced field‐induced ion enrichment and thickened EDL observed on bare Zn reveal the mechanistic origin of concentration polarization, corrosion, and unstable deposition. By introducing a PEO/PIM‐1 hybrid polymer as an artificial SEI, we decouple ion accumulation from the metal surface through a confined ion‐storage mechanism, thereby suppressing EDL‐induced heterogeneity and enabling uniform Zn nucleation and plating. The resulting inorganic–organic gradient SEI substantially enhances interfacial robustness, supporting long‐life operation in both symmetric Zn cells and high‐rate 4e⁻ Zn|I₂ batteries. This work not only visualizes ion concentration layer behavior experimentally but also provides a generalizable interfacial‐engineering strategy for stabilizing metal anodes in aqueous energy‐storage systems.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74049‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.