Layered solid Brønsted acid for dynamic interfacial pH regulation toward durable zinc anodes

Abstract

Aqueous zinc-ion batteries (AZIBs) are promising candidates for large-scale grid energy storage due to their inherent safety, durability, and low cost. However, their practical performance is hampered by the hydrogen evolution reaction (HER) on the Zn anode, which causes unstable Zn/electrolyte interfacial pH values, resulting in the formation of byproducts and uncontrollable Zn dendrite growth. To address these issues, we developed a layered solid Brønsted acid HNbMoO₆·H₂O (HNM) for interfacial pH regulation and Zn anode protection. Density functional theory (DFT) calculations suggested that the strong adsorption of OH⁻ ions by HNM (E_ads = −4.15 eV) and the presence of abundant interlayer hydrated protons in HNM facilitated effective adsorption and neutralization of OH⁻ ions, thereby offering stable interfacial pH values, preventing alkaline byproduct formation and suppressing tip-induced dendrite growth. Moreover, the layered HNM established stable ion transport channels, enabling ordered Zn²⁺ flux and homogeneous Zn²⁺ deposition. Notably, HNM simultaneously inhibited the HER and accelerated the Zn²⁺/Zn plating/stripping kinetics. Consequently, HNM@Zn enabled an excellent Coulombic efficiency of 99.7% (over 1000 cycles) in asymmetrical cells, an exceptional Zn²⁺ transference number of 0.79 and stable cycling for over 1750 hours in symmetrical cells, retaining a capacity of 130 mAh g⁻¹ after 1000 cycles in HNM@Zn||α-MnO₂ full cells. This work provides insights into multifunctional anode engineering for interfacial pH regulation towards high-performance AZIBs.

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New concepts

Layered solid Brønsted acid HNbMoO₆·H₂O (HNM) is introduced as a coating layer for the Zn anode. HNM simultaneously regulates interfacial pH values, inhibits the hydrogen evolution reaction, facilitates Zn²⁺ desolvation, and accelerates Zn²⁺/Zn plating/stripping kinetics. As a result, HNM@Zn achieves a high Zn²⁺ transference number (0.79) in symmetrical cells, an excellent Coulombic efficiency of 99.74% (over 1000 cycles) in asymmetrical cells, stable cycling for over 1750 hours in symmetrical cells, and superior capacity retention and cycling stability over 1000 cycles in HNM@Zn||α-MnO₂ full cells. This work provides critical insights into multifunctional Zn anode engineering for interfacial pH regulation towards high-performance aqueous zinc-ion batteries.

Introduction

The continuously increasing global energy demand and the need for renewable energy sources have necessitated the exploration of energy storage technologies to address the intermittency and instability of energy supply from renewable sources.¹ The electrochemical energy storage market has long been dominated by lithium-ion batteries (LIBs); however, the scarce Li reserves on the Earth's crust, formation of Li dendrites during overcharging and thermal runaway under high temperatures, compression, or short circuits pose severe supply risks and safety concerns on LIBs.^2,3 Hence, alternative safe, economical, and sustainable energy storage systems, e.g. sodium-ion batteries and zinc-ion batteries, are highly desirable.^4,5 Aqueous zinc-ion batteries (AZIBs) with non-flammable aqueous electrolytes inherently pose lower risks of fire or explosion, and the abundant and low-cost zinc anode makes AZIBs economically viable. Additionally, the Zn anode boasts impressive theoretical capacities (820 mAh g⁻¹ and 5855 mAh cm⁻³) and an appropriate redox potential (−0.76 V vs. standard hydrogen electrode SHE),^6,7 offering efficient energy storage capabilities.^8–12 Despite these advantages, AZIBs suffer from the side reactions at the Zn anodes, such as uncontrolled dendrite growth, the hydrogen evolution reaction (HER), and passivation reactions, impairing the stability and reversibility of AZIBs.^13,14

Zn anode undergoes plating/stripping during charging/discharging of AZIBs.¹⁵ Nonetheless, inherent defects or inhomogeneity on a Zn anode can lead to uneven electric field/Zn²⁺ ion concentration/current distribution at the Zn/electrolyte interface, leading to irregular Zn deposition.^16,17 Furthermore, electrochemical hydrogen evolution (EHE) and corrosion hydrogen evolution (CHE) reactions can occur at the Zn anode,^18–21 rendering increased OH⁻ concentration (i.e., higher pH) at the Zn/electrolyte interface, which in turn accelerates the formation of passivation layers and byproducts.^22–26 Therefore, a uniform Zn/electrolyte interface with well-maintained pH should be guaranteed in AZIBs. In order to enhance the stability and reversibility of the Zn anode, various approaches, including separator design (e.g. grafting metal–organic frameworks MOFs and covalent organic frameworks COFs),^11,27–29 electrolyte engineering (e.g. introducing electrolyte additives, high concentration salts, co-solvent or polymeric networks),^30–34 and Zn anode modification (e.g. artificial interface layer coating and crystal facet engineering),^35–37 have been adopted, among which coating an artificial interface layer on the zinc anode offers operational flexibility and scalability. For instance, Liu et al.²⁶ constructed a CuCK interfacial layer for zinc anode protection using copper gluconate, carboxymethyl chitosan, and kaolin, which achieved uniform Zn²⁺ deposition by the in situ induction of gradient alloy sites for Zn²⁺ nucleation and enhancing Zn²⁺ desolvation, realizing superior stability in asymmetrical (average Coulombic efficiency CE of 99.6% over 2100 hours), symmetrical (3600 h at the current density of 2 mA cm⁻²), and full cells (excellent capacity retention between −30 and 60 °C). Although these methods can mitigate side reactions and dendrite growth on the Zn anode, less attention has been paid to pH regulation at the Zn anode/electrolyte interface.

The most straightforward way for interfacial pH regulation is to directly introduce acidic substances. For example, Wang et al.³⁸ employed zwitterionic N-tris(hydroxymethyl)methyl glycine (TMG) as an electrolyte additive. The cationic part (–NH₂⁺–) of TMG forms a hydrophobic layer on the zinc surface, while the anionic part (–COOH–) dynamically responds to pH fluctuations, thereby regulating interfacial pH and hindering side reactions. Similarly, Zhou et al.¹³ introduced L-carnosine (L-car) into the electrolyte, and the abundant dual pH buffering N and O sites in L-car can form strong hydrogen bonds with protons, effectively hindering interfacial proton transport and significantly reducing the HER. Brønsted acids, which commonly serve as proton donors in catalytic reactions, also have significant application potential for batteries.³⁹ Nian et al. employed a type of soluble Brønsted acid, bis(trifluoromethanesulfonyl)imide (HTFSI) as an electrolyte additive. The proton-releasing HTFSI reduces insoluble alkaline by-product accumulation while the hydrophobic TFSI⁻ can form a protective layer on the zinc surface, thereby enabling uniform zinc deposition and suppressing side reactions and HER. Nonetheless, although acidic additives can inhibit OH⁻ aggregation, they are less effective at facilitating interfacial Zn²⁺ flux and blocking the direct contact between H⁺ and Zn anode. Unlike acidic additives that inevitably cause global electrolyte acidification, a solid acid coating releases protons to dynamically combine with the in situ-generated OH⁻ at the zinc surface, regulating interfacial pH and suppressing alkaline byproducts. Coupled with a large interlayer spacing, ensuring uniform ion flux, dual-functional layered solid acid coatings are envisioned to significantly enhance the long-term cycling stability of zinc anodes. To the best of our knowledge, no prior study has directly utilized solid-state acids as protective coatings for zinc anodes, distinguishing this approach from conventional bulk acidification strategies.

Herein, we propose an artificial pH-regulating interface layer on the zinc anode with a layered solid Brønsted acid, HNbMoO₆·H₂O (HNM). HNM has a layered tri-rutile structure (AB₂O₆),⁴⁰ large specific surface area and abundant porous structure, providing abundant active sites in catalytic reactions.^41–43 The HNM coating serves as a physical barrier between the Zn anode and H⁺; the layered structure of HNM can facilitate Zn²⁺ conduction, while the acidic sites can release H⁺ and regulate interfacial pH, rendering the superior reversibility of the Zn anode. DFT calculations have revealed the excellent OH⁻-absorption capability of HNM (E_ads = −4.15 eV), and in situ pH measurements have verified the pH-regulating effect of HNM. As a result, HNM-coated Zn anodes (HNM@Zn) exhibit low charge-transfer resistance and a high Zn²⁺ transference number (0.79), realizing excellent reversibility in asymmetrical HNM@Zn||Cu cells (average CE of 99.7% over 1000 cycles), and stable cycling in both symmetrical cells (over 1750 h at 1 mA cm⁻²) and HNM@Zn||α-MnO₂ full cells (capacity of 130 mAh g⁻¹ after 1000 cycles). This work highlights the importance of interfacial pH regulation and offers a practical pH-regulating coating layer on the Zn anode towards highly efficient and advanced AZIBs.

Results and discussion

HNM was synthesized following the procedure reported previously (see Experimental section, SI for details).⁴⁰ MoO₃, Li₂CO₃, and Nb₂O₅ were subjected to a solid-state reaction at 580 °C for 24 h (with one intermediate grinding) to obtain LiNbMoO₆ (hereafter referred to as LiNM), which was then protonated in HNO₃ to yield HNbMoO₆·H₂O. The crystal structures of HNM and LiNM are shown in Fig. 1a and b, where the protonation process replaces Li⁺ with H₃O⁺, endowing HNM with a larger interlayer spacing and more abundant acidic active sites within the layers. As confirmed by XRD analysis in Fig. 1c, both LiNM and the protonated HNM belong to the body-centered tetragonal phase, with the respective unit cell parameters of a = 4.701 Å, c = 9.248 Å for LiNM, and a = 4.690 Å, c = 26.75 Å for HNM (PDF #00-050-0886).⁴⁰ Compared with LiNM (PDF #00-050-0884), the characteristic peaks of HNM in the c direction show a significant shift to smaller angles, indicating the expanded interlayer spacing with H₃O⁺ protonation, which can facilitate the rapid and homogeneous transport of Zn²⁺ and enable the uniform deposition of Zn²⁺. X-ray photoelectron spectroscopy (XPS) was also performed on HNM to investigate the surface chemical state and composition. The Nb 3d spectrum (Fig. S1a) is deconvoluted into Nb 3d_3/2 and Nb 3d_5/2 centered at 209.8 eV and 207.1 eV, respectively, indicating that Nb retains the oxidation state of Nb⁵⁺ during synthesis.⁴¹ The high-resolution spectrum of Mo 3d (Fig. S1b) contains two peaks at 235.6 eV and 232.5 eV (corresponding to 3d_3/2 and 3d_5/2), indicating that Mo exists in its highest oxidation state, Mo⁶⁺.^41,42 As for the O 1s spectrum (Fig. 1d), in addition to the main O 1s signal peak at 530.1 eV representing the primary lattice oxygen,⁴⁴ the second characteristic peak at 530.5 eV is attributed to the O²⁻ ions in MoO₆ and Nb₂O₅,⁴⁵ while the minor peak at 531.7 eV is associated with adsorbed hydroxyl.^46,47 The molar ratio of Nb : Mo : O in HNM was also quantified to be approximately 1 : 1 : 5.7, approaching the stoichiometric ratio of HNM. In short, the XPS results are consistent with previous reports,⁴¹ indicating the successful synthesis of HNM.

Fig. 1 Characterization of HNM. (a and b) Schematic of the atomic structures of HNM and LiNM. (c) X-ray diffraction (XRD) patterns and PDF card curves of HNM and LiNM. (d) High-resolution O 1s X-ray photoelectron spectra (XPS) of HNM. (e) Scanning electron microscopy (SEM), (f) transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of HNM.

The morphology of HNM was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Fig. 1e, f and Fig. S2), showing the layered nanosheet morphology of both HNM and LiNM, while the HNM nanosheets have lateral sizes ranging from several hundred nanometers to a few micrometers. High-resolution transmission electron microscopy (HRTEM) of HNM revealed lattice fringes with an interlayer spacing of 13.28 Å, corresponding to the (002) plane of HNM. Further HRTEM observations (Fig. S3a) also showed lattice spacings of 1.63 Å assigned to the (220) plane of HNM, and lattice spacings of 3.69 Å and 2.65 Å (Fig. S3b) assigned to the (023) and (300) planes of LiNM,⁴⁸ respectively, confirming the successful synthesis of both LiNM and HNM. The corresponding energy-dispersive X-ray spectroscopy (EDS) mappings (Fig. S4) confirmed the uniform distribution of Nb, Mo, and O within HNM.

The Zn anode protection mechanism of the HNM interfacial layer is intuitively demonstrated in Fig. 2a. The layered HNM solid Brønsted acids bring the advantages of ordered zinc deposition, suppressed by-product formation, and inhibited OH⁻ agglomeration. When AZIBs are in operation, the exposure of bare Zn to aqueous electrolyte can induce side reactions such as EHE and CHE, producing abundant OH⁻ on the Zn anode surface.^49,50

Zn + 2H₂O → Zn²⁺ + 2OH⁻ + H₂↑

Fig. 2 pH-Regulating behavior of HNM. (a) Impact of OH⁻ enrichment on the zinc anode surface, and the working principle of the HNM coating. Density functional theory (DFT) calculation of OH⁻ absorption energy in HNM and on the bare Zn anode. Adsorption models of HNM@Zn electrode (b) before and (c) after adsorbing OH⁻, and bare Zn electrode (d) before and (e) after adsorbing OH⁻. (f) In situ pH measurements in symmetrical cells charged/discharged at 10 mA cm⁻²/10 mAh cm⁻² for 12 hours with HNM@Zn and bare Zn electrodes, respectively.

These accumulated OH⁻ can combine with Zn²⁺ to form the random precipitation of non-conductive by-products such as ZnSO₄(OH)₆·xH₂O (ZSH) on the Zn anode, causing non-uniform zinc metal deposition. This significantly promotes zinc dendrite growth and exacerbates side reactions, leading to a sharp decline in battery life.⁵¹ Fortunately, HNM as solid Brønsted acids contains abundant H₃O⁺ acidic sites within the interlayers, showing active proton-releasing capabilities and thus inhibiting the formation of by-products such as ZSH.^42,52–54 Specifically, the solid-state Brønsted acid HNM forms a physical barrier to avoid direct contact between the Zn anode and aqueous electrolyte, and HNM has most of its H⁺ in the form of H₃O⁺ within its layered structure, without free H⁺ to directly react with the Zn anode, thus avoiding both EHE and CHE. Moreover, HNM can release H⁺ to compensate for the agglomerated OH⁻ produced by EHE and CHE. At the same time, the acidic sites within the HNM layers can adsorb and consume OH⁻, preventing OH⁻ from reaching the zinc surface, thus inhibiting the formation of disordered precipitates and stabilizing pH at the Zn/electrolyte interface.

To verify the pH-regulating capability of HNM, density functional theory (DFT) calculations were employed to investigate the interactions of HNM and bare Zn with OH⁻ ions. The models of HNM and bare Zn before and after adsorbing OH⁻ are depicted in Fig. 2b–e. The calculated adsorption energies elucidate the affinity of HNM and bare zinc surfaces for OH⁻ ions. Notably, the active site H₃O⁺ in HNM exhibits a higher OH⁻ adsorption energy of −4.15 eV than that of bare Zn (−3.98 eV), corroborating that HNM can anchor OH⁻ ions and release H⁺ to compensate OH⁻ ions. The DFT calculations confirmed that HNM can suppress OH⁻ accumulation on the zinc anode surface, thereby extending the cycling stability of the Zn anode and AZIBs.

To further explain the pH-regulating capability of the HNM layer, an in situ pH monitoring cell was designed following a previous report (Fig. S5).³⁸ Detailed measurement procedures are provided in the Experimental section. Since the pH monitoring system was scaled up from a coin cell to an electrolytic cell, a higher current density (10 mA cm⁻²) was selected for the plating/stripping. As shown in Fig. 2f, during the cycling process, the pH value of the electrolyte in the symmetrical cell with HNM@Zn electrode is relatively stable, increasing from an initial value of 3.8 to 4.4, with a minimum value of 3.4 and a maximum of 4.6. The generally periodic pH value variation can be ascribed to the proton release and OH⁻ adsorption of HNM during charging/discharging cycles. In contrast, the pH values in symmetrical cells with a bare Zn electrode show a consistent increase across the cycling process, significantly increasing from 3.92 to 5.47. This indicates continuous OH⁻ generation in the solution, promoting the formation and agglomeration of ZSH. The significant pH increase suggests that the bare Zn anode experienced pronounced HER and corrosion reactions. These findings clearly confirmed the pH-regulating capability of HNM, thereby reducing byproduct formation, inhibiting dendrite growth, and enhancing battery life.

The Zn (002) plane, as a close-packed facet with the lowest surface energy, theoretically enables uniform planar Zn²⁺ deposition.⁵⁵ However, practical battery operation inherently involves dynamic deposition fluctuations and side reaction-induced protrusions. These irregularities create nucleation sites on higher-surface-energy planes such as (100) and (101), thereby accelerating dendritic growth.⁵⁶ To address this, strategies for promoting (002)-oriented deposition include pre-deposited Zn,^57,58 protective coatings,⁵⁹ and electrolyte additives that selectively passivate non-(002) facets.^60,61 Crucially, the HNM coating achieves dynamic interfacial pH regulation, suppressing anomalous protrusions from parasitic products, which reduces heterogeneous nucleation sites and consequently enhances the Zn (002) facet exposure on cycled anodes. Considering the superior pH-regulating capability and higher OH⁻ absorption energy of HNM, symmetrical cells assembled with HNM@Zn and bare Zn were subjected to 50 cycles at 0.5 mA cm⁻²/0.5 mAh cm⁻². As shown in the XRD patterns (Fig. S6), distinct peaks corresponding to ZSH by-products are evident on the cycled bare Zn electrode, while no by-products were detected on the cycled HNM@Zn. Moreover, the relative intensity of the Zn (002) crystal plane on the cycled HNM@Zn is obviously higher than that of the cycled bare Zn anode.

This indicates that HNM can suppress the formation of the ZSH by-products and induce the oriented and uniform (002) deposition of Zn²⁺ ions. The Zn deposition behavior was further observed via SEM and confocal laser scanning microscopy (CLSM) images (Fig. 3). The initially smooth surface of bare Zn evolved into a disordered morphology with the formation of dendrites and by-products (Fig. 3a and b). In comparison, the Zn surface in cycled HNM@Zn (HNM layer removed) showed the smooth deposition of Zn²⁺ without dendrite formation (Fig. 3d and e), promoting planar Zn(002) deposition and thereby increasing the cycle life of AZIBs. CLSM also revealed a similar trend (Fig. 3c and f). Cycled bare zinc becomes rough with large protrusions, which can pierce the separator and cause a cell short circuit. In stark contrast, the structure of the HNM coating remained relatively intact after cycling (Fig. S7) and exhibited a smoother morphology. To further investigate the ability of HNM to guide the Zn²⁺ deposition, in situ optical microscopy observations (Fig. 4a) were conducted for 60 minutes at a relatively high plating current density of 10 mA cm⁻². After 10 minutes, the surface of bare zinc began to exhibit distinct irregular protrusions with the “tip effect”, which gradually evolved into a large number of irregular moss-like zinc dendrites, rendering a non-uniform Zn surface and possibly short-circuit and even cell failure. In contrast, HNM@Zn maintained a smooth surface throughout the 60-minute deposition period, without dendrites being observed. The planar and uniform Zn (002) growth on HNM@Zn was further elucidated by the Zn²⁺ nucleation and growth behavior through chronoamperometry (CA) testing (Fig. 4b). The continuously increased current response in the bare zinc electrode indicates that adsorbed Zn²⁺ aggregates through disordered diffusion to form dendrites, demonstrating a dominant 2D diffusion process on the bare Zn surface. However, following an initial Zn nucleation process, HNM@Zn showed a relatively stable current response, indicating a dominant 3D diffusion. Such 3D diffusion is conducive to regulating uniform Zn deposition,¹¹ confirming that HNM can induce homogeneous and planar (002) deposition of Zn²⁺ and inhibit the formation of Zn dendrites.

Fig. 3 Characterization of the zinc anode surface before and after 50 cycles at 0.5 mA cm⁻²/0.5 mAh cm⁻². SEM iamges of bare Zn electrode (a) before and (b) after cycling. (c) Confocal laser scanning microscope (CLSM) image of the cycled bare zinc electrode. SEM images of (d) pristine HNM@Zn and (e) HNM@Zn after 50 cycles (with HNM layer removed). (f) CLSM image of the cycled HNM@Zn.

Fig. 4 (a) In situ optical microscope images of HNM@Zn and Zn electrodes during plating at 10 mA cm⁻². (b) CA curves of HNM@Zn and Zn electrodes. (c and d) Contact angle measurements for Zn and HNM@Zn. (e) EIS of HNM@Zn||HNM@Zn and Zn||Zn symmetrical cells. (f) Tafel curves, (g) LSV curves, and (h) plating/stripping activation energies of HNM@Zn and bare Zn electrodes. (i) Zinc ion transference number (tZn²⁺) of HNM@Zn-, LiNM@Zn-, and bare zinc-based symmetrical cells.

The wettability of the anode was characterized by contact angle measurements with the electrolyte. With the HNM coating, the contact angle of the Zn surface decreased significantly from 95.89° (bare Zn) to 73.4° (Fig. 4c and d). The enhanced wettability is indicative of the affinity between HNM and Zn²⁺, promoting the contact between the electrolyte and HNM@Zn, thereby improving the charge transfer kinetics. Fig. 4e compares the electrochemical impedance spectroscopy (EIS) of symmetrical cells with HNM@Zn and bare Zn electrodes. The semicircles observed in the high-frequency region correspond to the charge transfer resistance (R_ct) at the Zn anode/electrolyte interface. The obviously smaller R_ct of the HNM@Zn-based cell compared to that of the bare Zn-based cell indicates the facilitated interfacial charge transfer brought about by HNM coating.

The corrosion resistance of different zinc anodes was analyzed using Tafel plots. As shown in Fig. 4f, with the introduction of HNM, the corrosion overpotential (E_corr) of the zinc anode increased from −14.3 mV to −9.8 mV (vs. Zn²⁺/Zn), while the corrosion current (J_corr) decreased significantly from 0.59 mA cm⁻² to 0.29 mA cm⁻². The higher E_corr and lower J_corr of HNM@Zn indicated the superior corrosion resistance brought by the HNM coating layer. The E_corr and J_corr correlate well with hydrogen evolution behavior. As revealed by linear sweep voltammetry (LSV) measurements (Fig. 4g) in Na₂SO₄ electrolyte (from −0.5 V to 0 V versus Zn/Zn²⁺), the HNM@Zn electrode exhibited a more negative HER onset potential and a much lower hydrogen evolution current density compared to bare zinc. Specifically, at 0.5 V, the HER current density of bare Zn (10.64 mA cm⁻²) was much higher than that of HNM@Zn (2.33 mA cm⁻²), further confirming the effectiveness of the HNM@Zn electrode in suppressing the HER.

It should be noted that HNM is obtained by protonation of LiNM, which also has a layered structure. Therefore, in order to highlight the advantages of HNM, the corrosion resistance of LiNM@Zn was also investigated. LiNM@Zn showed a similar J_corr (0.29 mA cm⁻²) and comparable HER inhibition effect (Fig. S8 and S9) to HNM@Zn, but the lower E_corr (−19.1 mV) of LiNM@Zn than that of HNM@Zn highlights the merits of a solid Brønsted acid with improved corrosion resistance.

The Zn²⁺ plating/stripping activation energy (E_a) of different electrodes was quantified using the Arrhenius equation⁶² by measuring the EIS within a temperature range of 30–70 °C (Fig. S10). Compared with bare zinc (33.66 kJ mol⁻¹) and LiNM@Zn (24.53 kJ mol⁻¹), HNM@Zn exhibited a lower activation energy (19.99 kJ mol⁻¹) (Fig. 4h and Fig. S11). The activation energy is typically governed by the charge transfer process, which is primarily influenced by the desolvation energy barrier.⁶³ Therefore, the lower activation energy indicates that the HNM layer significantly enhances the Zn²⁺ desolvation, thereby accelerating the Zn²⁺ transfer and migration. The transference number of Zn²⁺ was thus further calculated by measuring the interfacial charge resistance before and after polarization (R₀ and R_s) and the initial and steady-state currents. (I₀ and I_s) of the symmetrical cell using chronoamperometry. As summarized in Fig. 4i and Fig. S12–S14, the Zn²⁺ transference numbers in HNM@Zn, LiNM@Zn, and bare zinc-based symmetrical cells were 0.79, 0.51, and 0.33, respectively. This indicates that the acidic sites within the HNM layers can capture anions such as OH⁻, promoting Zn²⁺ desolvation and directing Zn²⁺ migration, thereby increasing the Zn²⁺ transference number and inducing rapid and uniform Zn²⁺ deposition onto the Zn anode.

To verify the reversibility and stability of HNM@Zn, asymmetrical cells of HNM@Zn||Cu, LiNM@Zn||Cu, and Zn||Cu were assembled. The reversibility of zinc plating/stripping was investigated by evaluating various key parameters, including CE, voltage hysteresis, and cycle life. As shown in Fig. 5a, the HNM@Zn electrode exhibited stable plating/stripping behavior for over 1000 cycles, with an impressive average CE of 99.74%, significantly outperforming LiNM (Fig. S15) and bare zinc, which experienced short-circuiting after only 160 and 90 cycles, respectively. The corresponding plating/stripping curves at 1 mA cm⁻² and 0.5 mAh cm⁻² (Fig. 5b and Fig. S16) indicate that HNM@Zn||Cu displayed a small voltage hysteresis of 53 mV, lower than that of Zn||Cu (65 mV) and slightly larger than LiNM@Zn||Cu (36 mV), demonstrating reduced polarization and enhanced reaction kinetics. The remarkable CE and low polarization potential confirmed the improved desolvation kinetics at the HNM@Zn/electrolyte interface.

Fig. 5 (a) CE profiles of the HNM@Zn||Cu and Zn||Cu asymmetrical cells at 1 mA cm⁻² and 0.5 mAh cm⁻². (b) Corresponding galvanostatic charge/discharge (GCD) curves of asymmetrical cells. (c–f) Voltage-time profiles for HNM@Zn and bare Zn-based symmetrical cells at 1 mA cm⁻²/1 mAh cm⁻², 2 mA cm⁻²/2 mAh cm⁻², 5 mA cm⁻²/5 mAh cm⁻², and 10 mA cm⁻²/2.5 mAh cm⁻², insets in (c) indicate the overpotential of HNM@Zn-based symmetrical cell. (g) Nucleation overpotentials of HNM@Zn and bare Zn upon first plating. (h) Rate performances of the symmetrical cells at different current densities.

The stability of the anode can also be evaluated by the plating/stripping cycling of symmetrical cells. Fig. 5c and Fig. S17 show that the HNM@Zn-based symmetrical cell exhibited stable cycling for 1750 h at a current density of 1 mA cm⁻², longer than that of LiNM@Zn (1056 h) and bare zinc-based symmetrical cells (67 h). Encouragingly, compared with LiNM@Zn (60 mV) and Zn (40 mV), HNM@Zn showed a much lower overpotential of approximately 23 mV, indicating that the HNM layer can effectively reduce the energy barrier for Zn plating/stripping. When the current density was increased to 2 mA cm⁻², the HNM@Zn-based symmetrical cell still demonstrated better cycling performance for 750 h compared to the other two counterparts (Fig. 5d and Fig. S18). Considering that the cycling performance of batteries at high current densities is crucial for practical applications, the zinc plating/stripping cycling of the HNM@Zn-based symmetrical cell was further evaluated under stringent conditions at current densities of 5 and 10 mA cm⁻², as shown in Fig. 5e and f. Encouragingly, the HNM@Zn-based symmetrical cell maintained stable cycling for 300 h at 5 mA cm⁻² and 180 h at 10 mA cm⁻². In contrast, neither LiNM (Fig. S19 and S20) nor bare zinc-based symmetrical cells achieved cycling stability exceeding 100 h under these conditions. The nucleation overpotential can reflect the reversibility and stability of the coated electrode. As shown in Fig. 5g and Fig. S21, the nucleation overpotentials of HNM@Zn, LiNM@Zn, and bare Zn were 28.1, 39.1, and 46.6 mV, respectively. The significantly lower nucleation overpotential observed for HNM@Zn indicates the facile nucleation on HNM@Zn, thus promoting uniform plating at later stages. Fig. 5h and Fig. S22 display the rate performances of the symmetrical cells when the current density was increased from 1 mA cm⁻² to 10 mA cm⁻². The bare Zn-based symmetrical cell only maintained stability at the low current density of 1 mA cm⁻²; when the current was increased to 5 mA cm⁻², the voltage profile immediately became unstable. In contrast, the HNM@Zn-based symmetrical cell exhibited stable voltage profiles, demonstrating excellent rate capability, high reversibility, and low voltage hysteresis.

To evaluate the application of the HNM@Zn anode in practical AZIBs, the HNM@Zn||α-MnO₂ full cell was assembled using α-MnO₂ nanorods synthesized by a simple hydrothermal method as the cathode (Fig. S23). The assembled HNM@Zn||α-MnO₂ full cell was subjected to cyclic voltammetry (CV) testing (Fig. 6a). The HNM@Zn||α-MnO₂ full cell exhibited similar CV curves to that of the Zn||α-MnO₂ cell, indicating that the HNM layer does not interfere with the charge/discharge mechanism of the Zn anode but rather enhances its electrochemical performance. Two distinct reduction peaks were identified from the CV curves, corresponding to the intercalation of H⁺ and Zn²⁺ ions, respectively. Moreover, the HNM@Zn||α-MnO₂ full cell has a lower potential difference between the oxidation and reduction peaks, suggesting that the HNM anode endows the battery with highly reversible redox kinetics. The EIS curves are depicted in Fig. 6b and Fig. S24. Compared with the R_ct of LiNM@Zn||α-MnO₂ (478.2 Ω) and Zn||α-MnO₂ (623.1 Ω), the HNM@Zn||α-MnO₂ cell exhibits a much lower R_ct of 337.2 Ω, indicating that the HNM protective layer significantly enhances the charge transfer capability at the interface and improves the plating/stripping kinetics of Zn. The rate performances of the three full cells at different current densities are shown in Fig. 6c and Fig. S25. At current densities of 0.2, 0.5, 1, 2, and 5 A g⁻¹, the HNM@Zn||α-MnO₂ full cell delivered specific capacities of 304.5, 289.4, 214.7, 145, and 74.1 mAh g⁻¹, respectively. Notably, when the final current density returned to the initial value of 0.2 A g⁻¹, a significant capacity retention rate of approximately 80.5% was observed. Fig. 6d and Fig. S26 illustrate the capacity of the three full cells at different current densities; the HNM@Zn||α-MnO₂ cell maintained the highest discharge specific capacity at various current densities. To verify the long-cycle performance, the three full cells were tested at a current density of 1 A g⁻¹ (Fig. 6e and Fig. S27). The HNM@Zn||α-MnO₂ full cell retained a capacity of 130 mAh g⁻¹ after 1000 cycles, significantly higher than that of Zn||α-MnO₂, and achieved a stable CE close to 100%. The increased cycling stability and higher capacity of HNM@Zn||α-MnO₂ can be attributed to the HNM coating. Specifically, prior to long-term cycling at 1 A g⁻¹, the full cells necessarily underwent an activation process at 0.1 A g⁻¹ (10 cycles). This prolonged activation process can inevitably induce Zn anode corrosion, electrolyte pH fluctuation, and α-MnO₂ dissolution, triggering fast capacity decay. Crucially, the HNM coating effectively mitigated these issues by regulating interfacial pH to maintain electrolyte stability, enabling the HNM@Zn||α-MnO₂ full cell to demonstrate superior capacity retention after the activation process. Therefore, HNM@Zn||α-MnO₂ delivered higher capacity than Zn||α-MnO₂ in Fig. 6e. Furthermore, the coating's suppression of dendrite formation significantly enhanced the cycling stability, preventing premature failure from dendrite-induced degradation. The HNM-coated symmetrical cells demonstrated significantly enhanced electrochemical performance compared to various previously reported battery systems (Table S1).

Fig. 6 (a) Cyclic voltammetry (CV) curves at a scan rate of 20 mV s⁻¹ and (b) Nyquist plots of HNM@Zn||α-MnO₂ and Zn||α-MnO₂ full cells. (c) Rate performances at different current densities ranging from 0.2 to 5 A g⁻¹. (d) Galvanostatic charge/discharge profiles of HNM@Zn||α-MnO₂. (e) Long-cycle performance of the HNM@Zn||α-MnO₂ full cell and the Zn||α-MnO₂ full cell at a current density of 1 A g⁻¹.

Conclusions

In summary, to enhance the electrochemical stability and reversibility of the zinc metal anode, a layered solid Brønsted acid HNM was introduced on the zinc anode to regulate the interfacial pH and Zn²⁺ flux. Density functional theory (DFT) calculations revealed that the protonated H₃O⁺ acid sites in HNM have a strong OH⁻ adsorption capability, with a higher adsorption energy (E_ads) of −4.15 eV than that of bare Zn (−3.98 eV), thus inhibiting OH⁻ agglomeration and suppressing the formation of parasitic alkaline byproducts. The pH-regulating capability of HNM was further verified via in situ pH monitoring in symmetrical cells. The HNM@Zn electrode exhibited accelerated interfacial charge transfer and facilitated Zn²⁺ migration. Specifically, the CE of the asymmetrical HNM@Zn||Cu cell reached 99.7% after 1000 cycles. The symmetrical HNM@Zn||HNM@Zn cell demonstrated enhanced cycling stability, operating stably for over 1750 hours at 1 mA cm⁻²/1 mAh cm⁻² and for more than 300 hours at 5 mA cm⁻²/5 mAh cm⁻². Furthermore, the HNM@Zn||α-MnO₂ full cell exhibited high specific capacity across multiple current densities, retaining a capacity of 130 mAh g⁻¹ after 1000 cycles. Our work highlights the critical role of the HNM coating in enhancing the stability and reversibility of AZIBs through dynamic pH regulation and OH⁻ adsorption.

Author contributions

Conceptualization: Jingwei Chen. Methodology: Jingwei Chen, Caofeng Niu. Software: Caofeng Niu. Investigation: Caofeng Niu, Bing Xu, Jiachen Tian, Tongzhuang He, Lizhu Li. Resources: Jingwei Chen. Writing – original draft: Caofeng Niu. Writing – review & editing: Jingwei Chen, Weiqian Tian, Jingyi Wu, Yue Zhu, Huanlei Wang. Supervision: Jingwei Chen, Lifeng Chen. Funding acquisition: Jingwei Chen, Lifeng Chen.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Additional datasets are available from the corresponding author on reasonable request.

Experimental details, characterization data and performance comparison are provided. See DOI: https://doi.org/10.1039/d5mh01026h.

Acknowledgements

The authors express their gratitude to the reviewers for their insightful suggestions. This work was supported financially by the National Natural Science Foundation of China (52202320, 22293044), National Natural Science Foundation of China–China Academy of Engineering Physics “NSAF” Joint Fund (U2230101), the National Natural Science Fund for Excellent Young Scientists Fund (Overseas) Program (GG2090007003), the National Key Research and Development Program of China (2021YFA0715700), Shandong Excellent Young Scientists Fund Program (Overseas, Grant No. 2023HWYQ-060), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0450402), the Anhui Provincial Major Science and Technology Project (202203a05020048), the Fundamental Research Funds for the Central Universities (WK2490000002), and the Joint Research Center for Multi-Energy Complementation and Conversion.

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