Modulating the solvation structure to enhance amorphous solid electrolyte interface formation for ultra-stable aqueous zinc anode

Abstract

Electrolyte additives are extensively validated effective in mitigating dendrite growth and parasitic reactions in aqueous zinc-ion batteries (AZIBs). Nonetheless, the mechanisms by which additives influence the formation and characteristics of the inorganic solid–electrolyte interphase (SEI) are not yet fully elucidated. Herein, we investigate how Zn(CF₃COO)₂ additives influence solvation structure and elucidate the mechanism by which these additives promote the dual reduction of anions. Through cryo-transmission electron microscopy analysis, we identified the SEI as a highly amorphous ZnS/ZnF₂ phase. This amorphous hybrid SEI demonstrates exceptional stability, mechanical robustness, and high Zn²⁺ conductivity, effectively mitigating parasitic reactions and enhancing Zn plating/stripping reversibility. Even under elevated current densities, the Zn anode exhibits ultra-stable longevity and ultra-high reversibility. This study provides a comprehensive understanding of the intrinsic mechanisms governing solvation structure modulation that lead to the formation of amorphous hybrid SEI, underscoring their efficacy in enhancing the performance and durability of AZIBs.

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Broader context

Aqueous zinc-ion batteries (AZIBs) are regarded highly promising due to their distinct advantages, such as low cost, high safety, and environmental friendliness. However, AZIBs encounter dendrite growth, hydrogen evolution reaction, and corrosion, which result in poor cyclability and low energy/power density. Electrolyte additive has shown potential as an effective and straightforward strategy to mitigate these issues. Nevertheless, the mechanism by which additives influence the formation and properties of the inorganic solid–electrolyte interphase (SEI) remains unclear. In this work, Zn(CF₃COO)₂ shows promise as a versatile additive for AZIBs. The –CF₃ group rebuilds the robust hydrogen bonds with H₂O and reduces the coordination number of H₂O with Zn²⁺. Most importantly, CF₃COO⁻ aids the dual reduction of anions to form an amorphous SEI on the Zn anode. This SEI regulates Zn plating/stripping and inhibits dendrite growth. Consequently, the Zn anode exhibits significantly enhanced long-term duration and improved reversibility, even under high current densities and high depth of discharge. This detailed mechanistic study offers valuable insights into the pivotal role of electrolyte strategies in advancing high-performance AZIBs.

Introduction

Rechargeable aqueous zinc-ion batteries (AZIBs) are increasingly favored for the development of behind-the-meter and front-of-meter stationary energy storage systems,^1–6 which are essential for integrating renewable energies into the grid. Their appeal stems from their potential low cost, abundant constituent materials, environmental sustainability, and inherent safety.^7–14 Zn metal, with a high capacity (820 mA h g⁻¹), widespread availability, and favorable redox potential (−0.762 V vs. SHE), presents itself as a viable anode material.^15–18 However, Zn anodes operating in weakly acidic electrolytes face significant challenges,^19–21 including uncontrolled dendrite formation, severe surface corrosion, hydrogen evolution reaction (HER), and byproduct precipitation. These issues become more severe under demanding operational conditions like high area capacity, elevated current density, low anode-to-cathode ratio (N/P), and substantial depth of discharge (DOD).^22–24 These factors severely impair the reversibility and stability of Zn metal during plating and stripping processes, underscoring the critical need to develop solutions for dendrite-free Zn anodes with high utilization rates (>80%).

To overcome these challenges, ongoing efforts are focused on optimizing the various components of the cell, including the development of advanced electrolytes, protective interface layers, three-dimensional (3D) anode structures, plating substrates, and modified membranes.^25–28 Among these strategies, one of the simplest and most promising is the use of electrolyte additives to modify the solvation structure and Zn/electrolyte interface. Previous studies have shown that proper inorganic additives can help inhibit dendrite formation through electrostatic shielding at dendrite protrusions.^29,30 Nevertheless, besides adding cost, using heavy metal ions as additives introduces concerns regarding recyclability and water contamination, which runs against the environmentally sustainable and pollution-free goals of AZIBs. Organic additives have also been explored to regulate solvation structures and improve Zn plating while suppressing side reactions.^31,32 However, despite their relative effectiveness, these organic solutions often require large doses of relatively costly additives, thus posing additional obstacles. Consequently, there is a pressing need for the development of green, cost-effective, and efficient electrolyte additives that align with the sustainability goals of AZIBs.

Understanding how additives function is crucial to optimizing the electrolyte in rechargeable AZIBs. However, research on organic additives often neglects to investigate the mechanisms through which these additives affect the evolution of the solid electrolyte interface (SEI).¹³ This gap is largely due to the challenges in directly observing the SEI, which is highly sensitive to the electron beam in transmission electron microscopy (TEM) studies.³³

Manipulating the electrolyte through the use of additives significantly impacts both the solvation structure and the microenvironment at the Zn metal/electrolyte interface. Effective additives should achieve multiple objectives: (i) reduce solvation interactions between Zn²⁺ and H₂O;³⁴ (ii) exhibit a high binding affinity with Zn²⁺ to displace coordinated H₂O;³⁵ (iii) form strong interaction with H₂O to reduce its activity in the HER;³⁶ and (iv) facilitate the in situ formation of a highly conductive SEI on Zn anodes.³⁷

Recent reports have highlighted the beneficial properties of additives that induce the in situ formation of a fluorinated and sulfide SEI.^38,39 These additives have been found to produce an SEI with a high mechanical modulus, favorable interfacial energy with the underlying metal anode, and rapid ion transport kinetics. A particularly promising advancement has been the development of amorphous SEI with isotropic ionic conductivity and mechanical properties. This type of SEI significantly enhances the reversibility of the anode while offering inspiration for further optimization strategies.⁴⁰

Clarifying the structure–performance relationship of the SEI is crucial for optimizing battery performance, especially given the significant benefits of using amorphous inorganic materials as SEI components. However, several uncertainties remain regarding the precise forms of these components, their states within the SEI, and the correlation between anion properties and the formed interface. Addressing these uncertainties is essential for constructing a stable Zn anode/electrolyte interface and improving SEI quality and charge transfer. Thus, investigating these aspects lays a critical foundation for advancing AZIB technology.

In this study, we investigated the influence mechanism of Zn(CF₃COO)₂ electrolyte additives on the in situ formation of inorganic SEI. By combining multi-scale characterization techniques, including cryo-TEM, 3D time-of-flight secondary ion mass spectrometry (ToF-SIMS), molecular dynamics (MD) simulations, and first-principles calculations, we elucidated the regulatory behavior of the additives on the geometric features of the solvation structure. We explored the impact of these additives on the structure and composition of the SEI, as well as their effects on Zn plating/stripping behavior. The unique structural properties of this amorphous hybrid SEI endow the Zn anode with exceptionally long cycling stability and high DOD. This work provides a comprehensive knowledge of the intrinsic mechanisms by which solvation structure regulation induces the formation of amorphous SEI, offering reliable references and guidance for the future tailoring of SEI on Zn anodes.

Results and discussion

Electrolyte optimization and solvent structures of the electrolytes

The baseline battery used the conventional 2 M ZnSO₄ as the electrolyte (labelled ZS). Additional batteries were prepared adding 10, 50, and 80 mM Zn(CF₃COO)₂, into the baseline 2 M ZnSO₄ and were labelled as ZSF-10, ZSF-50, and ZSF-80, respectively. See the optical images of the electrolytes in Fig. S1 (ESI†). At about 80 mM, the solution became saturated, preventing further dissolution of Zn(CF₃COO)₂. The conductivity of the electrolyte increased from 34.8 to 38.1 mS cm⁻¹ as the Zn(CF₃COO)₂ concentration rose from 0 to 80 mM (Fig. S2, ESI†). Besides, wettability tests on the Zn anode (Fig. S3, ESI†) showed a significant decrease in contact angle from 81.5° to 32.6° with increasing Zn(CF₃COO)₂ concentration up to 80 mM. This improved wettability is related to both the accumulation of CF₃COO⁻ at the Zn surface and the strong electron-withdrawing character of the –CF₃ group in CF₃COO⁻, which is expected to interact with H₂O molecules to form intermolecular hydrogen bonds. This strong interaction also reduces the activity of H₂O molecules in the electrolyte solution potentially resulting in a lower HER activity.⁴¹ The pH values of the electrolytes are presented in Fig. S4 (ESI†). As the additive concentration increases, the pH rises significantly from 4.26 to 5.37. Compared to the mildly acidic electrolyte (ZS), the near-neutral ZSF electrolyte provides a more favorable environment for the electroplating of the Zn metal anode.

To verify this interaction, we employed Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectra to elucidate the effect of Zn(CF₃COO)₂ on the electrolyte solvation structure. As shown in Fig. 1a and Fig. S5a, b (ESI†), the weakened FTIR peaks corresponding to the O–H bending (1631 cm⁻¹) and stretching (3170–3400 cm⁻¹) of H₂O were observed to shift towards higher wavenumbers with increasing Zn(CF₃COO)₂ ratio. This shift indicates a reduction in the quantity of free H₂O molecules. Additionally, Zn(CF₃COO)₂ displays a strong C–F bond vibration, with its intensity increasing with the Zn(CF₃COO)₂ ratio, typically observed near 1199 cm⁻¹ (Fig. 1b). The ¹H NMR spectra (Fig. 1c) show the O–H bond peak shifting to a higher field with increasing Zn(CF₃COO)₂ concentration. This shift indicates the formation of hydrogen bonds between –CF₃ and H₂O (–CF₃⋯H–O), with the F atom in –CF₃ acting as a hydrogen bond acceptor due to its lone pair of electrons.⁴² The reconstructed hydrogen bonding environment reduces the activity of the H₂O solvent, thereby suppressing side reactions (e.g. HER) in AZIBs.

Fig. 1 (a–c) Solvation structure of Zn²⁺ with the addition of Zn(CF₃COO)₂. FTIR spectra (a) and (b) and NMR spectra (c) of ZS and ZSF-x electrolytes. (d) Electrostatic potential mapping of H₂O, SO₄²⁻, and CF₃COO⁻. (e) Binding energies of Zn²⁺–H₂O, Zn²⁺–SO₄²⁻, and Zn²⁺–CF₃COO⁻. (f) The structure and electrostatic potential of [Zn(H₂O)₅SO₄] and [Zn(CF₃COO)(H₂O)₄SO₄]⁻. (g) and (h) Solvation structure of ZSF-80 electrolyte (g) with its corresponding RDF and coordination number (h) obtained from MD simulations.

Given the strong interaction between CF₃COO⁻ and H₂O, we combined first-principles calculations and MD simulations to understand the additive-induced solvation structure variation of Zn²⁺ (structures of elementary units in Table S1, ESI†). The electrostatic potential (ESP) profiles of H₂O, SO₄²⁻, and CF₃COO⁻ were first calculated (Fig. 1d). The O atoms in SO₄²⁻ and CF₃COO– exhibit more negative ESP values (blue areas), indicating a tendency to coordinate with Zn²⁺ in the solvation sheath. Fig. 1e and Fig. S6a–c (ESI†) compare the binding energies of Zn²⁺ with three possible coordinated pairs in the electrolyte. The binding of Zn²⁺–SO₄²⁻ (−3.44 eV) and Zn²⁺–CF₃COO⁻ (−2.36 eV) are significantly stronger than that of Zn²⁺–H₂O (−0.21 eV), indicating that Zn²⁺ prefers to coordinate with SO₄²⁻ and CF₃COO⁻ rather than H₂O. This suggests the addition of Zn(CF₃COO)₂ will induce the displacement of H₂O molecules with CF₃COO⁻ in the solvation sheath, while the strong affinity of SO₄²⁻ towards Zn²⁺ ensures its stable coordination.

The distribution of the Zn²⁺ solvation structure and its neighboring molecules was simulated using MD calculations. In the ZS system (Fig. S7a and b, ESI†), the radial distribution function (RDF) results and the integrated coordination numbers indicate that part of the Zn²⁺ tends to coordinate with 5 H₂O molecules and 1 SO₄²⁻ to form [Zn(H₂O)₅SO₄] (structure in Fig. 1f). In contrast, in ZSF-x electrolytes (Fig. 1g and Fig. S8a and S9a, ESI†), CF₃COO⁻ permeates the solvation sheath and displaces one of the coordinated H₂O molecules. Distinct peaks corresponding to Zn²⁺–O (CF₃COO⁻) are observed at ∼0.17 nm in the RDF (Fig. 1h and Fig. S7b and S8b, ESI†), indicating the involvement of CF₃COO⁻ anions in Zn²⁺ solvation despite the low content of the Zn(CF₃COO)₂ additive. This participation results in the formation of [Zn(CF₃COO)(H₂O)₄SO₄]⁻ (structure in Fig. 1f) in the ZSF-x electrolytes. Compared to [Zn(H₂O)₅SO₄], the overall electrostatic potential of [Zn(CF₃COO)(H₂O)₄SO₄]⁻ is notably reduced and inhomogeneous, leading to a more unbalanced charge distribution. This variation is anticipated to reduce the adjacent electrostatic action towards Zn²⁺ and benefit the construction of hydrogen bonds in the electrolyte, consequently accelerating the diffusion kinetics of Zn²⁺.⁴³ The activation energy (E_a) for the desolvation kinetics of Zn²⁺ in ZS and ZSF is calculated based on the Arrhenius equation (Fig. S10a–c, ESI†). With the incorporation of the additive, the E_a decreases from 43.5 to 35.7 kJ mol⁻¹, indicating a lower desolvation energy barrier for [Zn(CF₃COO)(H₂O)₄SO₄]⁻. Based on these comparisons, ZSF-80, with the highest Zn(CF₃COO)₂ content, was selected for further analysis. From this point forward, it will be referred to simply as ZSF.

Formation, composition, and structure of the SEI

Since the structure and composition of the SEI are crucial factors influencing Zn plating/stripping, we investigated the effect of Zn(CF₃COO)₂ on SEI formation using in-depth X-ray photoelectron spectroscopy (XPS) combined with Ar⁺ sputtering. This allowed us to examine the depth-dependent elemental and valence state variation of the SEI components on Zn anode surfaces cycled in ZS and ZSF electrolytes (100 cycles). In the Zn 2p spectrum of the ZS case (Fig. 2a), the cycled anode exhibited strong Zn–OH signals at 1025.0 and 1048.1 eV, indicating the presence of the byproduct Zn₄SO₄(OH)₆·5H₂O (ZSH). These signals remained strong and consistent even with increased sputtering depth. In contrast, for the ZSF case, deconvoluted characteristic peaks corresponding to Zn⁰ (1020.0 and 1043.1 eV) and Zn²⁺ (1022.0 and 1045.1 eV) were observed (Fig. 2b), with no detectable Zn–OH peaks. The Zn²⁺ signals are attributed to inorganic components in the SEI, tentatively identified as ZnS and ZnF₂, which will be further confirmed in subsequent characterizations. The absence of Zn–OH in the ZSF group suggests that the addition of Zn(CF₃COO)₂ effectively inhibits the formation of the byproduct ZSH, thereby extending the lifespan of the Zn anode. For the anions, the possible elements within the SEI, inferred from the electrolyte composition, include S²⁻ (present in both ZS and ZSF) and F⁻ (present only in ZSF). Consequently, we initially focused on the S 2p XPS spectra for both the ZS and ZSF groups, and complementary using ToF-SIMS to analyze the 3D distribution of S⁻ groups. Besides, we analyzed the F 1s XPS spectra and the 3D distribution of F⁻ groups only for the ZSF group, as the ZS group does not contain the F element.

Fig. 2 In-depth XPS and ToF-SIMS characterization of Zn anodes after 100 cycles. (a) and (b) Zn 2p XPS spectra of Zn anodes in ZS (a) and ZSF (b) electrolytes. (c) and (d) S 2p XPS spectra of Zn anodes in ZS (c) and ZSF (d) electrolytes. (e) and (f) S⁻ 3D and 2D spatial distribution on Zn anodes in ZS (e) and ZSF (f) electrolytes detected by ToF-SIMS. (g) F 1s XPS spectra of the surface of a Zn anode in ZSF electrolyte. (h) F⁻ 3D and 2D spatial distribution on the surface of a Zn anode in ZSF electrolyte.

In the S 2p XPS spectra (Fig. 2c and d), the Zn anodes cycled in ZS and ZSF show recognizable SO₄²⁻ (168.8 and 170.0 eV) signals at all etching depths. We attribute these signals to the formation of byproduct ZSH (ZS electrolyte) and residual ZnSO₄ from the electrolytes (both ZS and ZSF). Notably, the ZnS peak is barely visible in the ZS case, whereas it is significantly more distinguishable in the ZSF (161.8 and 163.0 eV), with its intensity increasing with deeper etching. This indicates the addition of Zn(CF₃COO)₂ promotes the decomposition of SO₄²⁻ to S²⁻, which is relatively difficult in ZS electrolyte,⁴⁴ leading to the formation of a ZnS-rich SEI on the Zn anode (ZSF electrolyte). Fig. 2e and f compares the S⁻ distribution in 3D and two-dimensional (2D) models. Although both ZSH and ZnS can be reflected as S⁻ in the ToF-SIMS characterization, their distributions are quite different. It is notable that the ZS case exhibits a much looser distribution of S⁻ (Fig. 2e), which indicates that the surface morphology of the Zn anode cycled in ZS is probably loose and porous. In contrast, the S distribution on the upper surface of the Zn anode cycled in ZSF is much smoother and more even (Fig. 2f), indicating the formation of a dense and thin ZnS-containing SEI.

The F 1s XPS spectra from the Zn anode cycled in the ZSF electrolyte (Fig. 2g) display a –CF₃ signal (687.4 eV) that diminishes and disappears with increasing etching depth. This is assigned to the presence of residual Zn(CF₃COO)₂ on the Zn surface. During the etching, the ZnF₂ signal (684.7 eV) remains consistent, indicating a ZnF₂-rich SEI derived from the Zn(CF₃COO)₂ additive. The amount of F⁻ detected on the Zn surface is lower compared to that of S⁻ (3D ToF-SIMS, Fig. 2h), which is related to the lower content of F source in the electrolyte. We speculate the SEI for the ZSF case to be a hybrid SEI formed by a mixture of ZnS/ZnF₂ phases. FTIR and Raman spectra were used to characterize the surface components on Zn anodes, further confirming the co-existence of ZnS/ZnF₂ (Fig. S11a and b, ESI†) on the Zn anode (ZSF case). Considering the source of the elements in the electrolyte, it is reasonable to assume that the addition of Zn(CF₃COO)₂ induces the reduction of both SO₄²⁻ and CF₃COO⁻, resulting in the formation of a hybrid ZnS/ZnF₂-rich SEI on the Zn anode.

The structural characteristics of the SEI significantly impact battery performance. Given the difficulty in directly characterizing the surface structure of Zn foil and the sensitivity of the SEI to the electron beam, we electroplated Zn metal onto a copper microgrid for cryo-TEM analysis. The Zn-plated microgrid, as the anode, was cycled in ZS and ZSF electrolytes (10 cycles, 1 mA cm⁻²). After cycling, in the case of ZS, the plated Zn metal exhibits an irregular agglomerated particle morphology (Fig. S12a, ESI†). Cryo-high-resolution TEM (cryo-HRTEM) identified the Zn (1 0 0) plane without any observable surface SEI structure. Energy-dispersive X-ray spectroscopy (EDX) element mapping shows the homogeneous distribution of Zn and S elements (Fig. S12b, ESI†). Based on the aforementioned XPS and ToF-SIMS results (Fig. 2a, c, and e), we attribute the S element to the presence of byproduct ZSH, generated during cycling in the ZS electrolyte. In contrast, for the Zn metal in the ZSF case (Fig. 3a), a distinct layered structure identified as the SEI was identified on the surface. Cryo-HRTEM analysis revealed the distribution and characteristics of the two phases: the crystalline Zn metal (Fig. 3a and Fig. S13, ESI†) with Zn (1 0 0) and Zn (1 0 1) crystal planes, and the amorphous SEI. Furthermore, EDX was used to determine the distribution of Zn, S, and F (Fig. 3b), indicating that the SEI is rich in S and F. Combined with the aforementioned characterizations of the Zn anode surface in the ZSF group, it is clear that the addition of the Zn(CF₃COO)₂ additive induces the formation of an amorphous hybrid ZnS/ZnF₂-rich SEI, as illustrated in Fig. 3c. The unique isotropic feature of the amorphous SEI architecture will benefit the ionic diffusion kinetics, suppressing dendrite growth and protecting the Zn anode from side reactions during electrolyte exposure.

Fig. 3 (a) and (b) Cryo-TEM and cryo-HRTEM images (a) and corresponding EDX mapping of the SEI (b) on the Zn metal cycled in the ZSF electrolyte. (c) Schematic illustration of the amorphous hybrid ZnS/ZnF₂-rich SEI on the Zn anode (ZSF). (d) HOMO and LUMO energy levels of H₂O, SO₄²⁻, and CF₃COO⁻. (e) Illustration of solvated and free SO₄²⁻. (f) Zn²⁺ solvation structures in ZS and ZSF electrolytes. (g) LUMO energy levels of SO₄²⁻ in various solvation structure environments.

The regulation of the solvation structure by Zn(CF₃COO)₂ is the primary factor in the formation of the unique amorphous SEI. According to theoretical calculations, CF₃COO⁻ and SO₄²⁻ have higher highest occupied molecular orbital (HOMO) energy levels compared to H₂O (Fig. 3d). This suggests that these anions, with their narrower band gaps, will preferentially adsorb on the Zn anode surface, further facilitating their involvement in SEI formation. Although the higher HOMO energy levels of SO₄²⁻ theoretically imply that they are more likely than H₂O to act as electron donors and coordinate with solvated Zn²⁺, in practice, not all SO₄²⁻ in the electrolyte can participate in the solvation structure. The aforementioned RDF results show that the coordination factor of Zn²⁺–SO₄²⁻ in the ZS electrolyte is merely 0.52 (Fig. S7, ESI†), which is much lower than the Zn²⁺/SO₄²⁻ ratio (1/1) in the electrolyte. This indicates that a significant portion of SO₄²⁻ exists in a free, non-solvated form (Fig. 3e).

Given the solvated [Zn(H₂O)₆]²⁺ forms an angle of approximately 90° with any H₂O–Zn²⁺–H₂O arrangement (Fig. 3f), the entry of SO₄²⁻ into the Zn²⁺ solvation sheath is greatly hindered by spatial constraints. Therefore, regulating the solvation structure through additives is crucial. Compared to CF₃COO⁻ as a source of ZnF₂, the higher LUMO value of free, non-solvated SO₄²⁻ (11.205 eV) makes it more difficult to reduce and form a ZnS-containing SEI. We compared the LUMO values of SO₄²⁻ in three different chemical microenvironments (Fig. 3g). The results show that solvated SO₄²⁻ has a much lower LUMO energy level (−2.62 eV) than free SO₄²⁻, indicating that the former is more easily reduced to form inorganic ZnS in the SEI. Thus, promoting the entry of SO₄²⁻ into the solvation sheath is particularly important. Besides, the involvement of CF₃COO⁻ in coordination further regulates the solvation structure and facilitates the reduction of SO₄²⁻ (LUMO = −7.179 eV). On one hand, the aforementioned FTIR and NMR results indicate the addition of Zn(CF₃COO)₂ forms strong hydrogen bonds with H₂O (Fig. 1a and c), thereby weakening the coordination of H₂O with Zn²⁺. This makes it easier for H₂O to be displaced from the inner solvation sheath (Fig. 3f), providing sufficient spatial conditions for SO₄²⁻ to enter the sheath. On the other hand, some CF₃COO⁻ directly participate in the coordination of the solvation structure, further adjusting the coordination geometry and space. RDF results show that the introduction of Zn(CF₃COO)₂ increases the Zn²⁺–SO₄²⁻ coordination in ZSF from 0.52 in ZS to 0.67 (Fig. 1h), demonstrating that the addition of additives effectively promotes the formation of the amorphous hybrid ZnS/ZnF₂-rich SEI.

Properties of the amorphous SEI

An ideal SEI layer should theoretically meet the following criteria: (i) be robust enough to withstand significant volume changes during the plating/stripping processes, ensuring structural integrity and preventing degradation; and (ii) be highly conductive to Zn²⁺ ions while being insulating to electrons, thereby facilitating efficient ion transport and minimizing parasitic reactions. As a crucial metric for evaluating the SEI's strength,⁴⁵ we examined the Young's modulus of Zn anode surfaces after 100 cycles. As illustrated in Fig. 4a, the average Young's modulus of the Zn anode cycled in the ZSF electrolyte (6.4 GPa) was significantly higher than that of the ZS group (4.7 GPa), which is attributed to the existence of amorphous hybrid ZnS/ZnF₂-rich SEI. The high-modulus SEI, combined with its uniform distribution across the Zn anode surface, could contribute to preventing vertical dendrite growth and result in more homogeneous Zn plating.

Fig. 4 (a) The Young's modulus of the cycled Zn anode. (b) Contact angles of ZS and ZSF electrolyte on the cycled Zn anodes with ZSH and ZnS/ZnF₂ surfaces. (c) Zn²⁺ transference numbers. (d) and (e) Adsorption energies (d) and diffusion energy barriers (e) of Zn²⁺ at different interfaces. (f) Rate performance. (g, h) Finite element simulated Zn plating behaviors in ZS (g) and ZSF (h) electrolytes.

The wettability of the electrolyte on the electrode significantly affects Zn²⁺ conduction at the solid–liquid interface. Considering the SEI in situ formed on the electrode surface is directly in contact with the electrolyte, we tested the contact angle of the baseline (ZS) and ZSF electrolytes on the surface of Zn anodes after 100 cycles (Fig. 4b): i.e. one covered with in situ formed ZSH by-product and the other having an amorphous hybrid SEI (ZnS/ZnF₂). The measured contact angle between ZS electrolyte and ZSH (92.5°) is much higher than that between ZSF electrolyte and ZnS/ZnF₂ (35.6°). This difference is due to the lower interfacial adsorption energy of the latter. This implies that the ZnS/ZnF₂ SEI can significantly enhance the electrolyte's wettability on the electrode, thereby promoting Zn²⁺ transport behavior at the interface. Thanks to the improved electrolyte wettability of the ZnS/ZnF₂ SEI, the measured Zn²⁺ transference number (t_Zn²⁺) increases from 0.39 to 0.77 after the addition of Zn(CF₃COO)₂ (Fig. 4c and Fig. S14a, and S14b, ESI†). The EIS curve of Zn||Zn symmetric cells in ZSF electrolyte for different polarization time is presented in Fig. S15 (ESI†). This enhancement in Zn²⁺ diffusion should reduce electrode polarization and promote uniform Zn nucleation.

The electrochemistry at the SEI during the Zn plating involves the desolvation of Zn²⁺, the adsorption and capture of Zn²⁺ by the SEI, and the inward diffusion of Zn²⁺ through the SEI. To simulate the adsorption-diffusion process of Zn²⁺ on the Zn anode surface after desolvation, we employed first-principles calculations to compare the affinity of different surface components for Zn²⁺ and the subsequent Zn²⁺ diffusion behavior.⁴⁶ As shown in Fig. 4d, the adsorption affinities of Zn²⁺ on ZnF₂ (−1.74 eV), ZnS (−1.44 eV), and ZnS/ZnF₂ (−2.08 eV) are stronger than that on the ZSH byproduct (−1.18 eV). Similarly, the subsequent Zn²⁺ diffusion shows a similar trend, with the lowest energy barrier (0.40 eV) within the ZnS/ZnF₂ SEI (Fig. 4e and Fig. S16–S18, ESI†). These results demonstrate the ZnS/ZnF₂ SEI simultaneously accelerates the desolvation process, adsorption, and transfer kinetics of Zn²⁺ at the SEI. Consequently, the rate capability of the ZSF electrolyte significantly surpasses that of the ZS electrolyte, maintaining stability even at an ultra-high current density of 60 mA cm⁻² (Fig. 4f).

Metal anode batteries commonly exhibit the “tip effect”,^47–49 where ions tend to preferentially deposit at sharp edges with a smaller radius of curvature due to the promoted electric field, leading to rapid development of dendrites from small protrusion defects on the anode surface. Here, we used finite element simulation to model dendrite growth in ZS and ZSF electrolytes by defining the compositional properties of the protrusion surface. As shown in Fig. 4g, in the ZS electrolyte, Zn²⁺ preferentially adsorb and plate along the direction perpendicular to the protrusion, causing continuous growth of the dendrite length. In contrast, the strong affinity of the ZnS/ZnF₂ SEI for Zn²⁺ keeps the surface in a Zn²⁺-rich state (Fig. 4h), while the electronic insulation of ZnS/ZnF₂ ensures these Zn²⁺ remain unreduced before passing through the SEI. These Zn²⁺, adsorbed on the SEI surface and diffusing within the SEI, also concentrate at the tips under the influence of the electric field, generating an electrostatic shielding repulsion that mitigates the “tip effect”. This significantly affects the deposition behavior on the Zn anode surface and the morphology of dendrite growth, thereby directly impacting the battery's lifespan.

Zn plating/stripping behaviors and side reactions

The plating morphology of the Zn anode significantly affects the lifespan and reliability of the AZIBs. We compared the Zn anode surface morphology evolution of Zn anodes cycled in ZS and ZSF electrolytes using ex situ scanning electron microscopy (SEM) and optical confocal microscopy (pristine morphology of Zn anode in Fig. S19a, b, ESI†). After the 1^st plating in ZS electrolyte, numerous randomly oriented, sharp, and thin flakes were observed on the Zn anode surface (Fig. 5a and b). Cross-sectional SEM images further confirmed this phenomenon (Fig. S20a, ESI†), which poses serious risks of short-circuiting and potential safety concerns during battery operation. In contrast, the surface of the anode cycled within the ZSF electrolyte was smooth, dense, and uniform without visible dendrites (Fig. 5c, 5d and Fig. S20b, ESI†), which should positively impact the lifespan and performance of the Zn anode. After 100 cycles, the anode cycled in ZS developed a loose and porous layer with inherited and further developed randomly oriented flakes (Fig. 5e–g). This structure creates numerous parasitic reaction sites and defects, which promote the nucleation and growth of more flake-like Zn grains as well. The further growth of dendrites, extending along the gaps in the glass fibers, ultimately leads to functional failure and safety hazards. Conversely, the anode cycled in ZSF maintained a smooth and dense surface layer with no visible dendrite growth even after 100 cycles (Fig. 5h–j), which is expected to result in a stable and reliable AZIB performance.

Fig. 5 (a)–(j) Morphologies of Zn dendrite growth in ZS and ZSF electrolytes. SEM and 3D height images of Zn anodes in ZS (a) and (b) and ZSF (c) and (d) electrolytes after the 1^st plating. SEM and 3D height images of Zn anodes in ZS (e) and (g) and ZSF (h) and (j) electrolytes after 100 cycles. (k) XRD patterns of the Zn anodes with ZS and ZSF electrolytes after 100 cycles. (l) CA test of Zn anodes in ZS and ZSF electrolytes at an overpotential of −200 mV. (m) Tafel plots of Zn anodes in ZS and ZSF electrolytes. (n) LSV curve.

The structure of the plated Zn metal was further characterized by X-ray diffraction (XRD) to analyze its surface texture and orientation anisotropy. As shown in Fig. 5k, the relative intensity ratio of the (0 0 2) to (1 0 0) peaks (I₍₀₀₂₎/I₍₁₀₀₎) increases from 1.2 (ZS) to 1.5 (ZSF), indicating the growth mode of the latter is no longer characterized by 〈1 0 0〉-oriented growth as in the former. Additionally, the content of byproduct ZSH, commonly found in AZIBs,^50–52 was significantly reduced in the ZSF case.

To further analyze the surface nucleation behaviors related to Zn²⁺ diffusion kinetics in ZS and ZSF electrolytes, chronoamperometric analysis (CA) was first performed on a symmetric cell, which allowed a more sensitive examination of the initial plating process and surface charges (Fig. 5l). After applying a potential of −0.2 V, the current density of Zn electroplating in the ZS electrolyte showed a continuous increase throughout the CA test. The current response was closely and positively correlated with the electrochemical reaction active area of the Zn anode/electrolyte interface, specifically the extent of dendritic growth.^53,54 During this process, the dendrites in ZS indicate 2D diffusion on the grain scale and porous structure at the Zn anode surface. An uneven electric field causes Zn to plate at the edges of pre-existing irregular crystal nuclei, leading to continuous 2D growth and ultimately resulting in uncontrollable dendrite formation. In contrast, the current response of Zn electroplating in the ZSF electrolyte remains stable after a transient increase in current density for 75 seconds (Fig. 5l). At this point, Zn grains begin to grow normally to the 2D epitaxial layer, resulting in 3D diffusion behavior dominating at the grain scale, which stabilizes the current response. This Zn grain growth behavior promotes a uniform and dense Zn plating layer. Additionally, the Zn nucleation overpotential in the ZSF electrolyte is higher than in the ZS electrolyte (Fig. S21 and S22, ESI†), corresponding to a smaller nucleation radius,⁵⁵ which is beneficial for uniform Zn plating.

Parasitic reactions (e.g. HER and the formation of ZSH) significantly limit the practical application of aqueous AZIBs.⁵⁶ To analyze their extent, Fig. 5m depicts the Tafel plot of Zn anodes in ZS and ZSF electrolytes. In the presence of the Zn(CF₃COO)₂ additive, the corrosion potential (vs. Ag/AgCl) of the electrolytes increases from −973.2 (ZS) to −970 mV (ZSF), and the corrosion current (j) decreases from 2.01 to 0.39 mA cm⁻², indicating that the additive effectively inhibits the corrosion of Zn metal anodes.⁵⁷ The HER of the electrolytes was analyzed using linear sweep voltammetry (LSV) curves. As depicted in Fig. 5n and Fig. S23 (ESI†), the HER potential decreased in the ZSF case, implying that CF₃COO⁻ significantly inhibits the electrochemical reduction of H₂O.⁴² Besides, we observed the symmetric Zn||Zn cells with ZS electrolyte generated significantly more gas and exhibited thicker bulges after 100 cycles (Fig. S24a and b, ESI†). Even more evident gas production was observed in the pouch-battery cell with the ZS electrolyte (inset of Fig. S23, ESI†).

We further investigated the spontaneous side reactions of Zn anodes in various ZSF-x electrolytes by soaking Zn anodes in the electrolytes for 7 days (Fig. S25–S28, ESI†). In the ZS electrolyte (Fig. S25, ESI†), the soaked Zn foil exhibited significant side reactions, evidenced by the distribution of sharp side reaction products. Elemental distribution and XPS results (Fig. S29a–d, ESI†) of these four groups of Zn foils indicate that these products are related to ZSH components. With the increase in Zn(CF₃COO)₂ content, the spontaneous side reactions on the Zn surface were alleviated, a transparent thin film gradually appeared on the Zn foil surface (Fig. S28, ESI†), and the S content decreased. This suggests that the additive reduces the formation of ZSH components and causes the Zn anode to spontaneously react with the ZSF-x electrolyte, spontaneously forming a SEI, even before electroplating. The XRD and XPS results (Fig. S29–S31, ESI†) indicate the suppression of ZHS generation and the amorphous characteristic of the SEIs. Notably, the even Zn element distributions detected in element mappings indicate the Zn(CF₃COO)₂ aids in the spontaneous formation of a dense SEI on the Zn anode surface, which promotes further uniform Zn plating.

Electrochemical performance

To investigate the impact of Zn(CF₃COO)₂ additive on the stability and reversibility of AZIBs, Zn||Zn and Zn||Cu cells were assembled and tested. Fig. 6a compares symmetric cells using ZS and ZSF electrolytes. The ZSF-based cells demonstrate an exceptionally stable cycling performance, maintaining stability for up to 6 months (4600 h) at 2 mA cm⁻², which corresponds to a cumulative plated capacity (CPC) of 4.6 Ah cm⁻². In contrast, symmetrical cells using ZS electrolyte fail rapidly, with the overpotential increasing significantly after just 72 h. A similar trend is observed at 10 mA cm⁻² with a capacity of 5 mA h cm⁻² (Fig. S32, ESI†). Even when a higher current rate of up to 40 mA cm⁻² was employed, the ZSF-based cell maintained ultra-stable electrochemical performance with a lifespan of 310 h (Fig. S33, ESI†). These results confirmed that the formed amorphous ZnS/ZnF₂ SEI has a significant effect on improving battery performance, enhancing both lifespan and rate capability.

Fig. 6 The electrochemical performance of symmetric cells. (a) and (b) Galvanostatic Zn plating/stripping in Zn||Zn symmetrical cells at 2 mA cm⁻² with a capacity of 1 mA h cm⁻² (a), and 60 mA cm⁻² with 15 mA h cm⁻² (b). (c) and (d) CE of Zn||Cu half cells at 10 mA cm⁻² with a capacity of 1 mA h cm⁻² (c) and 60 mA cm⁻² with 15 mA h cm⁻² (d). (e) Long-term electrochemical performance of Zn||V₂O₅ coin cells at 10 A g⁻¹. (f) Cycling performance of Zn||V₂O₅ pouch-battery cells with an N/P ratio of 1.13 at 1 mA cm⁻².

Since Zn anodes used in laboratory tests are usually in excess, the excellent performance obtained often cannot be proportionally scaled up to practical applications. Therefore, the DOD becomes a particularly important parameter for measuring Zn anode utilization.⁵⁸ In this study, we employed symmetric cells with an ultrathin Zn anode (30 μm) and a DOD of 87.1% to test battery performance. As shown in Fig. 6b, the ZSF-based symmetric cell maintains stable cycling for 180 h at an ultrahigh current density of 60 mA cm⁻². We analyzed the morphologies of the Zn anode under such a high DOD. As shown in Fig. S34 (ESI†), the Zn anode cycled in the ZS electrolyte exhibits numerous cracks, indicating significant structural degradation. In contrast, the Zn anode cycled in the ZSF electrolyte remains smooth and flat, reflecting enhanced stability. Furthermore, XRD analysis of the cycled Zn reveals that the ZS sample contains a substantial amount of ZSH, as evidenced by prominent XRD peaks (Fig. S35, ESI†). This exceptional stability is attributed to improved Zn²⁺ transport kinetics within the amorphous ZnS/ZnF₂ hybrid SEI. We traced and compared the reported CPC of Zn||Zn cells in Fig. S36a, b, and Table S2 (ESI†). Our findings demonstrate not only an unexpectedly high CPC, but also impressive performance metrics such as high DOD, high current density, and high j × C. These results indicate that our Zn anodes exhibit some of the best-reported performances for AZIBs.

Furthermore, we evaluated the coulombic efficiency (CE) of Zn||Cu batteries to assess the reversibility of Zn anodes. The ZSF-based Zn||Cu cells demonstrated average CEs of 99.8% over 3500 cycles (Fig. S37 (ESI†), 1 mA cm⁻², 1 mA h cm⁻²) and 99.7% over 5500 cycles (Fig. 6c, 10 mA cm⁻², 1 mA h cm⁻²). In contrast, the CEs of the ZS-based cells rapidly declined after just 50 cycles (1 mA cm⁻², 1 mA h cm⁻²) and 153 cycles (10 mA cm⁻², 1 mA h cm⁻²). This rapid decay is attributed to dendrite growth and side reactions leading to failure without SEI protection. Additionally, the ZSF case presents less severe potential hysteresis (inset of Fig. 6c) compared to the ZS case (Fig. S38, ESI†), indicating that SEI enhances electrochemical kinetics and reduces polarization. More detailed profiles can be distinguished in the capacity–voltage curve observed during the 4000–5000 cycle range (Fig. S39a–j, ESI†) and the time–voltage curve (Fig. S39k, ESI†). Under harsh conditions (DOD = 87.1%, 60 mA cm⁻²), the Zn||Cu cell with ZSF electrolyte exhibited an initial CE (ICE) of 98.3% (Fig. 6d), maintaining a high CE of over 99% after 50 cycles. Conversely, the Zn||Cu cell with ZS electrolyte showed an ICE of 85.9%, which dropped to 69.0% after just 7 cycles due to severe fluctuations. Compared to existing literature, the cycling performance of Zn||Cu batteries with ZSF electrolytes significantly outperforms other reported systems (Fig. S40 and Table S3, ESI†), highlighting the beneficial impact of the Zn(CF₃COO)₂-induced amorphous hybrid ZnS/ZnF₂ SEI.

For practical applications, anode should be combined with a proper cathode in a full-battery cell.^59,60 In this study, we utilized V₂O₅ as the cathode material and assembled Zn||V₂O₅ full-battery cells using ZS and ZSF electrolytes. The V₂O₅ material was synthesized via a hydrothermal method, resulting in a flake-like morphology (Fig. S41 and S42a–d, ESI†). Fig. S43a and S43b (ESI†) display the CV curves of full-battery cells at various scan rates for both ZS and ZSF cases (after activation for 3 cycles). To qualitatively evaluate the Zn²⁺ diffusion kinetics in full-battery cells, we fitted the slopes using the Randles–Ševčík eqn (1):^61,62

I_p = 2.69 × 10⁵n^3/2AD^1/2v^1/2C₀ (1)

where I_p denotes the peak current, n represents the number of electrons transferred per molecule during the redox reaction (2 for Zn²⁺), A is the surface area, D is the diffusion coefficient, v is the scan rate, and C₀ is the concentration of Zn²⁺. ZSF shows a steeper fitted slope compared to ZS (Fig. S43c, ESI†), indicating a faster Zn²⁺ diffusion rate in the ZSF battery. This is attributed to two aspects: (i) the unique structure of the amorphous ZnS/ZnF₂ SEI, which promotes rapid Zn²⁺ diffusion on the Zn anode side; (ii) the solvation structure regulation, which benefits the desolvation of Zn²⁺, thereby enhancing the electrochemical reaction kinetics.

In terms of short-term cycling performance (Fig. S44a, ESI†), the Zn||V₂O₅ full-battery cells with ZSF electrolyte demonstrate outstanding cycle stability, retaining 99.97% capacity over 100 cycles at 1 A g⁻¹, significantly surpassing the ZS-based cells. The capacity–voltage curves of the ZSF-based cells after numerous cycles (Fig. S44b, ESI†) show much higher reproducibility than those of the ZS-based cells (Fig. S44c, ESI†), indicating superior electrochemical reversibility. At a high current density of 10 A g⁻¹ (Fig. 6e), the ZSF-based cell operated stably for 4500 cycles with a low average decay of 0.005% per cycle. In contrast, the ZS case failed after 1269 cycles, attributed to a short circuit caused by Zn dendrite growth in the ZS electrolyte. After 1000 cycles, the full-battery cells were disassembled and their components analyzed. SEM observations of the Zn anodes revealed that the ZS electrolyte resulted in a much more severe dendrite growth (Fig. S45a and b, ESI†). Some dendrites even penetrated the separator, causing part of the fibers to detach from the separator during disassembly. In contrast, the Zn anode cycled in ZSF showed a relatively smooth surface.

A low capacity ratio of anode to cathode (N/P ratio) is crucially important for the practical applications of battery packs.⁶³ In this regard, we fabricated pouch-battery cells with a low N/P ratio to evaluate the potential of our strategy for practical applications. As shown in Fig. 6f, at a low N/P ratio of 1.13, the Zn(CF₃COO)₂ additive enabled the Zn||V₂O₅ pouch-battery cell to significantly extend the battery lifespan to 400 cycles with a capacity retention of 87% (1 mA cm⁻²). The comprehensive performance of the full-battery cells significantly surpasses those reported in existing literature (Fig. S46 and Table S4, ESI†), illustrating the remarkable efficacy of the solvation structure engineering strategy. After 400 cycles, four series-connected pouch-battery cells can still provide stable power to a 5 V-rated light-emitting diode array (Fig. S47 and Video S1, ESI†). These results confirm the great potential of Zn(CF₃COO)₂ additives in promoting the commercial viability of AZIBs.

Conclusions

The conclusions section should come in this section at the end of the article, before the acknowledgements. In summary, by engineering the solvation structure via Zn(CF₃COO)₂ additive, we successfully induced the formation of an amorphous ZnS/ZnF₂-rich hybrid SEI on the Zn anode. Through a combination of theoretical calculations and experimental characterization, we elucidated the intrinsic mechanism by which solvation structure regulation induces SEI formation and clarified the role of this SEI in promoting electrochemical reaction kinetics. The SEI promoted uniform Zn plating, enhanced ionic transport kinetics, and inhibited dendrite growth. This engineered SEI contributed to ultra-stable cycling stability of over 6 months and impressive high-current performance, maintaining operation for over 180 h at 60 mA cm⁻² with a high DOD. Additionally, assembled pouch full-battery cells demonstrated excellent cycling stability, sustaining 87% capacity retention over 400 cycles at a low N/P ratio (1.13). Overall, this study highlights the potential of solvation structure engineering to address the critical challenges faced by Zn anodes AZIBs, paving the way for the development of next-generation energy storage systems with enhanced safety and eco-friendliness.

Author contributions

Q. S. directed the research project. G. Z. conceived the idea and designed the experiments. G. Z. and Q. S. co-wrote the manuscript. S. H. and M. I. supported the TEM characterization. P. R., J. L., and S. W. conceived material characterizations. P. W. conceived the finite element simulation. Y. T., L. C., and A. C. reviewed and edited the manuscript. All authors discussed the results and provided comments on the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the Joint Fund of Henan Province Science and Technology R&D Program (235200810097) and the Generalitat de Catalunya (2021SGR01581). This research was supported by the Scientific Service Units (SSU) of ISTA Austria through resources provided by the Electron Microscopy Facility (EMF) and the Nanofabrication Facility (NFF). G. Z. and J. L. thank the China Scholarship Council (CSC) for the scholarship support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee03750b

‡ These authors contributed to this work equally.

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