Interfacial molecular layer induced by trace chlorogenic acid for highly stable zinc anodes

Abstract

Chlorogenic acid (CGA) is investigated as an electrolyte additive to achieve superior stability for zinc anodes. This trace molecular additive not only alters the solvation structure but also forms an interfacial molecular layer that continuously protects the zinc anode during repeated cycles. As a result, this leads to an extended cycle life (5340 h) and high coulombic efficiency (99.76%) in (a)symmetric zinc cells, while also enhancing the performance of full cells.

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Aqueous zinc batteries (AZBs) are promising candidates for energy storage due to their advantages exhibited: abundant resources, inherent safety, high theoretical capacity (820 mAh g⁻¹), low redox potential (−0.762 V vs. SHE), and low toxicity.^1–4 However, as shown in Fig. 1a, the Zn anode suffers from corrosion, hydrogen evolution reaction (HER), and uneven Zn²⁺ deposition during cycling due to its surface heterogeneity and the aggressive nature of H₂O.² These processes degrade interfacial reactions, promoting severe dendrite growth and compromising the Zn anode’s intrinsic advantages, ultimately hindering further development.⁵

Fig. 1 (a) Schematic illustration of the detrimental interfacial reactions occurring at the Zn anode in the ZnSO₄ electrolyte. (b) The interfacial molecular layer regulates the interface chemistry. (c) Calculated MESP distributions of H₂O and CGA molecules. (d) LUMO and HOMO energy levels of H₂O and CGA molecules. (e) Adsorption modes and (f) energy of H₂O, and –COOH, –OH, –PhOH groups of CGA on the Zn(002) surface.

To address these issues, various strategies, such as constructing artificial interfacial layers, optimizing electrode design, modifying separators, and engineering electrolytes, have been developed to enable uniform Zn deposition and stabilize Zn anodes.⁶ Among these strategies, electrolyte engineering is particularly appealing as it simplifies preparation and reduces both material and time costs.⁷ Compared to gel and salt-in-water electrolytes, electrolyte additives have attracted significant attention due to their superior effectiveness and cost efficiency, demonstrating strong potential for future applications.^8,9

However, current electrolyte additive research still faces key challenges: (1) overuse of additives, (2) unstable molecular adsorption on Zn surfaces for protective layer formation, and (3) limited functionality leading to poor Zn nucleation/deposition.¹⁰ These issues lead to both economic and performance drawbacks in Zn anode protection. Thus, developing trace additives that form stable protective layers to enable uniform Zn nucleation/deposition is crucial.

Herein, trace chlorogenic acid (CGA) was introduced to simultaneously modify the solvation structure and form an interfacial molecular layer at the Zn interface, which promotes the uniform deposition of Zn²⁺ ions and retains its stability throughout prolonged cycling. It effectively mitigates damage during the aging process, providing comprehensive and enduring protection to the Zn anode (Fig. 1b). This optimized electrolyte formulation facilitates significant enhancements in both electrochemical performance and practical applicability.

CGA contains multiple polar functional groups (–COOH, –OH, –PhOH), as confirmed by Fourier transform infrared (FTIR) (Fig. S1, ESI†). The molecular electrostatic potential (MESP) of CGA is more negative than H₂O, indicating abundant Zn-affinity sites that enhance Zn²⁺ adsorption and interfacial stabilization (Fig. 1c).¹¹ The energy levels of the lowest occupied molecular orbitals (LUMO) and highest unoccupied molecular orbitals (HOMO) of H₂O and CGA were studied to evaluate their charge accumulation (Fig. 1d).¹¹ CGA's narrower energy gap (4.12 eV vs. H₂O's 8.92 eV) facilitates electron transfer and Zn surface adsorption, attributed to its polar functional groups' electron-withdrawing capability. Furthermore, the contact angles of different electrolytes on the Zn foil further confirm that the incorporation of CGA molecules enhances the Zn affinity of the interface (Fig. S2, ESI†). The adsorption energies of –COOH (−1.74 eV), –OH (−1.49 eV), and –PhOH (−1.90 eV) in CGA on the Zn(002) surface are significantly higher than that of H₂O (−0.26 eV), further corroborating that CGA molecules preferentially adsorb at the Zn interface (Fig. 1e and f). This results in the formation of a protective molecular layer that shields the interface from water intrusion and enhances the interfacial chemistry of the Zn anode.⁴

To explore the function of the CGA, FTIR analysis of the ZnSO₄ electrolyte with CGA concentration gradients was firstly employed to provide insights into the solvation structure evolution. With the CGA content increasing, the O−H bending (1530–1740 cm⁻¹) and O−H stretching (2900–3700 cm⁻¹) bands are blue-shifted. These findings suggest that CGA molecules occupy the active sites of H₂O within the solvation structure, leading to an increase in the amount of free H₂O and thereby effectively altering the solvation structure. (Fig. S3, ESI†).¹² In ZnSO₄ electrolyte, the ion associations in solvated aqueous ZnSO₄ solutions can be classified into solvent-separated ion pairs (SSIP, Zn²⁺(H₂O)₆SO₄²⁻) and contact ion pairs (CIP, Zn²⁺(H₂O)·OSO₃²⁻), based on their degree of association.¹³ The SSIP and CIP species account for 20.48% and 79.52% (Fig. S4, ESI†), respectively, as determined from the peak area ratio. Upon the introduction of CGA, the v(SO₄²⁻) band shifts to a lower frequency, with 55.29% of SSIP and 44.71% of CIP present, indicating that SO₄²⁻ faces difficulties integrating into the solvation structure of Zn²⁺ ions within CGA/ZnSO₄ electrolyte, owing to the strong interaction between CGA and Zn²⁺ ions. This interaction effectively suppresses the formation of by-products (Zn₄SO₄(OH)₆·xH₂O). Moreover, the O–H stretching vibrations within the range of 3000–3700 cm⁻¹ can be divided into three characteristic peaks, corresponding to strong, medium, and weak H-bonds, respectively.¹ Compared to the ZnSO₄ electrolyte (Fig. 2a), the CGA/ZnSO₄ electrolyte exhibits enhanced strong and weak H-bonds, while the medium H-bond is diminished (Fig. 2b and c). This indicates that the CGA additive can reconstruct the H-bond network of the ZnSO₄ electrolyte, thereby facilitating changes in the solvated structure.^1,13 This optimized solvation structure facilitates the enhancement of the electrolyte environment and promotes beneficial interfacial reactions.⁸ The activation energy tests confirmed that the optimized solvation structure in the CGA/ZnSO₄ electrolyte, exhibits a reduced de-solvation energy, thereby enabling the rapid deposition of Zn²⁺ ions at the interface (Fig. S5, ESI†). To analyse the stability of CGA in the repeated plating/stripping and the influence of the CGA on the solid-electrolyte interphase (SEI) formation, X-ray photoelectron spectroscopy (XPS) coupled with Ar⁺ sputtering was utilized to examine the Zn anode after 50 cycles at 1 mA cm⁻² and 1 mA h cm⁻². Energy calibration was performed using the C 1s peak at 284.8 eV as the reference (Fig. 2d). The C=O (289.6 eV) and C–O (286 eV) signals could be attributed to the CGA molecules adsorbed on the Zn anode surface.¹⁴ The O 1s spectrum reveals SO₄²⁻ (532.1 eV) and ZnO/Zn(OH)₂ (530.4 eV), resulting from zinc salt precipitation in the ZnSO₄-based electrolyte (Fig. 2e). The S 2p spectrum provided evidence for the existence of ZnSO₄ (169.4 eV). Interestingly, the signal for ZnS (162.2 eV) increased with increasing interphase depth (Fig. 2f). Notably, the XPS depth profile indicated a continuous decrease in the signal intensity of the C 1s, O 1s, and S 2p spectra as the sputtering duration was prolonged, while the binding energy positions remained stable, suggesting the stability of CGA molecules and the uniformity of the ZnS-rich SEI on the Zn anodes that had been cycled with the CGA/ZnSO₄ electrolyte.¹⁵

Fig. 2 Raman spectra of v-OH for (a) ZnSO₄ electrolyte and (b) CGA/ZnSO₄ electrolyte and (c) their comparison. XPS depth profiles of (d) C 1s, (e) O 1s and (f) S 2p after Ar⁺ sputtering of the Zn anode after 50 plating/stripping cycles at 1 mA cm⁻² and 1 mA h cm⁻² in CGA/ZnSO₄ electrolyte.

Furthermore, differential capacitance (DC) measurements were conducted to validate the adsorption behaviour of CGA molecules on the Zn electrode via assembling Zn‖Cu asymmetrical cells.¹⁶ Compared to the ZnSO₄ electrolyte, the DC value significantly decreased with the addition of CGA, which could be attributed to the effective adsorption of CGA molecules on the Zn anode, thereby forming the interfacial molecular layer (Fig. 3a). This protective effect of the CGA molecular layer was confirmed through both HER and anti-corrosion tests. In linear polarization tests, under the same corrosion current conditions, the addition of CGA caused the potential to shift from −0.132 V to −0.272 V (Fig. 3b), significantly enhancing the ability to suppress the HER. Similarly, the Tafel curves yielded consistent results (Fig. 3c), demonstrating that the introduction of CGA leads to the formation of a robust adsorbed molecular layer, thereby effectively suppressing the corrosion tendency and rate of the Zn anode.¹³ Further investigations of the CGA molecular adsorption layer on regulating the deposition mode and kinetics of Zn²⁺ ions at the Zn interface were conducted. Evidence from chronoamperometry (CA) (Fig. S6, ESI†), nucleation overpotential tests (Fig. S7, ESI†), and exchange current density assessments (Fig. S8, ESI†) confirmed that the CGA molecular layer not only promotes the rapid 3D diffusion of Zn²⁺ ions, enabling uniform deposition but also lowers the interfacial nucleation energy barrier, thereby fostering robust nucleation and rapid interfacial deposition reaction kinetics.¹⁷ This protective effect was further corroborated by in situ optical microscopy (Fig. 3d), which demonstrated that the Zn surface in the CGA-enhanced electrolyte was notably more uniform, smooth, and flat, compared to that in the ZnSO₄ electrolyte. 3D confocal laser scanning microscopy (CLSM) of the cycled Zn anode further revealed that the CGA molecular layer effectively safeguarded the Zn anode (Fig. 3e), ensuring uniform deposition (Fig. S9, ESI†).² Moreover, the X-ray diffraction (XRD) patterns of the cycled Zn anode provide further evidence that this CGA molecular layer directs Zn²⁺ ions to deposit along the parallel (002) crystal plane of the Zn surface and reduces by-products (Fig. S10, ESI†), in contrast to the uneven deposition observed in the ZnSO₄ electrolyte. SEM images of the cycled Zn anodes also affirmed this conclusion (Fig. S11 and S12, ESI†).

Fig. 3 (a) The differential capacitance–potential curves, (b) LSV curves and (c) Tafel polarization curve for Zn foil in different electrolytes. (d) In situ optical microscopy images capture the Zn deposition process at 1 mA cm⁻² and 1 mA h cm⁻². (e) 3D CLSM images of Zn anodes after 15 cycles at 1 mA cm⁻² and 1 mA h cm⁻². DRT analysis of Zn‖Zn symmetric cells with (f) CGA/ZnSO₄ and (g) ZnSO₄ electrolyte cycled at 1 mA cm⁻² and 1 mA h cm⁻². (h) Nucleation overpotential of Zn‖Cu asymmetric cells at different cycle numbers.

A durable molecular layer at the Zn interface that remains functional during repeated cycling to mitigate both cyclic and calendar aging is crucial for long-term anode protection.¹⁸In situ electrochemical impedance spectroscopy (EIS) was employed to investigate the Zn²⁺ kinetics in the continuous plating/stripping at 1 mA cm⁻² and 1 mA h cm⁻² (Fig. S13, ESI†), and corresponding distribution of relaxation times (DRT) analysis demonstrated a steady charge transfer process (R_ct) in the CGA/ZnSO₄ electrolyte (Fig. 3f), while an obvious fluctuation of R_ct coupled with the slow diffusion of Zn²⁺ (R_diff) occurs in the ZnSO₄ electrolyte (Fig. 3g), suggesting an irreversible Zn plating/stripping process resulting from side reaction-induced Zn loss with increasing cycle number, which also provide evidence that the CGA molecular layer continuously maintains the advantages throughout the cycling process. Additionally, by monitoring the nucleation overpotentials during cycling (Fig. 3g and Fig. S14, ESI†), it is observed that the CGA molecular layer consistently promotes the uniform deposition of Zn²⁺ ions during repeated cycles, resulting in a homogeneous interface. Furthermore, testing of the interface impedance after different resting periods reveals that the CGA molecular layer not only resists cycling-induced aging but also demonstrates a notable resistance to calendar aging (Fig. S15, ESI†).¹⁹

By testing the rate performance of ZnSO₄ electrolytes with different proportions of CGA, it was determined that the Zn anode protection effect is best when the CGA proportion is 1.0% of the solid mass of ZnSO₄ (Fig. S16, ESI†). Zn‖Cu asymmetric cells with CGA exhibit stability over 1000 cycles at 5 mA cm⁻²/1 mA h cm⁻², corresponding to a CE of 99.76% (Fig. 4a). However, in ZnSO₄ electrolyte, the CE is unable and significant short-circuit phenomena occur at the 261th cycle. This underscores the remarkable reversibility of Zn plating/stripping following the addition of CGA, a feature further corroborated by the accompanying stable voltage profiles (Fig. S17 and S18, ESI†).¹⁹ Furthermore, even under higher current densities (50 mA cm⁻²), the CGA/ZnSO₄ electrolyte retains a remarkable 98.69% high reversibility after 700 cycles, thereby confirming the robust protection and reversibility provided by the CGA molecular layer (Fig. S19–S21, ESI†).¹⁹ Additionally, the Zn symmetric cells with ZnSO₄ electrolyte exhibit a sharp voltage drop at 5.5 mA cm⁻² and 5.5 mA h cm⁻² in the incremental current density test, signalling a short circuit of the cell. In contrast, the voltage plateau of the CGA/ZnSO₄ electrolyte increases linearly as the current density increased to 15.5 mA cm⁻² and 15.5 mA h cm⁻² (Fig. 4b). Moreover, owing to the protective effect of the CGA molecular layer, the Zn symmetric cells exhibit exceptional rate performance across both low (1–10 mA cm⁻², Fig. S22, ESI†) and high (10–50 mA cm⁻², Fig. 4c) current densities. Zn symmetric cells with CGA exhibit an impressive cycle life exceeding 5340 h at 1 mA cm⁻²/1 mA h cm⁻² (Fig. 4d). Compared with previously reported works, the current work demonstrates a significant advantage in improving the lifespan of AZBs (Fig. S23 and Table S1, ESI†). To validate the resistance of the CGA molecular layer to more stringent cycle aging, further long-term cycling tests were conducted under higher current densities and areal capacities. Under testing conditions of 5–20 mA cm⁻² and 1–20 mA h cm⁻², the introduction of CGA significantly enhanced the cycling lifespan of the Zn symmetric cells with different Zn utilization depending on the depth of discharge (DOD) from 1.7% to 34.2% (Fig. S24–S26, ESI†).²⁰ Additionally, by considering both cycling and calendar aging effects on the cell, shelving-recovery performance tests were employed to assess the durability of the CGA molecular layer. It was observed that the optimized CGA electrolyte exhibited significantly longer cycling times compared to the ZnSO₄ electrolyte (Fig. S27, ESI†).

Fig. 4 (a) CE of Zn‖Cu asymmetric cells. (b) The potential evolution of Zn‖Zn symmetric cells under stepwise increases in current densities. (c) Rate performance from 10 to 50 mA cm⁻² with 2 mA h cm⁻² and (d) galvanostatic plating/stripping of Zn‖Zn symmetric cells in different electrolytes. (e) and (f) Self-discharge tests and (g) long-term cycling stability of Zn‖MnO₂ full cells.

Furthermore, a Zn‖MnO₂ full cell was assembled to validate the practical applicability of the CGA molecular layer (Fig. S28, ESI†). First, cyclic voltammetry (CV) testing with different scan rates showed that MnO₂ in both electrolytes featured two redox peaks (Fig. S29, ESI†). Moreover, the self-discharge characteristics of the full cells were investigated. After a 48-h resting period, the CGA/ZnSO₄ electrolyte holds 87.84% of its initial capacity, significantly surpassing the ZnSO₄ electrolyte (67.57%). (Fig. 4e and f). Additionally, the Zn‖MnO₂ full cell also displayed an improved rate performance (Fig. S30, ESI†). Moreover, the full cell with CGA/ZnSO₄ electrolyte at a high N/P ratio of 6.1 demonstrated stable capacity fading (Fig. S31, ESI†). To meet the practical application conditions, full cells with a low N/P ratio of 3.03 were further evaluated (Fig. 4g). After 1000 cycles at 1 A g⁻¹, the capacity retention of the full cell with CGA reached 66.20% and a stable CE of close to 100% was achieved, much higher than that for the cell without the additive (26.58%). To further confirm the robust and durable molecular layer also maintaining long-term optimization effects within the full cell system, in situ EIS was employed to monitor the impedance variations throughout the cycling process (Fig. S32, ESI†). The impedance of the full cell with CGA progressively decreased with continuous cycling, facilitating enhanced transfer and deposition kinetics.

In conclusion, trace CGA was introduced as an additive to regulate the solvation structure and form a robust molecular adsorption layer at the Zn anode interface. A series of characterizations and analyses confirmed that CGA molecules facilitate the uniform deposition of Zn²⁺ ions and resist damage during the aging process, offering comprehensive and lasting protection to the Zn anode. As a result, the cells with the CGA-modified electrolyte demonstrate exceptional long-term cycling stability. This study offers dependable theoretical guidance for identifying optimal additives for the enhancement of high-performance AZBs.

This work was financially supported by the National Natural Science Foundation of China (52203083); Youth Science and Technology Top Notch Talents Project of Guizhou Provincial Department of Education ([2024]314); Guizhou Province Science and Technology Achievement Application and Industrialization Plan (Major Project) (2023-010); Guizhou Province Basic Research Program (ZK [2024]078); Guizhou University Student Innovation and Entrepreneurship Plan Project (gzugc2023004).

Data availability

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

Conflicts of interest

The authors declare that there are no conflicts of interest.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc00327j

‡ These authors contributed equally.

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