A thiol-modified solid electrolyte interphase enhances the stability of zinc anodes under high depths of discharge

Abstract

Aqueous zinc-ion batteries (AZIBs) have gained considerable attention within the growing energy storage sector. However, dendritic growth and side reactions on zinc anodes significantly degrade battery stability, especially at high depths of discharge (DODs). These issues present a substantial challenge in attaining long-term cycling performance. In this study, a new electrolyte additive, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TMPEI), was employed to optimize the interface between the zinc electrode and electrolyte, effectively addressing these issues. The thiol groups in TMPEI strongly interact with zinc metal, forming stable Zn–S bonds through coordination interactions. This interaction results in the formation of a reliable and durable solid electrolyte interphase (SEI) layer on the zinc surface. This SEI layer not only controls zinc-ion deposition and inhibits dendrite growth but also decreases the energy barrier for zinc deposition. Furthermore, the hydrophobic properties of TMPEI help repel water molecules from the zinc anode, significantly limiting the hydrogen evolution reaction (HER) and corrosion processes. This effective interfacial protection enables Zn‖Zn symmetric cells to achieve stable cycling for up to 2000 h at 5 mA cm⁻² and 5 mA h cm⁻². Even under a high current density of 10 mA cm⁻² and a high DOD of 56.93%, the battery still demonstrates a cycling lifespan of 500 h. This research provides an efficient design strategy to enhance the cycling stability of zinc anodes in AZIBs under extreme conditions.

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Introduction

Aqueous zinc-ion batteries (AZIBs) have received considerable attention in recent years because of their high safety, low cost, environmental sustainability, and relatively high theoretical capacity. Zinc is abundant and affordable, while aqueous electrolytes improve the safety of batteries by reducing the risk of combustion and explosion. Additionally, zinc offers a theoretical capacity of 820 mA h g⁻¹ and a low redox potential of −0.76 V vs. SHE, making AZIBs a promising choice for renewable energy storage.^1–3 Although AZIBs show great potential on paper, the uncontrolled formation of zinc dendrites and side reactions at the electrode/electrolyte interface, particularly within the electric double layer (EDL), significantly limit their practical application. Specifically, Zn²⁺ ions in the electrolyte often coordinate with water molecules, forming Zn [(H₂O)₆]²⁺ complexes. During deposition, Zn²⁺ must first release some of its coordinated water molecules before it can be deposited. This process results in a significant amount of free water in the inner Helmholtz plane (IHP) of the EDL, which promotes corrosion and triggers the hydrogen evolution reaction (HER).^4,5 Moreover, due to microscopic surface irregularities on the electrode and uneven diffusion of Zn²⁺ ions, deposition tends to occur preferentially in protruding regions or areas with higher Zn²⁺ concentrations. This leads to an increase in the local electric field strength, contributing to the “tip effect” and accelerating dendrite growth.^6,7

The solid electrolyte interphase (SEI) layer is a thin solid film that forms at the electrode/electrolyte interface, preventing direct contact between the two, reducing side reactions, and improving Zn²⁺ deposition behavior through selective ion conduction.^8–10 The construction of SEI layers, as an effective strategy for regulating the electrode/electrolyte interface, involves two approaches: electrode modification and electrolyte modification.^11,12 Electrode modification (e.g., Bi/Bi₂O₃ hybrid interfaces, zinc-philic ZnS layers, and monolayer graphene) entails pre-treatment of the zinc anode surface to form artificial SEI layers.^13–15 These layers protect the electrode from erosion by water molecules and other reactive species, suppress corrosion and hydrogen evolution reactions (HER), and improve the mechanical strength of the electrode to accommodate volume changes during plating/stripping cycles.^16,17 Electrolyte modification (e.g., chondroitin sulfate (CS), triethyl phosphate (TEP), and nitrilotriacetic acid (NTA)) enables in situ formation of SEI layers on the zinc anode during battery cycling.^18–20 These SEI layers help block water molecules and regulate Zn²⁺ deposition behavior, ensuring highly selective Zn²⁺ transport.^21,22 Despite considerable progress in mitigating zinc dendrites and side reactions, current SEI layers still exhibit limitations. Most electrode modification techniques involve coating, resulting in weak adhesion between the SEI layer and the zinc anode, which can lead to cracking or detachment due to mechanical fatigue during extended cycling.^23–25 Similarly, electrolyte modification faces challenges in ensuring strong adhesion of the additives to the zinc anode surface. In particular, under prolonged cycling or high depth of discharge (DOD) conditions, SEI layers are prone to peeling or failure.^26,27 Inadequate adhesion impedes the full effectiveness of the SEI layer in suppressing zinc dendrites and side reactions. Under high current densities and areal capacities (≥5 mA cm⁻²), the cycling lifespan of Zn‖Zn symmetric cells regulated by most reported SEI layers typically falls short of 1000 h.^28–30 Therefore, further research is required to develop stable, durable, and efficient SEI layers on zinc anodes to enhance the cycling stability of AZIBs under long-term cycling and high DOD conditions.

In recent years, thiol-based materials have gained widespread attention due to their distinctive structures and functional properties. Thiol groups (–SH) exhibit both nucleophilic and metal-affinity behaviors, enabling them to form strong bonds with metal surfaces through coordination adsorption or the creation of metal sulfides. These characteristics have proven effective in metal corrosion protection and surface modification.^31–33 By utilizing their strong adsorption capabilities, incorporating thiol-based materials into zinc anodes to construct SEI layers offers a novel strategy for enhancing the cycling stability of AZIBs under high DOD conditions. In this study, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TMPEI) was introduced, and two approaches—electrode modification and electrolyte modification—were employed to construct artificial SEI layers on the electrode surface and in situ SEI layers, respectively. Their performance was then evaluated. The molecular structure and properties of TMPEI are described in Fig. S1.† Experimental findings and theoretical calculations showed that TMPEI forms stable Zn–S bonds with zinc via coordination interactions, protecting the zinc anode, preventing water molecule penetration, and significantly suppressing HER and corrosion reactions. The orderly adsorption of TMPEI effectively regulated the behavior of zinc-ion deposition, lowered the migration energy barrier for zinc ions, and improved the reversibility of electrode reactions. Additionally, the zinc-philic and hydrophobic nature of the TMPEI SEI layer enhanced Zn²⁺ diffusion and further suppressed side reactions. The results demonstrated that both methods notably improved the cycling stability of AZIBs. Among them, the electrolyte-modified approach showed the best performance, allowing Zn‖Zn symmetric cells to operate continuously for up to 2000 h at 5 mA cm⁻² and 5 mA h cm⁻². Even under extreme conditions, such as a high current density of 10 mA cm⁻² and a high depth of discharge of 56.93%, Zn‖Zn symmetric cells maintained stable cycling for 500 h. Furthermore, Zn‖Cu half-cells exhibited excellent cycling reversibility, with an average coulombic efficiency (CE) of 99.27% over 2100 cycles.

Results and discussion

Working mechanism of the TMPEI SEI layer

By incorporating TMPEI, modified electrodes and electrolytes were prepared, as illustrated in Fig. S2.† Specifically, TMPEI was added as an additive to a 1 M zinc trifluoromethanesulfonate (Zn(OTF)₂) electrolyte, labeled as ZSO and TMPEI-n, where “n” represents the mass fraction of TMPEI in the solution. Simultaneously, Zn foil was immersed in an ethanol solution containing TMPEI to fabricate TMPEI-n@Zn modified electrodes.

To examine the adsorption characteristics of TMPEI on zinc anodes, density functional theory (DFT) calculations were conducted to determine the adsorption energies (E_ads) of H₂O and TMPEI on distinct zinc crystal planes (100, 101, and 002). The findings (Fig. 1a) indicated that the E_ads of TMPEI on all three crystal planes was considerably higher than that of water molecules, suggesting a preferential adsorption of TMPEI on the zinc anode surface. This effect inhibits the adsorption of free water and reduces the likelihood of HER occurrence.^20,34 Further analysis demonstrated that TMPEI exhibited greater E_ads on the 100 and 101 planes compared to the 002 plane. This variance in adsorption facilitates the passivation of the 100 and 101 planes, thereby limiting Zn²⁺ deposition on these planes and kinetically favoring Zn²⁺ deposition on the 002 plane. Given the higher atomic density and lower surface energy of the 002 plane, this deposition pattern mitigates disordered zinc dendrite growth and promotes the orderly deposition of Zn²⁺.³⁵ Furthermore, the binding energies (E_bind) of H₂O and Zn²⁺ with TMPEI were determined to be −0.14 eV and −1.57 eV, respectively (Fig. 1b), indicating that TMPEI possesses a significantly stronger binding affinity for Zn²⁺ than for water molecules, which highlights its zinc-philic and hydrophobic nature. This characteristic minimizes direct interaction between water molecules and the zinc anode while ensuring uniform diffusion of Zn²⁺ at the electrode/electrolyte interface, thus providing a kinetic advantage for the orderly deposition of Zn²⁺ on the zinc anode.³⁶

Fig. 1 (a) E_ads of TMPEI and H₂O on the Zn (002), (101), and (100) crystal planes. (b) E_bind of TMPEI with H₂O and Zn²⁺, respectively. (c) XRD spectra of zinc anodes after 100 h of cycling at 1 mA cm⁻² and 1 mA h cm⁻² for Zn‖Zn symmetric cells. (d) Water contact angles of zinc foil and TMPEI-n@Zn modified electrodes. (e) XPS spectra of the zinc anode in the TMPEI-0.01 cell. (f) S 2p spectra of the zinc anode in the TMPEI-0.01 cell. (g) Voltage–time curves of the TMPEI-0.01 and ZSO + TMPEI-1@Zn cells after 100 h of galvanostatic cycling. (h) Raman spectra of the zinc anodes in the TMPEI-0.01 and ZSO + TMPEI-1@Zn cells. (i) Schematic of the working mechanism of TMPEI.

XRD analysis was conducted to characterize the crystal orientation of the zinc anode following 100 h of constant current cycling. As presented in Fig. 1c, the intensity ratio of the diffraction peaks corresponding to the 002 and 101 planes was determined by calculating the I₍₀₀₂₎/I₍₁₀₀₎ values, which were found to be 1.28, 1.54, and 1.49 for the ZSO, ZSO + TMPEI-1@Zn, and TMPEI-0.01 cells, respectively. These results align with the predictions of DFT analysis, confirming that the introduction of TMPEI notably increased the deposition ratio of Zn²⁺ on the 002 plane. This effect contributed to the formation of a flatter and more uniform deposition layer on the zinc anode, effectively inhibiting the growth of zinc dendrites.³⁷ Furthermore, water contact angle measurements and surface energy experiments were performed on zinc metal. As depicted in Fig. 1d, TMPEI-modified Zn foil exhibited a greater hydrophobic contact angle compared to pure Zn foil, with the contact angle increasing progressively as the TMPEI concentration increased. Surface energy calculations using the Owens–Wendt equation further support this conclusion, with detailed experimental validation presented in Fig. S3† and the corresponding analysis. These findings further corroborate the predictions of the DFT analysis, demonstrating that the incorporation of TMPEI enhances the hydrophobicity of the zinc anode, thereby effectively mitigating side reactions.

Energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to investigate the surface elemental composition and chemical states of the zinc anode after constant current cycling.^38–40 EDS analysis (Fig. S4 and S5†) indicated that in TMPEI-modified cells, the zinc anode surface exhibited a uniform distribution of the elemental constituents of TMPEI molecules—specifically C, N, O, and S. XPS results (Fig. 1e, f, and S6–S8†) further verified the presence of characteristic peaks corresponding to these elements on the zinc anode surface of TMPEI-modified cells. High-resolution spectral analysis of O 1s, C 1s, and S 2p revealed the distinct covalent bonding characteristics associated with TMPEI molecules. Notably, in comparison to ZSO cells, the high-resolution S 2p spectrum of the zinc anode in TMPEI-modified cells exhibited a characteristic Zn–S bond peak at 163.4 eV. This finding suggests that the thiol groups in TMPEI molecules established stable chemical bonds with the zinc anode surface through coordination adsorption.

R–SH + Zn → R–S–Zn + H⁺

The Zn–S bonds formed through coordination adsorption securely anchor TMPEI onto the zinc anode surface, significantly improving the stability of the zinc anode interface. Moreover, Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) (Fig. S9 and S10†) revealed that, in comparison to pure Zn foil, the TMPEI-1@Zn electrode exhibited distinct vibrational peaks corresponding to TMPEI-specific chemical bonds, including C–H, C=O, C–N, S–H, and Zn–S.^18,41 These findings indicate that TMPEI can effectively adsorb onto the zinc anode surface through a simple immersion process, demonstrating strong adhesion capability. The experimental results confirm that TMPEI adsorption onto the zinc anode surface enables the formation of a stable TMPEI SEI layer through chemical interactions with zinc, thereby contributing to stable battery operation.

Given that the solvation structure and the electrode/electrolyte interface are critical factors in optimizing zinc deposition behavior, FTIR, nuclear magnetic resonance (NMR), and Raman spectroscopy were utilized to analyze the TMPEI-0.01 electrolyte (see ESI, Fig. S11–S13†). Analysis of these spectral data indicates that the introduction of TMPEI does not significantly alter the solvation structure of zinc ions or the water activity in the solution.^18,42,43 This finding further confirms that the enhancement in electrode performance due to TMPEI primarily results from its adsorption on the zinc anode surface. To assess the adsorption stability of TMPEI, Zn‖Zn symmetric cells were assembled and subjected to constant current cycling tests under demanding conditions of 5 mA cm⁻² and 20 mA h cm⁻². As shown in Fig. 1g, both cells maintained stable cycling for over 100 h. XPS and FTIR analyses were performed on the zinc anodes after cycling. As presented in Fig. S14,† even after high-area-capacity cycling, the S 2p spectrum of the TMPEI-modified zinc anode continued to exhibit characteristic Zn–S bond peaks. Raman analysis (Fig. 1h) further displayed stretching vibrational peaks corresponding to TMPEI chemical bonds. These findings provide clear evidence of the strong adsorption stability of TMPEI on the zinc anode surface to ensure long-term stable operation of zinc anodes under high DOD conditions. In addition, Raman spectroscopy was performed on the zinc anodes of Zn‖Zn symmetric cells that underwent over 2000 hours of cycling. The test results (Fig. S15†) show that the characteristic chemical bond vibrations of TMPEI remain stable, with no formation of new chemical bonds. Therefore, it can be inferred that during long-term cycling, TMPEI remains stable without degradation and does not react with other electrolyte components. This further highlights its strong chemical and adsorption stability, ensuring the long-term stability of the SEI layer during cycling.

To visually represent the mechanism by which TMPEI enhances the performance of zinc anodes, a schematic diagram is presented (Fig. 1i). In the ZSO system, the non-uniform migration of zinc ions leads to variations in local deposition rates, resulting in surface protrusions. These protrusions induce the “tip effect”, where an increase in local electric field intensity accelerates zinc ion migration and deposition toward the tip region, ultimately facilitating dendrite formation and growth. Furthermore, the interaction between the zinc anode and water molecules triggers the HER, leading to the accumulation of hydrogen gas on the anode surface, which disrupts surface smoothness and exacerbates dendrite growth. Additionally, the local pH rise caused by the HER decreases the solubility of zinc salts, leading to the formation of insoluble byproducts that hinder zinc ion transport and deposition.^44,45 In contrast, the introduction of TMPEI results in the coordination of thiol groups with zinc to form Zn–S bonds, anchoring TMPEI onto the zinc anode surface and establishing a robust TMPEI SEI layer. The inner region of this interfacial layer utilizes the adsorption properties of thiol groups to occupy water molecule attachment sites, while the outer region leverages the hydrophobicity of thiol groups to further prevent direct contact between water molecules and the zinc anode, thereby significantly mitigating side reactions. Moreover, the uniform distribution of TMPEI on the zinc anode surface not only reduces the migration energy barrier of Zn²⁺ but also enhances the Zn²⁺ deposition ratio on the 002 plane by preferentially adsorbing onto the 101 and 100 planes, thereby promoting uniform Zn²⁺ deposition.

Morphological characteristics of electrodeposited zinc anodes

To examine the regulatory effect of the TMPEI SEI layer on the zinc anode plating/stripping process, in situ optical microscopy (OM) was utilized to dynamically monitor the interfacial evolution during zinc deposition at a current density of 1 mA cm⁻². In the ZSO electrolyte (Fig. 2a), irregular dendrite growth progressively emerged on the zinc surface as electrodeposition proceeded, continuing to expand and extend. By 80 min, bubble formation due to the HER became apparent, further disrupting the deposition process and intensifying deposition nonuniformity. In contrast, with the introduction of the TMPEI modification strategy (Fig. 2b and c), the zinc deposition becomes much more uniform and smooth, without dendrite growth or bubble formation. Further observations of the surface morphology of the zinc anodes after electroplating were performed using OM (Fig. S16†). The results clearly show that the TMPEI-modified zinc anode has a flatter and smoother surface. These findings confirm that TMPEI-modified electrodes and electrolytes effectively regulate zinc deposition behavior, suppress dendrite formation and the HER, and substantially enhance the deposition uniformity and interfacial stability of zinc anodes.

Fig. 2 (a and c) Interface evolution of the zinc deposition process on zinc foil in ZSO and TMPEI-0.01 electrolytes, respectively. (b) Interface evolution of the zinc deposition process on TMPEI-1@Zn in ZSO electrolyte. (d and e) OM and SEM observations of the zinc anode after 100 h of galvanostatic cycling at 1 mA cm⁻² and 1 mA h cm⁻² in Zn‖Zn symmetric cells. (f–h) Morphological observations of the zinc anodes using an optical profilometer.

A Zn‖Zn symmetric cell was assembled and subjected to constant current cycling tests at 1 mA cm⁻² and 1 mA h cm⁻² for 100 h. Post-cycling analysis revealed that in the cell utilizing ZSO electrolyte, a substantial accumulation of dendrites was observed on the Zn surface, along with severe damage to the separator (Fig. S17†). In contrast, in the cell incorporating the TMPEI modification strategy, the Zn surface exhibited a smooth and uniform deposition morphology. Further observations using OM and scanning electron microscopy (SEM) (Fig. 2d and e) demonstrated that in the ZSO cell, the Zn surface displayed extensive dendrite growth, uneven plating distribution, and significant structural defects. This irregular deposition behavior led to an increase in internal resistance and accelerated cell performance degradation.^46,47 In comparison, the Zn anode in the TMPEI-modified cell exhibited a flatter and more compact plating structure, with no visible dendrite growth. These results further confirm that the TMPEI SEI layer effectively regulates Zn ion deposition dynamics, facilitating uniform Zn ion distribution on the electrode surface.

Additionally, the deposition morphology and surface roughness of the Zn anode were analyzed using an optical profilometer. In the ZSO cell (Fig. 2f and S18†), pronounced dendrite growth was observed, characterized by randomly distributed sharp protrusions and relatively high surface roughness. In comparison, the deposition morphology in the ZSO + TMPEI-1@Zn cell exhibited some improvement, though noticeable dendritic growth remained present (Fig. 2g). In contrast, the Zn anode in the TMPEI-0.01 cell (Fig. 2h) displayed a smoother and more compact deposition structure, with dendrite growth effectively suppressed and surface roughness significantly reduced. These findings indicate that TMPEI, as an additive, exhibits excellent regulatory capabilities in optimizing localized zinc deposition behavior, thereby markedly enhancing the smoothness and uniformity of the electrode interface.

Suppression of side reactions and electrochemical properties of zinc anodes

To assess the corrosion resistance of different zinc anodes, Zn foil and TMPEI-1@Zn were immersed in ZSO electrolyte, while another Zn foil was immersed in TMPEI-0.01 electrolyte for 8 days. Observations (Fig. S19†) indicated that unmodified Zn foil in ZSO solution generated a significant amount of impurities, attributed to uneven zinc deposition and detachment of the loose deposition layer. In contrast, in the two electrolytes containing TMPEI, only a minimal amount of impurities was observed. OM and SEM analysis of the immersed Zn foils (Fig. S20†) revealed that the ZSO Zn foil exhibited numerous disordered dendritic structures on its surface, whereas the surfaces of TMPEI-1@Zn and TMPEI-0.01 Zn foils appeared much smoother and more uniform. Tafel tests conducted on Zn‖Zn symmetric cells (Fig. 3a and S21†) showed that the corrosion current and corrosion potential of the ZSO cell were 0.87 mA cm⁻² and −0.97 V, respectively. Following TMPEI modification, the corrosion current was significantly reduced, and the corrosion potential increased, indicating that the TMPEI SEI layer effectively mitigated the corrosion tendency of zinc and slowed the overall corrosion rate.⁴⁸ Additionally, after 100 h of constant current cycling, X-ray diffraction (XRD) analysis was performed on the Zn anode surface of Zn‖Zn symmetric cells. The results (Fig. 3b) demonstrated that in the ZSO cell, a substantial amount of byproducts, primarily basic zinc salts (Zn_xOTF_y(OH)_2x−y·nH₂O), accumulated on the Zn anode surface.⁴⁹ In contrast, in TMPEI-modified cells, byproduct formation was significantly reduced, with cells utilizing TMPEI-0.01 electrolyte showing almost no detectable byproduct diffraction peaks. These findings confirm that the incorporation of TMPEI effectively inhibits corrosion reactions and substantially enhances the corrosion resistance of Zn anodes.

Fig. 3 (a) Corrosion potential and corrosion current of zinc anodes in Zn‖Zn symmetric cells measured from Tafel curves. (b) XRD patterns of corrosion products on zinc anodes in three types of batteries. (c) The LSV curves of the HER for the three types of batteries were tested at a scan rate of 5 mV s⁻¹. (d) CA curves of the three types of batteries obtained by chronoamperometry and corresponding tZn²⁺ calculated using the Bruce–Vincent formula. (e) Change in R_ct before and after CA testing of the TMPEI-0.01 cell. (f) Nyquist EIS plots of the TMPEI-0.01 cell at different temperatures (273–313 K). (g) Calculated E_a for Zn²⁺ diffusion in the three types of batteries using the Arrhenius equation. (h) Galvanostatic cycling performance of Zn‖Cu half-cells at 1 mA cm⁻² and 0.5 mA h cm⁻².

To examine the influence of the TMPEI SEI layer on the HER of Zn anodes, linear sweep voltammetry (LSV) tests were conducted. The results (Fig. 3c) indicated that at −5 mA cm⁻², the HER potential of the ZSO cell was −0.946 V (vs. Ag/AgCl). In comparison, the HER potentials of TMPEI-0.01 and ZSO + TMPEI-1@Zn cells were reduced by 0.266 V and 0.345 V, respectively. The observed reduction in potential suggests that the HER was effectively suppressed, thereby minimizing side reactions on the Zn anode. This suppression is attributed to the adsorption of TMPEI on the Zn anode surface, which effectively reduces the exposure of active sites and inhibits the reduction of H⁺.⁵⁰ Additionally, thickness variations of Zn‖Zn symmetric cells before and after 100 h of cycling were measured (Fig. S22†). The results showed that the thickness of the ZSO cell increased from 3.1 mm to 4.4 mm, exhibiting a significant bulging phenomenon, indicative of the severe HER occurring within the cell. In contrast, the thickness of the TMPEI-modified cell remained nearly unchanged before and after cycling, further validating the exceptional effectiveness of the TMPEI SEI layer in suppressing the HER.

Chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) tests were performed to evaluate the influence of the TMPEI SEI layer on the electrochemical properties of Zn anodes. Fig. 3d, e, and S23† present the CA curves and charge transfer resistance (R_ct) of the cells before and after CA testing. The results demonstrated that in the ZSO cell, the current continuously increased within 500 s, which was attributed to the uneven deposition of Zn²⁺ and rapid dendrite growth. This process enlarged the contact area between the electrode and the electrolyte, allowing more zinc ions to participate in the electrochemical reaction, leading to a continuous rise in current.⁵¹ In contrast, the CA curves of TMPEI-modified cells exhibited more stable current variations, indicating significantly improved Zn²⁺ deposition behavior. Further calculations of the zinc ion transference number (tZn²⁺) revealed values of 0.36, 0.41, and 0.45 for ZSO, ZSO + TMPEI-1@Zn, and TMPEI-0.01 cells, respectively. The increase in tZn²⁺ effectively reduced ion concentration polarization and facilitated uniform Zn²⁺ diffusion.⁵² Additionally, EIS tests (Fig. 3f and S24†) indicated that compared to the ZSO cell, the introduction of TMPEI significantly lowered R_ct, reflecting reduced reaction resistance and improved reaction efficiency.⁵³ The calculated activation energy (E_a) values (Fig. 3g) for ZSO, ZSO + TMPEI-1@Zn, and TMPEI-0.01 cells were 25.29 kJ mol⁻¹, 21.27 kJ mol⁻¹, and 17.70 kJ mol⁻¹, respectively, demonstrating a substantial reduction in the energy barrier for Zn²⁺ deposition and improved electrochemical reaction kinetics.⁵⁴ These results confirm that the TMPEI SEI layer effectively lowered the interfacial energy barrier for zinc deposition and optimized the charge transfer pathway. From a thermodynamic perspective, based on Gibbs free energy principles, systems tend to reach a more stable state by minimizing free energy.⁵⁵ The incorporation of TMPEI facilitated the formation of a more stable and ordered interfacial structure between the electrode and the electrolyte, regulating zinc deposition behavior, effectively reducing the system's free energy, and ultimately lowering the zinc deposition energy barrier.

To evaluate the influence of the TMPEI SEI layer on electrode reversibility, constant current cycling tests were performed on Zn‖Cu half-cells. As shown in Fig. 3h and S25,† the ZSO cell exhibited significant fluctuations in coulombic efficiency (CE) and failed after only 290 cycles. In contrast, the ZSO + TMPEI-1@Zn cell maintained an average CE of 98.83% after 1600 cycles, while the TMPEI-0.01 cell achieved an average CE of 99.27% after 2100 cycles. This substantial improvement is attributed to the introduction of TMPEI, which effectively regulates the uniform and ordered deposition of Zn²⁺, thereby significantly enhancing the cycling reversibility of the electrode redox processes.⁵⁶ Furthermore, cycling tests were conducted on Zn‖Cu half-cells at a current density of 5 mA cm⁻². The results (Fig. S26†) indicated that the TMPEI-0.01 cell maintained an average CE of 98.73% over 400 cycles, demonstrating the exceptional effectiveness of the TMPEI SEI layer in enhancing electrode cycling reversibility under high current density conditions.

Electrochemical performance of Zn‖Zn symmetric cells

By varying the concentration of TMPEI in modified electrodes and electrolytes, Zn‖Zn symmetric cells were tested to assess the optimization effect of the TMPEI SEI layer in enhancing the cycling stability of zinc anodes. As illustrated in Fig. 4a, under constant current cycling at 1 mA cm⁻² and 1 mA h cm⁻², the ZSO cell failed abruptly after only 75 h, likely due to uneven zinc deposition triggering dendrite growth, which subsequently damaged the separator.⁵⁷ In contrast, the introduction of TMPEI as an additive significantly improved cycling performance. At various TMPEI concentrations, the cycle life of the cells exceeded 4200 h, representing an enhancement of over 50 times compared to the ZSO cell. Additionally, TMPEI-modified electrodes effectively enhanced cell durability, with the ZSO + TMPEI-3@Zn cell achieving a cycle life of 3600 h (Fig. S27†). A comparison of nucleation overpotentials among different cells (Fig. S28†) revealed that the TMPEI-0.01 cell exhibited a nucleation overpotential of 56.3 mV, significantly lower than the 87.1 mV and 102.6 mV observed for the ZSO + TMPEI-1@Zn and ZSO cells, respectively. The reduction in nucleation overpotential effectively mitigated the formation of localized high electric fields and promoted uniform Zn²⁺ nucleation, facilitating smooth zinc deposition while suppressing side reactions.^46,58 The plating/stripping curves indicated that ZSO + TMPEI-n@Zn cells exhibited higher polarization voltage and nucleation overpotential compared to TMPEI-n cells. In conjunction with previous Tafel and EIS experimental findings (Fig. 3), it can be inferred that the TMPEI-n@Zn modified electrode, prepared via the soaking method, formed a denser SEI protective layer, effectively suppressing interfacial side reactions. However, this also increased zinc ion migration resistance, thereby weakening the regulatory effect of TMPEI on Zn²⁺ deposition. Conversely, when TMPEI was directly introduced into the electrolyte as an additive, adjusting its concentration allowed for the formation of an SEI layer with a more rational density distribution on the zinc anode surface, thereby optimizing Zn²⁺ deposition regulation.

Fig. 4 (a) Cycling performance of TMPEI-n cells at 1 mA cm⁻² and 1 mA h cm⁻² (insets show enlarged voltage–time curves). (b) Cycling performance of TMPEI-n cells at 5 mA cm⁻² and 5 mA h cm⁻². (c) Cycling performance of TMPEI-n cells at 10 mA cm⁻² and 10 mA h cm⁻². (d) Rate performance of Zn‖Zn symmetric cells. (e) Comparison of the working performance of Zn‖Zn symmetric cells at 5 mA cm⁻².

Constant current cycling tests were further conducted on different cells at 5 mA cm⁻² and 5 mA h cm⁻², as shown in Fig. 4b and S29.† The results indicated that the cycling curve of the ZSO cell exhibited irregular fluctuations after only 50 h, eventually leading to failure due to a short circuit. In contrast, the introduction of TMPEI demonstrated remarkable regulatory effects, particularly in the TMPEI-0.02 cell, which maintained stable cycling for up to 2000 h, highlighting the substantial enhancement in cell performance under high current density conditions enabled by the TMPEI SEI layer. Notably, most published studies typically employ excess zinc with a low DOD (<10%), which significantly deviates from the high-performance requirements of practical battery applications.^46,59 To address this limitation, a zinc foil with a thickness of only 30 μm was utilized as the anode to evaluate the regulatory capability of TMPEI under extreme conditions (Fig. 4c and S30†). Under harsh conditions of 10 mA cm⁻² current density, 10 mA h cm⁻² areal capacity, and a high DOD of 56.93%, the TMPEI-0.02 cell was still able to sustain stable cycling for up to 500 h. This result fully validates the potential of TMPEI to significantly enhance cycling performance under demanding conditions, further demonstrating its exceptional adsorption stability and regulatory capability.

The above experiments indicate that in the TMPEI-modified strategies, TMPEI-3@Zn and TMPEI-0.02 represent the optimal TMPEI concentrations for modified electrodes and electrolytes, respectively. To further assess the performance of the batteries, rate capability tests were carried out at various current densities and areal capacities ranging from 1 to 20 mA cm⁻². The results (Fig. 4d) revealed that the ZSO cell exhibited poor stability, failing quickly during cycling tests due to fluctuations in current density. In contrast, the TMPEI-modified cells demonstrated stable cycling performance even at an ultra-high current density of 20 mA cm⁻². The cycling performance of TMPEI-modified cells was compared to those from other zinc anode modification strategies reported in the literature, as shown in Fig. 4e, S31 and Table S2.† The results showed that TMPEI outperformed most of the other reported strategies for modifying zinc anodes, providing strong evidence for the potential of the TMPEI SEI layer in enhancing the stability of zinc anodes under high current density and high DOD conditions. Additionally, a comprehensive comparison was made between TMPEI and other SEI methods in terms of lifespan, efficiency, and practical feasibility. This comparison further emphasizes the superior performance of the TMPEI SEI layer and its significant advantage in simplifying the preparation process, thereby improving the practical feasibility of the method (Table S3†).^60–65

Electrochemical performance of Zn‖AlVO-NMP full cells

Vanadium-based cathode materials have garnered considerable attention owing to their high capacity, energy density, and rapid ion and electron transport characteristics.⁶⁶ Following a previously established method,⁶⁷ AlVO-NMP cathodes were successfully synthesized, with the characterization results shown in Fig. S32.† To fully assess the effect of TMPEI incorporation on the electrochemical properties of the battery, Zn‖AlVO-NMP full cells were assembled and subjected to cyclic voltammetry (CV) testing. The results (Fig. 5a) indicated that the CV curves for the various cells displayed similar redox peaks, suggesting that the incorporation of TMPEI did not lead to significant side reactions or disturbances in the battery's electrochemical environment.⁶⁸ Importantly, the TMPEI-modified full cells exhibited a larger CV area, indicating that the addition of TMPEI allowed the battery to achieve greater capacity.⁶⁹ EIS results for the full cells (Fig. 5b) revealed that the TMPEI-modified cells had a lower R_ct compared to the ZSO cell, indicating that the TMPEI SEI layer significantly enhanced the electrochemical reaction kinetics at the electrode surface, facilitating more efficient charge transport. Additionally, rate capability tests of the full cells at different charge/discharge rates (Fig. 5c) demonstrated that the TMPEI-modified cells exhibited higher specific capacities across all rates compared to the ZSO cell. This further supports that the introduction of TMPEI effectively improved the charge transfer capability and ion diffusion performance of the electrode under high-rate conditions.⁷⁰

Fig. 5 (a) CV curves of Zn‖AlVO-NMP full cells. (b) Nyquist EIS plots of Zn‖AlVO-NMP full cells. (c) Rate performance of Zn‖AlVO-NMP full cells. (d) Cycling performance of Zn‖AlVO-NMP full cells at 5 A g⁻¹. (e) GCD curves of Zn‖AlVO-NMP full cells at 5 A g⁻¹ after the first and 4000th cycles. (f and g) Self-discharge performance of Zn‖AlVO-NMP full cells using ZSO and TMPEI-0.02 electrolytes, respectively. (h) Demonstration of a pouch cell with TMPEI-0.02 electrolyte lighting a bulb.

To evaluate the influence of TMPEI incorporation on the cycling performance of Zn‖AlVO-NMP full cells, galvanostatic charge–discharge (GCD) cycling tests were conducted at a current density of 1 A g⁻¹. As shown in Fig. S33 and S34,† the ZSO cell, initially starting with a specific capacity of 200 mA h g⁻¹, exhibited significant fluctuations in both specific capacity and CE after 290 cycles, leading to rapid failure. In contrast, the TMPEI-modified cells showed improved cycling stability, with the TMPEI-0.02 cell maintaining a capacity retention of 86.92% after 1000 cycles, indicating its exceptional long-term stability. Additionally, GCD cycling tests were performed on Zn‖AlVO-NMP full cells at a higher current density of 5 A g⁻¹. As shown in Fig. 5d and e, with an initial specific capacity of 150 mA h g⁻¹, after 4000 cycles, the capacity retentions of the ZSO cell, ZSO + TMPEI-3@Zn cell, and TMPEI-0.02 cell were 47.46%, 63.01%, and 79.53%, respectively. These results demonstrate that the TMPEI SEI layer significantly enhanced the cycling stability of the cells under high current density, with the TMPEI-0.02 cell showing the best long-cycle performance, reflecting its excellent structural integrity and resistance to degradation. The increase in capacity during the early cycles of the TMPEI-0.02 battery and the later cycles of the ZSO + TMPEI-3@Zn battery can be attributed to the re-adsorption of TMPEI (see the discussion section in Table S3†). This process stabilizes the SEI layer, improving the efficiency of ion deposition and dissolution on the zinc anode surface, ultimately enhancing the overall performance of the battery.

To evaluate the self-discharge behavior of the batteries, static self-discharge tests were conducted on Zn‖AlVO-NMP full cells.⁷¹ As shown in Fig. 5f, g, and S35,† after 72 h of rest, the capacity retentions for the ZSO cell, TMPEI-0.02 cell, and ZSO + TMPEI-3@Zn cell were 71.6%, 82.8%, and 77.2%, respectively. These results indicate that the TMPEI SEI layer significantly improved the resting stability of the batteries, effectively mitigating self-discharge, and further enhancing their practical application potential. Lastly, a pouch cell was assembled with the TMPEI-0.02 electrolyte, and its practical use was demonstrated by powering a small light bulb (Fig. 5h).

Conclusion

This study presented TMPEI as a material for modifying the electrode/electrolyte interface to form an SEI layer with outstanding interfacial stability on the zinc anode surface, greatly enhancing its electrochemical performance. The improvement in the zinc anode performance due to TMPEI is primarily evident in the following aspects: (1) TMPEI can adhere to the zinc metal surface and form Zn–S bonds through coordination adsorption, thus establishing a durable protective interface. This enables efficient regulation of zinc anode cycling performance even under high-current conditions; (2) the protective adsorption of TMPEI, along with the hydrophobic characteristics of its thiol terminal groups, effectively prevents water molecules from reaching the zinc anode, significantly inhibiting the HER and corrosion; (3) the even distribution of TMPEI stabilizes and organizes the interfacial structure, lowering the migration energy barrier of Zn²⁺ and promoting its uniform deposition on the (002) plane, effectively suppressing dendrite formation. Thanks to the superior regulation of the TMPEI SEI layer, Zn‖Zn symmetric cells achieved stable cycling for up to 2000 h at a current density of 5 mA cm⁻² and an areal capacity of 5 mA h cm⁻². Even under harsh conditions of a current density of 10 mA cm⁻² and a depth of discharge of 56.93%, the battery retained a cycle life of 500 h. This study utilized an innovative interfacial regulation strategy that greatly enhanced the electrochemical stability of zinc anodes under extreme conditions, providing a new design pathway for developing high-performance AZIBs.

Data availability

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

Author contributions

Jie Liu: conceptualization, methodology, investigation, formal analysis, data curation, writing – original draft, and writing – review & editing. Peng Wang: supervision, resources, project administration, funding acquisition, and writing – review & editing. Xiaoyu Yang: visualization and writing – review & editing. Zinan Wang: methodology and validation. Hangyu Miao: software. Zhe Li: investigation. Wei Duan: project administration, supervision, and validation. Ying Yue: project administration, supervision, and validation. Yunpeng Liu: resources, conceptualization, and formal analysis. Yang Ju: resources and formal analysis.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the Distinguished Young Scholars of Hebei Province (E2024502077), the National Major Scientific and Technological Projects (2024ZD0803100), the Natural Science Fund for Beijing Natural Science Foundation (3232054), the National Nature Science Foundation of China (51977079), the Key Laboratory of Icing and Anti/De-icing of CARDC (Grant No. IADL 20210401), the Central Guidance on Local Science and Technology Development Fund of Hebei Province (226Z1204G), the Basic Research Project of Baoding City (2272P002), the Top Young Innovative Talents of Colleges and universities of Higher Learning Institutions of Hebei (BJ2021095) and the Fundamental Research Funds for the Central Universities (2023MS131).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01051a

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