Regulating the solvation structure and adsorption behavior in zinc anodes using polar organic molecules to achieve durable dendrite-free zinc metal anodes for aqueous zinc-ion batteries

Abstract

Aqueous zinc ion batteries (AZIBs) have attracted much attention because of their environmental friendliness, high theoretical capacity and low cost. However, zinc metal anodes face challenges of zinc dendrite formation and by-product generation during electrochemical reactions. Herein, the non-toxic cyclic organic compound 1,4,7,10-tetraazecyclododecane (Cy) was utilized as an additive to optimize a ZnSO₄ (ZS) electrolyte system, aiming to inhibit side reactions and dendrite growth on the zinc metal surface. Cy molecules could form coordination complexes with Zn²⁺ ions, thereby entering the solvated sheath of Zn²⁺ and reducing the activity of H₂O molecules. In addition, the contact between the active molecules of H₂O and the zinc metal anode was minimized, and hydrogen evolution potential was decreased as Cy adsorbed more preferentially to the surface of zinc metal than H₂O, thus avoiding local alkaline enhancement and effectively inhibiting side reactions. After incorporating 10 g L⁻¹ Cy into ZS electrolyte solution, a cycle life exceeding 4000 h was achieved for a Zn||Zn symmetric battery at 2 mA cm⁻²/1 mA h cm⁻². Additionally, stable cycling performance over 3000 cycles with an average CE of 99.45% was attained for a Zn||Cu asymmetric battery in the modified electrolyte system. Moreover, for a Zn||VO₂ full battery with the Cy-added ZS electrolyte, a capacity retention rate of 75.5% was obtained after 2000 cycles. This work proposes a high-efficiency electrolyte additive to suppress dendrite growth and side reactions on the surface of zinc metal by tailoring the solvated structure of Zn²⁺ and the interface between the Zn metal and the electrolyte.

---

1. Introduction

With the development of science and technology, electric energy has attracted significant attention as an inexpensive and environmentally friendly energy.^1–5 However, the commonly used lithium-ion batteries for electrical energy storage face challenges, such as the scarcity of lithium resources and safety concerns associated with organic electrolytes, highlighting the need for the development of low-cost and high-safety rechargeable batteries.^6,7 Among various aqueous batteries, aqueous zinc ion batteries (AZIBs) stand out because of their high energy density (820 mA h g⁻¹, 5855 mA h cm⁻³), low oxidation–reduction potential (−0.76 V) and environmental friendliness.^8–13 However, zinc metal anodes (Zn) face some serious challenges, such as the formation of zinc dendrites, hydrogen evolution reactions and corrosion reactions on the surface of Zn metal.^14–16 Obviously, inhibiting dendrite growth, hydrogen evolution and corrosion is crucial to prolonging the cycle life of AZIBs.

Recently, several strategies have been proposed to inhibit zinc dendrite growth as well as hydrogen evolution and corrosion reactions on the surface of the zinc anode, including (1) surface modification of zinc metal to isolate water molecules to suppress hydrogen evolution and corrosion reactions,¹⁷ (2) structural design of the zinc anode to increase the contact area between the electrolyte and the anode to provide more deposition sites,^18,19 and (3) electrolyte design and optimization to regulate the solvated structure of Zn²⁺ and interface characteristics of Zn metal and electrolyte to inhibit hydrogen evolution and corrosion reactions.^20–22 Among these strategies, electrolyte design and optimization is the most convenient solution, with the potential for scaling from laboratory settings to practical applications because of its excellent repeatability, universality and diversity.^7,23–27 Compared to inorganic salt additives, organic additives can more adjustably regulate the solvation structure of zinc ions and the electrode/electrolyte interface properties, making them widely favored. For example, Liu et al.²⁸ introduced cetyltrimethylammonium bromide (CTAB) into the electrolyte and found that CTAB is adsorbed onto the surface of zinc anode to regulate the deposition direction of Zn²⁺ and inhibit the formation of dendrites. Zhou et al.²⁹ used phenylalanine (Phe) to cooperate with Zn²⁺ ions to optimize the solvation environment of Zn²⁺ for extending the cycle life of AZIBs. In addition, N,N-dimethylacetamide (DMA),³⁰ formamide³¹ and β-cyclodextrin³² can be used to inhibit zinc dendrite growth and side reactions. It should be noted that 1,4,7,10-tetraazecyclododecane (Cy) has abundant C–N polar groups, which may lead to a higher bond energy between Cy and Zn²⁺ than between Zn²⁺ and H₂O, thus achieving significant management of solvation structure and electrode/electrolyte interface properties.

In this context, a non-toxic cyclic organic compound, Cy, has been integrated into the ZnSO₄ (ZS) electrolyte as an effective additive to mitigate the growth of zinc dendrites and curb the side reactions on the Zn metal surface. The Cy molecules are capable of substituting some of the H₂O molecules in the solvation shell of hydrated zinc ions (Zn²⁺) due to their stronger binding affinity with Zn²⁺ than H₂O. This substitution leads to a decrease in the quantity of electrochemically active water molecules on the Zn metal anode, thereby diminishing the reactivity of water molecules. Furthermore, Cy exhibits a higher adsorption energy on the Zn anode surface than water, allowing it to preferentially adsorb onto the Zn metal surface. This adsorption effectively shields water molecules from the Zn metal, reducing the potential for hydrogen evolution and preventing the formation of a highly corrosive, localized alkaline environment. Consequently, the incorporation of 10 g L⁻¹ Cy into the ZnSO₄ electrolyte has been shown to significantly enhance the cycle life of Zn||Zn symmetric batteries to over 4000 hours at a current density of 2 mA cm⁻² and an areal capacity of 1 mA h cm⁻². Additionally, the Zn||Cu asymmetric battery demonstrates stable cycling for more than 3000 cycles with an average coulombic efficiency (CE) of 99.45%.

2. Results and discussion

See the ESI† for full experimental details. As shown in Fig. 1(a), in the ZnSO₄ (ZS) electrolyte, Zn²⁺ ions do not exist as individual ions in aqueous solutions, but combines 6 water molecules to form a hydrated Zn ion of [Zn(H₂O)₆]²⁺. During cycling, hydrated Zn ions near the Zn anode release 6 highly chemically active H₂O, which is easily reduced to H₂. Meanwhile, the local pH value is increased, and thus, a by-product, Zn₄SO₄(OH)₆·xH₂O (ZSH), is generated. A large number of by-products aggravate the non-uniformity of the zinc anode surface, resulting in uneven electric field distribution, and eventually induce rampant zinc dendrites. However, in a Cy-added ZS electrolyte, Cy molecules have a stronger binding energy with Zn²⁺ than water molecules and, thus, replace partial water molecules in the solvation sheath of Zn²⁺ to form [Zn(H₂O)₂Cy]²⁺ molecules, as shown in Fig. 1(b). This reduction of chemically active H₂O prevents corrosion reactions and hydrogen evolution. Simultaneously, Cy molecules are more easily adsorbed on the surface of zinc metal, thus isolating the contact between water molecules and zinc metal and inhibiting the occurrence of side reactions.

Fig. 1 Schematic of zinc ions in (a) ZS electrolyte and (b) Cy-ZS electrolyte.

To verify the corrosion resistance of the Cy additive, the zinc foil was soaked for 5 days in the ZS electrolytes without/containing 10 g L⁻¹ Cy, as shown in Fig. S1 (ESI†). From Fig. S1a (ESI†), the surface of the zinc foil immersed in the ZS electrolytes exhibits numerous sharp nanosheets vertically/tilted, and the surface is loosely pored, indicating severe corrosion reactions. However, the surface of zinc foil immersed in 10 g L⁻¹ Cy-added ZS electrolyte is relatively smooth and dense, and no obvious protrusions are observed, suggesting that the Cy effectively inhibits the corrosion reaction on the surface of Zn metal, thus greatly reducing the production of by-products (Fig. S1b, ESI†). In addition, XRD was used to detect the zinc foil after soaking and cycling, as shown in Fig. S2 (ESI†). For the zinc foil immersed in the ZS electrolyte without 10 g L⁻¹ Cy, the strong characteristic peaks of the by-product Zn₄SO₄(OH)₆·5H₂O (PDF#39-0688) appear; however, no obvious characteristic peaks of the co-product can be detected on the surface of the zinc film dipped in the electrolyte containing 10 g L⁻¹ Cy. The Zn||Zn symmetric batteries assembled with different electrolyte solutions are cycled at 1 mA cm⁻² and 1 mA h cm⁻² for 10 h, 20 h and 50 h, respectively, to evaluate the effect of Cy on the inhibition of zinc dendrite growth, and zinc anode surface morphologies are observed by SEM, as shown in Fig. 2(a)–(f). Clear dendrites perpendicular to the zinc foil surface are observed for the zinc foil cycled for 10 h in the ZS electrolyte, and with the extension of cycle time, increasingly more larger zinc dendrites are generated, displaying a loose, porous, and disorderly surface. On the contrary, after cycling in the 10 g L⁻¹ Cy-added ZS electrolyte for 10 h, 20 h and 50 h, no obvious dendrites appear on the surface of zinc foil, giving a flat, smooth and dense surface. From Fig. 2(g) and (h), the zinc foil cycled in the ZS has obvious cracks and pores, but after the addition of 10 g L⁻¹ Cy, the surface of the zinc foil cycled remains structurally intact, with almost no significant protrusions throughout the electrode, which is similar to that of the original bare zinc sheet without cycling (Fig. S3, ESI†). Fig. S4 and S5 (ESI†) show the SEM image and elemental distribution of the zinc surface cycled for 50 h. After cycling in the ZS electrolytes for 50 h, a large amount of oxygen-containing or/and sulfur-containing substances are detected on the surface of zinc foil. In contrast, after 50 h of cycling in the 10 g L⁻¹ ZS electrolyte with added Cy, the colour of oxygen and sulfur elements on the zinc foil surface is not as deep as in the blank sample, indicating that oxygen and sulfur elements on the surface of the zinc anode become lower after the Cy is added. As shown in Fig. S5, oxygen-containing or/and sulfur-containing substances are much less after the addition of Cy. Similarly, on the surface of the zinc foil treated for 50 hours in the ZS electrolyte, there are clear XRD peaks of the ZSH by-product, whereas no ZSH by-product can be detected on the surface of the zinc foil treated in the ZS electrolyte with 10 g L⁻¹ Cy Z added (Fig. S6b, ESI†). The results show that the growth of zinc dendrites and the formation of by-products are significantly inhibited by the addition of Cy. In order to intuitively evaluate the inhibition effect of the Cy additive on zinc dendrites, the electrochemical behavior of Zn²⁺ in different ZnSO₄ electrolyte solutions was observed in situ at a current density of 10 mA cm⁻², as shown in Fig. 2(i) and (j). The electrochemical behavior of Zn²⁺ in different ZnSO₄ electrolyte solutions was observed in situ at a current density of 10 mA cm⁻², as shown in Fig. 2(j). The surface of the zinc film in the ZS electrolytes will gradually get rough with bigger or smaller unevenness. As shown in Fig. 2(i), as the deposition time increases, the surface of the zinc foil becomes more uneven and the deposited zinc becomes loose and porous. However, from Fig. 2(j), in the ZS electrolyte with 10 g L⁻¹ Cy, zinc deposition on the surface of zinc foil is dense and smooth over time. With the extension of cycle time, the zinc sheet still maintains a dense and flat state without obvious bulges. It is clear that the addition of Cy is effective in inhibiting dendrite growth and promoting the deposition of Zn²⁺ in dense instead of loose and porous zinc films.

Fig. 2 (a)–(c) SEM images of the surface of zinc anodes cycled in a ZS electrolyte for 10 h, 20 h and 50 h. (d)–(f) SEM images of the surface of zinc anodes cycled in 10 g L⁻¹ Cy-ZS electrolyte for 10 h, 20 h and 50 h. (g) and (h) SEM images of the cross section of zinc anodes cycled in a ZS electrolyte without/with 10 g L⁻¹ Cy for 50 h. In situ optical microscopic photographs of (i) galvanization in ZS electrolyte and (j) galvanization in ZS electrolyte containing 10 g L⁻¹ Cy for 90 minutes (the interval is 10 minutes).

The nucleation overpotential of Zn metals in the ZnSO₄ electrolytes without or with 10 g L⁻¹ Cy is shown in Fig. 3(a). Compared to the ZnSO₄ electrolyte, the nucleation overpotential of the Zn metal in the ZnSO₄ electrolyte containing Cy is increased by 23 mV, leading to a stronger nucleation driving force and smaller Zn nuclei, which is conducive to the preferential growth of uniform nucleation orientation, thus inhibiting the two-dimensional diffusion of Zn²⁺ and avoiding the “tip effect”. The LSV response and Tafel curves of Zn metals in ZS and 10 g L⁻¹ Cy-added electrolytes at 5 mV s⁻¹ were tested, and the results are shown in Fig. 3(b) and (c). At a current density of 75 mA cm⁻², the hydrogen evolution potential of the zinc foil in the ZS electrolyte is −1.53 mV (based on Ag/AgCl), substantially higher than that in 10 g L⁻¹ Cy-added ZS electrolyte (−1.69 mV), indicating that the hydrogen evolution reaction is effectively suppressed after the introduction of Cy. As shown in Fig. 3(c), after adding 10 g L⁻¹ Cy, the corrosion potential of the Zn metal increases significantly, suggesting that Cy effectively inhibits corrosion reactions, which is consistent with the above-mentioned XRD results (Fig. S2 and S6, ESI†). To avoid the effect of Zn deposition on the hydrogen evolution potential, the electrochemical window in a 2 M Na₂SO₄ solution was monitored (Fig. S7, ESI†). After introducing Cy, the electrochemical window was expanded by 33.6 mV. The CA curves also revealed the mechanism of the different deposition behaviors of Zn²⁺ in different electrolytes (Fig. 3(d)). When a constant overpotential of −150 mV was applied to the Zn electrode in the ZnSO₄ electrolyte, indicating a continuous two-dimensional diffusion process, the current density continued to increase for more than 200 s, which did not favour the even nucleation of Zn²⁺. However, in the ZnSO₄ electrolyte containing 10 g L⁻¹ Cy, the current density of the Zn electrode hardly increased after 5 s, indicating that in the ZS electrolyte containing Cy, initial Zn nucleation and two-dimensional diffusion began within 5 s, followed by a stable three-dimensional diffusion process, which was very favorable for the uniform nucleation of Zn²⁺. The EIS spectra of the Zn||Zn-symmetric batteries in the ZS electrolyte and 10 g L⁻¹ Cy-added ZS electrolyte are shown in Fig. 3(e). Obviously, the impedance of the symmetric battery increased after adding 10 g L⁻¹ Cy, indicating that Cy increases the interface resistance due to its non-conductive nature. Furthermore, the addition of Cy led to an increase in the Zn²⁺ transfer number t_Zn²⁺ from 0.178 to 0.593 (Fig. S8, ESI†), which indicated that the addition of Cy reduced the conductivity of the electrolyte (Fig. S9, ESI†). In order to evaluate the coulombic efficiency of Zn electrodes, the Zn||Cu asymmetric batteries were assembled. The CEs of the Zn||Cu asymmetric batteries in the 2 M ZnSO₄ without/with 10 g L⁻¹ Cy are shown in Fig. 3(f)–(g). From Fig. 3(f) and (g), it can be observed that the battery with the original ZnSO₄ electrolyte has unstable cycling, with the CEs of fluctuations and failure after 300 cycles. However, for the battery in the ZnSO₄ electrolyte with 10 g L⁻¹ Cy, a high initial CE of 80.2% was obtained and an excellent average CE of 99.45% was achieved over 3000 cycles (Fig. 3(g) and (h)). The rate performance of the Zn||Cu cells in the ZS electrolytes with and without 10 g L⁻¹ Cy at different current densities is shown in Fig. S10 (ESI†). From Fig. S10a (ESI†), it can be observed that when the Zn||Cu asymmetric battery cycling in the ZS electrolyte reached a current density of 10 mA cm⁻², the battery had a short circuit and the voltage fluctuated intensely; and when the current density was further increased to 20 mA cm⁻², a short circuit occurred and the battery failed. However, as shown in Fig. S10b (ESI†), an asymmetric battery with 10 g L⁻¹ Cy is cyclically stable for more than 70 h at all current densities.

Fig. 3 (a) CV profiles of Zn foils for Zn nucleation on Ti foil in ZS and 10 g L⁻¹ Cy-added ZS electrolytes at 1.0 mV s⁻¹. (b) LSV response curves of Zn metal in ZS and 10 g L⁻¹ Cy-added electrolytes at 5 mV s⁻¹. (c) Tafel plots of Zn foils in different electrolytes at 5 mV s⁻¹ based on a three-electrode system. (d) CA curves of the Zn²⁺ plating process in different electrolytes. (e) EIS spectra of Zn||Zn symmetric cells in the ZS electrolyte and the designed electrolyte with 10 g L⁻¹ Cy. (f)–(h) CE of Zn||Cu cells in 2 M ZnSO₄ with and without 10 g L⁻¹ Cy.

The Zn||Zn symmetric batteries with different concentrations of Cy were assembled to evaluate the effects of Cy on the electrochemical cycling stability and rate performance at different current densities, as shown in Fig. 4 and Fig. S11 (ESI†). To determine the optimal concentration of Cy, the Zn||Zn symmetric batteries were tested in the ZS electrolytes with 0 g L⁻¹, 5 g L⁻¹, 10 g L⁻¹, and 15 g L⁻¹ Cy concentrations at a current density of 10 mA cm⁻²/10 mA h cm⁻². From Fig. 4(a), the battery cycling in the blank electrolyte fails at 70 h, and the lives of the Zn||Zn symmetric battery cycling in the electrolytes with 5 g L⁻¹ Cy or 15 g L⁻¹ Cy are slightly increased to more than 100 h. The exciting thing is that the battery containing 10 g L⁻¹ Cy is particularly prominent, providing a cycle life of over 100 hours, which is much longer than that of the batteries containing 0 g L⁻¹ (70 h), 5 g L⁻¹ Cy (110 h), or 15 g L⁻¹ (100 h). In other words, the optimum Cy concentration is 10 g L⁻¹. From Fig. 4(b), the symmetric battery with 10 g L⁻¹ Cy exhibits a life of more than 4000 h, which is about 50 times (80 h) longer than that in the ZS electrolyte. As shown in Fig. 4(c), the cycle life of the symmetrical battery with 10 g L⁻¹ Cy electrolyte at a current density of 0.5 mA cm⁻²/0.25 mA h cm⁻² is as long as 5500 h, much longer than the cycle life of the battery in the ZS electrolyte (153 hours). Moreover, at 1 mA cm⁻²/0.5 mA h cm⁻², the Zn||Zn symmetric battery in the ZS electrolyte can only cycle stably for 97 h, which is significantly shorter than that with 10 g L⁻¹ Cy electrolyte (4800 h), as shown in Fig. S11 (ESI†). Fig. 4(d) shows that the stable cycle time of the battery does not exceed 130 hours when cycling in the ZS electrolyte, but when cycling in 10 g L⁻¹ Cy electrolyte, the Zn||Zn symmetric battery has good stability with the change in the current density. The addition of Cy significantly increased the cycle life of the Zn||Zn-symmetric batteries, as shown in Fig. 4(e) and Table S1 (ESI†). Clearly, the cycle life and cumulative capacity of the Zn||Zn-symmetric batteries in the 10 g L⁻¹ Cy-added ZS electrolyte are comparable or superior to those in the recently reported electrolytes.

Fig. 4 Electrochemical properties of Zn||Zn symmetrical batteries with different electrolytes: (a) long-term constant current cycling at 10 mA cm⁻²/10 mA h cm⁻² for Zn||Zn symmetric batteries with different concentrations of Cy additives; (b) long-term cycling for Zn||Zn symmetric batteries with ZS electrolyte without/with 10 g L⁻¹ Cy at a current density of 2 mA cm⁻²/1 mA h cm⁻²; (c) long-term cycling of Zn||Zn symmetric batteries with different electrolytes at 0.5 mA cm⁻²/0.25 mA h cm⁻²; (d) rate performance of Zn||Zn symmetric batteries with different electrolytes at current densities ranging from 1 to 10 mA cm⁻²; (e) comparison of the cycle time and cumulative capacity of Zn||Zn-symmetric batteries with 10 g L⁻¹ Cy with other state-of-the-art Zn||Zn-symmetric batteries reported recently.^{17,28,33–45}

Based on the theoretical calculation and experimental characterization, the mechanism of inhibiting zinc dendrite growth and side reaction by Cy additives is explored. First, molecular dynamics (MD) simulations were performed to simulate the solvation structure of Zn²⁺ ions in the ZS and Cy-added electrolytes. As shown in Fig. 5(a), in the ZS electrolyte, 6H₂O molecules and one Zn²⁺ molecule form the primary solvation structure of Zn²⁺. In contrast, the solvated structure of Zn²⁺ is significantly altered after the addition of Cy molecules in the Cy-added electrolyte, as Cy molecules can enter the solvated sheath structure of hydrated Zn²⁺ and replace the original four water molecules (Fig. 5(c)). In addition, the radial distribution function (RDFs) and coordination number of Zn²⁺ are shown in Fig. 5(b) and (d). In the ZS electrolyte, there are close to 6 water molecules near Zn²⁺ (Fig. 5(b)) with Zn²⁺–O peak appearing at 2.15 Å near Zn²⁺. Similarly, in the Cy electrolyte, the Zn²⁺–O peak at 2.15 Å near Zn²⁺ decreases and the Zn²⁺–N peak appears at 2.15 Å near Zn²⁺, as shown in Fig. 5(d) and (f), which shows that the water molecules in the solvated structure of the hydrated Zn²⁺ are replaced by molecules of Cy. The interaction between Zn²⁺ and H₂O or Cy was explored by quantum chemical calculations. As can be seen in Fig. 5(e), the binding energy between Zn²⁺ and Cy molecules (−217.39 kcal mol⁻¹) is much higher than the binding energy between Zn²⁺ and H₂O molecules (−66.24 kcal mol⁻¹), suggesting that the addition of Cy can replace water in the solvated sheath structure of hydrated zinc ions and form a new solvated structure. Moreover, when a Cy molecule was introduced into the original [Zn(H₂O)₆]²⁺ solvated structure to replace H₂O, the electrostatic potential value was significantly reduced (Fig. 5(g)), indicating that the electrostatic repulsion around Zn²⁺ can be alleviated, which is conducive to the rapid transport of Zn²⁺. The mechanism of inhibiting zinc dendrite growth and side reactions by Cy additive was characterized using nuclear magnetic, infrared and Raman spectra, as shown in Fig. S12 and S13 (ESI†). Nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy were performed to study the effects of Cy on the solvation structure of hydrated zinc ions in the ZS electrolyte. As shown in Fig. S12a (ESI†), the ²H peak of pure D₂O appears at 4.6570 ppm, and after adding 2 M ZnSO₄, the ²H peak moves to 4.7168 ppm, indicating that the strong bond is formed by Zn²⁺ and the O atom of D₂O reduces the amount of active D₂O. When different concentrations of Cy (5 g L⁻¹, 10 g L⁻¹ and 15 g L⁻¹) were added to the ZS electrolyte, the ²H peak of D₂O shifted to 4.7152 ppm, 4.7140 ppm and 4.7112 ppm, respectively, indicating that part of D₂O in the solvated structure of hydrated zinc ions were replaced by Cy molecules. Furthermore, as shown in Fig. S12b–d (ESI†), FT-IR experiments also show that the addition of Cy regulates the solvated shell structure of hydrated Zn²⁺. The tensile vibration of SO₄²⁻ (1078–1092 cm⁻¹) shows a significant blue shift with the increase in the level of Cy, as shown in Fig. S12b (ESI†), indicating that the strong hydrogen bond interaction of H₂O and the significant weakening of SO₄²⁻ confinement weaken the electrostatic coupling between Zn²⁺ and SO₄²⁻, thus affecting the original solvated shell of Zn²⁺. Moreover, from Fig. S12c (ESI†), with the increase in Cy content, the stretching vibration of water molecules shows a blue shift, indicating that water molecules in the electrolyte increase. Fig. S12d (ESI†) shows that as the Cy content increases, the O–H stretching band of the water moves to a higher wavenumber due to the weakened hydrogen bond between the water molecules. From the Raman spectra in Fig. S13a (ESI†), the O–H stretching vibration at 3200–3650 cm⁻¹ shifts to a higher wave number after the addition of Cy, indicating that hydrogen bonds are formed between Cy molecules and water molecules, while the original hydrogen bond network is destroyed, thus weakening the activity of water. In Fig. S13b (ESI†), the vibration of the S–O symmetric stretching band of the SO₄²⁻ anion located between 970 and 1020 cm⁻¹ has undergone a blue shift after the addition of Cy molecules, indicating the formation of a contact ion pair (Zn²⁺–OSO₃²⁻). The OSO₃²⁻ participates in the inner layer of the Zn²⁺ solvation sheath by closely binding with Zn²⁺. The formation of contact ion pairs indicates that the interaction between Cy and water molecules is stronger than that between Zn²⁺ and water molecules, regulating the solvation sheath structure of Zn²⁺. To sum up, the Cy molecules can replace some of the chemically active H₂O molecules by entering the solvated shell of hydrated Zn²⁺ and isolate the contact between the active water molecules and the zinc anode, which effectively reduce the hydrogen evolution reaction and significantly inhibit the zinc dendrites and side reactions during electrochemical reactions. The adsorption capacities of H₂O or Cy on the Zn(002) crystal plane were calculated and compared by density functional theory (Fig. 5(h) and Fig. S14, ESI†). The adsorption energy of water molecules on the plane of Zn(002) is −26.05 kcal mol⁻¹, while the adsorption energy of Cy molecules on the plane of Zn(002) is −190.08 kcal mol⁻¹, indicating that the Cy molecules are reverently adsorbed onto the surface of Zn(002) compared to H₂O, thus guiding the homogeneous deposition of Zn²⁺. By studying the molecular orbital levels of different ligands (Fig. S15, ESI†), the highest occupied molecular orbital (HOMO) of Cy is higher than that of water molecules (−6.01641 eV vs. −8.91572 eV), indicating that the Cy additive exhibits a higher tendency to undergo electron loss upon adsorption onto a Zn foil, thereby facilitating the formation of efficient charge transport pathways.^46–48

Fig. 5 (a) and (c) 3D snapshots of ZS and Cy obtained via MD simulation, (b) and (d) RDFs of Zn²⁺–O(H₂O) in ZS and Cy electrolytes, (e) binding energy of Zn with different compounds (H₂O and Cy), (f) RDFs of Zn²⁺–N(Cy) obtained from the MD simulation of Cy electrolyte, (g) electrostatic potential data for [Zn(H₂O)₆]²⁺ and [Zn(H₂O)₂Cy]²⁺, and (h) adsorption energy of H₂O and Cy on the Zn(002) crystal surface.

To prove the practicability of the Cy electrolyte additive, the electrochemical performance of the Zn||VO₂ full battery assembled with ZnSO₄ electrolytes without or with 10 g L⁻¹ Cy using VO₂ nanosheets as the cathode, zinc foil as the anode and glass fiber as the diaphragm is shown in Fig. 6 and Fig. S16–S21 (ESI†). Fig. S16 (ESI†) shows the SEM image and XRD pattern of VO₂, indicating the successful synthesis of VO₂. In the ZS or Cy-added ZS electrolytes, the CV curves of the Zn||VO₂ full cells show similar redox peaks, indicating that the addition of 10 g L⁻¹ Cy had no effect on the redox reaction of Zn and VO₂ (Fig. 6(a)). The specific capacity of the Zn||VO₂ battery is much higher in the 10 g L⁻¹ Cy-added electrolyte than in the ZS electrolyte, as shown in Fig. 6(b)–(d). The specific capacities of complete batteries assembled with 10 g L⁻¹ Cy as electrolyte are 217, 202, 170, 126, 84, 130, 174, 209 and 258 mA h g⁻¹ at current densities of 0.5, 1, 2, 5, 10, 5, 2, 1 and 0.5 A g⁻¹, significantly larger than those in the ZS electrolyte (189, 139, 106, 70, 41, 70, 111, 142 and 170 mA h g⁻¹ at 0.5, 1, 2, 5, 10, 5, 2, 1 and 0.5 A g⁻¹, respectively). As shown in Fig. 6(e), the impedance of the battery does not change much after adding 10 g L⁻¹ Cy. As shown in Fig. S17 (ESI†), after adding 10 g L⁻¹ Cy, the impedance of the battery with different cycles does not change much. Fig. 6(f) shows the long-term cycling performance of the Zn||VO₂ full cells without/with 10 g L⁻¹ Cy at 5 A g⁻¹. After 2000 cycles, the capacity retention rate of the battery containing 10 g L⁻¹ Cy is 75.5%, which is significantly higher than that in the ZS electrolyte (24.9%). Fig. S18 (ESI†) shows the SEM image of the zinc anode of the full battery after 100 cycles. In the ZS electrolyte, the anode presents a dendritic cluster surface, showing severe dendrite growth and side reactions during the electrochemical reaction, corresponding to XRD (Fig. S19, ESI†), while in the 10 g L⁻¹ Cy electrolyte, the Zn anode of the full cell presents a very flat, dense and smooth surface after 100 cycles. Fig. S20 and S21 (ESI†) show the XRD and SEM images of the full cell cathode after 100 cycles. The morphology of the VO₂ cathode material did not change significantly after cycling in the Cy electrolyte. However, since Cy molecules are larger than water molecules, they may have an impact on the positive electrode material when plating/stripping.

Fig. 6 Electrochemical performance of Zn||VO₂ full cells: (a) cyclic voltammetry curves in electrolytes without/with 10 g L⁻¹ Cy at a scan rate of 0.1 mV s⁻¹; (b) rate performance at current densities ranging from 0.5 to 10 A g⁻¹; (c) and (d) charge–discharge curves in ZS electrolytes without/with 10 g L⁻¹ Cy at different current densities; (e) EIS plots in different electrolytes; (f) long-term cycling performance in ZS electrolytes without/with 10 g L⁻¹ Cy at 5 A g⁻¹.

3. Conclusions

In summary, the cyclic organic compound 1,4,7,10-tetraazecyclododecane (Cy), a non-toxic additive, has been proposed as an advanced electrolyte additive for AZIBs. Cy can form coordination complexes with Zn²⁺ ions and enter the solvation shell of hydrated Zn²⁺, replacing some chemically active H₂O molecules; moreover, Cy has a higher adsorption energy on the Zn anode than water, and thus, can be preferentially adsorbed onto the surface of the zinc anode to isolate water molecules from Zn metal. Thanks to the effective modulation of the solvation structure of the hydrated Zn²⁺ and Zn metal/electrolyte interface caused by Cy, zinc dendrite growth and side reactions during electrochemical reactions were greatly suppressed. Therefore, a cycle life exceeding 4000 h for a Zn||Zn symmetric battery in 10 g L⁻¹ Cy-added ZS electrolyte at 2 mA cm⁻²/1 mA h cm⁻² was achieved, while a stable cycling performance over 3000 cycles with an average CE of 99.45% was attainable for a Zn||Cu asymmetric battery in 10 g L⁻¹ Cy-added ZS electrolyte. Moreover, for a Zn||VO₂ full battery with a Cy-added ZS electrolyte, a capacity retention rate of 75.5% was obtained after 2000 cycles. This work shows that 1,4,7,10-tetraazecyclododecane is a high-efficiency electrolyte additive to suppress dendrite growth and side reactions on the surface of zinc metal by regulating the solvated structure of Zn²⁺ and the interface of Zn metal and the electrolyte.

Author contributions

Xiaoqin Zhang: investigation, methodology, conceptualization, writing – original draft, software. Haoran Lang: software, visualization, formal analysis. Chao Li: software. Min Li: visualization. Bin Xie: data curation, visualization. Ji Chen: visualization. Yuxiang Che: formal analysis. Yu Huo: software, formal analysis. Lin Li: software, visualization. Qiaoji Zheng: software, formal analysis. Xin Tan: software, formal analysis. Heng Zhang, software. Dunmin Lin: resources, writing – review & editing, supervision.

Data availability

The authors will provide the relevant data in response to reasonable request.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by Sichuan Science and Technology Program (2023NSFSC0439). The authors also appreciate the Shiyanjia Lab (www.shiyanjia.com) for the SEM tests. We appreciate eceshi (https://www.eceshi.com) for the FI-TR tests. The authors also appreciate the Beijing Nordson Technology Co., LTD (https://www.kexingtest.com) for the SEM tests.

References

1. J. Yang, B. Yin, Y. Sun, H. Pan, W. Sun, B. Jia, S. Zhang and T. Ma, Zinc Anode for Mild Aqueous Zinc-Ion Batteries: Challenges, Strategies, and Perspectives, Nano-Micro Lett., 2022, 14, 42.
2. Y. Bai, L. Luo, W. Song, S. Man, H. Zhang and C. M. Li, Nitrogen-Vacancy-Rich VN Clusters Embedded in Carbon Matrix for High-Performance Zinc Ion Batteries. Advanced, Science, 2024, 11(19), 2308668.
3. Y. Bai, Y. Qin, J. Hao, H. Zhang and C. M. Li, Advances and Perspectives of Ion-Intercalated Vanadium Oxide Cathodes for High-Performance Aqueous Zinc Ion Battery, Adv. Funct. Mater., 2024, 34(11), 2310393.
4. Y. Bai, H. Zhang, H. Song, C. Zhu, L. Yan, Q. Hu and C. M. Li, Engineering anion defects of ternary VS-Se layered cathodes for ultrafast zinc ion storage, Nano Energy, 2024, 120, 109090.
5. Y. Bai, H. Zhang, W. Liang, C. Zhu, L. Yan and C. Li, Advances of Zn metal-free “Rocking-Chair”-type zinc ion batteries: recent developments and future perspectives, Small, 2024, 20(8), 2306111.
6. J. Wang, Y. Yamada, K. Sodeyama, E. Watanabe, K. Takada, Y. Tateyama and A. Yamada, Fire-extinguishing organic electrolytes for safe batteries, Nat. Energy, 2017, 3(1), 22–29.
7. B. Liu, X. Yuan and Y. Li, Colossal Capacity Loss during Calendar Aging of Zn Battery Chemistries, ACS Energy Lett., 2023, 8(9), 3820–3828.
8. J. Yang, B. Yin, Y. Sun, H. Pan, W. Sun, B. Jia, S. Zhang and T. Ma, Zinc Anode for Mild Aqueous Zinc-Ion Batteries: Challenges, Strategies, and Perspectives, Nano-Micro Lett., 2022, 14(1), 42.
9. Y. Li, Y. Liu, J. Chen, Q. Zheng, Y. Huo, F. Xie and D. Lin, Polyaniline intercalation induced great enhancement of electrochemical properties in ammonium vanadate nanosheets as an advanced cathode for high-performance aqueous zinc-ion batteries, Chem. Eng. J., 2022, 448, 137681.
10. P. Ruan, S. Liang, B. Lu, H. J. Fan and J. Zhou, Design Strategies for High-Energy-Density Aqueous Zinc Batteries, Angew. Chem., Int. Ed., 2022, 61(17), e202200598.
11. D. Wang, Q. Li, Y. Zhao, H. Hong, H. Li, Z. Huang, G. Liang, Q. Yang and C. Zhi, Insight on Organic Molecules in Aqueous Zn-Ion Batteries with an Emphasis on the Zn Anode Regulation, Adv. Energy Mater., 2022, 12(9), 2102707.
12. Y. Liu, X. Lu, F. Lai, T. Liu, P. R. Shearing, I. P. Parkin, G. He and D. J. L. Brett, Rechargeable aqueous Zn-based energy storage devices, Joule, 2021, 5(11), 2845–2903.
13. J. Cao, D. Zhang, X. Zhang, Z. Zeng, J. Qin and Y. Huang, Strategies of regulating Zn²⁺ solvation structures for dendrite-free and side reaction-suppressed zinc-ion batteries, Energy Environ. Sci., 2022, 15(2), 499–528.
14. Q. Zhang, J. Luan, Y. Tang, X. Ji and H. Wang, Interfacial Design of Dendrite-Free Zinc Anodes for Aqueous Zinc-Ion Batteries, Angew. Chem., Int. Ed., 2020, 59(32), 13180–13191.
15. Z. Xing, C. Huang and Z. Hu, Advances and strategies in electrolyte regulation for aqueous zinc-based batteries, Coord. Chem. Rev., 2022, 452, 214299.
16. J. Zhou, M. Xie, F. Wu, Y. Mei, Y. Hao, R. Huang, G. Wei, A. Liu, L. Li and R. Chen, Ultrathin Surface Coating of Nitrogen-Doped Graphene Enables Stable Zinc Anodes for Aqueous Zinc-Ion Batteries, Adv. Mater., 2021, 33(33), e2101649.
17. Z. Shi, M. Yang, Y. Ren, Y. Wang, J. Guo, J. Yin, F. Lai, W. Zhang, S. Chen, H. N. Alshareef and T. Liu, Highly Reversible Zn Anodes Achieved by Enhancing Ion-Transport Kinetics and Modulating Zn (002) Deposition, ACS Nano, 2023, 17(21), 21893–21904.
18. X. Wang, J. Meng, X. Lin, Y. Yang, S. Zhou, Y. Wang and A. Pan, Stable Zinc Metal Anodes with Textured Crystal Faces and Functional Zinc Compound Coatings, Adv. Funct. Mater., 2021, 31(48), 2106114.
19. H. Liu, J. Li, D. Wei, X. Liu, Z. Cai, H. Zhang, Z. Lv, L. Chen, H. Li, H. Luo, Y. Zhao, H. Yu, X. Wang, F. Chen, G. Zhang and H. Duan, Biomimetic Honeycomb Zn Anode Enabled Multi-Field Regulation toward Highly Stable Flexible Zn-Ion Batteries, Adv. Funct. Mater., 2023, 33(25), 2300419.
20. Y. Dai, C. Zhang, W. Zhang, L. Cui, C. Ye, X. Hong, J. Li, R. Chen, W. Zong, X. Gao, J. Zhu, P. Jiang, Q. An, D. J. L. Brett, I. P. Parkin, G. He and L. Mai, Reversible Zn Metal Anodes Enabled by Trace Amounts of Underpotential Deposition Initiators, Angew. Chem., Int. Ed., 2023, 62(18), e202301192.
21. C. Chang, S. Hu, T. Li, F. Zeng, D. Wang, S. Guo, M. Xu, G. Liang, Y. Tang, H. Li, C. Han and H.-M. Cheng, A robust gradient solid electrolyte interphase enables fast Zn dissolution and deposition dynamics, Energy Environ. Sci., 2024, 17(2), 680–694.
22. R. Wang, Q. Ma, L. Zhang, Z. Liu, J. Wan, J. Mao, H. Li, S. Zhang, J. Hao, L. Zhang and C. Zhang, An Aqueous Electrolyte Regulator for Highly Stable Zinc Anode Under −35 to 65 °C, Adv. Energy Mater., 2023, 13(40), 2302543.
23. M. Peng, X. Tang, K. Xiao, T. Hu, K. Yuan and Y. Chen, Polycation-Regulated Electrolyte and Interfacial Electric Fields for Stable Zinc Metal Batteries, Angew. Chem., Int. Ed., 2023, 62(27), e202302701.
24. Y. Liang, M. Qiu, P. Sun and W. Mai, Comprehensive Review of Electrolyte Modification Strategies for Stabilizing Zn Metal Anodes, Adv. Funct. Mater., 2023, 33(51), 2304878.
25. Z. Zhang, Y. Zhang, M. Ye, Z. Wen, Y. Tang, X. Liu and C. C. Li, Lithium Bis(oxalate)borate Additive for Self-repairing Zincophilic Solid Electrolyte Interphases towards Ultrahigh-rate and Ultra-stable Zinc Anodes, Angew. Chem., Int. Ed., 2023, 62, e202311032.
26. H. Li, R. Zhao, W. Zhou, L. Wang, W. Li, D. Zhao and D. Chao, Trade-off between Zincophilicity and Zincophobicity: Toward Stable Zn-Based Aqueous Batteries, JACS Au, 2023, 3(8), 2107–2116.
27. Y. F. Tian, S. J. Tan, Z. Y. Lu, D. X. Xu, H. X. Chen, C. H. Zhang, X. S. Zhang, G. Li, Y. M. Zhao, W. P. Chen, Q. Xu, R. Wen, J. Zhang and Y. G. Guo, Insights into Anion-Solvent Interactions to Boost Stable Operation of Ether-Based Electrolytes in Pure-SiO_x||LiNi_0.8Mn_0.1Co_0.1O₂ Full Cells, Angew. Chem., Int. Ed., 2023, 62(33), e202305988.
28. Z. Liu, R. Wang, Y. Gao, S. Zhang, J. Wan, J. Mao, L. Zhang, H. Li, J. Hao, G. Li, L. Zhang and C. Zhang, Low-Cost Multi-Function Electrolyte Additive Enabling Highly Stable Interfacial Chemical Environment for Highly Reversible Aqueous Zinc Ion Batteries, Adv. Funct. Mater., 2023, 33(49), 2308463.
29. A. Zhou, H. Wang, F. Zhang, X. Hu, Z. Song, Y. Chen, Y. Huang, Y. Cui, Y. Cui, L. Li, F. Wu and R. Chen, Amphipathic Phenylalanine-Induced Nucleophilic–Hydrophobic Interface Toward Highly Reversible Zn Anode, Nano-Micro Lett., 2024, 16(1), 1–15.
30. F. Wu, Y. Chen, Y. Chen, R. Yin, Y. Feng, D. Zheng, X. Xu, W. Shi, W. Liu and X. Cao, Achieving Highly Reversible Zinc Anodes via N,N-Dimethylacetamide Enabled Zn-Ion Solvation Regulation, Small, 2022, 18(27), 2202363.
31. C. You, R. Wu, X. Yuan, L. Liu, J. Ye, L. Fu, P. Han and Y. Wu, An inexpensive electrolyte with double-site hydrogen bonding and a regulated Zn²⁺ solvation structure for aqueous Zn-ion batteries capable of high-rate and ultra-long low-temperature operation, Energy Environ. Sci., 2023, 16(11), 5096–5107.
32. C. Meng, W. He, L. Jiang, Y. Huang, J. Zhang, H. Liu and J. J. Wang, Ultra-Stable Aqueous Zinc Batteries Enabled by β-Cyclodextrin: Preferred Zinc Deposition and Suppressed Parasitic Reactions, Adv. Funct. Mater., 2022, 32(47), 2207732.
33. W. Fan, P. Li, J. Shi, J. Chen, W. Tian, H. Wang, J. Wu and G. Yu, Atomic Zincophilic Sites Regulating Microspace Electric Fields for Dendrite-Free Zinc Anode, Adv. Mater., 2023, 36(1), 2307219.
34. L. Cao, D. Li, E. Hu, J. Xu, T. Deng, L. Ma, Y. Wang, X. Q. Yang and C. Wang, Solvation Structure Design for Aqueous Zn Metal Batteries, J. Am. Chem. Soc., 2020, 142(51), 21404–21409.
35. X. Bai, Y. Nan, K. Yang, B. Deng, J. Shao, W. Hu and X. Pu, Zn Ionophores to Suppress Hydrogen Evolution and Promote Uniform Zn Deposition in Aqueous Zn Batteries, Adv. Funct. Mater., 2023, 33(42), 2307595.
36. Z. Li, Z. Shu, Z. Shen, Y. Liu, Y. Ji, L. Luo, R. Li, Y. Cai, H. Ian, J. Xie and G. Hong, Dissolution Mechanism for Dendrite-Free Aqueous Zinc-Ions Batteries, Adv. Energy Mater., 2024, 2400572.
37. J. Zhang, Y. Liu, Y. Wang, Z. Zhu and Z. Yang, Zwitterionic Organic Multifunctional Additive Stabilizes Electrodes for Reversible Aqueous Zn-Ion Batteries, Adv. Funct. Mater., 2024, 2401889.
38. K. Qiu, G. Ma, Y. Wang, M. Liu, M. Zhang, X. Li, X. Qu, W. Yuan, X. Nie and N. Zhang, Highly Compact Zinc Metal Anode and Wide-Temperature Aqueous Electrolyte Enabled by Acetamide Additives for Deep Cycling Zn Batteries, Adv. Funct. Mater., 2024, 2313358.
39. S. Yang, A. Chen, Z. Tang, Z. Wu, P. Li, Y. Wang, X. Wang, X. Jin, S. Bai and C. Zhi, Regulating the electrochemical reduction kinetics by the steric hindrance effect for a robust Zn metal anode, Energy Environ. Sci., 2024, 17(3), 1095–1106.
40. Y. Xia, R. Tong, J. Zhang, M. Xu, G. Shao, H. Wang, Y. Dong and C.-A. Wang, Polarizable Additive with Intermediate Chelation Strength for Stable Aqueous Zinc-Ion Batteries, Nano-Micro Lett., 2024, 16(1), 82.
41. X. Zhao, Y. Wang, C. Huang, Y. Gao, M. Huang, Y. Ding, X. Wang, Z. Si, D. Zhou and F. Kang, Tetraphenylporphyrin-based Chelating Ligand Additive as a Molecular Sieving Interfacial Barrier toward Durable Aqueous Zinc Metal Batteries, Angew. Chem., Int. Ed., 2023, 62(46), e202312193.
42. S. Wang, Y. Ying, S. Chen, H. Wang, K. K. K. Cheung, C. Peng, H. Huang, L. Ma and J. A. Zapien, Highly reversible zinc metal anode enabled by zinc fluoroborate salt-based hydrous organic electrolyte, Energy Storage Mater., 2023, 63, 102971.
43. X. Wang, K. Feng, B. Sang, G. Li, Z. Zhang, G. Zhou, B. Xi, X. An and S. Xiong, Highly Reversible Zinc Metal Anodes Enabled by Solvation Structure and Interface Chemistry Modulation, Adv. Energy Mater., 2023, 13(36), 2301670.
44. X. Wu, Y. Xia, S. Chen, Z. Luo, X. Zhang, Y. Lu, H. Pan, B. B. Xu, M. Yan and Y. Jiang, Transient Zwitterions Dynamics Empowered Adaptive Interface for Ultra-Stable Zn Plating/Stripping, Small, 2023, 20(8), 2306739.
45. K. Xiao, L. Yang, M. Peng, X. Jiang, T. Hu, K. Yuan and Y. Chen, Unlocking the Effect of Chain Length and Terminal Group on Ethylene Glycol Ether Family Toward Advanced Aqueous Electrolytes, Small, 2023, 20(12), 2306808.
46. P. Sun, L. Ma, W. Zhou, M. Qiu, Z. Wang, D. Chao and W. Mai, Simultaneous Regulation on Solvation Shell and Electrode Interface for Dendrite-Free Zn Ion Batteries Achieved by a Low-Cost Glucose Additive, Angew. Chem., Int. Ed., 2021, 60(33), 18247–18255.
47. J. Yang, H. Yan, H. Hao, Y. Song, Y. Li, Q. Liu and A. Tang, Synergetic Modulation on Solvation Structure and Electrode Interface Enables a Highly Reversible Zinc Anode for Zinc–Iron Flow Batteries, ACS Energy Lett., 2022, 7(7), 2331–2339.
48. B. Xie, Q. Hu, X. Liao, X. Zhang, H. Lang, R. Zhao, Q. Zheng, Y. Huo, J. Zhao, D. Lin and X. L. Wu, Multifunctional Electrolyte toward Long-Life Zinc-Ion Batteries: Synchronous Regulation of Solvation, Cathode and Anode Interfaces, Adv. Funct. Mater., 2023, 34(6), 2311961.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03833a

---