Enabling highly reversible Zn anode via an interfacial preferentially adsorbed additive containing nucleophilic groups

INTRODUCTION

Currently, constructing a cleaner and more efficient society requires greater utilization of renewable energy sources to replace fossil fuels^[1-3]. Due to the unstable nature of renewable energy, there is an urgent need to develop large-scale electrochemical energy storage systems (ESS) to cope with grid fluctuations^[4-7]. Considering the strict requirements for safety and economy, aqueous zinc-ion batteries (AZIBs) are considered the most promising alternatives for grid energy storage. The Zn anode possesses core advantages including environmental friendliness, low cost, high energy density (820 mAh g^-1), and appropriate redox potential (-0.76 V vs. the standard hydrogen electrode)^[8-11]. However, the cycle lifespan of the zinc metal anode is seriously insufficient to satisfy the demands of commercial application, which is owing to the numerous complex reactions occurring at the interface of zinc metal anode in aqueous electrolytes^[12,13]. It is well known that during electrodeposition, the Zn anode undergoes uneven initial nucleation of zinc. Under the “tip effect”, zinc dendrites grow rapidly, eventually piercing the separator and causing a sudden short circuit^[14-16]. In addition, the Zn anode is in a thermodynamically unstable state in aqueous solutions, where active water molecules in the electrolyte can compete with Zn²⁺ cations to capture electrons, leading to a hydrogen evolution reaction (HER)^[17-19]. The occurrence of HER is accompanied by a local increase in pH at the anode surface, which in turn leads to surface hydroxide sulfate passivation. Dendrite growth and parasitic side reactions resonate with each other, severely limiting the practicality of AZIBs as a feasible and effective ESS.

The deposition morphology and uniformity of zinc are determined by the deposition/dissolution behavior of Zn^[20,21]. Altering the chemical environment at the surface of zinc metal and regulating the distribution and transport of Zn²⁺ at the anode/electrolyte interface (AEI) are key to continuously optimizing the morphological evolution of the zinc metal anode^[22-24]. Unlike traditional methods of constructing an artificial solid electrolyte interface (SEI) on zinc metal to regulate the deposition kinetics of Zn²⁺, the use of electrolyte optimization strategies has been proven to be a simple and direct approach to control the deposition behavior of Zn^2+[25,26]. Some organic compounds containing polar groups, such as alcohols, ketones, and ethers, can optimize the deposition behavior of zinc and suppress side reactions by altering the solvation structure of zinc ions or reconstructing the hydrogen bonding network in the electrolyte^[27,28]. Recently, ester-based organic molecules, due to their strong adsorption capacity for zinc, have been used to regulate the interfacial chemistry of the zinc metal anode^[29]. For example, Wang et al. used the interaction of trimethyl phosphate (TMP) molecules to increase the electron density of water protons, thereby suppressing water activity and enabling symmetrical cells to operate for up to 1,600 h^[3]. Liu et al. constructed a mixture solvent using propylene carbonate (PC) and aqueous solution, effectively reducing the free water molecules in the Zn²⁺-solvent cations, enabling the full cells to exhibit a high specific capacity of 183 mAh g^-1 over 300 cycles^[30]. However, current research on ester components mostly involves adding them as co-solvents to aqueous electrolytes, which significantly increases the cost of the electrolyte and decreases the ionic conductivity. Therefore, exploring the use of ester molecules in trace addition, and elucidating the underlying mechanisms by which ester molecules regulate the zinc deposition process is crucial for the application of ester organic compounds in AZIBs.

In this study, we present methyl acetoacetate (MA), a water-soluble ester, as a novel additive for AZIBs to improve the stability and reversibility of zinc electrodes. The calculated and experimental results indicate that the MA molecule possesses two oxygen atoms with high charge density, which can preferentially adsorb at AEI, effectively controlling the nucleation and diffusion processes of zinc ions. This molecular adsorption layer also promotes the growth of the preferred Zn (002) crystal facet and limits the generation of zinc dendrites. Furthermore, the adsorbed MA molecules create an electrical double layer (EDL) with reduced water content at AEI. This unique water-poor EDL prevents direct contact between zinc foils and active H₂O, restraining the Zn corrosion reaction [Scheme 1]. As a consequence, modulated Zn deposition behaviors by MA additives greatly prolong the cycling lifespan of symmetric cells over 1,500 h (2 mA cm^-2 with a capacity of 2 mAh cm^-2) and the Zn//Cu cell achieves stable cycling for 1,400 cycles at 2 mA cm^-2. Moreover, at 1 A g^-1, the Zn//NVO full cell using MA exhibits an improved reversible specific capacity retention of 92.8% after 600 cycles. This research offers a guideline of utilizing ester-based additives to enable highly reversible Zn anodes for high-performance AZIBs.

Scheme 1. Zn plating behavior under the effect of MA additive.

MATERIALS AND METHODS

RESULTS AND DISCUSSION

The molecular structure of MA with two nucleophilic carbonyl groups is shown in Figure 1A. The surface ESP is utilized to investigate the contribution of MA molecules to electrochemical processes. As displayed in Figure 1B, the results evidence that electronegative carbonyl oxygen atoms in MA molecules exhibit more negative ESP value than other atoms, suggesting that zinc metal anode and positively charged Zn²⁺ ions interact more strongly with nucleophilic carbonyl groups on MA. This interaction favors the adsorption of MA onto the zinc foil surface in the electrolyte, thus modulating the distribution, transmission, and deposition dynamics of Zn²⁺ at the AEI. To explore the adsorption behavior of MA on zinc anode, the Eads of MA molecules, Zn atoms and H₂O molecules on the Zn (002) crystal plane was investigated via DFT calculations [Figure 1C], and the corresponding optimized structures are presented in Figure 1D. MA molecules demonstrate an Eads of -3.56 eV on the Zn (002) plane, which is much lower than that of Zn atoms (-2.79 eV) and H₂O molecules (-3.02 eV), proving the domain occupation of MA at AEI, which manipulates the kinetics of Zn²⁺ ions and reduces the proportion of adsorbed water molecule. In addition, the elemental composition and functional groups on the electrode reveal the adsorption of MA in XPS characterization and Raman spectrum. Supplementary Figure 1 shows the full survey spectra of the zinc foils soaked in different electrolytes for five days. In the high-resolution C 1s spectra [Figure 1E], C-C, C-O and C=O species can be detected for the zinc metal anode soaked in the solution with MA. Besides, the O 1s spectra in Figure 1F show a higher proportion of characteristic peaks of C=O and C-O bonds, which verify the presence of MA molecules on the anode surface. As depicted in the Raman spectra [Figure 1G], the characteristic peaks of MA can be observed for the zinc foil soaked in MA-contained electrolyte, where the -CH₃ band at 2,963 cm^-1, -CH₂ at 2,935 cm^-1, and -COOH at 1,735 cm^-1 are consistent with the molecular structure of MA, indicating the strong adsorption between the MA molecules and zinc anode surfaces.

Figure 1. (A) Molecular structure and (B) electrostatic potential mapping of MA. (C) The adsorption energy of the zinc atom, water molecule and MA molecule on Zn (002) crystal plane. The corresponding structures of the (D) MA molecule, Zn atom and water molecule adsorbed on zinc. (E) C 1s, (F) N 1s XPS spectrum, and (G) Raman spectra of zinc foils immersed in electrolyte with/without MA for 5 days.

Because an electrolyte with 10 mM MA can maximize the cycling life of symmetric cells, this concentration of MA additives was selected as the optimal condition for the optimized electrolyte [Supplementary Figure 2]. Furthermore, some basic kinetics of interfacial stability for the electrolyte were conducted. First, the contact angle measurements were executed. As displayed in Figure 2A, with the addition of MA, the contact angle is reduced. This result demonstrates that the MA molecules significantly influenced the AEI and enhanced the wettability to the anode, which is crucial for achieving a uniform distribution of zinc ions. The nucleation overpotential is inherently connected to the formation of critical atom clusters. In Figure 2B, the overpotential for zinc nucleation at the zinc electrode in the MA-containing electrolyte is markedly higher than that observed in pure ZnSO₄ electrolyte. This indicates a substantial driving force that favors the formation of finer Zn nuclei and a greater nuclei density, which is further supported by the collected CV curves of symmetric cells using different electrolytes [Figure 2C]. It is generally believed that the formation of fine Zn nuclei is beneficial for homogeneous and dense Zn deposition, which can further enhance the cycling stability of zinc electrodes^[31,32]. The coulombic efficiency (CE) value is used to characterize the reversibility of zinc anode during repeated deposition/dissolution processes. As presented in Figure 2D and E, at 6 mA cm^-2, the Zn//Cu asymmetric cell with MA maintains a higher and more stable CE in 800 cycles. On the contrary, in the electrolyte without MA, regular deposition/dissolution can only sustain less than 110 cycles. After 100 cycles, the voltage profile becomes erratic and the CE drops dramatically, indicating the presence of an internal short circuit [Supplementary Figure 3]. Additionally, the Zn//Cu asymmetric battery using MA-contained electrolyte performs steadily for over 1,400 cycles [Figure 2F] at a lower current density of 2 mA cm^-2. These results indicate that the stability and reversibility of zinc deposition/dissolution are significantly enhanced.

Figure 2. (A) Contact angle measurements of different electrolytes on zinc metal foil. (B) Nucleation overpotential at 1 mA cm^-2 using different electrolytes. (C) CV curves of symmetric cells in different electrolytes. CE of asymmetric cells in different electrolytes at (D) 6 mA cm^-2 and (F) 2 mA cm^-2. (E) Voltage profiles of Zn//Cu asymmetric cell with MA at 6 mA cm^-2.

We believe that the nucleophilic carbonyl groups in MA act as an excellent hydrogen bond acceptor. Consequently, when MA molecules are adsorbed onto the Zn electrode surface, they can establish a MA-H₂O H-bond network. This network would confine water molecules and restrict their intermolecular interactions, thereby reducing water activity and restraining the decomposition of water^[33,34]. To confirm this hypothesis, we conducted further analysis using Fourier transform infrared (FTIR) spectroscopy to track the evolution of hydrogen bonds within the electrolyte and to assess the ability of MA to inhibit corrosion of anode. As shown in the FTIR spectra [Figure 3A-C], the introduction of MA results in a redshift of the H-O stretching vibrations at 3,000-3,400 cm^-1. This shift signifies a coordination interaction between H₂O and MA, weakening the H-bond network among H₂O molecules. As a result, the number of active H₂O molecules can be reduced, which in turn suppresses parasitic reactions triggered by decomposition of water. Simultaneously, the addition of MA causes a noticeable blue shift in the stretching vibration of SO₄^2- at approximately 1,100 cm^-1 in the FTIR spectra, which suggests a weaker interaction between Zn²⁺ and SO₄^2- mediated by MA^[35-37]. In addition, the linear sweep voltammetry (LSV) curves [Figure 3D] provide clear insights into the HER performance. The addition of MA causes a negative shift in the current response, which confirms the inhibition of H₂ evolution. Typically, Zn electrodes exist in a thermodynamically unstable state within aqueous solutions, leading to continuous chemical corrosion. This corrosion process results in electrode passivation, which in turn diminishes the electrical conductivity^[38-40]. Corrosion suppression capability of MA additive is also validated by immersing zinc foils in electrolytes with/without MA [Figure 3E and Supplementary Figure 4]. After only 5 days of immersion in a blank electrolyte, the Zn electrode becomes dark and turns from silvery white into black. As certified by the SEM images, it shows an extremely bumpy surface with an accumulation of uneven flakes. Conversely, the surface of the zinc electrode soaked in MA-contained electrolyte remains flat with a metallic luster, and only a few nanoparticles are visible in the SEM image. Furthermore, XRD patterns were utilized to examine the composition of by-products at the zinc foil surfaces. As illustrated in Supplementary Figure 5, the loosely and haphazardly accumulated flake layers are identified as primarily Zn₄SO₄(OH)₆·5H₂O by-product, and the Zn anode exhibits significantly stronger diffraction intensity in the MA-free electrolyte. During the cycling process, due to the coexistence of electrochemical corrosion and chemical corrosion, the corrosion on the anode surface will be more severe^[41-43]. It is generally assumed that among the various crystal planes in the hexagonal close-packed structure, the (002) plane is advantageous for the even deposition of Zn²⁺ ions along a horizontal direction^[44,45]. This crystal plane can also enhance the corrosion resistance of the anode and help curb the HER. Therefore, we also used XRD to detect the zinc electrodes after cycling in various electrolytes. As displayed in Figure 3F, the intensity ratios of (002) peak to (101) peak are 1.54 and 2.75 for Zn anode cycled in blank and MA-contained electrolyte, respectively, suggesting the increased (002) planes for zinc deposits in electrolyte with MA. Hydrogen evolution and surface passivation are common occurrences in batteries, whether during periods of inactivity or operation. To assess this, we evaluated the shelf life and recovery performance of symmetric cells under a regimen of alternating cycling and resting, as displayed in Figure 3G. The Zn//Zn cell using MA-contained electrolyte maintains stable electrochemical cycling for 2,500 h. On the contrary, the cell using the blank electrolyte shows substantial polarization voltage fluctuations, resulting in cell failure less than 200 h. Thus, the addition of MA can significantly stabilize the zinc metal electrode in aqueous electrolytes by reducing water activity and suppressing the Zn corrosion reaction.

Figure 3. (A-C) FTIR spectra of different electrolytes. (D) LSV curves using NaSO₄ solutions with/without MA. (E) Optical images, corresponding SEM images and (F) XRD patterns of zinc electrodes after 50 cycles using various electrolytes. (G) Electrochemical performance of the symmetric cells using different electrolytes when enduring alternate cycling and resting process.

Benefiting from the significant role of MA in stabilizing the zinc metal electrode, the enhanced electrochemical performance of cells using MA-contained electrolyte can be expected. When cycled at 5 mA cm^-2, the symmetric battery using MA addition achieves a long cycling life reach of 600 h [Figure 4A], while the cell in blank electrolyte displays an inferior cycle stability of less than 150 h because of the short circuit resulting from the zinc dendrites. Noticeably, at a low current density of 2 mA cm^-2, the battery with MA can steadily operate for 1,500 h [Figure 4B]. The corresponding enlarged voltage profiles are displayed in Figure 4C, demonstrating a gradually decreasing polarization voltage, due to the continuous activation of the anode surface^[46-48]. Even when cycled under a high current density with higher capacities of 10 mA cm^-2 and 5 mAh cm^-2, the excellent stability of 600 h can also be realized with the effect of MA [Supplementary Figure 6].Figure 4D and E shows the rate performance of the symmetric batteries with/without MA at different current densities from 1 to 10 mA cm^-2. The cell using a MA-contained electrolyte displays a steadier cycle profile with a smaller polarization voltage than that in the blank electrolyte. Furthermore, the limit capacity tests, conducted through a set deposition/dissolution duration of 1 h and a progressive increase in current density by 0.2 mA cm^-2, also highlight the electrochemical reversibility in the MA-containing electrolyte [Supplementary Figure 7]. These findings further confirm that the interfacial preferentially adsorbed MA can create an adsorption layer on Zn interface, which efficiently prevents zinc dendrite formation and parasitic side reactions, thereby markedly improving the reversibility and stability of zinc electrodes.

Figure 4. (A) Current-voltage profiles of the Zn//Zn symmetrical batteries with different electrolytes at 5 mA cm^-2 with 1 mAh cm^-2. (B) Enlarged voltage profiles of the symmetrical cell. (C) Current-voltage profiles of symmetrical cells with different electrolytes at 2 mA cm^-2 and 2 mAh cm^-2. (D) The rate performance of symmetrical cells with a fixed capacity of 1 mAh cm^-2. (E) Corresponding polarization voltage values of symmetrical cells at various current densities.

The enhanced electrochemical reversibility means an outstanding improvement in the stability of zinc electrodes, and also proves that the growth process of zinc dendrites is inhibited. The macroscale images of zinc metal deposition in different electrolytes are depicted in Figure 5A-F, showcasing a pronounced difference in the deposition morphology. In addition, the 3D confocal laser scanning microscopy (CLSM) images indicate that the Zn anode cycled in the MA-free electrolyte has a rough surface with high average surface roughness. Conversely, the anode cycled in the MA-containing electrolyte shows a markedly smoother and more compact surface, with a significantly lower average surface roughness. Furthermore, the Zn//NVO full batteries were assembled with electrolytes without/with MA to evaluate the practical application potential. Attributed to the enhanced reversibility of the zinc electrode in the MA-contained electrolyte, the Zn//NVO full cell delivers outstanding cycling stability with a high specific capacity of 138.4 mAh g^-1 and a high capacity retention of 92.8% at 1 A g^-1 [Figure 5G and H]. Conversely, the cell with blank electrolyte displays a fast capacity decay accompanied by a lower capacity retention of 63.2% [Figure 5G and Supplementary Figure 8], due to the uncontrolled growth of Zn dendrites in the absence of MA^[49,50]. Furthermore, the Zn//NVO full battery incorporating MA demonstrates a significantly reduced interface impedance, signifying excellent interfacial compatibility between the anode and the electrolyte [Figure 5I], which is beneficial for enhancing the cycling stability of full batteries^[51,52]. The surface morphology of zinc electrodes after fifty cycles at 1 A g^-1 was examined [Figure 5J]. It is evident that the zinc electrode cycled in the blank electrolyte exhibits numerous irregular dendrites and mossy Zn deposits. On the contrary, the zinc electrode cycled in the MA-contained electrolyte retains a compact and dense deposition structure without obvious corrosion pits. In addition, we assembled a Zn//I₂ full cell using iodine as the cathode. Surprisingly, the Zn//I₂ full cell can stably run for 300 cycles with a specific capacity of 3.97 mAh at a high loading of 25.4 mg cm^-2 [Supplementary Figure 9]. The above results underscore the effectiveness of the MA additive electrolyte strategy in stabilizing the zinc metal electrode and enabling the development of high-performance AZIBs.

Figure 5. CLSM images and the corresponding surface roughness curve of the Zn electrodes after fifty cycles in electrolytes (A-C) without and (D-F) with MA addition. Electrochemical performance of Zn//NVO full cells using electrolytes with/without MA: (G) Cycling performance at 1 A g^-1; (H) Selected GCD curves; (I) EIS curves; (J) SEM images of the anodes of full batteries after 50 cycles.

CONCLUSIONS

To summarize, we utilized MA with nucleophilic groups as an interface stabilizer to enhance the cycling performance of the Zn anode. Combining the calculation and experiment results, MA molecules preferentially adsorb on the Zn/electrolyte interface, owing to their strong affinity to the zinc metal anode. Moderate electron-withdrawing properties of carbonyl groups allow them to redistribute the charge, thereby modulating the nucleation and growth of Zn²⁺, which leads to the achievement of a uniform and compact Zn deposition layer^[53]. Moreover, the adsorbed MA molecules establish a H₂O-poor EDL structure which diminishes side reactions and surface passivation at the AEI due to the decomposition of active water. Consequently, the symmetric cell using MA-contained electrolyte delivers an enhanced stability of 1,500 h and promotes high Zn plating/striping reversibility over 1,400 cycles in Zn//Cu asymmetric cell at 2 mA cm^-2. Additionally, the Zn//NVO full cell with the incorporation of MA achieves a high capacity retention of 92.8% for 600 cycles at 1 A g^-1. This finding provides an efficient strategy for AEI through ester-based additives with nucleophilic groups for fulfilling highly reversible Zn anodes.