Stereoisomerism of multi-functional electrolyte additives for initially anodeless aqueous zinc metal batteries

Abstract

Stereoisomerism, arising from the distinctive spatial arrangements of atoms despite identical molecular formulae, often displays different chemical reactivities. Herein, we demonstrate how geometric isomerism of multifunctional electrolyte additives affects aqueous zinc metal batteries. Inspired by natural bacteria, we compared trans-butenedioic acid (fumaric acid) and its cis-isomer (maleic acid), revealing different hydrogen bonding dynamics and solvation environments, as confirmed by femtosecond transient absorption spectroscopy and computational simulations. The trans-isomer promotes the formation of favorable interfacial structures and ion pathways, improving Zn deposition reversibility and cycling stability. As a result, Zn symmetric cells showed stable plating/stripping for over 6150 h at 1 mA cm⁻² and 1 mAh cm⁻² and 1500 h at 5 mA cm⁻² and 5 mAh cm⁻². The Zn-predeposited Cu||MnVO full cells exhibited a capacity retention exceeding 70% after 1000 cycles at 2 A g⁻¹, ultimately achieving over 270 cycles for initially anodeless Cu||zincated MnVO cells at a high current density of 30 mA cm⁻². The application of the isomerism concept on the design of new electrolyte materials and associated solvated and interphasial chemistries offers a new pathway to the next generation of batteries.

Introduction

Isomers are defined as molecules or polyatomic ions with identical formulas that have different spatial arrangements of atoms. In particular, stereoisomers, where the atom connectivity remains the same, but the relative positions of atoms vary, may display different physical properties and chemical reactivities despite the same molecular formula¹. Cis–trans geometric isomers, a subset of stereoisomers, exhibit diverse configurations in a three-dimensional space, where the interactions or reactions with other molecules often proceed in different manners due to their respective electron distribution and steric hindrance, leading to the distinctive (electro)chemical kinetics and products^2,3. For instance, certain bacteria in nature possess the capability to regulate the membrane hydrophobicity through different modes of interactions by controlling the cis–trans isomerism of the carbon chain within the lipid bilayers (Fig. 1a)^4,5. So far, the application of the stereoisomerism concept on energy storage has been confined solely to the electrode materials, such as organic positive electrode^6,7 and negative electrode^8,9. However, the regulation of the solvation structure of working ions and the associated interaction chemistry by isomeric molecular structures in the electrolyte field has yet to be explored.

Fig. 1: Concept and interaction chemistry by stereoisomers.

a Schematic illustration of the multifunctional effects of the FU additive in ZMBs. White, gray, and red spheres represent H, C, and O atoms, respectively. b Binding energy of Zn²⁺ with H₂O, FU⁻, FU²⁻, MA⁻. Pink, gray, light blue, and dark blue spheres represent H, C, O, and Zn atoms, respectively. c Radial distribution functions and corresponding coordination numbers of Zn²⁺–O (FU/MA) in ZnSO₄/FU and ZnSO₄/MA. d Electrostatic potential maps of Zn²⁺(H₂O)₄FU²⁻ and Zn²⁺(H₂O)₅MA⁻. Pink, gray, light blue, and dark blue spheres represent H, C, O, and Zn atoms, respectively. e FT-IR spectra, f ¹H NMR spectra, and g ¹³C NMR spectra of ZnSO₄, ZnSO₄/FU and ZnSO₄/MA.

Inspired by the fact that cis–trans isomers exhibit different effects on the solvation structure and hydrogen bonding (HB) networks, it is postulated that the electrochemical properties and corresponding interface of aqueous electrolytes could be modulated by the stereotype of isomers^10,11. Herein, we demonstrate the stereoisomerism of butenedioic acid as a multifunctional electrolyte additive, bringing distinct effects on aqueous zinc metal batteries (ZMBs). Recently, aqueous ZMBs have received significant attention as next-generation batteries owing to their high theoretical capacities (820 mAh g⁻¹ and 5855 mAh cm⁻³ as normalized against Zn⁰ negative electrode), safety, and low cost^12,13. Since the redox potential (−0.76 V vs SHE) of Zn negative electrode is compatible with the electrochemical stability window of most aqueous electrolytes, ZMBs could be made safe and cheap by manufacturing under a mild environment that requires no stringent dry atmosphere¹⁴. However, the practical deployment of aqueous ZMB is still prevented by Zn dendrite formation, sustained corrosion, and the accompanying hydrogen evolution reactions (HER), which deteriorate the electrochemical performance and cycling stability of ZMBs^15,16,17. To address these challenges, electrolyte additive engineering has been investigated as a cost-effective and efficient approach. Some additives were designed to participate in manipulating either the solvation structure to lower water activity or the charge transfer process through the reduced desolvation energy of Zn(H₂O)₆^2+18,19,20. Others mitigate “tip effects” on the Zn negative electrode via electrostatic shielding or interfacial adsorption, thereby curbing the growth of dendrites^21,22,23. Functional additives are also explored with the purpose of forming a solid-electrolyte-interphase (SEI) on the Zn negative electrode, thus improving the plating/stripping efficiency^24,25,26.

In order to resolve the aforementioned limitations without sacrificing the intrinsic merits of the aqueous electrolytes, in this work, multifunctional additives are rationally designed. We chose geometric isomers of butenedioic acid, such as trans fumaric acid (FU) and cis maleic acid (MA), as a proof of concept, where the solvation and interfacial structures are expected to differ due to the difference in ion-additive and water-additive interactions^27,28. That is, distinctive polarities and configurations of two isomers through the cis–trans geometries around the C=C double bond confer upon FU and MA to display different behaviors in their HB interaction, zincophilicity, and SEI chemistry (Fig. 1a)²⁹. In particular, the trans-isomer FU shields the interferences of intramolecular HB network, enables two-step ionization, and generates divalent FU²⁻ ions, which result in attenuating water activity, achieving strong zincophilicity, and participating in the modified solvation structure of Zn²⁺. Furthermore, this multifunctional FU additive achieves the stable formation of ion channels and ionic conductive SEIs, facilitating the charge transfer reaction and preventing unnecessary side reactions. Leveraging the dual-ionization capability of FU, the Zn||Zn symmetric cells were stably operated over 6150 h at 1 mA cm⁻² and 1 mAh cm⁻², and over 1500 h at 5 mA cm⁻² and 5 mAh cm⁻². Pairing with manganese vanadate (MnVO) positive electrodes, our ZMB full cells achieved robust durability and reversibility, showcasing a high capacity retention of 82.5% over 2000 cycles at 2 A g⁻¹. Notably, the Zn-predeposited Cu||MnVO full cells demonstrated a high capacity retention of >70% and Coulombic efficiency (CE) of nearly 100% even after 1000 cycles at 2 A g⁻¹, and ultimately achieved over 270 cycles for high mass loading (15 mg cm⁻²) initially anodeless full cells at a high current density of 30 mA cm⁻².

Results

Stereoisomeric induction of regional regulation divergence

The solubilities of FU and MA according to dosage have been exploited to understand the effect of cis–trans isomerism on the chemical property, as shown in the digital images of ZnSO₄ (Supplementary Fig. 1). The two carboxyl groups of trans FU are distributed on the opposite side of the C=C double bond, leading to a smaller molecular polarity and dipole moment than those of MA. According to the like-dissolves-like rule and high polarity, the solubility of MA in water is greater than that of FU. When the concentration of FU reached 40 mM, solubility could be saturated, so further addition of FU led to a macroscopic phase separation of the electrolyte solution. On the other hand, MA was completely dissolved owing to its good solubility, even when it was added up to 400 mM. This also affects the pK_a values. The pK_a1 and pK_a2 values of FU are 3.02 and 4.39, respectively, while they are 1.92 and 6.23 for MA³⁰. Considering the K_a1/K_a2 ratios of 23 and 20417 for FU and MA, respectively, the former readily forms divalent anions, while the latter is much more difficult to do so. The pH of 40 mM FU and MA solutions was tested to be 2.762 and 2.104, respectively. Given the diagrams of ionized species under the current pH conditions³⁰, FU can form divalent anions, whereas MA can be monovalent.

In order to clarify the effect of stereoisomerism on the Zn²⁺ coordination chemistry, theoretical calculations were employed using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The MD statistical analysis reveals the state of HBs in each electrolyte, where the average number of H–O (water) in bulk ZnSO₄ is obviously reduced after adding FU (Supplementary Table 1). Furthermore, DFT analyzed the chemical interactions between Zn²⁺ with H₂O, FU⁻/FU²⁻ ion, and MA⁻ ion as shown in Fig. 1b (Supplementary Table 2 and Supplementary Data 1–4). The binding energies for Zn²⁺–H₂O, Zn²⁺–FU⁻, and Zn²⁺–MA⁻ are calculated as −4.41 eV, −18.05 eV, and −18.52 eV, respectively. Particularly, the calculation implies that the binding energy of Zn²⁺–FU²⁻ reaches the most negative value of −25.62 eV. Considering the strongest binding energy with FU²⁻, the products of FU additive tend to form a stronger interaction with Zn²⁺ compared to those of MA additive, strengthening its ability to alter the solvation structure of hydrated Zn²⁺ ion.

As expected, MD simulations could elucidate that the stereoisomeric additives differently alter the coordination chemistry of hydrated Zn²⁺. The snapshot of bulk ZnSO₄ electrolyte exhibits the typical coordination of six H₂O molecules per Zn²⁺, corresponding to Zn(H₂O)₆²⁺ (Supplementary Fig. 2 and Supplementary Data 5 and 6). In the ZnSO₄/FU electrolyte, FU²⁻ enters the primary solvation layer and then, replaces two H₂O molecules, reconstructing the solvation structure into Zn²⁺(H₂O)₄FU²⁻ (Supplementary Fig. 3 and Supplementary Data 7 and 8). In contrast, the addition of MA only replaces one H₂O molecule to reconstruct Zn²⁺(H₂O)₅MA⁻ (Supplementary Fig. 4 and Supplementary Data 9 and 10). These simulation findings indicate that the trans configuration of FU²⁻ exhibits stronger binding with Zn²⁺ compared to MA⁻, resulting in a more pronounced modification of the Zn²⁺ solvation structure in which the former participates. To further investigate the differences in solvation coordination induced by the stereoisomers, radial distribution functions (RDFs) and coordination numbers (CNs) were calculated (Fig. 1c and Supplementary Fig. 5). In bulk ZnSO₄, an obvious peak of Zn–O by H₂O at ~0.2 nm implies the structured arrangement of H₂O in the nearest solvation layer around Zn²⁺. Furthermore, the average CN of Zn–H₂O in the primary hydration layer of bulk ZnSO₄ is 5.44. In ZnSO₄/FU electrolyte, two peaks of Zn–O by FU²⁻ are observed at 0.17 nm and 0.20 nm, respectively, indicating that the two O atoms of FU are coordinated with Zn²⁺ in the solvation shell. The average CN of Zn with H₂O in the first hydration layer of ZnSO₄/FU electrolyte is approximately 5.13, while that of FU²⁻ is nearly 0.04, further confirming the bidentate coordination of FU²⁻ with Zn. On the other hand, the average CNs of Zn with H₂O and MA⁻ in the primary hydration layer of ZnSO₄/MA are around 4.88 and 0.01, respectively.

Given the highly efficient low additive concentration in this work, the stronger ability of FU²⁻ to regulate the local solvation structure suggests a more significant role at the interface, which will be discussed in detail in our subsequent analysis. Figure 1d (Supplementary Data 11 and 12) and Supplementary Fig. 6 (Supplementary Data 13) display the electrostatic potential (ESP) maps of three types of solvation structures. Compared to ESP of bulk ZnSO₄, the values significantly decrease after the addition of FU, which is consistent with the binding energy results (Fig. 1b)³¹. This implies that the regulation of solvation structure by additives weakens the electrostatic repulsion around the Zn²⁺ ions, facilitating ion transport and enhancing the ability for desolvation³². Furthermore, the trans configuration and intermolecular interaction of FU enable it to alter the solvation shell of Zn²⁺ much more intensely than MA does, demonstrating the stronger coordination with Zn²⁺ ions³³.

Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy were employed to investigate the influence of stereoisomeric interaction chemistry on the HB network. As shown in the FT-IR spectra in Fig. 1e, the peak of H–O stretching vibration at around 3400 cm⁻¹ exhibits a noticeable blueshift when FU is added into bulk ZnSO₄ electrolytes. Moreover, an increased blueshift of the water peak is observed with increasing the amount of added FU (Supplementary Fig. 7). This shift indicates a weakening of HB between free H₂O molecules and a homogenized chemical environment³⁴. As shown in Raman spectra, the three deconvoluted bands at 3260 cm⁻¹, 3450 cm⁻¹, and 3620 cm⁻¹ are assigned to strong, medium, and weak HBs, respectively (Supplementary Fig. 8). Compared to bulk ZnSO₄, the proportion of strong HB in ZnSO₄/FU electrolyte increased from 32.6% to 40.9%, which further corroborates the substantial reduction in free water molecules³⁵.

As shown in ¹H NMR in Fig. 1f, the peak of H₂O in bulk ZnSO₄ appears at 5.1028 ppm. Upon the addition of 40 mM FU, this peak shifts to 5.0858 ppm in the up-field direction, due to the intermolecular HB between the additive and H₂O, leading to an increase in electron density and the proton shielding effect³⁶. For a deeper understanding of the interaction of FU and MA in an aqueous ZnSO₄ electrolyte, the ¹³C NMR spectra were collected as shown in Fig. 1g. The –CH=CH– peak in ZnSO₄/FU is observed at a lower field compared to that in ZnSO₄/MA, while the –COO⁻ peak is captured at a higher field. This suggests that in MA, the cis conformation leads to a significant proportion of intramolecular HB, resulting in its less impact on the HB network than in FU³⁷.

Local reconstruction energetics and dynamics of electrolyte by stereoisomeric additives

In this work, femtosecond transient absorption spectroscopy (FSTAS) was first applied in the ZMB electrolyte field to investigate the local kinetic behaviors of additives and Zn²⁺ in solution, as well as the different interaction (dynamic energy exchange) routes at extremely short time scales. The oscillatory dynamics within solvation structures can be broadly categorized into intermolecular motions, such as fluxional molecular exchange (1–100 ps) and vibrational energy redistribution between bound molecules (~10 ps), and intramolecular motions, including bond vibrations (<1 ps) and internal molecular rotations (<1000 ps), each occurring on distinct timescales^38,39,40,41. Therefore, this femtosecond-level characterization amplifies the differences between cis/trans stereoisomeric additives, providing insights that were not obtained from the conventional spectroscopies widely used in previous studies on ZMB additives. Figure 2a elucidates the excited state absorption (ESA) features of FU/MA additives, displaying oscillatory decay at multiple frequencies. These oscillations indicate different interaction pathways between the cis and trans isomers, involving distinctive dynamic energy exchange mechanisms. Notably, further Fourier-transform analysis of the ESA oscillatory decays reveals intriguing changes after the addition of ZnSO₄ salt. As depicted in the Fourier-transform spectra in Fig. 2b, contrasting the additive-containing solution before and after the addition of ZnSO₄, it is observed that the intervention of ZnSO₄ salt leads to the emergence of high-intensity oscillation modes in low-frequency (10⁻⁴THz) and an intensity reduction in high-frequency (10⁻² to 10⁻³THz) modes. This phenomenon suggests that ZnSO₄ salt provides additional energy exchange pathways on longer timescales (~ns). The differences can be monitored by comparing cis and trans additives. The interaction of trans FU is governed by both medium-frequency (10⁻³THz) and low-frequency (10⁻⁴THz) interactions, while high-frequency (10⁻²THz) interactions are significantly reduced after adding salt. Conversely, cis MA is primarily controlled by low-frequency interactions, with no significant high-frequency interactions regardless of the presence of ZnSO₄ salt. This difference highlights the distinct dynamics and energetics of the cis and trans isomeric additives. The interaction of MA additives with Zn²⁺ (only low-frequency) is weaker than that of FU additives (medium + low frequency), which is consistent with DFT results. Therefore, we reasonably speculate that these differential behaviors are fundamentally driven by the formation of intramolecular HB and steric hindrance in the cis configuration of ring-like MA. The strong interaction of Zn²⁺ with trans FU results in a smaller additive-ion contact distance and more restricted additive motion³⁸. Meanwhile, in the absence of Zn²⁺, the lack of intramolecular HB in FU leads to a relatively open molecular structure, which can facilitate the interactions with surrounding water molecules or others to achieve a stronger energy exchange at mid to high frequency ranges. In contrast, the intramolecular HB in MA affects its electronic structure and vibrational modes, causing an energy exchange to occur more readily at low frequencies, indicating a slower energy exchange. This finding indicates that the presence of intramolecular HB restricts the vibration and rotation of the molecule, thereby further inhibiting intermolecular HB interactions between MA and free water molecules and resulting in energy absorption and transfer at lower frequencies.

Fig. 2: Femtosecond energetics and kinetics influenced by stereoisomerism.

a ESA feature of different electrolytes. b Fourier-transform analysis of oscillatory decays in ESA Spectra. c Light absorbance spectra for aqueous solutions of ZnSO₄, additives, and their mixture. Pseudocolor plots of pump-probe spectra from light absorbance spectra for d ZnSO₄, e ZnSO₄/FU, and f ZnSO₄/MA.

Figure 2c displays the light absorption spectra of aqueous solutions containing ZnSO₄, additives, and their mixtures. Notably, the absorbance of ZnSO₄ was amplified by a factor of 10 due to the difficulty in observing its signal. While the aqueous ZnSO₄ solution itself shows negligible absorption, slight absorption edge shifts are observed for the additives. The interaction of cis MA with Zn²⁺ results in a redshift (or upshift of the Fermi level from dipolar interaction), while the interaction of trans FU with Zn²⁺ leads to a blueshift (or downshift of the Fermi level from dipolar interaction). Compared to the cis MA additives, the trans FU additives shift to higher wavelengths regardless of the presence of ZnSO₄ salt. This supports our hypothesis that FU²⁻ has a stronger conjugation effect compared to MA⁻. Theoretically, the changes after adding ZnSO₄ should result in a blueshift for both additives due to the electron-withdrawing effect of the positively charged cations. However, the actual observation shows a blueshift for FU additives and a redshift for MA additives, respectively. We reasonably speculate that this is because the linear FU²⁻ coordinates with Zn²⁺ through one of its carboxyl groups, which increases the electron-withdrawing effect and pulls the electrons closer to the n orbital, thus increasing the n → π^* transition energy and causing a blueshift in the absorption peak. In contrast, the spatial proximity of both carboxyl groups of MA⁻ to Zn²⁺ results in a more uniform distribution of electrons, which decreases the n → π^* transition energy and thus causes a redshift in the absorption peak. This observation is consistent with the ESP mapping results, bond length⁴², as further corroborated by the peak variations observed in the high-resolution FT-IR spectra⁴³ (Supplementary Figs. 9 and 10, Supplementary Data 14–17 and Supplementary Notes 1 and 2).

We further illustrate pseudocolor plots of pump-probe spectra for these solutions in Fig. 2d–f (Supplementary Figs. 11 and 12). The pump wavelength is fixed at 310 nm to selectively excite the additives. Time zero is within the pump-probe delay of 10⁻¹ ps to 10⁰ps (pump illumination). Before time zero, no signals are observed. Once excited (pumped), both trans and cis additives exhibit rapid multi-frequency oscillatory decay, with different excited state energy levels and relaxation pathways due to their respective configurations. These oscillations are attributed to the dynamic exchange (going back and forth) of energy between the excited additives and water molecules in the solvation environment. Upon adding ZnSO₄ salt, high-frequency oscillations weaken, and simultaneously, high-intensity and low-frequency patterns emerge. This indicates the appearance of a new, slower dynamic interaction (between Zn²⁺ and additives), compensating for the pre-existing interaction between additives and water. Interestingly, for cis MA additives, the peak at ~2800 ps shows a stronger ΔmOD (milli Optical Density Difference) compared to the peak at ~3500 ps, while for trans FU additives, the peak at ~2800 ps shows a weaker ΔmOD compared to the peak at ~3500 ps. These changes on the timescale can be correlated to the rotational motion of the molecules. Once the free additives bind with Zn²⁺, the rotational features are expected to appear over longer periods due to restricted molecular motion in the bound state. This restriction arises from the influence of the isomers’ size, shape, and interactions with the surrounding environment on rotational dynamics^40,41. This restricted motion leads to longer rotational correlation times, which can be observed in pump-probe experiments as slower decay components in transient absorption signals. Thus, we confirm again that the restricted molecular rotational motion is due to the higher degree of binding between FU additives and Zn²⁺.

Interfacial modification by stereoisomeric additives

The change in the interfacial structure by the addition of FU/MA was investigated by calculating the adsorption energies between the Zn₍₀₀₂₎ plane and different components through a DFT method (Fig. 3a and Supplementary Data 18–21). The adsorption energy between the Zn₍₀₀₂₎ plane and water is −0.45 eV, which is lower than −3.43 eV and −3.47 eV with monovalent FU^- and MA^- ions, respectively. However, the divalent FU²⁻ ion demonstrates the highest adsorption energy (−4.09 eV), indicating its stronger adsorption ability on the Zn surface compared to MA²⁻. Figure 3b illustrates the capacitance-potential profile of three electrolytes, further supporting the adsorption energy results. Particularly, the capacitance of ZnSO₄/FU is much lower than that of ZnSO₄/MA, confirming that FU²⁻ possesses stronger adsorption capabilities. This serves as the most robust empirical evidence affirming the superior adsorption capacity of FU²⁻, as well as the difficulty in generating MA²⁻ due to intramolecular HB (Supplementary Table 3). According to previous studies, the modulation of the interface by adsorbed FU²⁻ may play a crucial role in restricting the occurrence of the HER and corrosion^44,45. Moreover, through MD simulations, we further demonstrated that the adsorbed FU²⁻ on the Zn surface can capture and accumulate Zn²⁺, forming ion channels during the plating process and guiding uniform deposition (Supplementary Fig. 13 and Supplementary Data 22 and 23). The function of this adsorption will be further validated and explored in subsequent interfacial MD simulations.

Fig. 3: Interfacial modification by stereoisomers.

a Adsorption energy of Zn₍₀₀₂₎ with H₂O, FU⁻, FU²⁻ and MA⁻. Pink, gray, light blue, and dark blue spheres represent H, C, O, and Zn atoms, respectively. b Capacitance-potential profiles of ZnSO₄, ZnSO₄/FU and ZnSO₄/MA. c Contact angle of different electrolytes. d Snapshots of electrolyte distribution at the interface in different electrolytes. Pink, gray, light blue, and dark blue spheres represent H, C, O, and Zn atoms, respectively. e Normalized density profiles along the z-axis perpendicular to the Zn surface. The density is normalized to the bulk electrolyte value and presented in arbitrary units.

The contact angle tests reflect the higher zincophilicity by the adsorbed isomeric additives (Fig. 3c and Supplementary Fig. 14). When the added amounts of FU are 20 mM and 40 mM, respectively, the contact angle between the electrolyte and Zn foil decreases from 101.9° to 90.8° and 86.9°, respectively, indicating the favorable adsorption of zincophilic FU on the Zn surface. The addition of 40 mM MA also moderately enhances the zincophilicity of the electrolyte, resulting in a contact angle reduction to 91.1°, which is higher than that of FU under the same conditions. The enhanced zincophilicity is associated with the modified interfacial chemistry by FU additives, facilitating the uniform Zn deposition process⁴⁶.

We conducted MD simulations to investigate the interfacial structure. The model provides a direct visualization of additive adsorption occupying the electrode surface, which in turn reduces the local concentration of other components (Fig. 3d, Supplementary Fig. 15, and Supplementary Data 24–29)⁴⁷. The same conclusion can be drawn from the static distribution analysis, where the adsorption of additives leads to a decrease in the density of various species at the electrode surface. Notably, the reduction in water molecules is particularly significant, as it plays a crucial role in suppressing HER (Fig. 3e). For Zn²⁺, a new peak emerges at ~7 Å in the ZnSO₄/FU system, indicating that the FU additive captures Zn²⁺ and facilitates its enrichment. In contrast, the MA additive, due to its cis configuration, does not exhibit this effect. Furthermore, we examined the interface under applied electric fields (Supplementary Fig. 15 and Supplementary Data 30–35). During the plating process, Zn²⁺ in the ZnSO₄/FU system consistently exhibits a peak at ~7 Å, confirming that the adsorption of FU^2- enables the formation of an ion channel, which can effectively guide Zn²⁺ deposition.

Interphasial chemistry and SEI structure

To investigate the potential impact of the two isomeric additives on SEI formation, we analyzed their energy gaps between the unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO) through DFT calculations (Supplementary Fig. 16 and Supplementary Data 36–38). The LUMO energy levels of H₂O, FU, and MA are 1.694 eV, −2.831 eV, and −3.169 eV, respectively. This confirms that both FU and MA are thermodynamically more prone to reduction upon several charge/discharge processes, which leads to the in situ formation of SEIs³⁶. The smaller HOMO-LUMO gap of MA compared to FU is attributed to the propensity of the cis configuration to undergo the reduction reaction, while the first hydrogen atom is lost⁴⁸.

In-depth analyses through XPS, XRD, and ToF-SIMS spectra confirm that FU contributes to the formation of an ionic conductive inorganic SEI layer on the Zn surface^49,50. As shown in Fig. 4a, the Zn surface after plating/stripping in bulk ZnSO₄ exhibits typical carbon signals due to exposure to air. After Ar⁺ etching for only 300 s, no carbon signal peak is detected. In contrast, the Zn surface after plating/stripping in ZnSO₄/FU shows a distinct carbon signal, which is not derived from air exposure, even after etching for 900 s (Fig. 4b). C 1s and O 1s spectra (Supplementary Fig. 17) were deconvoluted into CO₃²⁻ and C=O, respectively, increasing the etching depth, which indicates the formation of a stable substance including CO₃²⁻ groups on the Zn surface. Compared to ZnSO₄/FU, ZnSO₄/MA shows a lower proportion of CO₃²⁻ and C–O signals when etched to deeper layers (Supplementary Fig. 18). This might be because FU²⁻ has a stronger adsorption energy with Zn compared to MA⁻, thereby promoting more efficient interfacial reaction kinetics and facilitating the formation of a more stable and uniform SEI layer on the Zn surface⁴⁶. The stronger adsorption of FU²⁻ promotes the formation of a denser and more stable SEI, whereas MA⁻ plays a weaker role, resulting in a less stable SEI.

Fig. 4: Interphasial chemistry and SEI structure.

In-depth XPS images of plated Zn after plating/stripping 20 times in a ZnSO₄ and b ZnSO₄/FU. c XRD images of plated Zn after plating/stripping 20 times in different electrolytes. Zn₅(CO₃)₂(OH)₆ (ZCO, code: JCPDS 01-072-1100, space group: C2/m), Zn₄SO₄(OH)₆·6H₂O (ZSO, code: JCPDS 00-039-0688, space group: P-1). d HR-TEM image of the crystalline Zn and ZCO SEI, and the EDX mapping of Zn, O, and C. The sample is plated with Zn after plating/stripping 20 times in ZnSO4/FU. e The Zn²⁺ trajectory in ZCO SEI. Pink, gray, light blue, and dark blue spheres represent H, C, O, and Zn atoms, respectively. In this figure, all the samples are collected under ~25 °C.

XRD results demonstrate that the addition of FU and MA contributes to forming an inorganic SEI of Zn₅(CO₃)₂(OH)₆ (ZCO, JCPDS 01-072-1100) (Fig. 4c). This SEI not only promotes ionic conduction for the improved Zn²⁺ transfer but also protects the Zn electrode isolating the solvent⁵¹. Moreover, Zn₄SO₄(OH)₆·6H₂O (ZSO, JCPDS 00-039-0688) by-product is absent in ZnSO₄/FU and ZnSO₄/MA, while bulk ZnSO₄ shows its existence, which means that the additives suppress the formation of alkaline by-products. In the ToF-SIMS spectra under the negative mode, signals of C⁻ (m/e = 12), CO₃²⁻ (m/e = 60), and ZnO⁻ (m/e = 81), arising from the bombardment-induced breakdown of ZCO, are demonstrated (Supplementary Fig. 19), which further substantiates the presence of this inorganic compound. Through TEM testing of the Zn electrode after FIB cutting, more information about the microstructure of SEI was obtained as shown in the HR-TEM image in Fig. 4d. SEI layers on the surface of Zn show a prominent lattice fringe spacing of 0.272 nm, corresponding to the (021) plane of ZCO, as verified by the analysis of the Fourier-transformed reflection. (Supplementary Fig. 20). All lattice spacings of Zn and ZSO by-product were absent, further supporting the presence of ZCO. The integrity of the Zn substrate is demonstrated by observing the lattice spacings of 0.248 nm for the Zn (002) plane and 0.209 nm for the Zn (101) plane. EDX mapping of the cross-section of the Zn electrode after FIB cutting, reveals distinct distributions of Zn, O, and C at the interface. This indicates the in situ growth of ZCO SEI derived from the FU additive on the surface of the Zn electrode to protect it in subsequent plating/stripping processes. Through MD simulations, Zn²⁺ migrates along possible trajectories on the (001) plane in the ZCO SEI (Fig. 4e and Supplementary Data 39–45). Zn²⁺ rapidly traverses the plane, further confirming that ZCO is a fast Zn²⁺ ion conductor for the facilitated interfacial ion transfer. Finite element method (FEM) with COMSOL Multiphysics 6.0 was employed to further assess the impact of SEI presence on Zn deposition behavior. At a potential difference of 20 mV, the concentration contours of Zn²⁺ at the boundaries of electrodes without SEI were relatively flat and did not conform to the shape of the edges, indicating non-uniform deposition for Zn dendrite formation (Supplementary Fig. 21a). On the other hand, the ZCO SEI shows that the shape of the Zn²⁺ concentration contours more closely matches with the interphase compared to that without SEI (Supplementary Fig. 21b). These results indicate that in the presence of SEI, Zn²⁺ ions are more likely to be distributed spatially uniformly, as consistent with the shape of the edges, thus facilitating more homogeneous Zn deposition for stable battery operation. As proposed in the SEI formation mechanism (Supplementary Fig. 22), the dicarboxylic acid additives can be preferentially dissociated on the electrode surface compared to water, forming monocarboxylic acid intermediates^52,53. These intermediates are subsequently dissociated into water-soluble CO₂, which reacts with H₂O and Zn²⁺ to form ZCO⁵⁴.

We investigated the Zn deposition behavior using XRD. The ratio of I₍₀₀₂₎/I₍₁₀₁₎ plane in the XRD was much higher for ZnSO₄/FU (0.32) than those for the bulk ZnSO₄ (0.06) and pure Zn foil (0.05) (Supplementary Figs. 23 and S24), indicating a greater preferential deposition on the Zn₍₀₀₂₎ plane by the addition of FU. This finding was further supported by the relative texture coefficient (RTC) results, where the addition of FU increased the RTC₍₀₀₂₎ by almost fivefold (Supplementary Fig. 25 and Supplementary Note 3). Moreover, an increase in the I₍₀₀₂₎/I₍₁₀₁₎ ratio was consistently observed across different deposition time scales and at high current densities (Supplementary Fig. 26). The preferential deposition on the Zn₍₀₀₂₎ plane results in a smoother Zn surface, which helps mitigate dendrite formation during repeated plating/stripping cycles⁵⁵. As shown in SEM images in (Supplementary Fig. 27a), Zn deposited in the bulk ZnSO₄ electrolyte exhibits large-sized vertical growth after 20 cycles at 1 mA cm⁻² and 1 mAh cm⁻². On the other hand, Zn deposited in the ZnSO₄/FU electrolyte shows a distinct horizontal stacked morphology owing to a uniform and compact deposition along the (002) plane, ensuring dendrite-free Zn deposition (Supplementary Fig. 27b).

LSCM images (Supplementary Figs. 28a, b) and related top-view images (Supplementary Figs. 28c, d) show a significant reduction in surface roughness with the addition of FU, with the S_a value (arithmetical mean height) of the FU-additive surface measured at only 0.930 μm, significantly lower than the 2.501 μm observed for bulk ZnSO₄, indicating a smoother interface that creates a uniform surface electric field, thereby avoiding the tip effect of Zn deposition and preventing a series of side effects. As shown in Zn deposition patterns visualized by in situ optical microscopy (Supplementary Figs. 29a, b), Zn deposition in bulk ZnSO₄ begins to exhibit irregular deposition after 15 min, showing tip effects after 60 min of deposition. Under the same conditions, no dendritic growth was observed in the ZnSO₄/FU electrolyte. This uniform and smooth Zn deposition in ZnSO₄/FU was further confirmed by collecting AFM images of Zn foils (Supplementary Figs. 30a, b). The AFM height intercept of the Zn foil in ZnSO₄/FU electrolyte (270 nm) is much lower than that in ZnSO₄ electrolyte (890 nm). Consequently, these findings demonstrate a uniform and dense deposition of Zn modulated by the FU additive.

Electrochemical behavior of Zn metal negative electrodes in electrolytes with FU/MA additives

The asymmetric Zn||Cu cells were assembled and operated at the low current density of 0.5 mA cm⁻² and high capacity of 3 mAh cm⁻² to evaluate the suppression of HER and corrosion (Fig. 5a and Supplementary Fig. 31). These operating conditions significantly amplify the impact of side reactions, providing a more rigorous assessment of electrolyte stability. Notably, the ZnSO₄/FU and ZnSO₄/MA electrolytes achieved high average Coulombic efficiencies of ~99.42% and ~99.32%, respectively, over an extended cycling duration of 840 h (70 cycles). By contrast, ZnSO₄ alone failed within just three cycles due to severe HER, highlighting the critical role of FU in mitigating side reactions and ensuring prolonged stability for practical operation.

Fig. 5: Electrochemistry and Zn deposition in stereoisomer-containing electrolytes.

a CEs of Zn||Cu asymmetric cells in different electrolytes at 0.5 mA cm⁻² for 3 mAh cm⁻². b CA curves and c activation energy of ZnSO₄, ZnSO₄/FU and ZnSO₄/MA. d Plating/stripping performances of Zn||Zn symmetric cells in different electrolytes at 1 mA cm⁻² for 1 mAh cm⁻². e Plating/stripping performances of Zn||Zn in ZnSO₄/FU at 5 mA cm⁻² for 5 mAh cm⁻². f In situ DRT analysis of symmetric cells (1 mA cm⁻² for 1 mAh cm⁻²) using different electrolytes. The testing temperature is ~25 °C. g Comparison of the current density, capacity, and plating/stripping time with recently reported Zn anodes using optimization strategies. h CEs of Zn||Cu asymmetric cells in different electrolytes (inset indicates zoomed image) and i the corresponding charge-discharge profiles in ZnSO₄/FU at 1 mA cm⁻² for 1 mAh cm⁻².

The LSV curves also show the HER inhibition from additives. Zn²⁺ was replaced by Na⁺ to exclude the influence of Zn deposition on HER during observation. The onset of HER was drastically delayed in Na₂SO₄, Na₂SO₄/FU, and Na₂SO₄/MA electrolytes (Supplementary Fig. 32a), demonstrating that the adsorption of the additive on the Zn surface effectively suppresses HER. As supported by the Raman spectra, the addition of FU transforms the HB environment in the bulk electrolyte from long-range order to isolation, thereby preventing the H₂ generation⁵⁶. In the range of −0.5 V to −1.3 V, the LSV curve with the additive exhibited a distinct peak that first increased and then decreased, as marked by the ellipse, which supports the formation of SEI. To further verify this, LSV tests were conducted on the Zn foils that had undergone plating/stripping in their respective Zn-based electrolytes to allow SEI formation (Supplementary Fig. 32b). The disappearance of this peak confirms its association with SEI formation. Moreover, the presence of SEI further improves HER suppression. To further substantiate the SEI formation, we also performed staircase potential distribution of relaxation times (DRT) analysis, with the detailed discussions provided in Supplementary Fig. 33 and Supplementary Note 4. Tafel curves further provide information about the corrosion kinetics of the Zn electrode (Supplementary Fig. 34). After the addition of FU and MA, the corrosion potential increases and the corrosion current decreases, indicating the enhanced corrosion resistance of the Zn electrode by the FU additives⁵⁷. We also tested the mass change of Zn immersed in different electrolytes and used the classical electrochemical noise (ECN) technology to confirm that FU helps inhibit corrosion (Supplementary Fig. 35). Thus, the trans FU effectively decelerates the corrosion kinetics of the Zn electrode and HER for the more reversible deposition kinetics due to the reconstruction of the solvation structure and the modified local interface as verified by the combined computational and spectroscopic results⁵⁸.

The chronoamperometry (CA) tests were further conducted to observe the diffusion behaviors of bulk and modified electrolytes (Fig. 5b). The corresponding CA curves exhibit a standard 3D diffusion behavior in both ZnSO₄/FU and ZnSO₄/MA electrolytes, manifesting in a short time. This further supports a more facile Zn deposition through the 3D diffusion process^59,60, which is mainly attributed to the adsorption layer at the EDL. By contrast, an elongated and extensive 2D ion diffusion process in the ZnSO₄ electrolyte is characterized by uneven Zn deposition. We also conducted tests on the activation energy of different electrolytes (Fig. 5c). The ZnSO₄/FU showed a lower activation energy of 27.7 KJ mol⁻¹ than 40.4 KJ mol⁻¹ and 29.0 KJ mol⁻¹ in ZnSO₄ and ZnSO₄/MA, respectively, reaffirming the role of ionic channels in accelerating dynamics of Zn²⁺ transfer. As demonstrated above, FU²⁻ ions are adsorbed on the Zn surface through an end-to-linear trans configuration, which could capture the Zn²⁺ by the negative charge of the unadsorbed portion to guide the uniform Zn deposition. On the other hand, MA^- ions are adsorbed in a ring-like configuration on the Zn surface, and their unadsorbed portion lacks a negative charge, making it unable to capture Zn^61,62. This closely resembles the configuration effect of lipids on the regulation of hydrophobicity intensity, indicating the feasibility of our bioinspired stereoisomerism.

The reversible and stable Zn deposition on the Zn negative electrode in ZnSO₄/FU electrolytes was investigated using symmetric Zn||Zn cells (Fig. 5d and Supplementary Fig. 36). At a current density of 1 mA cm⁻² and an areal capacity of 1 mA h cm⁻², symmetric Zn||Zn cells in bulk ZnSO₄ electrolyte failed in plating/stripping after 122 h due to Zn dendrites for internal short circuits. Under the same condition, symmetric Zn||Zn cells were tested in ZnSO₄/FUXX electrolyte, where XX means the concentration of FU in ZnSO₄/FU electrolyte, varying the concentration of FU from 10 mM to 100 mM. The same notation applies to ZnSO₄/MA. They achieved stable and reversible Zn deposition for approximately 2000 h (ZnSO₄/FU10), 4930 h (ZnSO₄/FU20), 6150 h (ZnSO₄/FU40), and 2315 h (ZnSO₄/FU100), respectively, much greater than that in bulk ZnSO₄. Thus, the optimal concentration of FU additive was determined to be 40 mM, where this optimal sample will be denoted as ZnSO₄/FU unless specified. Considering the minimal amount of additive required, as well as its commercial cost compared to other similar dicarboxylic acids, FU additive is thought to have high economic and sustainable efficiency in ZMBs (Supplementary Table 4). The symmetric cell in ZnSO₄/FU100 exhibited noticeable voltage fluctuations, due to the presence of undissolved FU. Particularly, Zn||Zn cells with ZnSO₄/FU40 electrolyte showed smaller overpotentials and longer cyclability than those of bulk ZnSO₄ and ZnSO₄/MA40, with high ionic conductivity and transference number of Zn²⁺ (Supplementary Figs. 37–40 and Supplementary Tables 5 and 6). Furthermore, the initial 10 plating/stripping cycles of the symmetric cell were zoomed to observe the nucleation overpotential (Supplementary Fig. 41). During the first plating, the nucleation overpotential was estimated to be 73.9 mV, 78.9 mV, and 87.8 mV for ZnSO₄, ZnSO₄/FU, and ZnSO₄/MA, respectively. This increase is attributed to the coordination of Zn²⁺ with FU²⁻ and MA⁻, resulting in a slight increase in desolvation energy. However, thanks to the formation of ZCO SEI with high ionic conductivity, the energy barrier for ion transfer at the electrode surface was significantly reduced, leading to a substantial decrease in Zn deposition overpotential in ZnSO₄/FU after 1st plating/stripping cycle. At the 10th plating/stripping cycle, the nucleation overpotential for ZnSO₄/FU was only 18.8 mV, much lower than the 34.8 mV for ZnSO₄. For further comparison, the electrochemical performance was evaluated under a more severe deposition/stripping condition at a current density of 5 mA cm⁻² and an areal capacity of 5 mA h cm⁻² (Fig. 5e). The symmetric cells in ZnSO₄/FU were more stably and reversibly operated for 1500 h, than those in bulk ZnSO₄ and ZnSO₄/MA (Supplementary Figs. 42 and 43), owing to the modified solvation structure, facilitated ionic transport of ion channel by FU addition, and ionically conductive SEIs.

The in situ EIS of Zn||Zn symmetric cells was conducted (Supplementary Fig. 44). The symmetric cell plated/stripped in bulk ZnSO₄ initially exhibited significant impedance, gradually leading to a short circuit after 50 h. With the addition of FU, the cell impedances significantly decreased. After 100 plating/stripping cycles, the cell sustained a stable overpotential. We fitted the in situ EIS spectra (Supplementary Figs. 45 and 46) and discussed each component of the impedance in Supplementary Table 7. We further conducted advanced in situ DRT analysis to assess the refined resistances influenced by the additive. As depicted in Fig. 5f, all curves exhibit mainly four peaks at logτ ≈ −5 s, −3 s, −1.8 s, and 1 s, corresponding to impedances of reactant adsorption, phase evolution, charge transfer, and product diffusion, respectively. Following plating/stripping, all resistances, particularly the impedances of adsorption and charge transfer in ZnSO₄/FU, are observed to be lower than those of the bulk electrolyte and ZnSO₄/MA. The original resistance value for Zn²⁺ adsorption is merely 0.68 Ω in ZnSO₄/FU, which is less than that of bulk ZnSO₄ (0.78 Ω) and ZnSO₄/MA (0.75 Ω). Similarly, the charge transfer resistance in ZnSO₄/FU is 10.96 Ω, significantly lower than that of bulk ZnSO₄ (26.80 Ω) and ZnSO₄/MA (19.27 Ω). These further signify enhanced Zn²⁺ transport and charge transport by FU.

Figure 5g presents the comparative plating/stripping performance of Zn||Zn symmetric cells at different current densities and deposition capacities with the recent literature about electrolyte engineering (Supplementary Table 8), highlighting the effectiveness of our additive under various testing conditions. At a current density of 1 mA cm⁻² and an areal capacity of 1 mAh cm⁻², the ZnSO₄/FU achieved 6150 h of operation, nearly doubling the average level. Similarly, at a current density of 5 mA cm⁻² and an areal capacity of 5 mA h cm⁻², the ZnSO₄/FU exhibited stable plating/stripping for 1500 h, also the highest under these conditions.

The CEs of Zn plating/stripping on current collectors was evaluated using asymmetric Zn||Cu cells (Fig. 5h). The Zn||Cu half-cell with ZnSO₄/FU reached a high average CE of ~99.9% after a long cycling for 2100 cycles due to the inhibition of Zn dendrites, HER, and resistive by-products. The voltage profiles for Zn deposition/dissolution in ZnSO₄/FU became quite stable after 50 cycles and achieved an extremely high CE of ~99.9% (Fig. 5i). On the other hand, the Zn||Cu cells in bulk ZnSO₄ electrolyte failed after approximately 110 cycles. Although ZnSO₄/MA enabled the cell to operate for approximately 1560 cycles owing to the formation of SEI, its low ionic conductivity, inability to form ionic channels, and remaining HER hindered further improvement. These results indicate that the addition of FU is most effective for reconstructing solvation structures, facilitating the kinetic properties at the electrolyte and interface, as well as constructing the ionic conductive and stable ZCO SEIs.

To further validate the high CEs, the Zn||Cu asymmetric cells were tested under the high capacity of 4 mAh cm⁻² at the current densities of 1 mA cm⁻² and 4 mA cm⁻², respectively. At 1 mA cm⁻² and 4 mAh cm⁻², the cells in ZnSO₄/FU electrolyte achieved cyclic stability over 100 cycles (800 h), while those in ZnSO₄/MA electrolyte started to exhibit instability after ~70 cycles (Supplementary Fig. 47). By contrast, the cell in ZnSO₄ electrolyte rapidly failed within only five cycles due to severe side reactions. At 4 mA cm⁻² and 4 mAh cm⁻², the cells in both ZnSO₄/FU and ZnSO₄/MA demonstrated high average CEs of ~99.8% (Supplementary Fig. 48). The cell in ZnSO₄/MA started to lose stability after approximately 270 cycles, whereas that in ZnSO₄/FU maintained stable cycling performance after 450 cycles. In comparison, the cell in ZnSO₄ failed within 50 cycles. Moreover, the Zn||Cu cells confirmed the superior stabilizing role of FU in suppressing detrimental side reactions and ensuring long-term stability at various current densities (Supplementary Fig. 49).

We characterized the deposition behavior of Zn²⁺ on heterogeneous substrates in different electrolytes using cyclic voltammetry (CV) in Supplementary Fig. 50. ZnSO₄/FU exhibited a smaller overpotential, which is consistent with the trend in symmetric cells, indicating that FU has a universal effect in enhancing overall performance.

Full cell performances of initially anodeless ZMBs

In order to demonstrate the superiority of the FU additive for practical applications, Zn||MnVO full cells were assembled to couple the Zn negative electrode with the MnVO positive electrode. The electrochemical behavior of Zn||MnVO cells in ZnSO4 was analyzed by measuring CV curves (Supplementary Fig. 51). The full cells in ZnSO₄/FU exhibited a larger CV area, corresponding to the larger capacity they achieved during cycling compared to ZnSO₄. At a specific current of 0.1 A g⁻¹, the maximum capacity of the ZnSO₄/FU system reached 315 mAh g⁻¹, much higher than the 280 mAh g⁻¹ of the ZnSO₄ system, approaching the theoretical specific capacity of graphite electrodes in LIBs (372 mAh g⁻¹). The rate performance (Fig. 6a) was evaluated by increasing the specific currents from 0.1 A g⁻¹ to 5 A g⁻¹. The superior rate capability of the Zn | |MnVO cells in ZnSO₄/FU was confirmed, demonstrating a high-rate capacity of 161 mAh g⁻¹ at 1 A g⁻¹ when the rate increased by 10-fold, and a substantial capacity of 86 mAh g⁻¹ even at 5 A g⁻¹. These values measured at various rates were higher than those in the blank ZnSO₄ electrolyte. In long-term cycling tests at a high specific current of 2 A g⁻¹, the Zn||MnVO cells in ZnSO₄/FU electrolyte achieved a high capacity retention of 82.5% and reversible capacity of 88 mAh g⁻¹ after 2000 cycles, along with nearly 100% of CE (Fig. 6b). In contrast, the Zn||MnVO cells in bulk ZnSO₄ electrolyte only showed low capacity retention of 51.0% and reversible capacity of 50 mAh g⁻¹ after 1700 cycles. We performed CV tests at different scan rates to investigate the potential of FU in enabling fast and reversible kinetics. The results show that the capacitive contribution in ZnSO₄/FU is consistently higher than that in ZnSO₄ (Supplementary Fig. 52). This indicates that FU further enhances charge storage kinetics, contributing to improved rate performance⁶³.

Fig. 6: Full cell performances in different electrolytes.

a Rate performances of Zn||MnVO in different electrolytes at 0.1, 0.2, 0.5, 1, 2, 5 A g⁻¹. b Cycling performances of Zn||MnVO in different electrolytes at 2 A g⁻¹. c Cycling performances of Zn-predeposited Cu||MnVO in different electrolytes at 2 A g⁻¹. d Cycling performances of Zn||AC in different electrolytes at 10 A g⁻¹. e Cycling performances of Zn-predeposited Cu||AC in ZnSO₄/FU under −10 °C and 60 °C. f schematic diagram of the composition difference between a regular battery and an initially anodeless battery. g Cycling performances of initially anodeless Cu||zincated MnVO in different electrolytes at 2 A g⁻¹.

We further used polytetrafluoroethylene (PTFE) binder to fabricate MnVO positive electrodes with a high loading of up to 25 mg cm⁻², for practical cycling performance under high areal capacity. The Zn||MnVO full cell in ZnSO₄ failed to operate properly within just 10 cycles at 0.2 A g⁻¹, whereas that in ZnSO₄/FU was stably operated for 100 cycles, delivering an initial discharge capacity of approximately 5 mAh cm⁻² (Supplementary Fig. 53). The full cell in ZnSO₄/FU delivered the high reversible capacity reducing polarization (Supplementary Figs. 54 and 55). As shown in Supplementary Fig. 56, the full cell in ZnSO₄ failed within 10 cycles, at a high specific current of 2 A g⁻¹. On the other hand, the full cell in ZnSO₄/FU extends the stable cycling over 1500 cycles with high capacity retention of 70%, delivering an initial discharge capacity of approximately 4 mAh cm⁻². FU allows the full cell to deliver a high reversible capacity while preserving high stability (Supplementary Figs. 57 and 58). These results demonstrate that FU enables stable operation of the full cells under practical conditions of high mass loading and large areal current density.

Zn negative electrodes further were combined with commercial phenazine (PNZ) organic positive electrodes to demonstrate the versatility of FU additive for the enhanced overall cell performance (Supplementary Fig. 59). The ZnSO₄/FU cell exhibited a capacity retention of 74% and stable capacity of ~96 mAh g⁻¹ after 1000 cycles at 1 A g⁻¹, along with high CE. In contrast, the Zn||PNZ cells in bulk ZnSO₄ experienced micro-short circuits at around 600th cycles and rapid capacity decay after 800 cycles. As shown in CV curves, the addition of FU contributed to increasing the capacity and reducing the polarization, indicating faster charge storage kinetics (Supplementary Fig. 60).

Considering the high CE of 99.9% in the asymmetric Zn||Cu cell with ZnSO₄/FU, a Zn-predeposited Cu||MnVO cell could be constructed to deliver the improved energy density (Fig. 6c). At a high specific current of 2 A g⁻¹, the Zn-predeposited Cu||MnVO cell with ZnSO₄/FU exhibited an initial capacity of 96.8 mAh g⁻¹, showing almost no noticeable decay comparable to the conventional Zn||MnVO cell. The galvanostatic charge-discharge (GCD) curves demonstrate the smaller polarization of ZnSO₄/FU (Supplementary Fig. 61). On the other hand, the Zn-predeposited Cu||MnVO cell with bulk ZnSO₄ showed an initial capacity of only 82 mAh g⁻¹. Furthermore, the Zn-predeposited Cu||MnVO cell with ZnSO₄/FU maintained a 70% capacity retention and nearly 100% CE even after 1000 cycles, which was much higher than 47% capacity retention and CE (~99.1%) in bulk ZnSO₄ electrolyte after 270 cycles. More importantly, our Zn-predeposited Cu||MnVO full cell achieved a high capacity retention of 70% over 1000 cycles at the high specific current of 2 A g⁻¹, far exceeding other ZMBs that utilize Zn-predeposited current collectors reported in previous literature, as shown in Supplementary Fig. 62 (Supplementary Table 7). We also conducted self-discharge tests and DRT techniques to demonstrate the superiority of FU, with detailed information discussed in Supplementary Figs. 63–66 and Supplementary Notes 5–8.

Hybrid Zn-ion capacitors (HZICs) were assembled by pairing Zn negative electrodes with commercial activated carbon (AC) positive electrodes. The HZIC cycled in bulk ZnSO₄ experienced a capacity decline after around 4000 cycles at a very high specific current of 10 A g⁻¹, and eventually failed after ~7330 cycles (Fig. 6d). On the other hand, the HZICs in ZnSO₄/FU electrolyte showed long-term cyclic stability over 60000 cycles with a capacity retention of 99.8% and a CE of ~100%, comparable to or better than many previously reported metal ion capacitors (Supplementary Table 9). Additionally, Zn-predeposited Cu||AC hybrid Zn-ion capacitors were configured to confirm stable operation in a wide range of temperatures as shown in Fig. 6e (Supplementary Fig. 67). As verified by the high capacity retention (~100% and ~95%) and high average CE (~99.9%) of Zn-predeposited Cu||AC HZICs over 900 and 700 cycles at −10 °C and 60 °C, respectively, the FU additives offer the versatile design for low n/p devices and stable operation under extreme conditions.

The initially anodeless Cu||zincated MnVO cells were assembled to demonstrate the full potential of the FU additive (Fig. 6f). Notably, the FU additive significantly enhances the cycling stability of initially anodeless Cu||zincated MnVO cells, ensuring cyclic stability over 270 cycles (Fig. 6g). Despite a high mass loading up to 15 mg cm⁻², high areal current density of 30 mA cm⁻², and the absence of Zn-rich ZnVO positive electrodes, our work have demonstrated strong competitiveness with the recently published state-of-the-art works (Supplementary Fig. 68 and Supplementary Table 10).

Discussion

We have investigated the unique interaction chemistry and interfacial structure resulting from the stereoisomerism of multifunctional electrolyte additives, which were ultimately applied in low n/p ratio full cells, utilizing Zn-predeposited Cu as the negative electrode, and even in initially anodeless Cu||zincated MnVO full cells. The cis–trans geometries around the C=C double bond were attributed to distinct polarities and configurations, resulting in the different energetics and dynamics of FU and MA additives. FSTAS first demonstrated that linear trans FU additives influence free water molecules through intermolecular HB, facilitating a rapid energy exchange and effectively mitigating the HER. Conversely, the ring-like cis MA additives formed intramolecular HB due to their configuration, sacrificing intermolecular HB interactions with the adjacent medium. Pump-probe spectroscopy, DFT calculations, and MD simulations confirm FU²⁻ stronger Zn²⁺ binding affinity, which is manifested in its bidentate coordination in contrast to the monodentate coordination of MA⁻. Regarding the interfacial structure, the divalent FU²⁻, resulting from two-step ionization, exhibits higher adsorption energy on the Zn surface compared to the MA⁻, which is monovalent because it undergoes only one-step ionization. This significantly impacts the local charge distribution and water activity in the EDL, thereby restricting parasitic reactions. MD simulations demonstrate that the formation of FU²⁻ ionic channels facilitates smooth Zn deposition, consistent with XRD and various electron microscopy results. Additionally, FU forms a highly ion-conductive SEI layer, further protecting the Zn electrode and promoting Zn²⁺ transfer. The enhanced kinetics result in lower overpotentials, enabling Zn||Zn symmetric cells to achieve highly reversible, dendrite-free Zn plating/stripping with stability exceeding 6150 h at 1 mA cm⁻² and 1500 h at 5 mA cm⁻². Full cells with Zn||MnVO exhibited an impressive capacity retention of 82.5% after over 2000 cycles at 2 A g⁻¹. The virtually 100% CE also facilitated high cycling performance in initially anodeless full cells. The versatility of the isomeric modification is further confirmed by the optimized performance of organic positive electrodes and temperature-flexible ZICs. Consequently, this work elucidates the significant impact of stereoisomerism of molecular additives on HB interactions, solvated structure, and interfacial characteristics, as well as provides new insights into the correlation between isomeric additives and solvation energetics/dynamics from the perspectives of femtosecond-scale characterization.

Methods

Electrolyte preparation

The bulk electrolyte of 2 M ZnSO₄ was formulated by dissolving ZnSO₄·H₂O (Alfa Aesar, >99.9%) in deionized water. For the cell electrolyte preparation, varying concentrations of FU (Alfa Aesar, ≥99.0%) and MA (Alfa Aesar, 98+%) were separately introduced into the initially prepared bulk electrolyte.

Positive electrode preparation

The MnVO was fabricated by adding 60 mg of V₂O₅ (Sigma-Aldrich, ≥98%) to 14 mL of water, followed by the addition of 1.5 mL of H₂O₂, and stirring until the solution became clear. Then, 30 mg of MnSO₄·H₂O was added and uniformly dissolved. The mixture was aged at 30 °C in the dark until complete precipitation occurred. After centrifugation, washing, and drying, MnVO was obtained. The positive electrodes were prepared by blending 70 wt% of ball-milled active materials, 20 wt% Ketjen Black (KB, Lion Specialty Chemicals Co., Ltd.), and 10 wt% polyvinylidene fluoride (PVDF, Sigma-Aldrich, average Mw ~1,80,000) in 1-methyl−2-pyrrolidone (NMP, DAEJUNG). The resulting slurry was cast onto a titanium foil (Sigma-Aldrich, thickness of 50 μm, purity 99.6 + %) using the Doctor blade method and dried at 80 °C overnight, yielding a MnVO loading of ~4.0 mg cm⁻² on each positive electrode.

For high loading (~15.0 mg cm⁻² and ~25.0 mg cm⁻²) MnVO positive electrodes, the materials were prepared by blending 70 wt% ball-milled active materials, 20 wt% carbon black (CB), and 10 wt% poly(tetrafluoroethylene) (PTFE, Sigma-Aldrich) in ethanol (DAEJUNG). Ethanol was added multiple times during the mixing process to ensure uniform dispersion of the mixture. The resulting slurry was then carefully cast onto hydrophobic carbon paper (WizMAC) using the Doctor blade method and dried overnight.

The activated charcoal (AC) positive electrodes were synthesized through 70 wt% of commercial AC powder (Sigma-Aldrich, 4–12 mesh particle size), 20 wt% CB, and 10 wt% PVDF in NMP. The slurry was cast on carbon paper and dried at 80 °C overnight. The AC loading is ~5.5 mg cm⁻² in each positive electrode. The PNZ positive electrodes were synthesized through 60 wt% of commercial PNZ powder (Sigma-Aldrich, 98%), 30 wt% CB, and 10 wt% PVDF in NMP. The slurry was cast on carbon paper using the Doctor blade method and dried at 80 °C overnight, resulting in a PNZ loading of ~4.0 mg cm⁻² in each positive electrode.

Cell fabrication

CR2032 coin-type cells were employed to assemble Zn||Zn symmetric cells under ambient atmospheric conditions. Zinc foils (Alfa Aesar, thickness 250 μm) underwent mechanical rolling to achieve a thickness of 20 μm and were punched into Φ10 mm disks to serve as electrodes. Glass fiber (Whatman, GF/A) was utilized as the separator (Φ16 mm), and 100 μL of electrolyte was added. The construction of Zn||Cu asymmetric cells mirrored that of the Zn||Zn symmetric cells, with the exception that the Zn positive electrode was substituted with a Cu positive electrode (Sigma-Aldrich, Φ10 mm, thickness of ~17 μm, purity 99.9%). Full cells were composed of Zn negative electrodes or pre-zincated Cu negative electrodes (with a theoretical Zn loading of 6 mAh cm⁻²), the aforementioned positive electrodes, and 100 μL of electrolyte. The materials of the case and spring are stainless steel (Wellcos Corp).

Material characterizations

For the electrolyte part, Fourier-transform infrared (FT-IR) spectra were recorded using FT-IR 4700s (JASCO) employing attenuated total reflection (ATR) methods. Raman spectra were obtained using a Confocal Raman Spectrometer (NT-MDT) with a wavelength of 532 nm. NMR samples were prepared by dissolving ZnSO₄ (and the additive) in deuterium oxide containing 0.75 wt% 3-(trimethylsilyl) propionic−2,2,3,3-d4 acid sodium salt (TSP) as the internal standard. ¹H NMR and ¹³C NMR spectra were recorded on a 700 MHz spectrometer (AVANCE III 700, Bruker). To evaluate the ionic conductivities and pH value of the electrolytes, a real-time conductivity/pH meter (SevenMulti, Mettler-Toledo) was employed. Contact angle measurements were carried out using a contact angle goniometer (SmartDrop_Plus, FEMTOBIOMED). The digital images of electrolytes were collected by an iPhone (13, Apple). The morphologies of Zn foils were examined through scanning electron microscopy (SEM, JSM-7000F, JEOL). The operando optical images were collected by an Olympus microscope with a Nikon lens. A 3D measuring laser confocal scanning microscope (3D LCSM, OLS5100, Olympus) was utilized for the three-dimensional imaging and reconstruction of post-plated zinc samples. Atomic force microscopy (AFM) measurements were conducted on NX10 (Park Systems). High-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEM-ARM300F, coupled with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. The samples were prepared using a focused ion beam (FIB, Helios 5 UX, Thermo Fisher Scientific). The data were processed using Gatan GMS 3 software. X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer (PANalytical, Almelo) with Cu Kα radiation (λ = 0.154 nm). The scan rate is 10° min⁻¹. For the characterization of the electrode surface components, X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Fisher Scientific) was employed. In-depth XPS spectra were obtained by etching the electrode surface with argon ion sputtering for varying durations (0 s, 300 s, 600 s, and 900 s). The pass energy was set to 20 eV for high-resolution scans, and the energy step size was 0.1 eV. The XPS spectra were calibrated against the C 1s peak from adventitious carbon at 284.6 eV. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was performed using a TOF.SIMS 5 (ION-TOF). A pulsed 25 keV Bi⁺ primary ion beam (1.000 pA) was used for analysis on a square region with a side length of 40 µm. For sputtering, a 1 keV Cs⁺ ion beam (50.000 nA) was applied to a square region measuring 150 µm on each side. Transient spectroscopic analysis of additive-ZnSO₄ solutions was carried about by using an ultrafast pump-probe spectrometer (HARPIA-TA, Light Conversion) consisting of a 6 W 2 kHz repetition rate femtosecond Yb:KGW amplifier laser (HYPERION, Ultrafast Systems, Inc.) and an optical parametric amplifier system (APPOLO-Y, Ultrafast Systems, Inc.) in Cryogenic Femtosecond Transient Kerr-Gate Spectro-microscopy Facility (NFEC−2025-03-305124). Excitation pulses centered at 310 nm were generated via the OPA and modulated by a variable neutral density filter to achieve a power density of approximately 60 μJ cm⁻². The probe beam consisted of a white-light supercontinuum, which was obtained by directing a fraction of the 1030 nm fundamental beam into either a sapphire crystal for the visible region or a YAG crystal for the near-infrared. The concentrations for the pump-probe studies are the same as the optimal concentrations in the experiment, with ZnSO₄ at 2 M and the additive at 40 mM. Pump and probe pulses were polarized at fixed angles (45 < θ < 90) to observe the oscillatory transient features from molecular motions. High-resolution FT-IR spectra were obtained using an FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA) equipped with a mercury-cadmium-telluride (MCT) detector cooled with liquid nitrogen in Kangwon Radiation Convergence Research Support Center of Korea Basic Science Institute (KBSI) at Kangwon National University. All samples were placed on a 10-times reflectance Ge ATR cell (incident angle: 45°, PIKE, USA). The background spectrum and all sample spectra are recorded with 2048 and 1024 scans with a 4 cm⁻¹ spectral resolution. All ex-situ measurement samples are properly stored.

Electrochemical measurement

The WonATech WBCS3000L battery test system and the Maccor Series 4000 battery test system were employed for testing (a)symmetric cells and full cells in a climate-controlled room (average temperature 25 ± 2 °C). A suite of electrochemical properties, including CV, electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and CA, was systematically investigated using the BioLogic VMP3 Multichannel Potentiostat. Conducting EIS measurements over a broad frequency range, spanning 10 Hz to 100 kHz with a voltage amplitude of 10 mV under a single sine mode, 6 points per decade. The measurement was conducted after stabilizing the cell at open-circuit voltage. Monitoring of the HER was performed using LSV in a 2 M Na₂SO₄ aqueous solution with/without 40 mM additive, at a sweep rate of 5 mV s⁻¹, from 0 V to −3 V. CV curves for full cells were collected at a sweep rate of 0.2 mV s⁻¹, and the potential range was the same as the voltage range of the full cells. The CA method was employed to record diffusion curves with an overpotential of −150 mV. Corrosion tests, within a three-electrode setup using Zn foils, were conducted in a 2 M ZnSO₄ electrolyte with/without a 40 mM additive. The working and counter electrodes were Zn foils. The reference electrode is silver/silver chloride (Ag/AgCl). The differential capacitance-potential curves were obtained using Zn||Cu cells with the Mott-Schottky method (0.3 V–1.0 V vs Zn/Zn²⁺). For testing CE using Zn||Cu cells, the voltage cut-off is set to 0.5 V. In this paper, all the CEs are calculated as the ratio of the plating capacity to the stripping capacity in the same cycle.

Calculation methods

First-principles calculations based on DFT were conducted using the Castep module of Materials Studio, employing the generalized gradient approximation (GGA) and Perdew, Burke Ernzerhof (PBE) exchange-correlation functional. The crystal structure model of Zn was established within a 3 × 3 × 1 supercell of a Zn unit cell with a 15 Å vacuum layer. Additives were placed on the Zn₍₀₀₂₎ frame, with the bottom layers fixed and the top layer allowed to relax. Energy calculations utilized an 8 × 8 × 1 k-point grid, and a plane-wave cutoff energy of 394 eV was set after convergence testing. Geometry optimization was achieved with a convergence tolerance of 1.0 × 10⁻⁵eV atom⁻¹ for energy, 2.0 × 10⁻²eV Å⁻¹ for maximum force, and 1.0 × 10⁻³Å for maximum displacement. The HOMO/LUMO levels and ESP were calculated using the Hybrid-B3LYP functional with the DND basis set (basis file: 3.5). SCF calculations were conducted with a convergence threshold of 1.0 × 10⁻⁶ a.u. for the energy difference between iterations, employing the direct inversion in the iterative subspace (DIIS) method to accelerate convergence.

MD simulations were performed using the Forcite analysis tool of Materials Studio to investigate the Zn²⁺ solvation structure. The electrolyte molecules were arranged in a periodic box to construct bulk systems according to their real ratios. The atomic electrostatic and van der Waals interactions were calculated by the particle mesh Ewald and cut-off method, respectively, with the same distance of 1.2 nm. All electrolyte systems were initially relaxed using a microcanonical ensemble (NVE) at 300.0 K for 500 ps. Subsequently, equilibrium density and volume were simulated using constant-pressure, constant-enthalpy ensemble (NPH) and canonical ensemble (NVT) MD for 1.0 ns at 300.0 K and 1 bar pressure for energy minimization of the initial structure. The final system structure was obtained through constant-pressure-constant-temperature (NPT) MD simulations for 5.0 ns with a 1.0 fs time step. RDFs and statistical summaries of hydrogen bonds were based on the final structures obtained using the Forcite analysis tool.

To conduct FEM simulations, the meshes (sizes controlled by the multi-physical field) have been constructed accordingly by COMSOL Multiphysics 6.0. The geometry models without and with SEI were constructed, respectively. The convex (protrusion) and concave regions at the interface are simulated with continuous semicircles (radii labeled in the figures). The concentrations of Zn²⁺ on the upper and the lower boundaries, C_upp and C_low, were set 1000 mol m⁻³ and 0 mol m⁻³, respectively. The electric potential on the upper and the lower boundaries, φ_upp and φ_low, were set as 20 mV and 0 mV, respectively, with the standard potential of Zn/Zn²⁺ set as reference. The model integrates the electrostatic field and dilute mass transfer field, with transient analysis conducted over 2000 ms in 100-ms time steps.