Water-modulated electrolyte enables the development of advanced low-temperature Zn-ion batteries

Abstract

Aqueous Zn-ion batteries face huge challenges at sub-zero temperatures due to electrolyte freezing and parasitic side reactions. Here, we present a low-temperature Zn-ion battery with a water-modulated Zn(BF₄)₂ electrolyte consisting of 1,3-dioxolane (DOL) and water in a 95 : 5 volume ratio. DOL was selected due to its low freezing point and ability to dissolve Zn(BF₄)₂ at high concentrations. A minimal water content is maintained to suppress ring-opening polymerisation of DOL, which prevents viscosity increment and conductivity losses, thereby preserving the electrolyte stability. By combining the computational simulations with experiments, the results show that DOL molecules disrupt the H-bond network of the remaining water and alter the Zn²⁺ solvation structure, which suppresses the hydrogen evolution reaction and facilitates dendrite-free Zn deposition even at low temperature. The water-modulated electrolyte exhibits excellent low-temperature performance. Specifically, the Zn‖MgVO full cell operates at −30 °C with over 90% capacity retention after 300 cycles at 1C, and it sustains over 2000 cycles at a high rate of 10C. This study confirms DOL modulation's key role in achieving hydrogen-free and dendrite-free Zn anodes, advancing understanding of intermolecular interactions in low-temperature electrolyte design.

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Introduction

Aqueous Zn-ion batteries (AZIBs) are attracting great interest as sustainable, large-scale energy storage systems due to the abundance and low cost of Zn and its high theoretical capacity (820 mA h g⁻¹) and relatively low redox potential (−0.76 V vs. the standard hydrogen electrode (SHE)).^1–6 In contrast to lithium-based batteries, AZIBs utilise water-based electrolytes and Zn metal anodes, offering improved safety and environmental benignity for grid-scale applications.^7–12 These advantages make AZIBs promising candidates for integrating renewable energy sources into reliable power supply systems.^13–15 However, in cold climates and high-latitude regions, energy storage systems are required to operate at sub-zero temperatures.^16–20 Conventional AZIBs with mild aqueous electrolytes (such as 2 M ZnSO₄) cannot stably operate, because freezing or boosted viscosity of the electrolyte at <0 °C significantly impedes Zn²⁺ transport and leads to capacity loss and cell failure.²¹ Looking for a potential low-temperature zinc salt and developing strategies is highly important to enable durable ZIB operation in sub-zero environments.

Zinc tetrafluoroborate (Zn(BF₄)₂) is considered to be a potential low-temperature Zn salt compared to other commonly used salts, such as ZnSO₄ and ZnCl₂.^22–24 The BF₄⁻ anion formed an OH⋯F hydrogen bond (H-bond) with water molecules, preventing the formation of the ice crystal lattice and enabling the electrolyte to remain liquid at lower temperatures. BF₄⁻ is a weakly coordinating anion that can readily dissociate and is highly soluble in both water and polar organic solvents.²⁵ This broad solvent compatibility enables the formulation of hybrid aqueous–organic electrolytes. In addition, Zn(BF₄)₂ is inexpensive and widely available – it is reported to cost only about AU$1.81 per gram (Fig. S1), lower than that of commonly used ZnSO₄ (AU$13.5) and much lower than that of specialty salts like Zn(TFSI)₂ (AU$129). This cost advantage makes it attractive for large-scale use.²⁶ Despite these appealing features, Zn(BF₄)₂ has seen limited use in conventional AZIBs owing to its drawbacks in pure water.²⁷ Due to the thermodynamic activity of pure water, BF₄⁻ readily reacts with H₂O to yield boric acid species and hydrofluoric acid (via BF₄⁻ + H₂O → BF₃(OH)⁻ + H⁺ + F⁻), introducing H⁺ that drives corrosion of Zn and F⁻ that can attack surfaces.²⁸ It results in violent hydrogen evolution reaction (HER), corrosion of Zn metal and metal-based current collectors, and continuous formation of byproducts like Zn(OH)₂ that passivate the electrode. These issues explain why Zn(BF₄)₂ is rarely employed in AZIB electrolytes to date, despite its potential advantages.

Recognising these challenges, recent research has focused on engineering Zn(BF₄)₂-based electrolytes that inhibit water's reactivity while harnessing the beneficial properties of BF₄⁻.^6,29 Wang et al. reported a hybrid electrolyte by dissolving Zn(BF₄)₂·4H₂O in the vinylene carbonate–water mixed solution.⁴ In this electrolyte, the hybrid electrolyte enabled the full cell with stable cycling with 85% capacity retention after >1300 cycles, whereas the same cell in an aqueous Zn(BF₄)₂ electrolyte died in <10 cycles. As another example, He et al. developed a dual-solvent electrolyte using hydrated Zn(BF₄)₂ in a mixture of ethylene carbonate and dimethyl carbonate.¹⁰ This dual-carbonate electrolyte effectively inhibited Zn dendrite growth and side reactions, achieving a high coulombic efficiency (CE) of 99.7% over 700 cycles in Zn‖Cu batteries at 1 mA cm⁻² without short-circuiting. Similarly, Wang et al. introduced a hydrated deep-eutectic electrolyte based on Zn(BF₄)₂ and acetamide, which provided exceptional protection against dendrites and side reactions. As a result, Zn‖polyaniline full cells exhibited good cycling stability at 1 A g⁻¹.²⁸ However, these studies have not addressed the interaction between water and the additive, which is highly important for designing high-performance Zn(BF₄)₂ electrolytes. Moreover, their behaviour under low-temperature conditions remains unexplored.

Here, we selected 1,3-dioxolane (DOL) from three commonly used cyclic ethers (1,4-dioxane, DOL and tetrahydrofuran (THF)) based on the solubility of Zn(BF₄)₂ and low temperature properties. Atomic-level analysis including DFT calculations and molecular dynamic simulation were conducted to reveal the interactions between water and DOL. Firstly, water molecules can terminate the Zn²⁺-initiated ring-opening polymerisation (ROP) of DOL, preventing uncontrolled solidification. Secondly, DOL disrupted the H-bond network between water molecules, markedly reducing the activity of free water in the electrolyte. In addition, DOL altered the solvation sheath of Zn²⁺, resulting in a lower de-solvation energy of desolating Zn²⁺. Benefiting from these, the HER, corrosion reactions, and dendrite growth are significantly suppressed, as confirmed by a series of experiments, such as in situ gas chromatography (GC), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thus, the modulated DOL–water electrolyte (95 : 5 vol%) enables stable operation at −30 °C. Using Mg²⁺-preintercalated vanadium oxide (MgVO) as a cathode, the Zn‖MgVO full cells at −30 °C demonstrate a high discharge capacity (150 mA h g⁻¹) with over 90% capacity retention after 300 cycles at 1C and run for over 2000 cycles at 10C with sustained cycling performance. By employing this water-modulated electrolyte, we achieve a marked improvement in battery longevity and safety at subzero temperatures. These results provide valuable insights for designing electrolytes in low-temperature AZIBs.

Results and discussion

We first surveyed various cyclic ethers to identify a suitable low-temperature solvent for Zn-based electrolytes. Fig. 1a and S2 compare the solubility of Zn(BF₄)₂·xH₂O in 1,4-dioxane, tetrahydrofuran (THF), and DOL vs. water. DOL exhibits the second-highest Zn salt solubility (5.8 mol L⁻¹), only surpassed by water, highlighting DOL's superior solvating ability for Zn²⁺. Combined with DOL's significantly low freezing point (−95 °C), the excellent solvating ability makes it an attractive primary solvent for a sub-zero Zn-ion electrolyte.³⁰ The thermogravimetric analysis (TGA) results show that the sample contains 4.4 water molecules in one hydraulic Zn(BF₄)₂ (Fig. S3). Without introducing additional water molecules, the result of dissolving Zn(BF₄)₂·4.4H₂O in 100% DOL showed that the liquid electrolyte gradually turned into a gel-like substance after 12 h of resting, indicating that the electrolyte undergoes self-solidification, which makes it unsuitable for practical use (Fig. S4). This is mainly because Zn²⁺ as a strong Lewis acid caused DOL to undergo a ROP, turning it into a long-chain polymer, which altered the solution's physicochemical properties and provided a mechanistic explanation for the observed solidification.^31–34

Fig. 1 (a) Solubility of Zn(BF₄)₂ in selected solvents (1,4-dioxane, THF, DOL and water). (b) Photograph of two kinds of electrolytes (hybrid 95% and pure water-based 1 m Zn(BF₄)₂); the Tyndall effect (laser light scattering) confirms the formation of ROP in pure electrolyte. (c) Reaction and termination mechanisms of ROP. (d) NCoor and g(r) of 1 m Zn(BF₄)₂ and hybrid 95% electrolyte between Zn²⁺ and the first solvation sheath (last 5 ns within equilibrium simulations). (e) ESP map of the Zn²⁺ first solvation sheath with 5H₂O–1DOL. (f) Surface area in each ESP range on the Zn²⁺ first solvation sheath surface of 6H₂O, 5H₂O–1DOL and 4H₂O–2DOL. Visual analysis of intermolecular interactions, (g) 1H₂O–1H₂O, (h) 1DOL–1H₂O and (i) 1DOL–2H₂O. Colour code: red, O; white, H; cyan, C.

When 5% water was introduced, this phenomenon disappeared (Fig. 1b). In the process of cationic polymerisation, if a water molecule approaches the end of a positively charged growing chain (oxonium ion), it can attack the positive centre as a nucleophile, thereby being protonated into H₃O⁺, and the original active chain is terminated (Fig. 1c). Additionally, water can also directly intercept the active centre of polymerisation and make the reaction inactivated. Accordingly, we formulated a “hybrid 95%” electrolyte of 95 vol% DOL + 5 vol% water, aiming to exploit DOL's low-temperature performance while using a minimal water fraction to assist ionic conduction and salt dissociation.

To understand how DOL molecules affect the Zn²⁺ solvation environment in 1 m Zn(BF₄)₂ solution systems, we conducted molecular dynamics simulations for both pure water-based 1 m Zn(BF₄)₂ and hybrid electrolytes (Fig. S5). The Zn–O radial distribution function (Fig. 1d) in the hybrid 95% electrolyte shows a well-defined first-shell peak at 2.1 Å, similar in position to that in pure water. But coordination number analysis reveals a distinct difference: each Zn²⁺ is typically coordinated by six H₂O molecules in the pure electrolyte. In contrast, in hybrid 95% electrolyte, one DOL can replace a water molecule to fill the Zn²⁺ coordination sphere. The introduction of DOL into the Zn²⁺ solvation sheath leads to a less polar solvation environment, as evidenced by the electrostatic potential (ESP) mapping of representative clusters. Fig. S6 shows the ESP distribution of Zn²⁺–6H₂O and Zn²⁺–4H₂O–2DOL clusters, both of them featuring strongly polarised regions (intense red and blue surfaces corresponding to high and low potential). By contrast, the ESP distribution of Zn²⁺–5H₂O–1DOL has more uniformly distributed surface potential, featuring less polarised regions (Fig. 1e). And the histogram of surface ESP values (Fig. 1f) quantifies this: the Zn²⁺–5H₂O–1DOL cluster exhibits a less central distribution of ESP compared to both Zn²⁺–6H₂O and Zn²⁺–4H₂O–2DOL clusters, indicating a reduced polarity and the lower de-solvation energy for desolating Zn²⁺.

Introducing DOL not only alters Zn²⁺–solvent electrostatics but also disrupts the H-bond network among free water molecules. Here, ‘free water’ refers to the effective availability of uncoordinated (or weakly coordinated) H₂O molecules that participate in H-bond percolation and interfacial reactions.³⁵ The independent gradient model based on Hirshfeld partition (IGMH) analysis provides a visual measure of interaction between two molecules. It shows regions of different interaction strengths, displayed through isosurfaces corresponding to different δ_g values (which indicates variations in the electron density gradient at points of interest). Additionally, the function sign(λ₂)e serves as a mapping tool to distinguish interaction types. Here, r represents the genuine electron density within the current system and a higher value signifies a stronger interaction. Moreover, λ₂ is the second largest eigenvalue of the electron density Hessian matrix, with “sign()” retrieving the sign of the value. Therefore, positive sign(λ₂) represents repulsion while negative sign(λ₂) represents attraction. Overall, the sign(λ₂)e value could be used to distinguish the interaction type and intensity. A pronounced blue peak appears in Fig. 1g, corresponding to strong O–H⋯O interactions between molecules (inset illustrates a representative water–water H-bond). As shown in Fig. 1h, this signature increases significantly with an intensity of 0.06 a.u., indicating that DOL has higher interaction strength with water molecules. This confirms that the DOL molecule is competitive with other water molecules and could disrupt the original H-bond network. Fig. 1i shows that a single DOL molecule could form two stable H-bonds with water molecules, which shows that DOL molecules have strong bonding ability to attract water molecules in the electrolyte.

To verify the DOL impact on the solution, characterization has been performed. As shown by contact angle measurements (Fig. 2a), a droplet of pristine electrolyte sits on Zn with a high contact angle (78.3°), indicating poor interfacial affinity. In contrast, the hybrid 95% electrolyte spreads readily, yielding a much lower contact angle of 34.8°. This 55% reduction in the contact angle suggests the significantly improved interfacial affinity of the electrolyte to Zn. Such improvement is beneficial for uniform Zn plating/stripping, which in turn regulates Zn²⁺ nucleation and growth, yielding smoother and more homogeneous Zn deposits. To evaluate the effect of the hybrid 95% electrolyte on the water activity, Fourier-transform infrared (FTIR) spectroscopy was conducted. Fig. 2b compares the O–H stretching region of the pure 1 m Zn(BF₄)₂ electrolyte and the hybrid 95% electrolyte. In the hybrid system, the O–H vibrational band is redshifted (to lower wavenumbers) and noticeably sharpened relative to that in aqueous solution, indicating that H-bonds among water molecules become fewer and weaker in the hybrid system. A similar conclusion can be obtained from the O–H bending band results in Fig. S7. It suggests that the DOL molecules effectively isolate water molecules from each other, breaking the continuous H-bond network. This H-bond network disruption is further confirmed by molecular dynamics simulations (Fig. S8). The average number of H-bonds per H₂O in 1 m Zn(BF₄)₂ is 2.33. As for the hybrid 95% electrolyte, the average number of H-bonds per water was reduced to 0.67, which was a 71% reduction. This confirmed that DOL can effectively reduce the water molecules' activity and thereby inhibit the HER.

Fig. 2 Fundamental experiments on two different electrolytes (hybrid 95% and pure water-based 1 m Zn(BF₄)₂). (a) Snapshots of contact angles of two electrolytes. (b) FTIR spectra of the two electrolytes. Raman spectra of (c) B–F bond and (d) Zn–O bond of the two electrolytes. (e) LSV curves of the two electrolytes. (f) Proportion of each H-bond type between water molecules of the two electrolytes. Schematic diagrams of the interphases between the Zn anode and (g) 1 m Zn(BF₄)₂ and (h) hybrid 95%. Colour code: red, O; white, H; black, C; blue, F; aqua, B; grey, Zn.

Raman spectroscopy was employed to probe the Zn²⁺ solvation environment and anion interactions in the hybrid electrolyte. Compared to pure 1 m Zn(BF₄)₂, the Raman peak of the B–F bond of BF₄⁻ in the hybrid electrolyte undergoes a blue shift and its peak intensity reduces (Fig. 2c). This indicates that BF₄⁻ is mostly solvated by water molecules in pure 1 m Zn(BF₄)₂, which can easily lead to hydrolysis. By contrast, in the hybrid 95% electrolyte, BF₄⁻ experiences a less free environment, which means that the hydrolysis reaction is inhibited, reducing the production of H⁺ and F⁻ ions in the solution. Fig. 2d shows the Raman spectra on the Zn–O stretching modes. In the aqueous electrolyte, Zn²⁺ exhibits a characteristic Zn–O vibration (originating from Zn(H₂O)₆²⁺ complexes). In hybrid 95%, this Zn–O bond blue shifts to a higher frequency and decreases in intensity, indicative of a changed Zn²⁺ coordination environment. It shows that fewer water molecules coordinate to Zn²⁺ in the hybrid 95% electrolyte, which proves that DOL can replace one water molecule and coordinate to Zn²⁺. This reorganised solvation structure is advantageous for Zn battery operation, as it limits the participation of water in deleterious reactions. Additionally, an obvious broad peak appeared in the O–H stretching vibration of water molecules at 3000–3700 cm⁻¹, and the number of water molecules with strong H-bonds decreased, indicating that DOL destroyed the H-bond network of water molecules (Fig. S9). Notably, a similar trend has been reported in other weakly solvating or co-solvent electrolytes, where replacing water in the Zn²⁺ solvation sheath with organics/anions suppresses water reactivity.³⁶

To verify the effect of DOL on the HER, linear sweep voltammetry (LSV) curves were measured. LSV profiles (Fig. 2e) demonstrate a significantly reduced cathodic current for the HER in the hybrid 95% electrolyte compared to 1 m Zn(BF₄)₂. A substantial increase in current is observed at around −0.66 V (vs. Zn/Zn²⁺) in 1 m Zn(BF₄)₂, corresponding to water that can be readily reduced to H₂. However, in the hybrid 95% electrolyte, the on-set potential is shifted to around −0.8 V (vs. Zn/Zn²⁺), which indicates that the introduction of DOL can significantly suppress the HER. Fig. S10 shows the classification of H-bond numbers between water molecules. For example, the 1 H-bond means that the water molecule only contributes to one H-bond and the 4-H-bond means that both H atoms and O atoms in the water molecule contribute to the formation of 4 H-bonds. In aqueous solution, stronger H-bonds between water molecules result in a robust H-bond network. Additionally, water molecules tend to accumulate on the surface of the negative electrode during the charging process, leading to the HER. By analysing the classification of different numbers of H-bonds between water molecules, the type of 1 H-bond and 2 H-bond is related to the weak H-bond, while the type of 3 H-bond and 4 H-bond is related to the strong H-bond. Fig. 2f reveals that in the 1 m Zn(BF₄)₂ electrolyte, water molecules forming 1 H-bonds and 2 H-bonds account for 31.97% and 42.18% of the population, respectively, while those with 3 H-bonds and 4 H-bonds represent 21.09% and 4.76%. For the hybrid 95% electrolyte, the share of 1 H-bond and 2 H-bond species jumps to 77.24% and 20.16%, whereas 3 H-bond and 4 H-bond species collapse to just 0.25% and 0%. Strong H-bonds promote a percolating water network that results in Grotthuss-type proton hopping and high interfacial H₂O accumulation.³⁷ In the hybrid 95% electrolyte, DOL has stronger interactions with water molecules, shifting a fraction of strong H-bonds to weak H-bonds, thereby lowering proton mobility and inhibiting the HER. This dramatic loss of strongly H-bonded clusters in the hybrid 95% electrolyte is consistent with our FTIR findings. By breaking the water–water H-bond network, the hybrid solvent suppresses overall water activity and isolates individual H₂O molecules.

In 1 m Zn(BF₄)₂ Zn electrolyte (Fig. 2g), Zn²⁺ are fully hydrated by water molecules (typically forming Zn²⁺–6(H₂O) complexes). This water-rich solvation environment makes the Zn anode prone to parasitic reactions. The HER competes with Zn²⁺ deposition, generating H₂ gas and abundant OH⁻ at the interface. The accumulation of OH⁻ raises the local pH, which corrodes the Zn surface by forming Zn(OH)₂/ZnO by-products. These alkaline by-products deposit loosely, and H₂ gas disrupts the electrode/electrolyte contact, resulting in an unstable interface. Meanwhile, Zn plating tends to occur unevenly – the metal sprouts into dendrites due to uncontrolled ion flux and the tip effect. These dendrites further induce side reactions by exposing fresh reactive Zn to the electrolyte, creating a self-reinforcing cycle of the HER, corrosion, and rough dendrite growth.³⁸ Ultimately, the aqueous Zn anode/electrolyte interface is dynamic and rough, characterised by pH fluctuations, continuous H₂ bubbling, and the accumulation of porous Zn hydroxides – all of which affect reversible Zn plating/stripping.

By contrast, in the hybrid 95% electrolyte (Fig. 2h), the Zn²⁺ solvation structure is fundamentally reorganised.³⁹ DOL molecules readily coordinate to Zn²⁺, replacing one water molecule in the cation's primary solvation shell. This reduced water activity weakens water-driven side reactions, and the onset potential of the HER is shifted to a significantly more negative value. Moreover, the hybrid 95% solvation sheath alters the Zn deposition kinetics: it raises the nucleation overpotential for Zn metal, promoting more uniform and dense Zn growth. Instead of disordered dendrites, Zn deposits as a compact layer, since ion transport and reduction occur in a more controlled manner. The hybrid electrolyte also regulates the electric double layer at the interface. DOL and BF₄⁻ participate in the inner Helmholtz layer, which helps homogenise the local electric field and Zn²⁺ flux, which is beneficial for the Zn²⁺ deposition.⁴⁰ In brief, by minimising water–water interactions, the hybrid 95% electrolyte lowers the “water activity” and weakens the Zn²⁺–solvent binding.

To evaluate the room temperature performance of the cells with two different electrolytes, Zn‖Zn symmetric cells were assembled to test the effect of adding DOL on the cycling life and stability of the Zn anode. As shown in Fig. 3a, the hybrid 95% electrolyte enables remarkably prolonged, dendrite-free cycling. The cell sustains stable Zn plating/stripping for over 350 h at 1 mA cm⁻² without short-circuit or a significant polarisation increase, whereas the Zn‖Zn cell in 1 m Zn(BF₄)₂ fails within only 70 h due to rampant dendrite growth. This dramatic improvement in cycle life arises from the regulated Zn²⁺ solvation environment in the hybrid electrolyte, which suppresses water-driven side reactions and uncontrolled Zn deposition. Electrochemical characterisation studies were performed to evaluate the CE of Zn stripping/plating with different electrolytes. The hybrid 95% electrolyte also delivers nearly ideal Zn plating/stripping reversibility. In Zn‖Cu half-cells (Fig. 3b), an average CE of 99.92% is achieved over 400 cycles, indicating that almost all deposited Zn is successfully stripped with negligible loss. This high CE is significantly greater than that of 1 m Zn(BF₄)₂ electrolyte systems, which confirms that the HER and Zn corrosion are effectively inhibited.

Fig. 3 Room-temperature electrochemical performance: (a) the cycling lifespan of Zn‖Zn symmetric cells in 1 m Zn(BF₄)₂ and hybrid 95% electrolyte at 1 mA cm⁻². (b) CE of Zn‖Cu cells in 1 m Zn(BF₄)₂ and hybrid 95% electrolyte at 1 mA cm⁻². (c) Tafel curves for the Zn‖Zn symmetrical cells 1 m Zn(BF₄)₂ and hybrid 95% electrolyte. (d) CV curves in the first 3 cycles of the Zn‖MgVO full cell. (e) EIS measurements of the Zn‖MgVO full cell with the hybrid 95% electrolyte.

Tafel analysis (Fig. 3c) shows that the hybrid 95% electrolyte shifts the corrosion potential (E_corr) to more positive values and increases the Zn²⁺/Zn exchange current density compared to the Tafel curve of the 1 m Zn(BF₄)₂ aqueous electrolyte, indicating markedly enhanced Zn²⁺ charge transfer kinetics. Moreover, the hydrogen-evolution branch exhibits lower cathodic current at a given overpotential than in pure water, evidencing kinetic suppression of the HER. To evaluate the electrochemical reversibility of full cells at room temperature, the Zn‖MgVO full cells were assembled and tested by cyclic voltammetry (CV) measurements as shown in Fig. 3d. The first three cycles display nearly overlapping redox peaks for the MgVO cathode, with slight shifts in peak potential and nearly constant peak currents. Such CV curves indicate that Zn²⁺ (de)intercalation in the MgVO cathode proceeds with minimal side reactions over initial cycles. In addition, the sharp and well-defined anodic/cathodic peaks persist without degradation, confirming a highly stable cathode/electrolyte interface. In addition, galvanostatic intermittent titration technique (GITT) analysis (Fig. S12) reveals markedly reduced polarisation in the hybrid electrolyte, signifying improved Zn²⁺ transport kinetics. Each current pulse induces only a slight transient voltage step, and the cell voltage quickly relaxes to its steady-state value, indicating minimal internal polarisation. The rapid attainment of near-equilibrium potential after each pulse demonstrates that Zn²⁺ insertion/extraction is highly reversible in the hybrid 95% electrolyte. This enhanced ion mobility is consistent with the low R_ct observed in EIS and stems from the solvent-separated Zn²⁺ solvation structure that minimises charge-transfer barriers.

Electrochemical impedance spectroscopy (EIS) measurements (Fig. 3e) further underscore the stable electrode/electrolyte interface afforded by the hybrid DOL–H₂O medium. Nyquist plots collected at various charge–discharge states exhibit similar semicircles, with only slight changes over cycling. The charge-transfer resistance (R_ct) remains low, indicating a stable impedance profile that suggests the hybrid electrolyte prevents the formation of thick resistive layers and maintains efficient ion transport over prolonged operation. The DRT contour map reveals a clear evolution of the cell's interfacial kinetics during cycling (Fig. S11).⁴¹ A prominent relaxation feature in the millisecond τ range (10⁻³–10⁻² s) emerges as the dominant peak, corresponding to the charge-transfer process at the electrode/electrolyte interface. Throughout the charge/discharge process, there are no new slow-time-constant peaks that appear. The high-frequency response stays nearly unchanged, suggesting that aside from the charge-transfer step, other processes such as ion transport remain stable, supporting the superior electrochemical performance (efficient charge transfer and rapid kinetics) enabled by the DOL–water hybrid electrolyte.

To verify the effects of these two electrolytes on the HER, in situ gas chromatography (GC) was conducted. The results confirm that the hybrid 95% electrolyte remarkably suppresses the HER during Zn plating/stripping. The pure aqueous electrolyte exhibits a pronounced H₂ signal by gas evolution data (Fig. 4a). In aqueous Zn electrolytes, a large excess of “free water” at the Zn interface readily participates in the HER, siphoning charge away from metal plating. Such free H₂O can easily adsorb on the Zn surface and get reduced to H₂, especially at high local alkalinity or nucleation sites on the freshly deposited metal. In contrast, the hybrid electrolyte shows virtually no detectable H₂ gas, confirming that the HER is effectively inhibited (Fig. 4b), as the two-dimensional GC curves show in Fig. S13. The stable liquid environment, combined with suppressed gas evolution, ensures that the Zn electrodeposits in the hybrid electrolyte maintain integrity and uniformity throughout the low-temperature operation. This difference highlights an advantage of this hybrid approach.

Fig. 4 Zn plating behaviour studies. In situ gas chromatography curves to dynamically evaluate the H₂ amount during the Zn plating/stripping in (a) 1 m Zn(BF₄)₂ and (b) hybrid 95% electrolyte. (c) XRD patterns of Zn electrodes after 50 cycles at −30 °C. (d) 3D laser confocal images of the cycled Zn foil with two kinds of electrolytes at −30 °C. SEM images of Zn electrodes in different electrolytes: after 50th plating in (e) 1 m Zn(BF₄)₂ and (f) hybrid 95% electrolyte at −30 °C.

It's critical to evaluate the influence of the electrolyte on the Zn plating behaviour at extremely low temperature. Zn electrodes stripped from cells with different electrolytes were evaluated after 50 cycles at −30 °C to explore the effect on the Zn reversibility. First, in Fig. S14, XRD pattern of MnVO shows that the strongest peak at 7.39° corresponds to a larger lattice spacing of 11.95 Å, suggesting that the introduction of Mn(II) with water together can expand the interlayer spacing further as reported in preinserted MgVO, which promotes efficient Zn²⁺ intercalation and deintercalation.⁴² In 1 m Zn(BF₄)₂, a pronounced diffraction peak appears at 31.9°, attributable to the (111) plane of ZnF₂, confirming extensive corrosion by-product formation (Fig. 4c). By contrast, this ZnF₂ peak disappeared in electrodes cycled in the 95% DOL-based hybrid electrolyte, indicating the suppression of corrosion products. Besides, for the Zn electrode in 1 m Zn(BF₄)₂, the peak intensity ratio of Zn(002)/Zn(101) was 31.93%. But for the Zn electrode in the hybrid 95% electrolyte, the peak intensity ratio of Zn(002)/Zn(101) increased to 45.16%, indicating that DOL changes the preferred orientation for Zn(002) deposition. In summary, the hybrid 95% electrolyte yields a smooth, dendrite-free Zn morphology by steering crystal nucleation and growth at the molecular level. These interfacial modifications are highly effective in maintaining Zn anode stability at −30 °C, highlighting the hybrid electrolyte's potential for enabling ultra-stable low-temperature Zn batteries.

In addition, electrodes stripped from cells with different electrolytes were evaluated after 50 cycles at −30 °C. 3D laser confocal images show that cycled Zn foil from 1 m Zn(BF₄)₂ at −30 °C produces a highly rough and dendritic surface, whereas the hybrid 95% electrolyte produces a relatively compact deposit (Fig. 4d). This confirms again that DOL contributes to the uniformity and compactness of Zn deposition behaviour at low temperature. In the 1 m Zn(BF₄)₂ electrolyte, the SEM image (Fig. 4e) displays ramified Zn structures with protruding flakes and voids, characteristic of uncontrolled dendrite growth under low-temperature conditions. In contrast, the hybrid 95% electrolyte promotes a uniform, densely packed metal layer with no obvious dendrites, indicating a dramatically more orderly Zn deposition process (Fig. 4f). This improvement is attributed to the molecular interactions at the interface in the presence of the organic co-solvent. Cyclic ethers like DOL are known to adsorb on the electrode surface and modulate Zn²⁺ plating kinetics, guiding the metal to deposit in a more planar, layer-by-layer fashion.⁴³ Such adsorption of the ether molecules on certain Zn facets likely increases the nucleation density and limits runaway growth at any single point. Therefore, the hybrid 95% electrolyte effectively regulates the Zn electrocrystallisation at −30 °C, yielding a compact anode interface free of the large protrusions seen in the pure aqueous system.

To confirm the reversibility of the Zn plating/stripping at an extreme temperature of −30 °C, electrochemical tests were performed. The results demonstrate that the hybrid electrolyte maintains excellent Zn anode reversibility and plating uniformity at −30 °C. The Zn‖Zn symmetric cell tests further highlight the excellent interfacial stability imparted by the hybrid electrolyte. As shown in Fig. 5a, the symmetric cell exhibits a consistently low polarisation over 1000 h of continuous plating/stripping at −30 °C. This stable behaviour implies that dendrite formation is effectively suppressed – there are no sudden voltage jumps or cell shorting, indicating uniform Zn plating without the growth of large, spiky deposits.

Fig. 5 Full battery electrochemical performance at −30 °C. (a) The cycling lifespan of Zn‖Zn symmetric cells in 1 m Zn(BF₄)₂ and hybrid 95% electrolyte at 0.2 mA cm⁻². (b) CE of Zn‖Cu cells in 1 m Zn(BF₄)₂ and hybrid 95% electrolyte at 0.2 mA cm⁻². (c) CV curves in the first cycle of Zn‖Cu batteries. (d) Schematic diagram of the Zn‖MgVO full cell. (e) GITT test of the Zn‖MgVO full cell with the hybrid 95% electrolyte. (f) Rate capability of the Zn‖MgVO full cell at different rates from 1C to 10C. (g) Cycle performance of Zn‖MgVO cells at −30 °C and 1C. (h) Cycle performance of Zn‖MgVO cells at −30 °C and 10C.

In Zn‖Cu half-cells, an average CE of 98.6% is achieved (Fig. 5b), indicating that nearly all the Zn deposited on Cu can be stripped back with only little losses to side reactions. This remarkable reversibility is maintained over 300 cycles at a plating capacity of 0.2 mA h cm⁻², demonstrating stable long-term Zn cycling under subzero conditions. As a result, Zn deposits more uniformly and reversible Zn²⁺/Zn redox chemistry is preserved in each cycle. Such performance is far beyond what pristine water electrolytes can sustain at low temperature – below 0 °C, the 1 m Zn(BF₄)₂ electrolyte suffers from reduced ionic conductivity, leading to poor Zn deposition efficiency. The CV response in Fig. 5c further confirms that Zn deposition/stripping kinetics remain fast at −30 °C. Clear cathodic and anodic peaks are observed corresponding to Zn²⁺ plating/stripping even in this cold environment. The preservation of a well-defined redox peak indicates that Zn²⁺ can efficiently transfer between the electrolyte and the Zn surface. The small peak separation in the CV suggests a low charge-transfer resistance and facile Zn²⁺ diffusion to the electrode.

Fig. 5d illustrates the configuration of the full cell, which is related to a Zn metal anode with a MgVO cathode in the hybrid 95% electrolyte. In this setup, the hybrid solvent environment serves a dual purpose: it protects the Zn anode by modulating the Zn deposition behaviour, and it facilitates Zn²⁺ transport by the DOL regulation of the first Zn²⁺ solvation structure. The predominance of DOL in the liquid phase ensures that the electrolyte remains unfrozen and maintains good wettability with both electrodes at −30 °C. This engineered solution environment results in an electrolyte that is both anti-freezing and ionically conductive, allowing the full cell to operate efficiently in the cold.

Quantitative analysis of Zn²⁺ kinetics in the full cell was carried out by the GITT. The voltage response during the GITT (solid discharge and charge curves in Fig. 5e) shows only mild polarisation for each current pulse, indicating that the cell equilibrates quickly – a first hint of fast ion transport. From the GITT data, the Zn²⁺ diffusion coefficient within the cathode was calculated at various states of charge and discharge. The diffusion coefficients remain in the order of 10⁻¹¹–10⁻¹² cm² s⁻¹ even at −30 °C, which highlights the efficacy of the hybrid electrolyte in preserving fast solid-state ion transport. Here, the GITT confirms that the hybrid electrolyte preserves good Zn²⁺ intercalation/deintercalation kinetics at −30 °C in tandem with its excellent anode interfacial stability.

The hybrid electrolyte's impact is clearly demonstrated in the capability of the full cell (Fig. 5f). The Zn‖MgVO cell was tested from 1C to 10C at −30 °C. Remarkably, the specific capacity remains high across this broad range of rates. At 1C, the cell delivers a specific capacity close to 150 mA h g⁻¹. As the current rate increases, the capacity drops gradually. At an extreme high rate of 10C, a considerable capacity is still obtained (50 mA h g⁻¹). Besides, when the rate is reduced back to 1C, the cell's specific capacity fully recovers, returning to its initial high value. The galvanostatic charge/discharge (GCD) curves show the electrochemical behaviour across cycling at different rates (Fig. S15). The recovery proves that there is no permanent damage that occurred during the high-rate cycling. Throughout the rate test, the CE stays nearly 100%, and no abnormal voltage behaviour is observed, underscoring the reversible nature of the fast Zn²⁺ insertion/extraction processes.

Beyond exceptional rate capability, the full cell also exhibits robust long-term cycling stability at −30 °C. The cell delivers a high specific capacity over hundreds of cycles with minimal decay at a discharge rate of 1C (Fig. 5g), retaining over 90% of its initial capacity after 300 cycles. The CE in this prolonged test averages 98.23%, indicating that each cycle incurs little irreversible loss of active Zn or the cathode material, and the stable capacity trend confirms that both electrodes are well-preserved by the electrolyte. In Fig. S16, the first-cycle curve (red) exhibits a well-defined charge/discharge plateau at around 1.4 V and a specific capacity of over 140 mA h g⁻¹, consistent with efficient Zn²⁺ intercalation into the MgVO cathode. Notably, the subsequent 50th, 200th, and 300th cycles display nearly overlapping profiles with minimal voltage hysteresis and no discernible degradation in capacity or polarisation. This remarkable overlap confirms the excellent reversibility and long-term stability of the hybrid electrolyte system. These results further support the hybrid electrolyte's ability to suppress parasitic side reactions and enable dendrite-free Zn plating/stripping, even under prolonged cycling at sub-zero temperatures.

Most impressively, the cell maintains its performance even under continuous ultra-high-rate operation. Fig. S17 and 5h show the long-term cycling of Zn‖MgVO full cells at 5C and a 10C rate at −30 °C. And GCD curves reveal a highly stable electrochemical behaviour across extended cycling in Fig. S18. The capacity curve stays relatively flat, demonstrating an extraordinary resilience to stress at both 5C and 10C. Simultaneously, the CE stays at around 99–100% for the entire duration (averaging 99.89% of 5C and 99.7% of 10C), which means that most Zn²⁺ ions inserted during charging are extracted on discharge under this high-rate condition. Here, the hybrid electrolyte's protective effect on the Zn anode and its promotion of fast kinetics allow the cell to avoid those failure modes. These findings demonstrate a significant step toward batteries that can function in harsh, subzero environments, expanding the potential for aqueous Zn-based energy storage in cold climates or high-power applications.

Conclusions

In conclusion, we demonstrated that a designed DOL–water (95 : 5 vol%) electrolyte with Zn(BF₄)₂ salt can overcome the persistent challenges of low-temperature AZIBs. By leveraging DOL's low freezing point and high Zn(BF₄)₂ salt solubility, the electrolyte remains fluid and ionically conductive at temperatures as low as −30 °C. Importantly, the small addition of water is tuned to inhibit unwanted polymerisation of DOL, striking a balance that maintains chemical stability. Atomic-level analysis uncovered the interactions between water and DOL. Firstly, water molecules can quench the Zn²⁺-initiated ring-opening polymerization of DOL, stopping the reaction before it turns into an uncontrolled gel. Secondly, DOL breaks up the H-bond network among water molecules, which sharply reduces the electrolyte's free-water activity. Thirdly, by modifying Zn²⁺ solvation shell, DOL lowers the energy required for the ion to shed its solvent molecules. This disruption of the aqueous structure dramatically lowers the activity of water in the electrolyte and suppresses side reactions during charge/discharge cycles. Zn‖MgVO full cells using this water modulated electrolyte retain more than 90% of their capacity over 300 charge/discharge cycles at −30 °C (1C). Even under extremely demanding conditions (10C high-rate cycling), the cells operated stably for more than 2000 cycles, highlighting the greatly enhanced reversibility of the Zn anode. This durability is attributed to the formation of a stable anode surface and reduced water reactivity, which enables Zn–metal batteries to deliver reliable performance in freezing temperatures, thus extending the usability of AZIBs to cold-climate applications. The cells operated stably for over 2000 cycles under extremely demanding 10C cycling conditions, demonstrating markedly enhanced Zn anode reversibility. The combination of a stable anode surface and suppressed water reactivity enables zinc–metal batteries to function reliably even in freezing environments, broadening the applicability of AZIBs in cold climates. We expect that this work will inspire further development of hybrid electrolytes to achieve safe, long-lasting, and all-climate AZIB storage solutions.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI) or from the author. Supplementary information: detailed descriptions of both experimental procedures and computational calculation details, supplementary figures illustrating intermediate results and additional analyses, and supplementary tables summarising extended datasets and parameter settings. These materials are intended to enhance transparency, reproducibility, and interpretation of the main text. See DOI: https://doi.org/10.1039/d5ta07679j.

Acknowledgements

Y. Zhu and X. Zhao contributed equally to this work. We acknowledge financial support from the Australian Research Council (CE230100032 and DE230100471). MD computations within this research were undertaken with the assistance of resources and services from the Phoenix High Performance Computing (HPC), which is supported by The University of Adelaide. DFT computations were undertaken with the assistance of resources and services from the National Computational Infrastructure (NCI), which was supported by the Australian Government.

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