Anion Engineering Toward Regulated Solvation Structure and Enhanced Performance in Hydrated Eutectic Electrolytes for Aluminum‐Ion Batteries

ABSTRACT

The practical deployment of rechargeable aluminum‐ion batteries (AIBs) is hindered by the absence of electrolytes that concurrently offer low cost, efficient ion transport, and stable interfacial electrochemistry. While hydrated deep eutectic electrolytes (HEEs) present a promising avenue, their advancement has been limited by high viscosity and poorly controlled interfacial reactions. This work establishing anion engineering, rather than solely focusing on ligand modulation, as a critical yet previously underappreciated design strategy. We systematically elucidate how the identity of the anion governs the coordination architecture within an ethylene glycol (EG)–based HEE. Crucially, the NO₃ ⁻ anion induces the formation of a homogeneous, inner‑sphere Al³⁺ solvation complex, which simultaneously reduces the dynamic viscosity, improves interfacial wettability, and elevates the cathodic stability limit. This tailored solvation structure lowers the desolvation energy barrier. As a result, the NO₃ ⁻‑HEE enables exceptional aluminum anode reversibility, evidenced by a high exchange current density and stable, dendrite‑free plating/stripping over 400 h. Full cells coupled with a CuHCF cathode deliver a specific capacity of ∼123 mAh·g⁻¹ and demonstrate outstanding capacity retention over 500 cycles. This study provides a foundational anion‑centric design principle for next‑generation, high‑performance eutectic electrolytes.

1. Introduction

The escalating demand for grid‐scale energy storage, driven by the rapid expansion of intermittent renewable sources like solar and wind, has intensified the search for post‐lithium battery technologies with low cost, high safety, and substantial capacity [1, 2]. Among the contenders, rechargeable aluminum‐ion batteries (AIBs) are particularly promising, leveraging the third most abundant element in the Earth's crust, aluminum, which offers an ultra‐high theoretical volumetric capacity (8040 mAh cm⁻³) [3, 4, 5].

The viability of any AIB technology is fundamentally dictated by its electrolyte [6, 7]. While conventional chloroaluminate‐based ionic liquids (ILs), typified by the AlCl₃/1‐ethyl‐3‐methylimidazolium chloride ([EMIm]Cl) system, enable reversible Al electrochemistry, their practical application is hampered by high cost, stringent moisture sensitivity, and inherent corrosiveness [8, 9]. Hydrated eutectic electrolytes (HEEs), formed through the complexation of a hydrogen bond acceptor with a low‐cost hydrogen bond donor, have emerged as a compelling alternative [10, 11, 12, 13]. Their attractive features, including ease of synthesis, nonflammability, and low toxicity, position them as ideal candidates for sustainable large‐scale storage. However, the development of HEEs is constrained by their high viscosity, moderate ionic conductivity, and insufficient stability at electrode interfaces.

To overcome these limitations, electrolyte engineering must focus on tailoring the coordination chemistry within the HEEs. A particularly innovative strategy involves the formulation of HEEs, where a controlled, minimal amount of water is deliberately introduced. While water is typically proscribed in metal batteries, its judicious incorporation can serve as a molecular‐level “lubricant,” effectively disrupting the dense hydrogen‐bonding network of the HEE [14, 15, 16, 17]. This leads to a dramatic reduction in viscosity and a concomitant boost in ionic conductivity, thereby addressing a primary bottleneck. The success of this strategy, however, hinges on a critical challenge: suppressing the deleterious reactivity of water at the highly reductive Al anode.

This challenge directs the focus to the strategic selection of the HBD. Ethylene glycol (EG) emerges as a preeminent candidate for constructing such a hydrated HEE [18, 19]. Its molecule, featuring dual hydroxyl groups, acts as a superior coordination modulator. EG can simultaneously chelate Al‐species and form stronger hydrogen bonds with water molecules than water does with itself. This effectively “locks” water into a tightly bound solvation cluster, significantly passivating its activity and shifting its reduction potential to a much lower level [11, 20, 21]. Consequently, an EG‐based hydrated HEE promises the dual benefit of enhanced ion transport kinetics and inherent interfacial stability.

Beyond the HBD and water content, the solvation structure—the ultimate governor of electrochemical behavior—is a complex ensemble of cations, HBDs, water, and anions [22, 23]. Yet, the influential role of the anion remains disproportionately unexplored in HEEs. The anion identity is paramount: it dictates the strength of cation‐anion interaction, influencing ion‐pair formation, cluster size, and thus cation mobility and desolvation kinetics [24, 25]. In a hydrated HEE, anions may also compete for coordination sites within the EG/water/Al^{3 +} network, thereby subtly influencing the overall solvation structure and the efficacy of water passivation.

In this study, we present a systematic investigation into anion regulation within a novel EG‐based hydrated eutectic electrolyte for high‐performance AIBs. This work aims to elucidate how the selection of anions (NO₃ ⁻, ClO₄ ⁻, and OTF⁻) governs the physicochemical properties, fine‐tunes the Al³⁺ solvation structure, and ultimately dictates the interfacial electrochemical behavior at both electrodes. Through an integrated approach combining spectroscopic analyses, electrochemical characterization, and theoretical computations, we reveal the underlying mechanism by which anions direct ion‐transport kinetics, which is critical for enabling dendrite‐free aluminum deposition. When coupled with a CuHCF cathode, the electrolyte optimized via anion engineering demonstrates remarkable rate performance and cycling stability. This study establishes an anion‐centric design principle for advanced hydrated eutectic electrolytes, providing a fundamental roadmap for their development toward practical, sustainable energy storage.

2. Results and Discussion

2.1. Physicochemical Properties and the Underlying Anion Effects

The successful synthesis of the HEE is confirmed by the formation of homogeneous, transparent liquids at ambient temperature for all three mixtures: NO₃‐HEE, OTF‐HEE, and ClO₄‐HEE (Figure S1). Prepared with a fixed Al‐salt to EG molar ratio of 1:8 and equivalent water content, this observed homogeneity signifies the establishment of a stable liquid phase, where crystallization is suppressed by an extensive hydrogen‐bonding network among Al^{3 +} cations, EG molecules, water, and the respective anions.

The fundamental physical properties of these hydrated DESs, which govern their practical handling and ion transport, are summarized in Figure 1a and reveal significant anion‐dependent trends. Critical for electrolyte engineering, the dynamic viscosity measured at 25°C follows the order: OTF‐HEE (35.8 mPa·s) > ClO₄‐HEE (28.4 mPa·s) > NO₃‐HEE (26.1 mPa·s). The elevated viscosity of OTF‐HEE is attributed to the substantial molar volume and strong coordinating ability of the trifluoromethanesulfonate anion, which increases intermolecular friction. In contrast, the lower viscosity of ClO₄‐HEE aligns with the weak coordination tendency and spherical symmetry of the perchlorate anion. Notably, NO₃‐HEE exhibits the lowest viscosity, indicating that the nitrate anion promotes a fluid microenvironment conducive to enhanced molecular mobility.

FIGURE 1.

Physicochemical properties of HEEs. (a) Viscosity and conductivity of HEEs. (b) TGA curves of HEEs. (c) Hygroscopic stability of HEEs versus EMImCl/AlCl₃. (d) Contact angles of the HEEs on an Al substrate. (e) DSC curves of the HEEs and EG. (f) Electrochemical stability windows of the HEEs measured by linear sweep voltammetry at a scan rate of 1 mV s⁻¹ using a three‐electrode configuration with a glassy carbon working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode.

The practical stability of an electrolyte encompasses both thermal and atmospheric resistance. Thermogravimetric analysis (TGA) curves (Figure 1b) indicate that all DESs undergo multi‐stage mass loss upon heating. The first decomposition step, occurring below 160°C, is attributed to the evolution of uncomplexed and coordinated water molecules. NO₃‐HEE completes this dehydration at the lowest temperature, implying a relatively labile association of water within its solvation sphere, which may be advantageous for preconditioning the electrolyte to a stable hydration state. Furthermore, the hygroscopic stability was evaluated by monitoring mass change under controlled ambient exposure (Figure 1c). The conventional 1‐ethyl‐3‐methylimidazolium chloride/aluminum chloride (EMImCl/AlCl₃) ionic liquid exhibited the most rapid moisture uptake. Among the HEE, NO₃‐HEE demonstrated the lowest hygroscopicity, highlighting its superior stability against atmospheric humidity—a vital characteristic for ease of processing and long‐term cell operation.

The interfacial wettability of the electrolytes on the aluminum electrode, a critical parameter governing charge‐transfer kinetics and deposition homogeneity was assessed by contact angle measurements (Figure 1d). Among the three systems, NO₃‐HEE exhibits the smallest contact angle (52.3°), indicating the most favorable wetting behavior. This superior wettability aligns with its lowest viscosity, suggesting that the fluid microstructure promoted by the nitrate anion facilitates intimate electrode–electrolyte contact. Such enhanced interfacial affinity favors a uniform distribution of the electric field and ion flux, which is essential for achieving smooth and dendrite‑free aluminum deposition.

The low‑temperature phase behavior of the hydrated eutectic electrolytes was characterized by differential scanning calorimetry (DSC) (Figure 1e). The DSC curves for the HEEs display clear glass‑transition events, confirming their amorphous nature within the measured range. In contrast, pure EG exhibits a crystallization and melting event, with a freezing point (T_l) around –13 °C. The absence of such crystallization exotherms in the HEEs, coupled with their low glass‑transition temperatures (T_m), demonstrates that the introduction of aluminum salts effectively suppresses solvent crystallization and stabilizes a deeply supercooled, glassy state.

The functional electrochemical stability of the electrolytes, as determined by their anodic and cathodic limits, is presented in Figure 1f. The oxidative stability follows OTF‐HEE > NO₃‐HEE > ClO₄‐HEE, consistent with the intrinsic electrochemical robustness of the anions. Conversely, the reductive stability, crucial for aluminum anode compatibility, exhibits the trend NO₃‐HEE < ClO₄‐HEE < OTF‐HEE. The most negative reduction potential observed for NO₃‐HEE indicates superior thermodynamic stability against reductive decomposition. This is ascribed to its unique solvation structure, which elevates the energy barrier for the reduction of active species. Consequently, this inherent interfacial stability is foundational to its exceptional electrochemical performance.

2.2. Solvation Structure: Decoding the Anion‐Specific Coordination Environment

To elucidate the anion‐dependent solvation structure within the hydrated eutectic electrolytes, the Raman spectral region characteristic of EG molecular fingerprints (990–1140 cm⁻¹)—encompassing C─C stretching, C─O stretching, and CH₂ rocking vibrations was examined (Figure 2a) [26]. The spectrum of pure EG displays sharp, well‐resolved characteristic peaks. Upon electrolyte formation, distinct spectral modifications emerge. In NO₃‐HEE, these peaks persist with only minor broadening, indicating that the nitrate anion induces minimal perturbation to the local bonding environment of EG. Greater spectral broadening is observed for ClO₄‐HEE, whereas in OTF‐HEE, the vibrational signatures of EG are markedly attenuated, evidencing substantial disordering of the solvent backbone due to the bulky OTF⁻anion.

FIGURE 2.

Spectroscopic characterization of solvation structures in HEEs. Raman spectra of HEEs and EG in the range of (a) 990–1140 cm⁻¹ and (b) 250–3750 cm⁻¹. (c) ²⁷Al NMR spectra of the HEEs. (d) ¹H NMR spectra of HEEs. FTIR spectra of HEEs and EG in the range of (e) 500–4000 cm⁻¹ and (f) 2000–3650⁻¹.

The O─H stretching region (3000–3600 cm⁻¹) in the full‐range spectrum (Figure 2b) reflects the condition of the hydrogen‐bonding network involving water and EG. In NO₃‐HEE, the O─H band remains relatively narrow and well‐defined, denoting a stabilized, ordered network—consistent with its low water‐binding energy observed by TGA. In stark contrast, the O─H band in OTF‐HEE exhibits pronounced broadening and a redshift, indicative of a strongly perturbed and disordered hydrogen‐bonding environment, which correlates with its elevated viscosity. The ClO₄‐HEE system again demonstrates an intermediate behavior.

The local coordination environment of Al³⁺ was directly probed using ²⁷Al nuclear magnetic resonance (NMR) spectroscopy (Figure 2c). All electrolytes exhibit a primary resonance near 0 ppm, which is characteristic of Al³⁺ in an octahedral, six‐coordinate geometry [27]. However, pronounced differences in the lineshapes provide critical insight into the homogeneity of this solvation sphere across the different anions. The spectrum of NO₃‐HEE is distinguished by a sharp, symmetric singlet. This narrow linewidth indicates a highly uniform chemical environment for all Al³⁺ ions, consistent with a single, predominant solvation structure. In contrast, the resonance for ClO₄‐HEE is noticeably broadened and exhibits clear asymmetry or a shoulder, suggesting a distribution of slightly different chemical shifts. This spectral feature implies the coexistence of multiple, distinct six‐coordinate Al³⁺ species within the electrolyte, indicative of a less homogeneous solvation environment. The most significant broadening is observed for OTF‐HEE, which displays a very broad and poorly resolved peak. The spectrum of NO₃‐HEE is distinguished by a sharp, symmetric singlet with a full width at half maximum (FWHM) of 120 Hz, indicating a highly uniform chemical environment for all Al³⁺ ions. In contrast, the resonance for ClO₄‐HEE exhibits significant broadening (FWHM = 350 Hz) with clear asymmetry, suggesting a distribution of distinct six‐coordinate species. The most substantial broadening is observed for OTF‐HEE, which displays a very broad peak (FWHM = 820 Hz), confirming a highly heterogeneous solvation environment. This structural gradient has direct functional implications. A uniform solvation shell, as in NO₃‐HEE, promotes consistent ion transport kinetics.

¹H NMR spectroscopy was employed to investigate the local environment of EG within the hydrated eutectic electrolytes, focusing on the methylene (‐CH₂, ∼3–4 ppm) and hydroxyl (─OH, ∼5–6 ppm) proton regions [28] (Figure 2d). The linewidth and resolution of these signals serve as direct indicators of the uniformity of the EG backbone's electronic environment and the hydrogen‐bonding network, respectively. In the ‐CH₂ region, spectral homogeneity follows the order NO₃‐HEE (sharp singlet) > ClO₄‐HEE (distinct doublet) > OTF‐HEE (severely broadened singlet), directly reflecting the coordinating character of the anions and fully consistent with the ²⁷Al NMR results.

Similarly, in the ‐OH region, the signal evolves from a well‐defined peak in NO₃‐HEE to a broad, diffuse resonance in OTF‐HEE, with ClO₄‐HEE again exhibiting an intermediate state. This progression quantitatively mirrors the increasing disorder in the hydrogen‐bonding network, as independently established by Raman spectroscopy. Fourier‐transform infrared spectroscopy (FTIR) confirm preservation of the EG molecular framework, as skeletal vibrations below 1500 cm⁻¹ remain largely intact (Figure 2e) [17]. Pronounced broadening appears specifically in the O─H stretching region (2000–3650 cm⁻¹, Figure 2f), indicating reorganization of the hydrogen‐bonding network that varies systematically with anion identity. Detailed examination of the O─H stretching region (3100–3650 cm⁻¹, Figure 2f) reveals a consistent redshift in all HEEs relative to neat EG. This shift reflects strengthened hydrogen‑bonding interactions resulting from coordination of EG hydroxyl groups to Al³⁺ and anions. The magnitude of the redshift correlates directly with the anion's coordinating strength, progressing from the mildest perturbation in NO₃‑HEE to the most pronounced in OTF‑HEE, with ClO₄‑HEE exhibiting intermediate behavior. These FTIR results align coherently with Raman and ²⁷Al NMR data. Together, they elucidate a competitive coordination mechanism where the NO₃ ⁻ anion plays a pivotal role in redistributing water molecules. By forming a strong inner‐sphere coordination with Al³⁺, NO₃ ⁻ induces the physical displacement of water molecules from the first solvation shell. These displaced water molecules are subsequently anchored within the EG‐dominated hydrogen‐bonding network, as evidenced by the narrowed O─H vibrational signatures. This anion‐mediated sequestration effectively deactivates the water molecules, ensuring that the Al³⁺ solvation environment remains homogeneous and thermodynamically stable against reductive decomposition, which is critical for achieving high‐reversibility aluminum plating/stripping.

Molecular dynamics (MD) simulations and the derived radial distribution functions (RDFs) and coordination numbers (CNs) provide quantitative evidence of anion‐specific coordination to Al³⁺ (Figures 3a–c). In ClO₄‐HEE, a weak Al─O (ClO₄ ⁻) peak yields a CN near 1, indicating minimal anion coordination. The OTF⁻‐HEE system exhibits a more distinct Al─O (OTF⁻) peak with a CN of ∼1.0, confirming partial yet definitive coordination. In contrast, NO₃‐HEE shows a sharp, intense Al─O (NO₃ ⁻) peak and a rapid rise in CN to ∼3 within 3 Å, demonstrating strong and prevalent nitrate coordination. Importantly, the high CN of NO₃ ⁻ corresponds to a marked decrease in water and EG coordination around Al³⁺, while the total CN remains ∼6. This quantifies the formation of a uniform, anion‐rich solvation shell consistent with the sharp ²⁷Al NMR signal, and reveals a structure in which water molecules are effectively displaced from the primary Al³⁺ environment.

FIGURE 3.

Molecular simulation and electronic interaction analysis of Al³⁺ solvation structures in HEEs. RDFs and CNs of Al³⁺ with anionic oxygen atoms for (a) ClO₄‐HEE, (b) OTF‐HEE and (c) NO₃‐HEE. Calculated ESP maps for the DFT‐optimized Al3+ solvation complexes: (d) [Al(EG)₃(H₂O)₂(ClO₄)]²⁺, (e) [Al(EG)₃(H₂O)₂(OTF)]²⁺ and (f) [Al(EG)₃(H₂O)(NO₃)₂]⁺.

To gain molecular‐level insight into the anion‑mediated solvation structure of Al^{3 +} in the HEEs, density functional theory (DFT) calculations were performed. The optimized representative solvation complexes were identified as [Al(EG)₃(H₂O)₂(ClO₄)]²⁺, [Al(EG)₃(H₂O)₂(OTF)]²⁺ and [Al(EG)₃(H₂O)(NO₃)₂]⁺ respectively (Figure S2). Electrostatic potential (ESP) analysis of these complexes reveals distinct electronic coupling between the anions and Al^{3 +}. In the ClO₄‑based complex, the negative potential localized on the perchlorate anion exhibits minimal spatial overlap with the Al^{3 +} center (Figure 3d), indicating weak electrostatic interaction. For the OTF‑based system, the negative potential is drawn closer to Al³⁺ (Figure 3e), reflecting moderate coupling. Most notably, in the nitrate‑based complex [Al(EG)₃(H₂O)(NO₃)₂]⁺, a pronounced and compact overlap of the negative potential (on the oxygen atoms of NO₃ ⁻) with the Al³⁺ core is observed (Figure 3f), signifying strong electronic interaction. The tightly coupled and uniform electrostatic environment in the nitrate‑containing complex suggests enhanced stabilization of the solvation shell, which is conducive to efficient ion transport and improved interfacial compatibility.

Projected density of states (PDOS) calculations were performed to probe the electronic interactions underpinning the distinct Al³⁺ solvation structures (Figure S3). For ClO₄‑HEE, weak Al 3d–O 2p orbital overlap indicates feeble hybridization, consistent with its loosely bound solvation structure and moderate interfacial stability. The PDOS of OTF‑HEE shows broad, disordered overlap, signifying strong yet mismatched coupling that contributes to its high desolvation barrier and poor kinetics. In contrast, NO₃‑HEE exhibits pronounced and well‐defined Al 3d–O 2p orbital hybridization within the –10 to –5 eV range. This efficient orbital interaction stabilizes the solvation structure while maintaining favorable ion mobility, Macroscopically, this theoretically verified structural uniformity significantly minimizes the energy penalty associated with the desolvation process. This provides a direct fundamental origin for the experimentally observed kinetic advantages, specifically the markedly lowered charge‐transfer resistance and the highest exchange current density.

2.3. Electrochemical Behavior and Al Anode

The interfacial compatibility between the aluminum anode and the electrolytes was first assessed through Tafel polarization analysis (Figure 4a), The corrosion potential (E _corr) follows the order: OTF‑HEE (‐0.18 V) > NO₃‑HEE (‐0.37 V) > ClO₄‑HEE (‐0.46 V) versus Ag/AgCl. The most positive potential of OTF‑HEE can be attributed to the strong coordinating ability of the OTF⁻ anion, which promotes the formation of a dense initial solvation layer at the interface, thereby passivating the surface. In contrast, the weak and non‑specific interactions of ClO₄ ⁻ with aluminum result in the highest corrosion tendency. However, the corrosion current density (i _corr) reflecting the kinetic rate of the parasitic corrosion reaction, is lowest for OTF‑HEE and highest for ClO₄‑HEE, with NO₃‑HEE exhibiting an intermediate value. This confirms that OTF‑HEE possesses the best overall corrosion resistance among the three systems.

FIGURE 4.

Interfacial electrochemical characterization and anode morphology analysis of Al anode with HEEs. (a) Tafel polarization curves of the Al electrode in different HEEs. b) Exchange current densities for the Al plating/stripping process derived from the Tafel plots. (c) Nyquist plots of pristine Al/Al symmetric cells via EIS. (d) Galvanostatic cycling curves of Al/Al symmetric cells over 400 h. (e) SEM images of Al anodes extracted from cycled Al/Al symmetric cells after 400 cycles.

However, a more critical metric for battery operation is the exchange current density (i ₀) for the reversible Al plating/stripping process, derived from the Tafel region (Figure 4b). Here, NO₃ ⁻‑HEE exhibits a significantly higher i ₀ than both OTF‑HEE and ClO₄‑HEE. This indicates the fastest intrinsic Faraday kinetics for the desired aluminum electrochemistry in the nitrate‑based electrolyte. The high i ₀ is a direct consequence of the homogeneous solvation structure induced by NO₃ ⁻, which facilitates synchronized ion transport and lowers the desolvation energy barrier. In contrast, the excessively strong coordination in OTF‑HEE, while beneficial for corrosion resistance, leads to a high kinetic barrier for the desired Al³⁺ reduction. The weak interaction in ClO₄‑HEE results in inefficient charge transfer, also yielding a low i ₀.

Electrochemical impedance spectroscopy (EIS) performed on pristine Al/Al symmetric cells provides independent verification of the anion‑dependent interfacial charge‑transfer kinetics. The fitted charge‑transfer resistance (R _ct) from the Nyquist plots (Figure 4c) reveals a direct correlation with the previously measured i ₀. Notably, NO₃‑HEE exhibits the lowest initial R _ct, this aligns perfectly with its highest i ₀ value, confirming the most favorable charge‑transfer kinetics for aluminum electrochemistry at its interface. This kinetic advantage originates directly from its homogeneous solvation structure, as revealed by ²⁷Al NMR and MD simulations. The uniform coordination environment in NO₃‐HEE ensures that all Al³⁺ ions experience a similar desolvation pathway, avoiding the kinetic dispersion that arises from multiple coexisting solvation species in ClO₄‐HEE and OTF‐HEE. Consequently, the average desolvation energy barrier is lowered, facilitating faster charge transfer at the electrode interface. The substantially lower R_ct for NO₃‐HEE is in excellent agreement with its highest exchange current density, quantitatively confirming that the homogeneous solvation structure effectively reduces the kinetic barrier for the Al³⁺/Al redox couple. This correlation integrates the two independent electrochemical measurements into a unified kinetic picture, where both the equilibrium exchange rate and the nonequilibrium charge‐transfer process consistently indicate the superior interfacial kinetics enabled by the NO₃ ⁻ anion.

The galvanostatic cycling performance of Al/Al symmetric cells, a direct measure of interfacial reversibility and stability, was systematically evaluated for the three HEEs over 400 h (Figure 4d). NO₃‑HEE demonstrates exceptional cycling stability, sustaining a minimal and stable voltage hysteresis (∼0.3 V) throughout the test. This low and steady polarization indicates highly reversible aluminum electrochemistry with negligible interfacial degradation. In contrast, cells employing ClO₄‑HEE and OTF‑HEE exhibit progressively increasing and unstable polarization, indicative of less robust interfacial kinetics and accumulating side reactions. This pronounced performance hierarchy stems directly from the anion‑dependent solvation structures elucidated in preceding sections. The homogeneous coordination environment in NO₃‐HEE facilitates efficient ion transport and enables uniform Al³⁺ desolvation at the electrode interface, thereby promoting dendrite‐free deposition and minimizing localized corrosion, which collectively underpin its superior cycling endurance.

To investigate the interfacial morphology of the aluminum anode after cycling, scanning electron microscopy (SEM) was performed on electrodes extracted from Al/Al symmetric cells after 400 cycles (Figure 4e). The aluminum anode cycled in OTF‑HEE exhibited minimal morphological change; its native oxide layer (Al₂O₃) remained largely intact and contributes to high interfacial resistance. This surface passivation is consistent with the large polarization observed during cycling. The electrode cycled in ClO₄‑HEE showed severe corrosive attack, with a rough, inhomogeneous surface featuring visible pitting and porous structures. This pronounced corrosion leading to sustained parasitic reactions and aluminum degradation. The anode cycled in NO₃‑HEE presented a markedly more uniform morphology with only localized, minor corrosion features. This suggests a balanced interfacial process in which the homogeneous solvation structure enables efficient and uniform aluminum plating/stripping, thereby achieving an optimal trade‑off between electrochemical activity and corrosion suppression.

2.4. Full Cell Performance

To validate the practical implications of the anion‐modulated solvation structures, full cells were constructed employing a CuHCF cathode, an aluminum metal anode, and the respective HEEs.

The kinetics and reversibility of Al^{3 +} intercalation/deintercalation in the CuHCF cathode were evaluated by cyclic voltammetry at a scan rate of 0.5 mV s^{− 1} (Figure 5a). The resulting CV curves reveal pronounced differences governed by the electrolyte composition. The cell employing NO₃‑HEE exhibited a pair of well‐defined, symmetric redox peaks with the smallest peak separation and the highest peak current density. This indicates highly reversible Al³⁺ storage with rapid reaction kinetics and minimal polarization. In contrast, the CV profiles for cells with ClO₄‑HEE and OTF‑HEE showed broader peaks and significantly larger peak separation, suggesting sluggish charge transfer and increased irreversibility. The superior kinetics in NO₃‑HEE are a direct consequence of its homogeneous solvation structure, which facilitates fast Al³⁺ desolvation and transport at the cathode interface.

FIGURE 5.

Electrochemical performance analysis of CuHCF//Al full cells with HEEs. (a) CV curves of full cells at a scan rate of 0.5 mV s⁻¹. (b) Galvanostatic charge‐discharge profiles of full cells. (c) Long‐term galvanostatic cycling performance of full cells over 500 cycles. (d) Calculated HOMO and LUMO energy levels of the dominant Al³⁺ solvation complexes.

Galvanostatic charge‐discharge profiles further elucidate the performance trade‐offs between capacity and energy efficiency (Figure 5b). ClO₄‑HEE delivered the highest initial discharge capacity (∼144 mAh g^{− 1}), likely due to its weak ion‐pairing and high population of mobile charge carriers facilitating initial cathode access. However, this is accompanied by a significant voltage hysteresis (∼1.1 V), indicating substantial polarization and energy loss during cycling—a manifestation of its inefficient interfacial charge transfer. Conversely, the profile for NO₃‑HEE, while showing a slightly lower initial capacity (∼123 mAh g^{− 1}), exhibits the most distinct voltage plateaus and the smallest polarization. This optimal balance highlights its key advantage: the kinetically favorable, homogeneous solvation structure enables highly efficient Al³⁺ transport with minimal overpotential, translating to superior voltage efficiency and energy retention per cycle. The cell with OTF‑HEE suffers from severely sloped profiles, very low capacity (∼63 mAh g^{− 1}), and extreme polarization, corroborating the profound kinetic limitations and resistive interface that dominate its electrochemical behavior.

The practical endurance of the full cells was evaluated through extended galvanostatic cycling over 500 cycles (Figure 5c). The cell with NO₃‑HEE demonstrates outstanding cycling stability, maintaining high‐capacity retention with a Coulombic efficiency consistently near 96%. As proven in Al/Al symmetric cell tests (Figure 4d), NO₃‑HEE sustains minimal polarization (∼0.3 V) over 400 h, indicating negligible interfacial degradation. This superior anode stability is visually confirmed by the uniform, minimally corroded morphology of the cycled aluminum electrode (Figure 4e). The homogeneous solvation structure of NO₃‑HEE thereby enables concurrent efficiency at both electrodes: it supports reversible Al plating/stripping at the anode while maintaining efficient Al³⁺ de/intercalation at the cathode, resulting in the observed high full‑cell cyclability. Conversely, the rapid capacity fade and low efficiency of cells using ClO₄‑HEE and OTF‑HEE are rooted in their failure to maintain a functional aluminum anode. The progressive voltage polarization and instability in their symmetric cell profiles (Figure 4d) foreshadow poor anode cycling in a full‑cell configuration. The SEM evidence (Figure 4e) provides the physical origin: ClO₄‑HEE leads to severe, non‑uniform anode corrosion, while OTF‑HEE results in a passivated, electrochemically inactive surface. These anode‑limited failure modes—continuous active material loss or severely hindered kinetics—directly manifest as the rapid and pronounced capacity decay observed in the respective full cells.

To provide an electronic structure perspective on the full cell performance, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the dominant Al³⁺ solvation complexes were calculated (Figure 5d). The significantly lower LUMO of the NO₃‐HEE‐based complex ([Al(EG)₃(H₂O)(NO₃)₂]⁺) indicates its highest thermodynamic propensity for reduction. This electronic structure feature is consistent with the observed electrochemical behavior, including the stable voltage profiles and uniform deposition morphology (Figure 4d,e). However, the primary origin of the enhanced performance in NO₃‐HEE is attributed to its homogeneous solvation structure, which facilitates efficient ion transport and reduces the desolvation energy barrier, as evidenced by the lowest R_ct and highest i₀ values. This distinctly low LUMO energy level ensures the preferential electronic accommodation of the nitrate‐coordinated complex, which effectively buffers the highly reductive Al interface and prevents the severe, uncontrolled decomposition of water. Experimentally, this theory‐predicted thermodynamic regulation directly accounts for the suppression of parasitic corrosion, translating to the highly uniform anode morphology and the exceptional low‐polarization cycling stability over 400 h. Conversely, its highest HOMO energy suggests a marginally lower oxidative stability. This provides an electronic‑structure rationale for the slightly reduced initial cathode capacity compared to ClO₄‑HEE, indicating mild, self‑limiting oxidative activity at the cathode interface.

3. Conclusions

In this work, we have systematically elucidated the pivotal yet previously overlooked role of anions in dictating the performance of HEEs for AIBs. Through a comparative study of NO₃ ⁻, ClO₄ ⁻, and OTF⁻ anions within an EG‐based HEE framework, we establish a clear anion‐dependent structure‐property‐performance relationship. Our findings demonstrate that the NO₃ ⁻ uniquely fosters a homogeneous and uniform solvation structure for Al³⁺, characterized by strong inner‐sphere coordination. This specific solvation environment, as unequivocally confirmed by multi‐modal spectroscopy and theoretical calculations, results in optimal physicochemical properties: lowest viscosity, best wettability, and superior reductive stability at the Al anode. Consequently, NO₃ ⁻‐HEE enables remarkably fast and reversible Al plating/stripping kinetics, as evidenced by the highest exchange current density and lowest interfacial resistance. It leading to dendrite‐free, uniform Al deposition and exceptional long‐term cyclability in symmetric cells. When integrated into full cells with a CuHCF cathode, the NO₃ ⁻‐HEE delivers a well‐balanced performance, combining appreciable capacity, minimal polarization, and outstanding cycling stability over 500 cycles. This work transcends the conventional focus on hydrogen bond donors in HEE design, introducing anion engineering as a fundamental and powerful strategy. We provide a molecular‐level blueprint for tailoring solvation structures and interfacial chemistry in multivalent battery electrolytes.

Author Contributions

X. K. was primarily responsible for writing the original draft and contributed to investigation and formal analysis. P. M. contributed to conceptualization, validation, resource provision, review & editing, supervision, and project administration. T. Y. was primarily responsible for computational analysis and simulations, and contributed to software, data curation, and review & editing. Y. L. participated in methodology design, investigation, validation, and review & editing. Z. Y. provided resources and assisted in supervision and analysis. C. F. contributed to investigation, validation, and data curation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting file 1: chem71060‐sup‐0001‐SuppMat.docx.