Strongly-coordinating organoborates with eccentric solvation structure enable secondary calcium metal battery

Abstract

Rechargeable calcium (Ca) metal batteries are promising candidates for next-generation energy storage owing to calcium’s abundance, divalent charge and low redox potential. However, achieving reversible Ca plating/stripping at room temperature remains challenging due to sluggish desolvation kinetics and unstable interfacial chemistry. Here, we design a class of reversible calcium electrolytes using eccentric solvation structure strategy by introducing strongly coordinating monovalent organoborates into calcium-based electrolytes. The tetra(trifluoroethanoloxy)-borate anion enables participation in the primary solvation sheath, while monovalent cations promote the formation of eccentric solvation structures via anion bridging. This solvation environment lowers both the solvation sheath reorganization energy and desolvation energy barrier of Ca²⁺, while simultaneously upshifting the Ca electrode potential, thereby facilitating the formation of an anion-derived CaO/CaH₂-rich polymeric solid electrolyte interphase layer ( ~ 29 nm). The synergy between fast desolvation kinetics and high interfacial stability enables calcium metal half-cells to achieve a coulombic efficiency of 97.65% under 2 mA cm⁻², along with dendrite-free deposition up to 20 mA h cm⁻². Full cells using de-sodiated layered oxide electrode exhibit stable cycling performance under a negative-to-positive ratio of 2.69 and high-rate conditions. This work highlights the role of solvation structure engineering in enabling secondary Ca metal chemistry, providing a rational design framework for advanced multivalent battery electrolytes.

Calcium metal batteries are limited by low reversibility and slow interfacial kinetics. Here, authors design an eccentric solvation structure by introducing monovalent cations and strongly coordinating organoborate anion, which enables fast desolvation and reversible calcium deposition.

Introduction

The growing demand for sustainable energy storage has intensified interest in alternatives to the conventional lithium-ion batteries (LIBs), whose scalability is constrained by limited lithium resources and rising cost^1–3. Multivalent metals such as Ca, Mg, Zn, and Al have garnered extensive attention due to their cost-effectiveness and high abundance in the Earth’s crust (e.g., Ca: ~36,300 ppm; Mg: ~20,900 ppm; Zn: 70 ppm; Al: 83,000 ppm versus Li: 20 ppm)^4–6. Among these, calcium (Ca) has emerged as a particularly promising alternative for post-lithium battery systems^7,8, combining a low redox potential (−2.87 V vs. standard hydrogen electrode) with a high volumetric specific capacity (2072 mAh cm⁻³). These attributes position calcium metal batteries as compelling candidates for high-energy, next-generation energy storage technologies^9,10.

Nonetheless, the development of secondary Ca batteries using a metallic Ca electrode is hindered by two main challenges: low reversibility and large overpotential, arising from the high reactivity of Ca metal and compact solvation sheath of Ca^2+11,12. Electrolytes with reactive components or high solvation free energy further increase the desolvation energy barrier of calcium ions, leading to sluggish interfacial kinetics while exacerbating the parasitic reactions. The as-formed thick solid-electrolyte interphase (SEI) impedes the Ca²⁺ transfer at the electrolyte/electrode interface, resulting in the low Ca reversibility and polarization-induced battery failure. Although previous studies have demonstrated that calcium metal can undergo reversible deposition reactions at elevated temperatures (>75 °C), the realization of practical calcium metal batteries operating at room temperature can be traced back to the pioneering discovery of calcium borohydride electrolytes in ether solvents^13,14. Based on these findings, researchers have developed various electrolyte systems through modifications of salts, solvents, and additives. For instance, calcium carborane (Ca(CB₁₁H₁₂)₂) and borohydride-derived fluoroalkoxy borates (Ca[B(hfip)₄]₂) have demonstrated a certain degree of reversibility toward Ca metal. However, they suffer from poor product consistency, high cost and inadequate performance under high current density^15–18. Substituting ether with chelating amine or high donor number solvent (DMAc) to form a less compact solvation sheath can reduce the desolvation energy barrier, though their compatibility with the positive electrodes requires further investigation^12,19. Incorporation of additives such as CaI₂, LiBr, LiBH₄, or NaBH₄ can decrease the overpotential by forming a halide-rich SEI layer or a high Ca-affinity substrate. However, the low anodic stability and corrosive property exclude halide/BH₄⁻-containing electrolytes from industrial applications^20–23. Therefore, developing a highly reversible, low-cost, non-corrosive electrolyte system with wide electrochemical windows is necessary for enabling practical and sustainable calcium metal batteries.

Herein, we propose an eccentric solvation structure strategy by incorporating monovalent organoborates with strongly coordinating anions into calcium electrolytes, thereby modulating the calcium deposition kinetics and composition of the SEI layer. Specifically, the commonly used weakly coordinating anion (hexafluoroisopropyloxy-borate anion, [B(hfip)₄]⁻) is replaced with a strongly coordinating alternative (trifluoroethanoloxy-borate anion, [B(tfe)₄]⁻) characterized by low fluorine substitution and small steric hindrance, enabling effective integration of the anion into the Ca solvation sheath. This modification reduces the thermodynamic electrode potential of Ca (E_Ca) while promoting the formation of an anion-derived SEI layer. Furthermore, the introduction of monovalent cations establishes a hybrid, eccentric solvation structure that mitigates electrostatic shielding effect on the Ca nucleus and lowers the desolvation energy barrier, thereby accelerating Ca²⁺ desolvation kinetics (Fig. 1a). As a result, the cells employing monovalent/Ca electrolytes form a thin, polycrystalline/polymeric SEI layer and exhibit a reversibility of 97.65%, along with stable plating/stripping overpotentials, robust performance under high current densities (up to 10 mA cm⁻²) and smooth deposition morphology over areal capacities of 5–20 mA h cm⁻². The combined merits of low overpotential and high coulombic efficiency (CE) enable high-loading calcium/sodium hybrid batteries with a negative-to-positive capacity ratio of 2.69 using a Ca metal electrode, delivering stable performance when paired with both layered and conversion-type positive electrodes. By carefully tuning the structure of the anions, various favorable electrolytes with an eccentric solvation structure for a calcium metal battery can be realized.

Fig. 1. Electrolyte design concept and Ca metal reversibility.

a Schematic illustration of the solvation structure design strategy for developing reversible calcium electrolytes, from weakly coordinating systems to strongly coordinating systems. The approach involves anion engineering and hetero-cation addition via monovalent strongly coordinating organoborates to modulate solvation kinetics and SEI formation. b The relationship between the concentration of impurity ions in raw materials and the Ca reversibility of prepared salts. c First-cycle deposition overpotential of Ca half-cells using different electrolytes under a current density of 0.25 mA cm⁻². (Electrolyte concentrations: Ca²⁺/monovalent ion = 0.1 M/0.8 M for Ca/Li/Btfe, Ca/K/Btfe, Ca/Cs/Btfe systems, and 0.3 M/0.4 M for Ca/Na/Btfe, Ca/Na/Bhfip systems; this ratio is used for all tests unless otherwise stated). d Cycling performance of Ca half-cells using electrolytes with different hetero-cations modification under 0.5 mA cm⁻² and 0.04 mA h cm⁻², the CE values annotated are average values over 1900 number of cycles. e Cycling performance of Ca half-cells using Ca/Na/Btfe and Ca/Na/Bhfip electrolytes under 2 mA cm⁻² and 0.33 mA h cm⁻². For Ca/Na/Btfe, data are presented as mean ± standard deviation with n = 3. The CE values annotated are average values over 300 cycles. f Voltage profiles of the cell using the Ca/Na/Btfe electrolyte in Fig. 1e.

Results

From serendipity to strategy: improved calcium reversibility

The inspiration of this work originates from repeated experiments that reveal the calcium salts (including Ca[B(hfip)₄]₂ and Ca[B(tfe)₄]₂) synthesized from different batches of raw materials exhibit markedly different reversibility in ether-based electrolytes (Fig. 1b and Supplementary Fig. 1). Subsequent inductively coupled plasma optical emission spectrometry (ICP-OES) analyses of the raw materials reveals the presence of residual metal ions originating from the borohydride synthesis process (Ag, K, Na, etc., Supplementary Fig. 2). Although these metal ions are present in low mass fractions, they appear to explain the performance discrepancies observed in Ca[B(hfip)₄]₂ system across different studies. In this work, Ca[B(hfip)₄]₂ and Ca[B(tfe)₄]₂ in 1,2-dimethoxyethane (DME) are set as baseline electrolytes (denoted as Ca/Bhfip and Ca/Btfe, 0.5 M), after which monovalent organoborate salts are introduced to elucidate the impact of heterogeneous cations and anion structure on calcium deposition.

The monovalent and calcium-based organoborate salts are synthesized via the reaction of corresponding borohydrides (pure CaBH₄, LiBH₄, NaBH₄, or KBH₄) with alcohols (trifluoroethanol or hexafluoroisopropanol). CsCB₁₁H₁₂ is used as a Cs⁺ source due to the lack of corresponding borohydride. Nuclear magnetic resonance (NMR) spectroscopy demonstrates the organoborate products are successfully synthesized without impurities (Supplementary Fig. 3). Then, electrolytes containing Ca[B(tfe)₄]₂/LiB(tfe)₄ (Ca/Li/Btfe), Ca[B(tfe)₄]₂/NaB(tfe)₄ (Ca/Na/Btfe)_, Ca[B(tfe)₄]₂/KB(tfe)₄ (Ca/K/Btfe)_, Ca[B(tfe)₄]₂/CsCB₁₁H₁₂ (Ca/Cs/Btfe) are prepared by dissolving the white salts into DME (Ca/Li/Bhfip and Ca/Na/Bhfip are defined in the same manner). Initial discharge voltage profiles of Ca||Cu half-cells (Fig. 1c) reveal that the Ca/Bhfip electrolyte with higher ionic conductivity exhibits lower overpotential. Moreover, hybrid electrolyte systems containing monovalent cations show significantly reduced overpotentials compared to the pure Ca/Btfe electrolytes, suggesting that the presence of monovalent cations primarily enhances interfacial charge transfer kinetics rather than bulk ion transport. Although Ca/Bhfip electrolytes exhibit smaller initial deposition overpotential than Ca/Btfe, prolonged cycling or large current density will lead to thickening of the SEI layer and dendrite formation, eventually resulting in polarization-induced failure or short-circuit (Supplementary Figs. 4 and 5). In contrast, the introduction of corresponding monovalent organoborate salts into low-reversibility Ca/Btfe and Ca/Bhfip systems enables all hybrid electrolytes to achieve reversible calcium plating/stripping with high coulombic efficiency at practical current densities (Supplementary Figs. 6 and 7). Meanwhile, Btfe-based electrolytes exhibit higher cycling reversibility when doped with Li^⁺ and Na^⁺ cations as compared to those doped with K^⁺ and Cs^⁺ (Fig. 1d). To ensure that the deposition of monovalent cations remains negligible, the optimal concentration for testing is set as Ca²⁺/monovalent ion = 0.1 M/0.8 M for Ca/Li/Btfe, Ca/K/Btfe, Ca/Cs/Btfe systems and 0.3 M/0.4 M for Ca/Na/Btfe, Ca/Na/Bhfip systems (Supplementary Fig. 8), this ratio is used for all experimental tests unless otherwise indicated. The thermodynamic electrode potentials of monovalent metals and calcium in the organic DME solvent are calculated according to the following equation²⁴:

$$E_{+}^o\left(\mathrm{non}-\mathrm{aq}\right)=E_{+}^o\left(\mathrm{aq}\right)+{\mathrm{\Delta }G}_t^o/z_{+}F$$ 1

Where $F$ is the Faraday constant and $z_{+}$ is the number of electrons involved in the electrode reaction. ${\mathrm{\Delta }G}_t^o\left({\mathrm{X}}^{\pm \mathrm{z}},\mathrm{W}\to \mathrm{S}\right)$ is the standard molar Gibbs free energy of transfer from water to solvent, which is obtained from density functional theory (DFT) calculation. The results show Ca²⁺/Ca couple exhibits a higher standard potential in DME compared to Li⁺|Li, Na⁺|Na, and K⁺|K (Supplementary Table 1 and Supplementary Data 1)^24,25. This indicates that Ca can be preferentially reduced in these electrolytes, which deviates from the well-established trend of standard electrode potentials referenced to the standard hydrogen electrode (SHE). Notably, Ca/Na/Btfe electrolyte exhibit better cycling stability than its Ca/Na/Bhfip counterpart (Fig. 1e), showing the lowest polarization overpotential and highest cumulative capacity compared with other electrolytes (Fig. 1f and Supplementary Figs. 9–12). Meanwhile, the Ca/Na/Btfe electrolyte also exhibits competitive performance among some recent relevant reports (Supplementary Figs. 13 and 14, Supplementary Table 4). Linear sweep voltammetry (LSV) results indicate that Ca/Na/Btfe electrolyte has a high anodic stability of 4.5 V (vs. Ca|Ca²⁺) towards common stainless steel counter electrodes (Supplementary Fig. 15), making it suitable for high voltage cycling.

Solvation structure and interfacial kinetics

Solvation structures play a pivotal role in determining the reversibility of electrochemical Ca deposition. We combine theoretical simulations and solvation cluster characterizations to elucidate the solvation sheath of different electrolyte systems. Molecular dynamics (MD) and density functional theory (DFT) simulations were first conducted to investigate the primary solvation environments in five representative electrolytes (Fig. 2a). For ease of comparison, the concentrations in the simulation boxes are uniformly set to Ca²⁺/monovalent ion = 0.1 M/0.8 M for calculation. The optimized coordinates of the five representative solvation structures are provided in Supplementary Data 2.

Fig. 2. Solvation sheath analysis and interfacial kinetics.

a Typical solvation structure and maximum end-to-end dimension of the primary Ca²⁺ solvation sheath obtained from MD simulations (electrolyte concentrations: Ca²⁺/monovalent ion = 0.1 M/0.8 M), color scheme of molecules: Ca, green; Na, yellow; Li, purple; K, blue; F, sky blue; O, red; C, black; B, brown; H is omitted in these structures. b Characterization of solvation structures across different electrolytes, classified based on the number of cations within the solvation sheath. SCaSS: single Ca²⁺ solvation structure (Ca²⁺ = 1); EcSS: eccentric solvation structure (monovalent ion+Ca²⁺ ≥ 2); Ca²⁺ clusters (Ca²⁺ ≥ 2). c Simultaneous small-angle and wide-angle X-ray scattering pattern of different electrolytes acquired at 25 °C, the purple, blue, and orange shaded regions represent the signal peak of clusters in Ca/Na/Btfe, Ca/Btfe, and Ca/Bhfip, respectively, within which the diameters of the representative solvation structures are located. d Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing of NaB(tfe)₄ single crystal obtained from Ca/Na/Btfe electrolyte, the complex forms a polymeric chain upon growing and packing. Color scheme of molecules: Na, pink; F, green; O, red; C, gray; B, blue; H, white. e Radial distribution function plots of the Ca–O_DME pair and Ca²⁺-solvent binding energy of different electrolytes. f Solvation sheath reorganization energy (λ) of different electrolytes (electrolyte concentrations: Ca²⁺/monovalent ion = 0.1 M/0.8 M). g Negative electrode potential (E_Ca) obtained from a three-electrode cell with ferrocene (~1 mM) as an internal reference.

Radial distribution function (RDF) analysis reveals that, regardless of monovalent organoborates addition, strong Ca–O(DME) interactions can be observed in Bhfip-based electrolytes with a coordination number of 8 (Supplementary Fig. 16). This indicates that the typical [Ca(DME)₄]²⁺ structure forms without B(hfip)₄⁻ anion participation, reflecting its weakly coordinating nature. Moreover, Ca²⁺ and monovalent cations form separate solvation shells, implying weak interactions between different primary solvation structures (Supplementary Fig. 17). In contrast, the Btfe-based electrolyte shows clear evidence of B(tfe)₄⁻ anions participating in the primary solvation sheath (Supplementary Fig. 18), showing its strongly coordinating ability. Upon the introduction of monovalent organoborates, both single Ca²⁺ solvation structure (SCaSS, one Ca²⁺ per solvation sheath) and Ca²⁺ solvation clusters (Ca²⁺ ≥ 2) are largely replaced by Ca²⁺/monovalent ion hybrid solvation structures (Fig. 2b). Among these, the Ca/monovalent ion ratio of 1:1 emerges as the most representative (Supplementary Fig. 19), where a B(tfe)₄⁻ anion bridges between a Ca²⁺ and a monovalent cation. To quantitatively define this structural pattern, we calculated the geometric center of each solvation cluster and evaluated the normalized offset ($\delta$) between Ca²⁺ and the center. Using this descriptor, single-ion Ca electrolytes exhibit nearly symmetric solvation ($\delta$ ≈ 0), whereas hybrid Ca/monovalent cations systems show strongly eccentric structures with $\delta$ approaching ~20% (Supplementary Fig. 20). Therefore, in this work, we define the structure containing Ca²⁺-Btfe⁻-monovalent ion as eccentric solvation structures (EcSS) since Ca²⁺ is no longer located at the center of the solvation structure.

Simultaneous small-angle and wide-angle X-ray scattering (simultaneous SAXS/WAXS) was employed to probe the microscopic solvation structures of different electrolytes (Fig. 2c)²⁶. The Ca/Bhfip electrolyte exhibits a distinct scattering signal at a Q value of 0.6–0.8 (orange part, corresponding to 10.5–7.8 Å), consistent with single [Ca(DME)₄]²⁺ solvation structures (8.9 Å, orange dashed line). Ca/Btfe electrolyte shows prominent signal at Q value of 0.35–0.6 (blue part, 18.0–10.5 Å), indicating the involvement of anions in the Ca²⁺ solvation structure (15.3 Å, blue dashed line). Notably, while pure monovalent salts with Btfe anion are dominated by scattering features below 15 Å (Supplementary Fig. 21), the hybrid Btfe-based electrolytes containing Li, Na, and K cations exhibit broad scattering features across 0.2–0.4 (purple part, 31.4–15.7 Å), with a strong signal located at 22.9 Å in the Ca/Na/Btfe electrolyte. This low-Q signature confirms the formation of bulky eccentric solvation structures. This hetero-cation interaction is further substantiated by ²³Na NMR (Supplementary Fig. 22), where a distinct upfield shift is observed in the hybrid system (unlike the non-shifting Bhfip control), verifying the incorporation of Na⁺ into the primary Ca²⁺ solvation shell. These results are consistent with the MD calculations. Single-crystal X-ray diffraction (SCXRD) of the Ca/Na/Btfe electrolyte confirms the bridging role of B(tfe)₄⁻ anion: two –OCH₂CF₃ branch coordinates with distinct Na⁺ ions, verifying the feasibility of bridging Ca²⁺ and monovalent ions within the solvation structure (Fig. 2d and Supplementary Fig. 23). RDF analysis and binding energy calculations (Fig. 2e and Supplementary Data 3) consistently show that the introduction of monovalent cations weakens the Ca–O(DME) interactions, which qualitatively corresponds to a reduced desolvation barrier for Ca²⁺. This indicates that, within the eccentric solvation structure, monovalent cations are capable of modulating the coordination environment of the Ca²⁺ solvation sheath by interacting with bridging B(tfe)₄⁻ anions. This conclusion is further supported by Raman spectroscopy, where a decrease in coordinated-DME peaks is observed in monovalent ion/Ca hybrid electrolytes compared with Ca/Btfe (Supplementary Fig. 24).

To experimentally validate the theoretical results, variable-temperature electrochemical impedance spectroscopy was performed on symmetric Ca||Ca cells using five representative electrolytes (Supplementary Fig. 25). Quantitative impedance decoupling analysis shows that while the addition of monovalent cations leads to a decrease in both R_SEI and R_ct, the absolute reduction in R_ct accounts for 97% of the total impedance drop (Supplementary Fig. 26). This demonstrates that the primary factor in reducing the overpotential is the acceleration of charge-transfer kinetics. This conclusion is further corroborated by the activation energy analysis (Supplementary Fig. 27). The desolvation activation energy (E_{a, desolvation}) is significantly reduced from 26.15 kJ mol⁻¹ (Ca/Btfe system) to 20.78, 21.49, and 23.15 kJ mol⁻¹ (Ca/Na/Btfe, Ca/Li/Btfe, Ca/K/Btfe system, respectively), all of which are markedly lower than the 41.28 kJ mol⁻¹ observed for the weakly-coordinating Ca/Bhfip electrolyte. Therefore, the introduction of monovalent cations serves as an effective strategy to lower the desolvation energy barrier and apparent impedance, particularly in scenarios where the coordinating capacity of the strong anion has reached its limit (Supplementary Fig. 28). These synergistic effects result in significantly reduced overpotentials and ensure the formation of a stable Btfe-derived SEI layer.

The emergence of such an eccentric solvation structure with anion-bridging effects will also impact the other thermodynamic and kinetic parameters associated with solvation, such as the negative electrode potential (E_Ca) and solvation sheath reorganization energy (λ). Herein, we employ an idealized calculation of λ as a qualitative comparative descriptor to evaluate the relative kinetic trends across different electrolytes. Drawing upon established methodologies in Mg battery research^12,27,28, we adopted specific simplifications, including the assumption of an ideal metal-liquid interface that neglects the specific influence of the SEI layer, while approximating the geometric parameter R (distance between the ion and its image) based on the distance between the solvated ion and electrode surface. The equation is given by:

$$\lambda =\frac{{\left(\mathrm{\Delta }e\right)}^2}{4\pi {\epsilon }_0}\left(\frac{1}{a_0}-\frac{1}{R}\right)\left(\frac{1}{{\epsilon }_{op}}-\frac{1}{{\epsilon }_s}\right)$$ 2

Where $\mathrm{\Delta }e$ is the charge transferred, ${\epsilon }_0$ is the vacuum permittivity, ${\epsilon }_{op}$ and ${\epsilon }_s$ are the optical dielectric constant and the static dielectric constant of DME, $a_0$ is the radius of the redox ion. Since the dielectric term remains constant due to the use of the same solvent, the variation in λ is primarily influenced by the distance-dependent term (1/a₀ − 1/R) (Supplementary Table 2). As $a_0$ increases from a single Ca²⁺ solvation structure to an eccentric solvation structure and separation distance decreases (surface film thickness, Supplementary Table 3), the distance term is reduced. Calculated results (Fig. 2f) reveal that the descriptor λ progressively decreases, transitioning from the single solvation structures (Ca/Bhfip, Ca/Btfe) to the eccentric solvation structure (Ca/Na/Btfe). This trend aligns well with the experimental measurements of desolvation activation energy and the observed reduction in overpotential, serving as a qualitative indicator of the accelerated charge-transfer kinetics.

The negative electrode potential in various electrolytes is measured using a three-electrode cell with ferrocene (~1 mM) as an internal reference (Supplementary Fig. 29)²⁹. For electrolytes with an eccentric solvation structure, cycled Ca foils are used as a reference electrode to establish the mixed-electrode potentials. The soaked Ca/ Ca cell indicates that the passivation layer has a negligible effect on the measurement of electrode potentials. Consistent with previous reports, strong cation-anion interactions in Btfe-based electrolyte systems shift the electrode potential to more positive values (weakening the reducing ability of Ca) compared with the Ca/Bhfip electrolyte, which further suppresses the reductive decomposition of the electrolyte and improves the Coulombic efficiency. (Fig. 2g and Supplementary Fig. 30).

Electrodeposition morphology and SEI structure analysis

The solvation structure directly governs the composition and properties of the SEI layer, thereby determining the Ca deposition morphology. Herein, digital imaging, scanning electron microscopy (SEM), and cryo-electron microscopy (cryo-EM) are employed to investigate the morphology of deposited Ca and the nanostructure of the resulting SEI layers. As shown in Supplementary Fig. 31, six electrolyte systems are evaluated by depositing 5 mAh cm⁻² of Ca metal on copper current collectors. Ca/Li/Btfe, Ca/Na/Btfe, and Ca/K/Btfe electrolytes achieve full deposition of 5 mAh cm⁻², resulting in uniform Ca layers with metallic luster. In contrast, deposits formed in Ca/Btfe, Ca/Bhfip, and Ca/Cs/Btfe electrolytes are uneven and dark, leading to short-circuiting during the plating process. SEM images further indicate that electrolytes with lower reversibility result in mossy, fine-grained calcium deposits (Fig. 3a), while those with higher reversibility yield larger, more uniform calcium grains (Fig. 3b, c). EDS images and cross-sectional SEM (Fig. 3d, e) reveal that Ca deposits obtained from Ca/Na/Btfe electrolyte exhibit a compact, dendrite-free morphology at the microscale, with a thickness of 25.44 μm—closely matching the theoretical value (1 mA h cm⁻² = 4.826 μm Ca). Using the Ca/Na/Btfe electrolyte, dendrite-free calcium deposition is achieved even at large areal capacities of 20 mA h cm⁻² (Supplementary Fig. 32). EDS analysis and ICP-OES results demonstrate that the capacity contribution in these systems mainly originates from metallic calcium, while the deposition contribution from monovalent cations is nearly negligible (Fig. 3f and Supplementary Figs. 33–36).

Fig. 3. Characterizations of electrodeposition morphology and SEI structure on the Ca electrode.

a–c SEM images and digital images (scale bar: 1 cm) of Ca metal deposits in Ca/Bhfip (a), Ca/Li/Btfe (b), Ca/Na/Btfe (c) electrolytes. Note that the digital images show Ca metal deposits adhered to the separators, which are detached from the Cu current collectors. d EDS mapping of the Ca deposits in Ca/Na/Btfe electrolyte. e Cross-sectional SEM image of deposited Ca with an areal capacity of 5 mA h cm⁻². f Capacity contributions of metal ions in 5 mA h cm⁻² deposits, determined from ICP-OES analyses. g Surface of deposited Ca in Ca/Bhfip electrolyte. h, i Surface (h) and zoomed-in nanostructure (i) of deposited Ca in Ca/Na/Btfe electrolyte. j Lattice fringes and FFT diffraction of passivation components in SEI layer. k Schematic illustration of (i), showing an outer SEI layer rich in nanocrystalline domains and an inner phase composed of crystalline Ca.

Cryo-EM is employed to analyze the structural and compositional features of the SEI. Deposits from Ca/Bhfip electrolyte show a thick and particulate by-product layer composed predominantly of organic species, along with few CaO, CaCO₃, and Ca(OH)₂ nanocrystals (Fig. 3g and Supplementary Fig. 37). In contrast, Ca deposits from Ca/Na/Btfe electrolyte exhibit polygonal morphology with smooth surfaces and few by-products (Fig. 3h). At higher magnification, the polycrystalline nature of SEI layer is detected (Fig. 3i). The lattice fringe of 0.323 nm is identified via fast Fourier transform (FFT), corresponding to the (111) plane of metallic Ca (Fig. 3j and Supplementary Fig. 38). A uniform SEI layer ~29.6 nm is observed on the metal surface, consisting of an amorphous organic matrix embedded with large number of inorganic nanocrystals. The outer layer is rich in CaO and CaH₂, while the inner layer contains sparse CaF₂ crystallites (Fig. 3k). The SEI structure and composition formed in Ca/Li/Btfe closely resemble those in Ca/Na/Btfe electrolyte, with CaO as the dominant inorganic component (Supplementary Fig. 39). Considering the Ca²⁺ diffusion barriers in CaO and CaH₂ are much lower than those of CaCO₃ and CaF₂³⁰, such a CaO-enriched SEI layer is expected to facilitate Ca²⁺ transport, thereby lowering the overpotential. The average SEI thickness for different electrolytes follows the following order: Ca/Na/Btfe <Ca/Li/Btfe <Ca/K/Btfe <Ca/Btfe <Ca/Bhfip, as shown in Supplementary Figs. 40 and 41 and Table 3.

SEI components analysis

The surfaces of cycled Ca electrodes are analyzed using X-ray photoelectron spectroscopy (XPS) to identify the decomposition products associated with different electrolytes. As shown in Fig. 4a and Supplementary Figs. 42 and 43, the strongly coordinating Ca/Na/Btfe electrolyte exhibits a lower content of organic components (C–F, C=O in F 1s and O 1s) and a higher content of inorganic CaO (Ca–O in O 1s) compared to the weakly coordinating Ca/Bhfip electrolyte. In contrast to the Ca/Btfe electrolyte, monovalent metal fluorides and oxides are found in SEI layers that are derived from Na or Li containing electrolytes, indicating that the monovalent-ion solvation sheath also contributes to SEI formation. To quantify the decomposition products, both organic and inorganic species are normalized to the Ca⁰ peak (345.9 eV), allowing comparison of their relative concentrations (Fig. 4b). The Btfe-based electrolytes exhibit significantly fewer decomposition products than the Ca/Bhfip electrolyte, suggesting reduced electrolyte decomposition and the formation of a thinner SEI layer, which facilitates Ca²⁺ transport at the electrode/electrolyte interface. These findings are consistent with the high coulombic efficiency in the half-cell and SEI observed in cryo-EM.

Fig. 4. Identifying the organic and inorganic composition at the Ca interface.

a XPS spectra of the cycled Ca electrode in Ca/Na/Btfe electrolyte after 20 cycles. b The relative concentrations of decomposition components in different electrolytes, the inorganic Ca²⁺ species, and organic byproducts are normalized to the peak of Ca⁰ to obtain the results. c–e TOF-SIMS spectra (c), inorganic (d), and organic (e) element intensity evolution of the Ca electrode in Ca/Na/Btfe electrolytes after 30 cycles, 3D scales: 10 μm in x and y directions; the z-axis is uncalibrated and scaled for visualization. f ATR–FTIR results of the cycled Ca electrode obtained from different electrolytes after 20 cycles. g, h, Schematic illustration of the structure and components of the SEI layer formed in Ca/Na(or Li)/Btfe (g) and Ca/Bhfip (h) electrolytes.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) further provides spatially resolved insights into the SEI composition (Fig. 4c and Supplementary Fig. 44). In the highly reversible Ca/Na/Btfe and Ca/Li/Btfe electrolytes, CaO is the dominant inorganic species and is uniformly distributed across the SEI surface (Fig. 4d). Among organic fragments, BO⁻ signals are markedly more abundant than hydrocarbon-based species such as C₂H⁻, indicating that the strongly coordinating anions play a major role in SEI formation (Fig. 4e). Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) is employed to gain a more detailed understanding of the organic components within the SEI (Fig. 4f). In the Ca/Bhfip electrolyte, C–O (alkoxide) signals³¹ appear at 1060 and 1019 cm⁻¹, corresponding to the monomeric byproducts formed from the reaction between DME and Ca metal. In contrast, the Btfe-based systems show -CH₂- stretching vibration signals at 2925 and 2850 cm⁻¹, along with characteristic peaks from B–O bonds and C–O–C group in aliphatic ethers^32,33. Considering that low-molecular-weight species are typically removed during DME rinsing, the detected -CH₂- and C–O–C signals are attributed to a relatively high-molecular-weight polyether. According to previous studies³⁴, we believe that the electron-deficient trisubstituted boron species formed from anion decomposition can react with DME decomposition intermediates (such as (RO)₂-Ca) to generate polymeric SEI layers containing—(B-O-CH₂-CH₂-O)_n—segments. Based on the above analysis, the strongly coordinating anion systems (e.g., Ca/Na/Btfe) are shown to form uniform and thin polymeric/polycrystalline SEI layers with monovalent cation-derived species adhered. In the weakly coordinating anion system (Ca/Bhfip), DME molecules in the solvation sheath mainly participate in film formation, resulting in a thicker, oligomeric SEI layer with lower inorganic content per unit area (Fig. 4g, h).

Based on the above experimental and theoretical findings, a comprehensive structure-property-performance relationship is established (Supplementary Fig. 45). The low-fluorine-substituted anion with small steric hindrance (E.g., B(tfe)₄⁻) promotes coordination with Ca²⁺, enabling its incorporation into the primary solvation sheath. Upon the introduction of monovalent cations, the strongly coordinating anions serve as bridges, linking calcium and monovalent solvation sheaths to form hybrid and eccentric solvation structures. These hybrid solvation environments are characterized by reduced DME coordination, lower desolvation energy barriers, and decreased reorganization energy. Collectively, these effects contribute to lower overpotential and elevated electrode potentials for the calcium electrode. As a result, electrolyte decomposition is suppressed, and anion-derived polymeric/polycrystalline SEI layers are formed, enabling stable Ca deposition and high reversibility. In comparison, in the Ca/Bhfip electrolyte system, neither the weakly coordinating anion (B(hfip)₄⁻) nor individual DME molecules are capable of bridging between different cations. This results in an independent solvation structure, which has minimal impact on the desolvation kinetics. Consequently, this exacerbates the decomposition of chelating DME molecules, ultimately leading to the formation of a poorly structured, solvent-derived oligomeric SEI and a shortened lifetime.

Electrochemical performance and prospects of eccentric solvation structures

We used symmetric Ca cells to evaluate the high-rate applicability of electrolytes featuring eccentric solvation structures. As shown in Fig. 5a and Supplementary Fig. 46, cells with Ca/Na/Btfe and Ca/Li/Btfe electrolytes exhibit stable plating/stripping stability at high current densities (up to 10 mA cm⁻²) without short-circuiting, demonstrating their suitability for high-power calcium metal batteries. Under harsh conditions of 4 mA cm⁻² and 1 mA h cm⁻² (Fig. 5b), the Ca/Na/Btfe electrolyte further delivers a low polarization (<500 mV) together with higher accumulated capacities (160 mA h cm⁻²).

Fig. 5. Cycling performance of Ca metal batteries and candidate anions for eccentric solvation structure.

a Rate performance of Ca symmetric cell under current densities ranged from 2, 3, 5, 8, 10 mA cm⁻² and the fixed capacity of 0.5 mA h cm⁻². b Long-term performance of Ca symmetric cell cycled under a high current density of 4 mA cm⁻². c Cycling performance of the Ca||de-sodiated Na_2/3Ni_1/4Cu_1/12Mn_2/3O₂ (NaNCM) hybrid cell using Ca/Na/Btfe electrolyte under 6.0 C (1.0 C = 170 mA g⁻¹). d Cycling performance of the Ca || high-loading NaNCM hybrid cells with a N/P ratio of 2.69 in Ca/Na/Btfe electrolyte under 1.0 C, two parallel cells are used. e Cycling performance of the Ca || CuS cell under 0.2 C in Ca/Na/Btfe electrolyte (1.0 C = 560 mA g⁻¹). f Classification of representative anions based on their calculated binding energy (E_b) with Ca²⁺ and their ionic diameters; the dashed lines divide the map into four quadrants according to anion size and binding energy. Strongly coordinating anions are typically located in the upper-left (large size, strong binding) and lower-right (small size, moderate binding) regions, whereas weakly coordinating anions fall into the upper-right region (large size, moderate binding). The green shading indicates the ideal organoborate anion region that can be used for eccentric solvation structure design. g Eccentric solvation structure strategy extended to other strongly coordinating Ca salts, Ca/Na/Btfp electrolyte enables highly reversible Ca deposition (0.25 mA cm⁻², 0.04 mA h cm⁻² for Ca/Na hybrid electrolytes, 0.05 mA cm⁻², 0.0025 mA h cm⁻² for single Ca electrolytes).

We employed the de-sodiated Na_2/3Ni_1/4Cu_1/12Mn_2/3O₂ (NaNCM) material as the positive electrode to investigate the high-voltage compatibility as well as Ca metal stability under varying ion concentrations in the electrolyte. The NaNCM was first charged to 3.9 V to obtain a Na-deficient condition. The Ca || de-sodiated NaNCM cell using the Ca/Na/Btfe electrolyte delivers an initial specific discharge capacity of 51.8 mA h g⁻¹ (Fig. 5c) for the first cycle and retains 89.4% of its capacity after 200 cycles under 6 C. The voltage profiles indicate minimal voltage fading throughout long-term cycling (Supplementary Fig. 47), suggesting the formation of a stable SEI layer on the Ca metal electrode. Furthermore, in order to evaluate the practical viability of the Ca metal electrode, a Ca || high-loading NaNCM full cell (active material loading: 11 mg cm⁻²) was assembled using electrochemically deposited Ca (2 mA h cm⁻²) and a de-sodiated NaNCM positive electrode with a capacity of ~0.74 mA h cm⁻² (negative-to-positive capacity ratio, N/P = 2.69). As shown in Fig. 5d, the cells deliver a high discharge capacity of ~0.75 mA h cm⁻² (67.7 mA h g⁻¹) and maintain stable cycling without sudden capacity drop after 55–60 cycles at 1 C/1 C rate. ICP-OES results reveal a hybrid Ca²⁺/Na⁺ intercalation mechanism (Supplementary Fig. 48), where Na⁺ ions provide the primary capacity (~91.5%) while Ca²⁺ ions contribute a small and highly reversible capacity (~8.5%). While the system operates as a hybrid Ca/Na chemistry, the successful cycling of these cells underscores the electrolyte’s ability to facilitate stable Ca stripping/plating even with variations in Na⁺ concentration present in the electrolyte.

Conversion-type positive electrodes such as CuS and sulfur are also tested to verify the general applicability. The Ca || CuS cell with Ca/Na/Btfe electrolyte exhibits a specific capacity of 128.3 mA h g⁻¹ (Fig. 5e) and maintains 59% capacity after 100 cycles. The Ca || S battery with a high sulfur loading (~3 mg_S, 70 wt% active material) delivers an initial specific capacity of 707 mA h g⁻¹ at room temperature, although its cycling stability remains to be improved (Supplementary Fig. 49).

Towards the end of this work, we evaluated the cost scalability and candidate anions for the eccentric solvation structure. Assuming an electrolyte consumption of 1.5 kg kWh⁻¹, current bill-of-materials (BOM) costs (4365–7061 US$ kWh⁻¹) are dominated by research-grade reagents (Supplementary Fig. 50). However, if the price of Ca(BH₄)₂ scales down to industrial level comparable to that of NaBH₄ (≈17 US$ kg⁻¹), the BOM cost for the Ca/Na/Btfe electrolyte drops to approximately 11.2 US$ kWh⁻¹, which approaches the cost benchmark of state-of-the-art lithium-ion battery electrolytes (~5 US$ kWh⁻¹). Regarding safety, while DME is an ideal low-viscosity model solvent for kinetics, partial substitution with high-flash-point glymes can further enhance safety for practical applications without compromising performance. Moreover, we also employ DFT calculations of Ca²⁺-anion binding energies and anion sizes to predict the coordination ability of a series of organoborates. The results reveal that shorter branched chains and lower degrees of fluorine substitution favor the formation of strongly coordinating anions (green region in Fig. 5f), thereby enabling the eccentric solvation structure strategy. Guided by this analysis, two representative anions were selected, one strongly coordinating (Btfp⁻) and one weakly coordinating (Bpfp⁻, Supplementary Fig. 51), to prepare Ca/Na hybrid electrolytes. As shown in Fig. 5g, both strongly coordinating anions and heterocation incorporation play crucial roles in enhancing the reversibility of Ca deposition, confirming the general applicability of the eccentric solvation structure strategy. These versatile combinations offer promising opportunities for the rational design of high-reversibility calcium-based electrolytes.

Discussion

In this work, we demonstrate that strongly coordinating monovalent organoborates effectively enhance calcium reversibility and reduce the overall cell overpotential by inducing an eccentric solvation structure. Strongly coordinating anions act as bridging ligands between monovalent cations and calcium ions, leading to the formation of large, complex clusters. This loosely bound solvation environment weakens the chelation between solvent molecules and calcium ions, resulting in a lower desolvation energy barrier, reduced solvation sheath reorganization energy, and an upshifted redox potential of the Ca electrode. Consequently, reductive electrolyte decomposition at the calcium surface is effectively suppressed, and a thin, CaO-rich polymeric/polycrystalline SEI layer is observed to form in Ca/Na/Btfe electrolytes. The combination of fast kinetics and high coulombic efficiency enables the realization of sustainable calcium/sodium hybrid batteries with a limited N/P ratio. Furthermore, by tuning the molecular structure of anions within the eccentric solvation structure, various electrolyte formulations can be rationally designed to promote reversible calcium electrodeposition.

Methods

General

Ca(BH₄)₂ (2021-2022 batch), Ca(BH₄)₂·THF (2021-2022 batch), pure Ca(BH₄)₂ (2024 batch) and CsCB₁₁H₁₂ were purchased from Sigma-Aldrich. NaBH₄, LiBH₄, and KBH₄ were purchased from Energy Chemical. Hexafluoroisopropanol and 2,2,2-trifluoroethanol were purchased from Adamas-beta. 3,3,3-trifluoro-1-propanol and 1,1,1,4,4,4-hexafluoro-2,3-bis(trifluoromethyl)butane-2,3-diol were purchased from Shanghai Macklin Biochemical Co. SuperP was purchased from XFNANO Co. Na tablets (99% purity, 0.45 mm) were provided by Changgao New Materials Co., Ltd. CuS was purchased from J&K and used as received. Na_2/3Ni_1/4Cu_1/12Mn_2/3O₂ (NaNCM), 1,2-Dimethoxyethane (DME), and tetrahydrofuran (THF) were purchased from Dodochem Co. Ca foils (99.9% purity) were purchased from zkynxc.com with thickness of 1 mm.

Synthesis of Ca/Li/Na/K organoborates

Ca[B(hfip)₄]₂, LiB(hfip)₄, NaB(hfip)₄: in a typical process, either 69.76 mg Ca(BH₄)₂, 43.56 mg LiBH₄, or 75.66 mg NaBH₄ powder was individually dissolved into 2 ml DME in a glass bottle, after which 1.428 g (8.5 mmol, excess) hexafluoroisopropanol was gradually added with stirring. The solution was stirred for 24 h for complete reaction. Finally, excessive hexane was added to the solution to precipitate the target solvates. After removing the supernatant, the white salts were vacuum-dried in the vacuum chamber of the glove box at room temperature.

Ca[B(tfe)₄]₂, LiB(tfe)₄, NaB(tfe)₄, KB(tfe)₄: in a typical process, either 69.76 mg Ca(BH₄)₂, 43.56 mg LiBH₄, 75.66 mg NaBH₄ or 107.88 mg KBH₄ powder was individually dissolved into 2 ml DME in a glass bottle, after which 0.85 g (8.5 mmol, excess) 2,2,2-trifluoroethanol was gradually added with stirring. The solution was stirred for 24 h for complete reaction. Finally, excessive hexane was added to the solution to precipitate the target solvates. After removing the supernatant, the white salts were vacuum-dried in the vacuum chamber of the glove box at room temperature.

Ca[B(tfp)₄]₂, NaB(tfp)₄: in a typical process, either 69.76 mg Ca(BH₄)₂ or 75.66 mg NaBH₄ powder was individually dissolved into 2 ml DME in a glass bottle, after which 0.969 g (8.5 mmol, excess) 3,3,3-trifluoro-1-propanol was gradually added with stirring. The solution was stirred for 36 h for complete reaction. Finally, excessive hexane was added to the solution to precipitate the target solvates. After removing the supernatant, the white Na salt and gel-like Ca solvate were vacuum-dried in the vacuum chamber of the glove box at room temperature.

Ca[B(pfp)₄]₂, NaB(pfp)₄: in a typical process, either 69.76 mg Ca(BH₄)₂ or 75.66 mg NaBH₄ powder was individually dissolved into 2 ml DME in a glass bottle, after which 1.419 g (4.25 mmol, excess) 1,1,1,4,4,4-hexafluoro-2,3-bis(trifluoromethyl)butane-2,3-diol was gradually added with stirring. The solution was stirred for 24 h for complete reaction. Finally, excessive hexane was added to the solution to precipitate the target solvates. After removing the supernatant, the white salts were vacuum-dried in the vacuum chamber of the glove box at room temperature.

Electrolyte preparation: For the B(hfip)₄⁻-based salts, the electrolyte concentration was calculated based on the molecular weight of the salt plus three equivalents of DME. For LiB(tfe)₄, NaB(tfe)₄, K B(tfe)₄, and Ca[B(tfe)₄]₂·DME salts, the concentration was determined according to their respective molecular weights. In order to fully dissolve the CsCB₁₁H₁₂, a THF/DME (1:3, v:v) mix solution is used in Ca/Cs/Btfe electrolyte. For Ca/Na/Btfp and Ca/Na/Bpfp-based electrolytes, concentration is set as follows: 0.25 M Ca[B(tfp)₄]₂, 0.5 M Ca[B(pfp)₄]₂, 0.1 M NaB(tfp)₄ + 0.2 M Ca[B(tfp)₄]₂, and 0.8 M NaB(pfp)₄ + 0.1 M Ca[B(pfp)₄]₂. The solvents, 1,2-dimethoxyethane and tetrahydrofuran, were used as received without further drying. The corresponding salts were added to the solvents in glass vials and dissolved via manual agitation (hand-shaking) until a clear, homogeneous solution was obtained. All liquid transfers were performed using micropipettes with polypropylene tips. The prepared electrolytes were stored in sealed glass bottles within the glovebox at room temperature.

Preparation of the electrode

The surface of the Ca metal (diameter: 11 mm; thickness: 1 mm) was mechanically polished with sandpaper until a shiny metallic luster was achieved. To prevent re-oxidation, the Ca electrodes were assembled into the cells immediately after the polishing process without any intermediate storage. The CuS electrode (mass loading ~3.2 mg cm⁻², active material loading ~2.5 mg cm⁻²) was prepared using CuS, SuperP, and polyvinylidene difluoride (PVDF) in an 8:1:1 mass ratio and cast on a Cu foil. The NaNCM electrode (mass loading ~2.4 and 13 mg cm⁻², active material loading ~ 2 and 11 mg cm⁻²) was prepared using NaNCM, SuperP, and polyvinylidene difluoride (PVDF) in an 8.5:0.5:1 mass ratio and cast on an Al foil. The S electrode (mass loading ~4.7 mg cm⁻², ~3 mg_S cm⁻²) was prepared using S/C composite, SuperP, and PVDF in an 8:1:1 mass ratio and cast on a carbon@Al foil. The above-mentioned solid mixture was manually ground in a mortar with a pestle for 0.5 h to ensure homogeneity. Subsequently, N-methyl-2-pyrrolidone (>99.9% purity, Dodochem Co.) was added to the mixture, and the resulting slurry was stirred at room temperature under an ambient atmosphere for 24 h. The slurry was then coated onto the current collector using a doctor blade. The as-prepared electrode sheets were dried in a vacuum oven at 80 °C for 12 h. After drying, the electrodes were cut into 11 mm discs using a manual disc cutter. The obtained electrodes underwent a secondary drying step at 60 °C for 6 h before being transferred directly into an Ar-filled glovebox for battery assembly.

Electrochemical measurement

The 2032-coin cells (Canrd Technology Co. Ltd.) were fabricated in the argon-filled glovebox with H₂O and O₂  <  0.1 ppm at 25 °C. Neware electrochemical testing systems (CT-4008T-5V10mA) were used to obtain the electrochemical performance of Ca metal-based cells at 25 °C. For Ca||Cu half-cells and Ca||Ca symmetrical cells, a glass fiber (GF-D, Whatman, 14 mm diameter, 680 µm thickness) separator was used for cycling. For half cells for Ca deposition, polypropylene (PP, Dodochem Co., 16 mm diameter, 25 µm thickness) separator was placed between the copper electrode and the GF-D glass fiber to avoid Ca adhesion. For half-cells and full-cells, the electrolyte amount was 100 µl and 200 µl. The CHI600E Workstation was used to evaluate the linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). LSV was performed from open circuit voltage (OCV) to 6 V at a scan rate of 35 mV s⁻¹. EIS measurements were carried out with a frequency range of 1 MHz to 0.01 Hz from 40 °C to 5 °C, using an AC amplitude of 5 mV. CV was performed from 2 V (vs Ca|Ca²⁺) to 6 V at a scan rate of 35 mV s⁻¹. Ca || de-sodiated Na_2/3Ni_1/4Cu_1/12Mn_2/3O₂ cell was cycled between 2.0 and 3.9 V. Ca || CuS cell was cycled between 0.5 and 3.0 V. Ca || S cell was cycled between 0.2 and 3.0 V. All the full cells were performed in constant-current mode. The Coulombic efficiency (CE) of the Ca metal half-cells was calculated as the ratio of the stripping capacity to the preceding plating capacity. Each electrochemical experiment was performed with 3–4 independent replicate cells. Some data are presented as the average values with error bars representing the standard deviation, as specified in the corresponding figures.

Characterization

All the Ca foils and Ca deposits for characterization are rinsed more than 3 times in pure DME solution for later testing. The morphologies and element mapping of the Ca electrode were detected by SEM SU-8010 (Hitachi) at 5–15 kV. Single-crystal X-ray diffraction (SCXRD) was performed on a Bruker D8 Venture Single Crystal X-ray Diffractometer. Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out on Perkin Elmer Avio 500. The Ca deposits with various capacities are obtained in the Ca||Cu cell for the first deposition. Nuclear magnetic resonance spectra (NMR) were performed on AVNEO500 (500 MHz). The surface composition of cycled Ca was examined by X-ray photoelectron spectroscopy (ESCALAB 250Xi, ThermoFisher Scientific). The Ca foils were obtained from Ca||Ca cells after 20 cycles under 0.2 mA cm⁻², 0.1 mAh cm⁻². Cryo-Electron Microscopy was performed on Talos F200C 200 kV using the Cu TEM digital grid (Zhongjingkeyi Film Technology Co., Ltd., Beijing) as substrate. For each electrolyte, 0.5 mAh cm⁻² Ca is firstly deposited on Cu grid in a Ca||Cu cell for cryo-EM observation, the as-deposited samples were retrieved from the cells and transferred to the cryo-transfer holder under the Ar atmosphere to avoid any exposure to air. Time of flight secondary ion mass spectrometry (TOF-SIMS) was analyzed by 5 iontof, PHI NanoTOFII, the Ca foils were obtained from Ca||Ca cells after 30 cycles under 2 mA cm⁻², 0.33 mAh cm⁻². Raman spectra were performed on a Renishaw InVia Raman microscope. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) was performed on Nicolet5700 under an atmosphere. The Ca electrodes were cycled in different electrolytes for 20 cycles in Ca||Ca cells to probe the organic components in SEI. During the ATR-FTIR test, the Ca metal was exposed to air for no more than 2 min. Simultaneous small-angle and wide-angle X-ray scattering (simultaneous SAXS/WAXS) was performed on XENOCS XEUSS 3.0* with excillum METALJET X-ray Source.

Simulation details

MD simulation

All MD simulations were conducted using the GROMACS 2022.2 program³⁵. The bonded parameters involving B were fitted by diagonal elements of the redundant internal coordinated Hessian method with Sobtop package³⁶. The Universal Force Field (UFF) force field was applied for the Lennard–Jones potential parameters of B element. And other solvation behaviors were predicted at the Generation Amber Force Field (GAFF) force field. The partial charges for all molecules were computed by fitting the RESP2 charge³⁷ of molecules at the B3LYP/ 6-311 + G** level via Multiwfn software³⁸. The electrostatic interactions in the systems were described by Particle-mesh Ewald summation. The integration for each MD step was set to 1 fs. All simulations were conducted under the NPT ensemble at 298.15 K. In each MD simulation, an initial equilibration period of 5 ns was used to equilibrate the system, followed by a 5 ns sampling period.

The definition of eccentric solvation structure: To further probe the internal symmetry and the deviation of the metal cation from the geometric center of the solvation shell, we introduce the cation-centroid displacement $d_{\mathrm{shift}}$. First, the geometric center (centroid, $r_c$) of the entire solvation cluster (comprising the central cation and all coordinating atoms) is calculated as the arithmetic mean of the coordinates of all N atoms involved:

$${\mathbf{r}}_c=\frac{1}{N}{\sum }_{i=1}^N{\mathbf{r}}_i$$

where ${\mathbf{r}}_i$ denotes the Cartesian coordinate vector of the $i-th$ atom.

Subsequently, the spatial deviation of the central Ca²⁺ from this geometric center is quantified by the Euclidean distance:

$${\mathbf{d}}_{shift}=\mathrm{\mid \mid }{r}_{Ca}-r_c\mathrm{\mid \mid }=\sqrt{{\left(x_{ca}-x_c\right)}^2+{\left(y_{ca}-y_c\right)}^2+{\left(z_{ca}-z_c\right)}^2}$$

where ${\mathbf{r}}_{ca}$ represents the position of the Ca²⁺.

Finally, the normalized offset ($\delta$) is defined as:

$$\delta =\frac{{\mathbf{d}}_{shift}}{\mathrm{L}}$$

where L corresponds to the maximum end-to-end dimension of the solvation cluster. We define solvation structures with $\delta$ > 10% as eccentric solvation structure.

DFT simulation

All DFT simulations were conducted using the ORCA 5.0.2 program³⁹. The solvation free energies $\left(\mathrm{\Delta }{G}_{\text{solv}}\right)$ of the metal cations (Li⁺, Na⁺, K⁺, Ca²⁺) in water (aq) and 1,2-dimethoxyethane (DME) were determined using a cluster-continuum model⁴⁰. First, the geometries of all species, including individual solvent molecules (H₂O, DME) and the ion-solvent clusters ${\left[M{\left(L\right)}_n\right]}^{z+}$ were fully optimized. This step was performed at the Density Functional Theory level using the B3LYP functional^41,42, the 6-31 + G* basis set⁴³ and the D3BJ dispersion correction^44,45. All optimizations employed the Conductor-like Polarizable Continuum Model (CPCM) for the respective solvent⁴⁶. Harmonic frequency calculations were run at this same B3LYP level to obtain thermochemical corrections (Gibbs free energy) and to verify all structures as true local minima.

Second, to achieve higher accuracy, refined electronic energies $\left(E_{\text{elec}}\right)$ were computed via single-point calculations on the optimized geometries. This refinement used the range-separated hybrid functional^44,47, wB97X-D3 paired with the larger def2-TZVP basis set⁴⁸.

The solvation free energy ($\mathrm{\Delta }{G}_{\text{solv}}$) was then derived from the thermodynamic cycle:

$$\mathrm{\Delta }{G}_{\text{solv}}\left(M^{z+}\right)=G_{\text{sol}}\left({\left[M{\left(L\right)}_n\right]}^{z+}\right)-n\cdot G_{\text{sol}}\left(L\right)-G_{\text{gas}}\left(M^{z+}\right)$$

Finally, the standard redox potential ($E^{\circ }$) was calculated for the DME solvent. This was achieved by computing the standard Gibbs free energy of ion transfer ($\mathrm{\Delta }{G}_t^{\circ }$) relative to water:

$$\mathrm{\Delta }{G}_t^{\circ }=\mathrm{\Delta }{G}_{\text{solv}}\left(DME\right)-\mathrm{\Delta }{G}_{\text{solv}}\left(aq\right)$$

For each ion, the $\mathrm{\Delta }{G}_{\text{solv}}\left(aq\right)$ value corresponding to the calculated water cluster model was used to establish the aqueous reference. The final potential was then calculated using the established thermodynamic cycle:

$$E^{\circ }\left(non-aq\right)=E^{\circ }\left(aq\right)+\frac{\mathrm{\Delta }{G}_t^{\circ }}{zF}$$

where $z$ is the number of electrons transferred (1 for Li, Na, K; 2 for Ca) and $F$ is the Faraday constant.

The binding energies were obtained by subtracting the total energies of the reactants from those of the corresponding Ca-solvent complexes. The final binding energy was then determined by linear interpolation based on the coordination numbers derived from MD simulations. The geometry of each compound was first optimized using the B3LYP functional⁴⁹ and the 6-31 + G* basis set. A dispersion correction was also considered using the Beck-Johnson damping function in all DFT calculations. The solvation effect was described with the conductor-like polarizable continuum model⁴⁶. The dielectric constant and refractive index were chosen as 7.2 and 1.9 to simulate the DME solution. The electronic single point energy was calculated using B2PLYP functional⁵⁰ and ma-def2-TZVP basis set.

Author contributions

S.L. and W.C. conceived the idea and designed the experiments. S.L. and J.Z. synthesized the electrolytes. F.M. performed the DFT/MD calculation. H.C. and Y. Z. synthesized and fabricated sulfur electrodes. Y.N., W.Z., and C.F. assisted in conducting characterization testing. L.F. and Y.(W.) Z. assisted in ATR-FTIR characterization. All authors participated in writing the paper. W.C. supervised the project.

Peer review

Peer review information

Nature Communications thanks Daniele Callegari, Ju-Sik Kim and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

We acknowledge the financial support from the Singapore Ministry of Education under its AcRF Tier 2 Grant No MOE-T2EP10224-0002 (W.C.) and Singapore National Research Foundation Investigatorship under Grant No NRF-NRFI08-2022-0009 (W.C.).

Data availability

All data that support the main findings are available in the main text and the Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Center, under deposition numbers CCDC 2487911 (Supplementary Fig. 23a) and 2551786 (Supplementary Fig. 23b). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/ Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-73484-4.