Enabling Ca plating and stripping by electrolyte manipulation in low-volatility solvents for Ca metal batteries

Abstract

Efficient Ca plating/stripping and the development of low-volatility electrolytes remain key challenges for the broad-scale application of Ca metal batteries. In this study, we demonstrate that CaBr₂ addition modifies the electrolyte environment, enabling highly reversible Ca plating/stripping in electrolytes based on low-volatility diglyme as a solvent previously considered nonfunctional.

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Ca metal batteries have gained considerable attention as a next-generation energy storage technology owing to the natural abundance, low reduction potential (−2.87 V vs. standard hydrogen electrode), and high volumetric capacity (2072 mA h cm⁻³) of Ca.¹ However, the practical implementation of Ca metal batteries is restricted because of the limitations in designing practical electrolytes that exhibit low volatility and allow efficient Ca plating/stripping.²

Recently, electrolytes containing weakly coordinating anions, such as Ca(B[hfip]₄)₂^3,4 and Ca(CB₁₁H₁₂)₂,^5,6 have gained increasing attention for enabling reversible Ca plating/stripping at room temperature. Ca(CB₁₁H₁₂)₂ is effective because of its fluorine-free nature, which prevents the formation of the Ca²⁺ diffusion–blocking CaF₂. In conventional electrolyte systems based on Ca(CB₁₁H₁₂)₂, 1,2-dimethoxyethane (DME) and tetrahydrofuran (THF) have been used as solvents because of their ability to weakly coordinate Ca²⁺ cations and resulting efficient desolvation. However, their low boiling points (DME: 82 °C, THF: 66 °C) lead to volatility and safety concerns. Despite these issues, no studies have yet explored the use of Ca(CB₁₁H₁₂)₂ with low-volatility solvents that exhibit reversible Ca plating/stripping.

To overcome these limitations, diglyme (G2) has been proposed as an alternative solvent, exhibiting a high boiling point (162 °C) while maintaining properties similar to those of DME (known as monoglyme, G1) and THF. However, the extended ether chain of G2 enhances Ca²⁺ solvation and may hinder desolvation. Studies on similar ether-based multivalent electrolytes^7,8 suggest that this effect increases the energy barrier for Ca²⁺ desolvation, resulting in increased overpotentials and sluggish electrochemical kinetics. Therefore, optimising solvation dynamics in G2-based electrolytes is crucial for efficient Ca plating/stripping.

Solvation manipulation using dual salt additives, explored in Mg and Ca systems, is a promising approach.^9–11 Among the potential additives, Br⁻ anions demonstrate a substantial effectiveness for modifying the solvation structure.^6,12 Br⁻ anions weaken excessive solvation interactions, facilitating desolvation and accelerating cation transport across the electrode interface.¹³ Additionally, Br⁻ anions suppress the generation of insulating byproducts, such as CaCO₃, thereby promoting the development of an ion-conductive solid electrolyte interphase (SEI) and enhancing long-term electrochemical stability.^14,15

In this study, we selected CaBr₂ as an additive because of its adequate solubility in G2, verified electrochemical activity, and compatibility with low-volatility electrolyte systems. Regarding other calcium halides, CaI₂ offers the advantages of a low Ca²⁺ diffusion barrier and favourable SEI potential but is poorly soluble in ethers,¹⁶ whereas CaCl₂ was found to be partially soluble but showed no Ca plating/stripping behaviour in our preliminary experiments. Hence, we prepared G2-based electrolytes containing 0.01, 0.03, or 0.05 M CaBr₂ and 0.4 M Ca(CB₁₁H₁₂)₂ to investigate their electrochemical performance and determine the mechanisms underlying solvation structure modulation and SEI formation. Higher concentrations were not used because of the solubility limit of CaBr₂ (0.06 M). The details of experimental methods are provided in the ESI.†

The Ca plating/stripping performance of the Ca(CB₁₁H₁₂)₂-based electrolytes with CaBr₂ was investigated through cyclic voltammetry (CV; Fig. 1a, b and Fig. S1, S2). Notably, the addition of a small amount of CaBr₂ (0.01–0.05 M) drastically improved Ca plating/stripping behaviour, whereas no distinct peaks were observed in the absence of CaBr₂ (Fig. 1a inset). These results suggest that the strong coordination between G2 and Ca²⁺ inhibits Ca plating/stripping.¹⁷ To examine electrolyte durability during Ca plating/stripping, we evaluated the overvoltage behaviour through galvanostatic cycling tests (Fig. 1c). At 0.01 M CaBr₂, the voltage gradually rose to 1 V, suggesting resistive-phase build-up due to electrolyte decomposition. In contrast, for 0.03 and 0.05 M CaBr₂, the overvoltage increase was substantially suppressed compared with that observed at 0.01 M CaBr₂. The final cycle overvoltage was 0.8 V at 0.03 M CaBr₂ and 0.6 V at 0.05 M CaBr₂, indicating an improvement in electrochemical performance. These enhancements were attributed to solvation structure modifications induced by Br⁻ anions and changes in the SEI composition. Ionic conductivity measurements (Fig. S3, ESI†) showed minimal impact from CaBr₂ addition, indicating a limited contribution to the performance enhancement.

Fig. 1 (a) Cyclic voltammograms of Ca plating/stripping for 0.4 M Ca(CB₁₁H₁₂)₂ with 0.05 M CaBr₂ in diglyme (G2) at the fourth cycle. Data for conditioning cycles are shown in Fig. S1 (ESI†). Inset: Presents the cyclic voltammogram recorded without CaBr₂. (b) Geometry of the CB₁₁H₁₂⁻ anion. (c) Galvanostatic Ca plating/stripping cycling performance of Ca/Ca symmetric cells at a current density of 5 μA cm⁻². Insets: Show enlarged voltage profiles at intermediate and final cycles.

The coordination environment of Ca²⁺ was investigated through nuclear magnetic resonance (NMR) and Raman spectroscopies. The ⁴³Ca NMR spectra featured sharp peaks at all CaBr₂ concentrations, indicating a fast exchange between coordination states and highly symmetrical average structure (Fig. 2a). The chemical shift change from −28.4 ppm at 0 M to −13.3 ppm at 0.05 M suggested a concomitant decrease in the shielding of the Ca nucleus. This trend was attributed to the substitution of coordinating G2 molecules by Br⁻ anions, which promoted the formation of contact ion pairs (CIPs) in place of solvent-separated ion pairs (SSIPs).^18,19 Separately, the addition of CaBr₂ had no noticeable effect on the ¹H and ¹¹B NMR spectra (Fig. S4, ESI†), which indicated that the interaction of this additive with the CB₁₁H₁₂⁻ anion was minimal.

Fig. 2 Coordination environment investigation of Ca²⁺ in 0.4 M Ca(CB₁₁H₁₂)₂ with 0, 0.01, 0.03, and 0.05 M CaBr₂ in G2. (a) ⁴³Ca NMR and (b) Raman spectra.

The results of Raman spectroscopy analysis further supported this structural evolution. For pure G2, characteristic vibrational bands appeared at 852, 826, and 808 cm⁻¹ (Fig. 2b and Fig. S5, ESI†). Upon the addition of Ca(CB₁₁H₁₂)₂, new peaks emerged at 877.5 and 835 cm⁻¹, corresponding to Ca²⁺–G2 coordination, as previously reported for G2-based K-ion systems with symmetric solvation structures.²⁰ As the CaBr₂ concentration increased from 0 to 0.05 M, the 877.5 cm⁻¹ band shifted to 872.8 cm⁻¹, which indicated a weakening of the Ca²⁺–G2 interaction, probably due to the progressive replacement of G2 molecules by Br⁻ anions.¹⁸ This vibrational shift was consistent with the NMR spectroscopy-observed solvation change and confirmed the progressive formation of CIPs in the presence of Br⁻ anions. In addition to the red shift, a minimal blue shift was observed in the C–O–C symmetric stretching region. The corresponding band shifted from 854.2 cm⁻¹ (free G2) to 854.4 cm⁻¹ (0 M CaBr₂) and further to 855.4 cm⁻¹ (0.05 M CaBr₂). The initial shift reflected the coordination of Ca²⁺ to G2, and the subsequent blue shift was attributed to the increased rigidity or symmetry in the Ca²⁺–G2 coordination environment induced by Br⁻. This interpretation is supported by the results of theoretical studies on glyme–metal ion complexes, where similar frequency shifts were correlated with changes in coordination structure and bond rigidity.²¹

To investigate the SEI formation changes induced by CaBr₂ addition, the Ca metal surface after 30 min of electrolyte soaking was analysed using ex situ X-ray photoelectron spectroscopy (XPS; Fig. 3a). The deconvoluted C 1s spectra featured three peaks attributable to alkyl carbon (C–C) at 284.5 eV, oxygen-bound carbon (C–O) at 286.5 eV, and electron-withdrawing group–associated carbon (COO) at 288.0 eV. At 0.05 M CaBr₂, the contents of C–O and COO species were lower than those at 0 M, which suggested a reduction in the amount of organic materials and the suppressed formation of G2-derived decomposition products. The O 1s spectra featured a dominant peak at 531 eV (C–O) and additional peaks at 533 eV (O–C=O) and 530 eV (Ca–O). At 0.05 M CaBr₂, the Ca–O peak gained intensity, whereas the O–C=O peak lost intensity, which indicated CaO formation and the suppression of organic species formation. Similarly, the Ca 2p spectra displayed a consistent trend, revealing an increase in the peak intensities for CaO (346.5 and 350 eV) and a decrease in those for CaCO₃ (347 and 350.5 eV) as the CaBr₂ concentration increased. As CaCO₃ hinders Ca²⁺ ion movement,^22,23 this compositional shift suggests potential improvements in ion transport. Similar trends were observed after galvanostatic Ca plating/stripping cycling (Fig. S6, ESI†), in line with the results shown in Fig. 1c. In the absence of Br⁻, post-CV XPS analysis indicated the loss of polymeric components and emergence of CaCO₃ and CaO (Fig. S7, ESI†), suggesting that the SEI had a limited stability. The B 1s spectra exhibited a dominant peak at 188.4 eV corresponding to the B–H moieties of the CB₁₁H₁₂⁻ anion, with no detectable peak at ∼191 eV corresponding to B–O species;⁶ this finding confirmed the chemical stability of the CB₁₁H₁₂⁻ anion on the Ca metal surface. This anion may be encapsulated within organic components and polymeric phases to afford a gel-like polymer electrolyte. The Br 3d spectrum showed peaks exclusively at 0.05 M CaBr₂, suggesting the formation of a Br-containing SEI. As shown in Fig. 3b, surface atomic composition analysis revealed a decrease in the atomic concentration of C and an increase in those of Ca and Br, indicating a transition from an organic-rich SEI to an inorganic-rich SEI.

Fig. 3 (a) C 1s, O 1s, Ca 2p, B 1s, and Br 3d spectra of Ca metal soaked for 30 min in 0.4 M Ca(CB₁₁H₁₂)₂ with 0 and 0.05 M CaBr₂ in G2. (b) Atomic concentration (atom%) of the passivation layer components on the Ca metal surface. (c) and (d) Schematics of solvation structure and SEI formation in electrolytes (c) without and (d) with CaBr₂.

Based on these results, the schematic illustrations in Fig. 3c and d provide a plausible explanation for the solvation structures and SEI formation mechanisms in the presence and absence of CaBr₂. In the absence of CaBr₂, the Ca metal surface forms a CaCO₃-rich SEI, which impedes Ca²⁺ conduction and results in a high overpotential. This behaviour can be attributed to the predominant presence of weakly coordinating CB₁₁H₁₂⁻ anions, which may promote the formation of SSIPs. During the desolvation process, solvent decomposition is expected to prevail over anion interactions, leading to the excessive formation of CaCO₃. Conversely, the introduction of CaBr₂ substantially alters SEI composition, resulting in the incorporation of CaO, CaBr₂, organic compounds, and polymeric structures. This SEI composition enhances Ca²⁺ transport and stabilises Ca plating/stripping, reducing overvoltage. The suppression of CaCO₃ formation is ascribed to CIP formation induced by Br⁻, which suppresses solvent decomposition during desolvation and promotes the formation of SEI structures that are more conducive to Ca²⁺ conduction.

To assess the viability of the developed electrolyte, an initial investigation was performed using a Ca–S battery (Fig. 4a), known for its notably high theoretical specific capacity.²⁴ A brief evaluation of oxidative stability revealed that all electrolytes were anodically stable above 3.3 V (Fig. S8, ESI†), which was sufficient for operation with sulfur cathodes. The initial discharge and charge capacities were determined as 536 and 512 mA h g⁻¹, respectively, demonstrating a reversible electrochemical behaviour (Fig. 4b). These values were lower than those achieved for Ca(CB₁₁H₁₂)₂ in DME/THF (805 and 705 mA h g⁻¹, respectively),⁵ possibly because the higher solubility of reaction intermediates in G2 led to active material loss and reduced capacity during cycling (Fig. S9, ESI†).

Fig. 4 (a) Schematic of a prototype Ca metal battery using an electrolyte of 0.4 M Ca(CB₁₁H₁₂)₂ with 0.05 M CaBr₂ in G2. (b) Voltage profile during the first cycle. (c) S 2p spectra of S/C electrodes in pristine (top) and discharged (bottom) states.

To investigate the conversion processes in the S/C cathode, we characterised pristine and discharged electrodes by XPS. The S 2p spectrum of the pristine cathode showed a spin–orbit doublet characteristic of elemental sulphur at 164.0 and 165.2 eV (Fig. 4c, top). After discharge to 0.5 V, the spectrum featured three doublets attributable to residual sulphur, CaS_n, and CaS (Fig. 4c, bottom).^24,25 The predominance of CaS indicated the effective conversion of sulphur to sulphides, validating the active participation of the sulphur cathode. This finding suggests that the capacity loss in G2 stems from CaS_n dissolution, not unreacted sulphur, and implies that the further optimisation of the solvent composition to suppress polysulfide dissolution could enhance the electrochemical performance.⁵ Compared with DME/THF,⁵ G2 facilitated the dissolution of CaS_n intermediates, leading to a decreased surface coverage by CaS and CaS_n and relatively enhanced visibility of these oxidised species in the XPS spectra.

In summary, this study revealed that the incorporation of CaBr₂ into Ca(CB₁₁H₁₂)₂-based electrolytes in G2 substantially enhanced Ca plating/stripping efficiency by modifying both solvation structure and SEI composition. The Br⁻ anions promoted CIP formation, suppressing solvent decomposition and minimising CaCO₃ formation, which hindered Ca²⁺ mobility. This improvement was reflected in the enhanced long-term electrochemical stability and reduced overvoltage. Furthermore, the optimised electrolyte enabled the successful operation of a Ca–S battery, demonstrating its potential for the development of high-energy-density systems. These findings highlight the role of anion engineering in advancing practical Ca metal batteries.

This work was supported by a JSPS KAKENHI Grant-in-Aid for Scientific Research B (No. 22H01803) and JST FOREST Program (Grant Number JPMJFR236F). Additional support was provided by Sumitomo Metal Mining Co., Ltd., the Murata Science and Education Foundation (No. M24AN132), and the Iketani Science and Technology Foundation (No. 0361179-A).

Data availability

All relevant experimental data within the article will be provided by the corresponding author on reasonable request.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Results of additional instrumental analyses. See DOI: https://doi.org/10.1039/d5cc01242b

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