Work

A Fluorine-Free Electrolyte for Calcium Metal Batteries

Year: 2025

Type: article

Abstract: Calcium metal batteries (CMBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), owing to their elemental abundance, lower dendrite risk, and potential for multi-electron redox processes. 1 Batteries based on calcium metal anode are particularly attractive, offering the lowest redox potential (−2.87 V vs. SHE) and a high volumetric capacity of 2073 mAh cm−3; comparable to that of lithium metal. 2 In addition, the lower charge density of Ca2+ compared to Mg2+ (another promising multivalent system based on abundant element and high theoretical capacity) resulting in lower coulombic interactions potentially favours power performance. 3 However, the standard reduction potential of Ca is atleast 500 mV negative to that of Mg, which makes practical calcium battery development a challenging task due to interfacial limitations- especially the formation of passivating layers in most conventional electrolytes that hinder Ca2+ transport and it's desolvation at the electrode/electrolyte inteface. Significant interest and technical improvements in this field were realised since the first report of reversible Ca plating/stripping in 2016 by Ponrouch et al. using Ca(BF4) 2 in an EC:PC solvent blend at elevated temperatures (70-100 °C). 4 This system formed a mixed organic-inorganic passivation layer that allowed Ca 2+ diffusion to some extent, with boron cross-linked polymeric matrices present in the passivation layer potentially playing an important role allowing for Ca plating. 5 The composition of the passivation layers can significantly affect Ca 2+ mobility. Density functional theory (DFT) calculations reveal that CaH 2 exhibits the lowest migration barrier (∼541 meV) among other major interphase components, like CaF 2 (∼2046 meV), CaCO 3 (∼1436 meV), CaO (∼997 meV), and favours Ca 2+ transport. 6 Room-temperature calcium plating was later realized using Ca(BH4) 2 -based electrolytes, which generate a CaH 2 -rich solid electrolyte interphase (SEI) that enabled sufficient Ca 2+ transport. 7 However, their limited anodic stability and high reactivity restrict compatibility with high-voltage cathodes. To address this, more anodically stable salts - such as Ca[B(hfip)4] 2 and Ca[Al(hfip)4] 2 - were developed, enabling operation above 3 V. 8 However, these salts are synthetically complex, expensive, and sensitive to impurities. As an alternative strategy, Hou et al. introduced a high-donor solvent, dimethylacetamide (DMAc), combined with Ca(TFSI) 2 . 9 Initially, the DMAc-based formulation reduced polarization and minimized the need for complex electrolyte engineering, facilitatating reversible Ca cycling at low overpotentials. However, during long term cycling, Ca(TFSI) 2 revealed increasing polarization, attributed to TFSI − decomposition and CaF 2 accumulation at the interface. Although TFSI − is bulky and charge-delocalized, it tends to form contact ion pairs (CIPs) above 0.5 M concentration - even in high donor number solvents like DMAc - leading to anion reduction and a brittle, ion-insulating passivation layer comprising mostly CaF 2 . 10 In response to these limitations and growing safety and environmental concerns over halogenated compounds, we synthesized Ca(MSI) 2 , a fluorine-free analogue of Ca(TFSI) 2 , via a one-step solid state mechanothermal synthesis process and paired it with DMAc. This new formulation demonstrated improved performance, achieving lower polarization (0.82 V vs. 0.95 V with TFSI − ) and stable cycling for over 1600 h at 0.02 mA cm −2 . Characterization showed that Ca(MSI) 2 produces a thinner, more uniform SEI with reduced interfacial decomposition. The absence of fluorine avoids CaF 2 -rich interphases and instead promotes a more uniform, ion-conductive passivation layer. The enhanced stability of Ca(MSI) 2 highlights that improved ionic mobility and SEI uniformity can reduce the need for inorganic interphase engineering. 9 Furthermore, its fluorine-free nature not only prevents CaF 2 formation but also aligns with sustainability goals by avoiding the energy-intensive synthesis of C–F bonds. This work highlights design rules for halogen-free Ca electrolytes with improved electrochemical reversibility, reduced environmental impact, and safer large-scale battery deployment, offering pathways applicable across emerging multivalent and beyond-lithium battery chemistries. References: (1) J. B. Goodenough et al. , J. Am. Chem. Soc., 2013, 135(4), 1167–1176 (2) B. Dunn et al. , Science, 2011, 334(6058), 928–935 (3) J. D. Forero-Saboya et al. , Adv. Mat. Interfaces, 2022, 9, 2101578 (4) A. Ponrouch et al. , Nat. Mater., 2016, 15, 169–172 (5) J. Forero-Saboya et al. , Energy Environ. Sci., 2020, 13, 3423–3431 (6) A. Shyamsunder et al. ACS Energy Lett., 2019, 4, 2271–2276 (7) D. Wang et al. Nat. Mater., 2018, 17, 16–20 (8) T. Pavčnik et al. J. Mater. Chem. A, 2023, 11, 14738–14747 (9) Z. Hou et al. Angew. Chem., Int. Ed., 2022, 61(50), e202214796 (10) J. D. Forero-Saboya et al. J. Phys. Chem. C, 2019, 123, 29524–29532 Figure 1

Source: EES batteries.

DOI: 10.1039/d5eb00162e