Reversible Calcium Plating and Stripping atRoom Temperature Using a Borate SaltAbhinandan Shyamsunder,†,‡ Lauren E. Blanc,†,‡ Abdeljalil Assoud,† and Linda F. Nazar*,†,‡

†Department of Chemistry and Waterloo Institute of Nanotechnology, University of Waterloo, 200 University Avenue West,Waterloo, Ontario N2L3G1, Canada‡Joint Center for Energy Storage Research, Argonne National Laboratory, Lemont, Illinois 60439, United States

*S Supporting Information

ABSTRACT: Interest has rekindled in reversible calcium plating and stripping,renewing hopes for the development of Ca-ion batteries. However, the developmentof an electrolyte that operates at room temperature and is stable to oxidation atpractical potentials remains a significant barrier. Here we report the synthesis andcrystal structure of a new fluorinated alkoxyborate Ca(B(Ohfip)4)2·4DME salt.Reversible plating and the dissolution of calcium from pure solutions of this salt indimethoxyethane are demonstrated at 25 °C with capacities of 1 mAh cm−2 at a rateof 0.5 mA cm−2 over 30−40 cycles, with an anodic stability of >4.1 V vs Ca/Ca2+ (andup to 4.9 V in dimethyltriflamide). The dominant product is calcium, accompaniedby CaF2 that forms by the reduction of the fluorinated anion. Whereas the cathodicstability requires improvement, this work shows that facile calcium plating andstripping at room temperature can be achieved using bulk electrodes.

The prospect of achieving high energy densities beyondthose offered by lithium ion batteries1 is leadingresearch efforts toward divalent ion batteries such as

those based on Mg or Ca.2,3 Although these divalent metals arehindered by lower gravimetric capacities compared with Li,they offer higher volumetric capacities (Mg, 3832 mAh·cc−3;Ca, 2072 mAh·cc−3) due to their ability to transfer multipleelectrons per ion. To date, most research efforts directedtoward the development of divalent battery systems havefocused on magnesium-based chemistries. However, incomparison with Mg, Ca offers (1) a lower reductive potential(−2.87 V vs standard hydrogen electrode (SHE) comparedwith −2.37 V vs SHE for Mg), (2) a higher earth abundance(4.2 vs 2.4% of the Earth’s crust), and (3) a lower chargedensity due to its larger size. These advantages are propellinginterest toward developing Ca ion batteries as an alternative toLi ion systems. Like any multivalent metal electrode, a majorproblem associated with the use of a calcium anode arises froma passivating layer that forms on the electrode surface duringcycling, which inhibits further plating and stripping. This issueis exacerbated in the case of Ca due to its highly reductivenature.4,5

Initial work on Ca electrodeposition dates to 1980, whenStaneiwicz examined calcium plating onto a Ni substrate in aCa/SOCl2 system and observed the formation of a thickpassivating CaCl2 layer that prohibited further electrochemicalactivity.6 Later, Aurbach et al. carried out seminal studies ofcalcium electrodes in typical battery solvent/salt combinations.They concluded that Ca deposition is impossible in thesystems they examined due to the passivation layers that were

formed which prevented any further ionic transport.7

Ponrouch et al. were the first to prove that Ca could bereversibly plated and stripped using a Ca(BF4)2/ethylenecarbonate/propylene carbonate (EC/PC) mixture at 100 °Con a stainless-steel (SS) substrate.8 The main productdeposited was calcium metal, along with CaF2, as confirmedthrough scanning electron microscopy (SEM) and X-raydiffraction (XRD) analysis. Ca batteries based on Ca/Snalloys9,10 have further highlighted the promise of Caelectrochemistry. Recently a breakthrough in calcium strippingand plating using a Ca(BH4)2 electrolyte was reported at roomtemperature with a Coulombic efficiency (CE) of 95% on a Ausubstrate.11 The high efficiency was attributed to the formationof CaH2 as a decomposition product, which, unlike otherionically insulating solid−electrolyte interphases (SEIs),promotes stable calcium electrodeposition and dissolution.Although attractive electrochemical performance was indi-cated, the Ca(BH4)2/tetrahydrofuran (THF) electrolytenaturally suffers from low anodic stability due to the highlyreducing nature of the borohydride anion. A viable electrolytewith a wider electrochemical window is capable of reversibleplating and stripping of calcium is desirable to enable high-voltage cathodes.Chemical similarities between calcium and magnesium

suggest that design principles employed for magnesium-ion

Received: July 19, 2019Accepted: August 22, 2019Published: August 22, 2019

Letterhttp://pubs.acs.org/journal/aelccpCite This: ACS Energy Lett. 2019, 4, 2271−2276

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batteries ought to provide a useful approach to developanalogous calcium electrolytes. Most of the previously reportedand stable electrolytes for magnesium batteries are based onGrignard reagents,12 but the apparent instabilities of theircalcium-based analogues13 limit the ability to mimic these.More recently, research on magnesium electrolytes has tendedtoward halide-free, anodically stable, and low-ion-pairingcompounds.14 These include the carborane-type materials15

and fluorinated alkoxyaluminate/borate salts.16−18 The latterare highly stable, weakly coordinating anions due to the

increased electron-withdrawing nature of their alkoxy ligands.19

The fluorinated alkoxy borate derivatives are comparativelymore stable than the alkoxy aluminates.20 The salts developedwith these anions are generally low-ion-pairing and providesufficient solubilities in conventional glyme-based solvents sothat one can investigate their electrochemical behavior insolution. Here we report the synthesis, crystal structure, andelectrochemistry of a new fluorinated alkoxy borate salt,Ca(B(Ohfip)4)2, which is based on the hexafluoroisopropoxy(Ohfip−) ligand, and investigate calcium stripping and plating

Figure 1. X-ray crystal structure of Ca(B(Ohfip)4)2·4DME, displaying the coordination environments of calcium and boron (Ca, blue; B,green; F, light blue; O, red).

Figure 2. (a) Fourth CV cycle with a 500 mM Ca(B(Ohfip)4)2 electrolyte on a Au substrate with calcium as the reference and counterelectrodes at a scan rate of 25 mV/s. The inset shows the charge passed on plating and stripping from the voltammogram. (b) Galvanostaticstripping and plating of calcium on a Au electrode with Ca counter and reference electrodes from a 500 mM Ca(B(Ohfip)4)2 electrolyte at0.2 mA·cm−2 to a capacity of 1 mAh·cm−2, with the inset showing the Coulombic efficiency. (c) Comparison of the 50th CV cycles with a 500mM Ca(B(Ohfip)4)2 electrolyte (red curve) and a 500 mM Ca(B(Ohfip)4)2 + 100 mM Bu4NCl electrolyte (blue curve). (d) Galvanostaticstripping and plating of calcium on a Au electrode with Ca counter and reference electrodes from a 500 mM Ca(B(Ohfip)4)2 + 100 mMBu4NCl electrolyte at 0.5 mA·cm−2 to a capacity of 1 mAh·cm−2. The inset shows the selected stripping and plating cycles with an efficiencyof 95%.

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at room temperature from solutions of this pure salt. In 1,2-dimethoxyethane (DME), we obtain capacities of 1 mAh·cm−2

at a rate of 0.2 mA·cm−2 and up to 0.5 mA·cm−2 with relativelylow polarization on stripping (∼170 mV), good Coulombicefficiency (92−95%), and over 35 cycles. In particular, the saltshows excellent anodic stability: up to 4.1 V in DME and 4.9 Vin N,N-dimethyltriflamide.Calcium hexafluoroisopropoxide (Ca(Ohfip)2·xTHF) and

tris-hexafluoroisopropoxy borate (B(Ohfip)3) were mixed inappropriate ratios in DME at 65 °C for 6 h (see SupportingInformation for details). The clear solution obtained was thenconcentrated under dynamic vacuum at room temperature toobtain the Ca(B(Ohfip)4)2·xDME salt. Nuclear magneticresonance (NMR) measurements (Figure S1) were carriedout on the salt and 11B NMR confirmed the formation of atetrahedral boron center by comparison to literature data.15 Toevaluate the number of DME molecules associated with thesalt and identify its structure, crystals were grown fromsolution through a slow evaporation technique. A single-crystalXRD solution of the structure of the salt yielded a unit cellindexed in the tetragonal space group I41/acd with a,b =19.6322(8) Å and c = 36.855(2) Å (Figure S2), revealing thatthe structure of Ca(B(Ohfip)4)2·4DME is similar to that of thepreviously reported magnesium analogue Mg(B(Ohfip)4)2·3DME.15 Boron is attached to four hexafluoroisopropoxyligands in a tetrahedral environment, whereas calcium ischelated to eight oxygen atoms from four DME molecules in apseudocubic environment (Figure 1). To minimize impuritiesand ensure consistent results for electrochemical studies, onlythe pure crystallized salt was used to prepare electrolytes.Electrochemical studies were conducted on the maximum

concentration of the Ca(B(Ohfip)4)2 salt in DME (0.5 M).Calcium plating and stripping were carried out by cyclicvoltammetry (CV) studies in three-electrode cells (12 mm indiameter) using Ca metal as a reference and counter electrode(see experimental methods in the Supporting Information fordetails). The CV plot (Figure 2a) exhibits the typical nature ofmetal plating and stripping on the cathodic and anodic scans.Calcium commences deposition at −550 mV on the cathodicscan, and in the anodic scan, the current starts to rise at ∼300mV, signaling the stripping of the plated calcium. On the basisof the amount of charge passed, the Coulombic efficiencyreaches ∼92% in the fourth cycle (inset, Figure 2a) and issteady until the 40th cycle. The first and the second CV cycles

along with the Coulombic efficiencies are shown in Figure S3.After this point, the current density decreases for bothprocesses, which is suggestive of the formation of a passivatinglayer on the deposited calcium. To further investigate thebehavior of the Ca(B(Ohfip)4)2·4DME salt in the DMEsolvent, we carried out galvanostatic cycling using three-electrode cells, with Ca metal as the counter and referenceelectrodes in the window from −1.5 to 1.2 V vs Ca/Ca2+.Stable stripping and plating of calcium to a capacity of 1 mAh·cm−2 (Figure 2b) was observed at a current density of 0.2 mA·cm−2 with overpotentials of 300 mV for plating and ∼180 mVfor stripping. However, a larger overpotential was exhibited onthe first plating. The initial round-trip efficiency is 62%, butthis increases to 92% after the fourth cycle due to conditioningof the electrode. These values are significantly better thanthose reported for the Ca(BF4)2 electrolyte, where elevatedtemperatures that promote passivating parasitic reactions wererequired to deposit calcium to a limited capacity of 0.165 mAh·cm−2.8 During plating in the 18th cycle, our cell shorted, andpost mortem analysis identified significant dendrite formationas the cause for cell failure (Figure S4). At a higher currentdensity (1 mA·cm−2) and the same capacity, the cell shortsearlier owing to accelerated dendritic growth (Figure S4b).SEM images indicate that these dendrites form becausecalcium does not deposit uniformly on the gold electrode.Furthermore, energy-dispersive X-ray spectroscopy (EDX)analysis of the deposits obtained shows a significant fractionof fluorine in addition to calcium (Figure S5). Slower rates(0.1, 0.05, and 0.01 mA·cm−2) were explored to investigate ifthe lower ionic conductivity (3.2 mS·cm−1) is responsible forthe patchy deposition observed. However, the decompositionof the anion competes with the deposition process at theseslow rates and thus restricts the lowest possible current densityto 0.2 mA·cm−2.Because higher current densities (>0.2 mA·cm−2) could not

be sustained due to the limited solubility of the Ca(B-(Ohfip)4)2·4DME salt in DME, Bu4NCl, which has previouslybeen shown to increase the diffusion coefficient for Ca(BH4)2in THF, was added to the electrolyte.21 Indeed, addition of thissalt at a 100 mM concentration increased the ionicconductivity from 3.2 to 6.7 mS·cm−1. As depicted in Figure2c, the addition of Bu4NCl furthermore enhances the cycle lifeof the Ca(B(Ohfip)4)2 electrolyte. Significant current wasobserved for the stripping process, even in the 50th cycle,

Figure 3. (a) Linear sweep voltammograms recorded on various substrates at a scan rate of 0.05 mV/s with Ca as both the reference andcounter electrodes for 0.2 M Ca(B(Ohfip)4)2 in DME electrolyte. (b) LSV recorded on a Al substrate for 0.2 M Ca(B(Ohfip)4)2 in N,N-dimethyltriflamide (DMT) solvent at 0.05 mV/s with Ca as both the reference and counter electrodes.

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whereas the electrolyte without the additive exhibited nocurrent on stripping. The positive impact of the additive is alsoreflected in the galvanostatic stripping and plating process,where stable cycling was observed at a current density of 0.5mA·cm−2 (Figure 2d), with overpotentials similar to those ofcycling studies conducted without the additive at a lowercurrent density (0.2 mA·cm−2). With Bu4NCl, after the firstfew cycles at 0.5 mA·cm−2, the Coulombic efficiency quicklyincreases to 95% and is maintained in excess of 30 cycles(Figure S6). Chloride-based additives for magnesium batteriesresult in an extended cycle life and high Coulombic efficiencyfor stripping and plating processes through the formation of asacrificial interphase on the deposited metal.22 Uniform platingensues as a result of the free chloride ions decorating thesurface of the electrode along with forming dimers with thesolvated magnesium species, which leads to the prevention ofparasitic processes at the interphase during charge transfer.However, different factors appear to be at play here becausecompletely uniform plating was not obtained on Au and othersubstrates, such as Cu and SS (Figure S7). The precise role ofBu4NCl (other than to increase the ionic conductivity), andthe exploration of other additives are under investigation in ourlaboratory and will be reported elsewhere.The anodic stability of the Ca(B(Ohfip)4)2/DME electrolyte

on typical electrode substrates such as Al, SS, Cu, and Au wasidentified through linear sweep voltammetry (LSV). Figure 3ademonstrates that the electrolyte has an oxidative stability of3.8 V on Au, significantly higher than the 3.0 V reported forthe Ca(BH4)2 electrolyte on Au. This is expected because thelatter was optimized for anodic stability. In comparison withAu, Ca(B(Ohfip)4)2/DME achieves its highest anodic stabilityof ∼4.1 V on an Al substrate, thereby allowing for theapplication of high-voltage cathode materials.23−25 In DME,

the anodic stability of the electrolyte is limited by thedecomposition potential of the solvent26 and not of the salt.When the salt is employed in a solvent stable to a highpotential such as N,N-dimethyltriflamide (5.7 V vs Li+/Li),27 itexhibits an anodic stability of 4.9 V vs Ca/Ca2+ on an Alsubstrate (Figure 3b). This anodic limit is >1 V higher thanthat of Ca(BF4)2 (∼3.5 V on a SS substrate).8

To confirm that the redox processes observed in CV scanscorrespond to Ca metal deposition and strippingand toidentify the nature of the passivating layer that forms ondepositiongalvanostatic deposition was carried out to acapacity of 6 mAh·cm−2 on a gold substrate from a 0.5 Melectrolyte with and without 100 mM Bu4NCl. The dark-graydeposits on the gold electrode were removed and collected forsynchrotron XRD experiments and SEM−EDX analysis. Full-profile fitting of the XRD pattern indicates that in both cases,(1) Ca deposits in two distinct phases (cubic Fm3m andorthorhombic Cmcm) and (2) the deposited material containscalcium fluoride along with Ca metal (Figure 4a,b). Rietveldrefinement of this experimental data to attempt to quantifyprecise weight fractions of each phase was not possible due tothe large degree of overlap of the broad XRD reflections in thedeposited samples. Therefore, the amount of CaF2 present inthe Ca deposits was estimated using EDX elemental analysis.To confirm the validity of this method, a pure CaF2 referencewas analyzed, showing a 1:2 ratio and verifying that EDX yieldsreliable relative atomic fractions of Ca and F withinexperimental error (Figure 4d). On average, EDX spectra ofthe deposited material exhibit a Ca/F ratio of 3:1,corresponding to a Ca/CaF2 ratio of 5:1 (Figures S8 andS9); however, elemental mapping shows that the depositedsamples contain distinct Ca-rich regions that are composed ofnearly 90% pure calcium metal (Figure 4c,e). In accord, SEM

Figure 4. Full-profile fits of XRD patterns of calcium deposited on gold from (a) 500 mM Ca(B(Ohfip)4)2 and (b) 500 mM Ca(B(Ohfip)4)2with 100 mM Bu4NCl showing the experimental data (black crosses) collected at a wavelength of 0.68926 Å, the fitted profile (red line), thedifference map between the observed and calculated data (blue line), and the Bragg positions of the phases present: Fm3 m Ca (red ticks),Cmcm Ca (blue ticks), Fm3 m CaF2 (dark-gray ticks), and Fm3 m Au (orange ticks). The Au was accidentally scraped from the electrodeduring sample preparation. (c) Elemental mapping of calcium deposited from 500 mM Ca(B(Ohfip)4)2. EDX spectra of (d) a CaF2 referenceand (e) the calcium-rich deposit (red circles).

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images of the Au electrode (Figure S8) reveal the presence ofisland-like deposits of calcium. Elemental mapping of the cross-sectional image also confirms the presence of Ca on theelectrode surface (Figure S9).The formation of CaF2 is the result of chemical reduction of

the hexafluoroisopropoxy ligand at the highly reducingpotential of Ca metal (−2.83 V vs SHE). No CaH2 isobserved in the deposited product, unlike the case ofdeposition of calcium from Ca(BH4)2 salt in THF.9 Theabsence of CaH2 may reflect a recent report which suggeststhat hydride abstraction is from the borohydride ion ratherthan the solvent (namely, THF).19 A recent report on anionicdecomposition pathways for divalent cation electrolytessuggests the formation of ion pairs between the Lewis acidiccation and the anion may be the culprit. In the case of Mg,these ion pairs could undergo partial reduction (Mg2+ → Mg+)at the interface, which could compete with the charge-transfermechanism and, in turn, activate the anion, leading to avehicular pathway for anionic decomposition.28 The CaF2impurity observed for the Ca(B(Ohfip)4)2 electrolyte is likelynot due to ion-pair formation, however, because Ca2+ is amuch weaker Lewis acid than Mg2+. We also note thatmagnesium deposition from the analogous Mg(B(Ohfip)4)2·3DME salt does not result in MgF2 formation.15 Thus thereason for the formation of CaF2 is probably due to the morereductive nature of Ca versus that of Mg. Further investigationsare currently being conducted to understand this phenomenonas well as the difference in plating behavior between Mg andCa in the presence of Cl−. Results of these studies, as well asdetailed investigations probing the structure of the electrolytebefore/after the addition of Cl−, will be communicated as aseparate work.In summary, we synthesized, structurally characterized, and

explored the electrochemical properties of a new andpromising calcium electrolyte salt based on a weaklycoordinating hexafluoroisopropoxy borate anion. In DMEsolutions, the salt exhibits reversible calcium stripping andplating at room temperature at capacities of 1.0 mAh·cm−2 andrates up to 0.5 mA·cm−2, with a high anodic stability up to 4.1V vs Ca/Ca2+ and up to 4.9 V in dimethyltriflamide. Whereasthe formation of CaF2 passivates the calcium metal anode onprolonged cycling, this may be addressed in the future by co-formation of a stable SEI such as CaH2. Electrolyte additivesthat could form ionically conductive interphases on the Cametal electrode to prevent anion reduction offer an alternativesolution to preserve the metal’s electrochemical activity.Nonetheless, we believe these results provide new insightinto the preparation of existing and future calcium electrolytesystems, which are vital to take calcium-ion batteries forward.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.9b01550.

Experimental methods, synthetic methods, list of figures,and single-crystal data (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: lfnazar@uwaterloo.ca.

ORCIDLinda F. Nazar: 0000-0002-3314-8197Author ContributionsA.S. and L.F.N. designed this study. A.S. carried out thesynthesis of the electrolyte and its spectrometric character-ization and performed all of the electrochemical measure-ments. L.E.B. carried out all of the SEM experiments, theanalysis, and the diffraction studies. A.A. carried out single-crystal refinement studies. A.S. and L.F.N. wrote themanuscript with contributions from L.E.B. and A.A.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the Joint Centre forEnergy Storage Research, an Energy Innovation Hub fundedby the US. Department of Energy, Office of Science, BasicEnergy Sciences. Synchrotron XRD measurements wereperformed using beamline 08B1-1 at the Canadian LightSource. We thank Dr. Joel Reid for assistance with collectingthis data. L.F.N. also thanks NSERC for platform supportthrough the Discovery Grant and Canada Research Chairprograms and Dr. Kevin Zavadil (Sandia National Labs) forhelpful discussions.

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■ NOTE ADDED IN PROOFDuring the revision stage of this paper a parallel work on thetitle compound was published online.29

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