Reversible Calcium Plating and Stripping at Room Temperature Using

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Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt Abhinandan Shyamsunder, Lauren E. Blanc, Abdeljalil Assoud, and Linda F. Nazar ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b01550 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Energy Letters

Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt Abhinandan Shyamsundera', Lauren E. Blanca', Abdeljalil Assouda, Linda F. Nazara'* aDepartment

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 AUTHOR INFORMATION Corresponding Author

* Email: [email protected]

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ABSTRACT

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

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The prospect of achieving high energy densities beyond those offered by lithium ion batteries1 is leaning research efforts towards divalent ion batteries such as those based on Mg or Ca.2,3 Although hindered by lower gravimetric capacities compared to Li, these divalent metals offer higher volumetric capacities (Mg; 3832 mAh/cc and Ca; 2072 mAh/cc) due to their ability to transfer multiple electrons per ion. To date, most research efforts directed toward the development of divalent battery systems have focused on magnesium-based chemistries. However, in comparison to Mg, Ca offers a (1) lower reductive potential (-2.87 V vs. SHE compared to -2.37 V vs. SHE for Mg), (2) higher earth abundance (4.2% vs. 2.4% of the Earth’s crust), and (3) decreased charge density due to its larger size. These advantages are propelling interest towards developing Ca ion batteries as an alternative to Li ion systems. Like any multivalent metal electrode, a major problem associated with the use of a Ca metal anode arises from a passivating layer that forms on the electrode surface during cycling which inhibits further plating and stripping of the metal. This issue is exacerbated in the case of Ca due to its highly reductive nature.4,5 Initial work on Ca electrodeposition dates to 1980 where Staneiwicz probed for calcium electrodepostion onto a Ni substrate in a Ca/SOCl2 system and observed the formation of a thick passivating CaCl2 layer that prohibits Ca electrodeposition.6 Later, Aurbach et al., carried out seminal studies of calcium electrodes in typical battery solvent/salt combinations. They concluded that Ca deposition is impossible in the systems they examined due to the passivation layers that were formed that prevent any further ionic transport.7 Ponrouch et al. were the first to prove that Ca could be reversibly plated and stripped using a Ca(BF4)2/EC/PC mixture at 100˚C on a SS substrate.8 The main deposited product was calcium metal along with CaF2 as confirmed through SEM and XRD analysis. Ca batteries based on Ca/Sn alloys9,10 have further highlighted the promise of Ca electrochemistry, and recently a breakthrough in calcium stripping and plating using


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a Ca(BH4)2 electrolyte was reported at room temperature with a coulombic efficiency of 95% on a Au substrate.11 The high efficiency obtained was attributed to the formation of CaH2 as a decomposition product which, unlike other ionically insulating SEI, promotes stable calcium electrodeposition and dissolution. Although an attractive efficiency for stripping and plating is obtained, the Ca(BH4)2/THF electrolyte naturally suffers from low anodic stability due to the highly reducing nature of the borohydride anion. A viable electrolyte with a wider electrochemical window that enables reversible plating and stripping of calcium is desirable to enable high voltage cathodes. Due to the chemical similarities between calcium and magnesium, design principles employed for magnesium ion batteries ought to provide a useful approach to develop analogous calcium electrolytes. Most of the previously reported and stable electrolytes for magnesium batteries are based on Grignard reagents,12 but the apparent instabilities of analogous calciumbased reagents13 limits the ability to mimic these. More recently, research on magnesium electrolytes has tended towards halide free, anodically stable and low-ion pairing compounds.14 These include the carborane type materials15 and fluorinated alkoxyaluminate/borate salts.16,17,18 The latter are highly stable, weakly coordinating anions due to the increased electron withdrawing nature of their alkoxy ligands.19 The fluorinated alkoxy borate derivatives are comparatively more stable than the alkoxy aluminates.20 The salts developed with these anions are generally low-ion pairing and provide sufficient solubilities in conventional glyme based solvents to investigate their electrochemical behavior in solution.

Here we report the synthesis, crystal structure and

electrochemistry 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 at room temperature from solutions of this pure salt. In DME, we obtain capacities of 1 mAh.cm-2 at a rate


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of 0.2 mA.cm-2 and up to 0.5 mA.cm-2 with relatively low polarization on stripping (170 mV), good coulombic efficiency (92 - 95%) and in excess of 35 cycles. In particular, the salt shows excellent anodic stability; up to 4.1 V in DME, and 4.9 V in N.N-dimethytriflamide. Calcium hexafluoroisopropoxide (Ca(Ohfip)2.xTHF) and tris-hexafluoroisopropoxy borate (B(Ohfip)3) were mixed in appropriate ratios in 1,2-dimethoxyethane (DME) at 65˚C for 6 hours (see Supporting Information for details). The clear solution obtained was then concentrated under dynamic vacuum at room temperature to obtain the Ca(B(Ohfip)4)2.xDME salt. NMR measurements (Fig. S1) were carried out on the synthesized salt;

11B

NMR confirmed the

formation of a tetrahedral boron center by comparison to literature data. 17 To evaluate the number of DME molecules associated with the salt and identify its structure, block-like crystals were grown from solution through a slow evaporation technique. Single crystal X-ray diffraction solution of the structure of the salt yielded a unit cell indexed in the tetragonal space group I41/acd with a,b = 19.6322(8) Å and c = 36.855(2) Å (Fig. S2), revealing that the structure of Ca(B(Ohfip)4)2•4DME

is

similar

to

the

previously

reported

magnesium

analogue

Mg(B(Ohfip)4)2•3DME.17 Boron is attached to four hexafluoroisopropoxy ligands in a tetrahedral environment while calcium is chelated to eight oxygens from four DME molecules in a pseudo-

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). ACS Paragon Plus Environment

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cubic environment (Fig. 1). In order to minimize impurities and ensure consistent results for electrochemical studies, only pure crystallized salt was used to prepare the electrolytes. Electrochemical studies were conducted on the maximal concentration of the Ca(B(Ohfip)4)2 salt in DME (0.5 M). Calcium plating and stripping was carried out by cyclic voltammetry (CV) in three electrode cells (12 mm in diameter) using Ca metal as reference and counter electrode (see Methods for details). The CV plot (Fig. 2a) exhibits the typical nature of

Figure 2. a) 4th CV cycle with a 500 mM Ca(B(Ohfip)4)2 electrolyte on an Au substrate with calcium as reference and counter electrodes at a scan rate of 25 mV/s. Inset shows charge passed on plating and stripping from the voltammogram.; b) Galvanostatic stripping and plating of calcium on an Au electrode with Ca counter and reference electrodes from a 500 mM Ca(B(Ohfip)4)2 electrolyte at 0.2 mA.cm-2 to a capacity of 1 mAh.cm-2 with inset showing the coulombic efficiency ; c) Comparison of the 50th CV cycles with a 500 mM Ca(B(Ohfip)4)2 electrolyte (red curve) and a 500mM Ca(B(Ohfip)4)2 + 100 mM Bu4NCl electrolyte (blue curve) and d) Galvanostatic stripping and plating of calcium on an Au electrode with Ca counter and reference electrodes from a 500 mM Ca(B(Ohfip)4)2 + 100mM Bu4NCl at 0.5 mA.cm-2 to a capacity of 1 mAh.cm-2, the inset shows selected stripping and plating cycles with an efficiency of 95%.


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metal plating and stripping on the cathodic and anodic scans. Calcium commences deposition at 550 mV on the cathodic scan, and in the anodic scan the current starts to rise at  300 mV, signaling stripping of the plated calcium. Based on the amount of charge passed, the coulombic efficiency reaches 92% at the 4th cycle (inset, Fig. 2a) and is steady until the 40th cycle. The 1st and the 2nd CV cycles along with the coulombic efficiencies are shown in Fig S3. After this point the current density decreases for both processes, suggestive of the formation of a passivating layer on the deposited calcium. To further investigate the behavior of the Ca(B(Ohfip)4)2.4DME salt in DME solvent, we carried out galvanostatic cycling using three-electrode cells with Ca metal as the counter and reference electrodes in the window from -1.5 V to 1.2 V versus Ca/Ca2+. Stable stripping and plating of calcium to a capacity of 1 mAh.cm-2 (Fig. 2b) was observed at a current density of 0.2 mA.cm-2 with overpotentials of 300 mV for plating and 180 mV for stripping, albeit with a larger overpotential on the first plating. The initial round-trip efficiency is 62%, but this increases to 92% after the fourth cycle due to conditioning of the electrode. These values are significantly better than those reported for the Ca(BF4)2 electrolyte where elevated temperatures – which promote passivating parasitic reactions – were required to deposit calcium to a limited capacity of 0.165 mAh.cm-2.8 During plating in the 18th cycle, the cell shorted and post mortem analysis identified significant dendrite formation as the cause for cell failure (Fig. S4). At a higher current density (1 mA.cm-2) and same capacity, the cell shorts sooner owing to accelerated dendritic growth (Fig. S4b). SEM images indicate that these dendrites form because calcium does not deposit uniformly, but rather only in certain areas of the gold electrode. Furthermore, EDX analysis of the deposits obtained also show a significant fraction of fluorine in addition to calcium (Fig. S5). Slower rates (0.1, 0.05, 0.01 mA.cm-2) were explored to investigate if the lower ionic conductivity (3.2 mS.cm-1) is responsible for the patchy deposition observed. However, the


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decomposition of the anion competes with the deposition process at these slow rates, and thus restricts the lowest possible current density to 0.2 mA.cm-2. Since higher current densities (over 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, 100 mM Bu4NCl – which has previously been shown to increase the diffusion coefficient for Ca(BH4)2 in THF - was added to the electrolyte. 21 Indeed, this increased the ionic conductivity from 3.2 mS.cm-1 to 6.7 mS.cm-1. As depicted in Fig. 2c, addition of Bu4NCl enhances the cycle life of the Ca(B(Ohfip)4)2 electrolyte. Significant currents were observed for the stripping process even at the 50th cycle while the electrolyte without the additive exhibits no current on stripping. The positive impact of the additive is also reflected in the galvanostatic stripping and plating process, where stable cycling was observed at a current density of 0.5 mA.cm-2 (Fig. 2d), with overpotentials similar to cycling studies conducted without the additive at a lower current density (0.2 mA.cm-2). With Bu4NCl, after the first few cycles 0.5 mA.cm-2 the coulombic efficiency quickly increases to 95% and is maintained in excess of 30 cycles (Fig. S6). Chloride based additives for magnesium batteries have resulted in extended cycle life and high coulombic efficiency for stripping and plating processes through the formation of a sacrificial interphase on the deposited metal.22 Uniform plating ensues as a result of the free chloride ions decorating the surface of the electrode, along with forming dimers with the solvated magnesium species that lead to the prevention of parasitic processes at the interphase during charge transfer. However, different factors appear to be at play here, since completely uniform plating was not obtained on Au and other substrates, such as Cu and SS (Fig S7). The precise role of Bu4NCl – other than to increase the ionic conductivity – and exploration of other additives are under investigation in our laboratory and will be reported elsewhere.


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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 and counter electrodes for a 0.2 M Ca(B(Ohfip)4)2 in DME electrolyte and b) LSV recorded on a Al substrate for 0.2 M Ca(B(Ohfip)4)2 in N,N-dimethyl triflamide (DMT) solvent at 0.05mV/s with Ca as both the reference and counter electrodes. The anodic stability of the Ca(B(Ohfip)4)2/DME electrolyte on traditional electrode substrates such as Al, stainless steel (SS), Cu and Au was identified through linear sweep voltammetry (LSV). Fig. 3a demonstrates that the electrolyte has an oxidative stability of 3.8 V on Au, significantly higher than 3.0 V reported for the Ca(BH4)2 electrolyte on Au. This is expected, since the latter was optimized for anodic stability. In comparison to Au, Ca(B(Ohfip)4)2/DME achieves its highest anodic stability of approximately 4.1 V on an Al substrate, thereby allowing for the application of high voltage cathode materials.23,24,25 In DME the anodic stability of the electrolyte is limited by the decomposition potential of the solvent26 and not of the salt. When the salt is employed in a solvent stable to high potential such as N,Ndimethyltriflamide (5.7 V vs Li+/Li),27 it exhibits an anodic stability of 4.9 V vs Ca/Ca2+ on an Al substrate (Fig. 3b). This anodic limit is over 1 V higher than that of Ca(BF4)2 ( 3.5 V on a SS substrate).8


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To confirm that the redox processes observed in CV scans correspond to Ca metal deposition and stripping and to identify the nature of the passivating layer that forms on deposition, galvanostatic deposition was carried out to a capacity of 6 mAh.cm-2 on a gold substrate from a 0.5 M electrolyte with and without 100 mM Bu4NCl. The dark gray deposits on the gold electrode were removed and collected for synchrotron X-ray diffraction 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 and orthorhombic Cmcm) and (2) the deposited material contains calcium fluoride along with Ca metal (Fig. 4a,b). Rietveld refinement of this experimental data in order to try to quantify precise weight fractions of each phase was not possible due to the large degree of overlap of the broad XRD reflections in the deposited samples. Therefore, the amount

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)2 with 100 mM Bu4NCl showing the experimental data (black crosses) collected at a wavelength of 0.68926 Å, fitted profile (red line), difference map between the observed and calculated data (blue line), and Bragg positions of the phases present: Fm3m Ca (red ticks), Cmcm Ca (blue ticks), Fm3m CaF2 (dark gray ticks), and Fm3m Au (orange ticks); the Au was accidentally scraped from the electrode during sample preparation. c) Elemental mapping of calcium deposited from 500 mM Ca(B(Ohfip)4)2, and EDX spectra of d) a CaF2 reference and e) the calcium rich deposit (red circles).


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of CaF2 present in the Ca deposits was estimated using EDX elemental analysis. To confirm the validity of this method, a pure CaF2 reference was analyzed, showing a 1:2 ratio and verifying that EDX yields reliable relative atomic fractions of Ca and F within experimental error (Fig. 4d). On average, EDX spectra of deposited material exhibit a Ca:F ratio of 3:1, corresponding to a Ca:CaF2 ratio of 5:1 (Fig. S8, S9); however, elemental mapping shows that the deposited samples contain distinct Ca rich regions which are comprised of nearly 90% pure calcium metal (Fig. 4c,e). In accord, SEM images of the Au electrode (Fig. S8) reveal the presence of island-like deposits of calcium. Elemental mapping of the cross sectional image also confirms the presence of Ca on the electrode surface (Fig. S9). The formation of CaF2 is the result of chemical reduction of the hexafluoroisopropoxy ligand at the highly reducing potential of Ca metal (-2.83 V vs SHE). No CaH2 is observed in the deposited product, unlike the case of deposition of calcium from Ca(BH4)2 salt in THF.11 The absence of CaH2 may reflect a recent report which suggests that hydride abstraction is from the borohydride ion rather than the solvent (namely, THF).21 A recent report on anionic-decomposition pathways for divalent cation-electrolytes suggests the formation of ion-pairs between the Lewis acidic cation 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 transfer mechanism and in turn activate the anion leading to a vehicular pathway for anionic decomposition.28 The CaF2 observed for the Ca(B(Ohfip)4)2 electrolyte is likely not due to ion pair formation, however, as Ca2+ is a much weaker Lewis acid than Mg2+. We also note that magnesium deposition from the analogous Mg(B(Ohfip)4)2•3DME salt does not result in MgF2 formation.17 Thus, the reason for the formation of CaF2 is probably due to the more reductive nature of Ca vs that of Mg. Further investigations are currently being conducted to understand this phenomenon


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as well as the difference in plating behavior between Mg and Ca in the presence of Cl-. Results of these studies, as well as detailed investigations probing the structure of the electrolyte before/after the addition of Cl- will be communicated as a separate work. In summary, we synthesized, structurally characterized, and explored the electrochemical properties of a new and promising calcium electrolyte salt based on a weakly coordinating hexafluoroisopropylborate anion. In DME solutions, the salt exhibits reversible calcium stripping and plating at room temperature at capacities of 1.0 mAh.cm-2 and rates up to 0.5 mA.cm-2, with a high anodic stability up to 4.1 V vs. Ca/Ca2+, and up to 4.9 V in dimethyltriflamide. While the formation of CaF2 passivates the calcium metal anode on prolonged cycling, this may be addressed in the future by co-forming a stable SEI such as CaH2. Electrolyte additives that could form ionically conductive interphases on the Ca metal electrode to prevent anion reduction offer an alternative solution to preserve the metal’s electrochemical activity. Nonetheless, we believe these results provide new insight into the preparation of existing, and future calcium electrolyte systems which are vital to take calcium-ion batteries forward. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental methods, Synthetic methods, List of figures, Single crystal data. Author contributions: AS and LFN designed this study. AS carried out the synthesis of the electrolyte, its spectrometric characterisation and carried out all the electrochemical measurements. LEB carried out all the SEM experiments, analysis and the diffraction studies. AA carried out single crystal refinement studies. AS and LFN wrote the manuscript with contributions from LEB and AA.


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AUTHOR INFORMATION Corresponding Author * Email: [email protected] ORCID Linda F.Nazar : 0000-0002-3314-8197 Abhinandan Shyamsunder: 0000-0003-4788-6809 Lauren E. Blanc: 0000-0001-8536-4214 Abdeljalil Assoud: 0000-0001-8536-4214 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the Joint Centre for Energy Storage Research, an Energy Innovation Hub funded by the US. Department of Energy, Office of Science, Basic Energy Sciences. Synchrotron XRD measurements were performed using beamline 08B1-1 at the Canadian Light Source. We would like to thank Dr. Joel Reid for assistance with collecting this data. L.F.N also thanks NSERC for platform support through the Discovery Grant and Canada Research Chair programs. REFERENCES

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ACS Energy Letters

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Reversible Calcium Plating and Stripping at Room Temperature Using

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