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Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries Kim Ta, Kimberly A. See, and Andrew A. Gewirth J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00835 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018
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Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries Kim Ta†,‡, Kimberly A. See†§, and Andrew A. Gewirth†,‡,* †Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States ‡Joint Center for Energy Storage Research, Argonne National Laboratory, Lemont, Illinois 60439, United States *Email:
[email protected] ABSTRACT Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI‒) and chloride-containing electrolyte in acetonitrile exhibit current densities which are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two electron‒transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical-electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg2(µ‒Cl)3·6THF]+ present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature.
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Current address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 USA ACS Paragon Plus Environment
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INTRODUCTION Li‒ion batteries (LIBs) are one of the most advanced technologies available for energy storage applications.1 However, safety concerns, the need for higher power density, and the desire for cheaper manufacturing costs has motivated the search for “beyond Li‒ion battery” materials and systems.2-3 Examples of new beyond Li chemistries involve deposition of multivalent metals resulting in a metal-based battery. Nonaqueous multivalent batteries such as Mg (theoretical capacity = 3833 Ah/L) and Zn (theoretical capacity = 5851 Ah/L) offer a significant enhancement in theoretical volumetric capacity compared to that of LIBs (2062 Ah/L). Moreover, both Mg and Zn metals are more abundant and less expensive than Li metal.2-4 Batteries based on multivalent metals, however, come with many issues. For Mg, only a limited number of ethereal‒based electrolyte solutions support reversible electrodeposition and stripping of Mg. Aurbach and coworkers introduced Grignard‒based electrolyte solutions that were anodically stable at potentials up to 3.3 V vs Mg.5-6 Even though these Grignard‒based solutions support efficient Mg electrodeposition, they are extremely reactive and highly sensitive to moisture. Another Mg electrolyte is based on alkoxides, and these solutions exhibit less sensitivity to moisture and air.7 A third electrolyte solution involves only MgCl2 and AlCl3 in tetrahydrofuran (THF).8-9 Other ethereal‒based electrolyte solutions are contain anions such as trifluoromethanesulfonyl
imide
(TFSI‒)
and
hexafluorophosphonate
(PF6)‒,10-11
monocarborane,12 borohydride,13-14 and hexamethyldisilazide15. These electrolytes feature large overpotentials (50 – 500 mV) and, in some cases, require conditioning following a period of quiescence.16-18 The burgeoning discovery of new electrolytes has not yielded as many corresponding insights into the mechanism of Mg electrodeposition. We previously suggested that Mg deposition from a
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series of electrolytes containing different salts proceeded according to a similar mechanism.19 In certain cases, the presence of a Mg dimer [Mg2(µ‒Cl)3·6THF]+ was correlated with the advent of efficient Mg electrodeposition.5,
8, 17, 20-21
The monomeric complexes, (MgCl2·4THF) and
(MgCl·5THF)+ resulting from the original dimer, were suggested to be the intermediate species present at the interface.21 Their existence at the electrode surface indicates that Mg electrodeposition may not proceed directly from the dimer species [Mg2(µ‒Cl)3·6THF]+ as previously suggested. It is still unclear, however, about the identity of the primary steps in the Mg electrodeposition process and their relation to processes occurring in other multivalent depositions. In contrast, Zn electrodeposition has much more facile kinetics relative to Mg in nonaqueous electrolyte solutions. A comprehensive study on Zn electrolyte solutions with various inorganic salts with anions such as TFSI‒, trifluoromethanesulfonate (OTF‒), hexafluorophosphate (PF6)‒, and tetrafluoroborate (BF4)‒ in various organic solvents including acetonitrile (MeCN), diglyme (G2), N’,N’‒dimethylformamide (DMF), and propylene carbonate (PC) was previously reported.22 The Zn electrolyte solutions exhibit excellent Zn electrodeposition/dissolution reversibility with high Coulombic efficiencies of >99% and highly stable anodic potentials up to ~3.8 V vs. Zn2+/Zn. Recently, our lab developed ZnAlxCo2‒xO4 spinels as cathode materials for nonaqueous Zn batteries that have an open circuit potential of 18 MΩ/cm) and annealed with a H2 flame. A 25−µm diameter Pt ultramicroelectrodes
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(UME) (CH instruments) were mechanically polished with a 400‒grit CarbiMet pad (Buehler) and a 800‒grit MicroCut disc (Buehler) then electrochemically polished in 0.1 M HNO3 by sweeping the potential from ‒0.5 V to 2.0 V (vs. Ag/AgCl) for 15 cycles before each experiment. Mg foil (GalliumSource, LLC) and Zn foil (Goodfellow, 99.95%) were polished with a razor blade before use as counter and reference electrodes. Voltammetric Simulations. COMSOL Multiphysics 5.3 (COMSOL Inc. Burlington, MA) was used to simulate cyclic voltammetry for both Zn and Mg electrodeposition/dissolution as described in the Supporting Information. RESULTS AND DISCUSSION
Figure 1: (a) Experimental CV of Zn electrodeposition in 0.5 mM Zn(TFSI)2 + 0.5 mM ZnCl2 in MeCN electrolyte solution at 90% C.E. (b) Simulated CV of Zn electrodeposition using COMSOL Multiphysics 5.3 at 100% C.E. Pt UME with a diameter of 25‒µm was used as the working electrode. Zn Electrodeposition/Stripping Analysis. Figure 1a reports CVs obtained from a Pt UME working electrode in a solution containing 0.5 mM Zn(TFSI)2 + 0.5 mM ZnCl2 in MeCN. ZnCl2 was added to enhance deposition at low Zn concentrations. Overlaid experimental and simulated CVs are presented in Figure S2 in the Supporting Information. Lower Zn concentrations were associated with higher deposition overpotentials in the absence of Cl addition.27 Starting at 0.8 V 6 ACS Paragon Plus Environment
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the cathodic scan shows little current until reaching a potential of ca. ‒0.2 V when Zn electrodeposition starts. The 0.2 V overpotential and hysteretic behavior on the anodic scan in Figure 1a is consistent with the presence of a nucleation overpotential as observed previously in Zn electrodeposition experiments.22, 28-29 As the potential is swept positive, the Zn electrodeposit is stripped from the electrode surface. The CV exhibits a scan rate dependence on the stripping potential with faster scan rates leading to more delayed stripping current. The Coulombic efficiency (CE) of deposition and stripping is found to be ca. 90% at the Pt UME. There are a few possible origins of the reduced CE observed relative to the macro electrode results described above. First, incomplete stripping from a putative Zn‒Pt surface alloy could reduce the CE. Formation of a Zn‒Pt surface alloy has been reported previously from both nonaqueous30-31 and aqueous32-33 electrolyte solutions. However, electrodeposition of Zn onto a Pt macroelectrode yields near unity CE, suggesting that alloy formation is not the origin of the reduced CE seen here. Second, resistance to charge transfer at the UME causes a reduction in potential at the center of the electrode surface while the edge of the UME is largely unaffected. This resistance leads to
greater flux and an increase in the magnitude of the potential
experienced by the edge of the electrode relative to that at the center of the electrode.34-35 This behavior is colloquially referred to as the ‘edge effect’. We associate the
