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A Study of Electrochemical Phenomena Observed at the Mg Metal/Electrolyte Interface Oscar Tutusaus, Rana Mohtadi, Nikhilendra Singh, Timothy S. Arthur, and Fuminori Mizuno ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00549 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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ACS Energy Letters

A Study of Electrochemical Phenomena Observed at the Mg Metal/Electrolyte Interface

Oscar Tutusaus, Rana Mohtadi, Nikhilendra Singh, Timothy S. Arthur, and Fuminori Mizuno*,1 Toyota Research Institute of North America, 1555 Woodridge Avenue, Ann Arbor, MI 48105, USA

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

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Current address: Toyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi 471-8572, JAPAN.

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The interface between Mg metal and electrolyte is a key factor affecting Mg battery performance. Switchable interfacial phenomena, involving apparent surface electrochemical inhibition under open circuit voltage and reactivation upon electrochemical polarization, were investigated with various Mg electrolyte systems, under both electrochemically static and dynamic conditions. Most notably, it was found that such behavior appears to be unique for the Mg system, implying that correct control of the interface is of considerable practical concern in Mg battery. This new challenge must be addressed in order to achieve high energy and high durability rechargeable Mg batteries.

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Battery technologies are becoming essential towards supporting a sustainable society. The first lithium-ion (Li-ion) battery was introduced in 1991 and has constantly evolved since, achieving widespread use in portable, automotive and grid applications. Li-ion battery energy density has been greatly improved over the years, but as this technology approaches its theoretical limits it becomes increasingly difficult to meet the energy needs of our portable electronic devices. In the search for systems with higher energy density, rechargeable Mg batteries have recently gained attention due to their potential application as post Li-ion systems. Compared with Li metal, a Mg metal anode features: (i) higher volumetric energy density, Mg (3833 mAh/cm3) vs. Li (2061 mAh/cm3); (ii) lower cost based on its higher Clarke number, Mg (1.93) vs. Li (0.006);1 and (iii) potential intrinsic safety due to absence of dendritic Mg growth, as opposed to observed dendritic Li metal deposition.2-10 The revival of practical and scientific interest in Mg batteries is highlighted by advances aimed at overcoming existing hurdles and mechanistic studies of factors governing its operation.2-19 Despite the unquestionable progress achieved in rechargeable Mg batteries, the number of available practical high voltage (>2 V vs. Mg) cathode materials displaying high Mg-ion mobility is still very limited. While some promising cathode materials have been reported,20-34 such as MnO2 polymorphs,21-23 V2O5,24-26 amorphous V2O5-P2O5,27 and MoO3,26 such advances still remain at an exploratory stage.5,6 One of the fundamental limitations in the exploration of high voltage cathodes resides in the availability of electrolyte systems compatible with Mg metal, providing a large electrochemical window and chemically compatible with cathode materials. Accordingly, a great deal of effort has been devoted to improving the properties of existing electrolytes (non-noble metal corrosivity, instability towards water, and incompatibility with high-voltage cathodes), so that they are suitable for use in rechargeable Mg batteries.35-45 In


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this context, we recently developed magnesium monocarborane (MMC) [Mg(CB11H12)2] as the first simple salt Mg battery electrolyte.17 MMC solutions in tetraglyme were shown to reversibly deposit and dissolve Mg, and display high oxidative stability (up to 3.5 V vs. Mg),46 while not corroding stainless steel at voltages above 2.5 V vs. Mg.17 These properties make MMC the first Mg electrolyte suitable for evaluating high voltage (>2.5 V vs. Mg) cathode candidates in standard stainless steel coin cells. Given the pressing issues associated with Mg cathode and electrolyte development, processes occurring at the Mg metal anode have received far less attention.13,14,47-53 Nevertheless, these processes also affect full cell performance and in many cases could have been misattributed to poor cathode performance. Hence, processes at the anode may be the real reason for some of the failures to translate cathode performance from three-electrode cells to two-electrode cells.5,6 Previous studies performed on a number of electrolytes compatible with Mg metal available at the time showed that impedance on the Mg metal interface increased under open circuit voltage (OCV), and drastically decreased under an applied voltage.13,14 Recently, we investigated the allethyl DCC electrolyte (EtMgCl-Et2AlCl/THF) and found a similar behavior.48 In this article, we further expand our study to other magnesium electrolytes, including the APC (PhMgClAlCl3/THF), the newly developed MMC electrolyte,9 as well as Mg(TFSI)2/DME and Mg(TFSI)2/ACN solutions. We employ electrochemical analyses to probe the interfacial phenomena that occur between Mg metal and selected electrolytes, under both OCV and an applied voltage using a two-electrode cell setup. With this knowledge, we aim at broadening our understanding of the origin and trends of such interfacial phenomena. We initially focused on the electrochemical performance of symmetrical Mg/Mg cells containing the MMC electrolyte (0.62 M Mg(CB11H12)2 in tetraglyme), galvanostatically. Cell

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voltage and impedance were monitored before and after each galvanostatic cycle (Figure 1). When cycling was started immediately after assembling the cell, an initial voltage spike extending to approx. +1.5 V was observed, followed by a voltage profile within a polarization of approx. ±52 mV at the 50th cycle (Figure 1a). As shown in the figure inset, this cell featured a relatively flat voltage plateau upon current reversal every 2 hours, indicating Mg deposition and dissolution processes. In contrast, when a symmetrical Mg/Mg cell containing the MMC electrolyte was kept at OCV for 50 h prior to galvanostatic cycling (Figure 1b), a higher initial voltage spike (approx. +3.0 V) was noted, and the voltage profile obtained during cycling had slightly higher polarization (about ±120 mV at the 50th cycle, Figure 1b inset).


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Figure 1. (a&b) Voltage response of a symmetrical Mg/Mg cell with MMC electrolyte without (a) and with (b) storage time for 50 h. Last two cycles of the chronopotentiogram were shown in the inset. (c&d) Nyquist plots under OCV of a symmetrical Mg/Mg cell with 50 h storage time at OCV, prior to (c) and during (d) electrochemical cycling. A magnified figure is shown in the inset. The frequency region for the EIS measurements was from 200 kHz to 10 mHz. Figures 1c and 1d display the Nyquist plots under OCV of the symmetrical Mg/Mg cell with MMC electrolyte (Figure 1b), during storage at OCV and during subsequent galvanostatic cycling, respectively. Similar to other electrolytes compatible with Mg metal,13,14,48 cell

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impedance recorded during rest at OCV (Figure 1c) displayed increasing interfacial cell resistance with storage time, reaching approx. 1000 kΩ cm2 (5 times its initial value) after 50 hours. Longer rest periods at OCV did not increase interfacial impedance further, indicating a possible stabilization of the interface. The large interfacial resistance developed during storage at OCV when compared to the cell without OCV storage time (Figure S1) accounts for the difference in initial voltage spike upon galvanostatic cycling (+3.0 V vs. +1.5 V, for the cell with and without storage time at OCV, respectively). In agreement with previous observations,13,14,48 cycling led to a substantial reduction of the interfacial resistance (Figure 1d). Impedance was found to stabilize at approx. 1.2 and 0.4 kΩ cm2 for the cell with and without storage time at OCV (Figures 1d and S1), respectively, roughly three orders of magnitude lower than their initial values. Accordingly, cell operation involving Mg deposition and dissolution, promotes disappearance of the initial large polarization due to formation of a new Mg metal/electrolyte interface.13 Note that, despite the relatively similar interfacial resistances during cycling in both cells, cell polarization was noticeably larger in the cell with rest at OCV (±120 mV vs. ±52 mV). The origin of this difference remains unclear at this moment. Impedance increase recorded on Mg/Mg symmetrical cells containing the MMC electrolyte during the initial 50 h could potentially indicate growth of an ion-blocking layer resulting from the reaction of Mg metal with reducible species present in the electrolyte. Comparison of scanning electron micrographs of the surface of Mg metal sample before and after soaking for 12 h in the MMC electrolyte (Figure S2) showed minimum alteration to the morphology. Further, XPS analysis of the soaked Mg sample excluded formation of any surface film as shown by presence of Mg metal (Mg 2p peak), and the lack of B 1s peak (Figure S2). Accordingly, impedance increase in MMC containing Mg/Mg symmetrical cells does not involve formation of a solid passivation layer.


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Further, cells containing another electrolyte compatible with Mg metal, APC (0.2 M PhMgClAlCl3 in THF), were also analyzed and displayed a similar behavior to MMC. In general, cells built using APC (Figures S3-S6) provided lower impedance values than those containing MMC electrolyte, leading to lower initial voltage spikes (+1.0 and +0.75 V, for cells with and without rest at OCV, respectively) and lower polarization upon galvanostatic cycling (±38 mV and ±35 mV, for cells with and without rest at OCV, respectively), even when using unpolished Mg electrodes (Figures S7-S8). It is possible to attribute these differences between MMC and APC electrolytes to the distinct nature of the adsorbed species at the interface within each electrolyte and Mg metal. Indeed, earlier studies have shown a strong dependence of the electrode impedance in Mg/Mg symmetrical cells when using structurally similar salts of composition RMgX (R=alkyl group, X=halide).13 As in the case of the MMC electrolyte, SEM and XPS analyses of a Mg metal sample soaked in the APC electrolyte revealed no solid passivation layer formation (Figure S2), implying that it is not the cause for the observed impedance increase. Moving away from electrolytes compatible with Mg metal such as MMC and APC, we also analyzed electrolytes based on Mg(TFSI)2 (TFSI=bis(trifluoromethanesulfonyl)imide) for which compatibility with Mg metal is still under controversy.54,55 Cells built using a Mg(TFSI)2/DME electrolyte showed significant differences to those containing electrolytes compatible with Mg metal (MMC and APC). In the absence of rest at OCV prior to cycling, cells were able to deposit and strip Mg albeit at a high overpotential (between ±350-450 mV, Figures S9-S10) and after an initial voltage spike of 1.9 V. Although addition of 50 h resting time under OCV before cycling caused a resistance buildup comparable to cells containing MMC and APC, cycling behavior was remarkably different (Figures S11-S12). Initially, cell polarization was high, starting at about ±1.9 V and sequentially decreasing until it stabilized at around ±80 mV after 17 cycles.

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Corresponding Nyquist plots during cycling exhibited a drastic decrease in the interfacial resistance from approx. 900 kΩ cm2 to 0.8 kΩ cm2. It is worth noting that the corresponding capacitive loop ended at a relatively high frequency (approx. 100 mHz) compared to APC or MMC (50 µm-thick Mg layer shuttled, based on the assumption of uniform Mg deposition/dissolution involving two-electron transfer. Similar behavior was observed when the same testing protocol was applied to a Mg/Mg symmetrical cell containing the APC electrolyte (Figure S24). In comparison, the current response for a symmetrical Li/Li cell with LiTFSI/PC (Figure S25) was very noisy beyond the 100th cycle, which corresponds to theoretically having only shuttled about a 50 µm-thick Li layer. Consequently, Mg metal can practically transport larger amounts of ions than Li metal under a bare metal surface condition, corroborating previous studies.10 The ability of the Mg metal/electrolyte interface to support continuous metal deposition/dissolution points to the possibility of achieving durable operation of the battery anode.


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Figure 2. (a) Snapshot of current/voltage responses for a symmetrical Mg/Mg cell with MMC electrolyte. Only the first 24 minutes of cycle number 100 is shown. Green arrow marks the end of +0.2 V voltage hold within the cycle; (b) Observed current densities at the end of each +0.2 V voltage hold and prior to impedance measurement at +0.2 V as a function of cycle number; (c&d) Nyquist plots of the symmetrical Mg/Mg cell as a function of cycle number under OCV and +0.2 V bias, respectively. A magnified figure is shown in the inset. The initial frequency was 200 kHz, while the last frequency was 1 Hz and 10 mHz for (c) and (d), respectively.

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Impedance spectra were also recorded during potentiostatic cycling, both under bias and at OCV, to probe the dynamic behavior at the Mg metal/electrolyte interface. Figures 2c and 2d show corresponding Nyquist plots at OCV and +0.2 V, respectively. Prior to cycling, the initial impedance at the interface between Mg metal and the MMC electrolyte was composed of a depressed semicircular capacitive loop (150 kΩ cm2, Figure 2c). Following the first +0.2 V polarization step, the impedance at that voltage drastically decreased to about 0.5 kΩ cm2 (Figure 2d), yielding a depressed semicircle at a higher frequency typical of a non-blocking interface with relatively small charge transfer resistance (Figures S26-S27, Table S1). After rest for 1 h, the impedance measured at OCV reverted to 185 kΩ cm2 (Figure 2c), higher than the impedance prior to cycling, yet maintaining its depressed semicircular shape. Hence, the interface between Mg metal and electrolyte is activated by an electrical field and deactivated during rest (also observed in Figure 1c and 1d). Thereafter, subsequent interfacial impedance spectra under a +0.2 V bias decreased until the 100th cycle (Figure 2d), coupled with an increase in measured current densities (Figure 2b), and remained relatively constant with a total resistance of about 0.12 kΩ cm2. Interfacial impedance spectra under OCV (Figure 2c) showed an analogous trend, with a curled spike appearing after 100 cycles, suggesting that time necessary for the interface relaxation was longer (Table S1). In this experiment, the surface roughness should be changing independently for the dissolution and deposition plane, so diffusion of chemical species would not be uniform for both. Such different surface conditions may eventually lead to the appearance of a curled spike in the low frequency region of the impedance spectra. At this point, we speculate that the diffusive component in impedance observed after OCV is similar in origin to the one observed in previous galvanostatic cycling experiments.


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Similar trends in the impedance values were noted when APC electrolyte was used in the same setup (Figure S28), but were not observed in the Li system (Figure S29), again suggesting such behavior is specific to the interface between Mg metal and an electrolyte compatible with Mg metal. A closer look at the impedance curves for the Li system reveals the source of the noise beyond 100th cycle noted in Figure S25. While impedance spectra recorded under bias constantly decreased throughout the experiment, those measured at OCV remained stable in the range 0.1-0.15 kΩ cm2 until ≈100th cycle and started to decrease thereafter, until only a noisy profile was observed at the 200th cycle. Such behavior is consistent with a temporary short circuit, likely a microshort, occurring inside the symmetrical Li/Li cell. To summarize, in the context of previous findings and considerations,13,14,19,49,50,57-60 the observed Mg anode/electrolyte switchable interfacial impedance in electrolytes compatible with Mg metal can be explained as follows: (1) electrochemically active species necessary for Mg deposition/dissolution are formed by electrochemical processes on Mg metal yielding a low interfacial impedance; (2) in the absence of an electrical field, electrochemically inactive species adsorb onto the Mg metal surface, inducing an increase in interfacial impedance; (3) newly formed interfaces between Mg metal and electrolyte improve on each subsequent cycle. For instance, recent studies of the interaction between chloride-containing electrolytes and Mg metal anode suggest that the switchable interfacial behavior emerges from the adsorption of Cl- anions at the Mg surface.57-60 Such “activation” of the Mg metal/electrolyte interface to achieve low impedance is slow and gradual, and believed to be currently masked by poor cathode performance (via C-rate) when a Mg battery is examined as full cell. Further, we found that the switchable interfacial behavior reported here is unique for the Mg metal anode and will become crucial for future investigations directed at material design and battery performance

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improvements. Additionally, the long-term stability of the Mg metal/electrolyte interface was also monitored and compared with that of Li metal/electrolyte by means of chronoamperometry. Mg metal as an anode showed superior tolerance as demonstrated by the absence of microshort formation at longer cycling times than a Li anode. Overall, it was recognized that the Mg metal/electrolyte interface displayed unique phenomena and had a significant impact on Mg metal electrode and battery performances. Thus, to enable high energy density and long-term durability of future Mg batteries, stabilization of the Mg metal/electrolyte interface is indispensable. Future investigations aimed at understanding the nature of the active species and potential interfaces, coupled with examining solvation/desolvation processes, would help in further understanding this phenomenon.

EXPERIMENTAL METHODS Except when noted otherwise, two-electrode cells were fabricated within a standard CR2032 coin cell made of stainless steel (SS316). Cells were assembled using a 435 µm thick sheet of glass microfiber filter (GF/A, Whatman) as a separator and 100 µL of electrolyte solution. Electrode materials were used as received, except for Mg metal (3N35, ESPI) and Li metal (Lectro® Max 100, FMC), which were polished prior to use unless stated otherwise. Tetraglyme (Sigma-Aldrich, 99%) was dried by elution through a column of neutral alumina (dried under vacuum for 15 h at 250 °C), followed by distillation from Na metal, and stirred in the presence of freshly prepared Mg shavings at 100 °C for 15 h. A variety of electrolytes were used in this study: (1) 0.62 M Mg(CB11H12)2 in anhydrous tetraglyme solution, which is abbreviated as MMC;17 (2) 0.2 M phenyl magnesium chloride (PhMgCl, Sigma-Aldrich, 99%) mixed with aluminum chloride (AlCl3, Sigma-Aldrich, 99.999%) in 2:1 ratio in tetrahydrofuran (THF,


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Sigma-Aldrich, anhydrous, inhibitor-free), which is abbreviated as APC;61 (3) 0.5 M Mg(TFSI)2 in 1,2-dimethoxyethane solution (DME, Sigma-Aldrich, anhydrous ), which is abbreviated as Mg(TFSI)2/DME; (4) 1.0 M Mg(TFSI)2 in acetonitrile solution (battery grade, Kishida Chemical), which is abbreviated as Mg(TFSI)2/ACN; (5) 0.1 mol/kg Mg(TFSI)2 (battery grade, Kishida Chemical) in N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (Kanto Corporation) solution, which is abbreviated as Mg(TFSI)2/PP13TFSI. For comparison, 1.0 M LiTFSI in propylene carbonate (battery grade, Kishida Chemical), which is abbreviated as LiTFSI/PC, was used for this study. All cells were assembled under Ar in a glove box (

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