Achieving High Cycling Rates via In-situ Generation of Active

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Achieving High Cycling Rates via In-situ Generation of Active Nanocomposite Metal Anodes Nikhilendra Singh, Timothy S. Arthur, Oscar Tutusaus, Jing Li, Kim Kisslinger, Huolin L. Xin, Eric A. Stach, Xudong Fan, and Rana Mohtadi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00794 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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ACS Applied Energy Materials

Achieving High Cycling Rates via In-situ Generation of Active Nanocomposite Metal Anodes Nikhilendra Singh1, Timothy S. Arthur1*, Oscar Tutusaus1, Jing Li2, Kim Kisslinger2, Huolin L. Xin2, Eric A. Stach2†, Xudong Fan3 and Rana Mohtadi1 1

Toyota Research Institute of North America, 1555 Woodridge Avenue, Ann Arbor, MI 48105

USA. 2

Center for Functional Nanomaterials, Brookhaven National Laboratory, Bldg. 735 – P.O. Box

5000, Upton, NY 11973 USA. 3

Center for Advanced Microscopy, 578 Wilson Road, Room B-6, CIPS Building, Michigan

State University, East Lansing, MI 48824 USA.

energy storage, magnesium, metal anodes, nano, interphase, operando, microscopy

The morphological control of electrochemically deposited metallic anodes, such as Li, Zn and Mg, under high applied rates is essential for the development of high energy-density batteries. For transportation applications, maximizing high rates and high energy-density is key to attaining viable customer acceleration and range expectations, respectively. In this work, the insitu generation of Mg nanocrystals allowed cycling under high rates (10 mA cm-2) and reduced temperature (0 oC) for the very first time. Through operando STEM analysis we discovered a

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highly functional SEI; a first of its kind, which enabled continuous deposition and dissolution of Mg without internal shorting. The unique morphology of the deposited Mg and the functional capability of the SEI are key to future development of practical metallic Mg anodes.

Energy storage systems will play a pivotal role in future energy production, grid storage and transportation demands. Since the commercialization of lithium ion (Li-ion) batteries in the 1990’s and the growing popularity of electric vehicles, incremental improvements to battery energy-density, capacity and cycle-life have generated confidence in a sustainable reduction of fossil fuel use.1−3 Recently, greater attention has been given to post Li-ion energy storage systems; e.g., solid-state, lithium-sulfur (Li-S), lithium-oxygen (Li-O2), and alternative ion systems (Na+, Ca2+, Zn2+, Al3+, etc.…).4−6 A particularly promising avenue is batteries based on magnesium (Mg) metal. Mg has a high negative reduction potential (-2.356 V versus NHE), lower cost based on its relative abundance in comparison to Li, potential safety advantages due to the absence of dendrites, and a significant advantage in volumetric energy density compared to the current graphite anode for Li-ion batteries, 3832 mA·h cm-3 vs. 777 mA·h cm-3, respectively.7−14 To date the highest performing Mg battery system remains the one demonstrated by Aurbach et al. in 2000, which involved the coupling of a Chevrel phase cathode (MgxMo6S8) and a Mg anode in an organohaloaluminate electrolyte.7 Since 2000, research has elevated the voltage and capacity of Mg battery cathodes to attain higher energy via investigation of non-corrosive, simple-salt electrolytes with wider electrochemical windows.15−26 The recent discovery of Mg monocarborane (MMC) as an outstanding electrolyte salt for Mg batteries was catalyzed by the emergence of Mg borohydride as a viable alternative to


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halogenated Mg organohaloaluminates.27,28 Magnesium electrolytes containing corrosive halide ions, such as Cl-, have been shown to have detrimental effects on the oxidative stability of the electrolyte in contact with metallic cell components.8 Learning from the chemical stability of closo-borane clusters in carboranyl Mg halide electrolytes, Mohtadi et al. discovered that MMC dissolved in tetraglyme (G4), and reversibly deposited and dissolved Mg metal while providing a high oxidative stability (3.8 V vs Mg).29 This breakthrough result was highlighted as the first simple-salt type Mg electrolyte suited for practical Mg batteries. Hence, investigating the basis of MMC/G4’s electroactivity, when all other simple salts [Mg(TFSI)2, Mg(PF6)2 and Mg(ClO4)2] have struggled, was critical to further improving and understanding the performance of Mg batteries. Additionally, the development of metallic anodes other than Mg, such as Zn and Li, has been severely hindered by the deposition and growth of dendrites on the anode surface, which create an internal short with the cathode. At high rates of charge (metal deposition), prolonged cycling and low operating temperatures, these observations are exacerbated, and thus must be considered as a challenge to device development. Compelling research exploiting various analytical tools which are expanding our knowledge of the solid electrolyte interphase (SEI) have concluded that Li dendrite formation is a product of the interphases formed on the surface of Li metal through a variety of proposed models. 30−41 Further, coupled with its non-reactivity towards moisture and non-corrosive nature, MMC/G4’s capability to maintain a non-dendritic morphology calls for a detailed analysis of the Mg electrode-MMC/G4 electrolyte interphase and serves as our motivation for this study utilizing advanced analytical techniques correlated with electrochemical data. Scanning transmission electron microscopy (STEM) has been a popular and important tool for investigating materials at the nanoscale, but has historically been limited in its capability to


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simulate real device function. However, with the advent of operando STEM, linking electrochemical results to morphological events is expected to expand our understanding of the true chemistry of such devices. 42−50 In this work, such investigations reveal a unique ability of the electrolyte to allow Mg metal cycling under unprecedented high current densities, even under extreme conditions. Results Electrochemical Identification of Interphase Formation from MMC/G4 The paramount prerequisite for a Mg electrolyte is the capability to deposit and dissolve Mg metal, ideally in a two-electrode symmetrical cell configuration mimicking practical battery rates and capacities. Figure 1a presents the first cycle potentiometric response to the galvanostatic cycling of Mg metal foil anodes at 0.1 mA cm-2, 0.5 mA cm-2, and 1.0 mA cm-2 current densities, limited to 0.5 mA·h of passed charge (for charge or discharge cycle), for 100 cycles. If the entirety of the charge passed is utilized to form Mg metal, we expect the deposition and dissolution of 737 nm of Mg. Li metal, in comparison, theoretically requires the movement of 1.372 µm of Li to achieve the equivalent capacity, due to the significant difference in density of Mg vs. Li (1.738 g cm-3 vs 0.534 g cm-3, respectively) and the 2e-/equivalent reduction of the Mg2+ ion. During the initial reduction, an initial voltage spike was observed and has previously been attributed to the activation of the Mg metal surface.9 The deposition and dissolution of Mg metal continued unperturbed for 100 cycles (Figure 1b), and interestingly, there was a ~50 % reduction in the overpotential after the first 10 cycles (Figure 1c). The reduced energy required for deposition and dissolution of Mg metal persisted throughout the remaining cycles. Thus, we


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define cycles 1-10 as the formation of the interphase on Mg metal while cycles 11-100 represent the reversible deposition/dissolution of Mg metal. The observed decrease in overpotential from the initial interphase formation to stable cycling is remarkable and requires further investigation. One explanation is the electrochemical formation of a more Mg-ion conductive solution, or alternatively, the formation of a more conductive interphase. Electrochemical impedance spectroscopy (EIS) can provide insights to the anode interface before, during and after galvanostatic cycling. As shown in the Nyquist plots in Figure 1d, the interfacial resistance (defined as the low-frequency intercept on the Z’-axis) was drastically lowered after the first cycle and continued to decrease as the anode was electrochemically cycled. The continued decrease in interfacial impedance illustrated the importance of the interphase in achieving low overpotentials for Mg deposition and dissolution. Additionally, the conductivity of the electrolyte solution was tested before and after electrochemical cycling, and was found to remain unchanged from the reported,27 initial value of 1.8 mS cm-1. However, while these results strongly implied the formation of a SEI layer on the surface of the Mg anode, distinguishing an electrode formed through SEI formation from one formed from surface roughening during Mg deposition must be confirmed through additional analysis of the Mg surface. To this end, further chemical and morphological analyses were obtained.


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Figure 1. a, 1st cycle potentiometric response of Mg/Mg symmetrical cells using MMC/G4 as an electrolyte at 0.1, 0.5 and 1 mA cm-2. b, 100th cycle potentiometric response of the same cells as in (a). c, A summary of the decrease in cycling overpotential noted from the 1st, 10th and 100th cycles for the same cells as in (a) and (b). d, Nyquist response representation to EIS measurements taken after the 1st and 100th cycle observed in (a) and (b).


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Ex-situ Analyses of MMC/G4 Interphase Composition and Deposit Morphologies Presented in Figure 2, XPS analysis was carried out to further study the implied formation of a SEI layer. Since distinguishing between metallic Mg formed via electrochemical reduction and the underlying substrate is challenging due to the kinetic energy similarities of the core Mg 2p and Mg 2s ejected electrons, secondary Auger electron ejection was utilized as a more reliable indication of surface changes. Here, the Mg KLL shells provide greater energy resolution between the Mg2+ and Mg0 oxidation states. Upon comparison of the Mg KLL peak of a mechanically polished bare Mg foil, to the surface of Mg metal after galvanostatic reduction at 0.1 mA cm-2 for 0.5 mA·h (Figure 2a), the formation of a functioning SEI was firmly demonstrated. First, a Mg2+ peak at 307.1 eV was found on both anode surfaces and attributed to the presence of Magnesium Oxide (MgO), indicating that mechanical polishing alone cannot remove the entire oxide layer from the surface of Mg metal. Additionally, a strong peak at 301.9 eV arising from the Mg0 substrate was present for both the bare Mg foil, and after the 1st deposition. We also observed an additional maximum at 304.7 eV and a shoulder at 299.5 eV attributed to the deposition of a Mg2+ and Mg0 species on the surface of the substrate, respectively. Considering that the minimum thickness of a pure Mg metal layer deposited on the surface at 0.5 mA·h (737 nm vide supra) is much greater than the ~10 nm probing depth of XPS, this result indicates a non-uniform deposition of Mg on the surface. Indeed, a SEM image of the surface after the 1st deposition (Figure 2b) showed the presence of large, hemispherical deposits scattered throughout the surface of the Mg foil (elemental mapping via EDX shown in Figure S1). In Figure 2c, further XPS analysis revealed that a large portion of the surface was covered by carbon and boron species, likely belonging to the SEI component of the deposited layer. Curve-fitting analysis of the high-resolution Mg 2p peak (Figure S2) confirm the presence of a


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Mg species attributed to the magnesium present in the SEI (MgSEI). The underlying substrate, fingerprinted by Mg0 and Mg2+ peaks, is present in all spectra due to the hemispherical deposition. Additionally, the MgSEI component increases intensely with cycle number up to 10 cycles, and may be defined as a surface species due to the loss of intensity after 10 minutes of Ar+ sputtering. The persistent presence of the C 1s and B 1s signals after 10 minutes of sputtering negated any concern that residual salt components, adsorbed to the surface of the Mg foil after rigorous washing, were responsible for the signal.

Figure 2. a, XPS HRES of the Mg KLL Auger peak of a polished Mg foil sample, and the Mg KLL Auger peak after the 1st deposition (Dep) at 0.1 mA cm-2 at 0.5 mA·h. b, SEM image representing the morphology of deposited Mg after the 1st deposition in (a). c, XPS sputtering profiles comparing the Mg 2p, C 1s and B 1s content of the 1st deposition in (a).

Upon continued galvanostatic cycling in MMC/G4, a black deposit was found bound to the glass fiber separator used in the symmetrical cells (Figure 3a – inset). TEM images of this black deposit suggest that the particles were a nanocomposite of ~10 nm Mg nanoparticles enveloped in an amorphous layer (Figure 3a). Selected area electron diffraction (SAED) was used to


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confirm that the particles were Mg metal, as evidenced by the 100, 110 and 200 reflections (Figure 3b). These results clearly support previous evidence for SEI formation with nanoparticle growth to form a porous, functioning electrode surface. Previously, the formation of a layer on the anode surface from electrolytes, such as Mg(TFSI)2, Mg(ClO4)2 and Mg(PF6)2 in Carbonate electrolytes, blocked the deposition and dissolution of Mg metal. However, traditional Mg organohaloaluminate electrolytes, such as APC (2:1 phenylmagnesium chloride:aluminum trichloride in tetrahydrofuran), have been shown to lack appreciable surface films which influence the deposition and dissolution of Mg (i.e. SEI-free Mg deposition).8 To investigate the difference in surface morphology and coulombic efficiency of the deposition and stripping process between MMC/G4 and APC electrolyte deposits, Mg/Cu half cells (Figure S3) were cycled at 0.1 mA cm-2 (to 0.5 mA·h). As reported by Aurbach et al, the coulombic efficiency for magnesium deposition and stripping in the APC electrolyte is 100 %, while the MMC/G4 electrolyte is increases from 68 % to 100 % within the first four cycles. The increase in coulombic efficiency with cycling is the formation of the interphase layer which enables efficient Mg-metal growth and dissolution. The resulting Mg deposits were compared via TEM. A copper TEM grid (without a formvar coating layer) was used on the surface of the Cu electrode to directly capture the interphases formed from these two electrolytes. After 9.5 cycles (ending after electrochemical reduction), the images illustrated vastly different morphologies for Mg deposited from both electrolytes. In MMC/G4 (Figure 3c), the electrochemical deposit was a composite of Mg nanoparticles enveloped in an amorphous layer, like the black particulate deposit captured in the separator. For APC (Figure 3e), the deposition showed a compact crystalline layer on the surface of the copper TEM grid, consistent with an SEI-free metal deposition. SAED (insets in Figures 3c and 3e) of the two deposits revealed that both layers were composed of metallic Mg0.


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After electrochemical oxidation (upon completion of the 10th cycle), the interphases remaining on the copper TEM grid were vastly different between MMC/G4 and APC (Figure 3d and 3f, respectively). There was clear SEI formation on the surface of the copper TEM grid after cycling in MMC/G4, while the copper TEM grid cycled in APC showed no presence of an SEI layer. Additional electrochemical evidence for the formation of SEI from MMC/G4 can be found via evaluation of the cycling coulombic efficiencies in the presented half cells (Figure S3c). Hence, interphase chemistry plays a large role in the capability of Mg electrolytes to deposit and dissolve Mg metal, as well as the resulting morphology of the deposited metal.

Figure 3. a, TEM image of a separator (inset) sample extracted from a Mg/Mg symmetrical cell after 100 cycles at 0.1 mA cm-2 (at 0.5 mA·h) in MMC/G4. b, The corresponding SAED pattern for the material circled in (a). c, TEM image of a Cu grid sample extracted after 9.5 cycles (after


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deposition of Mg) at 0.1 mA cm-2 (at 0.5 mA·h) in MMC/G4. d, TEM image of a Cu grid sample extracted after 10 cycles (after dissolution of Mg) at 0.1 mA cm-2 (at 0.5 mA·h) in MMC/G4. e, Repetition of experiment (c) except with APC electrolyte. f, Repetition of experiment (d) except with APC electrolyte. Please note that the light contrast fringe in (f) is due to a slightly under focused TEM image, and not a SEI layer.

During our analysis efforts, especially after the 1st deposition of Mg, we found that it was easy to mechanically remove surface precipitates/deposits during coin-cell disassembly. This observation inspired our exploration of additional alternative analysis techniques to further probe the deposition of Mg from MMC/G4. As noted above, the drastic difference in morphology of the electrochemically deposited Mg metal from different electrolytes, and recent results showing the effect of the separator on the morphology of metal deposits,51 led to our development and use of “separator-free” coin-cells. A schematic of the cell and galvanostatic cycling can be seen in Figure S4. As shown in Figure 4a, the hemispherical deposits at 0.1 mA cm-2 (0.5 mA·h) were not generated through the restricted diffusion paths in the glass-fiber separator. Further, by using separator-free coin cells, sections of the deposited layer could now be selectively removed through a focused-ion beam (FIB) process, without interference from the glass-fiber separator fibrils, for further analysis. It must be noted that the optimum comparative morphologies for deposited Mg with and without a separator were observed at 0.1 mA cm-2 for the 1st deposition (as highlighted in Figure S5), and thus these conditions were selected for this FIB-based interface study. Spot 1 and Spot 2 were selected as points of interest to analyze the bulk and interface of the deposition, respectively. Figure 4b summarizes the energy dispersive X-ray spectra (EDX) of the FIB sections, showing the presence of both boron and carbon phases within


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the interface region of deposition (spot 2), but only Mg within the bulk of the deposit (spot 1). The thin section recovered by FIB at spot 2 revealed two sections illustrating differences in morphology. The area closest to the Mg foil substrate (green outline) showed a random, polycrystalline morphology while the region extending (transitioning upward from spot 2) into the bulk of the Mg deposit contained a textured, columnar grain structure. This columnar structure extends into the bulk of the deposit, where SAED showed the presence of only Mg metal. Additionally, the presence of a second crystalline phase, indexed as a best match to MgB2O5 (Figure S6) was convincing evidence that boron and carbon are significant contributors to the formation of the SEI at the interface. However, since the B containing phase is an oxide material, one must note the factors required in the processing of the FIB sample; exposure to air upon introduction of the sample into the scanning electron microscope (SEM), and the highenergy Ga+ ion beam used. Hence, MgB2O5 may not be the actual phase present in the original sample (pre-sample processing). Similar results were also collected for Mg deposited at higher rates (Figure S7).


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Figure 4. a, SEM image highlighting the representative areas of Spot 1 and Spot 2, from where FIB samples were processed for EDX and TEM analysis. b, EDX spectra for the FIB samples obtained from Spot 1 and Spot 2 in (a). c, TEM image of the FIB sample obtained from Spot 2 in (a), displaying a variation in sample morphology/texture through the interface of deposition from the Mg foil substrate, extending into the newly deposited Mg from MMC/G4.


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In-situ Analysis of MMC/G4 Interphase Composition and Deposit Morphologies To eliminate such variables (ex-situ sample preparation and microscopy) operando STEM was utilized to observe Mg deposition from MMC/G4 and support the existing ex-situ data discussed above. Figure 5a is the potentiometric response to a 1.0 mA cm-2 galvanostatic deposition for 30 min (0.5 mA·h) as recorded from the operando STEM liquid electrochemistry holder (Figure 5a inset). Figure 5b provides relevant information for Figures 5c-i. Figure 5c is a snapshot of the edge of the glassy carbon (GC) current collector at 0 minutes (or prior to the experiment starting). Figures 5d-f are snapshots taken at 6 minutes (0.1 mA·h) into the deposition of Mg, while Figures 5g-i are snapshots taken at 30 minutes (0.5 mA·h) into the deposition of Mg. Upon close inspection of the interface between GC and the MMC/G4 electrolyte (Figures 5e-f), we observe the lack of a deposit (observed as dark contrast) at the GC edge. However, we also observe the presence of a dispersed/particulate deposit at a slight distance away from the GC edge (observed as light contrast particles). This suggests the formation of a thin SEI film (dark contrast) directly on the surface of GC, prior to the deposition of Mg (light contrast). A schematic representation of Figure 5e is provided in Figure S8 for additional clarification. With additional deposition time the dispersed/particulate deposit was observed to spread out from the GC electrode (Figures 5g-i). From the XPS analysis, the SEI components are likely formed from decomposition of the MMC/G4 electrolyte, thus concluding on a clear contrast between the electrolyte and SEI was unachievable. However, the growth of the nanocomposite morphology up to 0.5 mA·h gives us key insights into interphase formation and growth of Mg metal from MMC/G4, and supports previous data recorded on the presence of a nanocomposite material in Figure 3.


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Figure 5. a, Potentiometric response captured from the operando STEM liquid electrochemistry holder upon the application of 1 mA cm-2 (at 0.5 mA·h) with schematic representation of the holder (inset). b, Relevant information regarding data collected during the experiment. c, Operando TEM image taken at 0 minutes (0.0 mA·h) Mg deposition time in (a). d-f, Operando


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TEM images taken at 6 minutes (0.1 mA·h) Mg deposition time in (a). g-i, Operando TEM image taken at 30 minutes (0.5 mA·h) Mg deposition time in (a).

Combinatorial Extreme Condition Testing of the MMC/G4 Electrolyte From a fundamental scientific perspective, the operation and understanding of Mg batteries at standard electrochemical conditions (e.g. 1 mA cm-2 and 25 °C) via electrochemical and analytical means is a necessity. However, electrochemical storage systems in automobiles must not only muster the capability to withstand standard electrochemical testing conditions, but also extreme electrochemical testing conditions; inclusive of variable physical conditions, such as temperature, pressure and physical shock. The observation of a nanocomposite morphology via operando STEM lead us to explore the potential applications for Mg batteries in the transportation sector, where high energy density may be utilized to provide an extended range for electric vehicles. To explore the capability of MMC/G4 to operate under extreme conditions, galvanostatic cycling of symmetrical Mg-Mg cells was performed at 10 mA cm-2 (25 oC), 0 oC (1 mA cm-2), as well as the combination of 0 oC and 10 mA cm-2. A comparison of the 1st and 100th cycle from each condition is shown in Figures 6a and 6b. The reduction at 10 mA cm-2 revealed that initial reduction of MMC/G4 on the anode surface required a large over-potential, as the ±10 V instrument limit was reached by 0.3 mA·h, however cycling proceeded smoothly after the initial cycle. Although the overpotential was greater (as expected from the Nernst equation), cycling at 0 oC showed a similar reduction in overpotential by the 100th cycle. The capability of MMC/G4 was found to further diminish if the two extreme conditions were combined, as the initial reduction only reached 0.09 mA·h, and decreased with every cycle to < 0.01 mA·h by the 100th cycle. SEM images were taken after each initial reduction as shown in Figures 6c (10 mA


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cm-2, 25 oC), 6d (1 mA cm-2, 0 oC), and 6e (10 mA cm-2, 0 oC). Upon comparison of the electrochemical and SEM data, we hypothesize that the initial high-overpotential observed at 10 mA cm-2 was the deposition of fresh Mg metal platelets onto the Mg foil surface, while the surface of Mg metal at 0 oC appeared as a composite of deposited metallic Mg encased in an amorphous SEI. Surprisingly, at 0 oC and 10 mA cm-2, the surface of Mg foil showed scattered regions of Mg platelets protruding from the surface of the substrate, possibly due to the higher current density dominating the kinetic limitations imposed by the low temperature. From these images, we hypothesize that the sequence of electrochemical/chemical reactions which govern the deposition of the functioning SEI onto the surface of Mg foil, is suppressed under the combined extreme conditions.


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Figure 6. a, 1st cycle potentiometric response comparisons at various operating conditions (10 mA cm-2, 25 °C – black circles, 10 mA cm-2, 0 °C – blue circles, 1 mA cm-2, 0 °C – red circles). b, 100th cycles potentiometric response comparisons at the same operating conditions as in (a). c, SEM image of the deposition observed at the operating condition of 10 mA cm-2, 25 °C. d, SEM image of the deposition observed at the operating condition of 1 mA cm-2, 0 °C. e, SEM image of the deposition observed at the operating condition of 10 mA cm-2, 0 °C.

Discussion The mechanism for Mg deposition from MMC/G4 is crucial to the design of functioning interphases for Mg batteries. From the analysis of the electrochemical and analytical information, the deposition follows an intricate path beginning at the manipulation of the Mg foil substrate. Scheme 1 shows the hypothesized steps for Mg deposition from MMC/G4 on the surface of Mg metal: i.

Removal of MgO: Mg foil obtained through commercial sources have a native MgO layer on the surface which is electrically and ionically blocking. XPS analysis of the surface has shown that mechanical polishing of Mg foil does not completely remove the entire MgO layer, but likely serves to non-uniformly thin the MgO film.

ii.

Mg2+ reduction from MMC/G4: MMC/G4 electrochemical reduction to form Mg metal is the first key step. The energy required to reduce MMC/G4 through the thin-MgO layer is the cause of the high-overpotential (or the electrochemical spike) observed during the 1st reduction on Mg metal foil, which can reach reductive potentials as high as ‒ 4.5 V (at 1 mA cm-2, 25 °C). All symmetrical cells in MMC/G4 show a spike in voltage (at applied


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current densities < 10 mA cm-2) or a high-overpotential plateau (at applied current density = 10 mA cm-2). iii.

Fresh Mg0 nucleation: The charge transfer through the thin MgO layer is labeled as equation (iii). The product of (iii) is a fresh, non-oxidized Mg surface, which is an essential step to the continued cycling and reduction of MMC/G4.

For MMC/G4

reduction on non-Mg surfaces, such as GC in the operando STEM experiments, oxide layers are often very thin or easily removed through mechanical and chemical techniques which are difficult to perform on an active alkali-earth metal surface. Therefore, these steps are unique to Mg metal foils. iv.

SEI formation: A fresh Mg0 surface lowers the energy required for interphase formation on Mg metal. Persson et al. show that the stability of solvents may be lowered through Mg2+ contact-ion pair formation on the surface of Mg foil.52 Although a thorough further investigation is required, SEI formation initiated through solvent decomposition (iv), at the fresh Mg0 surface is the next key step. From XPS and TEM analysis, we propose that the SEI is amorphous and composed of carbon and boron species. Interestingly, SEI formation does not occur if the temperature is 0 oC and the current density is 10 mA cm-2, as Mg metal deposition and dissolution is diminished.

v.

Mg nanoparticle (MgNP) nucleation: The formation of a Mg-ion conductive SEI is a necessary precursor to form in-situ nanocrystalline Mg in a composite morphology, as observed in the TEM results. As proposed in (v), the presence of the SEI is pivotal to form the active nanoparticles. The nanocrystalline form of Mg in the composite


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morphology yields a high surface area anode and is extremely active for continued Mg0 reduction. vi.

Mg growth: Finally, in (vi), the nucleated nanocrystalline Mg has the capability of forming large spherical deposits of Mg metal at reduction overpotentials. FIB samples taken from the interface of deposition clearly show a transition from a composite morphology near the substrate containing SEI components to a textured deposition of only Mg metal towards the bulk of the electrolyte. Therefore, the SEI growth from MMC/G4 is kinetically hindered, and the non-dendritic growth of Mg metal, observed by previous researchers, is the preferred electrochemical reaction.


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Scheme 1. A mechanistic breakdown of the processes involved in the deposition of Mg from MMC/G4. (i) Removal of MgO, (ii) Mg2+ reduction from MMC/G4, (iii) fresh Mg0 nucleation, (iv) SEI formation, (v) Mg nanoparticle nucleation, and (vi) Mg growth.


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Conclusions Through in-depth interphase analysis, MMC/G4 has been shown to deposit and dissolve Mg metal through a mechanism where a SEI and nanocrystalline Mg metal are critical to electrochemical cycling at high-rates and low temperatures. Learning from Li metal research, where SEI formation is linked to dendritic growth and eventual shorting of the battery, further continued analyses of Mg deposition and dissolution are required to ensure non-dendritic growth. Additionally, uniform removal of the MgO film from the Mg metal surface is a fruitful avenue of research for the Mg anode. Finally, harnessing the low energy reduction and high-rate capability of the Mg nanocomposite morphology through known Mg nanoparticle chemical synthesis can minimize any detrimental effects of solvent decomposition in MMC/G4. Further development of methods for the controlled formation of nanocomposite morphologies and SEI for Mg metal, as opposed to in-situ formation of the same via electrolyte decomposition, should allow the community to delve deeper into interface/interphase properties. As the search for high-voltage, high-energy cathodes endure for Mg batteries, promoting and improving the performance of the Mg anode continues to be a worthwhile and important endeavor.

Methods 1. MMC/G4 Synthesis: Electrolyte synthesis and preparation was conducted in an argon filled glovebox. (HNEt3)(CB11H12) was purchased from Katchem (Czech Republic) and used as received. Mg powder (99.5 %, 325 mesh), anhydrous 1,2-dimethoxyethane (DME) and tetraglyme were


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purchased from Sigma Aldrich. DME was distilled from Na metal and stored over activated 3 Å molecular sieves. Tetraglyme was distilled from Na metal, and stirred in the presence of freshly prepared Mg shavings at 100 °C for 15 h, where it was stored until use. [Mg(DME)3](CB11H12)2 has been reported previously,27,53 but was prepared per a modified procedure described below: Synthesis of [Mg(DME)3](CB11H12)2: (HNEt3)[CB11H12] (5 g, 20.5 mmol) and Mg powder (1.32 g, 54.3 mmol) were weighed in a 125 ml flat bottom flask and anhydrous DME (60 ml) was added. The dark grey suspension was vigorously stirred until H2 evolution ceased and suspension turned light gray, typically taking about 3 h at room temperature when stirring is vigorous enough. The suspension was cooled down externally until original dark grey color was recovered. Unreacted Mg powder was filtered off using a fine frit and the filtrate was immediately heated up to 60 °C for a minimum of 1 h to induce crystallization. The solid was collected by filtration, washed with warm (50 °C) anhydrous DME (10 ml + 2 ml) and dried under vacuum. This resulted in 5.66 g (95 % yield) of white microcrystalline powder. NMR characterization was found to match the one reported previously.27 General procedure for the preparation of 0.45 M MMC/G4 electrolyte solution: [Mg(DME)3](CB11H12)2 (927 mg, 1.6 mmol) was added G4 (3.192 µl) and the mixture was stirred until dissolution. The clear solution was further stirred under vacuum for 30 minutes to remove DME solvent. 2. Preparation of standard coin-cells: Mg/Mg symmetrical cells were constructed using standard CR-2032 coin-cells. Each coin-cell utilized 15 mm diameter Mg foil pieces, which were punched from a 100 µm thick Mg foil stock supply purchased from ESPI Metals (3N purity). Each punched Mg foil piece was polished to


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remove surface oxides using common sandpaper, prior to use. A glass fiber separator was utilized to separate the two Mg foil pieces, and soaked in 100 µl of electrolyte for each test. Coin-cells were sealed using a standard Hoshen coin-cell crimper, and dismantled for analysis using a standard Hoshen manual coin-cell de-crimper. 3. Preparation of separator-free coin-cells: Mg/Mg separator-free coin cells were constructed using the standard CR-2032 apparatus. However, the glass fiber separator was replaced with a Teflon/PEEK spacer (0.281” ID, 0.5” OD). The electrolyte used for each test was placed within the spacer well. Further, the area of Mg foil pieces which encountered the electrolyte was controlled by covering the polished 15 mm Mg foil pieces with pre-punched (5 mm diameter punch) Teflon/PEEK tape. The final assembly process for the separator-free coin cells can be found in Figure S4. All modification materials for this setup were purchased from McMaster-Carr and used as received. 4. Electrochemical Evaluation (cycling and EIS) of coin-cells: All electrochemical evaluations were carried out on a Bio-Logic VMP3 Potentiostat with EIS measurement capabilities. EIS measurements were carried out at 0 V by scanning from 100 kHz to 100 mHz for all samples. Temperature variations for electrochemical studies were carried out using various ESPEC temperature and humidity chambers. Studies with references to “room temperature” denote 25 °C. 5. Ex-situ SEM Imaging: Field emission scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were utilized to characterize the morphology, homogeneity and composition of the cycled


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Mg/Mg symmetrical cells. Mg samples extracted from disassembled coin-cells (using a standard Hoshen coin-cell de-crimper) within an Ar-filled glove box, were first washed thoroughly in a 1:1 (volume) solution of THF:DME and allowed to dry. Once dry, representative samples were cut and mounted on a SEM stub contained within an air-free sample transfer vessel. A JEOL 7800FLV SEM outfitted with an Oxford EDX system, was operated at 5-20 kV, for all samples. 6. Ex-situ XPS Measurements: X-ray photoelectron spectroscopy (XPS) was collected using a Phi 5600 and analyzed using the Multipack software. Spectra were aligned to the C1s signal at 284.7 eV. After electrochemical measurements, the separator was removed and the magnesium foil was washed thoroughly in a 1:1 (volume) solution of THF:DME and allowed to dry. All processes were performed in an Arfilled glove box. Dried samples were loaded into a XPS chamber in a sealed transfer vessel. Finally, the Mg foil was analyzed after Ar+ sputtering for the reported times intervals. 7. Ex-situ Cu Grid TEM: This transmission electron microscopy (TEM) work was done using a JEM-2200FS microscope with an in-column energy filter operated at 200 kV. The Cu grid samples was extracted from the coin-cell as described in (2), washed thoroughly in a 1:1 (volume) solution of THF:DME and allowed to dry prior to microscopy. In experiments where the separator components were observed via TEM, the separator was washed thoroughly in a 1:1 (volume) solution of THF:DME allowing materials of interest to form a suspension in the THF:DME solution. This suspension was then drop-cast on to standard Cu grids and studied in the TEM.


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8. FIB-SEM/EDX: The cross-sectional TEM samples were prepared via the in-situ lift-out technique in a focused ion beam (FIB) system (FEI Nanolab 600 Helios) with final milling performed at 2 keV. 9. Ex-situ TEM for FIB Samples: The structure, morphology and chemical analysis of the FIB samples were done on JEOL 2100F TEM with a field-emission electron gun, operating at 200 kV and equipped with EDX capability. 10. In-situ TEM: All in-situ TEM experiments were carried out on a FEI Talos F200X scanning transmission electron microscope (STEM), using a Type-II Liquid Electrochemistry Holder designed by Hummingbird Scientific. This in-situ holder utilized glassy carbon coated platinum electrochemistry chips, standard spacer chips and chemically resistant O-rings which were all procured from Hummingbird Scientific. Mg was deposited from the MMC/G4 electrolyte at 1 mA cm-2 at room temperature while pumping the electrolyte through the holder at 2 µl/min. Electrochemical measurements on this holder were carried out by using a Bio-Logic SP-200 Potentiostat.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Definitions and eight supplementary figures (PDF).


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AUTHOR INFORMATION Corresponding Author * Email: [email protected] Present Addresses † Department of Materials Science and Engineering, University of Pennsylvania, Laboratory for Research on the Structure of Matter, Singh Center for Nanotechnology, 3231 Walnut Street, Philadelphia, PA 19104 USA. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conceptualization, N.S. and T.S.A.; Methodology, N.S. and T.S.A.; Validation, N.S. and T.S.A.; Formal Analysis, N.S., J.L., K.K., X.F., H.L.X., E.A.S. and T.S.A.; Investigation, N.S. and T.S.A.; Resources, O.T. and R.M.; Data Curation, N.S. and T.S.A.; Writing – Original Draft, N.S. and T.S.A.; Writing – Review & Editing, N.S., T.S.A., O.T., H.L.X., E.A.S., and R.M.; Visualization, N.S. and T.S.A.; Supervision, E.A.S., N.S. and T.S.A.; Project Administration, N.S. and T.S.A.; Funding Acquisition, R.M. Notes The authors declare no competing interests. Funding Sources This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704.


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ACKNOWLEDGMENT The authors would like to thank Sydney Sharpe, Julio A. Rodriguez Manzo, Norman J. Salmon and Daan Hein Alsem from Hummingbird Scientific, for their guidance and support during the operation of operando STEM experiments utilizing the liquid electrochemistry STEM holder.

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Achieving High Cycling Rates via In-situ Generation of Active

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