Stabilization of Lithium Metal Anodes by Hybrid Artificial Solid

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Stabilization of Lithium Metal Anodes by Hybrid Artificial Solid Electrolyte Interphase Alexander C. Kozen,⊥,†,‡,∥ Chuan-Fu Lin,⊥,†,‡ Oliver Zhao,† Sang Bok Lee,§ Gary W. Rubloff,†,‡ and Malachi Noked*,†,‡ †

Department of Materials Science & Engineering, ‡Institute for Systems Research, and §Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Li metal is among the most attractive anode materials for secondary batteries, with a theoretical specific capacity > 3800 mAh g−1. However, its extremely low electrochemical potential is associated with high chemical reactivity that results in undesirable reduction of electrolyte species on the lithium surface, leading to spontaneous formation of a solid electrolyte interphase (SEI) with uncontrolled composition, morphology, and physicochemical properties. Here, we demonstrate a new approach to stabilize Li metal anodes using a hybrid organic/inorganic artificial solid electrolyte interphase (ASEI) deposited directly on the Li metal surface by self-healing electrochemical polymerization (EP) and atomic layer deposition (ALD). This hybrid protection layer is thin, flexible, ionically conductive, and electrically insulating. We show that Li metal protected by the hybrid protection layer gives rise to very stable cycling performance for over 300 cycles at current density 1 mA/cm2 and over 110 cycles at current density 2 mA/cm2, well above the threshold for dendrite growth at unprotected Li. Our strategy for protecting Li metal anodes by hybrid organic/inorganic ASEI represents a new approach to mitigating or eliminating dendrite formation at reactive metal anodes illustrated here for Liand may expedite the realization of a “beyond-Li-ion” battery technology employing Li metal anodes (e.g., Li−S).

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INTRODUCTION The ever-increasing demand for next generation batteries challenges the scientific community to improve both the energy density and operational lifetime of battery electrode materials. Lithium metal is a promising anode material, due to its high capacity and low electrochemical potential. However, the nature of Li electrodeposition during charging can lead to Li dendrite growth upon repeated lithium deposition and dissolution. This may result in shorting of the cells, producing potentially dangerous battery failures. Despite these concerns, the potential benefits of Li metal anodes provide a strong incentive to establish new approaches to address the lithium anode shortcomings via integration of ASEI protection layers onto Li metal anodes. Solid Electrolyte Interphase on Li Metal Anodes. The nature of the solid electrolyte interphase (SEI) has been thoroughly investigated for rechargeable Li-ion batteries, where SEI formation on graphite anodes has been the focus of studies by many leading groups in the fields of electrochemistry and surface science.1−7 From this body of research, it is clear that SEI composition and structure are both key factors in determining battery performance and degradation. In commercial Li-ion batteries, SEI formed on the graphite anode stabilizes the electrode/electrolyte interface by preventing continual © 2017 American Chemical Society

deposition of reduced electrolyte species on the anode at low potentials while still allowing the diffusion of Li+ ions at reasonable rates. The formation of SEI on graphite electrodes is usually done in situ and galvanostatically during the last stages of battery manufacturing,1,4 with the more sensitive species reacting first at a higher potential via selective electrochemical processes. Additionally, the change in the volume of the graphite anodes is small upon cycling (1 mA/cm2 threshold for significant “mossy dendrite” formation36) for over 100 cycles, whereas neither an elastomeric layer nor a thin ceramic layer alone provides comparable Li surface protection, due to the drawbacks inherent with each individual component. These results present a new direction premiering the use of hybrid materials with tunable properties to enable stabilization of metallic anodes in rechargeable batteries.

induces low Coulombic efficiency, and causes irreversible capacity losses. This reduction process occurs preferentially at localized high electrochemical field regions on the anode surface through asperities or cracks in the SEI, exacerbated by the mechanical instability induced by large volume changes induced by plating/ stripping of Li metal during charge/discharge of the battery. These volumetric changes deform the natural SEI which grows on the Li surface by electrolyte decomposition, causing microscopic cracks in the SEI, which allows further electrolyte decomposition. During long-term cycling, this nonuniform Li deposition and dissolution will further increase the anode surface area, facilitating dendritic growth of Li and accelerating cell failure.2,4,8−12 Stabilization of Li Metal Anodes. Many efforts have been made to sufficiently protect the Li metal anode including the application of electrolyte chemistries,2,4 electrolyte additives,13 artificial interlayers between the electrolyte and the Li metal,14−19 and tuning the surface energy.20 Despite some progress, Li anode batteries still cannot compete with the cycle life of systems containing graphite anodes.21 Organic Protection Layers. One of the electrolyte solvents discovered to be more suitable for use with Li metal anodes is 1,3-dioxolane (DOL).4 As Li metal is cycled in DOL, the DOL is reduced and polymerized on the Li surface, forming a flexible elastomeric layer that can accommodate the surface morphology changes of the anode upon cycling during the deposition and dissolution of a large quantity of Li. DOL in the electrolyte solution polymerizes on exposed regions of Li, forming short chain oligomers of poly(ethylene oxide) (the “elastomeric” SEI) originated from electrochemical polymerization (EP) of the DOL.4 DOL has been used as an electrolyte additive in Li/MnO2 batteries, and many reports in the literature of Li−S batteries with reasonable cycle life use DOL as a component of the electrolyte solution.22 Despite the self-healing nature of the DOL-based SEI layer, DOL cannot alone mitigate the limited cycle life or completely prevent the reaction of the Li anode with spurious species in the electrolyte. To address this shortcoming, additional studies explored the utilization of additives to the solution such as LiNO3 or P2S5.23 Furthermore, elastomeric layers are chemically unstable in alkyl carbonate electrolyte solutions, resulting in etching and dissolution of some of the inorganic components of the elastomeric layer. DOL may introduce additional challenges related to electrolyte viscosity and SEI instability at the cathode/electrolyte interface due to poor anodic stability and the subsequent polymerization of DOL. The benefits realized using DOL as an electrolyte component, along with the elastomeric properties of its SEI, make it an attractive but incomplete solution to the problem of Li metal anode protection. Inorganic Protection LayersAtomic and Molecular Deposition. The synthesis of inorganic artificial solid electrolyte interphase (ASEI) layers on various cathode materials in rechargeable battery systems has been demonstrated previously by chimical vapor deposition (CVD), atomic layer deposition (ALD), and other types of syntheses.1,24−28 ALD for inorganics and its counterpart molecular layer deposition (MLD) for organics29,30 have played an increasingly dominant role for their capability to control thickness on the atomic scale for a wide variety of materials and over 3-D topography. Studies of ALD protection layers indicate that,

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RESULTS Formation of Elastomeric Protection Layer. We electrochemically synthesize the elastomeric layer on Li metal by cycling symmetric Li/Li cells in 0.35 M LiTFSI in 1:1 DME:DOL electrolyte (1% w/w LiNO3). These symmetric coin cells were cycled for 100 discharge/charge cycles of 1 h per 6299

DOI: 10.1021/acs.chemmater.7b01496 Chem. Mater. 2017, 29, 6298−6307

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Chemistry of Materials cycle at 0.03 mA/cm2 to polymerize the DOL on the lithium surface, creating an elastomeric layer on the surface of the Li metal. Electrochemical impedance spectroscopy (EIS) measured during formation of elastomer layer exhibits no increase in cell impedance (Supporting Information), indicating good Li ion mobility through the elastomeric layer. The final thickness of the layer under these conditions is ∼800 nm, as illustrated in the cross-sectional scanning electron microscopy (SEM) image in Figure 1a. We confirmed the elastomer layer thickness using

After synthesis of the elastomeric layer the electrodes were tested in conventional alkyl-carbonate electrolyte solutions. In operando imaging of the surface evolution of Li metal in 1 M LiPF6 in EC/DEC was conducted to evaluate the efficacy of the elastomeric layer alone in accommodating the volumetric changes accompanied by lithium deposition, under high current densities to examine the effect of dendrite growth. As the duration of these cycles is relatively short in comparison to battery design lifetimes, this approach does not elucidate the long-term chemical stability of the elastomeric layer on the Li metal. Figure 2 shows sequential images taken initially, after 3, 10, 20, and 30 min during Li plating at a current of −2 mA/ cm2, with the top row of images (Figure 2a−e) shown for bare Li foil during Li plating and the bottom row (Figure 2f−j) shown for an elastomer protected Li foil undergoing the same Li plating conditions. Significant surface changes were observed optically in the case of the bare Li metal (Figure 2a−e), including the appearance of bubbles attributed to electrolyte decomposition. Surprisingly, when the Li metal was pretreated electrochemically in DOL-containing electrolyte to form the elastomeric protection layer, the changes in the surface with plating were minimal, producing a stable surface in the alkyl carbonate electrolyte in currents as high as −2 mA/cm2 (Figure 2f−j). As observed, the morphology of the Li is well controlled and does not show dendritic type growth, suggesting even plating of Li over the entire surface of the electrode. We emphasize that our approach utilizes the DOL as a pretreatment in fabricating and configuring the Li metal anode, in contrast to previous reports which employed DOL as an additive in the electrolyte solution of the battery cell. Thus, our DOL pretreatment forms the elastomeric layer and avoids adverse effects that accompany the use of DOL in electrolyte solutions (e.g., anodic stability, viscosity, and lower ionic conductivity compared to alkyl carbonates). Surface Morphology and ChemistryHybrid Protection Layer. The formed elastomeric protection layer is known to be unstable in polar aprotic solvents once the DOL is removed,7 resulting in subsequent dissolution of the lithiumcontaining compounds in the elastomeric layer, leading to a

Figure 1. Cross-sectional SEM (a) and ToF-SIMS analysis (b) of the elastomeric layer on lithium metal.

time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurement of the Li intensity. The low but nonzero Li concentration in the elastomer layer indicates formation of a Licontaining solid electrolyte interphase, which likely contributes to the Li-ion conducting nature.

Figure 2. Effect of elastomer-only protection. Optical images (magnification 100×) taken in operando during Li deposition onto both bare (a−e) and elastomer-protected (f−j) Li metal foil at −2 mA/cm2. Images were collected in situ using a custom-designed electrochemical cell with a quartz window to facilitate in operando imaging. Full videos can be found in Supporting Information. 6300

DOI: 10.1021/acs.chemmater.7b01496 Chem. Mater. 2017, 29, 6298−6307

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Chemistry of Materials

Figure 3. (Top) In situ XPS survey spectra of as-deposited ALD LiPON on elastomer-only protected Li metal at 150 °C; (bottom) high-resolution XPS scans of the O 1s, N 1s, P 2p, and Li 1s photoelectron peaks from left to right, respectively.

Figure 4. Surface evolution SEM images of (a) bare Li, (b) elastomer-only protected Li, and (c) hybrid protected Li after 100 cycles of deposition/ dissolution at 1 mA/cm2.

decrease in ionic conductivity, void formation, and uneven electric fields on the surface of the lithium anode. This instability motivated us to apply an additional ALD LiPON protection layer on the top of the elastomer to mitigate its degradation, while enabling fast enough Li ion mobility so as not to impede transport rates. To address this challenge we deposited ALD LiPON directly onto the elastomeric layer. Our previously reported high temperature (250 °C) ALD LiPON process34 is above the melting point of Li metal (180 °C), making it unsuitable for this application. We thus modified our ALD LiPON process to reduce the deposition temperature to 150 °C. With this reduction in temperature comes a commensurate reduction in growth rate, from ∼1 Å/cycle to ∼0.7 Å/cycle. Figure 3 shows the survey and high-resolution XPS spectra of the ALD LiPON deposited at 150 °C on the surface of the pretreated Li metal. Reduction of the process temperature results in an approximate 1% carbon inclusion,

seen on Figure 3a. This approximate 1% carbon inclusion was confirmed by XPS spectra conducted on Si substrates in the chamber during the same ALD deposition (not shown), so the carbon is likely incorporated into the ALD films. The F 1s peak shown on the survey spectrum is the result of vaporization of lithium salts inside the ALD deposition chamber and cannot be avoided when elastomeric pretreated lithium pieces are used as deposition substrates. The high-resolution peaks (Figure 3b−e) are identical in peak shape and location to ALD LiPON deposited at 250 °C,29 indicating that the low temperature ALD LiPON is not only similar in chemical composition but also completely covers the elastomeric layer. However, XPS is only able to chemically probe the top ∼8 nm of the LiPON surface, so we cannot probe the chemistry of the elastomer−LiPON interface. To further investigate the efficacy of the hybrid protection layer as surface stabilizer for metallic Li anodes, we conducted a 6301

DOI: 10.1021/acs.chemmater.7b01496 Chem. Mater. 2017, 29, 6298−6307

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Figure 5. (a) Bare Li metal surface and Li metal surface after 100 stripping and plating cycles at 500 μA/cm2 in a symmetric Li−Li coin cell using 1 M LiPF6 in 1:1 EC:DEC electrolyte ending on the (b) Li plating cycle and (c) Li stripping cycle. (d) Li metal coated with hybrid protection layer before cycling and Li metal surface after 100 stripping and plating cycles at 500 μA/cm2 in a symmetric Li−Li coin cell using 1 M LiPF6 in 1:1 EC:DEC electrolyte ending on the (e) Li plating cycle and (f) Li stripping cycle.

Figure 6. Long-term cycling on symmetric Li/Li (red) and single-sided Li/hybrid Li cells at current density 1 mA/cm2 and capacity 1 mAh/cm2. (a) Full voltage profiles (cycle life) of long-term cycling of Li anodes; (b) early cycle life; (c) cycle life at Li/Li cells showing unstable profiles (Li anode degraded); and (d) cycle life at Li/Li cells’ failure.

series of battery cycling measurements of stripping/plating with hybrid protected Li and compared that with behavior of bare Li metal under the same electrochemical conditions. The long-term stability of bare Li metal, the elastomeric layer alone, and our ASEI protected electrode was tested in symmetric Li/Li cell with various current densities that mimic operation of a conventional full cell in EC/DEC based electrolyte. Figure 4 compares SEM images after 100 cycles for the bare Li surface (Figure 4a) and for Li protected only by the elastomer (Figure 4b). Comparison of these images demonstrates that the elastomer alone is not stable enough

to accommodate 100 cycles of deposition/dissolution at 1 mA/ cm2 in the conventional electrolyte but rather develops microscale cracks and dendritic surface evolution. We attribute this instability to partial etching and dissolution of the protection layer in the alkyl carbonate electrolyte upon longterm cycling. Our ASEI-protected electrodes exhibit a vastly improved surface morphology over both bare and elastomerprotected Li metal, with smooth, stable surfaces under identical electrochemical cycling conditions (Figure 4c). Starting with the initial Li surface, shown in Figure 5a, we cycle symmetric Li−Li cells for 100 galvanostatic deposition/ 6302

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Figure 7. Long-term cycling on symmetric Li/Li (red) and hybrid Li/hybrid Li cells at current density 2 mA/cm2 and capacity 2 mAh/cm2. (a) Full voltage profiles (cycle life) of long-term cycling of Li anodes; (b) early cycle life; (c) cycle life at Li/Li cells showed unstable profiles (Li anode degraded); and (d) cycle life at Li/Li cells’ failure.

Figure 8. SEM images of bare Li metal (top row, a−c) and hybrid-protected Li metal (bottom row, d−f) cycled in 1:1 EC:DEC for 100 cycles with varied current densities. In (c), a dendrite example is circled. Mechanistic schematics of comparison for (g) bare lithium and (h) hybrid elastomer/ LiPON protection showing Li metal surface layer evolution after 100 cycles at current densities of 2 mA/cm2, above the threshold for Li dendrite formation on unprotected Li metal.

dissolution cycles at 500 μA/cm2 lasting 1 h per cycle. Figure 5b,c shows the bare Li surface after cycling ending on a Li plating cycle and a Li stripping cycling, respectively. Despite cycling at current densities below the previously reported dendrite formation threshold (1 mA/cm2), after 100 charge− discharge cycles the surface of the lithium metal after a deposition cycle exhibits the characteristic structure indicating the early stages of dendrite formation. Figure 5d shows the initial surface of the Li metal directly after application of the hybrid protection layer but before any further electrochemical treatment. The surface cracking apparent in the protection layer is likely due to the difference in coefficient of thermal expansion (during the 150 °C ALD process) between the underlying elastomer and the topmost ALD LiPON layer. Figure 5e,f shows the hybrid protected Li surface after cycling ending on a Li plating cycle and a Li stripping cycle, respectively. In both cases, the majority of the

anode surface is covered by inert, relatively flat LiPON protection plates. Electrochemical Cycling. We conducted long-term cycling of the bare Li and hybrid protected Li metals as shown in Figure 6 and Figure 7. The symmetric coin cells were cycled with 1 M LiPF6 in 1:1 EC/DEC electrolyte. Figure 6a plots the cycling performance under 1 mA/cm2 current density for 1 h of discharge followed by 1 h of charge, which delivered a capacity of 1 mAh/cm2. At the early stages of cycling (Figure 6b), both bare Li and hybrid protected Li cells have stable voltage profiles with low overpotentials. However, after ∼220 cycles the symmetric bare Li cells start to degrade (Figure 6c), and after ∼250 cycles (Figure 6d), the bare Li cells (control, red) failed by generating a large overpotential generally associated with fast electrolyte consumption on an unstable Li surface.13 In contrast, the symmetric hybrid protected Li cells (black) displayed stable voltage profiles even after 280 cycles of 6303

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Chemistry of Materials

In Figure 8g, dendrites form randomly on the surface of unprotected Li due to surface irregularities formed during cycling and the process of SEI formation. Figure 8h depicts our hybrid protection layer, where excess Li deposition and SEI formation occurs between cracks of the LiPON plates on the surface. While the individual LiPON plates should not be flexible, they are spaced such that the entire surface is able to flex upon Li stripping and plating. In a heterogeneous surface as shown in Figure 8d−f, Li will preferentially strip and plate through the path of least ionic resistance. At first, this should be at the spaces between the LiPON plates. However, during the initial few cycles, the Li will react with the carbonate electrolyte to form an ionically insulating “natural” SEI layer covering and protecting the exposed Li areas between the LiPON plates. Once the impedance at these regions increases sufficiently to be higher than that at the LiPON plates, Li ions will preferentially travel through the LiPON rather than the cracks during Li plating and stripping, limiting the formation of further natural SEI at the cracks. The LiPON plates then maintain a stable structure on the electrode surface, preventing massive runaway dendrite formation upon cycling. In addition, these solid electrolyte plates prevent electrolyte decomposition over large regions of the Li metal surface, reducing consumption of cell electrolyte by SEI formation processes and resulting in longer cycle lifetimes.37

operation, which demonstrates the hybrid protection layer on Li metal surface stabilizes the Li/electrolyte interface to mitigate the degradation of Li metal upon cycling and to prevent the electrolyte decomposition and consumption. We further increased the current density and associated capacity to 2 mA/cm2 and 2 mAh/cm2, respectively. Voltage profiles for this high current cycling well above the dendrite formation threshold are presented in Figure 7. Severe dendrite formation and undesired SEI formation are expected at these cycling conditions. Indeed, in Figure 7a,c, early degradation is observed after only 55 cycles of operation for bare Li cells, and a failure occurred after 80 cycles (Figure 7d, red). However, the symmetric hybrid protected cells demonstrate stable cycling behavior (black profiles in Figure 7c,d) up to over 110 cycles. The promising cycling performance of the ASEI protected Li metal demonstrates the advantage of our approach and illustrates the significant improvement to be gained in Li anode stability through applying hybrid ASEI protection layer. Mechanisms for Hybrid Elastomer/LiPON Protection of Li Anodes. The detailed morphology of the hybridprotected Li surface demonstrates efficacy in suppressing dendrite formation and provides suggestions for the mechanisms responsible. SEM images of the surfaces of these electrodes ending on a Li plating cycle after 100 cycles, each lasting 60 min at 0.5−2 mA/cm2, are shown in Figure 8. The surface of the bare Li (Figure 8a−c) has a pitted texture, highlighting the inhomogeneous dissolution and deposition of Li on the anode surface. Figure 8c, an anode cycled at 2 mA/ cm2, clearly shows Li dendrites on the anode surface. While not large enough to break through a separator with thickness on the order of 50 μm, these dendrites will continue to grow far past 100 cycles and could eventually short the battery through a separator. In addition, the increase in surface area of the Li anode results in a continuous consumption of liquid electrolyte, which risks running the cell completely dry and consequent cell failure. In contrast to bare Li metal, Li protected with our hybrid protection layer demonstrates a stable, robust surface morphology. At 0.5 mA/cm2, Figure 8d, Li deposits can be seen near cracks in the LiPON plates; however, no significant asperities can be identified. At rates near the 1 mA/cm2 threshold required for mossy dendrite formation, Figure 8e, the image resembles but is more distinct than at lower current in Figure 6b, appearing as a set of plates separated somewhat from each other. From other XPS results discussed below, we associate the plates with the LiPON protection layer and the material between them with plating of the Li anode. Figure 8f shows an image of the hybrid protected Li cycled above the threshold required for dendrite formation, but this reveals nothing suggestive of dendrite formation. Instead, the shapes of the flakes seem to have changed, becoming more rounded, and the separations between the flakes appear to have narrowed. Both this surface topography (specifically the plate structure) and the absence of dendrites in hybrid-protected Li suggest that the plates are likely the ALD LiPON layer, having disassembled into plates by the mechanical forces generated by plating and stripping of Li and mediated by the thick elastomer layer underneath. A proposed mechanism for the efficacy of the hybrid protection layer on Li is depicted schematically in Figure 8g,h, where the surface morphology of the metal is depicted after cycling above the current density threshold required for dendrite formation on unprotected Li metal, representative of the conditions experienced during our experiments.

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DISCUSSION We have considered analytical approaches to test this mechanistic picture, with particular obstacles arising from the presence of the same elements (Li, C, P, N) in the materials involved (LiPON, elastomer, and electrolyte solution). For example, we chose to investigate the electrochemistry using the most commonly used electrolyte solutions, so the majority of this work was conducted by using LiPF6-based electrolyte in alkyl carbonate. However, the use of LiPF6 as the salt impeded our ability to provide chemical proof for LiPON stability on the surface of Li since the appearance of phosphorus was not uniquely indicative of LiPON. Attempts to confirm the chemical composition and to monitor the chemical (rather than morphological) evolution of our hybrid protection layer, using EDS mapping and XPS, have not led to conclusive results. To circumvent this obstacle, we also fabricated and tested similar cells using a LiTFSI-based electrolytedevoid of phosphorusand monitored the phosphorus after prolonged cycling in a Li/S cell with hybrid-protected Li anode. Using this methodology, we were able to capitalize on our capability to carry out in situ XPS both before and after charge/discharge cycling, using UHV transfer to the XPS without incurring exposure of the synthesized hybrid protection layers to reactive contaminants in air. As indicated in the SEM images shown in Figure 8, the surface morphology, and thus composition, is dominated by the micrometer-sized plates covering most of the surface area. High resolution XPS results in Figure 9 for the hybrid-protected Li anode surface before (Figure 9a) and after (Figure 9b) 100 cycles show a clear P 2p peak at the expected binding energy appropriate for LiPON. We consider this peak a signature of LiPON because the LTFSI electrolyte contains no phosphorus, so any phosphorus on the surface must be from the ALD LiPON. These data strongly suggest that the ALD LiPON protection layer is maintained after cycling in a real 6304

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pressed to the surface of stainless steel carrier disks to prevent direct handling of the Li metal surface. These Li disks were then assembled into symmetric coin cells with a separator (Celgard) and 90 μL of 0.35 M LiTFSI in 1:1 DME:DOL electrolyte (1% w/w LiNO3). These symmetric coin cells were cycled for 100 discharge/charge cycles of 1 h per cycle @ low currents of 0.03 mA/cm2 to polymerize the DOL on the lithium surface, creating an elastomeric layer on the surface of the Li metal. The Nyquist plot of the cell was taken every 10 cycles until stabilization of the Li interface was obtained. After the Li surface pretreatment, the coin cells were disassembled and the anodes removed in our glovebox. These anodes are washed with DME, vacuum-dried overnight at 10−6 Torr in a high vacuum transfer chamber loaded directly from the glovebox, and then transferred directly to our ALD system (Cambridge Nanotech Fiji F200) for ALD. ALD LiPON was deposited at 150 °C from four precursors: lithium tert-butoxide (LiOtBu, Sigma-Aldrich 97%), DI water, trimethylphosphate (Sigma-Aldrich 99.9%), and nitrogen plasma (Airgas grade 5.0). The ALD process used the following pulse−purge sequence: LiOtBu/ purge/H2O/purge/TMP/purge/N2/purge for 3 s/20 s/0.06 s/20 s/ 0.4 s/20 s/10 s/5 s, respectively. The LiOtBu precursor was kept at 165 °C, while the TMP precursor was kept at 70 °C. In this work, all depositions were 300 cycles, with a growth rate of ∼0.7 A/cycle, lower than the growth rate of our previously published ALD LiPON process due to the lower chamber temperature.34 The growth rate of LiPON is measured by ex situ spectroscopic ellipsometry (J.A. Woollam M2000D) using a B-spline model on a piece of monitor Si attached to the same carrier wafer as the Li anodes. We infer LiPON thickness on the Li anodes from the measuring of the LiPON thickness on the Si using spectroscopic ellipsometry; however, we acknowledge there may be some discrepancy between the thicknesses of LiPON films grown on different substrates. However, LiPON films on Li > 10 nm do not show any metallic Li 1s photoelectron peak, indicating the LiPON layer is beyond the escape depth of metal photoelectrons and the thickness measured on the Si is likely similar. Post-deposition, lithium was transferred via UHV transfer to an XPS (Kratos Axis Ultra DLD) system for surface chemical analysis. Survey spectra were collected using a 12 kV monochromatic Al Kα X-ray source in hybrid lens mode with a step size of 1 eV and pass energy of 160 eV. High-resolution spectra were collected using a 12 kV monochromatic Al Kα X-ray source in hybrid lens mode with a step size of 0.1 eV and pass energy of 20 eV. No charge neutralization was used on any of the samples. Next, the anodes were transferred back to the glovebox and reassembled into CR2032 coin cells (MTI Corp.) using a Celgard separator and 90 μL of 1.5 M LiPF6 in 1:1 EC:DEC electrolyte (Novolyte, BASF). These coin cells were cycled for 100 cycles using an Arbin battery tester for galvanostatic testing at 0.5, 1, and 2 mA/ cm2 for 1 h per discharge and 1 h per charge. After 100 cycles, the coin cells were disassembled in the glovebox. Li metal anodes were pumped down overnight at 10−6 Torr. Samples were transferred again to our XPS for surface characterization and then were moved back into the glovebox, loaded onto an SEM stage, and sealed in plastic bags for transfer to an external SEM. Samples were removed from the sealed bags and loaded into the SEM chamber with minimal (