Views of SEI − Battery Materials in the TEM,28 32 Allowing for Atomic- Species and Their Role on Anode Stability

Letter

http://pubs.acs.org/journal/aelccp Resolving Nanoscopic and Mesoscopic Heterogeneity of Fluorinated Species in Battery Solid-Electrolyte Interphases by Cryogenic Electron Microscopy William Huang, Hansen Wang, David T. Boyle, Yuzhang Li, and Yi Cui*

Cite This: ACS Energy Lett. 2020, 5, 1128−1135 Read Online

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ABSTRACT: The stability of lithium batteries is tied to the physicochemical properties of the solid-electrolyte interphase (SEI). Owing to the difficulty in characterizing this sensitive interphase, the nanoscale distribution of SEI components is poorly understood. Here, we use cryogenic scanning transmission electron microscopy (cryo-STEM) to map the spatial distribution of SEI components across the metallic Li anode. We reveal that LiF, an SEI component widely believed to play an important role in battery passivation, is absent within the compact SEI film (∼15 nm); instead, LiF particles (100−400 nm) precipitate across the electrode surface. We term this larger length scale as the indirect SEI regime. On the basis of these observations, we conclude that LiF cannot be a dominant contribution to anode passivation nor does it influence Li+ transport across the compact SEI film. We refine the traditional SEI structure derived from ensemble-averaged characterizations and nuance the role of SEI components on battery performance.

− ithium (Li)-based batteries are critical to decarbonizing chemistry of the SEI.8 12 These methods oﬀer high spectral transportation owing to their high energy densities.1 resolution and allow speciation of chemical components of the L Despite their importance, a fundamental understanding SEI, but in-plane spatial resolution is limited to the micrometer of all the structures within the battery and their function scale. These studies have revealed SEI components common to

Downloaded via STANFORD UNIV on March 18, 2020 at 17:18:52 (UTC). remains lacking. Most prominent among these structures is the high-performance electrolyte chemistries, such as the presence solid-electrolyte interphase (SEI), a passivation layer that of LiF in the SEI arising from decomposition of fluorinated forms on the negative electrode (anode) that arises from electrolyte components and its correlation to improved battery 2,3 − electrolyte reduction at the low potential anode. The SEI cyclability.13 16 The beneficial role of LiF has been See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. kinetically stabilizes the anode/electrolyte interface by rationalized by its passivation properties; with a low electronic preventing further reduction of solvent molecules, while also conductivity, large band gap, and high electrochemical stability regulating Li-ion transfer from the electrolyte onto the at the anode, LiF has been proposed to be an excellent 4,5 − anode. At the same time, the SEI is a source of irreversible component in passivating the anode.17 19 Driven by this capacity loss in the closed battery system and is the principal 6 positive correlation between LiF and high battery cyclability, means of capacity loss in lithium-ion batteries. A fundamental the battery community has pursued numerous LiF-based understanding of SEI properties and structure is paramount to artificial SEI coatings to enable the metallic Li anode, a next- increasing the energy density and cyclability of lithium generation anode that may potentially enable 500 Wh kg−1 batteries. 20−24 battery chemistries. Despite its significance, the SEI has been referred to as the “most important and the least understood” aspect of the battery.7 This arises from the difficulty in characterizing the Received: January 27, 2020 structure and chemistry of this sensitive interphase with Accepted: March 16, 2020 nanoscale resolution. As the SEI is considered a surficial film Published: March 16, 2020 on the anode, surface analytical techniques such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) are frequently used to probe the

Figure 1. Discrepancies between XPS and Cryo-TEM. (a) O and F XPS of EC/DEC + 10% FEC 1.0 M LiPF6 at various sputter times. Depth fi pro ling reveals increased concentrations of inorganic SEI species such as LiF and Li2O close to the anode surface. (b) SEI structure derived from XPS depth profiling. (c) Li metal plated on a Cu TEM grid. (d) Cryo-HRTEM of the compact SEI directly interfaced with Li metal in fl the same EC/DEC 10% FEC 1.0 M LiPF6 electrolyte. The image is 60 nm wide. (e) Fast Fourier transform (FFT) of 1d. Re ections are fl matched to Li2O and Li, without any LiF re ections present. (f) Inverse FFT of 1e showing Li2O enrichment in the SEI, particularly at the inner and outer interfaces.

Despite the importance of the SEI composition, the surfaces of the battery anode, we resolve the discrepancy arrangement of its components is poorly understood. Nano- between large area surface characterizations such as XPS with scale structural and chemical characterization of the interphase atomic-scale characterizations such as cryo-TEM. In a highly necessitates the use of transmission electron microscopy fluorinated electrolyte system, we find that LiF does not − (TEM) and spectroscopy;25 27 however, the sensitivity of precipitate in the compact SEI directly interfaced with the battery materials and interphases to electron beam irradiation anode surface. Instead, we define a new SEI length scale that and air exposure from sample insertion has hindered atomic- extends outside of the compact SEI, in which LiF is observed resolution characterization of the SEI in its native state. Recent by cryo-STEM to precipitate across the electrode. These advances in cryogenic transmission electron microscopy have findings provide a new perspective of the SEI structure across enabled the stabilization of beam-sensitive materials including multiple length scales and may revise traditional views of SEI − battery materials in the TEM,28 32 allowing for atomic- species and their role on anode stability. resolution images of the compact SEI on the carbon,33 Discrepancies between XPS and Cryo-TEM. XPS is a standard − silicon,34 and metallic Li anode.35 37 Surprisingly, despite the means of SEI characterization and offers rich chemical use of fluorinated electrolytes in these studies, LiF was not information at the cost of in-plane spatial resolution. While observed within the compact SEI directly on the active the spatial resolution in the out-of-plane direction is high, material surface. In this work, we utilize cryogenic scanning deriving an SEI structure from XPS alone is challenging as it TEM (cryo-STEM) and electron energy loss spectroscopy fails to capture in-plane heterogeneity at the submicron scale. (EELS) to identify the location of LiF in the SEI on the To illustrate this, we perform XPS depth profiling on a highly metallic Li anode. Through analysis of active and inactive fluorinated electrolyte system, ethylene carbonate/diethyl

1129 https://dx.doi.org/10.1021/acsenergylett.0c00194 ACS Energy Lett. 2020, 5, 1128−1135 ACS Energy Letters http://pubs.acs.org/journal/aelccp Letter fl carbonate (EC/DEC) 1.0 M LiPF6 with 10 vol % uoro- ethylene carbonate (FEC) as an electrolyte additive. Fluorinated solvents such as FEC, a fluorinated analogue to EC, are known to stabilize Si and Li metal cycling,18,19,38,39 with benefits attributed either to the generation of LiF during electrochemical reduction18,40 or to the polymerization of its organic reduction product.41,42 In addition to LiF originating fl from the de uorination of FEC, the LiPF6 salt easily hydrolyzes in the presence of trace water to form HF, which may react at the anode to yield LiF.43,44 Depth profiling at the O and F binding energy of a cycled Li metal electrode reveals a principally organic outer surface layer, rich in carbon−oxygen bonding characteristic of the SEI derived from the carbonate solvent (Figure 1a). With further depth profiling, the O and F spectra yield an increased concentration in Li2O and LiF, while the C signal decreases (Figure S1), suggesting enrichment of inorganic SEI species at the inner SEI. These findings are consistent with previous XPS studies of the SEI on the Li metal anode and have ultimately led to the standard picture of SEI, which consists of a “compact” SEI consisting of an inner inorganic layer and an outer organic layer.45 The XPS depth profile gives the SEI structure shown in Figure 1b, without prior knowledge of the in-plane heterogeneity of the SEI. In this work, we refine this model to yield a picture of the SEI on Li metal that bridges the nanoscale and microscale. Leveraging recent advances in cryo-TEM, we plated Li on a Cu TEM grid for HRTEM analysis of the compact SEI. In carbonate electrolytes, Li plates as nanoscale filaments, ensuring electron transparency (Figure 1c). Cryogenic high- resolution TEM (cryo-HRTEM) imaging of a 60 nm length of the compact SEI on the Li metal surface reveals that the SEI in Figure 2. Large area cryo-STEM EELS mapping of the Li metal SEI fl in EC/DEC + 10% FEC 1.0 M LiPF . (a) Mapping of Li, O, and F the same highly uorinated FEC and LiPF6 containing 6 of a representative Li metal filament region without any F signal. electrolyte is free of crystalline LiF and instead rich in Li2O × (Figure 1d). This is further corroborated by the fast Fourier The window is 270 nm 640 nm. (b) Mapping of Li, O, and F of a Li metal filament region with a single LiF particle. F and composite transform (FFT) of the image, which reveals all the maps were spatially rebinned to increase signal. The window is crystallographic data present in the image (Figure 1e). The 2100 nm × 340 nm. lattice spacing correlating to the highest intensity reflection of crystalline LiF is absent, and only reflections corresponding to Li2O and metallic Li are present. Mapping of the Li2O and Li reflections from reciprocal space back to real space illustrates Li, only a single LiF particle is observed. A single particle of Cu the localization of Li2O in the SEI, with enrichment of Li2Oat is also present, confirmed from EELS mapping of the Cu L- the Li/SEI interface and the outer surface of the SEI, edge (Figure S3), likely due to Cu dissolution from the current consistent with the layered SEI nanostructure observed in collector and its redeposition once Li is plated. These micron- previous works (Figure 1f).28,35 scale maps confirm that the cryo-HRTEM observation is Cryo-HRTEM is an effective methodology for identifying representative of the SEI; LiF is not incorporated into the the presence of SEI components; however, it is limited to a compact SEI that is passivating the active material, even in a relatively small region for analysis, and sensitive to only highly fluorinated FEC-containing electrolyte. Owing to the crystalline components within the SEI. In order to probe a higher solubility of LiF relative to Li2Oincarbonate larger length scale of the Li metal SEI and include sensitivity to electrolytes,46,47 LiF formed at the Li metal surface does not amorphous phases, we performed cryo-STEM EELS mapping precipitate directly into the compact SEI; instead, LiF of the Li, O, and F K-edges over micron-scale regions across precipitates as nanoparticles with sparse coverage of the active the Li metal filament. The annular dark-field (ADF) cryo- material and cannot be a dominant source of anode fi STEM image shows low intensity from the Li lament core, passivation. Li2O has the lowest solubility of all SEI with higher intensity arising from the surficial SEI layer owing components and is consequently the primary inorganic species to the higher atomic number (Z) of carbon and oxygen- within the compact SEI. This precipitation of large LiF containing SEI species (Figure 2a). The EELS maps show particles on the anode surface has been suggested previ- many regions of Li are completely free of F, with only signals ously,15,48,49 but our cryo-HRTEM and cryo-STEM EELS data from Li and a surficial O layer arising from the SEI. The EELS further suggest an absence of LiF in the compact SEI. As the spectra from the SEI shows no F signal, consistent with an compact SEI directly interfaced with the active material is the absence of amorphous LiF (Figure S2). In other regions, highly primary regulator of Li+ dissolution from the anode, it is localized F signals are found on the Li metal filament, unlikely that LiF plays a direct role in Li metal stripping suggesting the sparse, localized precipitation of LiF on the efficiency.4,35 Instead, the organic reduction products of EC, μ active material surface (Figure 2b). Within this 2 m length of FEC, Li2O, and their respective nanostructure are likely the

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Figure 3. Cryo-STEM mapping of the current collector. (a) Schematic of SEI formation on an amorphous carbon film TEM grid current collector. (b) Electrochemical protocol of SEI formation. (c) Mapping of Li, O, and F in a 10% FEC electrolyte without LiPF6. The window μ × μ μ × μ is 9 m 9 m. (d) Mapping of Li, O, and F in an electrolyte with 1.0 M LiPF6 and without FEC. The window is 13 m 13 m. key to uniform Li dissolution and preventing the formation of The cryo-STEM ADF image shows a highly heterogeneous electrochemically disconnected (dead) Li.35 surface on the current collector after SEI formation, with many Deposition of LiF on the Current Collector. Our observation of nanoparticles decorating the surface (Figure 3c). EELS a sparse LiF grain on the previous Li filament contrasts with mapping reveals most of these particles are rich in Li, O, the high LiF content observed in XPS. LiF can deposit on any and F, occurring in high areal concentration owing to the conductive surface at the anode; we hypothesize that LiF may relatively low surface area of the current collector. This deposit on the Li plating substrate as well, which is typically a suggests the coprecipitation of Li2O and LiF during SEI Cu current collector in Li || Cu half-cells50 and anode-free formation. In batteries with porous electrodes or plated Li, batteries.51,52 This current collector is the first surface on the where the surface area is much higher, the areal LiF battery negative electrode to form SEI37 while also remaining a distribution likely becomes much more dilute, explaining the stable surface during the highly dynamic Li stripping and low concentration observed in TEM on plated Li. Two regimes plating process, making it a likely location for significant LiF of particle sizes are observed, large particles with a diameter of deposition. In order to test this hypothesis, we constructed approximately 400 nm and small particles with a diameter of half-cells using an amorphous carbon film coated TEM grid as 100−200 nm. This likely occurs due to the SEI formation the working electrode (Figure 3a). These TEM grids are flat process; during the slow voltammetric sweep to the cutoff and conductive, mimicking the Cu current collector, while potential of 10 mV, LiF particles grow slowly once the remaining electron transparent for TEM analysis of any surface solubility limit is reached,53 favoring the growth of larger LiF layers formed on the current collector. We formed a SEI on the particles. After the LSV is finished, the SEI is stabilized using a C film TEM grid through linear sweep voltammetry (LSV) potentiostatic hold, a common step for SEI formation in (Figure S4) to a cutoff potential of 10 mV vs Li/Li+, above the lithium-ion batteries.54 During the constant voltage step at 10 Li plating potential, and stabilized the SEI using a mV, the overpotential for electrochemical formation and potentiostatic hold (Figure 3b). In order to examine the LiF nucleation of LiF is greater than 1 V,55 favoring smaller LiF contribution arising from the fluorinated solvent (FEC) alone, particles through classical electrochemical nucleation fl 36,50 without any LiF arising from the uorinated salt (LiPF6), we theory. Similar morphologies are observed for the case of form an SEI in a similar EC/DEC + 10 vol % FEC electrolyte, the FEC-free electrolyte, where the only F source is 1.0 M but with LiClO4 as the salt. LiPF6 (Figure 3d). The current collector surface is covered

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Figure 4. Nanostructure of LiF deposits. (a) Cryo-STEM ADF image of LiF nanoparticle. (b) Cryo-STEM EELS mapping of the same region fl in (a). (c) SAED pattern of a single LiF particle. LiF and Li2Ore ections are visible. (d) HRTEM of LiF nanoparticle with Li2O shell. Inset: fi fl FFT of (d). Low magni cation image is available in the Supporting Information. (e) Inverse FFT of LiF and Li2Ore ections from (d). primarily by O-rich particles, with sparse LiF coverage. LiF in predominantly single orientation, with some spreading of the fl fl LiPF6-containing electrolytes, in the absence of uorinated {200} re ection in reciprocal space. This suggests texturing of additives, arises primarily from the hydrolysis of the PF6 anion the LiF particle; the LiF particle may be polycrystalline with a into HF and PF5, with HF in commercial solvents generally preferred orientation for growth. Additional rings correspond- 56 fi less than 100 ppm. As a result, despite the relatively high ing to polycrystalline Li2O are present as well, con rming Li2O fluorine concentration in the electrolyte, few LiF particles are contributes to the oxygen-rich layer. Cryo-HRTEM supports observed. The fluoride ions available for LiF formation in the cryo-STEM structure of a F-rich core and O-rich shell; LiPF6-containing electrolyte arise mainly from the HF crystalline LiF is observed inside the particle, with an outer fl ff impurity, whereas in the uorinated solvent case, each FEC Li2O-rich layer (Figure 4d,e). Through the common ion e ect, molecule may provide an F− anion for LiF formation.42 LiF precipitation of LiF should be tied to the precipitation of other fi 40 does not form a neat lm in either case, rather precipitating as Li salts such as Li2O, consistent with our observation of the individual nanoparticles on the current collector and Li metal core@shell LiF@Li2O particle nanostructure. surface. Our observations may be generalized to lithium-ion The union of our microscale and nanoscale cryo-(S)TEM batteries using carbon or silicon as the anode; LiF nano- observations yields a more detailed picture of the SEI particles likely precipitate along the anode surface as well. nanostructure, depicted in Figure 5. LiF, considered to be a Nanostructure of LiF Deposits. To characterize the morphol- key component within the inner inorganic SEI passivating the ogy of individual LiF nanoparticles, we performed cryo-STEM anode, is instead found to precipitate outside of the compact EELS mapping at higher spatial resolution (Figure 4a). The SEI, in an “indirect SEI”. The inorganic components within the Cryo-STEM ADF image shows the representative structure of compact SEI are primarily Li2O, which is the dominant a single LiF nanoparticle; bright intensity arises from the passivating species in the SEI. This compact SEI is the primary particle core, with a lower intensity surface layer covering the regulator of lithium dissolution in Li metal systems owing to nanoparticle. Li, C, and O mapping show the entire particle is the radial stripping behavior of Li; on the basis of our rich in Li, as expected for SEI species, but F is primarily observation of LiF precipitation in the indirect SEI regime, LiF confined to the core and O is rich in the shell (Figure 4b). cannot be a principal factor for homogenizing lithium Selected-area electron diffraction (SAED) confirms the dissolution. However, as lithium metal growth occurs structure of the particle as crystalline LiF (Figure 4c), which basally,57,58 LiF on the current collector may play a role in is also supported by comparison of the F K-edge fine structure Li metal plating microstructure, and consequently on the from EELS to a LiF reference (Figure S5). The LiF particle is cycling stability. LiF, beyond its passivating properties, has

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beneficial role of fluorinated additives such as FEC likely arises from their rapid defluorination at the anode and subsequent polymerization, rather than solely from the generation of LiF. ■ ASSOCIATED CONTENT *sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c00194. Methods, XPS depth profiles, linear sweep voltammetry, EELS spectra, additional TEM images, and Coulombic Figure 5. Schematic of the SEI structure derived from Cryo- efficiency data (PDF) (S)TEM. ■ AUTHOR INFORMATION been proposed to stabilize Li electrodeposition by increasing Li+ surface diffusivity17 or homogenizing the Li+ diffusion field Corresponding Author 15 Yi Cui − Department of Materials Science and Engineering, gradient. The core@shell nanostructure of LiF@Li2O may ff + Stanford University, Stanford, California 94305, United States; increase di usivity of Li along the LiF/Li2O interface owing to a high defect concentration.59 Stanford Institute for Materials and Energy Sciences, SLAC To verify if LiF in the indirect SEI plays a role in the National Accelerator Laboratory, Menlo Park, California cyclability of Li metal, we preformed a LiF-rich SEI on the Cu 94025, United States; orcid.org/0000-0002-6103-6352; current collector in a highly fluorinated FEC-containing Email: [email protected] electrolyte (EC/DEC + 10 vol % FEC 1.0 M LiPF6), Authors disassembled the cell, and replaced the electrolyte with an William Huang − Department of Materials Science and FEC-free electrolyte (EC/DEC 1.0 M LiPF6). Cycling of Li Engineering, Stanford University, Stanford, California 94305, metal in the FEC-free electrolyte with the LiF-rich preformed United States; orcid.org/0000-0001-8717-5337 Cu SEI decouples the Li metal stability contribution of any LiF Hansen Wang − Department of Materials Science and on the current collector from the contribution of organic FEC Engineering, Stanford University, Stanford, California 94305, reduction products in the compact SEI on Li. Comparison of United States; orcid.org/0000-0002-6738-1659 ffi the Coulombic e ciency (CE) of Li/Cu half cells shows that David T. Boyle − Department of Chemistry, Stanford the Li cycled in EC/DEC with a preformed LiF-rich SEI is University, Stanford, California 94305, United States; slightly more stable than Li in EC/DEC without a preformed orcid.org/0000-0002-0452-275X LiF-rich SEI, but cycling of Li in EC/DEC + 10% FEC Yuzhang Li − Department of Materials Science and Engineering, electrolyte exhibits the greatest stability (Figure S7). LiF in the Stanford University, Stanford, California 94305, United States; indirect SEI thus has a contributing role toward Li metal orcid.org/0000-0002-1502-7869 cyclability, but in the case of the fluorinated solvent FEC, the greater contribution arises from the organic reduction product Complete contact information is available at: of FEC. LiF in the indirect SEI may contribute by promoting https://pubs.acs.org/10.1021/acsenergylett.0c00194 plating of a less porous Li metal layer but does not affect Li Author Contributions dissolution through the compact SEI, leading to significant Li loss through disconnected, dead Li. Plating of Li in the W.H. and Y.C. conceived the project and designed the presence of available FEC molecules allows for their experiments. W.H. carried out TEM. H.W. performed XPS defluorination and polymerization into the compact SEI,42 measurements. D.T.B. performed electrolyte synthesis. Y.L. which homogenizes the SEI nanostructure and Li+ dissolution, aided data analysis. W.H. and Y.C. co-wrote the manuscript, preventing dead Li. with input from all authors. Using cryo-(S)TEM, we examine the spatial distribution of Notes fi SEI species such as LiF and Li2O within the SEI of Li metal. The authors declare no competing nancial interest. Despite the abundance of LiF in the SEI of fluorinated The data that support the findings of this study are available electrolyte systems as measured through XPS, we find that the from the corresponding author upon reasonable request. compact SEI interfaced with the anode material is devoid of LiF, confirmed through cryo-HRTEM in conjunction with ■ ACKNOWLEDGMENTS cryo-STEM EELS. Instead, the compact SEI, which regulates Y.C. acknowledges the support to initiate Cryo EM research + ffi Li dissolution from Li metal, is rich in Li2O, owing to its low from the Assistant Secretary for Energy E ciency and solubility in the electrolyte. LiF may deposit as an indirect SEI Renewable Energy, Office of Vehicle Technologies, of the on any conductive surface, including the current collector and U.S. Department of Energy under the Battery Materials sparsely on Li metal. Owing to the moderate solubility of LiF Research (BMR) Program and the Battery500 Consortium in the electrolyte, LiF precipitates as large nanoparticles with program. Y.C. acknowledges further Cryo EM support from diameters greater than 100 nm, and an outer shell rich in the Department of Energy, Office of Basic Energy Sciences, oxygen and Li2O. The presence of LiF deposits on the current Division of Materials Science and Engineering under contract collector likely aid Li plating uniformity to some degree; DE-AC02-76SF00515. Part of this work was performed at the however, they do not incorporate into the compact SEI and do Stanford Nano Shared Facilities (SNSF), supported by the not play a significant role in the passivation of Li metal, nor the National Science Foundation under award ECCS-1542152. transport of Li+ through the SEI. Therefore, the role of LiF in Y.L. acknowledges the Intelligence Community Postdoctoral the SEI is more nuanced than commonly believed. The Research Fellowship for funding.

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