Formation of Reversible Solid Electrolyte Interface on Graphite

Letter pubs.acs.org/NanoLett

Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes Dongping Lu,*,† Jinhui Tao,‡ Pengfei Yan,§ Wesley A. Henderson,† Qiuyan Li,† Yuyan Shao,† Monte L. Helm,‡ Oleg Borodin,∥ Gordon L. Graff,† Bryant Polzin,⊥ Chong-Min Wang,§ Mark Engelhard,§ Ji-Guang Zhang,† James J. De Yoreo,‡ Jun Liu,† and Jie Xiao*,†,# †

Electrochemical Materials & Systems Group, Energy & Environment Directorate, ‡Physical Sciences Division, Fundamental & Computational Sciences Directorate, and §Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States ∥ Electrochemistry Branch, Sensor & Electron Devices Directorate, United States Army Research Laboratory (ARL), Adelphi, Maryland 20783, United States ⊥ Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the battery’s life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode−electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs. KEYWORDS: Solid electrolyte interface, concentrated electrolyte, electrochemistry, Li-ion battery, graphite

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SEI layer results in reduced battery service life and even safety concerns.12 The addition of selected electrolyte additives modifies the SEI’s composition, properties, and stability,13 but most of these additives are sacrificial, and, once depleted, their benefits to cycling stability are degenerated. In addition, the presence of the protective SEI adds to the battery’s impedance (resistance to ion transport), thus limiting the available energy and power. Beside protection from solvent and additive decomposition, it is interesting to find that solute concentration also plays a critical role for graphite stabilization. Superconcentrated solutions such as lithium bis(trifluoromethylsulphonyl)imide in dimethoxyethane (DME) with very limited free solvent can enhance the stability of reversible Li+ intercalation and extraction into and from graphite without using any additional additive.14−18 We also demonstrated stable cycling of Li-ion sulfur batteries by using

nterfacial reactions govern diverse phenomena such as functional-material synthesis processes and reactions for heterogeneous catalysis and energy storage and conversion. Crucial to the development of lithium-ion (Li-ion) batteries was the revelation that the graphite−electrolyte interface, inherently thermodynamically unstable, could be passivated by the formation of a solid electrolyte interphase (SEI),1−5a surface layer formed from the reaction of the exposed carbon with the electrolyte components such as ethylene carbonate (EC). Solvent selection was critical for formation of a stable SEI that minimizes the degradation of active material (carbon) and leads to high stability and low resistance to Li+ cation transport.6−10 In fact, EC has become an essential constituent of nearly all state-of-the-art electrolytes used for Li-ion batteries.11 Other solvents, such as propylene carbonate (PC) (despite its structural similarity to EC), do not produce effective SEI layers on graphite.11 Even with EC, however, the stability of the SEI layer is limited. The SEI slowly fractures and reforms, thus eventually leading to loss of the electrolyte; a process accelerated at elevated temperature. The instability of © 2017 American Chemical Society

Received: November 14, 2016 Revised: January 28, 2017 Published: February 6, 2017 1602

DOI: 10.1021/acs.nanolett.6b04766 Nano Lett. 2017, 17, 1602−1609

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room temperature (23 °C) with a Nanoscope 8 (Digital Instruments E scanner, Bruker) AFM. The AFM probes consist of silicon tips on silicon nitride cantilevers (HYDRA triangular lever, k = 2 N m−1, tip radius 9 nm; resonance frequency 70 kHz in air; Asylum Research). Highly ordered pyrolytic graphite (HOPG) (grade ZYA, Ted Pella) was cleaved along the basal plane by adhesive tape immediately before the AFM experiment. The AFM experiments without an applied voltage were completed on the HOPG surfaces in 100 μL of electrolyte solution sealed in a closed chamber. To evaluate the role of an external electric field on the behavior of the electrode− electrolyte interface, in situ electrochemical AFM analyses were conducted. The imaging was performed in a liquid cell with HOPG as both the substrate and working electrode, a Li metal wire as both the counter and reference electrodes, and 5 Ṁ LiTFSI-DOL as the electrolyte. A Solartron SI1287 electrochemical interface was used to control the potential of the working electrode during the AFM measurements. For typical imaging conditions, images were collected at scanning rates of 1−2 Hz. The deflection set point was carefully tuned to minimize the average loading force of ∼50 pN or less during the stable imaging. The images were analyzed using the image processing software package SPIP 5.1.4 (Image Metrology A/S, Hørsholm, Denmark). Characterization. Cycled graphite electrodes were harvested from the coin cells for characterization by transmission electron microscopy (TEM), scanning transmission electron microscopy−energy dispersive spectroscopy (STEM−EDS), scanning transmission electron microscopy−electron energy-loss spectroscopy (STEM−EELS) and X-ray photoelectron spectroscopy (XPS) analyses. Before the characterization, the electrodes were immersed in either pure DOL or DMC, depending on the electrolytes used for cycling, overnight and then rinsed with fresh solvent before drying under vacuum. For the TEM and STEM−EELS microanalyses, the graphite electrodes before and after cycling in various electrolytes were dusted on a lacy carbon TEM grid. Conventional TEM imaging and selective area electron diffraction (SAED) were conducted using a Titan 80−300 microscope operated at 300 kV. The microscope is equipped with an image Cs corrector for objective lens. Energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) were acquired using a Gatan Image filter (GIF, Quantum 965) with an electron beam convergence angle of 17.8 mrad and a collection semiangle of about ∼50 mrad. XPS measurements were performed on a Physical Electronics Quantera Scanning X-ray Microprobe. This system uses a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 32 element multichannel detection system. A 100 W X-ray beam focused to a 100 μm diameter was rastered over a 1.4 mm × 0.1 mm rectangular portion of the sample. The X-ray beam was incident normal to the sample, and the photoelectron detector was 45° off-normal. High-energy resolution (narrow scan) X-ray photoemission spectra were collected using a passenergy of 69.0 eV with a step size of 0.125 eV. All of the spectra were charge referenced using the C 1s line at 285.0 eV for comparison purposes. Sputtering depth and speed were calibrated with SiO2/Si with a known oxide layer thickness (ca. 2 nm per sputtering cycle). A pair of spectra were acquired at two different positions for each sputtering. To avoid electrode contamination or side reactions with atmospheric moisture and oxygen, the samples were transferred to the SEM and XPS in sealed vessels that were filled with Ar gas.

concentrated LiTFSI in dioxolane (DOL) and graphite anodes.19 This sheds new light on development of electrolyte with simplified composition but improved performance. Thus, fundamental understanding on the properties of concentrated electrolytes and electrode and electrolyte interfacial behaviors are crucial. On the basis of studies of different solute and solvent combinations with different instrumentations, functional mechanisms of the concentrated electrolytes are generally ascribed to (1) organic-components-based SEI generated from solvent polymerization and decomposition,20 (2) inorganic-components-based SEI derived from solute anion decomposition,16,18 and (3) preferential Li+ desolvation and intercalation due to improved chemical potential of Li+-glyme solvate with very limited free solvent molecules.21 Here, we demonstrate a fundamentally different anode shielding mechanism in which a reversible passivation layer is induced by an electric field on the electrode surface in contact with a highly concentrated electrolyte, regardless of the electrolyte’s specific composition, and plays a key role for electrode protection. Once the electric field is withdrawn (unpolarized), the surface reverts to its original structure that is essentially SEI-free. This surface layer thereby effectively isolates (shields) the charged surface from the electrolyte solvent molecules, preventing significant electrolyte decomposition. Experimental Methods. Materials and Electrolyte Preparation. The thick graphite-based composite electrode (5 mg cm−2 of graphite) was provided by Argonne National Laboratory. The composition and chemistry of the electrode is noted in Table S1. The Li metal (MTI Corporation, 99.9%) electrodes have a diameter of 15.6 mm and a thickness of 0.45 mm. Electrolyte-grade salts, lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2 or LiTFSI), were obtained from BASF Corporation. Electrolyte-grade solvents, 1,3-dioxolane (DOL), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC), were also obtained from BASF. Electrolytes were prepared by dissolving the salts in the solvents to obtain specific compositions with the desired ratios of moles of salt to volume of solvent. The compositions are designated in units of Ṁ to differentiate them from molarity (moles of salt to volume of solution), as it is difficult in practice to accurately prepare concentrated electrolytes in small volumes based upon molarity. The EC/EMC mixture was 40:60 (w/w). The materials were stored and handled in an argon-filled glovebox (MBraun). The concentrations of the electrolytes in terms of moles of solvent/mol of salt are given in Table S2. The electrolyte conductivity and viscosity were measured by WP 600 Series Meters (Oakton) and Ostwald viscometer, respectively, at room temperature. Electrochemical Measurements. Graphite/lithium (G/Li) half-cells were evaluated with CR2032 coin-type cell (MTI Corporation). The separators are Celgard 2500 and glass fiber (Whatman, GE) for 1 Ṁ and 5 Ṁ electrolytes, respectively. The electrochemical discharge performance was measured galvanostatically at various C rates (0.1, 0.5, 1, 2 and 3 C correspond to a full cell discharge in 10 h, 2 h, 1 h, 30 and 20 min, respectively) on an Arbin BT-2000 battery tester at room temperature and 0 °C, while the cyclic voltammetry (CV) testing was performed using a CHI660D electrochemical station. AFM Imaging. Atomic force microscope (AFM) images with and without electric fields were obtained in tapping mode at 1603


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Figure 1. AFM topographic images of an HOPG electrode immersed in an 5 Ṁ LiTFSI-DOL electrolyte with and without electrode polarization. (A) Before polarization (at open circuit potential). Polarized at: (B) 1.0, (C) 0.5, (D) 0.2, and (E) 0.1 V. Time after removal of polarization: (F) 10.4, (G) 17.2, and (H) 20.5 min. Time after the addition of free DOL in (H): (I) 24.2 min. (J) Schematic diagram of electrode-concentrated electrolyte interface with and without electrode polarization.

electrode to 1.0 V (versus Li+/Li) from the open circuit potential (OCP) at 2.85 V, the HOPG surface transforms from the ordered, self-assembled organization (Figures 1A and S1) into a compact surface layer (Figure 1B), denoting the fast response of the surface structuring to an electric field. Shortly thereafter, a nonuniform layer of crystal-like aggregates began to grow above the layer at the same potential. Polarization to more negative potentials and for longer periods of time result in the continuous growth of the aggregates (Figures 1C−E and S2). Upon removal of the field, i.e., at OCP, the height of the aggregates keeps on decreasing (Figures 1F,G and S2) and changes into very thin and porous layer after a 20 min rest period (Figure 1H). The formation of these layers at low potential is reversible, indicating that their identities differ markedly from that of a traditional insoluble SEI layer (formed from degraded electrode and electrolyte components).28 Further, when additional DOL solvent is added to the in situ cell (diluting the electrolyte), this layer almost disappears exposing the HOPG basal plane and very limited residues with a thickness around 3 nm (Figures 1I and S3). Notably, a significant restructuring of the surface (from that at OCP) is evident even at 1.0 V (Figure 1A,B), a potential at which there is a negligible current (Figure 2H), confirming that electro-

Results and Discussion. Highly ordered and oriented pyrolytic graphite (HOPG) and battery-grade graphite were selected as standard electrodes for this study because of their well-understood intercalation chemistry. Initial testing used a highly concentrated 5 Ṁ LiTFSI-DOL electrolyte as this has been recently found by our group to enable the stable operation of Li-ion sulfur cells with graphite as the anode.19 In situ AFM, a high-resolution probing technique enabling direct observation of the solid−liquid interface,20,22−24 was used to study a freshly cleaved HOPG surface immersed in this electrolyte with and without an electric field (Figure 1). All AFM images were acquired in tapping mode to minimize scratching from tip. In the absence of the field, the HOPG basal planes are clearly observed (Figures 1B and S1A). A highly magnified view of the basal planes reveals a highly ordered surface structure covering the surface of the HOPG (Figure S1C). Such a surface organization is presumably created by surface charging on the carbon and anion ordering within the adjacent liquid layer, as has been observed for graphite and HOPG surfaces immersed in surfactant solutions.25−27 The application and then removal of the polarization (electric field) to the HOPG electrode (working electrode in the in situ AFM cell) lead to profound changes in the electrode surface (Figure 1). Immediately upon polarization of the 1604


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Figure 2. CV scans (first, second, and third) of graphite/Li half-cells with different electrolyte solutions. (A) 1 Ṁ LiPF6-EC/EMC. (B) 1 Ṁ LiPF6PC. (C) 1 Ṁ LiTFSI-PC. (D) 1 Ṁ LiTFSI-DOL. (E) 5 Ṁ LiPF6-EC/EMC. (F) 5 Ṁ LiPF6-PC. (G) 5 Ṁ LiTFSI-PC. (H) 5 Ṁ LiTFSI-DOL.

Figure 3. Morphology and structure of graphite electrodes. HR-TEM images of the graphite electrodes after five CV cycles (0.05 mV s−1 cycled between 2.0 and 0 V). (A) 1 Ṁ LiPF6-EC/EMC. (B) 1 Ṁ LiPF6-PC. (C) 1 Ṁ LiTFSI-PC. (D) 1 Ṁ LiTFSI-DOL. (E) 5 Ṁ LiPF6-EC/EMC. (F) 5 Ṁ LiPF6-PC. (G) 5 Ṁ LiTFSI-PC. (H) 5 Ṁ LiTFSI-DOL.

chemically solvent and anion reduction reactions are not the origin of the morphology transformation. The observed reversible electrode shielding is found to be applicable to several other concentrated electrolyte solutions. Figure 2 compares the CVs of graphite cycled in different dilute electrolytes. For the state-of-the-art electrolyte, e.g., 1 Ṁ LiPF6EC/EMC (Figure 2A) with ethylene carbonate and ethyl methyl carbonate solvents, a small irreversible reduction peak (ca. 0.7 V) is present due to the intercalation of solvated Li+ cations and EC decomposition.28−31 When the solvent is switched to propylene carbonate (PC), which is known to be incompatible with graphite, a large reduction peak is observed

(onset near 1.0 V) with LiPF6 or LiTFSI salts (Figures 2B,C and S4B,C) due to the intercalation of PC-solvated Li+ cations followed by PC decomposition and graphite exfoliation.20,32 The above results indicate that the SEI derived from PC in 1 Ṁ electrolytes is not able to protect graphite effectively. When a 1 Ṁ LiTFSI-DOL electrolyte is used, there are two irreversible reduction peaks observed during the first cathodic scan (Figure 2D). This indicates DOL-derived SEI, although not as effective as the EC-generated one, does temporarily protect the graphite surface to some extent, enabling the limited reversible cycling of the graphite anode (Figure S4D).33 When highly concentrated (5 Ṁ ) electrolytes, which have essentially little to no 1605


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Figure 4. Structure and energy spectrum analysis of graphite surface. STEM−EELS image of a graphite electrode after five CV cycles in the 5Ṁ LiTFSI-DOL electrolyte. (A) STEM image with mapping of (B) C, (C) O, and (D) C-to-O ratio. (E) X-ray near-edge fine structure of the elements C, N, O, F, and Li.

in the solution. This is supported by limited cyclability of graphite in 1 M LiTFSI-DOL, although the irreversible capacity is large (Figure 2D and S4D). However, in 5 Ṁ LiTFSI-DOL electrolyte, the DOL polymerization should be reduced because most of the DOL molecules are coordinated with Li+ cations. As shown in Figure 1H,I, the precipitates disappear essentially after withdraw of the electric field or adding additional DOL solvents, excluding significant irreversible polymerization. Recently, Yamada et al. studied possible functional mechanism of concentrated electrolyte and found inorganic SEI layers derived from anion decomposition enable reversible Li + intercalation. In the present study, XPS was used to analyze the surface of the graphite electrodes before (in contact with the electrolyte but not cycled) and after cycling in the 5 Ṁ LiTFSI-DOL electrolyte. Elements of C, N, O, F, and S were identified on the surfaces of both electrodes (Figures S5A and S6A). These signals may come from electrolyte residuals, PVDF binder, Super P carbon, electrolyte decomposition. or a combination of several of these. After shallow sputtering (ca. 2 nm in depth) of the cycled electrode surface, the signals from these elements almost disappeared leaving C as the main component (Figure S6B). The F 1s XPS signal at around 687 eV is detectable during the whole-depth sputtering, which is ascribed to F of PVDF binder used in the electrode34 or LiTFSI residual (Figure S7). These results are comparable to those for pristine graphite (Figure S5B), suggesting that the surface layer on the cycled graphite is very thin. This is consistent with the very small reduction peak between 0.8 and 0.6 V during the first cathodic scan (Figure 2H), assigned to electrolyte decomposition. Electrolyte decomposition on the electrode is, therefore, still present in the concentrated electrolytes at low potentials but is dramatically reduced compared to that in dilute electrolytes. Of note, XPS is a localized analysis tool and the detection of certain elements does not necessarily mean

uncoordinated solvent present (Table S2), are used instead, very different CV results are obtained. For the cell with the 5 Ṁ LiPF6-EC/EMC electrolyte, the cathodic current begins to increase substantially below 0.2 V, and a corresponding oxidation peak appears in the 0.1−0.4 V range during the anodic scan due to the deintercalation of the Li+ cations (Figure 2E). Importantly, for the 5Ṁ LiPF6-PC and 5Ṁ LiTFSI-PC electrolytes (Figure 2F,G), PC decomposition is significantly suppressed during the first cycle with only a small reduction peak at 0.7 V notable. The same holds true for the 5Ṁ LiTFSIDOL electrolyte (Figure 2H).These results are strongly supported by the TEM characterization. Graphite surface roughening clearly occurs with the 1Ṁ LiPF6-EC/EMC (Figure 3A) and 1Ṁ LiTFSI-DOL (Figure 3D) electrolytes due to SEI formation and partial graphite exfoliation, whereas for the 1Ṁ electrolytes with PC the graphite undergoes extensive exfoliation (Figure 3B,C), in agreement with the CV results. In contrast, an essentially unaltered (from the original graphite) surface structure is evident from the high-resolution TEM images of the cycled graphite in all of the concentrated electrolytes (Figure 3E−H). Of note, all of the electrodes were treated in exactly the same manner prior to the TEM characterization. Compared to the performance of dilute electrolyte, significant improvement is observed for graphite in concentrated electrolytes despite their specific compositions. One possible reason for the improvement is SEI passivation layer generated from either solvent polymerization or salt decomposition. Jeong et al. studied interfacial reactions of graphite in concentrated LiTFSI-PC electrolyte and suggested that a layer (ca. 8 nm) of polymer-like precipitates may form and prevent solvent cointercalation into graphite. In diluted DOL-based electrolyte (e.g., 1 M LiTFSI-DOL), polymerization of DOL is possible due to the presence of excessive amount of free DOL 1606


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(Figure S12). These solvent molecules are highly susceptible to reduction by the electrons on the polarized surface.35 Highly concentrated electrolytes, however, have few solvent molecules (relative to the number of ions) and the majority of these are coordinated to the Li+ cations.36,37 Such mixtures resemble (see Figure S12), to some extent, ionic liquids composed of the same anions and organic cations. The negative polarization of a graphite surface exposed to an ionic liquid results in a very different surface structure than for dilute electrolytes containing organic solvents.35 Polarization in the concentrated electrolytes is expected to result in further exclusion of the solvent from the surface (relative to the bulk concentration) due to preferential adsorption of the Li+ cations as the potential is decreased (Figure 1J). The exclusion of the solvent molecules from the electrode surface in the restructured double-layer explains the largely reduced solvent decomposition in the concentrated electrolytes upon polarization, consistent with the observations in Figures 1 and 2. The surface layer on the HOPG noted upon initial negative polarization during the in situ AFM measurements (Figure 1B) is therefore likely to consist of the precipitated LiTFSI salt (which is partially solvated). The formation of the surface layer is critical for the protection of graphite during the Li+ intercalation and deintercalation processes. Within the layer, the few solvent molecules near the surface will predominantly be coordinated to Li+ cations, and thus, charge transfer (reduction) may preferentially occur to the readily available cations rather than the solvent molecules. This further concentration of the already highly concentrated electrolyte via negative polarization of the electrode leads to the growth of “crystalline solid” on the electrode surface (Figure 1B−1E), which further protects electrode from solvent cointercalation. The 5 Ṁ LiTFSI-DOL electrolyte has a DOL-to-LiTFSI molar ratio of 2.86:1 (Table S1). Once concentration becomes higher than 7 Ṁ , white solids crystallize that are identified to be (DOL)1/LiTFSI crystalline solvate by single-crystal XRD analysis (Figures S13 and S14). At these extreme conditions, the activity of the free DOL molecules is very limited and thus solvent co-intercalation was greatly suppressed. Upon depolarization of the anode (removal of the negative charge), these layers dissolve back into electrolyte, i.e., the electrolyte regains its uniform composition. The conventional SEI layer formed on graphite with electrolytes such as 1 Ṁ LiPF6-EC/EMC (Figure 3A) is wellknown to contribute significantly to the cell impedance, especially at low temperatures, resulting in poor cell performance.38,39 This is exemplified by a significant decline in the cell discharge capacity when the temperature decreases from 25 to 0 °C (Figures 5 and S15). When the 5 Ṁ LiTFSI-DOL electrolyte was instead used with the same graphite anode, a superior rate performance at 0 °C was exhibited, comparable to the room-temperature performance. The Li+ cation staging reactions within the graphite were also clearly evident (Figure S15).31 The measured viscosity and ionic conductivity of 5 Ṁ LiTFSI-DOL electrolyte are 14.6 mPa·s and 3.35 mS cm−1, respectively, indicating enhanced bulk solution conductivity upon significant increase of viscosity in comparison to those of 1 Ṁ LiTFSI-DOL (1.3 mPa·s and 1.87 mS cm−1, respectively). In addition, negligible difference was observed when fastcharging the same graphite electrode at different C rates with (Figure 5) or without (Figure S15) formation cycles at low current density, excluding electrode wetting issue for this concentrated electrolyte with graphite electrodes. Because (1) the reversibly formed electrode coating (Figure 1B) does not

that the entire electrode is fully covered by those decomposition products. To investigate whether those decomposition products can function as effective SEI and protect the graphite, we cycled graphite electrode for three cycles in 5 Ṁ LiTFSIDOL at 0.1 C, attempting to obtain preformed “SEI”, and then switched the electrolyte to 0.5 Ṁ LiTFSI-DOL without cleaning the graphite electrode. As shown in Figure S8, the electrode fails very quickly if cycled in 0.5 Ṁ LiTFSI-DOL directly. Even with three preformation cycles in 5 Ṁ LiTFSIDOL, the electrode still shows very limited capacities for only 15 cycles (Figure S9), which is much worse than that cycled in 5 Ṁ LiTFSI-DOL (Figure S10). These results indicate that the traditional electrolyte decomposition products cannot form effective “SEI” to protect graphite, which agrees well with a recent study by Watanabe et al.21 This is also consistent to the TEM observation on cycled graphite (Figure 3H). If inorganicbased SEI (e.g., LiF) is formed on graphite, it can be identified by TEM or EELS due to its outstanding chemical stability and insolubility in the electrolyte solvent.15 In other words, other “protecting” layers originated from concentrated electrolytes must coexist on the graphite surface to prevent the lattice exfoliation. To gather additional information about the “clean” surface of the graphite cycled in the 5 Ṁ LiTFSI-DOL electrolyte, STEM−EELS, a microanalysis technique for trace amounts of elements, was also employed. The principal element detectable on the graphite surface was C, with a lesser amount of residual O (Figure 4) present. No F or N was identifiable. This indicates that there is probably no significant TFSI− anion decomposition on the surface of the graphite in the concentrated electrolytes. The above results indicate that electrolyte (salt and solvent) decomposition in 5 Ṁ LiTFSI-DOL is largely suppressed and cannot form effective SEI to protect graphite. The inconsistency between the observed clean graphite surface from the HRTEM and STEM−EELS and the electrolyte decomposition components from the XPS analysis necessitated the verification of the source of these “SEI” components. Figure S11 shows STEM−EDS on cycled graphite electrode; integrated small particles from either the PVDF binder or Super P carbon are always found stuck on a large graphite particle. Electrolyte decomposition could happen on the surfaces of either graphite, PVDF binder, or Super P particles. Therefore, an EDS analysis in areal mode was performed on these three separated positions corresponding to the different electrode components. Besides the major C component, N, O, and F were also detected in areas 1 and 2 with low counts, but these were nearly negligible at area 3. This suggests that the electrolyte may decompose mainly on the high-surface-area PVDF binder or Super P, resulting in the source of the XPS N, O, and F signals. In addition, smooth and clean graphite surfaces were always identified during a careful HRTEM scrutiny of the different particles from different graphite electrodes cycled in the same electrolyte. To explain the observed behaviors of graphite electrode in the highly concentrated electrolyte, the effect of electric polarization on the electrode−electrolyte interface must be considered. The application of the electric field generates an electric double-layer restructuring of the electrolyte constituents next to the electrode surface (Figure 1J). For dilute electrolytes, MD simulations indicate that much of this doublelayer consists of polarized uncoordinated solvent molecules, dominated by the more polarizable solvent for solvent mixtures 1607


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04766. Tables showing component compositions, AFM images, height changes in scan areas, lithiation and delithiation results, XPS analysis results, charge and discharge curves, cycling stability, electrode characteristics, MD simulation boxes, ion and solvent coordination, crystal packing diagrams, charging capacities, and crystallographic information. (PDF)

Figure 5. Charge (lithiation) capacity of thick graphite electrodes. G/ Li half-cells cycled with 1 Ṁ LiPF6-EC/EMC (black) and 5Ṁ LiTFSIDOL (red) electrolytes at 25 °C (circles) and 0 °C (squares). Cells were discharged (delithiation) at a 0.1 C rate for all tests.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

continuously accumulate on the graphite surface upon cycling (as a degradation product) and (2) both local concentration and transference number of Li+ cations are further increased near the interface within the compact coating, the flux of Li+ cations is much higher at the graphite−electrolyte interface, even at low temperatures. At 25 °C, an almost-identical performance was noted for cells with the 1 Ṁ LiPF6-EC/EMC and 5 Ṁ LiTFSI-DOL electrolytes, further suggesting that the increased viscosity of bulk concentrated electrolyte (14.6 mPa· s) does not negatively affect the performance of the cells (Figures 5, S15, and S16). Conclusions. In summary, a fundamentally new surface protection mechanism is reported for graphite anode with concentrated electrolytes. Upon the negative polarization of the anode, the anions and partially solvated cations precipitate into a Li+ cation conducting layers near the electrode surface, which effectively prevent the cointercalation of solvent molecules into graphite. Unlike for the conventional SEI layers generated when dilute (1 M) electrolytes are used, upon polarization of highly concentrated electrolytes, the spontaneous self-assembly of reversibly formed protective layers shield the electrode from the bulk electrolyte. This can also explain the significantly improved oxidation stability of solvent molecules at high potential,40 the improved reduction stability of solvent molecules at graphite and Li metal electrodes,36,41,42 and the absence of Al corrosion for LiTFSI-based electrolytes.37,43 The work presented here, as well as other recent intriguing reports about the characteristics of concentrated electrolytes,44−46 exemplifies the dramatic different interfacial properties associated with such concentrated solvent-salt mixtures relative to commonly used dilute electrolytes. The identification of this reversible surface layer may lead to wide application of highly concentrated electrolyte and thus provides exciting opportunities for electrolyte tailoring for diverse electrochemical technologies. For industry applications, close attention should be paid to the high viscosity and related electrode-wetting problems if directly using highly concentrated electrolytes for high-loading electrodes in practical cells. In addition, chemical compatibility between electrodes and concentrated electrolytes as well as salt cost should be considered on a case-by-case basis for specified applications. This work discusses a very different SEI formation mechanism discovered in the concentrated electrolytes and provides new insights to inspire more revolutionary solutions to address the interface stability challenges.

ORCID

Dongping Lu: 0000-0001-9597-8500 Ji-Guang Zhang: 0000-0001-7343-4609 Present Address #

Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, United States Author Contributions

The manuscript was prepared by D.L. and J.X. with contributions from other coauthors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Energy Efficiency and Renewable Energy (EERE) Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under contract nos. DEAC02-05CH11231 and DEAC02-98CH10886 for the Advanced Battery Materials Research (BMR) Program. The SEM, XPS, TEM, and STEM−EELS characterization was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). PNNL is operated by Battelle for the DOE under contract no. DE-AC05-76RLO1830.

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REFERENCES

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Formation of Reversible Solid Electrolyte Interface on Graphite

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