Letter pubs.acs.org/NanoLett
Atomic Force Microscopy Studies on Molybdenum Disulfide Flakes as Sodium-Ion Anodes Steven D. Lacey,† Jiayu Wan,† Arthur von Wald Cresce,‡ Selena M. Russell,‡ Jiaqi Dai,† Wenzhong Bao,† Kang Xu,‡ and Liangbing Hu*,† †
Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States Electrochemistry Branch, Power and Energy Division, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States
‡
ABSTRACT: A microscale battery comprised of mechanically exfoliated molybdenum disulfide (MoS2) flakes with copper connections and a sodium metal reference was created and investigated as an intercalation model using in situ atomic force microscopy in a dry room environment. While an ethylene carbonate-based electrolyte with a low vapor pressure allowed topographical observations in an open cell configuration, the planar microbattery was used to conduct in situ measurements to understand the structural changes and the concomitant solid electrolyte interphase (SEI) formation at the nanoscale. Topographical observations demonstrated permanent wrinkling behavior of MoS2 electrodes upon sodiation at 0.4 V. SEI formation occurred quickly on both flake edges and planes at voltages before sodium intercalation. Force spectroscopy measurements provided quantitative data on the SEI thickness for MoS2 electrodes in sodium-ion batteries for the first time. KEYWORDS: In situ AFM, MoS2, wrinkling, SEI, sodium-ion battery, microbattery
S
MoS2 as a sodium intercalation host for room temperature NIBs have been conducted. Park et al. recently reported the electrochemical properties of MoS2 as an intercalation host for room temperature NIBs and suggested a two-step reaction mechanism where distortion of the MoS2 structure occurs in the low plateau region due to induced microstrains from insertion of 1.1 Na ions.15 However, no topographical observations of the passivation layer or the surface of sodiated MoS2 electrodes have yet been made. One key unknown in the Na/MoS2 system is whether the MoS2 surface develops a solid electrolyte interphase (SEI) in response to cathodic polarization during the sodiation process. The SEI is a thin layer of composite materials formed on the anode due to electrolyte decomposition at low cell potentials. Electrolyte solvents and salts are reductively decomposed by the anode to form the SEI during the initial charging phase, which consumes electrolyte and alkali metal ions during its formation. The formation of an SEI therefore results in an irreversible capacity loss during the initial charge cycle. The SEI layer plays an important role in battery performance by stabilizing the reactive interface between the active electrode and the liquid electrolyte, and its performance directly affects battery properties such as columbic efficiency, reversibility, and
odium-ion batteries (NIBs) have been increasingly investigated as potential low-cost energy storage alternatives due to the abundance of sodium in the Earth’s crust compared with lithium. A range of electrode anode materials have been investigated, including various types of carbon,1−7 tin,8,9 and red phosphorus.10 While many concepts can be transplanted from lithium-ion studies in sodium-ion research, the electrochemistry of sodium-ion turns out to be quite different in many aspects, which require unique design of electrode structures, new electrolyte compositions, and so forth. Compared with the lithium ion, the sodium ion is larger by approximately 39% in radius, which can lead to more severe electrode volume changes and slower charge/discharge kinetics.8,11,12 Molybdenum disulfide (MoS2) is one of the most stable and versatile members of the metal sulfides and is a particularly attractive high capacity intercalation host for sodium due to its dichalcogenide structure and large interlayer spacing.13−17 MoS2 is composed of layers of molybdenum atoms coordinated to six sulfur atoms and has an interlayer spacing of approximately 6.2 Å compared to the 3.35 Å spacing of graphite, a structural analog which is the most common commercial lithium-ion battery (LIB) anode.18−24 As an anode material for LIBs, MoS2 has a high reversible capacity and good cycling stability.18−23 Because of the laminar nature of MoS2 bonded by weak van der Waals forces and its inherent interlayer spacing, this metal sulfide can accommodate other metal cations in addition to lithium.25−27 At present, few studies of © 2014 American Chemical Society
Received: October 8, 2014 Revised: December 27, 2014 Published: December 30, 2014 1018
DOI: 10.1021/nl503871s Nano Lett. 2015, 15, 1018−1024
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battery was to allow in situ/in operando observation of the MoS2 surface during sodiation. The complete electrochemical system included the microscale MoS2 battery with a thin film Cu current collector and a Na metal reference electrode, all submerged in an electrolyte composed of 1.75 M NaClO4 in ethylene carbonate (EC). A photograph of the liquid electrochemical cell is shown in Figure 2b. EC is a universal solvent in commercial lithium ion electrolyte formulations. As a single solvent, it has been shown to be the main contributor to SEI formation because EC is a favored solvation member for Li+.33 Neat EC-based electrolytes with graphite anodes are known to closely simulate what occurs in actual LIBs with good cycling performance, which is typically attributed to the formation of stable anode SEI.34 EC has also been used to minimize solvent evaporation and to provide AFM probe stability.33 MoS2 is a 2D material analogous to graphite which suggests that a neat EC-based electrolyte will have similar effects regarding the formation of stable anode SEI for the Na/MoS2 microbattery due to the similar intercalation potentials of Na and Li. Hence, an SEI layer was expected to form on MoS2 from the reductive composition of the EC-based electrolyte. Figure 3 shows the topographical differences between pristine MoS2 and sodiated MoS2 at a cutoff potential of 0.4 V. The former image shows the characteristic topography of MoS2 flakes (Figure 3a). Multiple flake edges were observed without any noticeable distortions on the flake plateaus. Line profiles across the step edges (solid line) as well as along the plateau of the pristine MoS2 (dotted line) are shown in Figure 3b,c. Note that there is little variation in height along the plateau surface before sodium insertion/extraction. The topography of the MoS2 flakes after sodiation is more variable (Figure 3d), where obvious wrinkling is observed for the MoS2 flakes (Figure 3d). Sodiation as well as lithiation causes 2D materials such as MoS2 and graphite to volumetrically expand due to the accommodation of alkali metal ions. This intercalation process introduces strain on the material. To compensate for these insertion/extraction forces, the surface of the material relaxes by wrinkling. Zhu et al. once described a similar structural wrinkling phenomenon due to the mechanical stresses associated with the large volume change of sodiated tincoated wood fibers.8 Park et al. also reported on the expansion of the interlayer along the c-axis of MoS2 due to sodium ion insertion, which causes distortion of the material by microstrains.15 To quantitatively characterize the structural changes, line profiles were again taken across the flake edges and along the plateau for the sodiated MoS2 flakes (Figure 3e,f). Figure 3f shows the peaks and valleys along the plateau. Height and full width at half-maximum (FWHM) values were extrapolated from this line profile to characterize the observed wrinkles. The height of these wrinkles varied between 7 to 36 nm with an average height of 22.7 ± 8.6 nm. The average FWHM for the wrinkles were 345.4 ± 81.4 nm. The surface roughness of the sodiated and pristine MoS2 flakes was measured using WSxM SPM data analysis software.35 The average surface roughness values for pristine and sodiated MoS2 were 84.3 ± 7.6 nm and 138.1 ± 27.2 nm, respectively. The significant difference in surface roughness cannot be solely attributed to the wrinkling effect. Increased surface roughness was also attributed to SEI formation on the MoS2 electrode during the initial sodiation process. By topographical observation, the wrinkling effect seems to continue along all of the MoS2 flake plateaus, not just
rate capability. However, investigating this fragile and reactive layer in its natural environment is difficult. In situ characterization tools can provide enhanced knowledge of individual electrode particles at the nanoscale, especially the structural evolution of electrode materials during the process of intercalation/deintercalation and passivation, which can lead to crucial information related to battery performance and failure mechanisms. In situ transmission electron microscopy (TEM) has been widely used to investigate battery anodes, including silicon nanowires28 as well as tin nanowires and nanoparticles.29 While TEM is a powerful tool that can accurately describe changes in electrode materials, electrolytes used in electron microscopy differ significantly from those currently used in practical batteries and may not accurately represent electrode−electrolyte interface behavior. Solid-state nanobatteries with silicon nanowires have been fabricated for fundamental electrode investigations recently.30 In situ investigations including X-ray, Raman, neutron scattering, and scanning probe microscopy can also be used to investigate battery electrode surfaces.31,32 In this study, a planar microscale battery with an open cell configuration was designed to investigate MoS2 for NIB anodes. MoS2 flakes were used as anodes, while Na metal served as a reference electrode. The planar microscale battery was comprised of mechanically exfoliated two-dimensional (2D) MoS2 flakes and copper (Cu) connections and was constructed and analyzed in a dry room environment. An atomic force microscope (AFM) coupled with a liquid electrochemical cell was used to image the MoS2 electrode surface in real time to observe the live insertion/extraction of sodium ions and the SEI formation during cycling. SEI thickness was also evaluated using quantitative force spectroscopy measurements. AFM analysis of battery materials in a planar configuration under electrolyte in a dry room environment can be used to investigate a range of other electrode−electrolyte systems. Figure 1 shows a schematic of the in situ AFM setup with the planar microscale battery. The microbattery consists of
Figure 1. A schematic of the planar microscale battery analyzed by AFM in a liquid electrolyte and dry room environment for in situ measurements.
mechanically exfoliated MoS2 flakes pressed onto a glass slide. The 2D flakes were cleaved under an optical microscope to the desired thickness of tens to hundreds of nanometers (Figure 2c). Thermally evaporated Cu was deposited strategically with shadow masks to create electrical connections with the MoS2 electrode (Figure 2a,c). The purpose of this planar microscale 1019
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Figure 2. (a) A schematic of the sample fabrication process for MoS2 flakes with Cu connections, (b) a digital image of the planar microscale battery within a liquid electrochemical cell which is used to conduct in situ measurements, and (c) an optical image of MoS2 flakes partially coated by Cu.
Figure 3. In situ AFM images and line profiles (a−c) before and (d−f) after passivation for the Na/MoS2 planar microbattery for the OCV to 0.4 V case. The scans were in contact mode as the tip moved downward. Line profiles were taken across both the pristine (b) flake edges, solid line in (a), and (c) plateau, dashed line in (a), as well as the sodiated MoS2 (e) flake edges, solid line in (d), and (f) plateau, dashed line in (d).
xNa + MoS2 → NaxMoS2 (x ≤ 0.5, no change of MoS2) and (2) 0.51Na + NaxMoS2 → NayMoS2 (0.5 ≤ y < 1.1, distortion of MoS2).15 The new topographical observations of the MoS2 planar microbattery reinforced this suggested charge/discharge mechanism due to the permanent wrinkling of the MoS2 flake surfaces upon sodiation at low voltage (Figure 3d). In order to differentiate the contributions to surface features from SEI formation and MoS2 phase changes due to sodiation, in situ experiments were repeated with the cutoff voltage raised to 1.0 V, where sodiation has not occurred. Figure 4a−c exhibit the pristine MoS2 flakes before electrochemical cycling and the
along one plateau in particular. Even after multiple AFM scans, the observed wrinkles remain, which suggests that these structural features are not attributed to the passivation layer. The distortion of sodiated MoS2 is confirmed by in situ AFM topographical observations and quantitative measurements of the wrinkles. After the planar microbattery was cycled, the distorted MoS2 structure does not completely recover to the previous pristine condition. The same conclusions drawn with the present in situ AFM study and that by Park et al. from irreversible structural changes suggest the following two-step charge/discharge reaction mechanism of the Na/MoS2 cell: (1) 1020
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Figure 4. In situ AFM images and line profiles (a−c) before and (d−f) after passivation for the Na/MoS2 planar microbattery with 1.0 V cutoff. The tip scanning direction is upward from the bottom of the image in both panel a and panel d with the scans done in contact mode. Line profiles were taken across both the pristine (b) flake edges, solid line in (a), and (c) plateau, dashed line in (a), as well as the passivated MoS2 (e) flake edges, solid line in (d), and (f) plateau, dashed line in (d).
Figure 5. (a) Zoomed out 15 μm × 15 μm topographical in situ AFM image of the passivation layer of a sodiated MoS2 electrode. A 10 μm × 10 μm area was repeatedly scanned in contact mode to remove the passivation layer. (b) A line profile across the repeatedly scanned area illustrates the height variation of the SEI.
corresponding line profiles of the flake edges and plateau. Figure 4d shows the live formation of an SEI on MoS2 by the EC-NaClO4 electrolyte during this process, which is rather similar to that observed on HOPG in lithium-containing systems,33 as well as lithiated MoS2. A coating of SEI became apparent to the upward-scanning AFM tip as the surface potential became increasingly negative with SEI material appearing around 1.5 V toward the bottom of the image. In Figure 4c,f, the absence of wrinkling is supported by the observation of rough features with very short length scales on the order of 10−20 nm. Therefore, the roughness in Figure 4e,f is considered as contribution only from SEI material coating the MoS2 surface. As comparison, no wrinkling occurred at the 1.0 V cutoff for the lithium system in our experiment. However, SEI material appeared around 1.8 V as the AFM tip scanned in the downward direction. Previous studies by others have shown that nonreversible reactions occur below 1.1 V versus Li/Li+
due to the formation of polysulfides.18 AFM scans at potentials below 1.1 V showed significant alteration of the MoS2 surface likely due to the formation of both SEI and MoS 2 decomposition products. Under the AFM’s optical microscope, the flake surface appeared black and the topographical images did not reveal any structural wrinkling, unlike the wrinkling observed in the sodium system. Topographical AFM scans in contact mode confirm the presence of a passivation layer on the MoS2 substrate (Figure 5). Figure 5a shows that soft SEI material can be removed by the AFM tip. The same behavior was observed for the lithium system. However, the silicon nitride tip used to probe the topography of MoS2 was not able to remove the hard portion of the SEI material closer to the sodiated (or lithiated) electrode surface. This situation is analogous to numerous studies of lithium electrolytes on graphite anodes, where a silicon nitride tip on a low-modulus (0.12 N/m) cantilever could not 1021
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In summary, in situ AFM was used to analyze MoS2 as a sodium electrode for the first time. A planar microscale battery was designed to conduct in situ measurements and understand the topographical changes of MoS2 during cycling. AFM topographical scans and surface roughness measurements demonstrated permanent structural wrinkling of the sodiated MoS2 flakes at low voltage where the wrinkles had an average height of 22.7 ± 8.6 nm and FWHM of 345.4 ± 81.4 nm. On the other hand, SEI on the MoS2 electrodes appeared around 1.5 V before the wrinkling effect was induced by sodiation. In the MoS2-lithium system, a potential of 1.8 V induced SEI formation and no structural wrinkling was observed at any potential between 3 and 0.4 V. Force spectroscopy measurements yielded an average SEI thickness for the sodiated MoS2 electrode of 20.4 nm ± 10.9 nm and the lithiated MoS2 electrode of 61.8 nm ± 29.8 nm. The in situ AFM technique and planar microbattery design reported in this study can be applied to investigate SEI properties and formation for numerous electrode−electrolyte systems for alkali metal ion batteries. Additional fundamental science studies on SEI properties could yield information regarding how to control the interfacial reactions and improve rechargeable battery performance by designing both electrode and electrolyte materials in present and future electrochemical energy systems.
penetrate or remove the lower passivation layer with hard/ saltlike character.33,36,37 As an analog to graphite, similar SEI composition was expected for sodiated MoS2 due to the comparable intercalation potentials of Li and Na as well as the use of EC as the single solvent in our electrolyte mixture. A topographical scan of sodiated MoS2 with deliberate removal of the soft portion of the SEI was done by taking multiple scans of a 10 μm × 10 μm area followed by a zoomed out 15 μm × 15 μm scan to reveal the cleared area (Figure 5a). Figure 5b is a line profile across the repeatedly scanned area, which delineates the AFM cleared area. There is a height variation of approximately 20 nm between the 10 μm × 10 μm and the 15 μm × 15 μm scanned areas, although this variation does not describe the thickness of the hard SEI38 due to tip−sample interactions as described previously.33,36,37 Force spectroscopy provides more accurate quantitative measurements on the SEI thickness for sodiated MoS2. The measurements were taken on nonscanned MoS2 flake areas to avoid measuring the thickness of AFM-altered surfaces. Force− displacement curves were taken using the AFM and extrapolated using a custom MATLAB script to obtain SEI thickness data with accompanying uncertainty. Figure 6 shows a
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METHODS Preparation of MoS2 Planar Microscale Battery. Mechanically exfoliated MoS2 flakes were pressed onto a clean glass slide using the modified scotch tape method. After deposition, the glass slide with MoS2 thin film (thickness ∼10 μm from a bulk MoS2 crystal) is transferred to a microscope (reflection mode) and diminished by blade to the desired thickness of tens to hundreds of nanometers. A shadow mask technique was applied for Cu electrode (thickness of 50 nm) deposition, where the masks were applied to the MoS2 flakes under a microscope. In Situ AFM. AFM experiments were conducted in a dry room (T ∼ 20 °C, Tdew ∼ −40 °C) to avoid excessive water dissolution into the electrolytes and slow the oxidation of the reference electrode. A liquid electrochemical cell was utilized for in situ AFM experiments. A sodium metal reference electrode was cut into a narrow ribbon, attached to a thin wire, and placed around the circumference of the electrochemical chamber to allow the AFM probe to reach the sample surface without hitting the reference electrode. The dry sample was then placed in the AFM. Silicon nitride (Si3N4; DNP-10; Bruker, Camarillo, CA) probes with a nominal tip radius of 20 nm were used and calibrated for cantilever stiffness on the glass portions of the dry sample prior to addition of the liquid electrolyte. After calibration, the electrolyte (1.75 M NaClO4 in EC) was added dropwise to the electrochemical cell until a multimeter registered a stable OCV. The MoS2 electrode was controlled in potentiostat mode with a rate of 5 mV/s from OCV to the cutoff voltage (i.e., 0.4 and 1.0 V). AFM scanning was performed during the initial potentostatic scan in order to image the nucleation and formation of the SEI. The MoS2 was then held at the cutoff voltage while additional AFM scans were performed. Force spectroscopy was performed by the AFM on surfaces that had not been scanned for topography. The AFM tip significantly interacts with the surface and has been shown to remove soft SEI material during the process of a topographical scan. Therefore, only pristine surfaces were used for force spectroscopy measurements, avoiding the
Figure 6. Force−displacement curve with MATLAB extrapolated line of best fit (shown in red).
typical force−displacement curve along with the approximation of a hard surface with no SEI coverage. The tip begins to deflect upon contact with soft SEI, and the force increases nonlinearly until the tip interacts with material stiff enough to deflect the cantilever. Thickness is calculated as the difference between the calculated hard interaction distance and the distance at which the tip registers with the top of the SEI. Multiple force curves were obtained from various undisturbed portions on the sodiated (and lithiated) MoS2 samples. The SEI thickness ranged between 8 and 54 nm corresponding to an average thickness of 20.4 ± 10.9 nm, significantly thinner than the 102 ± 119 nm thickness observed by Cresce et al. for lithiated graphite.33 For the MoS2−lithium system, the SEI thickness ranged between 22 and 154 nm with an average thickness of 61.8 nm ± 29.8 nm based on our control experiment. The combination of the MoS2 substrate and NaClO4 electrolyte could potentially be less reducing toward EC, leading to a relatively thinner soft SEI compared to graphite. As an alternate hypothesis, the combination of EC, Na, and the MoS2 electrode could form an electrically insulating layer more readily than the EC/Li/graphite system, limiting the growth of additional SEI material and resulting in a thinner observed SEI. Further investigation is needed to accept or reject either hypothesis. 1022
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potential for measuring surfaces altered by topographical AFM scans. Electrolyte. Electrolyte solvents were chosen based on their ability to solvate NaClO4 as well as their SEI forming ability. The loosely bound sheet structure of MoS2 is isostructural with graphite, which made EC a logical solvent choice. A salt concentration of 1.75 M of NaClO4 allowed the EC-based electrolytes to remain liquid at the test temperature of 20 °C in the dry room. NaClO4 salts were selected to avoid HF damage to the 316 stainless steel sample stage. Similarly, 1.75 M LiClO4 in EC was used in the MoS2-lithium system, which also remained liquid at 20 °C.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the support from NSF CBET-1335979/ 1335944. Partial financial support from Department of Energy Applied Battery Research (DOE-ABR) Program is appreciated. S.R. was supported by an appointment to the U.S. Army Research Laboratory Postdoctoral Fellowship Program administered by the Oak Ridge Associated Universities through a cooperative agreement with the U.S. Army Research Laboratory.
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