Letter pubs.acs.org/NanoLett
In Situ and Quantitative Characterization of Solid Electrolyte Interphases Arthur v. Cresce,† Selena M. Russell,† David R. Baker,† Karen J. Gaskell,‡ and Kang Xu*,† †
Electrochemistry Branch, Power and Energy Division, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States ‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: Despite its importance in dictating electrochemical reversibility and cell chemistry kinetics, the solid electrolyte interphase (SEI) on graphitic anodes remains the least understood component in Li ion batteries due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative tools to characterize it. This work summarizes our systematic approach to understand SEI live formation, via in situ electrochemical atomic force microscopy, which provides topographic images and quantitative information about the structure, hierarchy, and thickness of interphases as function of electrolyte composition. Complemented by an ex situ chemical analysis, a comprehensive and dynamic picture of interphase formation during the first lithiation cycle of the graphitic anode is described. This combined approach provides an in situ and quantitative tool to conduct quality control of formed interphases. KEYWORDS: Interphase, SEI, in situ characterization, HOPG, Li ion battery, electrolyte, AFM
I
spectroscopy (XPS), Fourier transform infrared or Raman spectroscopy (FTIR, FT-Raman), and scanning or transmission electron microscopy (SEM, TEM), which provide rich information about interphasial chemistry and morphology but inevitably introduce significant disturbances to the delicate SEI due to exposure to environment or high energy beams, as well as mechanical and chemical deterioration through sample preparation and handling. On the other hand, in situ investigations of SEI formation have been restricted by instrumentation, which are either too phenomenological in nature to reveal key characteristics, as in the case of electrochemical impedance spectroscopy,2 or are subject to excessive constraints in simulating an actual electrochemical cell, as possible with ellipsometry and neutron reflectometry. In situ analysis by atomic force microscopy (AFM) with electrochemical control on the target surface allows direct probing of SEI formation and structure, while maintaining conditions that closely simulate real-life Li ion devices with minimal destructive impact. The quantitative nature of AFM provides the capability to directly monitor changes in the mechanical strength of interphasial layers as a function of applied electrochemical potentials or electrolyte compositions/ additives. Therefore, AFM is not only an in situ imaging tool, but also potentially a standard diagnostic method to evaluate the quality of SEI thus formed.9 This paper represents part of a
n state-of-the-art lithium ion batteries, the graphitic anode typically operates at potentials (−3.03 V vs SHE) where nearly no organic electrolyte component could remain thermodynamically stable against electrochemical reduction.1 To enable reversible Li+ intercalation/deintercalation with graphitic anodes, electrolyte solvents are chosen to maximize lithium salt solubility and Li+ conductivity, but more importantly for their ability to stabilize the electrolyte−graphite interface through the formation of a new subcomponent.2 This new phase, consisting of sacrificial reduction products from the electrolyte solution, prevents continuous electrolyte decomposition and is known as the solid electrolyte interphase (SEI) due to its electrolyte-like nature.3 The physical and chemical properties of the SEI not only ensure the reversibility of Li+ intercalation at the graphitic anode but also strongly influence the kinetics of Li+ transport across the electrolyte−electrode interface.4 The former manifests as cycle/calendar life stability of Li ion batteries as well as resilience against chemical and thermal degradation; the latter dictates rate capability and power density. Given the importance of the SEI, numerous efforts are dedicated to understanding its chemistry, physical properties, and formation process. Many aspects of the interphase are still under debate, including the electrochemical potential at which it forms, whether it consists of a singular or multiple components from different mechanisms, or if these components are arranged in a homogeneous manner or distributed hierarchically in a multilayer structure.5−8 Most such studies rely on ex situ instrumentation, such as X-ray photoelectron © 2014 American Chemical Society
Received: December 3, 2013 Revised: January 22, 2014 Published: January 29, 2014 1405
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systematic approach using in situ AFM to understand the SEI live-formation process on graphitic surfaces. In addition, ex situ surface analysis by XPS was used as a complementary means to provide chemical insights. Highly oriented pyrolytic graphite (HOPG) served as a compromise between satisfying imaging capabilities and closely simulating real graphitic anode surfaces used in Li ion batteries, due to the smooth and chemically homogeneous surface of HOPG that displays graphite basal planes with edge sites.10 Instead of a typical electrolyte mixture, pure ethylene carbonate (EC) was used as the single solvent to avoid solvent evaporation effects on solution concentration and AFM probe stability during experiments. The absence of linear carbonates is not expected to significantly alter interphasial chemistry, as EC has been shown to be the main contributor to SEI formation.5,11 Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) was used in place of the industry-standard lithium hexafluorophosphate (LiPF6) to circumvent the degradation of AFM components by HF, with the understanding that the TFSI− anion might participate in SEI formation chemistry to a larger extent than PF6− does, leaving its chemical signature of Sand F-containing species at the anode surface. LiTFSI was added at a high concentration (1.32−1.5 M) to maintain the electrolyte in a liquid state at room temperature. All listed potentials are given vs lithium. Details of both the AFM and XPS experiments and additional data are given in the Supporting Information (SI) section. Figure 1 presents a typical live formation of interphases on HOPG as its potential decreased from open circuit voltage (OCV, ∼2.5 V) to lithiation potential (∼0.0 V) in the control electrolyte (1.5 M LiTFSI/EC). During this first cathodic scan, the HOPG surface was continuously imaged as a function of decreasing potential, moving left to right across Figure 1. A low contact force (Fc) was used so that disturbance to SEI formation was minimized. From OCV (V1) to 2.1 V (V2), the HOPG surface appeared to be interphase-free, with both basal planes and edge sites clearly exposed. In the voltammogram accompanying the AFM image in Figure 1, the V2 process likely arose from reduction of dissolved O2 contamination in the electrolyte.12−17 The surface morphology changed significantly below ∼1.5 V, where pronounced accumulation of interphasial species occurred along edge sites, as highlighted by the yellow color in regions between V2 and V3 of Figure 1 that outline the edge profiles. This process likely corresponded to the initial intercalation of solvated Li+ into the top graphene layers of HOPG, and the subsequent reduction of EC molecules coordinated to intercalated Li+.18,19 Around 1.0 V (V3), surface species engulfed the basal planes. V3 correlates to typical reduction of carbonate species such as EC.2 The implication of these observations is that the visible surface deposits, most likely SEI precursors, appeared on graphite surfaces well before Li+ intercalation occurred. Between 0.90 V (V3) and 0.60 V, the decrease in cathodic current indicates that the surface might be electronically passivated, and the substantial increase of cathodic current below 0.60 V marks the start of HOPG lithiation by unsolvated Li+, likely accompanied with further reduction of EC molecules or precursors formed above 0.95 V. The extensive presence of surface species below 0.6 V obscured the view of both basal planes and edge sites. None of the peaks above 0.5 V were observed in subsequent voltammetry cycles of the HOPG anode (Figure S1), consistent with the HOPG surface being covered with a functional SEI after the first lithiation cycle.
Figure 1. SEI live formation on a HOPG surface in 1.5 M LiTFSI dissolved in EC (control electrolyte) imaged (top) during electrochemical potential sweep (bottom) from OCV at V1 to 0 V at V4, at a rate of 5 mV/s. Vertical lines correspond to key features described in the text. The arrow indicates the slow scan direction (rotated 90° from the vertical imaging convention shown elsewhere in this paper); thus image lines to the right are at lower potential and later time than lines to the left. Image parameters: 3.5 × 4.5 μm2, Fc,HOPG = 1.2 nN, 922 × 717 pixels. The collected topographic image was differentiated to highlight height variations with strong contrast, in which the leading edges and rough areas appear more yellow and descending edges and smooth areas appear more blue. The TOC figure was generated from the data presented here.
The interphase formed on HOPG appeared heterogeneously structured, with little correlation to the underlying HOPG features, as illustrated in Figures 1 and 2. Streaking during imaging indicated that the probe interacted with the surface species; hence even with minimal contact force the probe disturbed the uppermost SEI. Figure 2c shows a magnification of the upper layer with streaking. Figure 2a presents the results of deliberate removal of the upper layer, in which an area (ca. 5 × 5 μm2) was repeatedly scanned until no further morphology changes were observed, revealing a lower robust SEI layer. The region surrounding the repeatedly scanned area in Figure 2a shows the nearly pristine interphase that was imaged once, while Figure 2c−d compares the upper and lower layer morphologies. The lower layer consisted of nanometer-sized, nodular features that decorate both basal planes and edge sites, termed nanotuffets hereafter. Thus, the SEI has a bilayer structure: an upper, pliable, and poorly adhered phase likely permeable to electrolyte, and a lower, hard, well-adhered phase near the HOPG surface. A heterogeneous multilayer SEI structure has been proposed by previous studies, including the preliminary hypothesis made by Peled et al.20 and observation by Lu et al.7 based on ex situ time-of-flight secondary-ion mass spectrometry (TOF-SIMS) analyses of SEI formed on a Cu substrate. The line profile in Figure 2a−b illustrates the bilayer structure of the SEI and appears to measure an upper layer thickness of 3−20 nm. Reports from previous AFM experi1406
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upper layer from the thickness measurement, either through collapse or loss of the delicate upper layer interphasial material through solvent removal or mechanical separation. To address the representative issue of localized measurement by line profile, and as the first step in attempting to differentiate the contribution to SEI thickness from upper/soft and lower/ hard layers, a statistical approach utilizing force-displacement spectroscopy (F-d) was employed on the basis of the method developed for interphases grown on a manganese oxide surface.9 Figure 3 provides as an example the experimental F-
Figure 3. Example force−distance approach curve of AFM probe toward a SEI formed in the control system. Inset: Magnification showing thickness determination. Same axes as the larger graph. Dotted lines show the probe behavior on an SEI-free HOPG surface in electrolyte at OCV, before SEI formation, i.e., behavior on a hard surface.
d approach curve recorded on a SEI in control electrolyte, in which repulsive interactions between the probe and the sample surface caused the cantilever to deflect near 0.25 μm. As the force increased (moving right to left in Figure 3) the probe pushed against the pliable upper layer, eventually penetrating it to reach the hard lower layer, indicated by the linear deflection below 0.12 μm. As noted earlier in the discussion of Figure 2, the probes used in this study cannot displace the lower layer SEI of nanotuffets; thus these F-d measurements only estimated the thickness of the upper layer section of SEI and not the thickness of the whole interphase. Therefore, the thickness of the upper layer interphase formed from control electrolyte ranges from 10 to 480 nm (102 ± 119 nm, N = 15), which is greater than previously published values based on line profile and spectroscopic methods. The wide variance of these values reflects the heterogeneous nature of the interphase and its uneven distribution across the graphite surface, and therefore these F-d values should be more reliable and representative than local or ex situ measurements provided by simple AFM line scans or XPS, TEM, and SIMS. Electrolyte additives have proven to be an economical and effective means to improve battery performance, which may significantly alter the interphasial chemistry on graphitic surfaces. Three compounds were selected that are known to improve battery performance by increasing first-cycle efficiency or reducing capacity fading during subsequent cycles by electrochemically reducing before EC, thus forming an SEI of a particular chemical signature. Vinylene carbonate (VC)31 and fluoroethylene carbonate (FEC)32,33 are well-described in the literature. Tris(hexafluoro-isopropyl)phosphate (HFiPP) was originally developed to stabilize electrolytes at high voltage
Figure 2. (a) Topographic AFM image at 0 V in the control electrolyte, after repeated scanning of the inner, ca. 5 × 5 μm2, region. (b) Line profile corresponding to the white line in (a) illustrating the height variation of the SEI. (c−d) Magnifications (black dotted square in a) showing the features of the upper (c) and lower (d) SEI layers. Image parameters: (a) 8.0 × 8.0 μm2, (c−d) 1.0 × 1.0 μm2, all topography and Fc ≈ 14−18 nN.
ments by Domi et al. also demonstrated the removal of soft material, exposing a rough surface, to evaluate SEI thickness.21 Line profiles from topographic AFM images like those in Figure 2b constituted the basis for thickness measurement of the interphase, which generated values ranging between 1 and 60 nm.21−26 However, there are several uncertainties associated with this method as line profiles provide neither the absolute height nor the absolute depth of the SEI because the probe constantly interacts with the viscoelastic upper layer and cannot penetrate the hard lower layer.17 In addition, the line profile in Figure 2b illustrates the varied height and roughness of both layers, increasing the uncertainty of thickness measurements derived from this method. On the other hand, SEI thickness values estimated from ex situ approaches ranged from 50 nm with TEM,27 2−90 nm found with XPS,10,28,29 and up to 230 nm determined by SIMS29 and were derived from assumed sputtering rates. Intrinsic inaccuracy occurred in all of these methodologies due to the heterogeneous nature of SEI,30 and it is very likely that ex situ techniques and the corresponding process of sample preparation excluded some, or all, of the 1407
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Figure 4. Representative AFM images comparing the upper and lower layers of the interphase formed on HOPG in electrolytes with or without additives at 0 V. Top: First image of a given region before removing the upper layer. Bottom: Image of the same region as in the top row after scanning multiple times until the lower layer was exposed, and no further topography changes were observed. Insets: Magnifications of the lower layers. Image parameters: 5.0 × 5.0 μm2, insets 1.0 × 1.0 μm2, all topography and Fc ≈ 14−18 nN, with excursions up to 30 nN.
(102 ± 119, N = 15) nm thick, while the additives generated an average upper layer thickness between 9 and 17 nm, with narrower standard deviations and smaller maximum thicknesses. The thickness values from all experiments are plotted in Figure 5; Table S2 details the information presented in this
cathode surfaces (LiNi0.5Mn1.5O4 or LiCoPO4) but has also been shown to form a protective SEI on graphite, even under severe exfoliating conditions using a propylene carbonate (PC)based electrolyte.34 Table S1 contains the structural and compositional details of each electrolyte system. Column (a) in Figure 4 shows the first and the last scan of a 5 × 5 μm2 region of SEI formed in the control electrolyte without additives, which is the same region as in Figure 2. While the upper layer initially appeared rough with several larger/taller formations, the lower layer looked smoother but still rougher than pristine HOPG. Each surface displays a diverse/permeable soft upper layer and a hard/impervious lower layer comprised of nanotuffets. Therefore, the electrolyte systems cannot be easily differentiated based on visual topography comparison alone, c.f. the representative images in Figure 4. Further analysis revealed slight morphological differences between the electrolyte systems in terms of the root-mean-square surface roughness (Rrms) and applied potential. The formed interphases increased the surface roughness by a factor of 5 ± 4 (N = 16) relative to the pristine HOPG surface between OCV and 0 V. Between OCV and 0.8 V, Rrms increased by a factor of 8 ± 6 in the control system, but only by 4 ± 3 with additives, compared to the typical HOPG surface. Detailed roughness data are presented in Figure S2 and Table S3. With or without additives, excepting FEC, and between 0.8 and 0 V, the upper layer Rrms did not significantly change, and the lower layer Rrms increased by a factor of 3 ± 1. With FEC, the lower layer Rrms changed dramatically, increasing by a factor of 8 ± 8 in the same voltage range. The variability in the data originated from the highly variable nature of the SEI. In addition, these additives substantially altered the upper layer interpahse thickness, compared to the control electrolyte, based on F-d measurements. All of the additives tested led to upper SEI layers an order of magnitude thinner than that of the control, implying that the presence of these additives participated in the formation of SEI and significantly interfered with the ECreduction chemistry. The control electrolyte upper layer was
Figure 5. SEI upper layer thickness derived from force−distance curves on HOPG with different additives at 0 V.
figure. In particular, FEC generated a conspicuously thinner upper layer, ranging from 2 to 24 nm (9 ± 4 nm, N = 22), while VC and HFiPP generated upper layer SEI heights of 17 ± 8 nm (N = 10) and 16 ± 7 nm (N = 7), respectively. Among these additives FEC is the outlier, as it was observed that the electrolyte with an FEC additive formed a thinner upper layer and a rougher lower layer than the layers generated in the other electrolyte systems. Because the lower SEI layer was impenetrable to the AFM probe, quantitative information about the thickness or mechanical properties of the hard lower SEI layer cannot be obtained from the F-d technique with the probe used in these studies. However, even without explicit thickness data, insight on this key SEI substructure can still be extracted by comparing the lower layers generated in control and additive-containing 1408
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Figure 6. C1s spectra of different HOPG anodes after SEI formation from different electrolyte additives without rinsing (top, i), after minimal rinsing (middle, ii), and extensive rinsing (bottom, iii) with DMC.
currently being performed, of which the significance in revealing additive decomposition chemistry merits a separate publication. Since AFM cannot provide chemical information, XPS was employed as a complementary tool to gain chemical insight into the observed SEI structures. SEI samples of each electrolyte origin were grown on HOPG surfaces, as in the AFM experiments. Using a progressively aggressive rinsing protocol, an approximate correlation between the resulting surfaces and the topographic images observed with AFM was established, with the unrinsed samples corresponding to those with an intact upper-layer and rinsed samples corresponding to those with a (partially or completely) stripped upper layer and an intact lower layer. DMC effectively removes residual electrolyte and polymer-like interphasial species from electrodes, but salt species, such as Li2CO3 or LiF, are negligibly soluble in DMC and would remain on the rinsed electrode surface. The solubility of semicarbonate species, in this case dilithium ethylene dicarbonate (LEDC, Table S1), lies somewhere between polymers and Li2CO3 or LiF.2,35−37 LEDC is the reduction product of EC via a single-electron pathway.38,39 The samples were transferred to the XPS chamber through an nitrogen-filled glovebag to prevent exposure to ambient atmosphere. Figure 6 shows the C1s spectra of HOPG surfaces cycled in various electrolytes and in different rinsed states. The interphase formed in control electrolyte and without rinsing gave rise to four peaks in the C1s spectrum (Figure 6a,i). A characteristic pair of peaks at ∼290.2 and ∼286.8 eV arose from
electrolytes. Compared to electrolytes with additives present, the SEI originating from the control electrolyte grew much thicker. Since the presence of additive in electrolyte systems led to much thinner upper layers, it is reasonable to assume that in the control electrolyte the hard lower layer must have been formed at a slower rate during the reductive decomposition of EC. These additives, FEC in particular, may very likely have accelerated the formation of an insulating lower layer before a thick (>30 nm) upper layer could accumulate through reduction by electrons from the anode. The SEI-limiting effect of additives correlates well with the increased first cycle Coulombic efficiency frequently reported with the use of additives in Li ion battery electrolytes. This inference suggests that the lower layer of the visualized interphase is responsible for the electrically insulating nature of the SEI and has a direct effect on the overall thickness of the SEI. The possibility also remains that additive participation in EC-based SEI chemistry could have led to products that were partially or completely soluble, resulting in a thinner upper layer. This would also rationalize the impedance reduction that has been often observed in the presence of additives. The dynamic equilibrium between the dissolved and the precipitated interphasial species and how additives affect it is still an under-explored area. The chemistry, morphology, thickness, and other mechanical properties of the lower layer should be the focus of future studies. Although observations of SEI structure were made at a few discrete voltage steps, an in-depth study of the effect of additives on the voltage evolution of the SEI structure are 1409
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additives. This conspicuous behavior of FEC is consistent with topography and F-d measurement results discussed earlier. After thorough rinsing, all samples were deprived of their organic upper layer, with comparable abundance of CO3 species at 290 eV and carbon at 284.8 eV as shown in their C1s spectra (cf. Figure 6iii, a and c−d). In conclusion, in situ AFM techniques were used in combination with ex situ XPS to study interphases on graphitic surfaces, allowing a comprehensive glimpse into the live formation of this elusive battery component as a function of electrode potential and electrolyte composition. AFM observations showed that interphasial species start to accumulate preferentially along graphite edge sites at ∼1.5 V, a process probably dictated by the intercalation of solvated Li+ and accompanied with early stage solvent reduction. As the potential became more negative, in situ AFM was able to detect that the interphase actually consists of two distinct parts: a soft/polymeric upper layer and a hard/salt-like lower layer, covering both edges and basal planes of graphite. Forcedisplacement spectroscopy enabled an accurate and statistical estimate of the thickness of the soft upper layer, which was measured to be as thick as 480 nm with significant variation in thickness over the sample surface. This thickness could be reduced through the use of available electrolyte additives, implying that additives might play a role in the earliest stages of SEI formation. Complementary ex situ XPS analysis of the two different SEI sublayers confirmed the organic nature of the soft upper layer and the salt-like nature of the hard under layer. This combined diagnostic approach could potentially serve as a universal quantitative tool to evaluate the properties and quality of interphases formed on different electrodes from diverse electrolytes. It is hoped that this technique, properly applied, will lead to a detailed and predictive understanding of the crucial electrolyte−electrode interaction in all advanced battery chemistries.
carbonyl (CO3) and ether (CO) species, respectively. The roughly 1:1 abundance ratio of CO3−CO, listed in Table S4, agrees well with the relative abundance of carbonyl and ethereal carbons in LEDC. The peak at ∼293 eV in Figure 6a,i was generated by CF3 groups from residual LiTFSI on the surface. F1s spectra (Figure S3) also revealed that CF3 species exist near the surface, indicating that the TFSI− anion may remain intact as residual salt. Also present in the F1s spectra was a slight contribution from F− species. The relative ratio of CF3−F decreased with the removal of the upper SEI layer by rinsing with DMC. In addition, the S2p spectra of well-rinsed samples (Figure S4) contain both sulfonyl and sulfide (S2−) species. Both F − and S 2− were likely generated by LiTFSI decomposition during cathodic cycling.40−43 After partial rinsing of identically cycled samples, the same four peaks in the C1s spectra were observed, although with different relative intensities, shown in Figure 6a,ii. The change in observed intensities was consistent with the thinning of upper interphase layer, along with the increased intensity of carbon (∼284.8 eV) and inorganic species. The CO3−CO abundance ratio in the control SEI rose to 2.9:1 with partial rinsing, corresponding to the appearance of Li2CO3 close to the anode surface with the possibility of residual semicarbonate LEDC. The increasing CO3−CO ratio was accompanied by a decreased presence of LiTSFI, also indicated by a CF3 species at 293 eV (Figure 6a,ii). XPS cannot distinguish between the carbonate contributions from LEDC or Li2CO3. In spite of this spectroscopic limitation, it was concluded that as the CO3−CO ratio increased above 1:1, the contribution from organic species, as indicated by CO, must have decreased. Upon examining both Figure 6 and Table S4, it can be seen that the abundance ratios between CO3 and CO for the unrinsed surfaces generated in the presence of VC and HFIPP additives, respectively, were also near 1:1, indicating an upper layer consisting mainly of LEDC as well. However, the surface generated in the presence of FEC significantly differed from the others with a much larger CO3−CO ratio, along with additional peaks arising from carboxylate and CF2 species. Partial rinsing removed significant amounts of the upper layer from all surfaces, as evidenced by the significant reduction of the CF3 peak (293 eV) and the corresponding increase of the carbon signal at 284.8 eV relative to the CO3 and CO species (Figure 6ii). Extensive rinsing resulted in the removal of the upper layer, with the surface likely appearing similar to the lower layers in Figure 4, bottom row. The XPS shown in Figure 6a,iii indicates that the sample surface lacked CF3 (LiTSFI) and had a significantly reduced presence of CO (286.8 eV), suggesting that the lower layer mainly consisted of inorganic species such as Li2CO3. O1s spectra (Figure S5) confirmed the major presence of Li2CO3 in the lower layer with a peak at ∼532.0 eV. Li2O (528.5 eV) species was only identified in one sample (Figure S5a). The relatively strong F− signals in the F1s spectra (Figure S3) demonstrated that LiF could coexist with Li2CO3 as a lower layer component. HFiPP generated the largest Li2CO3−LiF ratio (14.2), and FEC generated the smallest ratio (2.6). The relative concentration of CO32−, S2−, O2−, and F− species in the lower layer are given in Table S5. The interphase generated in the presence FEC differed in that the abundance of CO3 species was relatively low. The appearance of fluorinated species in the lower layer along with the closer ratio between CO and CO3 species suggests that FEC is more involved in forming the lower layer interphase than are other
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ASSOCIATED CONTENT
S Supporting Information *
Electrolyte composition, structure of additives, instrumentation, and methods used; cyclic voltammetry, thickness, and roughness data, and XPS F1s, S2p, and O1s spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
A.v.C. and D.R.B. originated the concept of the study, designed the framework test protocol, and collected part of the data used. S.M.R. contributed significant AFM expertise in data acquisition, analysis, and validation and advanced test methods, as well as figure preparation and writing assistance. K.J.G. performed XPS data acquisition and contributed her written analysis to this document. K.X. provided senior leadership, expert advice, and contributed significantly to the writing of this paper. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Kyle N. Grew (ARL) for assistance in composing a MATLAB routine to analyze AFM F-d data. 1410
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Partial financial support from Department of Energy Applied Battery Research (DOE-ABR) Program is appreciated. This research was supported in part 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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