Mechanical Properties and Chemical Reactivity of LixSiOy Thin FilmsYun Xu,† Caleb Stetson,‡,§ Kevin Wood,† Eric Sivonxay,∥ Chunsheng Jiang,† Glenn Teeter,†

Svitlana Pylypenko,§ Sang-Don Han,‡ Kristin A. Persson,∥,⊥ Anthony Burrell,‡

and Andriy Zakutayev*,†

†Materials Science Center and ‡Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado80401, United States§Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States∥Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States⊥Department of Materials Science & Engineering, University of California, Berkeley, California 94720-1760, United States

*S Supporting Information

ABSTRACT: Silicon (Si) is a commonly studied candidatematerial for next-generation anodes in Li-ion batteries. A nativeoxide SiO2 on Si is often inevitable. However, it is not clear if thislayer has a positive or negative effect on the battery performance.This understanding is complicated by the lack of knowledge aboutthe physical properties of the SiO2 lithiation products and by theconvolution of chemical and electrochemical effects during theanode lithiation process. In this study, LixSiOy thin films as modelmaterials for lithiated SiO2 were deposited by magnetron sputteringat ambient temperature, with the goal of (1) decoupling chemical reactivity from electrochemical reactivity and (2) evaluatingthe physical and electrochemical properties of LixSiOy. X-ray photoemission spectroscopy analysis of the deposited thin filmsdemonstrate that a composition close to previous experimental reports of lithiated native SiO2 can be achieved throughsputtering. Our density functional theory calculations also confirm that the possible phases formed by lithiating SiO2 are veryclose to the measured film compositions. Scanning probe microscopy measurements show that the mechanical properties of thefilm are strongly dependent on lithium concentration, with a ductile behavior at a higher Li content and a brittle behavior at alower Li content. The chemical reactivity of the thin films was investigated by measuring the AC impedance evolution,suggesting that LixSiOy continuously reacts with the electrolyte, in part because of the high electronic conductivity of the filmdetermined from solid-state impedance measurements. The electrochemical cycling data of the sputter-deposited LixSiOy/Sifilms also suggest that LixSiOy is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorablemechanical properties.

KEYWORDS: mechanical properties, chemical reactivity, solid electrolyte interphases, LixSiOy, lithium ion batteries

■ INTRODUCTION

Silicon (Si) is considered to be one of the leading candidates fornext-generation anode materials in Li-ion batteries, as it featuresa theoretical capacity more than 10 times higher than graphite.1

However, one of the major challenges of silicon is the largevolume change during the lithiation and delithiation processes.This volume change eventually leads to the breakdown of thesolid electrolyte interphase (SEI) and further reactions thatcause increased electrolyte consumption, higher interphaseresistance, and the ultimate failure of the cell.2

There have been extensive studies on the SEI formation onthe surface of silicon.3 Although various reports on the overallcomposition of the SEI are reaching some consensus, theunderstanding of its physical properties and chemical reactivityis still limited. Most work is focused on the SEI compositionthrough characterization by Fourier transform infraredreflectance and X-ray photoemission spectroscopy (XPS),4−9

whereas less research has been done to separately study the SEI

properties independent of the electrode.10 Theoretically, a goodSEI should be flexible or soft enough to accommodate the largevolumetric change of the silicon. In addition to the mechanicalproperties of the SEI, high ionic conductivity and low electronicconductivity are also the important factors determining if acertain phase is beneficial to electrode stability.It has been hypothesized that one of the most important SEI

components for Si anodes in Li-ion batteries is LixSiOy, whichoriginates from the lithiation of the native oxide on Si (SiO2).The role of SiO2 on a Si anode is controversial: on one hand, itconfines the volume expansion of Si, whereas on the other handit induces interfacial reactions that consume lithium in theelectrolyte. The lithiation of SiO2 is a complicated process, andthe products depend on the charge state of the battery.11−13

Received: July 2, 2018Accepted: October 15, 2018Published: October 15, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 38558−38564

© 2018 American Chemical Society 38558 DOI: 10.1021/acsami.8b10895ACS Appl. Mater. Interfaces 2018, 10, 38558−38564

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The general findings are that SiO2 can be reduced to thepossible products of Li2Si2O5, Li4SiO4, Li2O, Si, and LixSi.

10

Nevertheless, most of the recent model system studies arefocused on the fully oxidized lithium silicates such as Li4SiO4,Li2SiO3, and Li2Si2O5.

7,14−16

In this work, the mechanical and electrochemical properties,as well as the chemical reactivity of the composite LixSiOy film,are investigated. For this purpose, a special combinatorialLixSiOy composite film with a composition close to that of thelithiated SiO2 was synthesized by reactive sputtering, allowingto study its mechanical properties without any effect from theelectrolyte decomposition products. According to the XPSresults, both the lithium-rich and silicon-rich areas consist oflithium silicates, some LixSiy, and very little SiO2. The filmcompositions agree well with the phase diagram predicted bydensity functional theory (DFT) calculations. It is found thatthe composite LixSiOy film exhibits a relatively high electronicconductivity and low hardness, based on the scanning probemicroscopy (SPM) results and impedance measurements. Thecomputed bulk moduli show the same trend as that of theexperimental results, specifically that higher Li content leads tolower moduli. To evaluate the chemical reactivity of thesematerials, a multilayer thin-film LixSiOy on Si was measuredusing impedance spectroscopy in a coin cell followed byelectrochemical charge/discharge cycling. The results indicatethat the composite film is unable to fully passivate the Si surfacebecause of its high electronic conductivity and despite itsductile mechanical properties.

■ METHODSLixSiOy thin films were synthesized by radio frequency (RF)magnetron sputtering (13.56 MHz) using a lithium target (Lesker,99.9%, 2″ diameter) and a silicon target (Lesker, 99.999%, 3″diameter). The lithium target was cleaned with hexane (99.9%) beforeuse to remove the residual mineral oil, which protects the lithiumtarget from being oxidized during the shipment. Furthermore, theoxidized film on top of the lithium target was periodically removed by aDremel tool with a copper brush. During the deposition, 30 W RFpower was applied to both the lithium and silicon targets. Copper foiland EAGLE XG glass were used as substrates for the electrochemicalmeasurement and SPM measurement, respectively. To generateplasma, Ar (99.999% purity) was introduced at a constant pressureof 3 mTorr into a vacuum chamber with a 3 × 10−8 Torr base pressure.The film was deposited at room temperature; so, it is expected to beamorphous (also confirmed by X-ray diffraction). After the synthesis,the samples were removed from the chamber to a glovebox. Airlesstransfer in a vacuum vessel to all characterization instruments wasadopted because of the high reactivity of LixSiOy. As a reference forcomparison, thin films of Li-free Si and LixSiOy/Si were made using thesame method.The XPS measurements were performed using a glovebox-

integrated PHI 5600 XPS system. The glovebox conditions werebetter than <10 ppm moisture and O2. The base pressure for the PHI5600 XPS system was below 7 × 10−10 Torr. Photoelectrons weregenerated using monochromatic Al Kα X-ray excitation (1486.7 eV).The spectrometer binding energy (BE) scale was calibrated bymeasuring the valence-band and core-level spectra from sputter-cleaned Au, Ag, and Cu foils (EF = 0.00 eV, Au 4f7/2 = 83.96 eV, Ag3d5/2 = 368.26 eV, and Cu 2p3/2 = 932.62 eV).17 Ar+ sputtering(incident energy of 3 keV) was used for depth profiling. Curve fittingand data processing were performed using Igor Pro with a customprogram adapted from the literature.18

SPM techniques have been used to evaluate the mechanical andelectrical properties of the thin films. Nanoindentation experimentswere performed in an argon-filled glovebox with an atomic forcemicroscope (Veeco D5000 and NanoScope V) using a Bruker DDESP

diamond-coated Si probe in contact mode. Changing the deflectionsetpoints applied to the tip allows the force to be controlled based onthe deflection versus z-distance calibration curves. For theseexperiments, four sites with different lithium concentrations wereindented with varied indentation forces. Each indentation was held fora duration of 10 s. After unloading the indentation, tapping-modeatomic force microscopy (AFM) surface morphology images weretaken and the indentation depths were measured.

The electrochemical properties were investigated by making coincells and performing impedance spectroscopy measurement andgalvanostatic cycling. The thin-film samples with 14 mm diameterwere punched out from the silicon-rich region and lithium-rich region.The electrolyte was 1.2 M LiPF6 in a 1:1 (volume) ethylene carbonate(EC) and dimethyl carbonate mixture. Lithium metal was used as thecounter/reference electrode. The cells were first charged to 1.5 V (vsLi+/Li hereafter) and then discharged to 50 mV. The impedanceevolution was investigated by collecting the spectrums of a freshelectrode and an exposed electrode to an electrolyte for 24 h. The ACimpedance data of a coin cell was collected in a frequency range of 100kHz to 10 mHz. The solid-state AC impedance data of thin filmsdeposited on interdigitated platinum electrodes were collected in afrequency range of 1 MHz to 1 Hz.

Phase stability was examined computationally by constructing aphase diagram from DFT calculations. The DFT calculations wereperformed, using the Vienna Ab-initio Simulation Package,19,20 onknown crystalline phases in the Li−Si−O system, found on theMaterials Project.19 The calculations were made using the projectoraugmented wave method with the generalized gradient approximationPerdew−Burke−Ernzerhof framework.21,22 The structure optimizationcalculations allowed all the atomic positions and lattice parameters torelax. Plane-wave cutoff of 520 eV and a minimum reciprocal lattice k-point density of 64/Å3 were used as parameters in DFT calculations.The Python Materials Genomics package (pymatgen) was used toconstruct the phase diagram of the LixSiOy system.23,24 Additionally,the bulk moduli were computed by fitting the Rose−Vinet equation25of state to the calculated energies and volumes of a series of deformedstructures for the amorphous materials.

■ RESULTS AND DISCUSSIONDeposition and Characterization. High-throughput

experimental combinatorial methods have been applied inmaterials science for the discovery and optimization of variousfunctional materials.26,27 These methods are also well-suited toelectrochemical applications such as fuel cells and lithium-ionbatteries.28,29 In this work, a combinatorial approach wasapplied to screen the properties of lithium silicon compoundswith different composition. The combinatorial chambergeometry is depicted in Figure 1. The lithium target was 8

Figure 1. Chamber geometry used to deposit LixSiOy thin films.

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cm away, facing the substrate, and the silicon target was 12 cmat an angle with respect to the substrate. Because of thisgeometry, the deposited films have a higher Si/Li ratio in theregion closer to the silicon source and a lower Si/Li ratio in theregion further from the silicon source. From these experiments,we found that the target-to-substrate distance has a strong effecton the oxygen content in the film: the shorter the target−substrate distance, the lower is the oxygen content. This trendcan be explained by the decreased reaction of lithium plasmaparticles with the oxygen contaminants in the chamber resultingfrom H2O base pressure. Further, the oxygen content in the filmcan be minimized by increasing the deposition rate byincreasing the power applied to the Li and Si targets. However,the power applied to the Li target cannot be too high to avoid Limelting.In Figure 2, depth profiles are shown for the Li 1s, Si 2p, and

O 1s core-level spectra, for both the lithium-rich and silicon-

rich regions of the thin film. From the XPS depth profile,significant oxidation was observed at the surface, even thoughthe samples were protected from ambient atmosphere duringthe transfer from the deposition to the characterizationinstrument. Figure S1 shows the experimental spectra (andthe corresponding peak fits) obtained from the bulk of the film.The compositions obtained via peak-fitting these spectra arelisted in Table S1. As can be seen from the XPS fitting results inTable S1, for both the lithium-rich and silicon-rich sides of thefilm, lithium silicate and lithium silicide are the maincomponents in the film, with some contribution from SiO2.The average compositions for the lithium-rich and silicon-richareas are Li4.88Si2.85O2.52 and Li1.58Si1.31O1.95, respectively.Hence, the thin film is subsequently referred to as a LixSiOycomposite film. We note that the lithium-rich area has morelithium and more O compared to the silicon-rich area. Thehigher oxygen content in the lithium-rich area results from thehigher affinity of lithium to oxygen.The ternary phase diagram in Figure 3 shows the stability of

crystalline compositions within the Li−Si−O phase space aspredicted by DFT calculations. The nodes represent thecompositions of stable crystalline phases, whereas the lines andtriangles depict the two-phase and three-phase regions,respectively. The “phase diagram” of the amorphous structuresis much more difficult to calculate, but it may be expected to besimilar. Following the dashed line, the phase diagram predicts

the possible lithiation products of SiO2 to be lithium silicides,lithium silicates, and Li2O depending on the degree oflithiation. For example, fully lithiated SiO2 is predicted as amixture of Li21Si5, Li, and Li2O. The films studied in this workare amorphous according to X-ray diffraction and thus can beconsidered as the lithiation products of the amorphous SiO2layer on the Si surface. The observed lithium-rich and silicon-rich areas represent different lithiation stages of the SiO2 film.

Scanning Probe Microscopy. SPM-based nanoindenta-tion measurements were performed on LixSiOy thin films and areference Si sample. The surface morphology is depicted inFigure S2a, indicating the film roughness of 20 nm in an area0.5 μm by 0.5 μm. Figure S2b shows the typical residualindentation imprints of the film reflecting the film behaviorupon different force loads.The mechanical responses from the lithium-rich and silicon-

rich LixSiOy are shown in Figure 4a. In the lithium-rich area, alinear increase of indentation depth with the load was observedin a force range of 5−55 μN, indicating ductile behavior. Incontrast, the silicon-rich area of LixSiOy shows a brittlebehavior: here, the probe easily broke through the film andthe cracks propagated, independent of the load in the forcerange. Figure 4b shows a scan in AFM height mode of the sixindentation points on both the silicon-rich and lithium-richregions. The magnified images in both the height and phaseAFM modes (Figure S3) also show the difference between theductile and brittle behaviors.To visualize the transition better, the indentation depth is

plotted against the changing lithium concentration in thecombinatorial sample in Figure S4. An inflection point, wherethe indentation depth increases sharply at a low Li content, isdue to the cracking of the film in a brittle manner. Thecomputational results for the different amorphous structureconfigurations (see Figure 5) confirm this trend: the higherlithium concentration both in Si and SiO2 causes a decrease ofthe bulk moduli. We note that the film is composed of bothlithium silicide and lithium silicates, and hence it combines themechanical properties of both materials. Incidentally, the filmcomposition falls close to the critical transition point of Li/Si =1:1. The reference crystalline Si showed a very hard behavior,that is, the indentation depth is 0 nm.

Electrochemical Impedance and Cycling. To obtainfurther insights into the chemical reactivity of the LixSiOycomposite films, a 50 nm thick sample was assembled into acoin cell and investigated by impedance spectroscopy and bygalvanostatic charge and discharge measurements (Figure 6).Capacity is not normalized in this work, as all the films have thesame thickness and area. As shown in Figure 6a, a low amount

Figure 2. XPS depth profile analysis of the LixSiOy composite thin filmon copper foil for (a) Si-rich region and (b) Li-rich region. The toppanels show the BE depth profile of O 1s (green), Si 2p (red), and Li1s (blue). The bottom panel shows atomic percentage as a function ofsputter depth.

Figure 3. Phase diagram of the Li−Si−O2 system at 0 K. The dashedblue line indicates the predicted lithiation trajectory from SiO2.

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of lithium could be extracted from the film even up to 1.5 Vduring the first charge cycle. This high open-circuit voltage(OCV) is due to the lower electrochemical potential of lithiumin LixSiOy which has a part of lithium bonded with oxygen. Toinvestigate the reaction of the LixSiOy composite film with anelectrolyte, it was measured by AC impedance at the OCV, asshown in Figure 6b. When the LixSiOy composite film comesinto contact with the electrolyte, the charge-transfer resistanceincreases with time. In contrast, for the silicon electrode, theimpedance is not time-dependent at OCV.

The higher charge-transfer resistance of LixSiOy, as comparedto Si, is likely to originate from the resulting LixSiOy surface−electrolyte interface. The higher the lithium concentration inthe compound, the lower will be the potential versus lithium,which promotes reduction reactions with the electrolyte.During charging, as SiO2 is lithiated to LixSiOy, this increasedreactivity may lead to further consumption of the electrolyte ifLixSiOy is exposed and not fully covered with the otherchemically stable electrolyte decomposition products. Therehas been a work in the literature30 discussing the reductionmechanisms of EC on the lithium silicide surface, which identifydifferent mechanisms depending on the degree of lithiation. Ahigher concentration of lithium was found to be more reactivebecause of the explicit interaction of EC molecules with Li+ ionsat the surface. In addition, recent experimental work by ourgroup found that both lithium silicide and lithium silicate reactwith the electrolyte and form fluoride and carbonate specieswhich are commonly seen in the SEI components.31,32

Computational works also demonstrate that chemistry-depend-ent interaction happens on the surface of lithium silicide withthe electrolyte.33 We note that the increased lithiation of SiO2leads to an increased reactivity with the electrolyte andconsumes lithium inventory.Impedance of the as-prepared LixSiOy thin film on an

interdigitated electrode (without any electrolyte exposure) isshown in Figure S5. This thin film behaves like a pure resistor,which is mainly because of the existence of lithium silicide in

Figure 4. (a) Indentation depth at different sites with increasing indentation force aligned with each valley in the curve. (b) AFM images of indentedfilm at both silicon-rich and lithium-rich regions.

Figure 5. Calculated bulk moduli of lithiated silicon and lithiated SiO2plotted against the degree of lithiation of these amorphous structures.The error bars correspond to the different configurations of theamorphous structures.

Figure 6. (a) Voltage profile of lithium-rich LixSiOy composite film and silicon-rich LixSiOy composite film. (b) Impedance evolution of lithium-richLixSiOy composite and silicon-rich LixSiOy composite as well as Si film.

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the film. Total conductivity is dominated by electronicconductivity. The total conductivity of a LixSiOy thin film iscalculated as

L R A/( )σ = ×

where σ is the conductivity, L is the distance between the Ptdigits, and A is the total contacting area. The conductivity σ iscalculated to be 2.7 S/cm, which is too high to be a good SEIlayer.To correlate the measured LixSiOy properties with the

electrochemical cycling performance of an electrode withLixSiOy at the surface, a double-layer sample was deposited withLixSiOy on top of a silicon thin film and compared to a Si film.The thickness of LixSiOy was set to 10 nm to emulate the nativeoxide thickness, and the thickness of the silicon thin film was setto 50 nm, as shown in the inset in Figure 7a. As shown in Figure7a, during the first cycle discharge profile, LixSiOy/Si exhibiteda lower OCV of only 1.5 V (compared to 2.5 V for Si) becauseof the existing lithium on the silicon surface. The difference ofthe CV profiles above 0.25 V (before Si starts lithiating) resultsfrom the surface reaction between LixSiOy and the electrolyte.Both the low potential of LixSiOy and its reaction productsmake the electrochemical reduction different from that in thesilicon case. The lower potential range profiles for both films arevery similar, as the same thickness of the silicon film wasdeposited. The cycling of LixSiOy/Si showed a lower coulombicefficiency and less stable performance as compared to that ofthe bare silicon (see Figure 7b). We note that the coulombic ef-ficiency of silicon is higher than that of the LixSiOy-coated Si,indicating that the LixSiOy film exhibits a greater reactivity withthe electrolyte. We hypothesize that the LixSiOy composite filmcontinuously reacts with the electrolyte, preventing thestabilization of SEI on the silicon electrode. These resultssuggest that SiO2 does not help stabilize the surface of the Sielectrode, as the lithiation products are not stable upon cycling.Previously, there has been a study34 on the effect of SiO2 withvarious thicknesses from 2, 7, to 10 nm on the anodeperformance of silicon nanowires. Only the 7 nm SiO2-coatedSi showed an improved performance compared to Si. This is acomplementary result from both mechanical property andreactivity. More studies of the reaction product betweenLixSiOy and the electrolyte are in progress to clarify the role ofLixSiOy in the functionality and SEI stabilization of the Sianode.

■ CONCLUSIONS

In summary, the mechanical and electrochemical properties, aswell as the chemical reactivity, of the LixSiOy composite thinfilm with the Li-to-Si ratio between 1.7 and 1.1 wereinvestigated. These LixSiOy films with Li-rich and Si-richcompositions were prepared by combinatorial sputtering as amodel system to study the initial products of lithiation of nativeoxide SiO2 layer on a Si anode, relevant for next-generation Li-ion batteries. The first-principles computational results showthat the behavior of both lithiated silicon and lithiated SiO2transition from brittle to ductile as lithiation happens. The as-synthesized composite LixSiOy film, which is a mixture oflithium silicides and lithium silicates, shows an equivalentbrittle-to-ductile transition. The intermediate product oflithiated SiO2, which contains lithium silicates and lithiumsilicates, is highly reactive with the electrolyte because of thelow potential resulting from the high lithium content. Thesecomposite LixSiOy with Li/Si = 1.7 and 1.1 showed a lowelectronic resistance and did not facilitate the passivation andstabilization of the Si films, despite their low resistance tofracture. Thus, a favorable ductile mechanical property is foundto be a desirable but insufficient criterion for stable SEIformation; low chemical reactivity and low electronicconductivity are also important for stable electrodes.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b10895.

XPS spectrum, AFM images, and impedance data (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: Andriy.Zakutayev@nrel.gov.

ORCIDYun Xu: 0000-0002-6680-519XChunsheng Jiang: 0000-0003-0230-7500Svitlana Pylypenko: 0000-0001-7982-734XSang-Don Han: 0000-0002-2931-659XKristin A. Persson: 0000-0003-2495-5509Andriy Zakutayev: 0000-0002-3054-5525

Figure 7. (a) Charge and discharge profiles of Si and LixSiOy/Si, with the inset picture showing the schematic of a double-layer thin film. (b) Cycleperformance and coulombic efficiency of double-layer thin film and pure silicon.

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Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was authored by the National Renewable EnergyLaboratory, operated by Alliance for Sustainable Energy, LLC,for the U.S. Department of Energy (DOE) under contract no.DE-AC36-08GO28308. Funding provided by Vehicle Tech-nologies Office, Hybrid Electric Systems Program. DavidHowell (Manager), Battery R&D, Brian Cunningham andPeter Faguy (Technology Managers), at the U.S. Departmentof Energy, Office of Energy Efficiency and Renewable Energy,are gratefully acknowledged. A portion of the research wasperformed using computational resources sponsored by theDepartment of Energy’s Office of Energy Efficiency andRenewable Energy and located at the National RenewableEnergy Laboratory. Computational resources of the NationalEnergy Research Scientific Computing Center (NERSC), aU.S. Department of Energy Office of Science User Facilityoperated under contract no. DE-AC02-05CH11231, aregratefully acknowledged. The views expressed in the articledo not necessarily represent the views of the DOE or the U.S.Government.

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