Volume Expansion during Lithiation of Amorphous Silicon Thin Film

Article pubs.acs.org/JPCC

Volume Expansion during Lithiation of Amorphous Silicon Thin Film Electrodes Studied by In-Operando Neutron Reflectometry B. Jerliu,† E. Hüger,† L. Dörrer,† B.-K. Seidlhofer,‡ R. Steitz,‡ V. Oberst,§ U. Geckle,§ M. Bruns,§ and H. Schmidt*,† †

Technische Universität Clausthal, Institut für Metallurgie, AG Mikrokinetik, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany ‡ Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany § Karlsruhe Institute of Technology, Institute for Applied Materials (IAM) and Karlsruhe Nano Micro Facility (KNMF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ABSTRACT: Amorphous silicon is a promising high-capacity anode material for the next generation of lithium-ion batteries. However, the enormous volume expansion of the active material during lithiation up to 400% (V/V0) is held responsible for capacity fading during cycling. In this study we measured continuously the volume modifications taking place during galvanostatic lithiation of amorphous silicon thin film electrodes by in-operando neutron reflectometry experiments. The results indicate (after initial effects) a linear increase in volume as a function of lithiation time and lithium content independent of current density and initial film thickness. The experimental results are in agreement with recent atomistic calculations.

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INTRODUCTION Rechargeable lithium-ion batteries are widely developed and used as power sources for portable electronic devices (smart phones, laptops, tablets) and will be essential in the field of automotive transportation (HEVs, PHVs, EVs).1−3 Especially for the latter applications, improvements in energy density and thus driving range/battery weight, power density, cycle life, and costs are required.4 One way to achieve these goals is the development of new electrode materials with a higher specific capacity than traditional materials, while the necessary cycle life is conserved.5 An interesting high capacity anode material for future applications is amorphous silicon with its enormous theoretical specific capacity of about 4200 mAh/g.3,5 During electrochemical lithiation at room temperature, lithium forms an amorphous alloy according to the reaction Si + xLi+ + xe− → LixSi (x ≤ 4.2), which involves bond breaking between silicon atoms, leading to drastic structural changes.6−10 The high number of Li atoms incorporated into the silicon electrode during lithiation causes large volume changes up to 400% (V/ V0). This often has the consequence of a decay of specific capacity during cycling that is attributed to mechanical fracture and irreversible side reactions that are invoked by the volume changes.5 Such capacity fading is especially found for high lithiation rates, which are highly desirable for commercial batteries. In particular, the large inhomogeneous volume expansion that occurs due to the coexistence of different phases and due to the crystallographic orientation dependent lithiation in crystalline materials is expected to be very detrimental for device operation. For noncrystalline materials like amorphous silicon more reversible morphology changes are found,11 © 2014 American Chemical Society

however, this topic is far from understood. Consequently, in order to improve battery device operation, a precise measurement and improved fundamental understanding of volume changes taking place during lithiation is required, best by inoperando studies. In the present work we demonstrate that neutron reflectometry (NR) can be used to explore volume changes of a thin film amorphous silicon electrode in-operando during cell operation. The results are discussed in the framework of literature.

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EXPERIMENTAL SECTION Electrochemical lithiation is done using a homemade closed three-electrode electrochemical cell for use at neutron facilities, as described in ref 12. A thin film arrangement prepared by r.f. magnetron sputtering is used to produce a large area working electrode necessary for NR experiments. The working electrode is made of a 1 cm thick support quartz block, which is coated by a thin palladium layer as a back contact and current collector. The electrode made of an amorphous silicon film is deposited on top of the palladium. A circular design with an electrode diameter of 40.5 mm (contact area to electrolyte) is used. Two different silicon electrode film thicknesses of about 40 and 140 nm were investigated, respectively. The counter and reference electrode are made of metallic lithium (1.5 mm foil, 99.9%, Alfa Aesar). Additionally, a separator with a thickness of 20 μm (Brückner Maschinenbau, Germany) is introduced between the two electrodes. As an electrolyte, propylene carbonate (SigmaReceived: March 5, 2014 Revised: April 14, 2014 Published: April 16, 2014 9395

dx.doi.org/10.1021/jp502261t | J. Phys. Chem. C 2014, 118, 9395−9399

The Journal of Physical Chemistry C

Article

Figure 1. Neutron reflectometry patterns (open circles) in the virgin state for the two types of electrodes with (a) 40 and (b) 140 nm thickness of the Si layer together with fitting results using Parratt32 (lines); (c, d) corresponding SLD profiles. The individual layers are indicated.

opposite side, and are detected afterward. 3He pencil detectors, offset from each other by 0.44° in 2θ, are used for recording the scattered intensity in the specular and background channel, respectively. The information obtained from NR is the reflected intensity perpendicular to the reflecting interface. The reflectivity data were corrected for footprint and background. Data analysis was performed on the basis of Parratt’s recursion algorithm using the Parratt32 software package.13

Aldrich, anhydrous, 99.7%) with 1 M LiClO4 (Sigma-Aldrich, battery grade) is used. The electrochemical cell was assembled within an argon-filled glovebox (water content < 0.1 ppm, oxygen content < 0.1 ppm). The thin film silicon working electrode was prepared by r.f. magnetron sputtering directly on the palladium coated quartz substrate using argon (6.0) sputter gas at an operating pressure of 0.3 Pa, and a sputtering power of 75 W at room temperature. Investigations with X-ray photoelectron spectroscopy combined with sputter depth profiling (Thermo Fisher Scientific KAlpha) indicated a homogeneous silicon layer and impurity concentrations (O, C) of less than 1% before lithiation. Grazing incidence X-ray diffraction (Bruker D5000, Co Kα, 40 kV) showed characteristic sharp Bragg peaks of palladium and a several degree broad reflection corresponding to amorphous silicon. Electrochemical lithiation was carried out using a computer controlled potentiostat (BioLogic, model SP-150). Lithium was galvanostatically inserted at a current of 100 (7.8 μA/cm2) and 25 μA (2.0 μA/cm2), respectively, resulting in lithiation potentials between 2.9 and 0.13 V versus Li. The neutron measurements were done using the θ/2θ mode and a monochromatic beam at a wavelength of 0.466 nm at the V6 reflectometer located at the Helmholtz-Zentrum Berlin, Germany. Data were recorded in the range from 0.06 up to 1.2° incident angle in the vertical scattering plane of the instrument. Here the quartz substrate is the incoming medium for neutrons. During a NR experiment, the collimated neutron beam (40 mm horizontal slits, 0.5 mm vertical slits) is directed through the side of the quartz block. The neutrons are reflected from the SiO2/Pd/Si/electrolyte interfaces, exit the quartz on the

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RESULTS AND DISCUSSION Figure 1a,b show neutron reflection patterns after assembling the cell and filling with electrolyte but before any charging was done (virgin state) for the two types of electrodes. The patterns show slight fringes, which result from the interference of neutrons reflected at the SiO2/Pd/Si/electrolyte interfaces. For the thicker electrode, the number of fringes is higher within the same qz range. The NR patterns are fitted by the program Parratt32 using a layer-model. The results are also shown in Figure 1a,b as lines. The corresponding scattering length density (SLD) profiles are given in Figure 1c,d. The individual layers are indicated. A thickness of 40 ± 2.5 and 140 ± 3 nm, respectively, is found for the amorphous silicon layers. The SLDs of the SiO2 support of the palladium layer and of the amorphous silicon layer are identical for the two types of electrodes. For the SLD of the electrolyte, we obtain slightly different values of (1.75 ± 0.13) × 10−4 nm−2 for the 40 nm Si layer and (1.90 ± 0.13) × 10−4 nm−2 for the 140 nm Si layer, which, however, is in good agreement to each other within error limits. The SLD of the electrolyte is not very sensitive to the fits, which explains also the relatively high error of about 7%. Details of the error calculation can be found in ref 12. 9396



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Figure 2. Typical examples of neutron reflectometry patterns (circles) recorded in-operando during lithiation for the two types of electrodes with (a) 40 nm and (b) 140 nm thickness, respectively. The charging time is indicated. Also shown are fitting results using Parratt32 (lines). Individual curves are shifted in intensity for clarity.

Figure 3. (a) Neutron reflectometry pattern (open circles) of the 40 nm electrode (100 μA) after 2.06 h of lithiation. Shown are also three different Parratt32 fits for models based on (1) a single LixSi layer with a homogeneous SLD, (2) a LixSi layer subdivided into four sublayers with different SLD, and (3) a SLD composed of two different sublayers with strongly differing SLD and rough interfaces. (b) Corresponding SLDs. The thickness of the LixSi layer as obtained from each fit is indicated. The roughness of Pd/Si interface is about 1.2 nm.

than that of the second case (109 min per run) due to faster lithiation driven by the higher current. These experimental settings are a compromise between the velocity at which lithiation takes place (given by the current density) and the time necessary for data acquisition (given by the instrument and the neutron flux). Consequently, for the 100 μA data less acquisition time per NR run has to be chosen in order to collect reliable data sets. The purpose of the present paper is to measure the thickness of the LixSi layer and its modification during lithiation. The layer thickness is extracted from the reflectivity measurements during lithiation by fitting the patterns with the program Parratt32. Results are also shown in Figure 2 as lines. The SLDs and thicknesses of the quartz substrate and of the palladium layer and the SLD of the electrolyte of the lithiated samples are identical to that of the virgin state. The thickness and SLD of the silicon electrodes systematically change during charging, resulting from the incorporation of lithium into the electrodes. However, a direct consequence of the limited data acquisition time for the measurements at 100 μA is that the Parratt32 fits of the patterns recorded at 100 μA are not unambiguous. As shown in Figure 3a, fits to the reflectivity can be nearly identical, while different SLD profiles correspond to each reflectivity pattern (Figure 3b). Three examples are compared in Figure 3 corresponding to (1) a single LixSi layer with a

The SLD of a certain layer is given by SLD =

Naρ M

∑ bi i

(1)

where M is the molecular weight of the compound or element, Na is Avogadro’s number, ρ is the mass density, and bi is the bound coherent scattering length of the ith type atom in the layer. Mass densities are calculated from the determined SLD for quartz, palladium, and amorphous silicon using eq 1, the known quantities bi, Na, and M, and the NIST online resource database.14 The results correspond very well to literature values (see ref 12). As is obvious, the electrodes in their initial state can be described very well by the fits to the neutron reflectivity data. Characteristic examples of neutron reflection patterns recorded in-operando continuously during lithiation are given in Figure 2 for a current of (a) 100 μA (40 nm Si layer) and (b) 25 μA (140 nm Si layer), respectively. As obvious, the neutron reflection patterns are modified during electrode charging, indicating an incorporation of lithium into the electrode and the formation of a LixSi layer, as demonstrated in ref 12. The scattering vector range, qz, covered by the measurements at 100 μA (0.03 to 0.33 nm−1) is lower than that at 25 μA (0.03 to 0.60 nm−1). This is due to the fact that for the first case the overall data acquisition time (26 min per run) has to be less 9397



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homogeneous SLD, (2) a LixSi layer subdivided into four sublayers with different SLD, and (3) a SLD composed of two different sublayers with strongly differing SLD and rough interfaces. Case (1) corresponds to a homogeneous lithiation process, while case (2) also shows a homogeneous Li distribution, but with transition layers at the interfaces. Finally, case (3) describes a heterogeneous lithiation process, that is, the penetration of a phase front with high lithium content and strongly roughened interfaces. Due to the limited available qz range, no unambiguous statement on the Li depth distribution can be given from the data presented here. However, the present data unambiguously allow extracting the thickness of the LixSi layer from the fits. As demonstrated in Figure 3b fits based on different model assumptions give the same thickness of the overall lithiated LixSi within error limits. This means that the determination of the thickness of the lithiated layer, L, from experimental data does not dependent on detailed model assumptions. Such a result was found for all patterns recorded in this study. Consequently, we use for fitting a simple model based on a homogeneous distribution of Li in the LixSi layer (case (1)) and considered the variation of the results generated by the different model assumptions in the error limits. We note that the chosen analysis does not necessarily require a homogeneous lithiation mechanism to be present (see Figure 3b). In Figure 4 the variation of the layer thickness as extracted from the fits is displayed as a function of lithiation time for the

For further analysis we assume that during Li incorporation the initial silicon layer is expanded in the direction perpendicular to the surface. Expansion in the direction parallel to the surface is strongly suppressed due to the adhesion of silicon to the substrate. A similar behavior is also indicated by AFM studies.11 Consequently, we assume a negligible length expansion parallel to the surface and the relative change of the thickness of the LixSi layer, L, is assumed to be identical to the volume change, V, according to (L − L0)/L0 = (V − V0)/V0, where the index 0 indicates the initial value (L0 = 40 or 140 nm). The relative volume change in the linear region of Figure 4 is plotted as a function of x (in LixSi) in Figure 5. The

Figure 5. Relative volume change (V/V0) as a function of Li content (x in LixSi) for different types of electrodes, charging current, and cycle number. The straight line is the result of atomistic calculations, as given in ref 18 for x > 0.5.

quantity x is obtained assuming a linear scaling with charging time and a final value of x = 4.2 at the end of the charging process. The complete lithiation curve was measured on the same type of electrode without simultaneous neutron experiments. In Figure 5 three data sets are shown: the first and the second charging cycle for the 40 nm electrode (current: 100 μA) and the first cycle for the 140 nm electrode (current: 25 μA). The data of the first cycle for the two types of electrodes are in good agreement with each other within error limits. The data for the second lithiation cycle of the 40 nm electrode are higher by about 5% and close to the limit of error estimation. The results of the third lithiation cycle for the 40 nm electrode are nearly identical to that of the second cycle and are not shown for clarity. A linear increase in volume (after initial effects) for the lithiation of patterned amorphous silicon layers magnetron sputtered on steel substrates is also found also by AFM measurements.11 In ref 18, the theoretically expected modification of the LixSi volume, V, as a function of x is calculated by atomistic simulations based on the activation-relaxation technique using the protocols given in ref 19. Also, shown in Figure 5 are theoretical volume expansion data of ref 18 in the linear region for x > 0.5. The experimental data found in this work are very close to the theoretical calculations. However, more important than absolute values is the slope of the linear dependence of the volume−composition curve. This slope corresponds to the volume increase per inserted lithium atom normalized to the initial volume. From the experiments we get a slope between 0.80 to 0.85 (error: 0.05) in good agreement for all data sets (also for the data of the second cycle for the 40 nm electrode) and also with the calculations of ref 18, where a value of 0.87

Figure 4. Thickness of the LixSi layer and Li/Li+ potential as a function of charging time for the 40 nm silicon electrode (100 μA).

lithiation at 100 μA (40 nm Si electrode). Also shown is the change of the cell potential measured between reference and working electrode. The potential decreases as a function of charging time from about 2.95 V (initial open circuit voltage) down to 0.16 V. At this point the cell is charged to about 40% of its maximum capacity and further charging is stopped in order to avoid a destruction of the electrode due to stress effects. We can separate two regions: an initial region (2.95−0.3 V) where no significant thickness modifications are found (t < 1.5 h) and a second region (0.3−0.16 V), where a continuous linear increase of the layer thickness, L, is visible (t > 1.5 h). In the first region no substantial change of the thickness is indicated and consequently significant incorporation of lithium is not expected here. The linear increase of thickness starts for potentials below 0.3 V, which is in agreement with reports from the literature, where Li insertion at potentials above 0.4 V was not observed by cyclovoltammetry measurements.15−17 9398



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Metals as Negative Electrodes for Li-Ion batteries. J. Mater. Chem. 2007, 17, 3759−3772. (7) Kasavajjula, U.; Wang, C. S.; Appleby, A. J. Nano- and BulkSilicon-Based Insertion Anodes for Lithium-Ion Secondary Cells. J. Power Sources 2007, 163, 1003−1039. (8) Zhang, W.-J. A Review of the Electrochemical Performance of Alloy Anodes for Lithium-ion Batteries. J. Power Sources 2011, 196, 13−24. (9) Obrovac, M. N.; Christensen, L. Structural Changes in Silicon Anodes during Lithium Insertion/Extraction. Electrochem. Solid-State Lett. 2004, A93−A96. (10) Limthongkul, P.; Jang, Y. I.; Dudney, N. J.; Chiang, Y.-M. Electrochemically-Driven Solid-State Amorphization in LithiumSilicon Alloys and Implications for Lithium Storage. Acta Mater. 2003, 51, 1103−1113. (11) Beaulieu, L. Y.; Hatchard, T. D.; Bonakdarpour, A.; Fleischauer, M. D.; Dahn, J. R. Reaction of Li with Alloy Thin Films Studied by In Situ AFM. J. Electrochem. Soc. 2003, 150, A1457−A1464. (12) Jerliu, B.; Doerrer, L.; Huger, E.; Borchardt, G.; Steitz, R.; Geckle, U.; Oberst, V.; Bruns, M.; Schneider, O.; Schmidt, H. Neutron Reflectometry Studies on the Lithiation of Amorphous Silicon Electrodes in Lithium-Ion Batteries. Phys. Chem. Chem. Phys. 2013, 15, 7777−7784. (13) Braun, C. Parratt32 or The Reflectivity Tool, Version 1.6.0, HMI Berlin Software: Germany, 1997−2002. (14) http://www.ncnr.nist.gov/resources/activation/. (15) Arreaga-Salas, D. E.; Sra, A. K.; Roodenko, K.; Chabal, Y. J.; Hinkle, C. L. Progression of Solid Electrolyte Interphase Formation on Hydrogenated Amorphous Silicon Anodes for Lithium-Ion Batteries. J. Phys. Chem. C 2012, 116, 9072−9077. (16) Schroder, K. W.; Celio, H.; Webb, L. J.; Stevenson, K. J. Examining Solid Electrolyte Interphase Formation on Crystalline Silicon Electrodes: Influence of Electrochemical Preparation and Ambient Exposure Conditions. J. Phys. Chem. C 2012, 116, 19737− 19747. (17) Kulova, T. L.; Skundin, A. M.; Pelskov, Yu. V.; Terukov, E. I.; Kon’kov, O. I. Lithium Insertion into Amorphous Silicon Thin-Film Electrodes. J. Electroanal. Chem. 2007, 600, 217−225. (18) Huang, S.; Zhu, T. Atomistic Mechanisms of Lithium Insertion in Amorphous Silicon. J. Power Sources 2021, 196, 3664−3668. (19) Chevrier, V. L.; Dahn, J. R. First Principles Model of Amorphous Silicon Lithiation. J. Electrochem. Soc. 2009, 156, A454− A458.

was found. Assuming an initial volume of a silicon atom of 21.1 Å3 (2.21 g/cm3),18 we get for the volume increase per inserted lithium atom a value of 17.3 Å3. These results indicate that the relative volume expansion is not only independent of the initial layer thickness and applied current density (for the parameters given), but also the differences between the first and second cycle are very small. After complete delithiation (first cycle) an electrode thickness of 60 ± 5 nm is determined. This corresponds to a volume increase of =V/V0 = 1.5, indicating that residual lithium is still present in the electrode due to irreversible processes. It should be emphasized that for the deviation of the relative volume change, in-operando studies have to be preferred to ex situ studies. Neutron reflectometry measurements conducted by our group after the interruption of the charging process indicate the existence of relaxation processes, which suggest that ex situ studies probably will not detect the correct volume expansion.

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CONCLUSION We used in-operando neutron reflectometry in order to determine the volume expansion of amorphous silicon thin film electrodes during electrochemical lithiation. We found a linear increase of the relative volume change (V/V0) for x > 0.5 (LixSi) with a slope of ∼0.8, corresponding the relative volume increase per inserted lithium atom, independent of initial silicon electrode thickness and charging current. The relative volume changes are compared to recent atomistic calculations, as given in literature.18 The relative volume increase per inserted lithium atom is in good agreement to the theoretical considerations and absolute values are higher by about 2−7% only. This indicates that an intrinsic quantity is measured.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +495323722094. E-mail: harald.schmidt@tu-clausthal. de. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft (DFG) in the framework of the focus program SPP 1473 (Schm 1569/23-1) is gratefully acknowledged. We thank the Helmholtz-Zentrum Berlin für Materialien und Energie for beamtime.

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REFERENCES

(1) Aifantis, K. E., Hackney, S. A., Kumar, R. V., Eds. High Energy Density Lithium Batteries. Materials, Engineering, Applications; WileyVCH: New York, 2010. (2) Yoshio, M., Brodd, R. J., Kozawa, A., Eds. Lithium-Ion Batteries: Science and Technologies; Springer: New York, 2010. (3) Yuan, X., Liu, H., Zhang, J., Eds. Lithium-Ion Batteries: Advanced Materials and Technologies; CRC Press, Inc.: Boca Raton, FL, 2011. (4) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. Electrochemistry and the Future of the Automobile. J. Phys. Chem. Lett. 2010, 1, 2204− 2219. (5) McDowell, M. T.; Lee, S. W.; Nix, W. D.; Cui, Y. Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium-Ion Batteries. Adv. Mater. 2013, 25, 4966−4985. (6) Larcher, D.; Beattie, S.; Morcrette, M.; Edstroem, K.; Jumas, J. C.; Tarascon, J. M. Recent Findings and Prospects in the Field of Pure 9399


Volume Expansion during Lithiation of Amorphous Silicon Thin Film

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