Size Dependent Pore Formation in Germanium Nanowires

Article pubs.acs.org/JPCC

Size Dependent Pore Formation in Germanium Nanowires Undergoing Reversible Delithiation Observed by In Situ TEM Xiaotang Lu,† Yang He,§ Scott X. Mao,§ Chong-min Wang,‡ and Brian A. Korgel*,† †

Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712-1062, United States ‡ Environmental Molecular Sciences Laboratory, Pacific Northwestern National Laboratory, Richland, Washington 99354, United States § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Germanium (Ge) nanowires coated with an amorphous silicon (Si) shell undergoing lithiation and delithiation were studied using in situ transmission electron microscopy (TEM). Delithiation creates pores in nanowires with diameters larger than ∼25 nm, but not in smaller diameter nanowires. The formation of pores in Ge nanowires undergoing delithiation has been observed before in in situ TEM experiments, but there has been no indication that a critical diameter exists below which pores do not form. Pore formation occurs as a result of fast lithium diffusion compared to vacancy migration. We propose that a short diffusion path for vacancies to the nanowire surface plays a role in limiting pore formation even when lithium diffusion is fast.

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INTRODUCTION More durable and powerful lithium ion batteries (LIBs) are needed to keep pace with rapidly developing portable electronic devices and electric vehicles.1−4 On the anode side of the LIB, much higher charge storage capacity could be achieved by replacing graphite with silicon (Si), germanium (Ge) or tin (Sn).5,6 All of these materials exhibit significant volume expansion upon lithiation; however, nanoscale Si, Ge and Sn, can tolerate these volume changes without significant mechanical failure and appropriate battery formulations of binder and electrolyte can be used to achieve relatively long battery life while retaining high charge storage capacities.7−16 In situ transmission electron microscopy (TEM) has provided a powerful technique to study new LIB materials in real time, enabling direct observation of LIB electrode materials as they undergo lithiation and delithiation.17,18 In situ TEM has revealed a variety of phenomena: for example, Ge nanowires expand isotropically during lithiation,19 whereas Si nanowires expand preferentially in certain crystallographic directions;20,21 there is a critical fracture size for crystalline Si nanoparticles;22 and rapid and reversible pore formation can occur in both Ge23 and Si24 nanowires during delithiation. These findings have helped provide a fundamental understanding of the structural evolution and damage accumulation mechanisms that occur in Ge- and Si-based anodes.25 Here, we report in situ TEM data for Ge nanowires of various diameter undergoing reversible lithiation and delithiation and find that pore formation is size-dependent. Ge © 2016 American Chemical Society

nanowires smaller than 23−27 nm in diameter do not form pores. This size effect originates from a competition between the rate of Li diffusion and the migration of vacancies to the nanowire surface. Vacancies have a shorter path to the surface of smaller diameter nanowires making vacancy migration fast enough to eliminate pore nucleation and growth.

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EXPERIMENTAL DETAILS Materials. Hydrogen tetrachloroaurate(III) trihydrate (≥99.9%, Aldrich), tetraoctylammonium bromide (98%, Aldrich), sodium borohydride (≥98.0%, Aldrich), anhydrous toluene (99.8%, Sigma-Aldrich), diphenylgermane (DPG, Gelest Inc.), and trisilane (Si3H8, Voltaix) were used as received without further purification. Gold (Au) nanocrystals approximately 2 nm in diameter capped with dodecanethiol were prepared as described in the literature.26,27 Germanium Nanowires Synthesis. Ge nanowires were synthesized by a Au nanocrystal-seeded supercritical fluid− liquid−solid (SFLS) reaction in a 10 mL titanium tubular reactor connected to a high performance liquid chromatography (HPLC) pump as outline in the literature.28 A 28 mL reactant solution of 25 μL of Au nanocrystal dispersion (20 mg/mL in toluene) and 190 μL of diphenyl germane (DPG) in anhydrous toluene was prepared in the glovebox. Prior to Received: October 8, 2016 Revised: December 1, 2016 Published: December 5, 2016 28825

DOI: 10.1021/acs.jpcc.6b10174 J. Phys. Chem. C 2016, 120, 28825−28831

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The Journal of Physical Chemistry C precursor injection, the titanium reactor was filled with N2 in the glovebox and then connected to the six-way valve and the back-pressure regulator at two ends. After the reactor was preheated to 380 °C and pressurized to 10.3 MPa with anhydrous toluene, nanowire growth was carried out with the reactant solution fed into the reactor at a rate of 0.5 mL/min for 40 min. The outlet pressure was maintained at 10.3 MPa. After completing the injection of the reactant solution, the reactor was sealed and cooled to 150 °C. An amorphous Si (aSi) shell was then deposited on the nanowires. A volume of 42 μL of trisilane diluted in 2 mL of toluene was loaded in a syringe and injected to the reactor via the six-way valve (Caution: trisilane is volatile, highly flammable and pyrophoric. Preparation of the dilute trisilane solution must be carried out inside an inert gas filled glovebox.). After injection of the trisilane solution was complete, the reactor was resealed and heated to 250 °C. After 12 h, the nanowire product was collected from the reactor and washed with a mixture of 4 mL of chloroform, 2 mL of toluene and 2 mL of ethanol, followed by centrifugation at 8000 rpm for 5 min. The purification procedure was repeated three times to remove unreacted reagent and molecular byproducts. In Situ Transmission Electron Microscopy (TEM). In situ electrochemical experiments were conducted in a Titan 80−300 scanning/transmission electron microscope using a Nanofactory TEM holder. The nanowires were drop cast onto a gold wire as the working electrode. Lithium (Li) metal on a tungsten wire was used as the counter electrode. A native Li2O layer forms on the Li metal surface during the transfer of the TEM holder from the glovebox to the TEM and serves as a solid-state electrolyte.20,24 Inside the TEM, the Li/Li2O electrode is moved by the piezo-positioner to touch the nanowire. Once contact is made, a bias of −2 V is applied to initiate lithiation. After the nanowire was fully lithiated, it was delithiated by applying a bias of 2 V. During the imaging, the beam intensity was minimized to avoid beam effects.29

of a crystalline Ge core with a relatively thin, rough amorphous Si shell. Figure 2a and b shows higher magnification TEM

Figure 2. (a,b) TEM images of a typical a-Si coated Ge nanowire studied by in situ TEM. (c) STEM HAADF image of the nanowire; (d) Ge distribution mapping by EDS; (e) Si distribution mapping by EDS. Elemental analysis shows that the nanowire core is composed of Ge and that the shell is Si.

images of a nanowire, which clearly shows the crystalline Ge core of the nanowire with much higher image contrast than the rough a-Si shell. Figure 2c shows a scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) image and corresponding EDS (energy-dispersive Xray spectroscopy) elemental maps in Figure 2d and e showing the Ge core and the Si coating. This nanowire structure with Ge core and Si shell is potentially interesting because of the combination of fast lithiation of the Ge core and high specific capacity of the Si shell.30 Compared to a crystalline shell, an amorphous shell has an advantage of creating significantly less tensile strain at the core−shell interface during nanowire expansion during lithiation, which is known to inhibit lithiation.10,31 Figure 3a shows a series of in situ TEM images of a Ge nanowire coated with a-Si as it undergoes an initial lithiation cycle. (See the accompanying Video S1 in the Supporting Information.) Lithiation occurs first along the Ge nanowire surface and then proceeds radially into core by the so-called core-into-shell lithiation mode.29 The Ge core lithiates before the amorphous Si shell, as it has a higher reaction voltage with Li than a-Si and is more electrically conductive.32,33 The observed progression of lithiation of the a-Si-coated Ge nanowires is similar to what has been observed for pure crystalline Ge nanowires:23 lithiation occurs by core-into-shell mechanism with negligible tapering. The a-Si coating does not adversely affect or change the behavior of Ge nanowires, as opposed to nanowires that have been coated with a crystalline Si shell, which showed significant inhibition of nanowire lithiation.34 Lithiation has increased the diameter of the nanowire in Figure 3a from 61 to 103 nm. Because the Ge core of the nanowire becomes amorphous upon lithiation, it is difficult to distinguish in the TEM image between the core and shell after the nanowire has lithiated. Figure 3b shows in situ TEM images of a Ge nanowire coated with a-Si with a twin defect in its core undergoing an

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RESULTS AND DISCUSSION Figure 1a and b shows SEM and TEM images, respectively, of the nanowires studied by in situ TEM. The nanowires consist

Figure 1. (a) SEM and (b) TEM images of Ge nanowires coated with amorphous Si (a-Si). (c) Illustration of the nanowire synthesis process. First, crystalline Ge nanowires are prepared by Au nanocrystal-seeded SFLS growth and then a-Si is deposited onto solvent-dispersed nanowires by thermal decomposition of trisilane. 28826

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Figure 3. (a) In situ TEM images of a Ge nanowire coated with an a-Si shell undergoing an initial lithiation cycle. (See accompanying Video S1 in the Supporting Information.) (b) In situ TEM images of a Ge nanowire coated with a-Si with a twin in its core undergoing an initial lithiation cycle. (See accompanying Video S2 in the Supporting Information.)

nanowire remains expanded after delithiation with a Ge core diameter of 36.4 nm (compared to 27 nm initially). Figure 5 shows a thinner nanowire undergoing delithiation. This nanowire had an initial diameter of the crystalline Ge core

initial lithiation cycle (see accompanying Video S2 in the Supporting Information). Electrochemically driven solid state amorphization (ESA) is observed to begin simultaneously at both the twin plane and the interface between the crystalline Ge core and the a-Si coating. Lithiation proceeds in the core and on the surface of the nanowire at about the same rate and the lithiating core eventually merges with the inward lithiation of the surface. This observation is consistent with predictions of faster Li diffusion along surfaces and grain boundaries compared to volume diffusion within the crystal.35−37 Figure 4 shows a lithiated Ge nanowire coated with a-Si as it delithiates (first delithiation cycle). The initial diameter of the

Figure 4. In situ TEM images of a lithiated Ge nanowire coated with a-Si as it delithiates. The Ge core diameter is 27 nm prior to lithiation. Pores form in the nanowire as it delithiates. The yellow arrows indicate the delithiation front, which proceeds in the direction opposite of Li diffusion/extraction from the nanowire.

Figure 5. In situ TEM images of a lithiated Ge nanowire coated with a-Si undergoing delithiation. The initial diameter of the crystalline Ge core prior to lithiation was 19 nm. Pore formation is not observed in this nanowire, even after multiple lithiation/delithiation cycles.

crystalline core of the Ge nanowire before lithiation was 27 nm. Delithiation occurs under a bias of +2 V. Pores form as Li diffuses out of the nanowire. Pore formation in delithiating Ge nanowires was first observed by in situ TEM by Liu et al.23 Pore formation results from the fast extraction of Li from the Ge core, which generates a high concentration of vacancies, which nuclate into pores. The average speed of the delithiation front, i.e., the boundary between the porous and nonporous region of the nanowire, is 13.7 nm/s. Now filled with the pores, the

of 19 nm prior to lithiation. This nanowire did not form pores during the delithiation cycle. Initially when the nanowire is lithiated there is little imaging contrast between the Ge core and the a-Si shell. After delithiation, the boundary between the Ge core and the Si shell again becomes distinct. In this case, the nanowire shrinks to a diameter of 20.1 nm, which is close to its initial diameter of 19 nm. Even after multiple lithation− delithiation cycles, no pores formed in this nanowire. 28827

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Figure 6. Comparison of in situ TEM results for Ge nanowires coated with a-Si with a range of diameters in the pristine state, the lithiated state and the delithiated state. Left column: nanowires that did not evolve pores upon delithiation. These nanowires had initial core diameters smaller than 23 nm. Right column: nanowires that have evolved pores. These nanowires had initial core diameters larger than 27 nm. All of the TEM images are shown with the same magnification. Vertical-section schematics and corresponding high-angle annular dark field (HAADF) images of two nanowires in the delithiated state are shown beside each column of TEM images.

Nanowires with a range of diameter were studied to determine if pore formation was affected by nanowire size. Figure 6 summarizes the experimental observations (see the Supporting Information for the complete experimental results in Figure S1). The critical diameter for pore formation was between 23 and 27 nm. Nanowires larger than 27 nm in diameter formed pores upon delithiation. Size-dependent pore formation has also been observed in dealloying Li−Sn alloys in which particles with diameter less than 300 nm did not evolve pores.38 A critical diameter of 12−15 nm for pore formation has been observed in Ni−Pt nanoparticles after etching Ni.39 Pores form when Li diffusion is fast compared to Ge selfdiffusion or, more accurately, the migration of Ge vacancies to the nanowire surface. The Li diffusion barrier in Ge is 0.44 eV.40 The activation energy for the self-diffusion of Ge is ∼3 eV, the energy to form a vacancy is 2 eV, and the activation energy for vacancy migration is another 1 eV.41 Therefore, Li diffusion is typically much faster than vacancy migration in Ge and pores readily evolve in Ge nanowires when they delithiate. Shell-into-core delithiation was observed. Similar to lithiation, delithiation occurs more rapidly on the surface of the nanowire than in the core, which evolves a surface layer, or crust, that extends significantly ahead of the porous region, as shown in Figure 7. (See also Figures S2 and S3 in the Supporting Information.) Pore formation during delithiation proceeds along the three stages illustrated in Figure 8: (1) vacancy generation, (2) pore nucleation, and (3) pore coarsening. Initially, very fast Li diffusion out of the nanowire40,42 generates a high concentration of vacancies in the nanowire. Some of the vacancies make it to the nanowire surface, leading to a shrinkage in diameter. Because the Li diffusion is so much faster on the

Figure 7. In situ TEM images of a Ge nanowire coated with a-Si undergoing delithiation. Pore nucleation and growth is observed in the images.

nanowire surface, the vacancies are formed and depleted more rapidly on the nanowire surface, which results in a dense Ge crust on the nanowire. This dense Ge crust subsequently creates a barrier to further vacancy migration to the nanowire surface. Since those vacancies cannot efficiently migrate to the 28828

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performance, i.e., that some types of heterojunction interfaces, such as the crystalline Ge/amorphous Si interface studied here, do not constrain the lithium uptake capacity of the material. Furthermore, a variety of size-related effects related to the structural integrity can occur in lithium ion battery electrode nanomaterials and the direct observation of materials behavior by in situ TEM observation can elucidate these effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10174. Statistics of the experimental results; additional TEM images (PDF) Video of in situ TEM experiment, lithiation (AVI) Video of in situ TEM experiment, lithiation 2 (AVI) Video of in situ TEM experiment, delithiation (AVI) Video of in situ TEM experiment, delithiation 2 (AVI)

Figure 8. Illustration of Ge nanowire cross sections showing how pores form in Ge nanowires as a result of delithiation when the diameter is larger than a critical size. The color-coding indicates the relative Li content in the nanowire with blue representing complete lithiation and red indicating pure Ge. Delithiation begins at the surface of the nanowire and initially a crust of Ge forms on the nanowire surface. As delithiation proceeds within the core of the nanowire, vacancies are generated. When the nanowire diameter is too thick, vacancies do not migrate to the surface before pores nucleate and grow. In the thin nanowires, vacancies migrate to the surface before pores can nucleate and form.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: 512-471-5633. Fax: 512-471-7060.

nanowire surface, they nucleate into pores, which coarsen as more Li diffuses out of the core and creates more vacancies. In the nanowires that are too small to form pores, the process occurs along the same path, except that surface area to volume ratio is high enough that after the formation of the Ge nanowire crust layer there are not enough vacancies formed within the nanowire core to lead to significant pore nucleation and growth. The higher surface curvature of the smaller diameter nanowires could also be providing a higher driving force for vacancy migration to the surface.43 The coating of a-Si does not evolve pores during delithiaion, similar to other published reports.36,44 Li diffusion is much slower in Si than Ge34,45 and the migration barrier for vacancies in Si is 0.45 eV,46 which is close to the diffusion barrier of Li in Si (0.47 eV),47 allowing for efficient vacancy migration to the surface without pore nucleation and growth. The stress induced by the Si shell on the Ge core could further slow down vacancy migration in the Ge core48,49 and shift the threshold for pore formation to even smaller size compared to nanowires without a shell. The critical diameter for pore formation is also probably influenced by the rate of discharging since the Li diffusion rate is competing with the rate of vacancy migration, which is not affected by the discharge rate.38 Faster discharging rates should result in a smaller critical diameter for pore formation.

ORCID

Brian A. Korgel: 0000-0001-6242-7526 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded in part by the program “Understanding Charge Separation and Transfer at Interfaces in Energy Materials (EFRC: CST),” an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, under Award No. DESC0001091 and the Robert A. Welch Foundation (Grant No. F-1464). C.-m.W. acknowledges the support of the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 6951379 under the advanced Batteries Materials Research (BMR) Program. In situ TEM was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830.

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CONCLUSIONS The in situ TEM studies on a range of sizes of Ge nanowires coated with a-Si revealed several aspects of the lithiation and delithiation behavior in those nanowires. First, the a-Si coating does not constrain the lithiation of the Ge core, which supports the rationale of using this kind of heterostructured nanowire in Li-ion batteries. Second, the interface between a-Si and the crystalline Ge core and grain boundaries in the Ge nanowire core provide the fastest Li diffusion paths. Finally, a critical diameter is observed for pore formation, in the range of 23−27 nm in diameter. The larger Ge nanowires exhibit pores after delithiation. The origin of this effect is the shorter migration path for vacancies to the surface of thinner nanowires. This work further shows that different kinds of heterostructured nanomaterials can exhibit significantly different battery

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DOI: 10.1021/acs.jpcc.6b10174 J. Phys. Chem. C 2016, 120, 28825−28831