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Reversible Li-ion Conversion Reaction for a TixGe Alloy in a Ti/Ge Multilayer Xiao Chen, Timothy T. Fister, Jennifer L. Esbenshade, Bing Shi, Xianyi Hu, Jinsong Wu, Andrew A. Gewirth, Michael J. Bedzyk, and Paul Fenter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14783 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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ACS Applied Materials & Interfaces

Reversible Li-ion Conversion Reaction for a TixGe Alloy in a Ti/Ge Multilayer

Xiao Chen1,2, Tim T. Fister1, Jennifer Esbenshade3, Bing Shi1, Xianyi Hu2, Jinsong Wu2, Andrew A. Gewirth3, Michael J. Bedzyk2, and Paul Fenter1* 1. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439 2. Applied Physics Program and Materials Science and Engineering Department, Northwestern University, Evanston, IL 60208 3. Chemistry Department, University of Illinois Urbana-Champaign, Champaign, IL 61801 *

Corresponding Author Email address: [email protected]

Abstract: Group IV inter-metallics electrochemically alloy with Li with stoichiometries as high as Li4.4M (M=Si, Ge, Sn or Pb). This provides the second highest known specific capacity (after pure lithium metal) for lithium ion batteries, but the dramatic volume change during cycling greatly limits their use as anodes in Li-ion batteries. We describe an approach to overcome this limitation by constructing electrodes using a Ge/Ti multilayer architecture. In operando X-ray reflectivity and ex situ transmission electron microscopy are used to characterize the hetero-layer structure at various lithium stoichiometries along a lithiation/delithiation cycle. The as-deposited multilayer spontaneously forms a one-dimensional TixGe/Ti/TixGe core-shell planar structure embedded in a Ge matrix. The interfacial TixGe alloy is observed to be electrochemically active and exhibits reversible phase separation (i.e. a conversion reaction). Including the germanium components, the overall multilayer structure exhibits a 2.3-fold reversible vertical expansion and contraction and is shown to have improved capacity and capacity retention with respect to a Ge film with equivalent active material thickness.

Keywords: Li-ion battery, Germanium, thin film, multilayer, X-ray reflectivity, Patterson function, Ge/Ti alloy

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Introduction Electronic mobile devices and electrical vehicles rely on increased energy capacity and power of batteries. Carbonaceous materials have been the primary choice for anodes in commercial Li-ion batteries for decades, primarily relying on intercalation in graphite to form LiC6. In comparison, conversion materials have much higher specific capacity, especially group IV intermetallics (4200 mAh/g for silicon and 1620 mAh/g for germanium compared to 372 mAh/g for LiC6). However, the intrinsically large volume change during cycling of group IV intermetallics (typically ~4-fold expansion) is problematic as it pulverizes the active materials, causing delamination from the current collector and conductive binders that leads to Li trapping and severe capacity fade. One common approach for solving this problem is to reduce the size of the electrode materials structure to the nanoscale, such as forming thin films,1-2 nanoparticles,3-4 mesoporous films5-6 and nano arrays.7 Such systems are designed to reduce overall strain and improve transport properties, but often suffer from poor Coulombic efficiency due to side reactions that scale with surface area. Previously it was demonstrated that multilayers consisting of alternating thin films of amorphous silicon and conducting adhesive metal layers allowed the intrinsically high Li-capacity of silicon to be combined with the reversibility of ultrathin silicon films and the conductivity of a composite material while maintaining a high Coulombic efficiency.8-10 Though Si is favored over other group IV elements, due to its high capacity, low cost and abundance, Ge is attractive due to its intrinsically high electronic conductivity at room temperature (2.1 S/m), which is 3 orders of magnitude higher than that of Si (1.6x103

S/m).11 Also Li diffusivity in Ge (6.25x10-12 cm2/s) is 2 orders of magnitude higher than Si (1.9x10-14

cm2/s).12 These electronic and ionic transport properties make Ge electrodes more suitable for high power anodes. Ge also benefits from its minimal surface native oxide, in contrast to silicon.1 that reacts with Li during the initial charging to form Li2O. This can lead to a large irreversible capacity for the first cycle for nanostructured anodes that have intrinsically large surface areas.


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Here, the lithiation of sputter-deposited Ge/Ti multilayer films were studied as a model nanostructured anode. Transmission electron microscopy (TEM) images and X-ray Reflectivity (XRR) data (using a Patterson function analysis) clearly show the development of TixGe alloy13 formation at the Ti-Ge interfaces within the as-deposited film. In operando XRR measurements observed that TixGe alloy layers are electrochemically active and exhibit a reversible conversion reaction to titanium plus lithiated germanium during lithiation/delithiation reactions. That is, the TixGe framework acts both as a binding material that prevents Ge layers from delaminating when undergoing the extremely large volume changes during lithiation reactions, and also as an active material for lithiation. This creates a hybrid electrode chemistry in which the lithiation of the Ge layers is a simple alloying reaction, while the lithiation of the TixGe layers is a conversion reaction that leads to reversible phase segregation leading to Ti layers and lithiated Ge. The results are discussed in the context of in operando x-ray analysis at different stages of lithiation/delithiation. Experimental Details: Sample Fabrication: Ge/Ti multilayers were deposited using DC magnetron sputtering. The rotary magnetron sputtering system was customized with two sputtering targets set horizontally. The base pressure was 5x10-7 Torr, the deposition pressure was 2.3 mTorr of Ar, and the power for both targets was 215 W. Target masks were designed to achieve uniform films. The growth rate was calibrated using XRR. The Ge/Ti multilayers were deposited on fused quartz at room temperature by translating the substrate holder between the Ti and Ge targets, to grow alternating layers of 20 Ti/Ge bilayers, with a bottom layer of Ti and a top layer of Ge. The sample used for XRR measurements had nominal film thicknesses of 20 Å Ti and 100 Å Ge, while that for the TEM results were 50 Å Ti and 100 Å Ge. Galvanostatic measurements of the extended cycling performance of the Ge/Ti multilayers were tested at a 1C rate (using a multichannel Arbin cycler) to probe the stability and performance of a 20 period multilayer with 100 Å Ge and 40 Å Ti layer thicknesses and is compared to a thin Ge film with the same overall thickness (2000 Å).



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Transmission Electron Microscopy: The cross-sectional TEM samples were prepared by a FEI Helois focused ion beam (FIB). After the thin section has been mounted on a FIB-grid, it is transferred into a TEM. The Ge/Ti multilayers thin film was then studied by a JEOL-2100F transmission electron microscope at 200 kV. High resolution electron microscopy (HREM) image showing phase contrast were taken to study the interface structure. X-ray Reflectivity Measurements: In Operando XRR measurements were performed at 33BM-C of the Advanced Photon Source (APS) in Argonne National Laboratory (ANL) using an X-ray photon energy of 20.00 keV with an incident flux of ~ 1010 photons per second. The X-ray beam (with cross-section of 2 x 0.2 mm2 and divergence of 40 µrad = 0.0005 Å along 2θ direction) illuminated a 2 mm-wide by 3 mmlong area on the sample. A specially-designed transmission electrochemistry cell8, 14 was used to study the multilayers samples as a function applied electrochemical potential. The sample size is 10 mm wide and 3 mm along the X-ray beam direction. The electrochemical potential of the working electrode and its associated currents were controlled by a CHI760D potentiostat. Li metal was used as both the counter and reference electrodes forming a half-cell, and all potentials are reported versus the Li/Li+ redox couple. Data were collected at fine intervals (∆q = 0.001Å ) using an area detector (Pilatus 100k). Each XRR scan took 5 minutes. In the present data, the statistical uncertainties in the experimental data are smaller than the systematic errors (e.g., associated with inhomogeneites in the sample thickness, for instance, as evidenced by the lack of Kiessig fringes), and therefore we have assigned a nominal and uniform relative uncertainty of 2% for each data point. X-ray Reflectivity and Patterson function analysis: Thin film X-ray reflectivity directly probes the vertical (i.e., laterally averaged) electron density profile of heterolayer structures, both within the layers and at the interfaces.9, 15-19 Specifically, the X-ray reflectivity signal, R(Q), is the fraction of the X-ray beam that is reflected from the surface as a function of the angle of incidence, θ, of the X-ray beam. For specular reflection, the scattering condition is characterized by a scattering vector magnitude, Q = 4πsin(θ)/λ, where λ is the X-ray wavelength. XRR is directly sensitive to the thickness, density, and interface


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roughness of interfaces in a multilayer structure. For simplicity we limit the measurements to the kinematical regime, where the reflectivity R

Ge

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