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On-chip lithium cells for electrical and structural characterization of single nanowire electrodes

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 265402 (http://iopscience.iop.org/0957-4484/25/26/265402) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 265402 (7pp)

doi:10.1088/0957-4484/25/26/265402

On-chip lithium cells for electrical and structural characterization of single nanowire electrodes A Subramanian1,2, N S Hudak2, J Y Huang2, Y Zhan3,4, J Lou3 and J P Sullivan2 1

Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA 2 Sandia National Laboratories, Albuquerque, NM 87185, USA 3 Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA E-mail: [email protected] Received 31 March 2014, revised 28 April 2014 Accepted for publication 7 May 2014 Published 12 June 2014 Abstract

We present a transmission electron microscopy (TEM)-compatible, hybrid nanomachined, onchip construct for probing the structural and electrical changes in individual nanowire electrodes during lithium insertion. We have assembled arrays of individual β-phase manganese dioxide (βMnO2) nanowires (NWs), which are employed as a model material system, into functional electrochemical cells through a combination of bottom-up (dielectrophoresis) and top-down (silicon nanomachining) unit processes. The on-chip NWs are electrochemically lithiated inside a helium-filled glovebox and their electrical conductivity is studied as a function of incremental lithium loading during initial lithiation. We observe a dramatic reduction in NW conductivity (on the order of two to three orders in magnitude), which is not reversed when the lithium is extracted from the nanoelectrode. This conductivity change is attributed to an increase in lattice disorder within the material, which is observed from TEM images of the lithiated NWs. Furthermore, electron energy loss spectroscopy (EELS) was employed to confirm the reduction in valence state of manganese, which occurs due to the transformation of MnO2 to LixMnO2. Keywords: Li-ion batteries, nanostructured electrodes, single nanowire characterization (Some figures may appear in colour only in the online journal) 1. Introduction

used in battery electrode systems [2]. The nanometer-scale dimensions present ultra-short pathways for lithium transport and support extremely high charge-discharge rates. In addition, the enhanced capability of low-dimensional nanomaterials to absorb the mechanical strain associated with volume expansion (in the case of alloying anodes), lattice disruptions (in the case of transition metal oxide cathodes), and phase changes (in the case of conversion-type electrodes) enables nano-constructs to accommodate lithium at capacities approaching their respective theoretical limits without structural degradation or failure. These attributes have been exploited in recent years to demonstrate the favorable lithium storage and cycling aspects of a variety of one-dimensional anodic and cathodic nanowires (NWs). Among anodic constructs (i.e. those for the negative electrode of a Li-ion

Lithium-ion (Li-ion) electrochemistry, with a favorable energy density (160 Wh kg−1) and high output voltage (∼3.5 V) [1], has emerged as the technology of choice for portable electronics and electric vehicle applications. Significant opportunities exist for further improvements and optimization of each of the primary components of a Li-ion battery: electrodes (material as well as geometry), electrolyte and the electrode/electrolyte interface. Among electrodes, important new data has been published pointing to the favorable attributes of nano-engineered architectures when 4

Current address: Institute of Photonics & Photo-Technology, Northwest University, Xi’an 710069, Shaanxi Province, People’s Republic of China.

0957-4484/14/265402+07$33.00

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© 2014 IOP Publishing Ltd Printed in the UK

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battery), investigations have highlighted the favorable performance of silicon NWs and nanotubes [3–5], germanium NWs [6], carbon nanotubes [7, 8] and tin oxide NWs [9, 10]. In the case of cathodic materials (i.e. those for the positive electrode), some investigations have focused on different phases of manganese dioxide NWs [11, 12], and on lithium cobalt oxide nanotubes [13]. The performance of nanomaterials as functional components in battery electrodes has been probed through two types of characterization approaches. In one approach, the nanomaterials are integrated into bulk-scale agglomerates through the use of additives, which are suitable for the material system under consideration, and their electrochemical performance is studied within conventional configurations such as Swagelokcells or coin cells. In the second approach, which has been enabled by accompanying advances in the area of experimental nanotechnology, electrochemical cells based on single NWs or nanoparticles are employed to study the transformation in the electrode material with atomic-to-nanoscale resolution. This technique has enabled new understanding of the changes occurring within the electrode microstructure due to lithium insertion and the impact that these changes produce in terms of electrode performance. While most single-NW electrode reports have constructed the electrochemical cell inside a transmission electron microscope (TEM) using TEM-STM probes [7, 10], there have only been isolated reports on the construction of on-chip, single NW-based electrochemical cells [14–16]. The integration of nanowire electrodes on silicon substrates enables their compatibility with silicon nanomachining processes and provides avenues to build architectures that can probe different material properties of relevance for electrode performance such as electronic conductivity, elastic modulus, chemical/electrochemical stability and long-term stability. For example, there has been an investigation of the electrolyte etching induced changes within single LiMn2O4 nanorods [17]. While this work looks at an important chemical failure mode, it still does not address the electrochemistry-induced changes within individual NWs. On the other hand, there have been two other reports on on-chip lithium cells to investigate electronic conductivity changes within silicon and vanadium oxide nanowire electrodes [14, 15]. We have previously employed on-chip electrochemistry to demonstrate lithiation-induced structural changes in tin oxide nanowire-based alloying anodes [16]. In this effort, we have employed the hybrid nanofabricated constructs to perform electrochemically correlated electrical and structural characterization of arrays of individual β-MnO2 NWs, which represent intercalation cathodes of interest for Li-ion batteries. Attributes of these demonstrated constructs include: (1) integration of arrays of individual NW electrodes in parallel through the use of nanoassembly techniques, while retaining the capability to probe their electrical and structural behavior individually, (2) material characterization without substrate effects (NWs are anchored in air between the assembly electrodes), and (3) creation of nanoscale viewing windows in silicon chips for compatibility with transmission electron

microscopy (TEM) and other advanced spectroscopic tools such as electron energy loss spectroscopy (EELS).

2. Device fabrication The hybrid nanofabrication procedure, which is employed to realize the on-chip devices, is schematically illustrated in figure 1 and has been described elsewhere in our earlier report [16]. Briefly, silicon wafers are subjected to a back-side, tetramethyl ammonium hydroxide (TMAH) etch, which reduces the substrate to a 100 μm thickness in the device regions from an initial thickness of 455 μm. Arrays of gold nanoelectrode pairs, which measure 5 μm wide and contain a 350 nm gap, are defined on the top-side using a combination of photo-and electron beam lithography. Next, the remaining silicon in the device regions is completely removed by a second TMAH etch from the top-side through e-beam lithography patterned arrays of rectangular openings (measuring 800 nm by 6 μm) in the silicon nitride. These rectangular holes in the nitride are designed to provide a viewing window for TEM in the device regions. Furthermore, this top-side etch through arrays of miniature windows results in nitride membranes with widths smaller than 10 μm and prevents their buckling due to thinfilm stresses. The β-MnO2 NWs are integrated on to the nanoelectrodes from a suspension in ethanol using ac dielectrophoresis (biasing configuration is highlighted in panel ‘e’). By optimizing the assembly parameters such as applied voltage, frequency, and NW concentration in solution, individual NWs may be assembled using this technique [18, 19]. The wiring of each assembled NW device to its own individual bonding pad on the right enables two terminal current–voltage (I–V) probing at the single nanowire-level (figure 1(e)). Scanning and TEM images of the nanofabricated constructs based on β-MnO2 NWs are shown in figure 2.

3. Device characterization and discussion Prior to studying the performance of NWs in the single-NW cell, we characterized the fundamental electrochemical behavior of the material system using a milligram-scale amount of the sample. The NWs were suspension-cast onto a carbon-coated aluminum substrate and sealed in a Swagelok-type cell with separator, electrolyte, and a lithiummetal counter/reference electrode. Galvanostatic cycling curves with a rate of 10 mA g−1 are shown in figure 3(a). These profiles are useful in estimating the lithiation capacity obtained at given voltages. Specific capacity measured from the first discharge is 150 ± 7 mAh g−1 (average and standard deviation of three cells), which is equivalent to Li0.49MnO2 stoichiometry. This capacity is greater than that observed for bulk crystalline β-MnO2 [20] and is lower than that of βMnO2 nanocrystals [21] or mesoporous, nanocrystalline βMnO2 [22]. Thus, the highly crystalline nature of the βMnO2 nanowire prevents the higher electrochemical accessibility demonstrated with other nanostructured β-MnO2 2

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Figure 1. Hybrid nanofabrication process. (a) Definition of trenches within the back-side of silicon chips by TMAH etching. Lithographically created windows in low-stress LPCVD silicon nitride serve as the mask for this etch. The silicon is thinned to ∼100 microns in the device regions at the end of this etch. (b) Definition of nanoelectrodes (Cr/Au) by electron beam lithography (EBL) and metal lift-off. (c) Dry etch to create openings in the nitride in the interelectrode-gap regions. (d) TMAH etching to create through holes in the chips in the inter-electrode gap regions. (e–f) Cross-sectional and top-views of a dielectrophoretically (DEP) assembled nanowire array.

Figure 2. (a) Nanoarray design. The contact design, where the NW array has a common DEP biasing contact on the left and separate bonding pads running to individual NW assembly locations on the right, can be seen in this SEM image. (b) A high magnification image showing a beta-MnO2 NW bridging a nanoelectrode pair. (c) Another nanoelectrode location with the brightness enhanced to illustrate the silicon nitride membrane and TEM window (which is an etched opening within the nitride) designs. (d) TEM image of a nanowire bridging two nanoelectrodes. Scale bars in panels (a–d) measure 400 um, 1 um, 4 um, and 50 nm, respectively.

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Figure 3. (a) Galvanostatic curves of 240 micrograms of beta-MnO2 NWs adsorbed on to an Al current collector. Voltage versus gravimetric capacity plots of the first three cycles is shown. (b) Schematic of the NW electrochemical measurement set-up that was realized using a probe station inside a glove-box. The NW array served as the working electrode and a gold-coated probe needle in contact with the common left contact pad was used to connect the array to the potentiostat. A few droplets of the electrolyte (1:1 EC:DMC with 1 M LiPF6) was placed on the chip to coat the entire NW array. A Li wire dipped into the electrolyte served as the counter electrode. (c) I–V curves of a NW at different levels of lithium insertion. For this NW, we performed lithium insertion under potential control and monitored its two-terminal I–V behavior at different stages of the lithiation process. A sequential reduction in NW conductance was observed as we proceeded from an initial, unlithiated state to lithiation potentials (versus Li+/Li) of 2.75 V, 2.5 V and 2 V, respectively. (d) A plot showing the change in NW resistance (normalized with respect to the un-lithiated state) as a function of lithium loading for three different NWs. It can be seen that trend is uniform at each of the NWs, resulting in a reduction in conductance by two orders of magnitude. In this plot, the NW resistance was computed at a bias of 3 V at each state. (e) I–V curves of a NW before and after the electrolyte dip-test, confirming that there is no chemical change in the NW due to immersion in the electrolyte and the change in impedance observed in panels (c–d) is purely due to an electrochemical effect. In this test, the NWs were dipped in the electrolyte for 30 min, and their I–V behavior was measured before and after the test.

materials (up to Li1.1MnO2 [23]). Other aspects that emerge from this figure are the reduction in MnO2 capacity and change in the lithiation voltage profile after the first discharge cycle, indicating the possibility of irreversible transformations in the electrode material upon initial lithiation. Next, we employ on-chip electrochemical characterization of single β-MnO2 NWs to gain further insights into the electrochemical behavior observed in the bulk samples of figure 3(a). The on-chip electrochemical characterization of these NWs (set-up illustrated in figure 3(b)) was performed inside a helium filled glove-box. The silicon chip was mounted on an electrical probe station and electrical contact to the nanowire array was made using a micromanipulator with gold-coated probe needles. A mixture of ethylene carbonate and dimethyl carbonate solution (1:1 ratio by volume) with 1 mol L−1 lithium hexafluorophosphate was used as the electrolyte. A droplet of the electrolyte was dispensed on the

chip to cover the nanowire array. A lithium wire (mounted onto a micromanipulator) was inserted into the electrolyte to complete the battery half-cell. Since the current signals during lithiation (estimated to be in the sub-pA range) are below the noise floor of our experimental configuration, we employed potentiodynamic scans to induce lithiation. We incrementally decreased the potential of the nanowire array versus Li+/Li to incrementally increase the amount of lithium inserted into the nanowire. After each stage of incremental lithium loading in the NW, as represented by lithiation potentials of 3 V, 2.75 V, 2.5 V and 2 V, we performed I–V transport measurements on individual NW electrodes in the array, in order to monitor their electronic conductivity changes with progressively increasing degree of lithiation. With this measurement, one can distinguish changes in electrode impedance due to changes in the electronic conductance of the materials themselves, i.e. changes in electrical conductivity of 4

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This is inconsistent with the large and progressive increase in impedance with incremental lithiation, which was observed in our case (figures 3(c), (d)). Furthermore, we have operated within a voltage window (between 2 V to 3.3 V), which is inaccessible for both, reductive and oxidative decomposition of the electrolyte to form an SEI layer. Formation of an SEI layer through reductive decomposition requires voltages below 2 V, while oxidative decomposition requires voltages above 4 V. While formation of an SEI layer through catalytic decomposition of the electrolyte cannot be ruled out, the absence of a step change in measured resistance, which is confined only to a single electrochemical potential supports out argument that the resistance change emerges from the disorder in the NW material. Next, we probed the structural and morphological changes using TEM imaging on representative NWs before and after lithiation. Figures 4(a), (b) show high resolution TEM images indicating the high crystallinity of the NWs prior to lithiation and the lattice disorder following lithiation. Despite the high defect density of the lithiated wire, the structure is still β-phase as determined by electron diffraction. Due to overlap between the Li K-shell and Mn M-shell energies, the Li content in the lithiated NWs is not readily measureable by transmission (EELS). However, another important signature of lithium insertion is an associated reduction in the valence state of manganese in LixMnO2. This reduction in the valence state manifests itself as a shift in the intensity (L3/L2) and branching (L3/(L3 + L2)) ratios of the Mn EELS spectra. This approach based on EELS analysis has been used as a confirmation of lithiation in both, MnO2 and LiCoO2 samples in previous studies [24, 25]. For example, in [24], the branching ratio was observed to reduce in γ-MnO2 from 0.72 to 0.68 for a corresponding reduction in valence state from Mn4+ to Mn3+. An example of the EELS spectra obtained from representative un-lithiated and lithiated β-MnO2 NWs in our case provides a shift in branching ratio from 0.58 to 0.55 (figure 4(c)), and this points to the presence of lithium in our NWs. The combined information from electrical transport, TEM imaging, and EELS spectroscopy measurements, shows that the β-MnO2 NWs undergo irreversible structural transformation due to lithium intercalation during the first cell discharge. Specifically, the NW lattice exhibits a significant degree of disorder with lithiation, which in turn results in a reduction in electronic conductivity by more than two orders of magnitude. Thus, from the second operational cycle of the battery half-cell, the structure and electrical character of the nanowire cathode is substantively different from the as-grown material. This irreversible transformation also explains the change from a flatter voltage profile in the first discharge cycle to the more sloped profile in the subsequent dischargecharge cycles (figure 3(a)). A similar change in the voltage profile was also observed in previous studies of β-MnO2/Li cells [20, 22] and was attributed to a change in lattice parameters (and concurrent expansion in unit cell volume) that occurs during initial lithium insertion [21]. Jiao and Bruce observed that the electrode material after the first discharge and after continued cycling consists of a mixture of pure

individual NWs, rather than changes in electronic conductance between particles, i.e. interfacial impedance. The present platform enables measurement of the intra-particle electronic conductance whereas measurements on mats of NWs include both intra-particle and interfacial electronic conductances. For instance, in the I–V plots shown in figure 3(c), the ‘black’ curve represents the transport characteristic of a nanowire in the assembled (un-lithiated) state. The ‘red’ curve represents the characteristic of the same wire after a potentiodynamic lithiation sweep from 3.3 V to 2.75 V. As can be seen from plot, the electronic conductivity of the nanowire drops by a factor of 2.1 with lithium loading up to a potential of 2.75 V. Further potentiodynamic scans up to lithiation potentials of 2.5 V and 2 V result in progressive reduction in electronic conductivity. The NW conductivity was observed to drop by more than two orders of magnitude upon full lithiation. We find this reduction in conductivity to be irreversible with subsequent charge-discharge cycles, indicating that the nanowire undergoes irreversible structural transformation in the first lithiation step. In addition, this behavior was repeatable across all the NWs in the array, and this can be seen from the normalized plot showing resistance change as a function of the lithiation state for three representative NWs in our sample (figure 3(d)). In panel ‘d’, the nanowire resistance is extracted from the respective I–V curves (for each lithiation state) at an applied of bias of 3 V. In order to confirm that the change in nanowire resistance is due to an electrochemical transformation and not due to a pure chemical reaction in the electrolyte (unlike the observation in [14]), we performed an electrolyte dip test. In this experiment, the NWs were dipped in the electrolyte for about 30 min and then, their electrical transport characteristic was compared with its native state. As can be seen in figure 3(e), there is very little change in NW resistance due to a pure dip in the electrolyte and confirms our argument that the changes in electronic conductivity is due to an electrochemical effect associated with lithium loading. We attribute this behavior to the high lattice disorder, which was observed in the lithiated NWs by TEM (discussed later). It is important to note that the I–V measurements have been performed in a two-terminal configuration, which has also been employed with previous reports on electrochemically correlated transport studies involving single NWs [4, 8]. In this configuration, the measurements include contributions from both, the NW-metal contact and the intrinsic NW resistances. However, we attribute the large changes in electrode impedance to changes in the NW material as opposed to changes in the contact resistance. This argument is supported by the fact that the NWmetal contact architecture remains unchanged with lithiation. SEM images of a representative NW, which were acquired before and after lithiation (figure 4(a)), show no changes within the resolution limits of SEM imaging. Another factor which may influence the contact resistance of the lithiated NW is the solid-electrolyte-interpahse (SEI) layer formation on its surface. However, the formation of an SEI layer typically occurs at a specific electrochemical potential and will cause a step change in NW resistance at this state-of-charge. 5

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Figure 4. (a) SEM images of the NW before (top) and after (bottom) lithiation indicating no visible changes in the NW-metal contact. (b) HRTEM image of a pristine MnO2 NW showing that it possesses a single crystalline structure. Diffraction pattern in the inset shows the NWs are in beta-phase. (c) Image of a fully lithiated NW (up to 2 V versus Li+/Li) showing that the NW contains a high degree of lattice disorder upon lithiation. (d) EELS spectra of the Mn L2,3 edge of the pristine (blue) and lithiated (green) nanowires. The spectra show a shift in the intensity (L3/L2) and branching (L3/L3 + L2) ratios caused by a lithiation-induced reduction in the valence state of Mn. The branching ratio for the lithiated NW shown here is 0.55, as opposed to the branching ratio of 0.58 for the un-lithiated NW.

β-MnO2 (electrochemically inaccessible) and β-LixMnO2 (which has the same crystal structure with an expanded a lattice parameter and slightly contracted c lattice parameter relative to β-MnO2) [21]. This is in agreement with spectroscopic measurements presented here showing β-phase material before and after initial lithiation. The sloping voltage profiles observed in subsequent cycles are evidence of continued single-phase lithium insertion/extraction in β-LixMnO2. Another important impact of the reduction in NW conductivity during the first discharge step is the observed capacity fading that is specific to the first cycle. In the case of data shown in figure 3(a), the capacity fading was observed to be 22.8% from the first to second discharge cycle (the capacity changes remained within a few percent in the subsequent cycles). A similar reduction in capacity has been observed in reports on Li-ion battery studies based on NW electrodes [4, 8], and our results show that irreversible structural changes upon first lithiation could be a potential source for this performance limiting behavior in NWs. Our observations are also in agreement with those of vanadium oxide NW electrodes, which exhibited decreased capacity induced by decreasing electronic conductivity [14].

4. Conclusions In summary, we have developed a hybrid nanomachined platform for performing Li-ion battery diagnostics on individual NWs. We have used β-MnO2 NWs as a case study to perform on-chip electrochemistry using conventional electrolytes inside a glovebox followed by atomic-scale structural characterization inside the TEM. Using a combination of electrical transport measurements, TEM imaging and spectroscopic techniques, we have provided insights into the irreversible, lithiation-induced structural transformation in the NWs at the end of the first cell discharge. Combined with our demonstrated capability to use the DEP-based nanoassembly technique for localizing the battery nanoelectrode, we anticipate this platform to be a powerful tool to interrogate and optimize the performance of nanostructured battery electrodes.

Acknowlegements This work was supported by a Laboratory Directed Research and Development (LDRD) project at Sandia National 6

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Laboratories and partly by the Science of Precision Multifunctional Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. In addition, this work was performed, in part, at the Sandia-Los Alamos Center for Integrated Nanotechnologies (CINT), a US Department of Energy, Office of Basic Energy Sciences user facility. The LDRD supported the development and fabrication of the MEMS platform and the development of TEM techniques. The NEES center supported some of the additional platform development and fabrication and materials characterization. CINT supported the TEM capability and the fabrication capabilities that were used for the TEM characterization and the synthesis of the unsealed platforms. This work was also partly supported by the Welch Foundation Grant C-1716 and by the National Science Foundation under Grant No. 1266438. We would also like to thank G Nagasubramanian, D Ingersoll, S Hearne, J Nogan, and D Huber for assistance with ideas, materials, techniques, and equipment. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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References [1] Tarascon J-M and Armand M 2001 Nature 414 359 [2] Arico A S, Bruce P, Scrosati B, Tarascon J-M and Schalkwijk W V 2005 Nat. Mater. 4 366 [3] Wen Z H, Lu G H, Mao S, Kim H, Cui S M, Yu K H, Huang X K, Hurley P T, Mao O and Chen J H 2013 Electrochem. Commun. 29 67

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On-chip lithium cells for electrical and structural characterization of single nanowire electrodes.

We present a transmission electron microscopy (TEM)-compatible, hybrid nanomachined, on-chip construct for probing the structural and electrical chang...
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