Matthew T. Janish

TEM in situ lithiation of tin nanoneedles for battery applications

Materials such as tin (Sn) and silicon that alloy with lithium (Li) have attracted renewed interest as anode materials in Li-ion batteries. Although their superior capacity to graphite and other intercalation materials has been known for decades, their mechanical instability due to extreme volume changes during cycling has traditionally limited their commercial viability. This limitation is changing as processes emerge that produce nanostructured electrodes. The nanostructures can accommodate the repeated expansion and contraction as Li is inserted and removed without failing mechanically. Recently, one such nano-manufacturing process, which is capable of depositing coatings of Sn “nanoneedles” at low temperature with no template and at industrial scales, has been described. The present work is concerned with observations of the lithiation and delithiation behavior of these Sn nanoneedles during in situ experiments in the transmission electron microscope, along with a brief review of how in situ TEM experiments have been used to study the lithiation of Li-alloying materials. Individual needles are successfully lithiated and delithiated in solid-state half-cells against a Li-metal counter-electrode. The microstructural evolution of the needles is discussed, including the transformation of one needle from single-crystal Sn to polycrystalline Sn–Li and back to single-crystal Sn.

Lithiation of Tin Nanoneedles Investigated by in-situ TEM

In the search for a better anode material for lithium-ion batteries (LIBs), elemental Sn has generated considerable interest due to its high theoretical specific capacity of 994 mAh/g. However, neither bulk material nor continuous films of Sn are useful in practice because the large volume change that occurs during lithiation and delithiation causes mechanical failure of the electrode [1]. Arrays of one-dimensional Sn nanostructures have been used to circumvent this, as they incorporate enough empty space into the structure to accommodate the expansion and contraction that occur during cycling. This reduces the associated stresses and prevents mechanical failure [2]. While most processes used to create nanostructures are slow and resource-intensive, a template-free, low-temperature, industry-scalable method for preparing nanostructured tin anodes has been reported [3]. The present study reports on the microstructural changes that these materials undergo during lithiation and delithiation as observed through in-situ experiments in the transmission electron microscope (TEM).

Erratum to: Template-free electrochemical synthesis of tin nanostructures

The images presented in Fig. 1a and b of Mackay et al. [1] were not obtained from samples produced using the specific conditions described in the Experimental section on page 1477. Replacement figures from samples prepared under the conditions described in the text [1] using a deposition current of 35 mA/cm are provided in this Erratum. Owen and Norton have recently shown [2] that nanoneedle structures can be produced using a range of deposition conditions; the material shown in the corrected figure was obtained with the best known conditions at the time of submission. The analysis of the structures and the way that these needles interact with, e.g., Li [3] are not affected by the error since the analysis in this paper was carried out on material prepared under the conditions described in [1].

Microtomy on Heat-Treated Electro-Spun TiO 2 Fibers

1 Dept of Physics and Astronomy. UTSA. One UTSA Circle, San Antonio, TX 78249 2 Dept of Chemical & Biomolecular Engng, U. of Connecticut, 191 Auditorium Rd, Storrs, CT 06269 3 Sandia National Laboratory, Materials Characterization Dept, MS 0886, Albuquerque, NM 87185 4 Dept of Materials Science & Engineering, U. of Connecticut, 191 Auditorium Rd, Storrs, CT 06269 5 Institute of Materials Science, U. of Connecticut, 97 North Eagleville Road, Storrs, CT 06269

Observations on Heavily Deformed Tantalum

The deformation of metals having the body-centered cubic (BCC) crystal lattice [1] is thought to occur by the glide of dislocations with a Burgers vector of 1 /2<111> [2, 3] or by twinning [4]. The present study is concerned with the deformation of the BCC, period-6 (third-series), transition metal, Tantalum [5, 6]. Although the slip planes have been discussed and modeled since before 1970 (see [3]), there are still many questions concerning the planes on which the screw dislocations glide and how the slip systems differ for different BCC metals. It is with these questions in mind that the authors are examining samples of Ta that have been heavily deformed under well controlled conditions. The material examined in the present study was obtained as a textured sputter target from the H.C. Starck company. Conventional tensile specimens were prepared from bulk disks and cut to shape using electro discharge machining (EDM) following the procedures used previously for Ta metal obtained from a different source [5]. The TEM analysis was performed on an FEI Tecnai F30 TEM operating at 300kV and on an FEI Titan G2 80-200 equipped with Chemi-STEM Technology operated at 200 kV. Part of the deformed sample that was used to extract FIB TEM specimens is shown in Figure 1a. The surface of the deformed material was quite uneven and showed effects of the EDM. However at a distance of 1 to 2 μm below the surface, the microstructure of the deformed Ta was clearly visible and consisted of very small grains with few dislocations in some parts (e.g., in Figure 1b), and relatively large grains in other parts containing both dislocations and sub-grain boundaries as illustrated in Figure 2; no contamination from the EDM was detected in these regions. Additional analysis using the chemical analysis capabilities of the Titan TEM was performed to determine the presence and location of any impurities, including oxygen, in the specimens. The structure of the Ta and of selected grain boundaries was determined using calibrated high-resolution imaging and diffraction pattern analysis. The initial results of this analysis, some of which are indeed surprising, will be presented.

Heat Treatment of TiO 2 /SiO 2 Electrospun Ceramic Fibers

Nano-TiO2 is currently one of the most interesting topics of study in materials science and beyond, and is being used in a wide variety of applications [1]. However, producing crystalline TiO2 nanostructures, other than simple powders, can pose significant challenges: growing such structures in the crystalline state tends to be slow and expensive, and while this can be overcome by fabricating amorphous structures quickly and cheaply, handling these materials after the subsequent heat treatment will reduce them to a powder. Dispersing TiO2 particles on a mechanically robust support is a common method for overcoming this issue. One processing pathway for doing so is to electrospin a TiO2/SiO2 solution into fibers followed by heat treatment, which causes the two immiscible materials to phase separate and the TiO2 to crystallize [2, 3]. A forthcoming publication describes how this process has recently been improved to drastically increase the TiO2 content of such fibers [4]; this work is concerned with the details of the phase separation of the TiO2 and SiO2 during heat treatment and the crystallization of the TiO2.

Using TEM Operando Methods to Understand Energy Storage

The structure and electrochemistry of battery technologies is well established but is inadequate for anticipated future needs (e.g., [1, 2]). One aspect that has not been examined thoroughly is exactly how the materials change during cycling between charging and discharging modes. Most studies of these processes rely on electrochemical measurements and indirect assumptions of the processes that are actually involved. The structural changes that occur have been assumed to make certain systems unusable but if the dimensions of the device are changed or if the morphology of the materials involved is reduced, these materials may become of interest, especially if combined with other candidate materials, e.g., in SnO2/Sn or Sn/C composites.

Microscopy of the Deformation of Tantalum

The deformation behavior of metals with the fcc structure is well-characterized and understood, but the same cannot be said of the bcc metals. This rather wide knowledge gap is attributable to the difference in structure and bond character of the two classes of materials. Plastic deformation in metals is achieved by the motion of dislocations, and the motion of dislocations in close-packed structures is relatively easy to predict. Because the bcc structure is not close packed (due to more complicated, partially covalent bonding), it stands to reason that dislocation motion, and therefore plastic deformation, is still not wellunderstood for the metals with this structure. Tantalum, a refractory bcc transition metal, is used in microelectronics for capacitors and resistors, in medical applications for surgical instruments or as an implant material, in X-ray lithography for masks, and in high-temperature structural applications, such as heat exchangers. While the bulk metal has the bcc structure, deposited thin films can be amorphous or have the beta-Ta or fcc structure [1, 2]. The fcc phase and small regions of a hcp phase have been identified in heavily deformed bulk Ta [3]. Twinning has been observed as a result of the indentation of thin films, but this has now been shown to be due to the formation of the fcc phase rather than twinning in the bcc phase [4]. In the present study we examine dislocations, in Ta, produced in different ways. First, thin films of Ta were deformed using nanoindentation. This method provides the model system; the thickness of the material can be varied and the films can be readily prepared for examination in the TEM. Then, different deformation processes were applied to bulk materials: indentation of pristine Ta, dynamical deformation of pristine Ta, and indentation of material already dynamically tested. These methods provide the “real material” for comparison to the model system; the latter is particularly interesting since the material is presumably already heavily deformed so that the pre-existing defects strongly influence the mechanical deformation. Figure 1 shows a low-magnification bright-field image of an indent in a thin Ta film on a Si substrate. The contrast in the image indicates that the film is polycrystalline with a somewhat columnar grain structure; this is a commonly observed microstructure in thin Ta films. The inset diffraction pattern identifies the bcc phase. Figure 2 is a high-resolution image from the indicated area in Fig. 1, where the strain is expected to be the highest. Although much of the image is dominated by {110} fringes from the bcc phase, the inset FFT from the indicated region of interest shows spots corresponding to the {111} planes of the fcc phase as reported in [2] and [3]. Figure 3 is an EBSD map of a dynamically deformed, indented Ta sample; the map was used to inform the location of FIB lift-outs and the TEM results will be presented. [6]

Observations on Orientation Relationships between Rutile and Brookite

TiO2 exists in nature in three different forms, namely rutile, anatase and brookite; it can also be prepared in the laboratory in each of these forms although the first two are by far the most common [1]. The transformation of anatase to rutile is well documented but that for brookite to rutile is not, in part because brookite is not often observed. It is common to find brookite and anatase together, where anatase is the major phase. These two phases could be easily taken for just one due to very similar diffraction patterns when analyzed by XRD. Anatase and brookite coexist at a consistent fraction until 600 oC, after which the fraction of brookite will decrease. Above 1000 oC both phases completely transform into rutile. In this context, understanding the mechanisms of the transformation process, the effect of impurities, and their relationship with the crystal structure in TiO2 polymorphs becomes relevant for new developments.