We Report on the Synthesis, Multifaceted Characterization, And

BNL-107893-2015-JA TOC Figure: We report on the synthesis, multifaceted characterization, and electrochemical activity of crystalline, chemically pure 200 nm lithium iron phosphate nanowires, mediated by using a seedless, surfactantless U-tube method. Sentence summary of work: We have developed a novel ambient preparative method for the fabrication of chemically pure, highly crystalline 200 nm lithium iron phosphate nanowires. 2 Ambient Synthesis, Characterization, and Electrochemical Activity of LiFePO4 Nanomaterials derived from Iron Phosphate Intermediates Jonathan M. Patete,1,⊥ Megan E. Scofield,1,⊥ Vyacheslav Volkov,2 Christopher Koenigsmann,1 Yiman Zhang,1 Amy C. Marschilok,1,3 Xiaoya Wang,1,4 Jianming Bai,5 Jinkyu Han,2 Lei Wang,1 Feng Wang,4 Yimei Zhu,2 Jason A. Graetz,4,# and Stanislaus S. Wong1,2,* Email: [email protected]; [email protected] 1Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400 2Condensed Matter Physics and Materials Sciences Department, Building 480, Brookhaven National Laboratory, Upton, NY 11973 3Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275 4Sustainable Energy Technologies Department, Building 815, Brookhaven National Laboratory, Upton, NY 11973 5National Synchrotron Light Source II, Building 741, Brookhaven National Laboratory, Upton, NY 11973 # Current address: Sensors and Materials Laboratory, HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, CA 90265-4797 ⊥These authors contributed equally to this work. *To whom correspondence should be addressed. 3 Abstract: LiFePO4 materials have become increasingly popular as a cathode material due to the many benefits they possess including thermal stability, durability, low cost, and long life span. Nevertheless, to broaden the general appeal of this material for practical electrochemical applications, it would be useful to develop a relatively mild, reasonably simple synthesis method of this cathode material. Herein, we describe a generalizable, 2-step methodology of sustainably synthesizing LiFePO4 by incorporating a template-based, ambient, surfactantless, seedless, U- tube protocol in order to generate size and morphologically tailored, crystalline, phase-pure nanowires. The purity, composition, crystallinity, and intrinsic quality of these wires were systematically assessed using TEM, HRTEM, SEM, XRD, SAED, EDAX, and high-resolution synchrotron XRD. From these techniques, we were able to determine that there is an absence of defects present in our wires, supporting the viability of our synthetic approach. Electrochemical analysis was also employed to assess their electrochemical activity. Although our nanowires do not contain any noticeable impurities, we attribute their less than optimal electrochemical rigor to differences in the chemical bonding between our LiFePO4 nanowires and their bulk-like counterparts. Specifically, we demonstrate for the first time experimentally that the Fe-O3 chemical bond plays an important role in determining the overall conductivity of the material, an assertion which is further supported by recent first principle calculations. Nonetheless, our ambient, solution-based synthesis technique is capable of generating highly crystalline and phase-pure energy-storage-relevant nanowires that can be tailored so as to fabricate different sized materials of reproducible, reliable morphology. Keywords: ambient synthesis; template synthesis; cathode material; lithium iron phosphate; nanostructures. 4 1. Introduction LiFePO4 materials have become increasingly popular as a cathode material, due to the many benefits they possess including thermal stability, durability, low cost, and long life span. Nevertheless, to broaden the general appeal of this material for practical electrochemical applications, it would be useful to develop a relatively mild, reasonably simple synthesis method 1, 2 of this cathode material. Since the seminal work performed by Goodenough and co-workers, olivine LiFePO4 has attracted the most interest due to its low cost, low toxicity, high thermal stability, and excellent electrochemical properties. Specifically, LiFePO4 exhibits a high, flat voltage profile, good cycle stability, and a high theoretical specific capacity (~170 mAh/g).3, 4 The material also possesses a relatively high lithium intercalation voltage of 3.5 V, relative to 3, 5 lithium metal. Moreover, the lifetime of a LiFePO4 battery has been estimated to extend to more than 2,000 cycles, which is key to producing commercial cells with high electrochemical durability and stability. As shown in Equation 1, the discharge of LiFePO4 involves the intercalation of Li+ along with the uptake of an equivalent number of electrons: + - FePO4 + Li + 1e → LiFePO4 E° = 3.5 V (1) The olivine crystal structure of LiFePO4 possesses a slightly distorted hexagonal-close 2+ packed array of oxygen atoms, wherein 50% of the octahedral sites are occupied by Fe and + 6 12.5% are occupied by Li . The FeO6 octahedra are corner shared and the LiO6 octahedra are edge shared with the Li+ ions, forming a continuous chain down the [010] crystallographic direction.6, 7 The olivine phase is uniquely advantageous, because the structural matrix formed by the iron-oxygen octahedral complex in LiFePO4 does not change significantly upon de- 8, 9 lithiation. By contrast, layered structures, such as LiCoO2, undergo significant structural reconfiguration, when the lithium ion content is decreased below a certain amount.10 In essence, 5 the olivine structure is anticipated to be more robust for long-term applications in Li-ion batteries, because the relatively stable structure should promote increased reversibility of the lithiation/de-lithiation process. Improvements to the lithium ion diffusion rate have also been successfully demonstrated by reducing the dimensions of the LiFePO4 material to the nanoscale regime. For instance, a reduction in particle size from the bulk to the nanoscale would minimize the path length for Li+ ion diffusion and facilitate electron transport through the material. It has also been suggested that nanoparticles maintain less mechanical strain, thereby enabling faster lithium ion diffusion into 4, 7 the material upon reversible intercalation, which would allow for improved cycle lifetimes. Nanostructured LiFePO4 also possesses increased surface area-to-volume ratios as compared with their bulk analogues, which facilitates electrochemical performance by increasing the 4, 6, 7 interface between the metal oxide and the electrolyte. As such, there have been extensive 11-19 reports regarding the preparation and characterization of LiFePO4 nanostructures. In particular, one-dimensional (1-D) nanomaterials, such as nanowires, nanotubes, nanorods, and nanoribbons, are expected to play a significant role in advancing LiFePO4 battery performance, due to their uniquely advantageous structural and electronic properties.3, 4, 20-32 For instance, computational analysis has shown that although there are three potential Li+ ion diffusion pathways, the preferred pathway is oriented along the b-axis (0.55 eV), wherein the + 33, 34 Li ions form a continuous chain through the FePO4 matrix. Therefore, an increased rate performance can be achieved in a nanowire system by selectively growing the material such that either the a- or c-axis is oriented along the anisotropic growth direction of the nanowire. Preferential growth along either the a- or c-axis would also enable the selective orientation of the b-axis and the Li+ ion channels along the radial aspect of the nanowire (e.g. parallel to the 6 diameter of the nanowire), which is confined to the nanoscale. This would effectively minimize the Li+ ion diffusion length through the material and also promote better performance at high rates of charge and discharge. Our synthesis method herein forms iron phosphate as the initial product, allowing for direct electrochemical evaluation of the FePO4 moiety. Prior reports have indicated that success in the electrochemical lithiation of iron phosphate materials can be very sensitive to specific structural properties, depending on the crystallinity and the phase of the FePO4 material. For example, a prior report yielded a cycle 2 specific discharge capacity of 76 mAh/g for an amorphous FePO4•2 H2O material, but only 18 mAh/g for a more crystalline hexagonal FePO4 35 material prepared at 500°C. Similarly, carbon nanotube-amorphous FePO4 core–shell nanowires realized a specific capacity of 175 mAh/g in lithium batteries36 and 120 mAh/g in 37 sodium batteries, respectively, but with ultra-thin amorphous coatings of FePO4 comprising only a few nm in thickness. A limitation of these prior studies was a lack of discernible X-ray diffraction patterns in each case. By contrast, herein, through directed control of synthesis properties, we can tailor the aspect ratio and size of FePO4 material, thereby providing for an opportunity to evaluate function with respect to electrochemical lithiation for nanowire FePO4 material relative to bulk-type granular FePO4 material. Recently, there have been a number of successful reports generating 1-D LiFePO4 nanomaterials primarily through hydrothermal and electrospinning-based techniques.3, 4, 21-23 By contrast, template-directed methods represent a conceptually straightforward approach for the synthesis of 1-D nanostructures.38 In general, the template acts as a structural framework for the nucleation and growth of materials

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