Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared

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Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared by Catalytic Decomposition of Fluoropolymer Vijayakumar Murugesan, Jong Soo Cho, Niranjan Govind, Amity Andersen, Matthew J Olszta, Kee Sung Han, Guosheng Li, Hongkyung Lee, David Reed, Vincent L. Sprenkle, Sung-Jin Cho, Satish K Nune, and Daiwon Choi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01983 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared by Catalytic Decomposition of Fluoropolymer Vijayakumar Murugesana, Jong Soo Chob, Niranjan Govinda, Amity Andersena, Matthew J. Olsztaa, Kee Sung Hana, Guosheng Lia, Hongkyung Leea, David M. Reeda, Vincent L. Sprenklea, Sungjin Chob, Satish K. Nunea,* and Daiwon Choia,* Pacific Northwest National Laboratory, 902 Battelle Boulevard, P. O. Box 999, Richland, WA 99354, USA. a

Joint School of Nanoscience and Nanoengineering, North Carolina Agricultural and Technical State University, Greensboro, NC 27401, USA b

*Corresponding author: [email protected]; phone: 509-375-4341; fax: 509-375-4448. [email protected]; phone: 509-371-6632; fax: 509-375-4448

Abstract Metal fluorides, with high redox potential and capacity from strong metal-fluoride bond and conversion reaction, makes them promising cathodic materials. However, detailed lithium insertion and extraction mechanisms have not yet been clearly understood and explained. Here we report low temperature synthesis of electrochemically active FeF3/FeF2 nanoparticles by catalytic decomposition of a fluoropolymer [perfluoropolyether (PFPE)] using a hydrated iron oxalate precursor both in air and in inert atmosphere. Freshly synthesized FeF3 nanoparticle delivered specific capacity above 210 mAh/g with decent cycling performance as a Li-ion battery cathode. Both in situ and ex situ characterization techniques were used to investigate the detailed PFPE decomposition and fluorination mechanisms leading to FeF3/FeF2 formation as well as the lithium insertion mechanism in a FeF3 cathode. Specifically, a detailed understanding was investigated using thermogravimetry–mass spectroscopy (TGA-MS), X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), and X-ray absorption near-edge structure (XANES). The novel synthesis route developed not only offers access to electrochemically active metal fluorides, but also offers a catalytic approach for decomposing highly inert fluoropolymers for environmental protection.

Keywords: Li-ion battery, cathode, iron fluoride, nanoparticle, fluorination, fluoropolymer, FeF3, FeF2.


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Introduction Recent concern over sustainability of the environment has engendered great interest in managing renewable energy resources by use of energy storage technologies.1-2 Among the many energy storage systems, the Li-ion battery is projected to be the leading technology for electric vehicles as well as stationary applications1-4 due to dramatic cost reduction along with steady improvement in energy density and cycle life in recent decades.5 Nevertheless, the specific capacity of the layered oxide cathodes widely used in commercial cells today limits the energy density from further advancement. Thus, a new strategy to increase the electrode capacity by storing more than one Li ion per transition metal atom via conversion reaction was proposed by the pioneering work of Tarascon and coworkers.6 In conversion reactions, the transition metal in initial metal nitrides,7 sulfides, fluorides,8-9 and oxides6, 9 is reduced by lithium to give LiX (X = H, N, O, F, P or S) and pure metal. Metal fluorides were the first conversion-type cathode material proposed by Arai et al.10 in the late 1990s and have been extensively studied by Amatucci and coworkers.11-14 The first reported FeF3 had a reversible capacity of 80 mAh/g,10 which was far below the theoretical capacity of 237 mAh/g from the Fe3+/Fe2+ redox couple, but attracted substantial interest due to a decent potential of 2.7 V vs. Li/Li+ and a large theoretical capacity of 712 mAh/g (1950 Wh/kg) from a complete three-electron reaction.9, 15-17 Transition metals at high oxidation states with strong ionic M-F bonds provide a high redox potential, but due to its insulating nature and structural irreversibility, FeF3 has been ignored as rechargeable cathode material. To overcome the sluggish kinetics and reversibility, particle size reduction to nanometer scale has been implemented to increase the surface area for better carbon mixing and to shorten the electron/ion paths.18-19 Various fabrication routes to obtain metal fluoride/carbon nanocomposites have been explored, including mechanical milling,17 precipitation,20-21 solvothermal,22-23 hydrothermal,23-24 ionic liquid,16, 25 thin film deposition,26 and heterogeneous nucleation on carbon nanotubes or graphene.20, 24, 27-29 In particular, a top-down approach by high-energy mechanical milling has been widely used to create nano-domains (10~30 nm) of FeF3, but often led to difficulties in controlling size as well as defect formation, causing undesirable side reactions that reduce reversibility during cycling. Recently, liquid precipitation routes have been widely used for better size control but require use of highly corrosive HF and an additional heat-treatment step to remove the lattice water.20, 23-24 In addition, surface protective coating and electrolyte optimization have significantly improved the cycling performance.30 Previously, surface fluorination of stainless steel leading to iron fluoride (FeF3) formation by use of perfluoropolyether (PFPE)-based lubricants was reported by NASA in 1989, but the detailed reaction mechanism has not been investigated.31-32 Similar to other perfluorinated compounds, PFPEs are low molecular weight, fluorinated oligomers that are chemically inert, thermally stable, and resistant to degradation under a wide range of conditions because of strong C-F bonds, which arise from the small covalent radius size and the high electronegativity of the fluorine atom. Because they are nonvolatile and their viscosities are nearly temperature independent, PFPEs have been commonly used as surface lubricants in severe environments such as in magnetic recording material, automotive applications, aerospace engines, satellite instruments, and machining.33 However, due to their environmental persistence and wide global distribution in wildlife and humans, PFPEs have been identified as long-lived environmental and atmospheric pollutants with



large global warming potentials.34 Hence, safer and easier methods for chemically manipulating fluorocarbons to either degrade or add values to these materials are being investigated. However, very limited work has been done on catalytic C-F activation by homogeneous metal complexes, highlighting the need to develop methods for materials from PFPEs. In the present work, a novel approach to form iron fluorides through thermochemical fluorination of hydrated iron oxalate precursors using fluoropolymers (PFPEs) for energy storage applications was explored. The iron oxalate decomposition and fluorination mechanisms have been studied under ambient air and argon atmospheres. Freshly synthesized FeF3/FeF2 nanoparticles were subjected to Li-ion battery cathode materials. The crystal structural and phase changes during initial lithiation of a FeF3 cathode have been investigated using ex situ X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) and comprehensive lithium insertion/extraction mechanisms are proposed for the first time. Experiment and Modeling Iron fluoride (FeF3 or FeF2) nanoparticles were synthesized using Fe(C2O4)•2H2O (reagent grade, Sigma) and Krytox grease (GPL 205, DuPont) (11% polytetrafluoroethylene [PTFE] oligomer and 89% PFPE). A precursor paste was made by mixing 3 g of Fe(C2O4)•2H2O with 4 g of Krytox grease in a Teflon Jar for 10 min using a high-energy mill (SPEX 8000M). The resultant precursor paste mixture was heat treated at 300°C for 10 h under air or Ar flow at a heating rate of 5°C/min. The reaction products obtained at various heat-treatment stages and conditions were characterized by using attenuated total reflectance (ATR, Pike MIRacle), which is in conjunction with Fouriertransform infrared (FTIR, Bruker 70) spectroscopy. Each ATR spectrum was accumulated for 128 scans with a resolution of 2 cm-1. 19F solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments were performed on a 600 MHz solid-state NMR spectrometer (Bruker, Germany) at a Larmor frequency of 564.66 MHz with a 2.5 mm HXY MAS NMR probe at a spinning speed of 25 kHz and at 293 K. The 90 degree pulse length and repetition delay were 5 µs and 1 s, respectively. The solid-state reactions during heat treatment of the Fe(C2O4)•2H2O and Krytox mixture in air/Ar were examined by thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) (TGA/DSC STA 449, Jupiter Netzsch instrument) equipped with a mass spectrometer (MS) (Aelos QMS 403C, 1 µ to 300 µ) using a standard electron impact ionization detector. The samples were loaded in alumina crucibles and transferred to the TGA-MS instrument and heated under ultrahigh pressure (UHP)-Ar/air flow up to 300°C at a heating rate of 5°C/min. The gases evolved during heating were passed through a heated fused silica capillary into the MS to obtain mass analysis during heating. The microstructures of the final iron fluorides were analyzed by a field-emission scanning electron microscope (FESEM, FEI Nova 600) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010). The crystal structural and phase evolution during different stages of heat treatments and electrochemical discharge/charge cycles of iron fluorides were characterized by using an X-ray diffractometer (XRD, Mini-Flex II, Rigaku Inc., Madison, WI) with a CuKα sealed tube (λ = 1.54178 Å). The Rietveld refinements on the obtained XRD patterns were carried out by using the X’pert Plus (PANalytical) program. The XANES data were collected at beamline (BL) 6.3.1.2 located at the Advanced Light Source, Lawrence Berkeley National Laboratory. The BL 6.3.1.2 with bend


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magnet offers a focused beam at 50 × 500 µm2 within an energy region of 250–2000 eV. The spectra were collected in both total electron yield (TEY) and total fluorescence yield (TFY) modes concurrently, at room temperature, and normalized by the current of the Au grid upstream of the beamline. All spectra were collected using respective powder/electrode samples loaded in a vacuum-chamber end-station to avoid atmospheric contamination. The collected spectra were subsequently processed with the ATHENA program to remove the background and apply appropriate normalization procedures. The theoretical XANES spectra were calculated using the restricted excitation window – time-dependent density functional theory (REW-TDDFT) approach, including multipole contributions to the oscillator strengths, as implemented in NWChem.35-36 This approach, which involves defining a restricted subspace of single excitations from the relevant core orbitals and no restrictions on the target unoccupied states, is valid because excitations from the deep core states are well separated from pure valence-level excitations. We have successfully used this approach in several studies over the last few years.37-38 In all our XANES calculations, the Sapporo-TZP-2012 all-electron basis set39 was used for the F absorbing center, while the other atoms were treated with the Stuttgart RLC ECP (Li, F)40 and Stuttgart RSCECP (Fe),41 respectively. The capping hydrogens were treated with the 6-31G* basis set.42 The exchange-correlation functional was treated with the PBE0 functional.43 For comparison with experimental values, the spectra were Lorentzian broadened by 1.5 eV. For electrochemical evaluation, a cathode electrode was prepared by mixing FeF3 powder with Super P carbon black in a planetary mill (Retsch 100CM) for 2 h at 400 rpm. The final active material, carbon black, and PVDF binder weight ratio was 80:10:10, with the active iron fluoride loading on Al foil being 1~2 mg/cm2. For studying the lithium insertion/extraction mechanisms using XRD and XANES characterizations, polyacrylic acid (PAA) binder was used to prevent fluorine signal from PVDF and discharge/charge current of 10 mA/g was applied. The FeF3 electrodes were electrochemically cycled in type 2325 coin cells (National Research Council, Canada) using an Arbin Battery Tester (BT-2000, Arbin Instruments, College Station, TX) at room temperature. A half-cell was assembled with 1 M LiPF6 in EC/DMC (ethyl carbonate/dimethyl carbonate, v/v 1:1) electrolyte and a Li metal anode. The Li-ion cells were tested between 2.0 and 4.5 V vs. Li+/Li at a charge/discharge rate of C/20 based on the theoretical capacity of 235 mAh/g. The FeF3 electrodes were collected from the coin cell at various discharged or charged states and washed four times in DMC to remove electrolyte salts before they were subjected to XRD and XANES characterizations.

Results and Discussion Iron Fluoride Formation Mechanism Pure crystalline FeF3 and FeF2 were synthesized by selective decomposition of PFPEs and in situ fluorination of iron oxalate via heat treatment at 300°C in air and argon atmospheres, respectively. Figure 1(a, b) shows the Rietveld refinements on the XRD patterns of both the FeF3 and FeF2 powders obtained. The FeF3 phase was refined using a trigonal R3c space group (a = b = 5.200 Å, c = 13.323 Å,  =  = 90°,  = 120°, JCPDS No. 00-033-0647) whereas the FeF2 phase was based on a tetragonal P42/mnm space group (a = b = 4.6974 Å, c = 3.3082 Å,  =  =  = 90°, JCPDS



No. 04-005-4204).44 Detailed structural information can be found in Table S1 of the electronic supplementary information (ESI).

Figure 1. Rietveld refinements of XRD patterns of as-prepared (a) FeF3 and (b) FeF2 nanoparticles. Microstructural FESEM and HRTEM images of the synthesized FeF3 and FeF2 particles are shown in Figure 2 (See also Supplementary Figure S1, ESI). They consist of primary nanoparticles (87% of the FeF2 theoretical capacity of 571 mAh/g during the first discharge, as shown in Figure S6 (b, c), (ESI). However, FeF3 particle surface seems to be significantly affected by the electrolyte LiPF6 salt, which is observed in TEY mode of XANES measurements. The electrochemical cycling results obtained here are from plain FeF3 and FeF2 materials without further synthesis optimization since the focus of our current study is mainly to understand the reaction mechanisms. Better carbon mixing, surface protective coating and electrode materials design would yield further improvement in electrochemical performance.30



Conclusion Iron fluoride (FeF3 and FeF2) nanoparticles were synthesized by a novel approach involving catalytic decomposition of a fluoropolymer (Krytox PFPE grease) at 300°C in both air and inert atmospheres. The detailed fluoropolymer decomposition and iron fluoride formation mechanisms have been proposed and discussed based on analyses conducted using FTIR, NMR, TGA-DSCMS, and XRD. The obtained FeF3 nanoparticle was electrochemically evaluated as a Li-ion battery cathode and the structural changes during discharge/charge was studied by using ex situ XRD and XANES. The initial lithiation and crystal structural changes in the FeF3 cathode occur through transformations involving two different phases. First, a phase transformation with crystal structure of intermediate triclinic Li0.5FeF3 phase is formed, followed by topotactic single-phase conversion reaction, giving a sloped voltage profile.

Acknowledgements Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated by Battelle for the U.S. Department of Energy (DOE) under Contract DE-AC05-76RL01830. This work was supported by the DOE Office of Electricity Delivery and Energy Reliability (OE) under Contract No. 57558. NMR investigations were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. The collection of XAS experimental spectra were supported by PNNL’s Laboratory Directed Research and Development (LDRD) program. This research used resources of the Advanced Light Source (ALS), which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231. Vijayakumar Murugesan thanks the ALS approved partner program (AP) for providing beamline access, and Dr. Jinghua Guo for support and help in XAS measurements at beamline 6.3.1.2.

Supporting Information Additional images of microstructures, TGA-MS spectra, crystal structures and performance data of iron fluorides are included with detailed crystal structural information used for XRD Rietveld refinements.

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Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared

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