In Situ XRD and TEM Studies of Sol-Gel-based. Synthesis of LiFePO4

Transcription

1 In Situ XRD and TEM Studies of Sol-Gel-based Synthesis of LiFePO4 Dominika A. Ziolkowska a,*, Jacek B. Jasinski b,*, Bartosz Hamankiewicz c, Krzysztof P. Korona a, She-Huang Wu d and Andrzej Czerwinski c,e a Faculty of Physics, University of Warsaw, Pasteura 5, Warsaw, Poland b Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY 40292, USA c Faculty of Chemistry, University of Warsaw, Pasteura 1, Warsaw, Poland d Department of Materials Engineering, Tatung University, No.40, 3rd Sec, Zhongshan N. Rd., Taipei 104, Taiwan e Biological and Chemical Research Centre, University of Warsaw, Zwirki i Wigury 101, Warsaw, Poland * Corresponding authors: dominika.ziolkowska1@gmail.com, jacek.jasinski@louisville.edu Supporting Information

2 porosity Figure S1. TEM analysis of a LiFePO4 precursor: SAED pattern showing an amorphous phase, TEM and HRTEM images showing an amorphous form and grain porosity (examples shown in circles), EELS spectrum of the C K-edge showing sharp fine structure lines from the organic component of the precursor material. 2

3 d101=4.27 Å Figure S2. TEM analysis of LiFePO4 material annealed in the furnace at 700 o C: TEM image showing grain porosity, HRTEM images revealing amorphous carbon structure and welldefined planes and lattice fringes of LiFePO4 crystallites (d101 = 4.27 Å), EELS spectra of the C K-edge typical for amorphous carbon. 3

4 Figure S3. SEM and XRD analysis of LiFePO4 material ex situ annealed in the furnace at 700 o C: SEM images show grain porosity and crystallite agglomeration, XRD pattern is typical for orthorhombic LiFePO4 Pnma crystallites (ICDD PDF # ). 4

5 Figure S4. Diffraction pattern of a sample annealed in an excess of oxygen showing two phases of α-fe2o3 (PDF) and Li3Fe2(PO4)3 (ICDD PDF hematite, PDF lithium iron (III) phosphate). 5

6 Figure S5. Temperature dependence of LiFePO4 unit cell (a) parameters and (b) volume during in situ XRD annealing (the values at 25 C were recorded after cooling the sample to room temperature). 6

7 Pore Volume dv/dw (cm³/g nm) precursor 700 C 800 C Pore width (nm) Figure S6. Pore distribution curves calculated from BET measurements showing the evolution of porosity in the precursor and materials annealed at 700 and 800 C. The BET surface area of the precursor is only 1.5 m 2 /g, whereas samples annealed at 700 C and 800 C give 17 and 25 m 2 /g, respectively. A small specific surface area of the precursor can be explained by assumpting that pores are closed in the xerogel (dry gel) particles. During annealing, when the decoposition of organic matrix and the development of LiFePO4 nanocrystals takes place, pores start to open and connect. The mesoporosity and overall increased specific surface area originates from the development of pores in the amorphos carbon, while LiFePO4 porosity increases with increasing temperature, in agreement with in situ TEM observation. The Brunauer-Emmett- Teller (BET) specific surface area was determined from nitrogen adsorption-desorption isotherms using a Micromeritics TriStar 3000 system. 7

8 Figure S7. Enlargement of Raman spectra of the samples annealed between 400 and 800 o C, showing a growth of a fully symmetric ν1 mode of LiFePO4 and the impurity phase of α- Fe2O3. 8

9 Figure S8. Electrochemical performance of the samples annealed at different temperatures showing the best performance for the material heated at 800 C. 9

10 Figure S9. Comparison of SAED and TEM images of LiFePO4 materials ex situ annealed at 700 C, 800 C and 900 C in the furnace: SAED images of the samples annealed at 700 C and 800 C contain slightly arced and multiplied spots which suggested that grains are textured policrystallites, whereas for the sample annealed at 900 C the number of diffraction spots is noticeably reduced, which indicates further significant coarsening of the grains at this temperature. These observations agreed with TEM images. 10

11 Figure S10. The first column in each box shows SEM images at different magnifications (scale bar from the top: 5 µm, nm, and 300 nm), while the second shows STEM and TEM images. SEM images present a general overview of the sample morphology, especially at 100k magnification where the difference between carbon (second image from the top) and LiFePO4 areas are shown. The porosity of this carbon developed while annealing at higher temperatures (notice a wide range of pore size at 900 C). STEM was performed to compare the porosity of LiFePO4 grains and crystallites in terms of annealing temperature. Both 700 and 800 C annealed samples showed a porous morphology through all crystallites. At 900 C crystallites became almost nonporous (which is also visible at the surface of the grain in the SEM image); only small fraction of them had still a porous morphology. Crystallites agglomerated into bigger (partially spherical) grains and separated from highly porous amorphous carbon. 11

12 Figure S11. Electrochemical impedance spectra (Nyquist plot) measured on the LiFePO4 samples annealed in the furnace at 700 C, 800 C and 900 C, respectively. The EIS measurements of the electrochemical cells were performed at open circuit voltage (OCV) and after the first and the tenth cycles. The comparison of the cells after the tenth cycle shows a persistent large difference in the resistivity of the sample annealed up to 900 C. 12