Research ArticleAPPLIED SCIENCES AND ENGINEERING

Localized concentration reversal of lithium during intercalation into nanoparticles

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Science Advances  12 Jan 2018:
Vol. 4, no. 1, eaao2608
DOI: 10.1126/sciadv.aao2608
  • Fig. 1 Experimental setup for in situ tracking of local Li intercalation within a single-crystalline nanoparticle.

    (A) Chemical image of the representative FP nanoparticles for in situ measurements (mapped via EELS; Fe: red, C: green). The particles are plate-like single crystals, thin in the [010] crystalline axis (inset); scale bar, 100 nm. (B) Schematic illustration of the in situ electrochemical cell containing a half-TEM grid loaded with FP nanoparticles, which enables in situ ED and HRTEM measurements at the single or subparticle level. The HRTEM imaging coupled with GPA (using a new algorithm as given in Materials and Methods) generates the spatial distribution of Li concentration in local areas of a single nanoparticle. STM, scanning tunneling microscope. (C) Demonstration of the high-rate capability of FP nanoparticles in the TEM grid–based electrodes. The voltage profiles are similar to those obtained from standard coin cells (fig. S2).

  • Fig. 2 In situ ED measurements indicating a solid-solution transformation in a single LixFePO4 nanoparticle.

    Representative ED patterns recorded from a b-oriented FP nanoparticle (shown in fig. S3) at (A) the pristine state and (B) an intermediate state after 113 s of lithiation. The splitting of the (200) and (001) diffraction spots indicates the transformation from FP (marked by black arrows) to LixFePO4 (red ones). (C) Evolution of intensity profiles of the (200) diffraction spots versus the distance from the center (000). Dots: original data; solid lines: fitted ones by weighted Gaussian and Lorentzian functions. (D) Evolution of relative change of lattice spacing a, defined by Embedded Image, where the lattice parameter aLixFePO4 was measured from the peak positions in the fitted profiles in (C) (being marked with red vertical lines), as a function of the elapsed time (shown by red triangles); the dashed curve is a guide to the eye. The error bars correspond to the standard deviation obtained from four diffraction spots ±(100) and ±(200). a.u. arbitrary unit.

  • Fig. 3 Visualization of the Li concentration evolution at sub-nanoscale within a single-crystalline LixFePO4 nanoparticle during lithiation (at a rate of ~18 C).

    (A) Bright-field image and expanded view of the local lattice image from a single-crystalline FP nanoparticle (about 113 nm in width and 120 nm in length) at the pristine state oriented along [010] (scale bars, 50 and 5 nm, respectively). (B to I) 2D maps of local Li concentration (x in LixFePO4) at a series of lithiated states for the selected area marked by the blue box in (A), which were derived from the lattice displacement of (100) planes relative to the pristine state (see fig. S6 for HRTEM images and Materials and Methods for the GPA processing). Color scale next to (B) indicates the lattice expansion Δa (relative to the lattice spacing a at the pristine state) and corresponding normalized Li concentration x. Scale bars, 5 nm. (J) Evolution of the averaged Li concentration (x) averaged over selected areas with different sizes [as marked by squares in (B) to (I)].

  • Fig. 4 Chemical potential as a function of Li concentration in fast-lithiating regions Ωα and surrounding regions Ωβ within a single particle.

    (A and B) Schematics showing the mechanism for concentration reversal in Ωα when the chemical potential function is the same for Ωα and Embedded Image. (C and D) Schematics showing the mechanism for concentration reversal in Embedded Image when its chemical potential function (μα) has larger values than that of Embedded Imageβ) near the upper spinodal point. The degrees of concentration reversal in the two cases are shown by the black arrows in (B) and (D). The evolution of Li concentration is indicated by the magenta and cyan arrows, respectively.

  • Fig. 5 Simulation of the localized concentration reversal within a single-crystalline particle using a phase-field model.

    (A) Chemical potential functions μα and μβ versus normalized Li concentration (x) for the fast-lithiating regions Ωα and surrounding regions Ωβ, respectively. (B to E) Snapshots of projected Li concentration along the [010] direction (left column) and the corresponding 3D view of the simulated concentration distribution (right column) at different stages of lithiation. The color scale represents Li concentration. Several cuboids marked by white boxes in (B) are assumed to be Ωα with the modified chemical potential function, μα. The normalized Li concentrations of the Ωα and Embedded Image in (A) are also labeled in (C) to (E). (F) Evolution of averaged Li concentration over different sampling sizes as labeled with domains 1 and 2 in (B), which was extracted from movies S2 and S3. Domain 1 is selected to be inside Ωα, whereas domain 2 consists of Embedded Image and a part of surrounding Embedded Image. The averaged concentration of domain 1 is plotted as the magenta curve that exhibits similar behavior as the experimentally examined FP nanoparticle during lithiation (Fig. 3J).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaao2608/DC1

    movie S1. An in situ TEM movie showing the evolution of the ED patterns projected along the [010] direction.

    movie S2. A simulation movie showing the evolution of projected Li concentration along the [010] direction.

    movie S3. A 3D view movie showing the evolution of simulated Li concentration distribution in different local regions.

    fig. S1. Morphology and structure of FP and LFP nanoparticles.

    fig. S2. Electrochemical performance of FP nanoparticles tested in regular coin cells.

    fig. S3. A bright-field TEM image showing a single-crystalline FP nanoparticle (in the central region), being tilted to [010] direction for in situ ED measurements (Fig. 2).

    fig. S4. The evolution of ED intensity profiles of diffraction spot (001) versus distance from the central spot (000), shown with original data (dots), and fitted ones (solid lines) by weighted Gaussian and Lorentzian functions.

    fig. S5. Electron energy-loss spectra of Fe-L2,3 and O-K edges from the FP nanoparticle (shown in fig. S3) before and after lithiation during in situ ED measurements.

    fig. S6. HRTEM images from a single particle (oriented along [010]) during lithiation (as in Fig. 3) taken at different times.

    fig. S7. Evolution of intensity profiles of (100) and (001) diffraction spots in the FFT patterns obtained from the HRTEM images in fig. S6, in comparison to that obtained from in situ ED measurements.

    fig. S8. Local lattice displacement in FP due to lithiation.

    fig. S9. Intensity profiles of the Li concentration distribution showing continuous variation of the Li concentration between the neighboring Li-rich and Li-poor regions.

    fig. S10. Evolution of Li concentration averaged at the lower left and upper left corners of region B in Fig. 3 (B to I).

    fig. S11. Synchrotron XRD patterns in comparison to that obtained by Rietveld refinements, from pristine LFP, chemically delithiated FP, and electrochemically lithiated LFP.

    fig. S12. Inhomogeneity of anti-site defects within the electrochemically lithiated nanoparticles (LFP).

    fig. S13. Calculated strain energy density on the surface of the particle and on the a-c plane at the center of the particle.

    fig. S14. Evolution of averaged Li concentration over different sampling sizes (as labeled with domains 1 and 2 in Fig. 5B), with low and high anisotropic ratios of Li mobility along the three crystalline directions.

    note S1. Nonuniformly distributed anti-site defects in LFP

    note S2. Concentration reversal in local regions within different anisotropic ratio of Li mobility along the three crystalline directions

    References (54, 55)

  • Supplementary Materials

    This PDF file includes:   

    • Legends for movies S1 to S3

    • fig. S1. Morphology and structure of FP and LFP nanoparticles.

    • fig. S2. Electrochemical performance of FP nanoparticles tested in regular coin cells.

    • fig. S3. A bright-field TEM image showing a single-crystalline FP nanoparticle (in the central region), being tilted to [010] direction for in situ ED measurements (Fig. 2).

    • fig. S4. The evolution of ED intensity profiles of diffraction spot (001) versus distance from the central spot (000), shown with original data (dots), and fitted ones (solid lines) by weighted Gaussian and Lorentzian functions.

    • fig. S5. Electron energy loss spectra of Fe-L2,3 and O-K edges from the FP nanoparticle (shown in fig. S3) before and after lithiation during in situ ED measurements.

    • fig. S6. HRTEM images from a single particle (oriented along [010]) during lithiation (as in Fig. 3) taken at different times.

    • fig. S7. Evolution of intensity profiles of (100) and (001) diffraction spots in the FFT patterns obtained from the HRTEM images in fig. S6, in comparison to that obtained from in situ ED measurements.

    • fig. S8. Local lattice displacement in FP due to lithiation.

    • fig. S9. Intensity profiles of the Li concentration distribution showing continuous variation of the Li concentration between the neighboring Li-rich and Li-poor regions.

    • fig. S10. Evolution of Li concentration averaged at the lower left and upper left corners of region B in Fig. 3 (B to I).

    • fig. S11. Synchrotron XRD patterns in comparison to that obtained by Rietveld refinements, from pristine LFP, chemically delithiated FP, and electrochemically lithiated LFP.

    • fig. S12. Inhomogeneity of anti-site defects within the electrochemically lithiated nanoparticles (LFP).

    • fig. S13. Calculated strain energy density on the surface of the particle and on the a-c plane at the center of the particle.

    • fig. S14. Evolution of averaged Li concentration over different sampling sizes (as labeled with domains 1 and 2 in Fig. 5B), with low and high anisotropic ratios of Li mobility along the three crystalline directions.

    • note S1. Nonuniformly distributed anti-site defects in LFP

    • note S2. Concentration reversal in local regions within different anisotropic ratio of Li mobility along the three crystalline directions

    • References (54, 55)
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    • Movie S1 -

      An in situ TEM movie showing the evolution of the ED patterns projected along the [010] direction.

    • Movie S2 -

      A simulation movie showing the evolution of projected Li concentration along the [010] direction.

    • Movie S3 -

      A 3D view movie showing the evolution of simulated Li concentration distribution in different local regions.

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