Spectroscopic X-ray Diffraction for Microfocus Inspection of Li-Ion

Aug 20, 2014 - ABSTRACT: We developed spectroscopic X-ray diffraction (XRD) analysis to visualize electrochemical reactions occurring at various locat...
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Spectroscopic X‑ray Diffraction for Microfocus Inspection of Li-Ion Batteries Haruno Murayama,*,† Koji Kitada,† Katsutoshi Fukuda,† Akio Mitsui,† Koji Ohara,† Hajime Arai,† Yoshiharu Uchimoto,‡ Zempachi Ogumi,† and Eiichiro Matsubara§ †

Office of Society-Academia Collaboration for Innovation, Kyoto University, Uji, Kyoto 611-0011, Japan Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan § Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ‡

S Supporting Information *

ABSTRACT: We developed spectroscopic X-ray diffraction (XRD) analysis to visualize electrochemical reactions occurring at various locations in Li-ion batteries (LIBs). Continuous irradiation with monochromatic X-rays in an energy region using a confocal setup provided a fixed observation position on the order of several tens of microns. Unlike three-dimensionally position sensitive XRD analyses, e.g., angle-scanning XRD and energydispersive XRD, this energy-scanning XRD analysis with angle-scanning of the monochromator instead of the detector-scanning has the advantage of profile resolution, position sensitivity, and time-resolution for mapping concentration gradients and diffusion of Li+ associated with the electrochemical properties of LIBs. The microscopic structural inhomogeneity in a sheet-like composite electrode of LiNi1/3Co1/3Mn1/3O2 with a thickness of 150 μm was successfully determined with a depth resolution of 50 μm during cell operation. This work demonstrates the potential of spectroscopic XRD as a nondestructive and pinpoint analysis method, thus contributing to the development of high-performance LIBs.

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INTRODUCTION Li-ion batteries (LIBs) have been extensively used in portable electronics, including laptop computers and mobile phones.1−3 Owing to strong, increasing demands in the automotive field,4,5 further improvements in power density are expected. The properties of LIBs, including energy density, capacity, response, and cycle life, are primarily governed by their electrical and ionic conductive networks.6−8 Most LIBs consist of versatile composite electrodes containing active materials, conductive agents, and binders coated on current collectors to promote smooth Li+ communication. However, this complex structure complicates an understanding of the inevitable microscopic inhomogeneities in the spatial variations of the Li+ concentration, i.e., the unreactive portion. Therefore, elucidating the mechanism by which inhomogeneities occur in the composite electrode during operation is critical for the development of LIBs. To visualize these kinetic phenomena in complex systems, advanced analyses must be developed. Electrochemical reactions occurring at the composite electrode have been intensively investigated by a variety of in situ techniques such as spectroscopic, microscopic, and diffractometric methods.9−11 In particular, the diffraction techniques using hard X-rays offer useful information on the Li+ concentration in the crystalline samples as a lattice constant and, thus, a state of charge (SOC) of a LIB can be understood by differences not only in the overall electron flow but also in the bulk crystal structure.12−14 These differences motivated us © 2014 American Chemical Society

to develop a rapid and high-resolution X-ray diffraction (XRD) imaging technique to map electrochemical reactions based on a novel concept. In the conventional angle-scanning XRD method, the detector angle is varied to detect a series of elastic scatterings diffracted from the sample, resulting in geometrical changes in the X-ray probe area. Recent achievements in a fast data acquisition of the XRD peaks are an installation of position-sensitive detectors to the XRD analysis, although these manners have principally no spatial resolution in the direction of the incident X-ray. These features are highly unsuitable for understanding reaction inhomogeneity. When a fixed observation position using a confocal setup is introduced to XRD measurements, the resulting information about the crystal structure at each observed position can be replaced with the spatial variations in the Li+ concentration in the composite electrode. The confocal XRD method, in which the incident beam and the detector remain fixed at the desired angles, yields Bragg reflections from a confocal point optically defined by collimating slits. This experimental setup is widely employed in the energy-dispersive XRD (EDXRD) method, which permits tomographic profile analyses of the internal features of bulky, dense materials such as metals, alloys, and battery cells covered by steel casings under working conditions.15−19 In this method, Received: March 24, 2014 Revised: August 20, 2014 Published: August 20, 2014 20750

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laminated aluminum film bag in an argon-filled glovebox. Prior to the in situ XRD measurements, the cell was preliminary conditioned by performing charge/discharge cycles between 3.0 and 4.3 V at CCs to ensure the electrochemical performance. In Situ Energy-Scanning Confocal XRD Measurements. Before in situ measurements using the energy-scanning confocal XRD method and synchrotron instruments, XRD data for LixNi1/3Co1/3Mn1/3O2 at several x values were collected using conventional X-ray diffractometer with Mo Kα radiation (Rigaku Corp., SmartLab). The d values of the (113) planes were elongated between ca. 1.355 and 1.370 Å via the cell operation from x = 0.5 to 1.0 in LixNi1/3Co1/3Mn1/3O2. The XRD measurements in this work were performed on the BL28XU of SPring-8 (Hyogo, Japan). A detailed schematic layout of the energy-scanning confocal XRD equipment is shown in Supporting Information, Figure S1, parts c and d. The synchrotron radiation X-ray from the storage ring is very intense and concentrated in narrow energy bands by an undulator. The incident beam was collimated by a pair of slits, and the diffracted beam was also collimated by another pair of slits at the front of the detector. Because one of the incident slits was set 17 m away from the other incident slit, the incident beam was extremely parallel. Thus, the brilliant and directional X-rays were roughly monochromated using a pair of horizontal mirrors (M1 and M2) coated with Rh, and only X-rays with the target wavelength were then selected using a Si111 channel-cut crystal with a narrow gap. The usable energy range of the beamline was between 20 and 30 keV with more than 0.8 × 1013 photons s−1 mm−2 of photon flux in this setup. By taking into consideration the fluctuation of the exit-beam height, H, and the transmittance of X-rays, the energy-scanning range was set between 20 and 30 keV in this measurement as mentioned in the Results. The incident beam was adjusted to 50 μm (vertical) × 200 μm (horizontal) using a pair of fourdimensional slits, and then the diffracted beam was also collimated by a pair of four-dimensional slits (50 μm vertical ×200 μm horizontal) on the diffractometer arm. Because the d value for the (113) planes of LixNi1/3Co1/3Mn1/3O2 varied between 1.355 and 1.370 Å when the x in LixNi1/3Co1/3Mn1/3O2 varied between 0.5 and 1.0, the timeresolved, energy-scanning confocal XRD spectra in the d-range between 1.346 and 1.381 Å were acquired at three positions in the cross section of the composite electrode during the electrochemical discharge process. The incident beam and detector angles were set to 9 degrees and 18 degrees, respectively. The diffraction was measured using a YAP (Ce+doped yttrium aluminum perovskit) scintillation detector system. Because the YAP scintillation detector is a highcounting-rate detector for high-energy X-rays, the intensity of the diffraction signals can be accurately measured without counting loss. The energy calibration was performed using the Bragg reflection of a standard CeO2 powder parched from NIST Standard References Material 674b. The electrochemical cell was laid on a horizontal stage in reflection mode and was connected to the charging/discharging device (see Supporting Information, Figure S1).

the white-radiation (polychromatic) X-ray beam is guided to the sample, and the energy-discriminating detector then analyzes the diffracted X-ray energies from the sample. However, EDXRD generally yields broader diffraction peaks compared to those produced by the angle-scanning XRD method, regardless of the intrinsic sample crystallinity, owing to the energy resolution, which depends on the performance of the detector.20−22 Such low peak resolution analysis is insensitive to the slight peak-shifts associated with the Li+ reaction and is limited to mainly qualitative analyses. If, instead of using white X-rays, continuous irradiation with monochromatic X-rays in an energy region was available in the setup, the diffraction peaks would appear in a spectrum as a function of X-ray energy. This energy-scanning XRD method may have suitable spatial and depth resolutions for the quantitative analysis of Li+ transport in LIBs. To develop energy-scanning confocal XRD analysis, the fluctuation of the beam position of the incident X-ray must be suppressed during the energy-scanning. A standard doublecrystal monochromator is typically used to keep the exit-beam position constant by adjusting the distance between the two crystals at each energy. This adjustment is rather timeconsuming. By contrast, a channel-cut crystal intrinsically has two crystal faces with a completely parallel geometry. Because this system uses a fixed gap, it is inevitable that the exit-beam position changes in response to the monochromator angle. Conventional wisdom, therefore, suggests that a channel-cut monochromator system is unsuitable for ensuring a constant exit-beam height during the energy-scanning. However, minimizing the gap and using collimator slit optics to extract the exit-beam may result in negligible changes in the beam position over a limited energy range. Here, we report that the successful use of spectroscopic confocal XRD analysis to characterize the microscopic electrochemical reaction occurring at a porous composite electrode composed of LiNi1/3Co1/3Mn1/3O2, which is one of the most common active materials for the positive electrode of LIBs, during battery operation and highlight the position sensitivity and the profile resolution of our XRD method.

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EXPERIMENTAL SECTION Electrochemical Cell Preparation and Electrochemical Measurement. A composite electrode with a thickness of 150 μm was prepared by slurrying commercial LiNi1/3Co1/3Mn1/3O2 powder (Toda Kogyo Corp.) with 5 wt % acetylene black and 5 wt % polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone solvent, after which the mixture was coated onto the aluminum current collector with a thickness of 20 μm. The porosity of the composite electrode, p, was prepared as 30% and was determined using the following relationship: p = (Vtotal − Vdense)/Vtotal

(1)

where Vtotal is the volume of the composite electrode and Vdense is the theoretical volume that the composite electrode would have if it were completely dense. Vdense is a summation of the volumes of LiNi1/3Co1/3Mn1/3O2, acetylene black, and PVDF, which are calculated using their weights and true densities. The working electrode was incorporated into an aluminum pouchtype cell (see Supporting Information, Figure S1, parts a and b) with a metallic lithium foil as the counter electrode and 1 M LiPF6 electrolyte in a 3:7 ethylene carbonate-ethyl methyl carbonate solvent. The cell was assembled and sealed in a

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RESULTS Spatial Resolution. Scanning the energy of the monochromatic X-rays for continuous irradiation in the energy range requires the rotation of a Si111 channel-cut crystal. The change in the exit-beam height, H, from such a double-crystal monochromator is estimated using the following equation: 20751

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defined by the goniometer radius and the slit system. In practice, the intensity profile of the direct beam (see Supporting Information, Figure S4) yielded a Δθ of 55.9 μrad. Then the instrumentally determined profile resolution is estimated as ca. 0.0004 using eq 5. Figure 1a shows the energy-

(2)

where gap is the distance between the Si111 channel-cut crystal and the monochromator angle, θm, which is the angle of the incident X-ray to the Si111 channel-cut crystal. A detail of the equation is shown in Supporting Information, S2. The estimated exit-beam height sharply decreases when θm exceeded 6 degrees in this study (see Supporting Information, Figure S3). The vertical widths of the collimating slits, w, must be placed in the spatially overlapping area of the two exit beams at the initial and destination θms. The stability of the resulting beam in terms of position and divergence was confirmed using a diffractometer with a goniometer radius of 1 m (see Supporting Information, Figure S4). The standard deviation of the peak positions represents the deflection of the beam position caused by the angle-scanning of the monochromator. This result indicates that the beam position is highly stable during the energy-scanning over the energy range from 20 to 30 keV. Compared to the accuracy of the diffractometer center, the fluctuation of the beam position is negligible. The spatial resolution, h, in a symmetrical confocal setup achieved by the incident and receiving slits that collimate the incident and diffracted X-rays is given in the following equation: h = w/cos(2θ /2)

Figure 1. XRD data for the cell containing the Li1Ni1/3Co1/3Mn1/3O2 electrode. (a) Energy-scanning confocal XRD spectrum and (b) anglescanning XRD pattern. The observed position was at a distance of 70 μm from the surface of the working electrode.

scanning XRD spectrum of the electrochemical cell containing Li1Ni1/3Co1/3Mn1/3O2 composite electrodes. The intensities of the spectrum were normalized by the incident beam intensity, the self-absorption, the detection efficiency of the YAP scintillation detector, which depends on the wavelength, and the Lorentz’s polarization factor. Peak-center positions were obtained to an accuracy of 0.001 Å by applying Gaussian function fitting. Judging from the Δd/d values obtained using this XRD analysis (ca. 0.0033), which were wider than the profile resolution estimated using eq 5 (see Supporting Information, Table S1), the experimentally obtained peak widths can be used as an indicator of the sample crystallinity. Importantly, these values and the peak intensities are essentially the same as those obtained from the angle-scanning XRD at constant energy (29 keV), as shown in Figure 1b. Demonstration of in Situ Observation of an Operant Electrode in a LIB. Prior to the in situ XRD measurements, the cell was charged from the fully discharged state at a constant current (CC) of 27.8 mA g−1 (0.1 C) for 5 h to adjust x in LixNi1/3Co1/3Mn1/3O2 to 0.5. The cell was then laid on the horizontal stage of a diffractometer to obtain the best spatial resolution and was discharged to 2.0 V at a CC of 139 mA g−1 (0.5 C) while the in situ energy-scanning XRD measurement was performed. A discharge curve of the electrochemical cell used in this experiment is shown in Figure 2. The discharge capacity, 67 mAh g−1, corresponds to 24% of the theoretical capacity of LiNi1/3Co1/3Mn1/3O2 (278 mAh g−1), indicating that x in LixNi1/3Co1/3Mn1/3O2 varied from 0.50 to 0.74 as a result of the discharge processes in this study. During the measurement, three positions at distances of 20 μm (A), 70 μm (B), and 120 μm (C) from the surface of the composite electrode (see Figure 3a) were repeatedly measured every 2 min by moving the stage in the vertical direction. In this setup, the detectable depth of the composite electrode was limited to 120 μm from the surface owing to the self-absorption effects shown in Figure S5. The time evolution of the 113 peak, which was selected as a measure of the electrochemical discharge and rest processes, is shown in Figure 3b. Before the electrochemical discharge, the 113 peaks obtained from the three positions showed the same interplanar spacing, d = 1.354 Å. These d values indicate that x = 0.5 in the

(3)

where w is the slit size and 2θ is the angle formed by the incident X-ray and detector directions. A detail of the equation is shown in Supporting Information, S2. A lozenge-shaped area is the confocal point created by the incident and receiving slits and dominates h, which becomes large when the θ is large. In addition to the spatial resolution, the self-absorption effect of the incident and diffracted X-ray paths in the sample increases when θ decreases because the absorbance is directly proportional to the beam path length, L, which is related to 1/sin θ. Furthermore, the relationship between θm and the interplanar spacing, dhkl, is given in eq 4 dhkl = dSi111 × sin θm/sin θ

(4)

where (hkl) are the Miller indices of the diffracting planes, θ is the scattering angle, and dSi111 is the interplanar spacing of the (111) plane of Si (3.1356 Å). A detail of the equation is shown in Supporting Information, S2. The incident beam and detector angles are determined based on the Bragg reflections, the spatial resolution, and the self-absorption. Angular Divergence of the X-ray Beam and Profile Resolution. The profile resolution is affected by the angular divergence of the incident beam and is thus able to detect tiny changes in the crystal structures of LixNi1/3Co1/3Mn1/3O2 at the different SOC. In general, the profile resolution (i.e., Δd/d) of the EDXRD is composed of the angular divergence of the X-ray beam (Δθ) and the energy resolution of monochromating device, e.g., energy-discriminating detector15 and monochromator, (ΔE) as follows: Δd ΔE = + |cot θ|Δθ d E

(5)

In our case, ΔE/E is the bandwidth of the monochromator, and the ΔE/E of the Si111 channel-cut monochromator typically offers an energy resolution on the order of 10−4, which is approximately 10 times better than that of the Ge solid-state detector usually used in EDXRD. The Δθ is composed of the intact divergence from the light source and the angle of view 20752

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Thus, the insertion of Li+ decreased in accordance with the increment of the distance from the counter electrode. During the rest process, shifts of the 113 peaks to smaller d values were observed on the counter electrode side and the center of the electrode. Conversely, the position of the 113 peak obtained from the current collector side shifted to larger d values. Finally, the d values at the three positions were homogenized, and the peaks were observed at approximately 1.362 Å, which corresponds to x = 0.74 in LixNi1/3Co1/3Mn1/3O2. The changes in the d value between the initially conditioned state and the discharged state after the rest process are consistent with the total quantity of electrons in the discharge volume.

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Figure 2. Performance of the LiNi1/3Co1/3Mn1/3O2 electrode. The discharge curve acquired during the in situ XRD measurement at a CC rate of 0.5 C. The inset shows the typical changes in the interplanar spacing of the (113) planes of a bulk LixNi1/3Co1/3Mn1/3O2 crystal as x in the cell charged from the fully discharged state at a very slow CC rate (0.07 C).

DISCUSSION The results presented here demonstrate that our spectroscopic XRD system utilized continuous irradiations of the monochromatic X-rays having a wide-range energy region. First, we will discuss the differences between the use of white X-rays and continuously monochromated X-rays. As the material used in this study has no absorption edge in the energy range of 20−30 keV, the spectroscopic XRD data had an excellent signal-tobackground ratio (S/B) and a profile resolution comparable to that of angle-scanning XRD. Even if the absorption edge of some component exists in the energy range of 20−30 keV, the spectroscopic XRD can avoid this concern by selecting an appropriate observation angle and energy range, as mentioned above. Conversely, the direct irradiation with white X-rays, which tend to include a wider range of X-ray photon energies, often excites the constituent atoms in the material used for LIB, yielding a large amount of X-ray fluorescence as a background signal. Another concern is that the monochromator commonly used in synchrotron facilities must be cooled using some refrigerant to reduce the thermal load induced by the irradiation of the white X-rays. It is clear what effect direct irradiation of LIBs with white X-rays would have on LIBs, whose performance is highly dependent on the thermal load. Thus, because our method does not use white X-rays, it may be superior to EDXRD in terms of the S/B ratio and decreased irradiation damage to LIBs. We also compared LIB inspection by the spectroscopic XRD system with an example of the fixed observation position analysis via EDXRD analyses from a viewpoint of the profile resolution. Ronci, F. and co-workers reported that a limitation of EDXRD analysis is the detection of neighboring peaks, e.g., the 107−009 and 108−110 pairs, in similar material owing to the performance of the detector.22 As shown in Figure 1a, these pairs were clearly separated in our experiment. In addition to analysis based on the 113 peak, an analysis based on the 110 peak that is independent of the greatly varying c-axis in the system is available, increasing the attractiveness of our method. As a result of the peak separation, a precise determination of the lattice constants, a = 2.8637(5) Å and c = 14.238(3) Å, was achieved. The accuracy of the lattice constants is nearly identical to that given by the angle-scanning XRD method shown in Figure 1b. Similar precision can be observed in the in situ analysis. On the basis of the Δx/Δd estimation shown in the inset of Figure 2, the energy-scanning confocal XRD system has a profile resolution of Δd/d and can track changes in Li+ concentration (Δx/d) on the order of 10 −2 x in LixNi1/3Co1/3Mn1/3O2. The data acquisition time for every spectrum in this study was approximately 40 s, corresponding to an electrochemical Li+ transport of Δx = 0.0056. Because this value is smaller than Δx/d, the results presented in Figure 3

Figure 3. In situ XRD observation for a LixNi1/3Co1/3Mn1/3O2 electrode. (a) Schematic view of the lozenge-shape probe gauge in the cross section of the electrode and (b) time evolution of the intensity and position of the 113 peak of LixNi1/3Co1/3Mn1/3O2 during the discharge reaction and rest processes. The incident beam and detector angles were set at θ and 2θ, respectively. The vertical slits widths are w, and the spatial resolution, which is given as the shorter diagonal of the lozenge-shaped gauge in the case of the low angle measurement, is h. The observed positions are referred to as the counter electrode side (A), the center of the composite electrode (B), and the current collector side (C), respectively. x in LixNi1/3Co1/3Mn1/3O2 was estimated from the relationship between the interplanar spacing of the (113) planes and the x given in the inset of Figure 2

LixNi1/3Co1/3Mn1/3O2 crystal. Increasing x in LixNi1/3Co1/3Mn1/3O2 through electrochemical discharge caused a gradual shift in the 113 peaks to larger d values until the end of the discharge process. At the end of the discharge process, the 113 peaks reached 1.367, 1.365, and 1.359 Å for the counter electrode side, the center of the electrode, and the current collector side, respectively. On the basis of the relationship between d and x, as shown in the inset of Figure 2, the x values in LixNi1/3Co1/3Mn1/3O2 on the counter electrode side, the center of the electrode, and the current collector side were 0.90, 0.84, and 0.66, respectively. 20753

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can be considered an instantaneous measurement; the profile resolution is sufficient for the purpose, and the measurement time can be reduced by increasing the energy of the incident Xrays. Using the spatial resolution of the energy-scanning XRD analysis, information on the electrochemical Li+ transport at the three observation portions of the composite electrode can be discussed. During the discharge process, the Li+ concentration at all observation positions increased in proportion to time as shown in Figure 3. Therefore, the rates of Li+ insertion (Δx/ Δt) at each position could be estimated. The rates were 0.83, 0.71, and 0.33 h−1 for the counter electrode side, the center of the electrode, and the current collector side, respectively. Compared to the bulk discharge rate of the cell, 0.5 C (=h−1), the rate of Li+ insertion near the surface of the composite electrode was faster than the discharge rate of the overall cell. The Li+ depletion at the surface of the composite electrode should function as resistance to rapidly increase the potential of the overall cell, resulting in a reduction in capacity. For the rest process, the driving force of Li+ transport apparently varied depending on the location. The Li+ concentration was homogenized within a few tens of minutes after discharging without an external bias. The results indicate that the delay in the Li+ supply on the current collector side at a discharging rate of 0.5 C created a Li+ gradient in the composite electrode. The Li+ supply depends on the penetration of the electrolyte into the composite electrode, which has a porosity of 30%. The reaction inhomogeneity observed in this study would be eliminated if the Li+ supply in the composite electrode was drastically improved by optimizing the electrode porosity. Thus, our spatially accurate XRD analysis can trace the Li+ transport associated with the cell properties, which might promote the further optimization of structures in the composite electrode. One such study is now underway. The spectroscopic XRD method developed in this study provides a reliable diffraction pattern comparable to that of angle-scanning XRD. The inevitable emergence of a loweremittance and higher-brilliance X-ray source will drastically improve the quality of data obtained from the spectroscopic XRD. We believe that the diffraction data obtained from the energy-scanning XRD method will play an active role in the structural analysis of microvolume materials through future collaboration with Rietveld refinement. This work offers a useful analytical tool for the optimization of the structure of composite electrodes and battery operation conditions and, therefore, greatly contributes to the evolution of technology associated with LIBs. This method is useful for the nondestructive monitoring of various materials, including other electrochemical cells.

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AUTHOR INFORMATION

Corresponding Author

*(H.M.) Telephone: +81-774-38-4968. E-mail: murayama@ saci.kyoto-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Research and Development Initiative for Science Innovation of New Generation Battery (RISING project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2010B1033, 2011B1036, 2012A7602, 2012B7602, and 2013A7602). The authors thank Mr. T. Kakei, for his contributions to sample preparation and electrochemical evaluation.

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ASSOCIATED CONTENT

* Supporting Information S

Sketch of setup of the aluminum pouch-type cell, configuration of the cell, schematic layout of the energy-scanning confocal XRD equipment on the BL28XU of SPring-8, additional explanation of equations, estimated changes in the exit-beam height from the Si111 channel-cut monochromator, intensity profiles of the direct beams through the incident and detecting slits, self-absorption of the sample, and Δd/d values obtained by the energy-scanning XRD spectrum and the conventional XRD pattern. This material is available free of charge via the Internet at http://pubs.acs.org. 20754

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