Quantitative Visualization of Salt Concentration Distributions in Lithium

Jan 15, 2018 - A fundamental understanding of concentrations of salts in lithium-ion battery electrolytes during battery operation is important for op...
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Communication Cite This: J. Am. Chem. Soc. 2018, 140, 1608−1611

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Quantitative Visualization of Salt Concentration Distributions in Lithium-Ion Battery Electrolytes during Battery Operation Using X‑ray Phase Imaging Daiko Takamatsu,* Akio Yoneyama, Yusuke Asari, and Tatsumi Hirano Research & Development Group, Hitachi, Ltd., Hitachi-shi, Ibaraki 319-1292, Japan S Supporting Information *

(ϕ) caused by an X-ray passing through a sample.7 The amount of ϕ is proportional to the density of the medium on the optical path, as described later. As the cross-section of ϕ is ∼1000times larger than that of the absorption in a hard X-ray region for light elements, the X-ray phase-contrast imaging has an extremely high density resolution compared with that of ordinary absorption-contrast imaging.8 Because of this advantage, it can be used to visualize small density differences in samples composed of light elements, such as biological soft tissues and organic materials.9 Because the electrolytes of LIBs are also composed of light elements, the X-ray phase imaging is expected to be effective in visualizing small density differences in an electrolyte during battery operation. Eastwood et al. recently demonstrated the use of X-ray phase imaging to visualize lithium−metal microstructure formation in a symmetric lithium/lithium cell upon application of a current by distinguishing qualitative levels of density.10 However, methods to quantitatively evaluate small and continuous changes in density using the X-ray phase imaging techniques have not yet been established, although the small and continuous changes are of importance to characterize the salt concentration distributions in the electrolyte. In this paper, we first develop an in operando X-ray phase imaging technique for visualizing and quantifying the salt concentration distributions in the electrolytes. We also demonstrate estimation of salt diffusivities from the observed concentration profiles. Figure 1a is a schematic view of the experimental setup of the X-ray phase imaging system. The system consists of a twocrystal (first/second crystals) X-ray interferometer to conduct fine imaging using synchrotron radiation, which is similar to those described elsewhere.11−14 The incident X-ray is divided at the first wafer (S), then reflected by the second (M1) and third (M2) wafers, and recombined to generate two interference beams at the fourth wafer (A). A spectro-electrochemical cell was installed as a sample on the optical path, and the cell was connected to a galvanostat (GS) to be charged and discharged. A schematic illustration of the cell used in this study is shown in Figure 1b. A two-electrode cell is composed of a LiFePO4 composite electrode as a working electrode (WE), lithium metal as a counter electrode (CE), and 1.0 mol dm−3 (M) LiClO4 in a 1:2 volumetric mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as an electrolyte. The distance between WE and CE was set as ∼1 mm in this study

ABSTRACT: A fundamental understanding of concentrations of salts in lithium-ion battery electrolytes during battery operation is important for optimal operation and design of lithium-ion batteries. However, there are few techniques that can be used to quantitatively characterize salt concentration distributions in the electrolytes during battery operation. In this paper, we demonstrate that in operando X-ray phase imaging can quantitatively visualize the salt concentration distributions that arise in electrolytes during battery operation. From quantitative evaluation of the concentration distributions at steady states, we obtained the salt diffusivities in electrolytes with different initial salt concentrations. Because of no restriction on samples and high temporal and spatial resolutions, X-ray phase imaging will be a versatile technique for evaluating electrolytes, both aqueous and nonaqueous, of many electrochemical systems.

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ptimal operation and design of lithium-ion batteries (LIBs), particularly in high-power applications such as vehicles, requires knowledge of transport properties in electrolytes during battery operation. As the electrolytes used in commercial LIBs exhibit large concentration polarization due to comparatively low Li+ transference number and salt diffusivity, the polarization is largely related to the transport properties.1,2 For example, it is known that internal resistance of the LIBs temporarily rises by repeating high-rate charge/ discharge cycles and that it decreases by stopping the battery operation and rest. Because such a reversible resistance rise occurs only during the battery operation, this phenomenon is speculated to be related to the salt concentration distribution in the electrolyte. However, there are few techniques that can be used to quantitatively characterize salt concentration distributions in the electrolytes during battery operation. In situ nuclear magnetic resonance (NMR)3,4 and magnetic resonance imaging (MRI)5,6 have been used for quantitatively characterizing electrolyte profiles. However, the cells used in NMR/MRI are in special shapes (e.g., extremely vertically long cells), and this may influence the ionic transport. For quantitatively characterizing the salt concentration distributions in the electrolytes during battery operation, it is necessary to establish a novel in operando technique that can accommodate cells similar to actual LIBs with high spatial and temporal resolutions. X-ray phase imaging is a technique that can be used to visualize density differences by detecting the X-ray phase shift © 2018 American Chemical Society

Received: December 18, 2017 Published: January 15, 2018 1608

DOI: 10.1021/jacs.7b13357 J. Am. Chem. Soc. 2018, 140, 1608−1611

Communication

Journal of the American Chemical Society

estimated as changes from the first image of t = 0 s (without an applied current), the phase-shift change (Δϕ) reflects the change in electrolyte density caused by charging from the initial state. Figure 2c displays Δϕ profiles along the x-axis of the images from the CE side to the WE side shown in Figure 2b. The Δϕ profiles shown in Figure 2c were averaged on the y-axis because they showed almost uniform distribution (see Figure S2b). Before applying current, variation of ϕ was significantly small (see Figure S2c), indicating that the electrolyte density is uniform in the cell. When constant-current charge began, the Δϕ in the vicinity of the WE increased while that near the CE decreased. From Figure 2c, the Δϕ profiles gradually increased until point D (t = 1700 s), whereas the profiles were linear and unchanged after point E (t = 3000 s). This suggests that the density gradient in the electrolyte became the steady state within 3000 s at the C/2 rate. Figure 3a displays the Δϕ profiles during constant-current charge at 0.06 mA (0.4 mA/cm2, C/2 rate) and 0.36 mA (2.4 mA/cm2, 3C rate), and Figure 3b illustrates the corresponding voltage profiles. The profiles at 3C rate show steeper slopes than those at the C/2 rate. Figure 3c shows the slopes Δϕ (rad/m) obtained by a linear fitting at the electrolyte bulk region (x = 300−700 μm). As shown in Figure 3c, the slope at the C/2 rate soon became constant (reaching steady-state conditions), whereas that at the 3C rate increased monotonically during the charging and reached the upper limit voltage before reaching steady-state conditions. The capacity at 3C rate charging (0.04 mAh) was as small as ∼30% of that at the C/2 rate (0.14 mAh), suggesting that the salt concentration gradient at high power application can be a limiting factor of capacity. The high temporal and spatial resolutions of our method enable us to visualize dynamics of electrolyte density during high-rate charge/discharge cycles (see Figure S3 and Movie S1). To evaluate the salt distribution during the charging from the X-ray phase imaging, we conducted a quantitative evaluation of the phase-shift changes at the steady-state conditions of the C/ 2 rate charging. In the X-ray phase imaging, the refractive index decrement δ is used as imaging contrast, and the ϕ caused by passing through the sample is given by

Figure 1. (a) Schematic view of the two-crystal X-ray interferometer setup. (b) Schematic illustration of the spectro-electrochemical cell used for in operando X-ray phase imaging consisting of a LiFePO4 composite electrode as a working electrode (WE), Li metal as a counter electrode (CE), and electrolyte. The cell is connected to a galvanostat (GS). Continuous phase-shift images of electrolyte in a square region of red dashed lines were analyzed.

(see Figure S1 for details of cell conditions and the experimental setup). To investigate the salt concentration distribution in the electrolyte during battery operation, we focused on the continuous phase-shift images of a red square region hereafter. Figure 2a shows a galvanostatic charge profile of the cell during the X-ray phase imaging by applying a current of 0.06 mA (0.4 mA/cm2, corresponding to C/2 rate). The upper horizontal axis denotes state of charge (SOC). The plateau appearing at 3.5 V (vs Li/Li+) indicates that the electrochemical delithiation of LiFePO4 proceeded well in the cell used in this study. Figure 2b shows X-ray phase-shift images of the electrolyte region in the charging process acquired at points A−F of Figure 2a (t = (A) 300, (B) 600, (C) 1000, (D) 1700, (E) 3000, and (F) 7500 s). Because the phase shifts are

Figure 2. (a) Constant current (0.06 mA, C/2 rate) charge profile and (b) corresponding extracted phase-shift change images of electrolyte. Because the phase shifts are estimated as changes from the first image of t = 0 s (without an applied current), the phase-shift change (Δϕ) reflects the change in electrolyte density caused by charging from the initial state. (c) Δϕ profiles along x-axis averaged on y-axis of (b). 1609

DOI: 10.1021/jacs.7b13357 J. Am. Chem. Soc. 2018, 140, 1608−1611

Communication

Journal of the American Chemical Society

distribution of the salt concentration, diffusivity of salt D is estimated to be 3.1 × 10−10 m2/s (see SI-4 for details). Although the salt diffusivity of the specific electrolyte used in this study (1.0 M LiClO4 in EC/EMC = 1/2) has not been reported, the estimated value is comparable to diffusivities in similar electrolytes reported in the literature: 2.6 × 10−10 and 1.0 × 10−10 m2/s at 1.0 M LiClO4 in propylene carbonate (PC)15,16 This supports the validity of the quantification on the salt distribution changes in this study. Finally, we demonstrated the determination of the salt diffusivities of different initial salt concentrations. Figure 4

Figure 4. Phase-shift changes (Δϕ) under the steady-state conditions during constant-current (0.06 mA, C/2 rate) charging for initial salt concentrations of 0.3, 1.0, and 3.0 M. The right vertical axis indicates estimated changes in the nominal salt concentrations (ΔC).

shows the phase-shift profiles under steady-state conditions during constant-current (0.06 mA, C/2 rate) charge of the electrolytes with initial salt concentrations of 0.3 (blue) and 3.0 M (red) with that of 1.0 (black) for comparison. The estimated ΔC using eq 3 is also shown on the right vertical axis of Figure 4. The slope of the concentration gradient was steeper for higher initial salt concentration electrolyte. The slopes give the ∂C/∂x of 0.044 and 0.120 M/m for C = 0.3 and 3.0 M, respectively. The estimated diffusivities were 6.6 × 10−10 and 2.4 × 10−10 m2/s for C = 0.3 and 3.0 M, respectively. It is clear that the diffusivity is smaller for the higher salt concentration. This dependency on the salt concentration is consistent with the trend obtained by other conventional methods.4,16 One of the advantages of the present technique is no limitation on the electrode materials and electrolyte salts. The reason to use LiClO4 as the electrolyte salt in this study was its chemical stability against contact with the atmosphere even if the cell sealing was incomplete. We confirmed that other salts, such as LiPF6, LiBF4, and LiBETI, can also be used (see Figure S4). Furthermore, even if a separator or a gel electrolyte is present between the electrodes, this method can be applied as long as X-ray can pass through them and their geometries remain unchanged. Temporal (a few seconds) and spatial (a few μm) resolutions of our system are sufficiently high compared with those of other conventional methods (see Table S2). In conclusion, we report for the first time, using in operando X-ray phase imaging, the quantitative evaluation of salt concentration distributions in the electrolytes during battery operation with high temporal and high spatial resolutions. This technique would be useful for optimizing operational conditions, including transient conditions and design of LIBs, particularly in high-power applications such as vehicles. We envisage that the X-ray phase imaging will become a versatile tool for evaluating electrolytes, both aqueous and nonaqueous, of many electrochemical systems, which will further our understanding of the dynamic behavior of electrolytes in actual applications.

Figure 3. (a) Phase-shift change (Δϕ) profiles along the x-axis at the points in (b). (b) Voltage profiles in constant-current charging at the C/2 rate (blue) and 3C rate (red). (c) Slopes of Δϕ at the electrolyte bulk region (x = 300−700 μm).

ϕ=

2πL δ λ

(1)

where λ is the wavelength of the X-ray and L is the sample thickness along the X-ray path.14 The Δϕ caused by charging from the initial state can be obtained as

Δϕ = Lreλ ∑ ΔniZi i

(2)

where re is the classical electron radius, ni is the number density in a unit volume, and Zi is the atomic number of each atoms contained in the medium. We use change in the nominal salt concentration (ΔC) from the initial state, and we consider that change in the density (Δni) is proportional to ΔC. From this assumption, we can obtain the correlation between Δϕ and ΔC as ⎛ Δn ⎞ Δϕ = A × ΔC , A = Lreλ ∑ ⎜ i ⎟Zi ⎝ ΔC ⎠ i

(3)

Herein, we consider A with only the salt concentration change at constant volume. Substituting concrete numerical values into the constant terms in eq 3, we obtain A = 15628 rad/M. The calculation procedure for obtaining A is described in SI-3. The experimentally observed phase-shift gradient (∂Δϕ/∂x) under the steady-state conditions was 1385 rad/m, as shown in Figure 2c. By substituting the observed ∂Δϕ/∂x into eq 3, one can determine the salt concentration gradient (∂C/∂x) as 0.089 M/m. Because of the steady-state conditions and the linear 1610

DOI: 10.1021/jacs.7b13357 J. Am. Chem. Soc. 2018, 140, 1608−1611

Communication

Journal of the American Chemical Society

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b13357. Cell assembly and experimental conditions, dynamic behavior during high-rate charge/discharge cycles, quantification of X-ray phase-shift imaging, diffusivity estimation, salt dependences, and comparison table among several analytical methods (PDF) Video visualizing dynamics of electrolyte density during high-rate charge/discharge cycles (AVI)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was carried out under Proposal Nos. 2012B7601 and 2013A7600, which were approved by the High Energy Accelerator Research Organization. The authors specially thank Dr. Yukinori Koyama (National Institute for Materials Science) for a careful reading of the manuscript and excellent editorial suggestions. We also acknowledge Dr. Haruo Akahoshi (Hitachi, Ltd.) for fruitful discussions, and we thank H. Haruna, M. Kanno, M. Hirooka, and K. Kitagawa (Hitachi, Ltd.) for their support of sample preparations.

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DOI: 10.1021/jacs.7b13357 J. Am. Chem. Soc. 2018, 140, 1608−1611