Evolution and Migration of Lithium-Deficient Phases during

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Evolution and Migration of Lithium-Deficient Phases during Electrochemical Delithiation of Large Single Crystals of LiFePO

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Toshiro Yamanaka, Taketoshi Minato, Ken-ichi Okazaki, Takeshi Abe, Koji Nishio, and Zempachi Ogumi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00246 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Energy Materials

Evolution and Migration of Lithium-Deficient Phases during Electrochemical Delithiation of Large Single Crystals of LiFePO4 Toshiro Yamanaka,†* Taketoshi Minato,† Ken-ichi Okazaki,† Takeshi Abe,‡ Koji Nishio,† Zempachi Ogumi†

†

Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan,

‡

Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

KEYWORDS: Battery; Electrode materials; LiFePO₄; ionic diffusion; olivine

ABSTRACT: In a lithium ion battery, lithiation and delithiation of active materials occur during charging and discharging, and diffusion of Li (or Li vacancy) and related phase transformation in the active material greatly affect battery performance. Evolution and migration of Li-deficient phases at surfaces of single crystals of LiFePO4 (2×3×6 mm in size) during electrochemical delithiation was studied by using Raman microscopy and in situ wide-view scanning Raman spectroscopy. Domains of phases with Li vacancies (LV phases) of a few micrometers or less in

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sizes formed in the LiFePO4 (LFP) phase during delithiation at a low potential of 4 V vs Li/Li+, and LV phases migrated at a rate corresponding to Li diffusivity of 3×10-9 cm2/s. In contrast, at a high potential of 4.5 V, an FePO4 (FP) phase grew in addition to the LV phase, and the rates of growth of the FP phase and migration of LV phases were characterized by quite different Li diffusivities of 1.4×10-11cm2/s and close to 10-7 cm2/s, respectively. The very different rates suggest mechanisms for diffusion strongly coupled to phase mixing/separation kinetics depending on the concentration of Li, which provides implications for the development of superior active materials for high-performance batteries. Such phenomena probably extend to other phase-separating battery materials. This work demonstrates a unique method for in situ observation of phases with dilute vacancies and their large-scale migration.

1. INTRODUCTION Elucidation of ionic diffusion and related phase evolution in solids is important in relation to fundamental science as well as applications because they are key phenomena in various components in devices to achieve high performance, including solid electrolytes in batteries, gas sensors, thermoelectric converters, capacitors, and solid oxide fuel cell and also active materials in batteries. LiFePO4 is one of the most extensively studied cathode active materials for a lithium ion battery1 due to its abundance in resources, low cost, excellent cycle stability and high rate performance.2 However, there is still much debate about the mechanisms of phase transformation and Li diffusion in this material. During delithiation, the LixFePO4 phaseseparates into two phases, LiαFePO4 (FP) and Li1-βFePO4 (LFP) phases (α≒ 0.1, β≒ 0.1), showing a miscibility gap at the Li concentration between them,3 and the gap becomes narrow as the particle size


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decreases to less than 40 nm.4-8 Although Li migrates along the [010] axis (b axis),9,10 propagation of the phase boundary along the [100] axis (a axis)11-13 as well as that along the b axis14 are thought to occur. Since the mixing energy of Li in LixFePO4 is small,15 a solid-solution phase16-20 and various other phases (super lattices21-23 and amorphous24) with Li concentration in the miscibility gap are also accessible during lithiation and delithiation, and formation and distribution of phases are sensitively dependent on structures,25-27 elastic effect,28 and rates of charge/discharge.29-31 Transfer of Li is an elementary process in the above-mentioned phase transformation. There is a large discrepancy among so far reported Li diffusivities. Li diffusivity has been estimated by electroanalytical methods mostly using composite electrodes containing secondary particles of a few micrometers in size that each consist of many carbon-coated primary nanoparticles of LiFePO4, and Li diffusivities ranging from 10-12 to 10-16 cm2/s were reported in many works. These values are much lower than the result of first principles calculation, 10-8 cm2/s in the [010] direction.9,32 Since delithiation of LiFePO4 particles in composite electrodes proceeds in a particle-by-particle manner33-35 or concurrently35 depending on rates, estimation of Li diffusivity using composite electrodes is not easy.36 Amin et al.37 used large LiFePO4 single crystals to estimate the chemical diffusion coefficient, Dδ, by polarization/depolarization measurements, and the obtained values were in the order of 10-9 to 10-8 cm2/s in the [010] direction at 450 to 490°C, which leads to a value of 10-13 cm2/s at room temperature when linear fitting of the values at 450 to 490°C is extrapolated to room temperature. This value is again much lower than the results of calculation.9,32 Weichert et al. observed propagation of boundaries between LFP and FP phases during chemical delithiation of large LiFePO4 single crystals by using an optical microscope, and Li diffusivity was estimated to be 10-11 to 10-12 cm2/s.38 This diffusivity was


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thought to be faster than that in pure bulk due to faster diffusion in pores of FP phases that were created due to lattice mismatch between FP and LFP phases. Sugiyama et al. measured muonspin rotation and relaxation (µ+SR) spectra, and Li diffusivity was estimated to be 3.6 × 10−10 cm2/s.39 Estimation of Li diffusivity using a single secondary particle containing many carboncoated nanoparticles was also reported.40 In the present study, evolution and propagation of Li-deficient phases at surfaces of large single crystals of LiFePO4 during electrochemical delithiation were directly observed by using Raman microscopy and in situ wide-view scanning Raman spectroscopy. Various new features of phase evolution characterized by very different Li diffusivities were observed.

2. EXPERIMENTAL Figure 1a shows the setup for electrochemical delithiation of an LiFePO4 single crystal. The size of the crystal was 2×3×6 mm and it was supplied by Oxide Corporation. The crystal was cut using diamond-based cutters by the company to avoid undesired reactions. A separator film made of porous polypropylene was inserted between an Li foil and a current collector made of a fine stainless mesh painted with slurry consisting of acetylene black and polyvinylidene difluoride. The current collector was attached to the b plane of the crystal. The separator film was wetted by an electrolyte solution of 1 mol/dm3 LiClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1. Delithiation was induced by applying potential (Vc) between the Li foil and current collector. Wide-view Raman mapping was conducted with a 532 nm laser beam of 40 µm in size, a scan step of 20 µm and a spatial resolution of 20 µm. Optical skin depth for opaque insulators is several hundreds of nanometers at this wavelength.


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Figure 1b shows Raman spectra of the a plane before delithiation. The peak at 325 cm-1 is assigned as Fe-O vibration, and detailed identification of other Raman scattering peaks of LiFePO4 and FePO4 is given in the literature.41-43 During delithiation at Vc = 4V, an additional peak at 305 cm-1 appeared as shown in Fig. 1c. The positions of this peak is very close to that in a Raman spectrum of FePO4 (coupled translation of Fe + PO43-) (See Fig. S7). Thus, the peak at 305 cm-1 is assigned as Raman scattering from a phase with dilute Li vacancy (LV). The area of the peak at 305 cm-1 in Fig. 1c is only 0.5% of that in a spectrum of FePO4 (see Fig. S7). At Vc = 4V, the rate of delithiation was small and an FP phase, which appeared at a higher Vc of 4.5 V as shown later, did not appear. The area of the peak at 305 cm-1 was used in the following mapping.

3. RESULTS AND DISCUSSION Figures 1e-1h show wide-view mapping of the LV phase in a region of 600 µm × 800µm in size on the a plane shown by red lines in Fig. 1a. Each scanning took about 12 hours. The timing of mapping and delithiation is shown in Fig. 1d. The left end (position in [010] = 0) corresponds to the b plane in contact with the current collector. Nearly top views and side views are shown in the left and right panels, respectively. Vc, was changed from open circuit voltage (OCV) to 4 V to induce delithiation, and Vc was changed from 4 V to OCV 42 hours later (Fig. 1d). Mapping shown in Fig. 1e was started at 36 hours after the starting time of delithiation. Peak areas were not uniform, indicating the formation of domains of LV phase of a few micrometers in size. Peak areas are large on the left side close to the b plane, where Li+ was extracted, and decrease as the distance from the b plane increases. Figures 1f and 1g show results obtained after one day and four days (Fig. 1d), respectively. As time passed, peak areas on the left side decreased and those


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on the right side slightly increased, indicating that LV phases migrated from the left side to the right side. Vc was again changed to 4 V to start delithiation. Figure 1h shows results of mapping at one day after the start of delithiation. The peak areas on the left side again increased, indicating that LV phases formed on the left side.


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Figure 1. Delithiation of an LiFePO4 single crystal at Vc = 4 V. (a) Setup for delithiation of the crystal, Raman spectra of the a plane of the crystal (b) before and (c) after delithiation, (d) timing of delithiation and scanning. (e)-(h) show wide-view Raman mapping in the region shown in the red rectangle in (a) obtained with the timing between scanning and delithiation shown in (d).

The value for the diffusion coefficient of Li, D, was estimated from analysis of the above results based on Fick’s laws, J = -D dc/dx, and dc/dt = D d2c/dx2, where J is diffusion flux and c is the concentration of Li. The current measured at Vc = 4 V was nearly constant and the rate of delithiation was very small. Thus, J at x = 0 was assumed to be constant, although it is difficult to determine the accurate value for J at x=0 due to an unknown dark current. The best fitting (dashed lines in Figs. 1e-1g) was obtained when D was assumed to be 3×10-9 cm2/s. Another LiFePO4 single crystal was delithiated at Vc = 4.5 V, which is sufficiently high to induce growth of the FP phase. Figure 2a shows a photograph of the crystal after delithiation at Vc = 4.5 V. A clear white band can be seen on the left side, indicating formation of an FP phase. In many cases, pristine LiFePO4 appears greenish gray when it is thin. The dark color of the LFP phase is due to the large thickness of 4 mm (See Fig. S1 in supporting information of ref 44).44 In situ wide-view mapping was conducted in the region indicated by the blue rectangle in Fig. 2a. The excitation beam was scanned from the left side and each scanning took about 14 hours. The top panel in Fig. 2b is results before delithiation, showing no significant contrast. Vc was changed from OCV to 4.5 V and scanning was started at the same time. Vc was kept at 4.5 V for 12 days and scanning was conducted every day. Results for the 1st day, 7th day and 12th day are shown in the lower panels in Fig. 2b. As time passed, the FP phase grew from the left side.


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Figure 2c shows zoomed mapping of the region indicated by blue rectangles in Fig. 2b. If the phase boundary between FP and LFP


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Figure 2. Delithiation of an LiFePO4 single crystal at Vc = 4.5 V. (a) Photograph of the single crystal after delithiation for 12 days, (b) wide-view Raman mapping in the region shown in the blue rectangle in (a), (c) zoomed mapping of the region shown by the blue rectangles in (b). (d) shows the same results of Raman mapping as in (b) with a magnified scale of the peak area. (e) is zoomed mapping in the region shown by the blue rectangle in (d). (f) shows a high-resolution Raman microscopy image around the position shown by a blue arrow in (a).

phases is defined by the half value (6000, yellow) of the maximum peak area (12000, dark brown), the results indicate that the phase boundary propagated 52 µm during a period of about 11 days. The rate of transport-controlled growth of a product in a solid state reaction is often expressed by the formula L=(2 Dt)1/2,45 where L and t are thickness of a growing phase and time, respectively. By applying this formula, D is estimated to be 1.42×10-11 cm2/s, which is very close to the value of 10-11 to 10-12 cm2/s obtained from propagation of phase boundaries during chemical delithiation of large single crystals.38 Figure 2d shows results of the same delithiation process as that shown in Fig. 2b with a magnified scale of the peak area. Extension of LV phases is clearly seen in the region of x < 500 µm, being indicated by bright blue (see definition of x in Fig. 2a). In addition, the results for the first day show that LV phases increased all over the scanned region, even at the b plane at the right side end (x≒1950 µm), which can be more clearly seen with a magnified scale of the peak area in Fig. 2e. Since LV domains are visible all over the sample even before delithiation, lithium vacancies at the right side end observed on the first day of delithiation may have come from nearby regions. However, in estimation of diffusivity, we can consider only the difference between the initial and final distributions of vacancies, and we cannot know which vacancies


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came from which positions. If diffusivity is estimated by the formula L=(2 Dt)1/2 using migration length of 2000 um (from x=0 um to x=2000 um in Fig. 2d) and corresponding scanning time of 14 hours, a value of 4×10-7 cm2/s is obtained. If migration length of 500 um (from x=0 um to x=500 um in Fig. 2d) and corresponding scanning time of 3.5 hours are used, a value of 1×10-7 cm2/s is obtained. The peak areas around the b plane on the right side continued to increase during delithiation at 4.5 V (7th and 12th days ), although peak areas in the region of 750 µm < x < 1850 µm did not increase significantly after the second day. This indicate that the Li vacancy preferentially segregated to the vicinity of the b plane. On the other hand, peak areas in the vicinity of the c plane (upper end, y ≒ 0) are rather small. After delithiation for 12 days, Vc was changed to OCV. Even at OCV, peak areas around the b plane on the right side continued to increase, while peak areas in the FP phase on the left side decreased (see Fig. S8). This suggests that Li (Li vacancy) migrated from the right (left) side end to the opposite side even at OCV. Figure 2f shows a high-resolution Raman image after delithiation around the position indicated by a bright blue arrow in Fig. 2a. The sizes of domains of LV phases are submicrometer to several micrometers. It is difficult to determine Li concentration (XLi) in the LV phase. The area of the peak at 305 cm-1 (Fig. 1c) of LV phases is about 5% of that of the FP phase in the highresolution Raman microscope measurements. If the peak area is assumed to be proportional to 1XLi and if XLi in FP phases is assumed to be about zero, XLi in LV phases is about 0.95. The results of the present study indicate a diffusion mode in which Li (or Li vacancy) migrates, resulting in the formation of micro-domains of LV phases that segregate to the b plane. This suggests that there is strong coupling between diffusion of Li (or Li vacancy) and phase formation kinetics.


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A possible interpretation of the various modes of phase evolution characterized by greatly different Li diffusivities is as follows. Figures 3a and 3b show illustrations of the dependence of free energy (G) of LixFePO4 on Li concentration, XLi.46,47 The curves consist of three parts, (i) δ2G/δXLi2>0, (ii) δ2G/δXLi20, having a maximum in (ii) and two minima in (i) and (iii), which leads to phase separation into two phases, FP phase around the minimum in (i) and LFP phase around the minimum in (iii).

Figure 3. Schematic illustration of dependence of G on Li concentration. Creation of phases and subsequent mixing kinetics during delithiation at (a) 4 V and (b) 4.5 V. Values for the second order differential of G to Li concentration, δ2G/δXLi2, are positive, negative and positive in parts (i), (ii) and (iii) of the curves, respectively.

When a small amount of Li is extracted from LiFePO4 (XLi=1, open circle), a new phase appears as shown by a closed circle in (a), which may correspond to LV phases in delithiation at Vc = 4V. This phase can easily mix with the environmental LiFePO4 phase as shown by blue arrows accompanied by migration of vacancies. Since δ2G/δX2 is positive, this mixing results in lowering of free energy (See the red arrow in Fig. 3b.), but this effect is small due to the small difference in XLi between the two phases shown in Fig. 3a. The obtained value for D of 3×10-9 cm2/s is close to the result of first principles calculation, 10-8 cm2/s.9,32


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At Vc = 4.5 V, FP phase is produced as shown by a closed square in Fig. 3b. Evolution of the FP phase is less easy because this evolution requires a decrease in XLi in the adjacent LFP phase from (iii) to (i), passing through the energy maximum in (ii). Thus, a smaller value for D of 1.42×10-11 cm2/s was obtained from propagation of the boundary between FP and LFP phases. This value is close to those previously derived from propagation of FP/LFP phase boundaries.38,48 Also, a large structural change from LFP to FP phases may result in a smaller value for D. The results for the first day of delithiation shown in Fig. 2b indicate that phases with various values of XLi appeared immediately after Vc was changed to 4.5 V. The colors (yellow green to dark violet) at the upper left (indicated by a blue arrow) suggest that phases with XLi from 0.55 to about 1 appeared if a linear relation between peak area and XLi is assumed (very rough approximation). In this case, a large driving force due to lowering of energy (red arrow in Fig. 3b) is operative for mixing of two phases (open and closed circles in region (iii) in Fig. 3b) with clearly different XLi values. Thus, a value for D as high as about 10-7 cm2/s was obtained. If Li0.55FePO4, which is in region (ii) in Fig. 3, is created, it will phase separate. During this phase separation process, some phases in region (iii) form and these phases mix with LiFePO4 being promoted by the driving force. In the yellow-green part in Fig. 2b (blue arrow), phase separation may be in progress, which cannot be observed by our method with low spatial and time resolutions. Zhu and Wang developed a method to estimate diffusivity of ions and the interface mobility in phase transformation electrodes in the two-phase region using a mixed-controlled model.49 This model will be useful to estimate the diffusivity in evolution and migration of phases observed in this study.


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4. CONCLUSIONS Evolution of Li-deficient phases at surfaces of single crystals of LiFePO4 during electrochemical delithiation was studied by using Raman microscopy and in situ wide-view scanning Raman spectroscopy. Domains of unique phases with Li vacancies (LV phases) of a few micrometers or less in size formed in the LiFePO4 (LFP) phase, and the LV phases migrated at a rate corresponding to Li diffusivity close to 10-7 cm2/s, being by three to five orders of magnitude larger than the diffusivity that was previously observed. The very different diffusivities suggest mechanisms for diffusion strongly coupled to phase mixing/separation kinetics depending on the concentration of Li, which provides implications for the development of superior active materials for high-performance batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of the electrochemical cell, experimental setup, Raman spectra, and wide-view Raman mapping at OCV after delithiation

AUTHOR INFORMATION Corresponding Author *[email protected]


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ACKNOWLEDGMENT This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under contract from the Research & Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2).

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