Visualization of Inhomogeneous Reaction Distribution in the Model

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Visualization of Inhomogeneous Reaction Distribution in the Model LiCoO Composite Electrode of Lithium Ion Batteries 2

Takashi Nakamura, Toshiki Watanabe, Yuta Kimura, Koji Amezawa, Kiyofumi Nitta, Hajime Tanida, Koji Ohara, Yoshiharu Uchimoto, and Zempachi Ogumi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12133 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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The Journal of Physical Chemistry

Visualization of Inhomogeneous Reaction Distribution in the Model LiCoO2 Composite Electrode of Lithium Ion Batteries Takashi Nakamuraa)*, Toshiki Watanabeb), Yuta Kimura a), Koji Amezawaa), Kiyofumi Nittac), Hajime Tanidac), Koji Oharac), Yoshiharu Uchimotoe), Zempachi Ogumid) a)

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1,

Katahira, Aoba-Ku, Sendai, 980-8577, Japan b)

Graduate School of Engineering, Tohoku University, 6-6-1 Aramaki-Aoba, Aoba-ku, Sendai

980-8579, Japan c)

Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo

679-5198, Japan. d)

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

Kyoto 611-0011, Japan e)

Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-Ku, Kyoto

606-8501, Japan Corresponding Author *Takashi Nakamura [email protected] +81-22-217-5341

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ABSTRACT

Two-dimensional X-ray absorption spectroscopy was carried out to observe the reaction distribution in a LiCoO2 composite electrode from the shift of the peak top energy in Co K-edge XAS spectra. The influence of ionic transportation to the inhomogeneous reaction was evaluated by using the model electrode, which sandwiched the LiCoO2 composite electrode between an aluminum foil and a polyimide ion blocking layer. When the model electrode was charged with the current of 6, 9 and 12 mA cm-2, the observed capacities were 51, 20 and 12 mAh g-1 and the charged area visualized from the shift of the peak top energy in Co K-edge XAS spectra were formed within ca. 700 µm, 500 µm and 200 µm from the edge of the electrode, respectively. The observed reaction distribution indicated that the electrochemically active region decreases with increasing the current density because of the large potential loss of the electrochemical processes.


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Introduction Lithium ion batteries are a promising candidate of high-capacity and high-power energy sources for electric vehicles and large-scale power storage applications. High rate characteristics become important for electric vehicle and large-scale power-storage applications, because of the strong needs for high power output and rapid charging.1-4 However, the present battery system cannot offer sufficient performance and stability under conditions for the high-rate uses.2-4 One of the most important issues is the formation of the reaction inhomogeneity during charge and discharge. Such a non-uniform charge/discharge behavior was predicted by the model calculations and experimentally evaluated by some techniques.5-16 During charge and discharge, highly active and inactive domains were observed in composite electrodes.6-8 Lithiation and delithiation reactions preferentially proceeded from a certain domains, and the reaction in the other domains followed after the reaction in the preferential domains. One possible cause of the inhomogeneity is the insufficient electronic conduction, i.e., poor connectivity of the conducting aids and the active materials. The reaction smoothly progresses in domains with the good electronic connection.7, 8 In the case of LiFePO4, in which charge/discharge reaction takes place through two-phase reaction processes, a reaction inhomogeneity was observed, that is, reacted and unreacted particles were independently observed, even if the state of charge (SOC) was macroscopically uniform in the electrode.9-11 Furthermore, under high-rate charge/discharge, the formation of the non-equilibrium metastable LixFePO4 phase was reported.12 As another type of the reaction inhomogeneity, the macroscopic gradual reaction distribution along the in-depth direction of the electrode was also reported.13-15 In ref. 13, it was found from micro X-ray diffraction that the SOC distribution was formed along the in-depth direction under a high-rate charge (18 C), while the almost uniform SOC distribution was confirmed under the low rate


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condition (0.11 C). The difference of the lithiation rate along the in-depth direction was also observed by energy-scanning confocal XRD under battery operation.14 Harris et al. reported the propagation of the charge/discharge reaction in graphite anode by observing the color change with the optical microscopy.15 From the time dependence of the reaction propagation, it was suggested that the reaction was diffusion controlling situation. In these works, the cause of the reaction distribution formation was suggested as the insufficient ionic conduction in liquid electrolyte during charge and discharge. The formation of the reaction distribution results in the loss of the capacity and the power output of the battery. Therefore, it is essential to understand the mechanism and the governing factors of the reaction distribution formation. The visualization of the propagation of the electrochemical reactions is very effective and useful for deep understandings of the complicated electrochemical processes. To achieve this, we have developed two-dimensional X-ray absorption spectroscopy (2D-XAS).16 In the present work, we demonstrated the visualization of the reaction distribution in the model composite electrode by using 2D-XAS technique and discussed the effect of the charging rate on the reaction distribution formation. The reaction distribution was evaluated by two-dimensionally observing the change of Co K-edge X-ray absorption spectra (XAS) of LiCoO2 as a function of position. The XAS measurements were carried out at the beam line of BL01B1 and BL28XU in SPring-8, Japan.

Experimental procedures The detailed procedures to fabricate the model electrode was shown in our previous paper.16 The cathode slurry was made by mixing LiCoO2:AB:PVDF = 75:15:10 and add a small amount of 1


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methyl-2 pirolydone. The slurry was printed on an Al foil current collector, and dried in an oven at 353 K for 80 h. The thickness of the composite cathode layer was about 50 µm. The model electrode was cut into 10×10 mm while cooling by liquid N2. A electrochemical cell was fabricated with an Li metal foil (Honjo Metal Co., Ltd.) as an anode, EC-DMC with 1 molL−1 of LiPF6 (EC:DMC = 1:1 in volume, Kishida Chemical Co., Ltd.) as an electrolyte, a polymer separator (Cell-Gard #2500, Polypore International, Inc.), and the LiCoO2 model electrode. After the cell fabrication, more than 5 h of aging time was taken for the infiltration of the liquid electrolyte into the electrode. After the aging procedure, the model electrode was charged with the current rate of 6, 9 and 12 mA cm-2. In each charging test, the cut-off voltage was set to 4.2 V vs. Li+/Li. After reaching the cut-off voltage, the model electrode was taken out from the electrochemical cell within 3 min. Then, the electrode was washed with salt free EC-DMC, dried and evaluated by the two-dimensional X-ray absorption spectroscopy. The X-ray absorption spectroscopy measurements at the Co K-edge were carried out at the beam line of BL01B1 and BL28XU in SPring-8, Japan. The area of about 600 × 1200 µm was observed with the spatial resolution of approximately 2 µm in the energy range of 7700-7740 eV with the energy step of 0.1-0.4 eV.

Results and discussion In order to study the reaction distribution formation in a porous composite cathode, we prepared the model composite electrode. In the model electrode studied here, a LiCoO2 composite cathode having about 50 µm of thickness on an aluminum foil current collector was covered by a polyimide film so that the direction of the ionic transportation was limited to the in-


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plane direction of the composite electrode. Detailed information of the model electrode was summarized in our previous paper.16 In the model electrode, the electronic conduction is required along in-depth direction (about 50 µm) while the ionic migration along in-plane direction (about 5,000 µm in maximum). Thus, sufficient electronic conduction and insufficient ionic migration are considered to be achieved. Then, the electrochemical reaction preferentially takes place near the edge of the electrode where ionic species (solvated Li+ and PF6-) can almost freely migrate in the electrolyte, rather than inside the electrode. As results, the reacted area expands along the inplane direction from the edge to the center of the model electrode. The influence of the ionic diffusion on the formation of the reaction inhomogeneity is selectively strengthened. One can quantitatively evaluate the key factors for the reaction distribution formation due to the slow ionic diffusion. The SEM image of the edge part of the model electrode was shown in Figure S1 in the supplementary information. A clear edge could be successfully formed by cutting the electrode in liquid N2. Although traces of small crack and delamination were observed, their effect on the control of the ionic diffusion was considered to be small because the electrode was mechanically compressed in the out-of-plane direction during charging. The permeability of the liquid electrolyte into the composite electrode was confirmed before the electrochemical and 2D-XAS measurements. When one edge of the model electrode was immersed to the electrolyte, the electrolytes was smoothly infiltrated into the composite electrode and reached to the other side of the electrode. The infiltration of the electrolyte was completed within a few minutes. After fabricating the electrochemical cell, more than 5 hours of aging time was taken before the electrochemical test. Therefore, the composite electrode in the model electrode was completely filled with the liquid electrolyte. To evaluate the distribution of LiCoO2 active materials in the


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model electrode, SEM and transmission X-ray images were observed. Figure S2 shows the SEM image of the top view of the LiCoO2 composite electrode without a polyimide film. As shown in the figure, LiCoO2 particles were basically distributed uniformly, although some agglomerations were observed in spots. In addition to SEM observation, the distribution of LiCoO2 was evaluated from two-dimensional images of the transmission X-ray before and after the absorption edge of Co, i.e., 7700 eV and 7740 eV. The division of these images shows the concentration distribution of LiCoO2, since the transmission X-ray is weakened at where LiCoO2 particles exist due to the absorption of X-ray by LiCoO2. Figure S3 in the supplementary information shows the division image around the center of the model electrode before and after the absorption edge of Co. This image indicated almost uniform distribution of LiCoO2 particles in the whole electrode. Figures S2 and S3 confirmed that the active materials were macroscopically distributed uniformly in the model electrode used in this work, although a little fluctuation of the LiCoO2 concentration existed from microscopic point of view. Figure 1-a shows the XAS spectra at Co K-edge of LixCoO2 composite electrodes prepared as the references of SOC. As LixCoO2 was charged, it was clearly observed that the shift of the peak top energy to the higher energy and the disappearance of small shoulder peak at around 7728 eV. These tendencies agree with those reported in the earlier XAS works.17, 18 The change in the Co K-edge absorption energy is caused by the change of the valence state of Co and the local structural change around Co. When Li ions are extracted from LixCoO2 by charging, the valence state of Co consequently increases to maintain the charge neutrality. Besides the valence change of Co, the local structure around Co also changes depending on the Li content. According to ref. 17, the shift of the peak top energy of the Co K-edge absorption spectra is mainly caused by the local structural change of LixCoO2. Because the shift of the peak top energy is the most apparent


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change in the XAS spectra, in this study, we estimated the SOC of LixCoO2 from the peak top energy of the Co K-edge absorption spectra. Figure 1-b shows the relation between the peak top energy of the Co K-edge absorption spectra and the lithium content of the reference LixCoO2 specimens. It was confirmed that the peak top energy has a one-to-one correlation with the lithium content of LixCoO2 except around x = 0.5 and 0.88 < x < 1.0. LixCoO2 has a two-phase coexisting state between 0.78 < x

Visualization of Inhomogeneous Reaction Distribution in the Model

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