Quantitative Analysis of Transition-Metal Migration Induced

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Quantitative Analysis of Transition-Metal-Migration Induced Electrochemically in "Lithium-Rich Layered Oxide Cathode" and Its Property to Contribution at High and Low Temperatures Ikuma Takahashi, Katsutoshi Fukuda, Tomoya Kawaguchi, Hideyuki Komatsu, Masatsugu Oishi, Haruno Murayama, Masaharu Hatano, Takayuki Terai, Hajime Arai, Yoshiharu Uchimoto, and Eiichiro Matsubara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08199 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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The Journal of Physical Chemistry

Quantitative Analysis of Transition-Metal-Migration Induced Electrochemically in "Lithium-Rich Layered Oxide Cathode" and Its Property to Contribution at High and Low Temperatures Ikuma Takahashi,*, †, ‡ Katsutoshi Fukuda, § Tomoya Kawaguchi, § Hideyuki Komatsu, § Masatsugu Oishi, § Haruno Murayama, § Masaharu Hatano, ‡ Takayuki Terai, † Hajime Arai, § Yoshiharu Uchimoto, ¶ Eiichiro Matsubara §, # †

Department of Nuclear Engineering and Management, School of Engineering, The University

of Tokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡

Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka-shi, Kanagawa

237-8523, Japan §

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

Kyoto 611-0011, Japan ¶

Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Honmachi,

Sakyo-ku, Kyoto 606-8501, Japan

ACS Paragon Plus Environment

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Department of Materials Science & Engineering, Kyoto University, Yoshida-Honmachi,

Sakyo-ku, 606-8501, Kyoto, Japan

ABSTRACT: Lithium-rich layered oxides (LLOs) have attracted much attention as high-capacity electrodes in lithium-ion batteries. Especially, the LLO is known to show a high performance at high temperature. And the transition metal (TM) migrates from the TM layer to the Li layer in the LLO active material during the charge-discharge cycle, which complicates our understanding of its electrochemical properties. In this study, we applied X-ray diffraction spectroscopy (XDS) for acquiring quantitative data of the TM migration depending on the crystallographic site in Li1.2-xNi0.13Co0.13Mn0.53O2, and have discussed their influence on the electrochemical properties at 40 °C and −10 °C. The XDS analysis shows that both Mn and Ni in the TM layer migrate to the Li layer during the charge process and return during the discharge process. This reversible migration, observed at 40 °C, corresponds to a high capacity. On the other hand, the operation at −10 °C decreases the degree of TM migration as well as the charge-discharge capacity. In particular, Mn and Ni hardly migrate to the TM layer and remain at the Li layer at the end of discharge. This clogged interlayer space, which would lower the Li+ diffusion, accounts for the capacity drop.


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1. Introduction Lithium-rich layered oxides (LLOs) have been intensively studied as attractive cathode materials for the next generation lithium battery 1-5 because of their very high capacity of more than 250 mAhg-1. In this system, the initial structure composed of microdomains of Li2MnO3 and LiMO2 (M = Ni, Co and Mn) 6-8 is modified after the first charge. Nevertheless, the electrochemical product formed works as the high capacity electrode during the subsequent charge-discharge cycles 9-13. Thus, the structural change of the initial charge and discharge is considered to provide the electrochemically active structure which is intimately associated a battery performance in practical use of electrical power sources for various devices. However, unique element migrations in the layered material occur during the charge-discharge cycles making the mechanism of charge compensation in these materials very complex. Transition metal (TM) ions, in particular, high compositional Mn ions and unstable Ni ions migrate from the host layer (TM site) to the interlayer gallery (Li site) 14-17. TMs migration often triggers a structural change from the original layered structure to the spinel structure 6-7, 18 and results in a decrease in capacity and operation potential restring device lifetimes. Also, TMs migration has reported to provoke the hysteresis behavior between charge and discharge process 17 , 19-21, which makes difficult to manage the state of charge on a device . Moreover, the interface state between active material and electrolyte is changed by TM migration from surface to bulk, which may provide the charge-discharge hysteresis 13. In addition, the excess amount of TMs at the Li site is believed to reduce the Li mobility associated with the rate capability of the cell 22-23. Thus, the electrochemical properties of the LLO system are related to the degree of TM migration. Several groups have reported that the rate capability of the LLO cathode strongly depends on the temperature as well as the presence of TMs at the Li site 13, 24-26. This fact indicates that the


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TMs migration can be regarded as an atomic movement phenomenon depending on the reaction temperature. Understanding the relationship between the TMs migration and the electrochemical properties under various temperatures, therefore, will help us to elucidate the charge /discharge mechanism of the LLO system. Although many analytical methods including TEM 6-7, 27-29 have been applied so far in order to probe the migrated TMs directly, to the best of our knowledge, quantitative analysis of the degree of TM migration has not been reported. More recently, the new synchrotron optics combined X-ray diffraction (XRD) with X-ray absorption spectroscopy (XAS) has been well established as a tool for material analysis. This combined new way of Xray diffraction spectroscopy (XDS) including diffraction anomalous fine structure (DAFS), which can provide information on the chemical state of a target element depending on the crystallographic site 30. Furthermore, the newly developed DAFS analysis method uses powder samples and the intelligible derivation of quantitative information on the migrated ion in multisite materials 31, further motivating us to use this method to investigate TM migration in LLOs during electrochemical operation at various temperatures. In this study, we deal with the temperature dependence of the amount of TM migration in practical use, i.e. high compositional Mn and unstable Ni in the TM layer and the Li layer of LLO, based on DAFS and site-selective X-ray absorption near edge structure (XANES) analyses. We also discuss the influence of the TMs migration on the charge-discharge properties.

2. Experimental 2.1 Sample preparation


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Li1.2Ni0.13Co0.13Mn0.53O2 purchased from Kusaka Rare Metal Co. as an active material was used in this study. The Li1.2Ni0.13Co0.13Mn0.53O2 powder, acetylene black as a conducting additive and polyvinylidene difluoride as a binder were mixed in a ratio of 75:20:5 weight% and coated on an aluminum current collector to form the working composite electrode. The thickness of the working electrode was 20–30 µm to eliminate the reaction distribution in the composite electrode immersed in the electrolyte. The aluminum pouch-type cell was composed of the Li1.2Ni0.13Co0.13Mn0.53O2 working electrode, a 1 mol dm-3 LiPF6 solution of ethylene carbonate and diethyl carbonate (3:7 volume ratio), and two metallic lithium foils as the counter and reference electrodes, respectively. A polypropylene separator was used for electrochemical measurements.

2.2 Electrochemical measurement For a stable charge-discharge cycle performance, the activation treatment was carried out prior to all electrochemical measurements for XDS 32. The activation treatment procedure is the charge-discharge cycle whose lower potential is 2.0 V vs. Li/Li+ and upper potential is gradually increased 4.5, 4.6, 4.7, and 4.8 V. This operation exposes the cell to conditions similar to those during practical use. As for the charge measurement in this study, the cell was completely discharged at 0.1 C to 2.0 V at room temperature, and then the temperature was set at either −10 °C (LT : Low Temperature) or 40 °C (HT : High Temperature). After the temperature was stabilized, the charge was started to 4.8 V at a 0.1 C rate (CC mode). As for the discharge measurement, the cell was completely discharged to 2.0 V, and then charged to 4.8 V at 0.1 C and keep voltage at 4.8 V for 30 minutes (CC-CV mode) under room temperature conditions.


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Subsequently, the temperature was set at either LT or HT, the discharging at 0.1 C was conducted to the cut-off potential of 2.0 V. A cell was used for a single measurement and not reused in this experiment.

2.3 XDS measurement To pick up the samples for XDS measurements from the cell, the charging or discharging was stopped at a certain capacity. Then, the cell was frozen with liquid nitrogen and opened to extract the electrode in a glove box under argon atmosphere. Subsequently, the active material was peeled from the working electrode and further mixed with carbon black for a dilution. A tablet obtained by pelletizing this composite was covered with a sealant Kapton film. The powder-XDS measurement 30 was carried out on the BL28XU at SPring-8, Hyogo, Japan. To know the absorption coefficient of the pellet, XANES measurement for the Mn-K edge and Ni-K edge was performed using the ionized chambers in a transmission mode. At the same time, the diffraction profile was observed at 003 and 104 in 3 using a two-dimensional detector (PILATUS Dectris Co.). After the activation treatment, it was clear that the active material has a layered structure related to 3 33 (see Figure S1). The DAFS spectra obtained by XDS offers a ratio of Transition metal in the Li layer and the TM layer on the basis of the intensity ratio of the edge jump between XANES-like spectrum in the Li layer (″ ) and in the TM layer ( ). Data acquisition and analysis are described in detail elsewhere 30. REX program package (Rigaku Co.) was used to deduce the edge jump of the XANES-like spectra in this study.


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3. Results 3.1 Electrochemical properties at HT and LT Figure 1(a) shows the charge curves of Li1.2-xNi0.13Co0.13Mn0.53O2 at HT and LT. The x value in Li1.2-xNi0.13Co0.13Mn0.53O2 can be estimated by comparing the obtained capacity with the theoretical maximum capacity (377 mAhg-1 for 1.2 Li per the chemical formula). In the charging process, the capacity values at HT and LT are 328 mAhg-1 and 250 mAhg-1, respectively, indicating that a lowering of the temperature yields about 25% decrease in capacity. In practice, the cell works for a longer time under HT, which can be confirmed as the slope in the region of x > 0.8. Another prominent difference between these curves is the upward behavior of the cell potential in the region of x < 0.3. To understand this difference better, the derivative values of the capacity Q per potential E, dQ/dE, were plotted versus E for the charge curves measured at LT

Figure 1 (a) Charge curves, and (b) dQ/dE plots of Li1.2Ni0.13Co0.13Mn0.53O2 at 40 °C (HT) and –10 °C (LT). The circles indicate the measurement point for XANES.


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and HT as shown in Figure 1(b). A broad upsurge could be observed from 3.0 V and above in the dQ/dE plots at HT, and it peaks at around 3.8 V. Subsequently, it starts to decay gradually until 4.2 V and becomes nearly constant. In the case of LT, a sharp peak, whose top is at 3.8 V, can be observed from 3.75 V to 4.0 V. The change in slope of the dQ/dE curve at 4.0 V reflects the rapid rise of the cell potential at x = 0.6 in Figure 1(a). Importantly, both the peak tops are nearly identical, indicating that a similar electrochemical reaction had occurred at these temperature modes less than x = 0.8. Judging from several factors, like the use of a thin electrode and the low-current-density operation, which rule out the possibility of diffusion limitation of Li+ or electrons in the composite electrode, this temperature dependence can be attributed to the bulk structural changes of the active material. Figure 2(a) displays the discharge curves at HT and LT. In case of the discharge process, x of the starting state is 0.99 because the charge capacity at 0.1 C to 4.8 V at room temperature was 310 mAhg-1 (377 mAhg-1 for x = 1.2) after the activation treatment. The discharge capacity of 312 mAhg-1 is comparable to the charge capacity, which indicates a fully discharged state at HT, i.e. complete restoration of Li+. On the contrary, the discharge capacity at LT is 197 mAhg-1, which is smaller than that at HT. This value corresponds to more than 35% loss in capacity compared to that at HT, being larger than that observed in the charge process. Moreover, similar temperature dependence in terms of the drop of the cell potential at an early stage with that of the charging was observed. In the dQ/dE plot from the discharge curve at HT (see Figure 2(b)), the downturn starts from more than 4.5 V, and the peaks are located at around 3.3 V. On the other hand, the discharge at LT exhibits two broad peaks, whose tops are at about 3.9 V and 3.3 V, respectively. The former peak is considered to be associated with the asymmetric peak profile of


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Figure 2 (a) Discharge curves, and (b) dQ/dE plots of Li1.2Ni0.13Co0.13Mn0.53O2 at 40 °C (HT) and –10 °C (LT). The circles indicate the measurement point for XANES.

the dQ/dE curve at HT. As a result, the decrease in the operation temperature results in a significant inhibition of the electrochemical reaction that occurs at around 3.3 V.

3.2 Charge-compensation behavior of TM in LLO Putting the multi-site environments of TM in LLO aside, a change in the chemical state of the TM can be roughly assessed by the transmission mode XANES. Figure 3(a) and (b) show the Mn-K edge and Ni-K edge XANES spectra, respectively, with x = 0, 0.45, 0.80 for LT, and 0, 0.45 and 1.04 for HT of Li1.2-xNi0.13Co0.13Mn0.53O2, in the charge process. These measurement points are denoted by circles in Figure 1(a). The Mn K-edge XANES spectrum of LLO at the starting state, x = 0, clearly shifts after the charge. Both the HT and LT spectra have a similar shape at x = 0.45. At further charging until x = 0.80 and 1.04 for HT and LT, respectively, a slight difference in the energy between the spectra can be seen depending on the x value in the active material. The global shifts of XANES spectra showed with the directions by three arrows


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Figure 3 XANES spectra of (a) Mn, and (b) Ni of x in Li1.2-xNi0.13Co0.13Mn0.53O2 during charging process at 40 °C (HT) and –10 °C (LT). The arrows reflect the change of XANES spectra in response to the charge process.

are reported to indicate the Mn oxidation

34-35

. The obtained XANES spectra, notably the

direction with second and third arrows, can also be shifted. This shift indicates that the electrochemical reaction from x = 0.80 to 1.04 can be achieved by Mn oxidation-related phenomena including the oxidation of Mn or Mn environment, where polyhedra consisting of Mn and O (abbreviated as Mn–O in this study) can contribute to the charge-compensation, at the high voltage. From these results, these changes related to Mn tend to proceed at all points of x. The energy of the Ni-K edge XANES spectra obtained in this study shifts to a higher energy with increasing the x value. Thanks to the simple shape of the Ni-K edge XANES spectrum in this material, a relationship between x and the absorption edge energy, E0, (the energy at half absorbance of the peak) shown in Table S1 can be used. With respect to x =0.45, E0 at LT is higher than that at HT, which means that Ni at LT is in a more oxidized state. Interestingly, both the oxidation states become nearly the same at the end of the charge process (x = 0.80 for LT and x = 1.04 for HT).


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Figure 4(a) and (b) show the Mn-K edge and Ni-K edge XANES spectra, respectively, acquired from LLO at x = 0.99, 0.51, 0.33 for LT, and 0.99, 0.51 and 0 for HT (These x values are denoted by circles in Figure 2(a)) of Li1.2-xNi0.13Co0.13Mn0.53O2 in the discharge process. The peak top shifts of Mn-K edge XANES spectra to a lower energy during the discharge, which can be mainly explained by the reduction of Mn. In particular, the peak energy of the x = 0 sample, which is about 6557 eV, is much lower than that of the x = 0.33 sample, indicating that the Mn or Mn–O reduction proceeds rapidly in the range of 0.33 > x > 0. As is the case with the charge process, a relationship between x and E0 of the Ni-K edge XANES spectra is given in Table S2. A drastic change in E0 between x = 0.99 and 0.51 indicates that Ni is almost reduced in this range.

Figure 4 XANES spectra of (a) Mn, and (b) Ni of x in Li1.2-xNi0.13Co0.13Mn0.53O2 during discharging process at 40 °C (HT) and –10 °C (LT). The arrows reflect the change of XANES spectra in response to the discharge process.


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3.3 Site-selective understanding of TM charge compensation The atomic ratio of Mn or Ni in the Li layer to that of LLO bulk can be obtained from the intensity ratio of the edge jump between XANES-like spectrum of TM layer ( ) and of Li layer ( ) extracted from DAFS analysis (see Figure S2–S5). The atomic ratio of Mn and Ni during the charging are shown in Figure 5(a) and (b), respectively. In Figure 5(a), the atomic ratio of Mn simply increases with the charge capacity. From this result, Mn at the TM layer migrates to the Li layer in the charging process, and about 9% of Mn has moved in the case of HT. At the end of the charging, the amount of Mn in the Li layer under the HT condition is a little larger than that in the LT condition. The percentage of Ni in the Li layer increases by about 1.5% in 0 < x < 0.45 and subsequently, changes little. This result indicates that the movement of Ni from the TM layer to the Li layer is complete at x = 0.45. Note that over 14% of Ni was already present in the Li layer after the activation treatment. Here, the atomic ratios of Mn and Ni in Li site during charging are summarized in Table S3.

Figure 5 The atomic ratio change of (a) Mn, and (b) Ni of x in Li1.2-xNi0.13Co0.13Mn0.53O2 in Li site during the charging process at 40 °C (HT) as diamonds and –10 °C (LT) as squares. The arrows are shown the charging direction.


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Figure 6 exhibits the atomic ratio of Mn and Ni at the Li layer during the discharge. Note that the capacity by CC mode charging at HT is larger than CC-CV mode charging at RT, preparation of the cell for discharging, however, the amount of TM migration is switched. Probably, CV charging is considered to impact to the amount of TM migration. The amount of Mn in the Li layer decreases with the decrease in x, and the difference reaches about 10% before and after the discharge at HT. In contrast to the case with the charge process, Mn at the Li layer migrated to the TM layer in the discharge process, implying that a reversible Mn migration

Figure 6 The atomic ratio change of (a) Mn, and (b) Ni of x in Li1.2-xNi0.13Co0.13Mn0.53O2 in Li site during discharging process at 40 °C (HT) as triangles and –10 °C (LT) as circles. The arrows are shown the discharging direction.

will occur during the charge/discharge cycle. There is no difference in the amount of migrating Mn between HT and LT at x = 0.51. At the cut-off potential of 2.0 V (discharge end), although the discharge end potential is same for both LT and HT, LT result showed double amount of Mn compared to HT. This temperature dependence seems to be related to the discharge capacity. The percentage of Ni slightly decreases by about 1% in the range of 0.99 > x > 0.51. Ni from the Li layer but seems to migrate dramatically to the TM layer in the range of less than x = 0.51. At x = 0.33, which is the end point of the discharge at LT, a 2.3% decrease was observed as compared


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to that of x = 0.51 for LT. As for HT, the difference is about 7%. Here, the atomic ratios of Mn and Ni in Li site during discharging are summarized in Table S4. The TM migration scheme of Li1.2-xNi0.13Co0.13Mn0.53O2 at the discharge states, notably a significant difference is seen between HT and LT, obtained from DAFS analysis are shown in Figure 7.

Figure 7 The TM migration scheme of Li1.2-xNi0.13Co0.13Mn0.53O2 at the discharge states for LT and HT.

4. Discussion Initially, we discuss the temperature dependence of the charge-discharge reaction based on the bulk information. In the charging process, the oxidation state of all the Mn with x = 0.45 at HT is the same as that at LT (see Figure 3), but the Ni oxidation state at HT is lower than that at LT, which indicates that other oxidation reactions for charge compensation might occur at HT. Co has reported to contribute to a capacity in the voltage slope region between 3 and 4.4 V vs. Li/Li+


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, which provides the most likely reaction is a preferential oxidation of Co in the LLO. In the

region of 0.45 < x < 0.8 at the LT charge, the Mn environment and Ni oxidations proceed to correspond to the charge capacity. At further charge region, until x = 1.04, in the HT condition, the Ni oxidation is negligible, which indicates that the Mn–O oxidation predominantly corresponds to the charge capacity 36. Here, we introduce the idea of the Mn or Ni migration into these reactions. It is known that the Mn oxidation in x < 0.45 corresponds to the oxidation for delithiation from the LiMnO2 component, which is formed by the initial charge-discharge process 13, 33, 36. In addition, the firstprinciples calculations of LiMnO2 predict that Mn migrates to the Li layer in response to delithiation during charging 37. These pioneering studies guide us to assume that the intense migration of Mn to the Li layer at the region of 0 < x < 0.45 (see Figure 5(a)) is promoted by Mn oxidation (Mn3+ → Mn4+), which corresponds to the delithiation from LiMnO2 component in x < 0.45 (E = 3.9 V). The Mn migration in this region has little temperature dependence. In the case of x > 0.45, the Mn–O oxidation corresponds to the oxidation of the Li2MnO3–related component 13, 33, 36, 38

. For example, the Li2MnO3 active material is known to show similar Mn migration

from TM layer to Li layer during charging 39. Therefore, we suppose that the TM migration observed in the charge process is accompanied by delithiation from the Li2MnO3 component in x > 0.45. The average oxidation state of Ni in LT at x = 0.45 is different from that in HT (Figure 3(b)) despite little change in the migrated amount of Ni (Figure 5(b)). Most of the residual Ni ions are rapidly oxidized in the region of 0.45 < x < 1.04 at HT, whereas the amount of Ni in the Li layer is nearly constant. Thus, the correlation of Ni between the oxidation and the migration seems weaker than that of Mn. In general, Ni in the layered material is known to show cation mixing between the TM layer and the Li layer 22, 28 owing to its ionic size and the valence which


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are similar to that of Li+. The Ni migration observed in this study may stem from vacancies associated with the extraction of interlayer Li+ from LiMO2 in the charge process. In the discharging process, the reduced states of TM with x = 0.51 exhibit a negligible difference between HT and LT (see Figure 4), indicating that the discharge reaction in the x region of more than 0.51 is probably common in HT and LT. Several groups have already reported that the reduction reactions of Co and Ni and Mn—O occur at more than 3.6 V, and the reduction reaction of Mn (Mn4+ → Mn3+) for lithiation to the MnO2 component formed by oxidation of Li2MnO3 occurs at less than 3.6 V 13, 25-26. Incidentally, if the correct amount of Mn3+ from Mn-K edge XANES spectra is estimated, the quantitative analysis of LiMnO2 components may be performed by the amount of Mn3+. Based on these explanations, the discharge capacity until x = 0.51 in the HT and LT conditions, whose potential is 3.49 V and 3.21 V, respectively (see Figure 2), can be attributed to the Ni reduction and a part of the Mn reduction. In other words, the peak of 3.9 V in the dQ/dE plot at LT should originate from the reduction reactions of Co and Ni, and the peak of 3.3 V originates from the Mn reduction reaction. By contrast, although the dQ/dE peak at HT is obscure compared to that at LT, the reduction reaction of Co and Ni at HT also proceeds around 3.9 V which is hidden under the broad reduction peak and shows a small temperature dependence. On the other hand, the reduction reaction of Mn, corresponding to the peak at around 3.3 V in this study, shows a significant dependence on the temperature. This dependence triggers the capacity loss when the operation is performed at LT. Here, the dQ/dE peaks observed in this study are rather broad, which may be due to the changes of the energetics of structure transformations. However, referring the relationship between the XANES spectra and dQ/dE plots, the main reaction observed as the dQ/dE peaks are considered to correspond to the redox reactions of Ni and Mn


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ions. The present discussion, therefore, comes to focus on a relationship between the Mn reduction and the Mn migration. As described above, the possible reversible Mn migration between the Li layer and the TM layer supports the electrochemical property in this system. In the discharge, the Mn migration from the Li layer to the TM layer may be assisted by the Li intercalation to the MnO2 component x = 0.51 (3.21 V for LT), which means that the Mn reduction has already started, resulting in the drop of the Mn content in the Li layer. This is supported by the XANES spectra. The large temperature dependence of the Mn reduction reaction influences the discharge capacity and the amount of Mn migration. The Ni migration is very small in the range of 1.04 > x > 0.51(Figure 6(b)), though all of the Ni was completely reduced in this region as shown in Figure 4(b). Then, Ni migrates without Ni reduction, indicating a weak correlation between the reduction reaction and the migration as in the case of the charging process. This Ni migration could be caused by the high compositional Mn migration. Here, the Ni ratio in the Li layer at the starting state (x = 0) for the charge process in Figure 5(b) is ~13%, but that produced by the complete discharge (x = 0) at HT is ~8%. This gap is derived from the different discharge temperature before the charge process. Here, it should be noted that the cell was pre-treated with the cycling charge-discharge at room temperature as the activation treatment, and the completely discharged cell at room temperature should show a unique Ni migration different from that of the cell after the discharge at HT or LT. The fact that the discharge at LT was accomplished at x = 0.33 indicates that the Ni migration is significantly sensitive to the Li content in this region (x < 0.33). In any case, Ni predominately migrates in the low x region of the charge/discharge cycle. We further discuss a possible reason for the drastic decrease in the discharge capacity at LT. The activation energy of the Mn reduction around 3.3 V is reported to be higher than that of Co


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and Ni reduction reactions around 3.9 V 24, 26. This estimation predicts that two ratedetermining steps for Li+ diffusion may occur in this material. At least, the reduction reactions of Co and Ni proceed at LT as well as HT, suggesting that their reactions hardly depend on the temperatures used in this study. In the low x region, where the reduction reaction of Mn occurs in response to the Li intercalation to the MnO2-related slabs, Mn migrates from the Li layer to the TM layer at HT, and Ni also migrates with increasing Li content, which result in the generation of vacancies in the interlayer space. As a result, Li+ can easily diffuse in the Li layer of LLO. However, a large amount of Mn and Ni remain in the Li layer during the LT operation. This indicates a temperature dependence of the structural change due to TM migration, which is considered to support the discussion 13 based on the relationship between charge-discharge hysteresis and temperature. Especially, influence of the high-valence Mn ions may be fatal for the smooth Li+ diffusion in the Li layer. Such a clogging interlayer obstructs the Li diffusion, and the capacity drastically decreases when the discharge rate is set to be constant, which experimentally underscores the importance of making sure the diffusion path for Li+ in the cation-mixing materials. 5. Conclusion We investigated the temperature dependence of the Mn and Ni migration between the TM layer and the Li layer in Li1.2Ni0.13Co0.13Mn0.53O2 by using DAFS analysis and discussed the influence of the Mn and Ni migration on the charge-discharge properties. Both the Mn and Ni migrate from the TM layer to the Li layer in the charge process, and vice versa in the discharge process. The amount of Mn migration relates to the Mn or Mn environment redox reaction and the Ni migration seems to have a strong correlation with the Li content in the active material. At the end of discharge, the TM migration from the Li layer to the TM layer can provide the


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vacancies for Li+ diffusion at high temperature, which results in a smooth Li diffusion in the Li layer. However, Mn and Ni hardly migrate to the TM layer and remain in the Li layer during low temperature operations, which obstructs the Li+ diffusion and results in a drastic decrease in capacity. Thus, the reversibility of the TM migration during charge-discharge cycle depends on the operation temperature, which governs the capacity and cyclability. This work demonstrates a great potential of using the reversible TM migration in a positive sense for energy storages and as pillars for structural stability in the exploration of novel electrodes.

ASSOCIATED CONTENT Supporting Information. XRD pattern of x in Li1.2-xNi0.13Co0.13Mn0.53O2 (x = 0) after activation, Ni K-edge absorption edge energies are plotted at HT and LT with x in Li1.2-xNi0.13Co0.13Mn0.53O2 in charging and discharging, XAFS-like spectra of Mn and Ni at TM site and Li site in charging and discharging process with x in Li1.2-xNi0.13Co0.13Mn0.53O2 . This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] Tel.: +81-46-867-5199, Fax: +81-46-865-5796


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Notes ACKNOWLEDGMENT This work was partly supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project under the auspices of New Energy and Industrial Technology Department Organization (NEDO (Japan))

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300

Discharge capacity / mAh g-1 150

0

40 °C -10 °C Activation and Charge

Pristine

Li ion MO6 (Mn, Co, Ni) ACS Paragon Plus Environment

Quantitative Analysis of Transition-Metal Migration Induced

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