Effect of Potential Profile on Battery Capacity Decrease during

Feb 27, 2017 - (2, 3) For widespread use of the EVs, further improvement of the LIB performances such as even higher energy density for drive distance...
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Effect of Potential Profile on Battery Capacity Decrease during Continuous Cycling Koji Kitada,†,§ Haruno Murayama,† Katsutoshi Fukuda,† Hajime Arai,*,†,⊥ Yoshiharu Uchimoto,‡ and Zempachi Ogumi† †

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

‡

ABSTRACT: The origin of the capacity decrease of lithiumion batteries during continuous charge/discharge cycling, as typified by battery operation in electric vehicles, is elucidated using LiNi1/3Co1/3Mn1/3O2 (NCM)-based composite electrodes. Electrochemical cycling tests without any rest process show the capacity decrease only during discharging. Reaction distribution analysis by operando energy-scanning confocal Xray diffraction indicates that considerable reaction inhomogeneity occurs on the lithiation of NCM during discharging, whereas the delithiation during charging proceeds homogeneously. It is shown that the reaction inhomogeneity caused by limited Li+ transportation in the composite electrode is relaxed during charging owing to the potential profile of the NCMbased electrode, whereas no such relaxation occurs during discharging. This result demonstrates that the optimization of the electrode potential profile is important for good cyclability of the batteries in continuous charge/discharge cycling, in addition to improving Li+ transportation within the battery.

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INTRODUCTION The appearance of lithium-ion batteries (LIBs) having high energy density, long cycle life, and low self discharge has greatly contributed to the development of portable devices such as cellular phones and laptops.1 To shift to low carbon society and to prevent global warming, high-performance LIBs have been applied also to power sources for electric vehicles (EVs) and become a strong trigger for the explosive spread of eco-friendly cars.2,3 For widespread use of the EVs, further improvement of the LIB performances such as even higher energy density for drive distance extension per one charging is required. The LIBs for the EVs require not only energy density but also power density.4 There are several essential ratedetermining processes that cause capacity decrease at high rates, such as interface reactions between an electrolyte and active materials,5,6 phase transitions within active materials,7,8 and insufficient supply of reacting species in composite electrodes.9−14 In particular, the lithiation/delithiation inhomogeneity in the cross-sectional direction of the composite electrode caused by slow transportation of reacting species is a critical issue for large-scale LIBs. This phenomenon has been reported by some researchers, for instance, by J. Liu et al. for the inhomogeneity of LiFePO4,15 T. Sasaki et al. for the behavior of two-layer electrodes consisting of LiNi0.80Co0.15Al0.05O2 and LiMn2O4,16 and H. Murayama et al. for the inhomogeneity of LiNi1/3Co1/3Mn1/3O2 (NCM).17 Their reports suggest that the inhomogeneous electrochemical reaction occurring in the cross-sectional direction of the © XXXX American Chemical Society

composite electrode significantly decreases the capacity in high rate operation. Furthermore, when accelerating and breaking are repeated in the EVs, as seen in traveling mountain passes, continuous charge/discharge cycling causes significant capacity decrease.18−20 However, the behavior has hardly been analyzed in previous studies since it is difficult to elucidate the electrode states under such nonequilibrium conditions. Deduced by a single charge or discharge test, it is expected that the observation of electrode inhomogeneity can clarify the origin of such capacity decrease during continuous charge/ discharge processes. In this study, we report the capacity decrease during continuous charge/discharge cycling at high rates using composite electrodes consisting of NCM, a typical cathode material for LIBs. The behavior was examined by electrochemical measurements and operando energy-scanning confocal X-ray diffraction (ES-XRD) that clarifies the reaction inhomogeneity in the cross-section of the electrode.17 The origin of the capacity decrease on continuous cycling is elucidated by ES-XRD that clarifies the spatial variation of Li+ concentrations in the active material. We also show electrode design based on the obtained results that suppress the capacity decrease under continuous cycling conditions. Received: December 23, 2016 Revised: February 19, 2017 Published: February 27, 2017 A

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Figure 1. Schematic view in the cross-section of the composite electrode with a confocal point for ES-XRD measurements.

Figure 2. Charge/discharge cycling profile of the NCM composite electrode at rate of (a) 0.05 C, (b) 0.1 C, (c) 1.0 C, and (d) 1.5 C. The black, red, and green lines correspond to first, second, and third charge/discharge cycling, respectively. The blue line is the fourth charging process.

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EXPERIMENTAL SECTION

conditioned by charge/discharge cycling processes between 3.0 and 4.3 V at a constant current of 0.2 C. Electrochemical and ES-XRD measurements during continuous charge/discharge cycling were performed between 2.0 and 4.2 V without rest processes at room temperature. In advance of all measurements, the cells were discharged at constant current of 0.05 C and maintained at constant voltage of 3.0 V for 3 h to set the composition x in LixNi1/3Co1/3Mn1/3O2 at 1.0. Additionally, to reach an equilibrium state of electrolyte concentration or active material composition, the cells were maintained under open-circuit (relaxation) conditions for more than 10 h. All the ES-XRD measurements were conducted at the SPring-8 BL28XU. To examine the inhomogeneity in the composite electrode during the electrochemical reaction with a high time resolution, we measured the ES-XRD spectra of the d-range around the (113) plane of NCM.22 Arrangement of the incident beam and detector was fixed at 7.1° and 14.2°, respectively, and a dimension of the lozenge-shaped probe area depending on the

Details of sample preparation and ES-XRD measurements have been described in the literature.17,21 NCM having an average secondary particle diameter of 8.9 μm was used as the active material of the positive composite electrode. To yield slurry for the composite electrode, NCM, acetylene black, and PVdF (dispersed in N-methyl-2-pyrrolidone) were mixed in a weight ratio of 90:5:5. The resulting slurry was cast onto an aluminum foil current collector and subsequently dried in a vacuum oven at 80 °C. The composite electrode layer was adjusted to thickness of 100 μm by carefully controlled calendaring processes. Aluminum pouch-type three-electrode cells were assembled in an Ar-filled glovebox. They consisted of the positive composite electrode (working electrode), lithium metal (reference and counter electrodes), and separator (polyolefin sheet). The electrolyte was 1 mol dm−3 LiPF6 in a 3:7 (by volume) solution of EC and EMC. The cells were first B

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Figure 3. Li+ concentrations in the NCM composite electrode during charge/discharge cycling at 1 C. The symbols blue circle, red triangle, green square, and purple diamond correspond to positions (A), (B), (C), and (D), respectively, at the cross-section of the composite electrode in Figure 1. The gray plots correspond to the change in cell potential. The broken lines show the switching points of charging and discharging processes.

collimated slits sizes was 30 μm (vertical) and 243 μm (horizontal), as shown in Figure 1. There were four positions for the ES-XRD measurements at distances of (A) 20 μm, (B) 50 μm, (C) 80 μm, and (D) 100 μm from the surface of the composite electrode. The obtained 113 peak profiles were curve-fitted with a Gaussian function, and the d value was extracted from the peak center. The resulting d values were converted into the local Li + concentrations in the active material using a preliminarily determined calibration plot of d-spacing vs Li+ concentration x in LixNi1/3Co1/3Mn1/3O2.17

rates, whereas such inhomogeneity is left during the continuous cycling process at high rates, leading to the capacity decrease. Since the influence of the local inhomogeneity on the subsequent cycles can hardly be elucidated by averaged information from the electrochemical tests, the reaction inhomogeneity during continuous cycling was examined by operando ES-XRD as shown in the following paragraph. We selected four different depths in the composite electrode (see (A)−(D) in Figure 1), and the variation of the local Li+ concentration in the active material was evaluated during the continuous cycling at 1 C, in which the discharging capacity decrease was observed. The results are shown in Figure 3. In the first charging process, the delithiation in the composite electrode uniformly progressed for the first 5 min. Subsequently, the difference in the Li+ concentration (i.e., inhomogeneity) between positions (A) and (D) increased and reached a maximum after 15 min from the start. The inhomogeneity was gradually shrunk after 15 min and eventually became uniform at the end of charging. In the following discharging process, the inhomogeneity simply expanded due to slow lithiation at the counter-electrode side such as position (D), and the cells reached the cutoff voltage with inhomogeneity, unlike the charging process. After the second charging started, this inhomogeneity immediately disappeared, and the delithiation homogeneously proceeded. During the second discharging, there was inhomogeneous lithiation, and the degree of inhomogeneity was even more significant than that during the first discharging. During the third charging process, Li+ was extracted from the active materials on average; however, unusual local Li+ insertion was observed at the current-collector side. The generation of the reaction inhomogeneity in the composite electrode can be expressed as follows.9,14,16,17 The electrochemical reaction requires both electrons and Li+ at the electrode/electrolyte interface and therefore the conductive additives and electrode pores containing the electrolyte play critical roles for transporting the electrons and Li+ to the interface, respectively. Since the electronic conductivity in the NCM composite electrode is generally sufficiently high,25,26 the electrochemical reaction in high rate operation is mainly governed by the ability to transport Li+ from the bulk electrolyte to the interface. The Li+ transportation is fast at the counter-electrode side facing to the electrolyte as the Li+supplying source, whereas tortuous Li+ pathways in the microstructural pores across the composite electrode slow down the Li+ transportation at the current-collector side and hence the electrochemical reaction. The Li+ transportation at

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RESULTS AND DISCUSSION We first performed repeated charge/discharge cycling at 0.05 C, 0.1 C, 1 C (defined as 278 mA g−1), and 1.5 C rates without rest periods to find typical operating conditions under which the cycle capacity decrease occurs. Excellent cycle characteristics were found at 0.05 C and 0.1 C rates, except for a small irreversible capacity loss observed in the first cycle, as shown in Figure 2(a) and (b). Since this first loss can be recovered by discharging the cell with a constant potential mode at sufficiently low potential such as 2.0 V vs Li/Li+, this loss is not due to the material degradation but presumably due to slow Li+ diffusion within the active material23 or slow Li+ charge transfer at the electrolyte/ active material interface near Li1.0Ni1/3Co1/3Mn1/3O2.24 At higher rates such as 1 C and 1.5 C, the capacity gradually decreased during the continuous cycling process in addition to the initial irreversible loss observed also at lower rates. At 1 C, the charging capacity of the second cycle reached the same capacity as the first discharging process, whereas the second discharging capacity decreased compared to the second charging capacity, as shown in Figure 2(c). After that, the capacity decrease was observed only in the discharging process in the repeated charge/discharge cycling. By further growth of the operating rate to 1.5 C, a significant capacity decrease was observed during discharging, and a small decrease was found also during charging, as shown in Figure 2(d). The lost capacity can be recovered by applying sufficiently long rest periods at the end of the charge or discharge process, indicating that the loss was not due to the material deterioration but to some kinetic reasons. This rate-dependent decrease of the capacity gives rise to suspicion of inhomogeneous lithiation/delithiation reactions occurring in the composite electrode during continuous cycling at high rates. Namely, it is supposed that the generated reaction inhomogeneity is relaxed in the composite electrode at low C

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these phenomena, direct observation of the Li+ concentration in the electrolyte is necessary, which is a future research topic. The unusual lithiation during charging that was locally observed in the beginning of charging can also be ascribed to potential relaxation phenomena. When the local Li+ concentration in the active materials at the current-collector side is significantly low at the end of discharging, lithiation continues during the initial charging period so that the level of Li+ balances the whole electrode potential. Meanwhile, delithiation occurs at the counter-electrode side, and thus the Li+ concentration soon becomes homogeneous in the whole electrode. At a faster rate of 1.5 C, the capacity decrease was seen also in the charging process. This suggests that the extended Li+ concentration gradient in the composite electrode during the fast discharging process cannot fully be recovered in the subsequent charging process. The results shown above suggest that electrode materials with large potential variation in their potential profiles are advantageous in promoting the relaxation of the inhomogeneity in the cross-section of the composite electrodes and preventing the capacity decrease caused by the inhomogeneity. A typical example is the linear potential profile of electrochemical capacitors.30,31 Indeed, they are known to have superior characteristics of fast cycling. Another example is amorphous V2O5, an insertion-type active material with a monotonous slope potential profile that shows a stable capacity for thousands of continuous cycles.32 These facts demonstrate that, in addition to improving the Li+ transportation within the battery, design of optimum potential profiles using material chemistry will actualize the electrodes and batteries that show stable capacity in continuous charge/discharge cycling.

the counter-electrode side is accelerated to meet the constant current flow as a whole. The asymmetric capacity decrease caused by homogeneous delithiation during charging and inhomogeneous lithiation during discharging can be ascribed to the nature of the potential profile of the active material. It has been reported that the Li+ concentration inhomogeneity in the composite electrode is relaxed by the local potential difference among the composite electrodes.21,27,28 Accordingly, the inhomogeneity generated in our NCM-based composite electrodes during the charge/ discharge processes can be relaxed by the potential difference among the composite electrodes, owing to its slanted potential profile. This is in contrast to the case of LiFePO4-based composite electrodes, in which the generated inhomogeneity is not relaxed due to its extremely flat charge/discharge potential profile in the whole Li composition range.29 To understand the effect of the local potential difference during the cell operation between x = 0.5 and 1.0 in LixNi1/3Co1/3Mn1/3O2, the local Li+ concentrations at the end of third discharging and fourth charging are plotted on the open-circuit potential (OCP) curve of NCM, as shown in Figure 4.

Figure 4. Open-circuit potential curve of the NCM composite electrode and the local Li+ concentrations x of the electrode at the end of the third discharging (filled symbols) and fourth charging (open symbols). The points designated as circle, triangle, square, and diamond shapes, respectively, correspond to the positions (A), (B), (C), and (D) in Figure 1.

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CONCLUSIONS

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AUTHOR INFORMATION

In this study we clarified the origin of the capacity decrease occurring during continuous cycling through the observation of the spatial variation of the Li+ concentration within the NCMbased composite electrode. When high rate charge/discharge cycling is employed using the NCM electrode without any rest, the capacity decrease is observed almost only on discharging. This can be ascribed to that the discharging and charging to flat and slanted potential regions, respectively, result in unrelaxing and relaxing the inhomogeneity generated during the battery operation. Gradual decrease in the discharging capacity can be attributed to the insufficient recovery of the Li+ concentration in the liquid phase at the current-collector side during charging, which causes further poor Li+ supply in the subsequent discharging. Restriction of the capacity decrease by the potential difference as the driving force for the relaxation of the reaction inhomogeneity suggests that the active materials having the monotonously slanted potential profile are suitable for high rate cycling as long as sufficient Li+ supply is ensured. Not only controlling the Li+ transportation within the composite electrode but also effectively utilizing the characteristics of the potential profile of the active material will suppress the inhomogeneity occurring in the composite electrode and can lead to the maximal use of the active material, even under complex cycling conditions as typified by EV operation.

The potentials in the four positions at the end of discharging and charging are, respectively, situated on the flat and slanted parts of the OCP curve. This is the origin of the slow Li+ concentration convergence at the end of discharging, when compared to the fast relaxation at the end of charging; namely, the potential gradient of 0.6 V/Li in the large x region (toward the end of discharging) resulted in large local Li+ concentration differences of 0.16 between the counter-electrode side and the current-collector side, whereas the potential gradient of 1.7 V/ Li in the small x region (toward the end of charging) caused the limited local Li+ concentration differences of 0.03 among the electrode. They naturally lead to the inhomogeneous lithiation and homogeneous delithiation in the composite electrode. As shown in Figure 3, the inhomogeneity generated during discharging gradually expanded with cycles, which caused gradual capacity decrease as can be seen in Figure 2(c) and (d). This can probably be attributed to that the Li+ depletion in the liquid phase (impregnated in the composite pores) at the current-collector side at the end of discharging cannot be fully recovered in the following charging process, causing gradual decrease in the Li+ concentration and insufficient Li+ supply in the subsequent discharging. For further detailed discussion of

Corresponding Author

*Tel.: +81-45-924-5406. E-mail: [email protected]. D

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Katsutoshi Fukuda: 0000-0002-7895-650X Hajime Arai: 0000-0001-6695-637X Present Addresses §

Mitsubishi Motors, Co., Ltd., 1 Nakashinkiri, Hashimecho, Okazaki, Aichi 444-8501 Japan. ⊥ School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259-G1-3, Nagatsuta, Midori-ku, Yokohama 226-8502 Japan. 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 at the BL28XU beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2013B7602, 2014A7602, 2014B7602, and 2015A7602). The authors thank Mr. T. Kakei for his contributions to the sample preparation and electrochemical evaluation.

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