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Improvements in the electrochemical performance of Li4Ti5O12-coated graphite anode materials for lithium-ion batteries by simple ball-milling
Accepted Manuscript Improvements in the electrochemical performance of Li4Ti5O12-coated graphite anode materials for lithium-ion batteries by simple ball-milling Ji-Yong Eom, Yong-Hoon Cho, Seong-In Kim, Dongwook Han, Dongrak Sohn PII:
S0925-8388(17)32209-0
DOI:
10.1016/j.jallcom.2017.06.210
Reference:
JALCOM 42279
To appear in:
Journal of Alloys and Compounds
Received Date: 29 March 2017 Revised Date:
19 June 2017
Accepted Date: 20 June 2017
Please cite this article as: J.-Y. Eom, Y.-H. Cho, S.-I. Kim, D. Han, D. Sohn, Improvements in the electrochemical performance of Li4Ti5O12-coated graphite anode materials for lithium-ion batteries by simple ball-milling, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.210. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Improvements in the Electrochemical Performance
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of Li4Ti5O12-coated Graphite Anode Materials for
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Lithium-ion Batteries by Simple Ball-milling
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Ji-Yong Eom,a,∗ Yong-Hoon Cho,a Seong-In Kim,a Dongwook Han,a and Dongrak Sohnb Energy Storage System R&D Center, Korea Automotive Technology Institute, Cheonan, Chungnam 31214, Republic of Korea b
Department of Materials Science and Engineering, Korea Advanced Institute of Science and
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ABSTRACT
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Technology, Daejeon 34141, Republic of Korea
Li4Ti5O12 (LTO)-coated graphite anode materials for lithium-ion batteries with superior rate-
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capability and cycling performance were prepared by simple ball-milling in a short time. LTO particles, uniformly coated on the surface of graphite active materials, improved the kinetics and stability on the surface of graphite particles on the basis of their high Li-ion diffusivity and structural stability. As a result, the LTO-coated graphite, which was ball-milled for 5 min, presented a high initial discharge capacity (324 mAh g-1 at 0.2 C), superior rate-capability (>260 mAh g-1 at 5 C), and excellent cycling performance (~94 % after 100 cycles at 0.2 C).
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ACCEPTED MANUSCRIPT KEYWORDS: Li4Ti5O12, graphite, ball-milling, anode, lithium-ion battery
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1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used as main power sources for portable electronics, electric vehicles (xEVs), and energy storage systems (ESSs) owing
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to their high energy density and long cycle life. In particular, a recent rapid growth of demands for xEVs has provoked the development of advanced LIBs with excellent power
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performance. However, it has been impeded by the poor rate-capability of graphite, one of the most well-known commercial anode materials for LIBs, in spite of the low cost and operating voltage of the graphite.
Surface coating has been considered as an effective strategy to improve the thermal
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stability and cycle life of graphite anode materials with specific functional carbonaceous materials and metal oxides.1-5 The previously reported metal oxides including ZnO, Bi2O3, Al2O3, SiO2, and Li3PO4 positively affected the capacity retention property of the active
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materials on cycling but had little effect on the rate-capability.6-11 Among various metal
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oxides, Li4Ti5O12 (LTO), a zero-strain anode material for LIBs, has no structural change during Li-ion insertion/extraction, which thereby contributed to an excellent cycling performance of LIBs.12-14 LTO has a higher Li-ion insertion/extraction voltage (~1.55 V vs. Li+/Li) compared with commercial graphite anode materials, so it can avoid both the reduction of electrolyte and the formation of solid-electrolyte interphase (SEI) layer under 1 V on the surface of LTO electrode.15 Also, high Li-ion diffusivity within LTO crystal structure facilitates the transport of Li-ions in the LTO active particles, which could significantly improve the rate-capability of anode materials by coating of LTO.16 Thus, it
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ACCEPTED MANUSCRIPT was expected that LTO would be appropriate for the surface modification of the graphite anode material. Recently, a few researches have been reported that the electrochemical performance of active materials can be improved by the surface modification with
consuming and harmful sol-gel or hydrothermal reactions.
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LTO.17-23 However, such LTO-coated electrode materials were prepared by time-
In the present study, we report a simple method to improve the cycling performance
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and rate-capability of LTO-coated graphite anode materials by ball-milling in a short time. Also, the morphology and crystal structure of the LTO-coated graphite were examined by
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scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, respectively, and the underlying mechanism for improvements in their electrochemical performance
2. EXPERIMENTAL
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was investigated by various electrochemical analysis methods.
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2.1. Material preparation and characterization Commercial natural graphite (Nippon Carbon Co., Ltd.) and LTO (Ishihara Sangyo
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Kaisha, Ltd.) were used as starting materials. The natural graphite was mixed with the LTO powder at a mass ratio of 95:5. The mixed powders were put into a stainless steel jar (80 mL) along with stainless steel balls (6 and 10 mm) where the ball to powder mass ratio was 15:1. Then, high-energy ball-milling (Across International's, VQ-N) was carried out with the vertical rotating speed of 1200 rpm for 5 or 10 min at room temperature. The crystal structures of the synthesized LTO-coated graphite samples were analyzed by powder X-ray diffraction (XRD, Rigaku, D/MAX-IIIC) with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 5° min‒1 from 10 to 90° and the morphologies of the samples were
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ACCEPTED MANUSCRIPT examined by scanning electron microscopy (SEM, Philips, XL 30 SFEG). The LTOcoated graphite samples prepared by ball-milling for 5 and 10 min were denoted as BM-5
2.2. Cell fabrication and electrochemical analysis
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and BM-10, respectively.
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The working electrodes were prepared by coating of a slurry mixture with each active material (90 wt %) and polyvinylidene fluoride (PVdF) binder (10 wt %) in N-
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methyl-2-pyrrolidene (NMP) solvent on a copper foil and dried at 120 °C in a vacuum oven for 6 h. Lithium metal foil was used as the counter electrode and 1 M LiPF6 dissolved in a 1:1:1 volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) was used as the liquid electrolyte (Panax Etec).
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The Li/graphite and Li/LTO-coated graphite cells of 2032 coin-type were assembled in an argon-filled glove box. Those cells were charged and discharged galvanostatically at various current density (1 C = 350 mA g-1) between 0.01 and 1.5 V using a battery
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cycler (WBCS3000, Won-A Tech). The cyclic voltammetry test was performed at a constant scan rate of 1 mV s-1 between 0.01 and 1.5 V using the battery cycler. The
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electrochemical impedance spectroscopy (EIS) measurement was carried out using FRA2 and PGSTAT20 system (Autolab) over the frequency range 100 kHz to 0.01 Hz.
3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-10) samples, respectively. The characteristic peaks for the graphite matched to those
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their XRD patterns was comprised of the main diffraction peak for the graphite and the rest of peaks attributed to the LTO particles which were coated on the graphite. Therefore, two different graphite and LTO phases coexisted in the LTO-coated graphite samples. Although
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the peak position of both LTO-coated graphite samples corresponded well, all the characteristic peaks for the graphite were more broaden and also their intensities were more
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reduced with an increase in the ball-milling time of the graphite and LTO mixture, indicating an amorphization and a reduction in the crystallite size of the graphite. The crystallite size could be estimated by a full width at half maximum (FWHM) of XRD peaks using the Scherrer equation.24 The crystallite size of the graphite and LTO-coated graphite (BM-5 and
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BM-10) samples was 69.2, 59.0 (BM-5), and 44.5 nm (BM-10) for the main diffraction peak of the graphite, respectively. It is expected that the LTO-coated graphite with smaller crystallite size would be favorable to the diffusion of Li-ions through the LTO-coated
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graphite particles, resulting in a better electrochemical performance. The SEM micrographs of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-
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10) samples are described in Fig. 2. As observed in Fig. 2(a) and 2(b), the particle sizes of the graphite and LTO were ~15 and ~4 µm, respectively. The LTO-coated graphite, where the LTO particles were well distributed on the surface of the graphite particles, was much smaller in the size (5~10 µm) than the graphite particles. Here, it is well known that both the particle size reduction and agglomeration of particles probably take place simultaneously during high-energy ball-milling process,25 which can induce the amorphization of the graphite particles (Fig. 1). In addition, it was found from the results of energy-dispersive X-ray
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ACCEPTED MANUSCRIPT spectroscopy (EDS) mapping for the LTO-coated graphite, as shown in Fig. 3 that C, O, and Ti elements were uniformly distributed throughout the LTO-coated graphite samples. Thus, it can be simply concluded that the LTO particles might be successfully coated on the surface
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of the graphite particles by ball-milling. Fig. 4 presents the initial charge/discharge profiles of the graphite and LTO-coated graphite (BM-5 and BM-10) samples. In the first charge curves, the voltage profiles of the
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LTO-coated graphite samples were distinguished from that of the graphite sample obviously. In the LTO-coated graphite samples, the voltage plateaus for the Li-ion insertion into the
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LTO particles on the surface of the graphite were observed at ~1.5 V. And the voltage plateaus around 0.8 V for the formation of SEI layer on the graphite surface,25 which are attributed to the surface area, were shown distinctly and the voltage plateau was lengthened with the ball-milling time. This indicates that the surface area of the graphite particles was
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increased dramatically by ball-milling. And then the voltage profiles of the LTO-coated graphite samples was gradually decreased, which similar to the Li-ion absorption process into the amorphous carbon materials.26 These results were consistent with those of the XRD
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analyses, indicating that the amorphization of the graphite was increased with the ball-milling time. Because the extraction of Li-ions inserted into the amorphous carbon structure was very
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difficult, the Li-ions were extracted gradually through the whole of voltage range during the discharge process, resulting in the large irreversible reaction.27,28 Therefore, although the charge capacities of the LTO-coated graphite samples during the first charge process were increased with the ball-milling time, the discharge capacities were decreased from 326 (graphite) to 318 (BM-5) and 310 mAh g-1 (BM-10), corresponding to the initial coulombic efficiency of 93, 50, and 36 %, respectively. From the second charge curves, the voltage
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ACCEPTED MANUSCRIPT profiles of all the samples presented almost similar behaviors, but still the gradual voltage behaviors were observed with the ball-milling time. The cyclic voltammetry curves for the graphite and LTO-coated graphite (BM-5 and
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BM-10) samples during the charge/discharge process are given in Fig. 5. The cyclic voltammetry curves for all the samples clearly showed the peaks, corresponding to the insertion/extraction of the Li-ions. The peaks at 1.5 V in the LTO-coated graphite samples
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appeared in the first charge and corresponded to the voltage plateaus for the Li-ion insertion into the LTO particles. And the peaks below 0.8 V, corresponding to the voltage plateaus
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around 0.8 V in the first charge curves shown in Fig. 4, were presumably due to the irreversible reaction in the Li-ion insertion into the graphite and LTO-coated graphite samples. As mentioned above, the irreversible reaction was resulted from the formation of SEI layer on the graphite surface and attributed to the surface area of the graphite. The
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surface area of the graphite particles were increased by the amorphization of the graphite with the ball-milling time and thus the amount of Li-ions consumed for the irreversible reaction in the LTO-coated graphite samples was also increased dramatically. In the first discharge
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curves, the peaks around 0.5 V corresponded to the extraction of the Li-ions inserted into the graphene layers of the graphite. The amount of Li-ions extracted from the graphene layers of
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the LTO-coated graphite was smaller than that of the graphite, because the amorphization of the graphite was increased by ball-milling. Therefore, the lower initial discharge capacity and coulombic efficiency of the LTO-coated graphite compared with the graphite were resulted from (i) the presence of inactive LTO coating layers on the surface of the graphite, (ii) amorphization of the graphite structure by ball-milling, and (iii) high initial irreversible charge capacity induced by large surface area of the LTO-coated graphite.29,30
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ACCEPTED MANUSCRIPT The rate-capabilities for the graphite and LTO-coated graphite (BM-5 and BM-10) samples were examined as a function of the applied current density (from 0.2 C to 5 C), as presented in Fig. 6. Although the graphite sample exhibited a drastic capacity decrease with
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an increase in the current density from 1 C to 5 C and could not restored its discharge capacity at all when cycled at 0.2 C again, the initial discharge capacity at 0.2 C of the LTOcoated graphite sample was maintained to ~80 % at the high current density of 5 C and could
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restored immediately to more than 90 % when cycled at 0.2 C again. It is expected that the superior rate-capability of the LTO-coated graphite to the graphite seemed to be resulted
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from the presence of the high-power LTO particles and the small crystallite size of the graphite particles. The crystallite size of the graphite particles was reduced and the LTO particles tended to be more uniformly distributed on the surface of the graphite with the ballmilling time, but the rate-capability of the BM-10 sample was inferior to that of the BM-5
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sample. This is because of the poor graphitic crystalline of the LTO-coated graphite sample followed by high irreversible reaction by ball-milling for a long time. The improved kinetic properties of the LTO-coated graphite samples were assessed by
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EIS measurements and compared with those of the graphite sample, as shown in Fig. 7. A semicircle in the high and intermediate frequency range of 100~0.01 kHz and a sloping line
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in the low frequency range of 10~0.01 Hz were observed in the Nyquist plots of EIS for the graphite and LTO-coated graphite samples at a fully charged state after the 3rd cycle at a current density of 0.2 C. The semicircle at the high and intermediate frequency corresponds to the migration of Li-ions through the passivation film, such as the SEI and to the charge transfer reaction on the surface of electrode, and the sloping line in the low frequency corresponds to the diffusion of Li-ions in the electrode as Warburg’s impedance.28 The charge transfer resistance, relating to the electronic resistance, of the LTO-coated graphite
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crystallite size of the graphite particles and LTO-coating on the surface of the graphite. In the further ball-milling, the poor graphitic crystalline of the LTO-coated graphite (BM-10) sample followed by high irreversible reaction resulted in an increase of the charge transfer
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resistance.
Improved cycling performance of the LTO-coated graphite (BM-5) sample was also
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demonstrated and compared with that of the graphite sample, as presented in Fig. 8. The LTO-coated graphite (BM-5) showed very stable and excellent cycling performance with the capacity retention of ~94 % after 100 cycles. On the other hand, the graphite exhibited much lower capacity retention than the LTO-coated graphite (BM-5). It is certain that the LTO-
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coating on the surface of the graphite made a positive effect on improving the cycling performance of the graphite. It has been reported that the LTO-coating layer on the surface of the graphite possibly minimizes unstable side reactions between the graphite and electrolyte
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during cycling, thereby enhancing the surface stability of the electrode materials.17 And also, the small crystallite size of the graphite by ball-milling enhanced the diffusion of Li-ions in
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the LTO-coated graphite. Therefore, it is believed that the surface modification of the graphite by the LTO particles is one of the most effective ways for improving the ratecapability and cycling performance of the graphite anode material for LIBs at a time.
4. CONCLUSIONS
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ACCEPTED MANUSCRIPT In the present study, we succeeded in the synthesis of the LTO-coated graphite anode materials for LIBs by simple ball-milling process in a short time, where the prepared highpower LTO particles were uniformly coated on the surface of the graphite particles. As a
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result, the electrochemical performance of the graphite anode material was improved by the surface coating with LTO particles. In particular, the LTO-coated graphite, which was prepared by ball-milling for 5 min, represented a high initial discharge capacity (324 mAh g-1
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at 0.2 C), superior rate-capability (>260 mAh g-1 at 5 C), and excellent cycling performance (~94 % after 100 cycles at 0.2 C). The enhanced rate-capability and cycling performance of
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the LTO-coated graphite were associated with improvements in the kinetics and stability on the surface of the graphite particles by the small crystallite size and the existence of the LTO particles with high Li-ion diffusivity.
Corresponding Author
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AUTHOR INFORMATION
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*Tel: +82-41-559-3355. Fax: +82-41-559-3181. E-mail: [email protected] (Ji-Yong Eom)
ACKNOWLEDGMENTS This work was supported by the Industrial Technology Innovation Program (Grant No. 10050568) funded by the Korea Ministry of Trade, Industry & Energy.
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FIGURES
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samples.
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Fig. 1. XRD patterns of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-10)
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Fig. 2. SEM images of the (a) graphite, (b) LTO, and LTO-coated graphite; (c) BM-5 and (d) samples.
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BM-10
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Fig. 3. EDS mapping images of carbon, oxygen, and titanium elements for the LTO-coated graphite; (a) BM-5 and (b) BM-10 samples.
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Fig. 4. Initial charge/discharge profiles of the (a) graphite and LTO-coated graphite; (b) BM5 and (c) BM-10 samples.
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Fig. 5. Cyclic voltammetry curves for the (a) graphite and LTO-coated graphite; (b) BM-5 and (c) BM-10 samples during the charge/discharge process.
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Fig. 6. Rate-capabilities for the graphite and LTO-coated graphite (BM-5 and BM-10)
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samples as a function of the applied current density.
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Fig. 7. Nyquist plots of EIS for the graphite and LTO-coated graphite (BM-5 and BM-10)
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samples at a fully charged state after the 3rd cycle at a current density of 0.2 C.
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Fig. 8. Cycling performance for the graphite and LTO-coated graphite (BM-5) samples at a
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current density of 0.2 C.
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Improvements in the Electrochemical Performance of Li4Ti5O12-coated Graphite Anode Materials for Lithium-ion Batteries by Simple Ball-milling Highlights
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The Li4Ti5O12 (LTO)-coated graphite samples were prepared by ball-milling. The Li-ion diffusivity is improved after LTO coating by ball-milling. LTO-coated graphite shows better electrochemical performance than pristine graphite.
S0925-8388(17)32209-0
DOI:
10.1016/j.jallcom.2017.06.210
Reference:
JALCOM 42279
To appear in:
Journal of Alloys and Compounds
Received Date: 29 March 2017 Revised Date:
19 June 2017
Accepted Date: 20 June 2017
Please cite this article as: J.-Y. Eom, Y.-H. Cho, S.-I. Kim, D. Han, D. Sohn, Improvements in the electrochemical performance of Li4Ti5O12-coated graphite anode materials for lithium-ion batteries by simple ball-milling, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.210. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Improvements in the Electrochemical Performance
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of Li4Ti5O12-coated Graphite Anode Materials for
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Lithium-ion Batteries by Simple Ball-milling
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Ji-Yong Eom,a,∗ Yong-Hoon Cho,a Seong-In Kim,a Dongwook Han,a and Dongrak Sohnb Energy Storage System R&D Center, Korea Automotive Technology Institute, Cheonan, Chungnam 31214, Republic of Korea b
Department of Materials Science and Engineering, Korea Advanced Institute of Science and
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ABSTRACT
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Technology, Daejeon 34141, Republic of Korea
Li4Ti5O12 (LTO)-coated graphite anode materials for lithium-ion batteries with superior rate-
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capability and cycling performance were prepared by simple ball-milling in a short time. LTO particles, uniformly coated on the surface of graphite active materials, improved the kinetics and stability on the surface of graphite particles on the basis of their high Li-ion diffusivity and structural stability. As a result, the LTO-coated graphite, which was ball-milled for 5 min, presented a high initial discharge capacity (324 mAh g-1 at 0.2 C), superior rate-capability (>260 mAh g-1 at 5 C), and excellent cycling performance (~94 % after 100 cycles at 0.2 C).
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ACCEPTED MANUSCRIPT KEYWORDS: Li4Ti5O12, graphite, ball-milling, anode, lithium-ion battery
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1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used as main power sources for portable electronics, electric vehicles (xEVs), and energy storage systems (ESSs) owing
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to their high energy density and long cycle life. In particular, a recent rapid growth of demands for xEVs has provoked the development of advanced LIBs with excellent power
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performance. However, it has been impeded by the poor rate-capability of graphite, one of the most well-known commercial anode materials for LIBs, in spite of the low cost and operating voltage of the graphite.
Surface coating has been considered as an effective strategy to improve the thermal
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stability and cycle life of graphite anode materials with specific functional carbonaceous materials and metal oxides.1-5 The previously reported metal oxides including ZnO, Bi2O3, Al2O3, SiO2, and Li3PO4 positively affected the capacity retention property of the active
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materials on cycling but had little effect on the rate-capability.6-11 Among various metal
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oxides, Li4Ti5O12 (LTO), a zero-strain anode material for LIBs, has no structural change during Li-ion insertion/extraction, which thereby contributed to an excellent cycling performance of LIBs.12-14 LTO has a higher Li-ion insertion/extraction voltage (~1.55 V vs. Li+/Li) compared with commercial graphite anode materials, so it can avoid both the reduction of electrolyte and the formation of solid-electrolyte interphase (SEI) layer under 1 V on the surface of LTO electrode.15 Also, high Li-ion diffusivity within LTO crystal structure facilitates the transport of Li-ions in the LTO active particles, which could significantly improve the rate-capability of anode materials by coating of LTO.16 Thus, it
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ACCEPTED MANUSCRIPT was expected that LTO would be appropriate for the surface modification of the graphite anode material. Recently, a few researches have been reported that the electrochemical performance of active materials can be improved by the surface modification with
consuming and harmful sol-gel or hydrothermal reactions.
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LTO.17-23 However, such LTO-coated electrode materials were prepared by time-
In the present study, we report a simple method to improve the cycling performance
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and rate-capability of LTO-coated graphite anode materials by ball-milling in a short time. Also, the morphology and crystal structure of the LTO-coated graphite were examined by
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scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, respectively, and the underlying mechanism for improvements in their electrochemical performance
2. EXPERIMENTAL
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was investigated by various electrochemical analysis methods.
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2.1. Material preparation and characterization Commercial natural graphite (Nippon Carbon Co., Ltd.) and LTO (Ishihara Sangyo
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Kaisha, Ltd.) were used as starting materials. The natural graphite was mixed with the LTO powder at a mass ratio of 95:5. The mixed powders were put into a stainless steel jar (80 mL) along with stainless steel balls (6 and 10 mm) where the ball to powder mass ratio was 15:1. Then, high-energy ball-milling (Across International's, VQ-N) was carried out with the vertical rotating speed of 1200 rpm for 5 or 10 min at room temperature. The crystal structures of the synthesized LTO-coated graphite samples were analyzed by powder X-ray diffraction (XRD, Rigaku, D/MAX-IIIC) with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 5° min‒1 from 10 to 90° and the morphologies of the samples were
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ACCEPTED MANUSCRIPT examined by scanning electron microscopy (SEM, Philips, XL 30 SFEG). The LTOcoated graphite samples prepared by ball-milling for 5 and 10 min were denoted as BM-5
2.2. Cell fabrication and electrochemical analysis
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and BM-10, respectively.
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The working electrodes were prepared by coating of a slurry mixture with each active material (90 wt %) and polyvinylidene fluoride (PVdF) binder (10 wt %) in N-
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methyl-2-pyrrolidene (NMP) solvent on a copper foil and dried at 120 °C in a vacuum oven for 6 h. Lithium metal foil was used as the counter electrode and 1 M LiPF6 dissolved in a 1:1:1 volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) was used as the liquid electrolyte (Panax Etec).
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The Li/graphite and Li/LTO-coated graphite cells of 2032 coin-type were assembled in an argon-filled glove box. Those cells were charged and discharged galvanostatically at various current density (1 C = 350 mA g-1) between 0.01 and 1.5 V using a battery
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cycler (WBCS3000, Won-A Tech). The cyclic voltammetry test was performed at a constant scan rate of 1 mV s-1 between 0.01 and 1.5 V using the battery cycler. The
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electrochemical impedance spectroscopy (EIS) measurement was carried out using FRA2 and PGSTAT20 system (Autolab) over the frequency range 100 kHz to 0.01 Hz.
3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-10) samples, respectively. The characteristic peaks for the graphite matched to those
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ACCEPTED MANUSCRIPT for the typical natural graphite. Also, the LTO sample exhibited its diffraction peaks, corresponding to the cubic spinel structured LTO with Fd3m space group (JCPDS No. 720426). In the cases of the LTO-coated graphite (BM-5 and BM-10) samples, a great part of
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their XRD patterns was comprised of the main diffraction peak for the graphite and the rest of peaks attributed to the LTO particles which were coated on the graphite. Therefore, two different graphite and LTO phases coexisted in the LTO-coated graphite samples. Although
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the peak position of both LTO-coated graphite samples corresponded well, all the characteristic peaks for the graphite were more broaden and also their intensities were more
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reduced with an increase in the ball-milling time of the graphite and LTO mixture, indicating an amorphization and a reduction in the crystallite size of the graphite. The crystallite size could be estimated by a full width at half maximum (FWHM) of XRD peaks using the Scherrer equation.24 The crystallite size of the graphite and LTO-coated graphite (BM-5 and
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BM-10) samples was 69.2, 59.0 (BM-5), and 44.5 nm (BM-10) for the main diffraction peak of the graphite, respectively. It is expected that the LTO-coated graphite with smaller crystallite size would be favorable to the diffusion of Li-ions through the LTO-coated
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graphite particles, resulting in a better electrochemical performance. The SEM micrographs of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-
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10) samples are described in Fig. 2. As observed in Fig. 2(a) and 2(b), the particle sizes of the graphite and LTO were ~15 and ~4 µm, respectively. The LTO-coated graphite, where the LTO particles were well distributed on the surface of the graphite particles, was much smaller in the size (5~10 µm) than the graphite particles. Here, it is well known that both the particle size reduction and agglomeration of particles probably take place simultaneously during high-energy ball-milling process,25 which can induce the amorphization of the graphite particles (Fig. 1). In addition, it was found from the results of energy-dispersive X-ray
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ACCEPTED MANUSCRIPT spectroscopy (EDS) mapping for the LTO-coated graphite, as shown in Fig. 3 that C, O, and Ti elements were uniformly distributed throughout the LTO-coated graphite samples. Thus, it can be simply concluded that the LTO particles might be successfully coated on the surface
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of the graphite particles by ball-milling. Fig. 4 presents the initial charge/discharge profiles of the graphite and LTO-coated graphite (BM-5 and BM-10) samples. In the first charge curves, the voltage profiles of the
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LTO-coated graphite samples were distinguished from that of the graphite sample obviously. In the LTO-coated graphite samples, the voltage plateaus for the Li-ion insertion into the
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LTO particles on the surface of the graphite were observed at ~1.5 V. And the voltage plateaus around 0.8 V for the formation of SEI layer on the graphite surface,25 which are attributed to the surface area, were shown distinctly and the voltage plateau was lengthened with the ball-milling time. This indicates that the surface area of the graphite particles was
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increased dramatically by ball-milling. And then the voltage profiles of the LTO-coated graphite samples was gradually decreased, which similar to the Li-ion absorption process into the amorphous carbon materials.26 These results were consistent with those of the XRD
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analyses, indicating that the amorphization of the graphite was increased with the ball-milling time. Because the extraction of Li-ions inserted into the amorphous carbon structure was very
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difficult, the Li-ions were extracted gradually through the whole of voltage range during the discharge process, resulting in the large irreversible reaction.27,28 Therefore, although the charge capacities of the LTO-coated graphite samples during the first charge process were increased with the ball-milling time, the discharge capacities were decreased from 326 (graphite) to 318 (BM-5) and 310 mAh g-1 (BM-10), corresponding to the initial coulombic efficiency of 93, 50, and 36 %, respectively. From the second charge curves, the voltage
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ACCEPTED MANUSCRIPT profiles of all the samples presented almost similar behaviors, but still the gradual voltage behaviors were observed with the ball-milling time. The cyclic voltammetry curves for the graphite and LTO-coated graphite (BM-5 and
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BM-10) samples during the charge/discharge process are given in Fig. 5. The cyclic voltammetry curves for all the samples clearly showed the peaks, corresponding to the insertion/extraction of the Li-ions. The peaks at 1.5 V in the LTO-coated graphite samples
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appeared in the first charge and corresponded to the voltage plateaus for the Li-ion insertion into the LTO particles. And the peaks below 0.8 V, corresponding to the voltage plateaus
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around 0.8 V in the first charge curves shown in Fig. 4, were presumably due to the irreversible reaction in the Li-ion insertion into the graphite and LTO-coated graphite samples. As mentioned above, the irreversible reaction was resulted from the formation of SEI layer on the graphite surface and attributed to the surface area of the graphite. The
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surface area of the graphite particles were increased by the amorphization of the graphite with the ball-milling time and thus the amount of Li-ions consumed for the irreversible reaction in the LTO-coated graphite samples was also increased dramatically. In the first discharge
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curves, the peaks around 0.5 V corresponded to the extraction of the Li-ions inserted into the graphene layers of the graphite. The amount of Li-ions extracted from the graphene layers of
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the LTO-coated graphite was smaller than that of the graphite, because the amorphization of the graphite was increased by ball-milling. Therefore, the lower initial discharge capacity and coulombic efficiency of the LTO-coated graphite compared with the graphite were resulted from (i) the presence of inactive LTO coating layers on the surface of the graphite, (ii) amorphization of the graphite structure by ball-milling, and (iii) high initial irreversible charge capacity induced by large surface area of the LTO-coated graphite.29,30
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ACCEPTED MANUSCRIPT The rate-capabilities for the graphite and LTO-coated graphite (BM-5 and BM-10) samples were examined as a function of the applied current density (from 0.2 C to 5 C), as presented in Fig. 6. Although the graphite sample exhibited a drastic capacity decrease with
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an increase in the current density from 1 C to 5 C and could not restored its discharge capacity at all when cycled at 0.2 C again, the initial discharge capacity at 0.2 C of the LTOcoated graphite sample was maintained to ~80 % at the high current density of 5 C and could
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restored immediately to more than 90 % when cycled at 0.2 C again. It is expected that the superior rate-capability of the LTO-coated graphite to the graphite seemed to be resulted
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from the presence of the high-power LTO particles and the small crystallite size of the graphite particles. The crystallite size of the graphite particles was reduced and the LTO particles tended to be more uniformly distributed on the surface of the graphite with the ballmilling time, but the rate-capability of the BM-10 sample was inferior to that of the BM-5
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sample. This is because of the poor graphitic crystalline of the LTO-coated graphite sample followed by high irreversible reaction by ball-milling for a long time. The improved kinetic properties of the LTO-coated graphite samples were assessed by
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EIS measurements and compared with those of the graphite sample, as shown in Fig. 7. A semicircle in the high and intermediate frequency range of 100~0.01 kHz and a sloping line
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in the low frequency range of 10~0.01 Hz were observed in the Nyquist plots of EIS for the graphite and LTO-coated graphite samples at a fully charged state after the 3rd cycle at a current density of 0.2 C. The semicircle at the high and intermediate frequency corresponds to the migration of Li-ions through the passivation film, such as the SEI and to the charge transfer reaction on the surface of electrode, and the sloping line in the low frequency corresponds to the diffusion of Li-ions in the electrode as Warburg’s impedance.28 The charge transfer resistance, relating to the electronic resistance, of the LTO-coated graphite
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ACCEPTED MANUSCRIPT (BM-5) sample at the intermediate frequency was much smaller than those of the graphite and another LTO-coated graphite (BM-10) samples. This indicates that the LTO-coated graphite (BM-5) sample could offer relatively high electron conductivity due to its small
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crystallite size of the graphite particles and LTO-coating on the surface of the graphite. In the further ball-milling, the poor graphitic crystalline of the LTO-coated graphite (BM-10) sample followed by high irreversible reaction resulted in an increase of the charge transfer
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resistance.
Improved cycling performance of the LTO-coated graphite (BM-5) sample was also
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demonstrated and compared with that of the graphite sample, as presented in Fig. 8. The LTO-coated graphite (BM-5) showed very stable and excellent cycling performance with the capacity retention of ~94 % after 100 cycles. On the other hand, the graphite exhibited much lower capacity retention than the LTO-coated graphite (BM-5). It is certain that the LTO-
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coating on the surface of the graphite made a positive effect on improving the cycling performance of the graphite. It has been reported that the LTO-coating layer on the surface of the graphite possibly minimizes unstable side reactions between the graphite and electrolyte
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during cycling, thereby enhancing the surface stability of the electrode materials.17 And also, the small crystallite size of the graphite by ball-milling enhanced the diffusion of Li-ions in
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the LTO-coated graphite. Therefore, it is believed that the surface modification of the graphite by the LTO particles is one of the most effective ways for improving the ratecapability and cycling performance of the graphite anode material for LIBs at a time.
4. CONCLUSIONS
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ACCEPTED MANUSCRIPT In the present study, we succeeded in the synthesis of the LTO-coated graphite anode materials for LIBs by simple ball-milling process in a short time, where the prepared highpower LTO particles were uniformly coated on the surface of the graphite particles. As a
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result, the electrochemical performance of the graphite anode material was improved by the surface coating with LTO particles. In particular, the LTO-coated graphite, which was prepared by ball-milling for 5 min, represented a high initial discharge capacity (324 mAh g-1
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at 0.2 C), superior rate-capability (>260 mAh g-1 at 5 C), and excellent cycling performance (~94 % after 100 cycles at 0.2 C). The enhanced rate-capability and cycling performance of
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the LTO-coated graphite were associated with improvements in the kinetics and stability on the surface of the graphite particles by the small crystallite size and the existence of the LTO particles with high Li-ion diffusivity.
Corresponding Author
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AUTHOR INFORMATION
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*Tel: +82-41-559-3355. Fax: +82-41-559-3181. E-mail: [email protected] (Ji-Yong Eom)
ACKNOWLEDGMENTS This work was supported by the Industrial Technology Innovation Program (Grant No. 10050568) funded by the Korea Ministry of Trade, Industry & Energy.
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FIGURES
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samples.
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Fig. 1. XRD patterns of the graphite, LTO, and LTO-coated graphite (BM-5 and BM-10)
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Fig. 2. SEM images of the (a) graphite, (b) LTO, and LTO-coated graphite; (c) BM-5 and (d) samples.
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BM-10
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Fig. 3. EDS mapping images of carbon, oxygen, and titanium elements for the LTO-coated graphite; (a) BM-5 and (b) BM-10 samples.
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Fig. 4. Initial charge/discharge profiles of the (a) graphite and LTO-coated graphite; (b) BM5 and (c) BM-10 samples.
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Fig. 5. Cyclic voltammetry curves for the (a) graphite and LTO-coated graphite; (b) BM-5 and (c) BM-10 samples during the charge/discharge process.
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Fig. 6. Rate-capabilities for the graphite and LTO-coated graphite (BM-5 and BM-10)
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samples as a function of the applied current density.
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Fig. 7. Nyquist plots of EIS for the graphite and LTO-coated graphite (BM-5 and BM-10)
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samples at a fully charged state after the 3rd cycle at a current density of 0.2 C.
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Fig. 8. Cycling performance for the graphite and LTO-coated graphite (BM-5) samples at a
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current density of 0.2 C.
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Improvements in the Electrochemical Performance of Li4Ti5O12-coated Graphite Anode Materials for Lithium-ion Batteries by Simple Ball-milling Highlights
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The Li4Ti5O12 (LTO)-coated graphite samples were prepared by ball-milling. The Li-ion diffusivity is improved after LTO coating by ball-milling. LTO-coated graphite shows better electrochemical performance than pristine graphite.