Applied Surface Science 258 (2012) 5938–5942
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Li4 Ti5 O12 -coated graphite as an anode material for lithium-ion batteries Meng-Lun Lee a , Yu-Han Li b , Shih-Chieh Liao b , Jing-Ming Chen b , Jien-Wei Yeh a , Han C. Shih a,c,∗ a
Department of Materials Science and Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan Materials and Chemical Engineering, Industrial Technology Research Institute, 195, Section 4, Chung-Hsing Road, Chutung 31040, Taiwan c Institute of Materials Science and Nanotechnology, Chinese Culture University, 55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 11114, Taiwan b
a r t i c l e
i n f o
Article history: Received 16 June 2011 Received in revised form 3 October 2011 Accepted 6 November 2011 Available online 15 November 2011 Keywords: Lithium-ion battery Graphite MCMB Li4 Ti5 O12
a b s t r a c t In this study, we have synthesized and characterized Li4 Ti5 O12 (LTO)-coated meso-carbon micro beads (MCMB) as an anode material for Li-batteries. The surface of MCMB powders was uniformly coated by the LTO nanoparticles to form a core–shell structure via a sol–gel process, followed by calcination. The average size of MCMB core was 20 m while the thickness of LTO shell was 80–120 nm. We found that LTOcoated MCMB has better rate-capability and cycle life, compared with the pristine MCMB. Electrochemical impedance spectroscopy (EIS) results showed that after 40 cycles, the cell resistance of the LTO-coated MCMB electrode increased slightly, while that of the pristine MCMB electrode increased signiﬁcantly. The enhanced performance of the LTO-coated MCMB electrode is attributed to the LTO coating, which suppresses the increase in the charge-transfer resistance during prolonged cycle. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction Rechargeable lithium-ion batteries have been widely used as a power source for many portable electric devices due to the high energy density. In the past two decades, various kinds of carbon such as carbon black graphite have been used as active material of negative electrode (anode) for rechargeable lithiumion batteries. Carbonaceous materials are widely employed as anodes in commercial lithium ion batteries because of their high capacity, stable voltage proﬁle and reasonable cost . Many researchers have been investigating the improvement of the reversible performance of the carbonaceous materials. Although structural change of these materials play an important role during the lithium intercalation–deintercalation process, the complex solid electrolyte interphase (SEI) formed on the surface of the carbon electrode is equally important [2–4]. The research in recent years has demonstrated that in liquid or gelled polymer electrolytes, a passivating layer generally addressed as a SEI is formed on the graphite or carbon anode during the ﬁrst charge . The surface phenomena of carbonaceous anodes have thus received increasing attention. Two models have been proposed to explain the formation of SEI. The one is that the SEI layer is formed through the co-intercalation of solvent molecules from a decomposed electrolyte, along with lithium ions, into the graphite anode . The
∗ Corresponding author at: Department of Materials Science and Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan. Tel.: +886 3 5715131x33845; fax: +886 3 5710290. E-mail address: [email protected]
other is that the SEI is formed by the decomposed electrolytes on the graphite surface . Nevertheless, the SEI represents a physical barrier between the lithiated carbon electrode, electrolyte and reacted binder . The characteristics of the SEI ﬁlm could be the most critical factors affecting the performance of carbon electrodes. Two persistent problems associated with the graphite or carbon anode are high irreversible capacity loss and poor cycling life. It is believed that the SEI ﬁlm on the surface of carbon can lead to the decomposition of electrolyte molecules and result in the high irreversible capacity and cycling performance deterioration [9–13]. Since the surface status of anode during electrochemical reaction in a lithium-ion battery is indeed a very complicated phenomenon, there is no absolute conclusion regarding the detailed mechanism up to now. Recent researches have shown that modiﬁcation of the graphite surface is an effective approach to improve its electrochemical performance [14–19]. Guo et al. improved cycling performance of natural graphite by coating poly-acrylonitrile on the surface of graphite particles via radiation-initiated polymerization . Zhao et al. modiﬁed the graphite by a carbon-ﬁlm encapsulation and reduced its BET surface area in order to minimize the irreversible capacity by reducing the formation of SEI ﬁlm during the ﬁrst intercalation process . Other researchers improved rate capability and cycling stability of cells by coating the carbon anode with metal oxides [21–23] to decrease the interfacial resistance on the electrode/electrolyte interface. Zhang et al. also reported that core–shell structured electrode materials could improve cycling behavior . In this research, we coated nanocrystalline Li4 Ti5 O12 (LTO) onto meso-carbon micro beads (MCMB) to form a core–shell structure by a sol–gel process followed by calcination. The spinel LTO is regarded
0169-4332/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.018
M.-L. Lee et al. / Applied Surface Science 258 (2012) 5938–5942
Fig. 1. Preparation of the LTO-coated MCMB.
as an ideal anode material with its long cycling stability due to zero strain or volume change during the lithium insertion and extraction process. In addition, the high voltage plateau of LTO (i.e.1.55 V) minimizes formation of the SEI on its surface. Various tactics such as metal-doping, carbon-coating and nano-sized particles have been used to improve the rate capability and cycling stability [25–29] of the LTO. In the present study, the coating morphology and crystalline structure of the composite material were investigated by SEM and XRD, respectively. The electrochemical properties of the prepared anode material were studied and the underlying mechanism will also be discussed. 2. Experimental The LTO-coated MCMB (MCMB–LTO) nanocomposite material was prepared via a sol–gel process followed by solid-state calcination in an Ar/H2 atmosphere at 800 ◦ C. The MCMB (Japan Carbon) powders were pretreated in 0.5 M sulfuric acid for 15 min. The powders were then ﬁltered and washed for 5 times with DI water. After being dried at 60 ◦ C for 8 h, 12 g of the surface-treated MCMB powders were dispersed in 15 ml of ethanol containing 0.4 g of lithium acetate (CH3 COOLi, 99%, Aldrich). The stoichiometric amount (i.e. Li/Ti = 4:5) of titanium isopropoxide (TTIP) (Aldrich) was then added into the ethanol solvent and stirred at 70 ◦ C for 5 h. The gel-like product was ﬁltered and dried in vacuum for 12 h and calcined in H2 /Ar atmosphere at 800 ◦ C for 2 h. The schematic of the process is shown in Fig. 1. The crystalline structure of the as-synthesized MCMB–LTO powders was studied by X-ray diffractometry (XRD, Philip PW-1700) operated at 40 kV and 40 mA with Cu K␣ radiation. The morphology and grain size of LTO coatings were examined with a ﬁeld emission scanning electron microscope (FESEM, JEOL JSM-6500F). The energy dispersive spectrometer (EDS) was used to analyze the chemical composition of the material. The working electrodes were made in the following way. A water-based slurry consisting of 90 wt.% of the MCMB–LTO powders, 8 wt.% of the aqueous polyacrylic latex binder (LA132), and 2 wt.% of the conductive carbon was coated onto the Cu foil by the comma coater (HIRANO TECSEED, TM-MC). The electrodes were then vacuum-dried overnight at 90 ◦ C. Galvanostatic charge–discharge measurements of the working electrodes were carried out using coin-type cells. The lithium metal was used as counter electrode in the cell. The 2032 coin cells were assembled using the copper foil as an anode in an argon glove box where both moisture and oxygen contents were <2 ppm. The mass of the electrodes was 23–25 mg. The electrolyte was composed of 1 M LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC). The electrochemical properties of the cell were tested with the multi-channel automatic battery cycler (Arbin) at a constant charging current from 0.05 C to 6 C. The cut-off voltages were set at 1.5 V and 0.01 V. Electrochemical impedance spectroscopy (EIS) has been widely used in the analysis of electrochemical intercalations of Li ions into carbonaceous materials and transition metal oxides. Lithium insertion involves a series of complex phenomena including lithium ion diffusion in the electrolyte, migration through the SEI ﬁlm covered on the electrode, charge transfer between the electrode and
the electrolyte, and the solid state diffusion in the material bulk, etc. [30–33]. In this research, EIS measurements of the cells were conducted using an electrochemical measurement unit (Solartron Instruments, SI1280B). The frequency range and voltage amplitude were set from 100 kHz to 0.01 Hz and at 5 mV, respectively. The Code ZView software was used to ﬁt the spectra to the possible equivalent circuit. 3. Results and discussion XRD proﬁles of the MCMB, pure Li4 Ti5 O12 (BTR Ltd., Shenzen), MCMB coated with the Li–Ti–O precursor, and ﬁnal product MCMB–LTO are illustrated in Fig. 2. It can be seen that the pattern of MCMB–Li–Ti–O precursor is almost the same as that of MCMB. After calcination, the MCMB coated with the Li–Ti–O precursor transformed into the core–shell MCMB–LTO with a trace of rutile TiO2 . Fig. 3 shows the morphology of the MCMB core, MCMB coated with the Li–Ti–O precursor and as-synthesized MCMB–LTO. It is seen that a continuous layer of the LTO almost completely covered the MCMB particle. Calcination leads to crystallization of LTO in the coating layer. The crystal size of the LTO coating was 80–120 nm, as shown in Fig. 3(d). The EDS result of spot 1 on the surface indicates the signal of carbon only (Fig. 3(e)), while that of spot 2 shows co-existence of the titanium, oxygen, and carbon (Fig. 3(f)). Fig. 4 shows the charge–discharge curves of the pure MCMB and MCMB–LTO at various C-rates. At the rate of 0.05 C, the charge capacity of pure MCMB was 362 mAh g−1 , compared with the theoretical capacity of 372 mAh g−1 for the graphite. The capacity decreased rapidly with the C-rate and was almost zero at the rate of 6 C. In contrast, the charge capacity of MCMB–LTO at 0.05 C was 358 mAh g−1 . In other words, the surface coating process did not change the structure and property of the MCMB, and the MCMB and MCMB–LTO had similar capacities at low C-rates (<0.2 C). As the C-rate increased, the advantage
Fig. 2. XRD proﬁles of the (a) MCMB–LTO (b) MCMB–Li–Ti–O precursor (c) MCMB (d) LTO. The open circle in (a) denotes the rutile TiO2 .
M.-L. Lee et al. / Applied Surface Science 258 (2012) 5938–5942
Fig. 3. FESEM images of the (a) MCMB, (b) MCMB coated with the Li–Ti–O precursor, (c) MCMB–LTO, (d) surface of MCMB–LTO at a higher magniﬁcation, and (e) and (f) EDX analysis of spots 1 and 2 on the surface.
of MCMB–LTO became prominent. Speciﬁcally, the capacity of MCMB–LTO was 179 mAh g−1 , 90 mAh g−1 and 40 mAh g−1 at 1-, 2- and 4-C-rates, respectively. The corresponding values for the MCMB were 65 mAh g−1 , 22 mAh g−1 and 13 mAh g−1 , as shown in Fig. 5. Fig. 6 shows the plot of charge capacity as a function of the cycle number for the pure MCMB and the MCMB–LTO samples. It is clear that the MCMB–LTO core–shell composite exhibits better
Fig. 5. Charge capacity of the pure MCMB and the MCMB–LTO at various C-rates.
Fig. 4. Galvanostatic charge–discharge curves of the (a) MCMB, and (b) MCMB–LTO.
Fig. 6. Cycle life testing of the MCMB and the MCMB–LTO. The samples were ﬁrst charged and discharged at various C-rates up to 8 C, followed by cycling at the constant charge–discharge rate of 0.2 C.
M.-L. Lee et al. / Applied Surface Science 258 (2012) 5938–5942
Fig. 7. The electrochemical impedance spectroscopy (EIS) measurement and the equivalent circuit of the anode.
cycle life performance, compared with the MCMB. We believe that the better rate capability and cycle life of the MCMB–LTO is due to the nanocrystalline LTO coating. The ac impedance of the MCMB and MCMB–LTO cells was measured in the ﬁrst and the 40th cycle. The typical EIS curve of the cells and the equivalent circuit used to ﬁt the impedance are shown in Fig. 7. The resistance measured at very high frequencies corresponds to the resistance of the ionic electrolyte Rs and is added in series to the circuit. The resistance (Rsei ) and capacitance of SEI layer in the high frequency semicircle are related to Li+ migration through the SEI layer. The capacitance of SEI ﬁlm is represented by a constant phase element (CPE) Qsei . The medium frequency semicircle is related to the charge-transfer resistance (Rct ) and capacitance of the double layer (Qdl ) on the particle surface [34–36]. And a CPE (Qd ) at the low frequency region is chosen to represent the bulk diffusion of lithium ions. This approach has been used to characterize the graphite and other active electrode materials and yielded a good agreement with the experimental data [37,38]. The expression for the admittance response of the CPE (Q) is 1 = Cωn cos Q
+ jCωn sin
where ω is the angular frequency and j is the imaginary unit. A CPE represents a resistor when n = 0, a capacitor with capacitance C while n = 1, an inductor when n = −1, and a Warburg resistance while n = 0.5. Fig. 8 shows the Nyquist plots of the cells in the discharged state (0.8 V) in the ﬁrst and the 40th cycle of charging–discharging processes. The corresponding impedance values of lithium-ion cells with the MCMB and the MCMB–LTO electrodes in the ﬁrst and the 40th cycles are listed in Table 1. In the ﬁrst cycle, the Rs , Rct and the total resistance (Rcell ) of MCMB cell are lower than those of the MCMB–LTO cell. This is due to the low conductivity of the LTO. In the 40th cycle, the Rcell of the MCMB cell increased 256% to 94.5 while that of the MCMB–LTO cell increased only 46% to 57.1 . We believe Table 1 Impedance parameters of lithium-ion cells with the MCMB and the MCMB–LTO electrodes in the ﬁrst and the 40th cycles.
MCMB 1st MCMB 40th MCMB–LTO 1st MCMB–LTO 40th
2.1 5.8 2.6 4.8
2.6 9.5 2.6 6.0
21.8 79.2 34.0 42.3
26.5 94.5 39.1 57.1
Fig. 8. Nyquist plots of the MCMB and the MCMB–LTO at the 1st and the 40th cycles.
that the better rate capability and cycle life of the MCMB–LTO cell is due to the lower cell resistance. It is known that the Rcell of the lithium-ion cell is mainly composed of the bulk resistance of lithium ion diffusion, solid electrolyte interphase resistance (Rsei ) and charge-transfer resistance (Rct ) . As indicated in Table 1, the Rct contributes most of the increase in Rcell of the cells after 40 cycles compared with Rs and Rsei . In other words, the cell impedance increasing is mainly caused by the charge-transfer resistance. Zhang et al.  studied the electrochemical impedance of cell with the lithium nickel based cathode and the graphite anode, and observed that Rct dominated Rcell during cycling. Therefore, our EIS results suggest that a nanocrystalline LTO coating on the MCMB surface signiﬁcantly suppressed the increase in the charge-transfer resistance during prolonged charging–discharging cycles and thus improved the electrochemical performance of the cell. In most cases, the SEI is a passivating ﬁlm on the surface of carbon and may hinder the electronic and ionic transports. Generally speaking, the formation of SEI is the most prominent at voltage less than 0.9 V . We believe that the underlying mechanism of the LTO coating suppressing the increase of the charge transfer resistance is due to its high redox potential (i.e. ∼1.5 V), which minimized formation of the SEI. Through the carefully controlled coating process, the standard deviation of Rct less than 1.8 was achieved and the effect of the LTO coating was accurately reproduced.
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