C composite as an anode material for Li-ion batteries

C composite as an anode material for Li-ion batteries

Solid State Ionics 274 (2015) 83–87 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi In-s...

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Solid State Ionics 274 (2015) 83–87

Contents lists available at ScienceDirect

Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

In-situ synthesis of nano-Li4Ti5O12/C composite as an anode material for Li-ion batteries Yurong Ren a,c, Peng Lu a, Xiaobing Huang b,⁎, Shibiao Zhou b, Yuandao Chen b, Beiping Liu b, Fuqiang Chu a,c, Jianning Ding a,c,⁎⁎ a b c

School of Materials and Engineering, Changzhou University, Changzhou 213164, China College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde 415000, China Jiangsu Early Phase Key Laboratory for Photovoltaic Engineering Science, Changzhou 213164, China

a r t i c l e

i n f o

Article history: Received 14 December 2014 Received in revised form 2 February 2015 Accepted 20 February 2015 Available online 20 March 2015 Keywords: Li-ion battery Anode material Li4Ti5O12 Li4Ti5O12/C composite PVP

a b s t r a c t In-situ coating approach using polyvinyl pyrrolidone (PVP) as carbon source is introduced in this work with the aim of getting high rate Li4Ti5O12/C composite. Li4Ti5O12/C composite, particle size of 50–200 nm in diameter, is well dispersed and the carbon layers are 2–4 nm in thickness. Li4Ti5O12/C composite delivers much higher electrochemical performance than Li4Ti5O12, in terms of reversible discharge capacity and rate performance. It exhibits high discharge capacities of 172 mAh g−1 and 141 mAh g−1 at 0.2 C and 10 C-rate, respectively. 95.7% of its initial capacity is retained after 200 cycles at 10 C, demonstrating good cycling stability. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) have been widely used as the major power sources for portable electronic devices and also regarded as a promising candidate of power sources for electric vehicles and large scale electricity storage stations [1]. Currently commercial LIB usually uses graphite as anode, which shows the low lithium intercalating voltage of approximately 100 mV (vs. Li+/Li), thus highly reactive metallic lithium is easily formed and deposited on the surface of electrode particles under a fast charge rate, leading to a high risk of reaction with the electrolyte or charged cathode [2–8]. It is therefore of great significance to find an anode material which can be used to achieve a high rate capability, long cycle life, and high safety [9]. Spinel Li4Ti5O12 has particular advantages of low toxicity, low cost, easy preparation and low-strain lithium intercalation/desintercalation, making it a promising candidate of lithium insertion anode materials for the next generation of Li-ion batteries [10,11]. Furthermore, it possesses a flat voltage plateau at approximately 1.55 V (vs. Li+/Li), which is higher than the reduction potential of most organic electrolytes, thus avoiding the formation of a solid electrolyte interface (SEI) [12]. Unfortunately, the low electronic conductivity of Li4Ti5O12 has been a main obstacle to its application ⁎ Corresponding author. Tel.: +86 18761162096. ⁎⁎ Correspondence to: J.N. Ding, School of Materials and Engineering, Changzhou University, Changzhou 213164, China. Tel.: +86 18761162096. E-mail addresses: [email protected] (X. Huang), [email protected] (J. Ding).

http://dx.doi.org/10.1016/j.ssi.2015.02.016 0167-2738/© 2015 Elsevier B.V. All rights reserved.

[9]. Some strategies including synthesizing the nanosized particles via various methods [13–15], surface modification by means of conductive matters (Au, Ag, Cu, carbon, etc) [16–19] and cation doping (Na+ [20], Ni2+ [21], Zn2+ [22], Al3+ [23], W6+ [24], Sr2+[25]) in Li and Ti sites have been proposed to address this issue. In recent years, combination of nanosized Li4Ti5O12 and carbon materials has been considered a reasonable approach to fabricate high performance Li4Ti5O12 in view of simplicity, effectivity and applicability [26]. Here, in this work, we explored a solid state approach that enables fabrication of nano-Li4Ti5O12/C composite. It is desirable that the addition of PVP can effectively enhance the electrochemical performance at high current densities.

2. Experimental Both samples were prepared by a solid-state reaction in the present work. Li4Ti5O12 was synthesized using CH3COOLi · 2H2O (99.99%) and TiO2 (99.5%) as raw materials. Firstly, the raw materials in a stoichiometric ratio were dissolved in distilled water and stirred magnetically to form a slurry mixture. In the following step, the obtained slurry was treated at 100 °C to form dried powder, which was then further heated in a horizontal quartz tube oven under flowing argon gas at 750 °C for 8 h to obtain the final Li4Ti5O12. The synthesis of Li4Ti5O12/C composite was similar to that of Li4Ti5O12, but adding PVP as carbon source mixed with the raw materials in distilled water.

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Y. Ren et al. / Solid State Ionics 274 (2015) 83–87 Table 1 BET surface area of both samples. BET surface area (m2 g−1) Li4Ti5O12 Li4Ti5O12/C

6.0 12.6

3. Results and discussion

Fig. 1. XRD profiles of as-obtained Li4Ti5O12 samples.

The crystallographic structural characterization of as-prepared samples was performed by X-ray powder diffraction analysis (XRD) with Cu Kα radiation (DX 2700). The particle morphologies were observed by scanning electron microscopy (SEM, JSM-6510LA) and transmission electron microscopy (TEM, JEOL JEM-2100F). The carbon content of Li4Ti5O12/C composite was confirmed by means of a carbon–sulfur analyzer. The electronic conductivity was measured using a four-point probe method (RTS-9, Guangzhou). The BET surface area of the samples was detected by nitrogen adsorption–desorption at − 196 °C using a Builder SSA-4200 apparatus. To fabricate the working electrodes, 85 wt.% of active material, 10 wt.% of Super-P carbon as conductive material and 5 wt.% of LA132 as binder were dispersed in water solvent to form homogeneous slurry. The slurry was then cast on an aluminum foil and dried at 100 °C for 10 h in a vacuum oven. Finally, CR2032 coin-type cells were assembled in an argon-filled glove box, using lithium foil as the counter electrode, Celgard 2400 as the separator, and 1 mol/L LiPF6 dissolved in a mixture of EC, DEC, DMC with a volume ratio of 1:1:1 as the electrolyte. Galvanostatic charge and discharge measurements were performed in a potential range of 1–3 V at room temperature. The AC impedance data were recorded in the frequency range 10− 2 Hz to 105 Hz using CHI760E electrochemical station (Shanghai Chenhua).

Fig. 1 displays the XRD patterns of the as-prepared samples. As can be seen, the main diffraction lines of both Li4Ti5O12 samples are in good agreement with the cubic spinel structure (Fd-3m space group). This indicates that the presence of carbon has no effect on the Li4Ti5O12 phase. Furthermore, the Li4Ti5O12 shows a relatively sharp peak with high intensity. On the other hand, the Li4Ti5O12/C composite indicates the decreasing intensity of the peaks and broadening diffraction peaks. The results above suggest that Li4Ti5O12/C composite probably has smaller crystallite size in comparison with the pristine one. It is well accepted that the presence of carbon could hinder the growth of Li4Ti5O12 particles during the calcination process, resulting in the smaller crystallite sizes. The carbon content in the Li4Ti5O12/C composite measured by carbon–sulfur analyzer is about 1.6%. The morphologies of the two samples are compared in Fig. 2. Clearly, the Li4Ti5O12 has larger particles as large as 200–500 nm in diameter, whereas the Li4Ti5O12/C has a smaller particle size of 50–200 nm in diameter. The data from the BET measurement shown in Table 1, gives a specific surface area of about 6.0 and 12.6 m2 g−1 for the crystalline material from Li4Ti5O12 and Li4Ti5O12/C composite, respectively. Apparently, Li4Ti5O12/C composite with the smaller particles promises large surface area, resulting in short lithium ion diffusion path and enough contact between the active material and electrolyte. In order to understand the carbon coating thickness and structure on the surface of Li4Ti5O12 particle, we conducted HRTEM analysis of Li4Ti5O12/C composite. As shown in Fig. 3, a very thin and uniform carbon layer with the thickness of 2–4 nm is clearly seen on the surface of Li4Ti5O12 particle. It implies that the PVP was successfully carbonized to carbon species in present experimental condition [27]. The data from the electronic conductivity measurements are given in Table 2. As can be seen, the conductivity of Li4Ti5O12/C composite is 9.75 × 10−3 S cm−1, about twelve times higher than that of Li4Ti5O12. Fig. 4 shows the impedance spectra of the Li4Ti5O12 and Li4Ti5O12/C electrodes measured at the stable voltage of 1.55 V, respectively. It is obvious that each Nyquist plots is composed of a depressed semicircle and a straight line. According to the literature, the intercept at the Z′ axis in the high frequency corresponds to the ohmic resistance (Re),

Fig. 2. SEM images of the samples: (A) Li4Ti5O12; (B) Li4Ti5O12/C composite.

Y. Ren et al. / Solid State Ionics 274 (2015) 83–87

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Fig. 3. TEM image of Li4Ti5O12/C composite. Fig. 4. Nyquist plots of Li4Ti5O12 and Li4Ti5O12/C electrodes.

representing the resistance of the electrolyte. The semicircle in the middle frequency range is attributed to the charge transfer resistance (Rct). The straight line in the low frequency is associated with lithium ion diffusion in Li4Ti5O12 [28–30], The lithium ion diffusion coefficient could be calculated from the low frequency plots according to the following equation [31–34]: 2 2

2 4

4 2

D ¼ R T =2A n F C σ

2

ð1Þ

where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidation, F is the Faraday constant, C is the concentration of lithium ion, and σ is the Warburg factor which is relative with Zre [31–34].

Z re ¼ RD þ RL þ σω

−1=2

ð2Þ

where ω is frequency. The relationships between Zre and the reciprocal square root of frequency in the low frequency are shown in Fig. 5. All the parameters obtained and calculated from EIS are summarized in Table 2. It can be seen that Li4Ti5O12/C composite exhibits the smaller chargetransfer resistance than that of the Li4Ti5O12. It can be confirmed that the decrease of charge transfer resistance is beneficial to the kinetic behaviors during charge/discharge process. This indicates that carbon coating Li4Ti5O12 using PVP as carbon source is favorable to improve the conductivity, leading to the improved rate performance [35]. Furthermore, Li4Ti5O12/C composite possesses the higher lithium diffusion coefficient compared with Li4Ti5O12. The above results suggest that Li4Ti5O12/C composite with narrow size distribution, high electronic conductivity and large lithium diffusion coefficient exhibited much better electrochemical performance than Li4Ti5O12 [35]. This coincides well with the discharge capacities as shown in Figs. 7 and 8. The charge–discharge profiles of Li4Ti5O12/C composite and pristine Li4Ti5O12 between 1 V and 3 V at 0.2 C rate are shown in Fig. 6. As seen, both samples show similar charge–discharge curves with potential plateaus, which correspond to the lithium deintercalation and intercalation process. Furthermore, the difference between the charge and discharge plateau potentials of the Li4Ti5O12/C composite is smaller than that of the pristine Li4Ti5O12. According to recent report [36], the small voltage between the charge and discharge plateaus is representative

of its good kinetics. Li4Ti5O12/C composite with smaller difference between the charge and discharge plateau potentials exhibits the better kinetics. Fig. 7 shows cycling performance of Li4Ti5O12 and Li4Ti5O12/C composite at various charge–discharge rates from 0.2 C to 10 C. It is clear that the discharge capacity of both samples gradually decreases with the increasing rate. This is because the utilization of the active material decreases as the rate increases [37]. However, the Li4Ti5O12/C composite manifested a higher discharge capacity, which is more and more obvious with the increasing current density. Li4Ti5O12/C composite and Li4Ti5O12 deliver the discharge capacities of 172 mAh g−1, 165 mAh g−1 at 0.2 C, respectively. At high rates, Li4Ti5O12/C composite delivers much higher reversible capacity than Li4Ti5O12. Li4Ti5O12/C composite has a discharge capacity of 141 mAh g− 1 at 10 C, While that of Li4Ti5O12 decreases to 80 mAh g− 1. Without a doubt, the Li4Ti5O12/C composite exhibits much better rate performance. The improved rate capability could probably be attributed to the following reasons: (1) Li4Ti5O12/C composite has the smaller particle size (see in Fig. 2), which results in the short diffusion path of lithium ion (see in Table 3); and (2) carbons obtained during pyrolysis of PVP assure good electronic conductivity (see in Table 2), thus promises much better electrochemical performance [22]. For evaluating the cycling stability of Li4Ti5O12/C composite, the cell was further cycled at a current rate of 10 C for another 200 cycles after 10 cycles at 0.2 C. The results are displayed in Fig. 8. As shown, an initial

Table 2 Electronic conductivity of the samples. Electronic conductivity (S cm−1) Li4Ti5O12 Li4Ti5O12/C composite

8.41 × 10−5 9.75 × 10−4

Fig. 5. The relationship curve between Zre and ω−1/2 in the low frequency.

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Y. Ren et al. / Solid State Ionics 274 (2015) 83–87 Table 3 Impedance parameters of the Li4Ti5O12 and Li4Ti5O12/C composite.

Li4Ti5O12 Li4Ti5O12/C

Re (Ω)

Rct (Ω)

σ (Ω s−1/2)

D (cm2 s−1)

2.0 1.8

202 102

24.7 19.3

6.11 × 10−11 1.00 × 10−10

Fig. 6. The third charge and discharge curves for Li4Ti5O12 and Li4Ti5O12/C composite.

discharge capacity of the Li4Ti5O12/C composite is up to 140 mAh g−1 and the capacity retention after 200 cycles is 95.7%. Meanwhile, the charge/discharge efficiency of Li4Ti5O12/C composite remains almost 100% from the 1st cycle to the 200th cycle, demonstrating good cycling performance. 4. Conclusions Carbon-coated Li4Ti5O12 was prepared by using PVP as carbon source through a facile solid-state reaction. SEM images showed the decreased particle size and EIS results confirmed the decreased charge-transfer resistance and improved Li+ diffusion coefficient after carbon coating. The as-prepared Li4Ti5O12/C composite exhibited much better electrochemical performance. It is convinced that the carbon layer on the surface of Li4Ti5O12 particles was 2–4 nm in thickness, which greatly contributes to the high conductivity. Furthermore, Li4Ti5O12/C composite has much smaller particle size, which could shorten the diffusion path of lithium ion and further expedite the ion transport, thus leading to the superior rate performance. Acknowledgments This project was financially supported by the National Natural Science Foundation of China (No. 51304077 and 21476031), the

Fig. 7. Cycling performance of Li4Ti5O12 and Li4Ti5O12/C composite at various rates.

Fig. 8. Cycling performance of Li4Ti5O12/C composites at 10 C.

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