Facile solution-based synthesis of spinel Li4Ti5O12 nanosheets and the application in lithium ion Batteries

Facile solution-based synthesis of spinel Li4Ti5O12 nanosheets and the application in lithium ion Batteries

Solid State Ionics 268 (2014) 131–134 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Fa...

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Solid State Ionics 268 (2014) 131–134

Contents lists available at ScienceDirect

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

Facile solution-based synthesis of spinel Li4Ti5O12 nanosheets and the application in lithium ion Batteries Junsheng Wang a, Baofeng Wang a,⁎, Jie Cao a, Yufeng Tang b,⁎ a b

College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090 China CAS Key Laboratory of Materials for Energy Conversion Shanghai Institute of Ceramics, CAS, Shanghai 200050, China

a r t i c l e

i n f o

Article history: Received 7 March 2014 Received in revised form 14 June 2014 Accepted 7 October 2014 Available online xxxx Keywords: Lithium ion batteries Anode material Li4Ti5O12 nanosheets Low-temperature synthesis

a b s t r a c t A simple low-temperature solution-synthesis method is developed for preparing nanostructured Li4Ti5O12. Compared with normal hydrothermal synthesis (temperature N 120 °C), this low-temperature (60 °C) approach is more suitable for fabricating pure Li4Ti5O12 nanomaterials on a large scale due to its low requirements of reaction conditions and equipments. The as-prepared Li4Ti5O12 nanosheets are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and cyclic voltammetry. Galvanostatic testing results show that the discharging capacity of the sample still remains 148.9mAhg−1 at 10C rates after 100 cycles, presenting excellent rate performance and cycle stability. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, spinel Li4Ti5O12 (LTO) has been considered as a promising alternative material to carbonaceous materials due to its excellent Li-ion insertion/extraction reversibility with zero-strain characteristics and a higher Li-insertion potential (1.55 V) to entirely avoid potential safety issues [1,2]. Although LTO has a theoretical specific capacity of 175mAhg− 1, its high rate capabilities are relatively low due to the poor electronic conductivity and ultra-slow Li-ion diffusion [3,4]. To overcome those disadvantages, many studies focused on the modification of the material [5–7], including surface conductive coatings, doping aliovalent metal ions and reducing particle size. Among these methods, reducing particle size is an effective way to increase the electrode/electrolyte contact area and shorten the diffusion length of lithium ions and electrons, thus significantly improving the rate capability [8,9]. Synthesis of nanostructured LTO is a promising strategy to enhance the high rate performance [10–13]. In previous works, solution-based methods, such as hydrothermal synthesis, provide a considerable approach to synthesize nanostructured LTO [12–15], as reported by Y.Q. Wang et al. [15], Rutile-TiO2 coated LTO nanosheets were synthesized by a hydrothermal method at 180 °C, which shows high specific capacities and high-rate performances. However, these hydrothermal approaches usually carried out at high temperature (N 120 °C), which suffers from the difficulties of mass production. Besides, we found that excessive high hydrothermal temperature was harm for fabricating pure LTO in our recently exploration. Herein, we ⁎ Corresponding authors. Tel./fax: +86 21 35303544. E-mail address: [email protected] (B. Wang).

http://dx.doi.org/10.1016/j.ssi.2014.10.005 0167-2738/© 2014 Elsevier B.V. All rights reserved.

provide a simple low-temperature (60 °C) solution-synthesis route to fabricate pure nanostructured LTO sheets, which is satisfying for largescale synthesis. When used as an anode material for lithium ion battery, the as-prepared LTO nanosheets presents high reversible capacity and good cycling stability even at high current density. 2. Experimental 2.1. Material preparation In a typical procedure, 4 mmol TiO2 colloids, obtained from the hydrolysis of 1.4 mL tetrabutyl titanate in ethanol/water mixed solution, was mixed with 20 mL 0.2 mol L−1 LiOH in an Erlenmeyer flask. The Erlenmeyer flask was maintained at 60 °C for 7 days and then cooled to room temperature naturally. For comparison, the experiment was also carried out at room temperature (25 °C) for 60 days and at 200 °C and 240 °C for 36 h in autoclave. The resulting white precipitate was recovered by centrifugation, washed with deionized water and ethanol thoroughly, and then dried in an oven at 60 °C. Finally, the asprepared samples were calcinated in a tube furnace at 550 °C for 6 h in the air. The four samples were labeled as LTO-60, LTO-25, LTO-200, LTO-240, correspondingly the synthesis temperature 60 °C, 25 °C, 200 °C, 240 °C respectively. 2.2. Physical characterization The crystal structures were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-RB using Cu Kα radiation) with Cu Kα radiation (λ = 0.1541 nm). The 2θ ranged from 5 to 70° with a scan

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rate of 6° min−1 and a step width of 0.02°, The morphology was examined by transmission electron microscopy (TEM, JEOL 2100 F), field emitting scanning electron microscopy (FE-SEM HITACHI S-4800). 2.3. Electrode preparation and electrochemical testing Electrochemical measurements were performed using coin-cells assembled in an argon-filled glove box (German, M. Braun Co., [O2] b 1 ppm, [H2O] b 1 ppm). The composite electrodes were made of the active materials powder (80 wt%), acetylene black (10 wt%) and polyvinylidene fluoride (PVDF) binder (10 wt%) homogeneously mixed in N-methyl pyrrolidinone (NMP) solvent and then coated uniformly on a copper foil. Pure lithium foil was used as a counter electrode. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (EC + DMC + EMC) (1:1:1 in volume). Galvanostatic cycling of the assembled cells was carried out using a Land CT2001A tester (Wuhan, China) between cutoff voltages of 2.5 and 1.0 V at room temperature. A CHI660e electrochemical workstation was used for cyclic voltammetry measurements with potential range between 1.0 and 2.5 V at a scan rate of 0.1 mV s−1. The second CV cures was adopted for discussion as the redox peaks in the first CV cures are asymmetric due to many side reactions at the surface during the initial charge/discharge.

TiO2 colloids will react with LiOH to form an orthorhombic hydrous lithium titanate phase by a hydrothermal process. The hydrous lithium titanate turn into Li4Ti5O12 by losing water and H3O+ located between the gaps of the lithium-titanate oxide layers after a further thermal treatment at intermediate temperature [16].In this work, we found TiO2 colloids could react with LiOH even at room temperature. As shown in Fig. 2a-1, the diffraction peaks of LTO-25 before calcination can also be indexed as hydrous lithium titanate. Compared with crystal TiO2, the fresh-made TiO2 colloids had active functional group for reaction [17,18], which made it easy to transfer to hydrous lithium titanate. The diffraction peaks of anatase TiO2 in Fig. 2a-2 may attribute to the insufficient reaction. Increasing temperature can accelerate the reaction, e.g. pure LTO can be obtained at 60 °C (Fig. 1). However, too high temperature will affect the purity of product. Fig. 2b shows the XRD profile of LTO-200 and LTO-240. In Fig. 2b, the main diffraction peak can describe as Li4Ti5O12, but the diffraction peaks of rutile TiO2 can also be found, which might be ascribed to the partial crystallization of TiO2 colloids under high hydrothermal temperature. The crystal TiO2 is not so easy to transfer to hydrous lithium titanate as TiO2 colloids. Above results manifested that a proper reaction temperature is important for preparing pure LTO. Considering the possibility of mass production, a low temperature (60 °C) reaction condition is more attractive.

3. Results and discussion 3.1. Phase characteristics of the LTO-60, LTO-25, LTO-200, and LTO-240

(a) Anatase TiO2

a-2 intensity(a.u.)

Fig. 1 shows XRD profile of the LTO-60 samples before and after calcination. Similar to the products via typical hydrothermal reaction [12], the diffraction peaks of the sample without calcination (Fig. 1a) can be indexed as hydrous lithium titanate (JCPDS No.47-0123). After heat treatment, such hydrothermal products can be transformed to spinel Li4Ti5O12. As shown in Fig. 1b, all the peaks of sintered sample could be ascribed to the cubic spinel Li4Ti5O12 (JCPDS No. 49-0207). It confirmed the successful synthesis of spinel Li4Ti5O12 by a low-temperature solution-synthesis and subsequent thermal treatment. The broadening of the diffraction pecks is ascribed to nanoscale structure of the material. As we know, TiO2 colloids can be obtained by tetrabutyl titanate hydrolyzing. The process can be presented as below:

a-1

hydrolysis

TiðC 4 H 9 OÞ4 þ 4H2 O → TiðOHÞ4 ↓ þ 4C 4 H 9 OH

10

20

30

40

50

60

70

2 < Theta > /degree

(b) *Rutile

111

TiO2

400 LTO-240

(b) 440 333 531

331 310

200 110

401 201

20

30

40

*

LTO-200

(a)

020 601 220 711 421 621 221 312

10

*

intensity(a.u.)

intensity(a.u.)

311

50

60

2 < Theta > /degree Fig. 1. XRD patterns of LTO-60 (a) before and (b) after calcination.

70

*

10

20

*

30

40

50

60

70

2 < Theta > /degree Fig. 2. XRD profile of LTO-25 (a-1) before and (a-2) after calcination; (b) XRD profile of LTO-200 and LTO-240.

J. Wang et al. / Solid State Ionics 268 (2014) 131–134

3.2. Morphology of LTO-60 and LTO-200 Fig. 3a and b show the FESEM images of the LTO-60 and LTO-200, both present sheet structure and stack irregularly. The desultory nanosheets can highly increase the specific suface area of the LTO, which can provide more transport channels and surfaces to rage for lithium to insert into the electrode material. And the nanosheets may shorten the diffusion distance of lithium ions. It can be seen in Fig. 3a and b that the LTO-60 has a uniform size (ca. 200 nm), while the LTO-200 has an irregular size (200 ~ 300 nm). Fig. 3c and d show the TEM image of the LTO-60 and LTO-200. Obviously, both of them are sheetlike structure with the thickness of ca. 10 nanometers. Typically, the size of the LTO-60 is smaller than that of LTO-200, which is in accordance with the SEM results. This indicates that the size of the sheet increases with the synthesis temperature increasing. Fig. 3f shows that the nanosheets of LTO-60 are composed of randomly oriented crystallites; this is concluded from the selected-area electron diffraction (SAED) patterns, where continuous circles were observed. The high resolution electron microscopy results (Fig. 3e) show that the d111 spacing of 0.48 nm is well in accordance with the bulk spinel Li4Ti5O12, which indicate the well-crystallized spinel phase in the nanostructured materials can be prepared in low temperature 3.3. Electrochemical performance of the LTO-60 and LTO-200 Cyclic voltammograms (CVs) of LTO-60 and LTO-200 electrodes are shown in Fig. 4a. In the cycle, one pair of redox peaks appears at ca.1.47

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(reduction) and ca.1.71 (oxidation) V in both samples. The redox peaks should be attributed to the oxidation and reduction of Ti3+/Ti4+ accompanied along with Li+ insertion/extraction in the materials. Compared to the LTO-200, the cathodic and anodic peaks of the LTO-60 are much higher and sharper, which may attribute to the smaller size, for electrode materials, a smaller particle size always means a shorter diffusion length of lithium ions and electrons. Rate capabilities of LTO-60 and LTO-200 electrodes have been investigated and the results are shown in Fig. 4b. The cell was cycled 20 times at 1C, 2C and 10C respectively. As shown in Fig. 4b, the LTO-60 electrode delivered a capacity of 170.6, 163, and 150.1 mAh g−1 at 1, 2 and 10C respectively. As for LTO-200 electrode, the capacity is 170.3 at 1C, which is close to the results of LTO-60. When rates are up to 2C and 10C, the LTO-200 electrodes only remains 160 and 139.1 mAh g− 1 respectively. The capacity retention of LTO-60 was about 90% at the high current density of 10C as compared to that at the low current density of 1C, while the LTO-200 was only about 82%. It suggests better rate capacity of the synthesized LTO-60 for rechargeable lithium battery than LTO-200. Galvanostatic curves of LTO-60 at each rate are given in Fig. 4c. At a discharge rate of 1C, high capacities of ca. 170 mAhg−1 were obtained for the samples due to the enough diffusion time for both Li+ and electron at this low current rate. When cycled at 1C, it delivers a pair of flat potential plateaus around 1.54 and 1.59 V (vs. Li/Li+), associating to the two phase equilibrium between Li4Ti5O12 and Li7Ti5O12. It can be observed that the flat potential plateaus of discharging curves are still maintained with the rates increasing, indicating that the electrodes

Fig. 3. SEM images of LTO-60 (a) and LTO-200 (b), TEM images of LTO-60 (c) and LTO-200 (d), and the HRTEM (e) and SAED pattern (f) of LTO-60.

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1.5

0.0 -0.5 -1.0 -1.5 1.0

10C 150

100

50

LTO-200 LTO-60

0 1.5

2.0

2.5

0

20

Potentail /V

2.5

60

200

(c)

1C 2C 10C

40 Cycle number

2.0

1.5

(d)

1C

Discharge capacity/mAhg -1

Cell potential/V

(b)

2C

-1

0.5

Current /mA

1C

Discharge capacity/mAhg

1.0

200

(a)

LTO-60 LTO-200

160

10C 120

80

40

1.0 0

0

50

100

150 Discharge capacity/mAhg-1

200

0

20

40 60 Cycle number

80

100

Fig. 4. (a) Cyclic voltammograms (CVs) of the LTO-60 and LTO-200 electrodes at a scan rate of 0.1 mV s−1. (b) Discharge capacity of the LTO-60 and LTO-200 cell from 1C to 10C for 20 cycles at each rate, (c) Galvanostatic curves of the LTO-60 cell at each rate, (d) The cycling performance of the LTO-60 cell at different discharge/charge rates.

polarization keeps staying at a low degree even at 10C. Fig. 4d shows the cycling performance of lithium insertion into and extraction from spinel LTO. The LTO-60 exhibits a stable reversible capacity of 163.4 mAhg−1 and 148.9 mAhg−1 after 100 cycles at 1 C and 10C, corresponding to a capacity loss of only 0.02% and 0.03% for each cycle respectively. Compared with flower-like LTO nanosheets acquired by Y.F. Tang et al. [12] and pure LTO nanosheets obtained by Y.Q. Wang et al. [15], LTO-60 presents an exceptional stability retention upon cycling and higher specific capacities. Above results demonstrate that the LTO nanosheets prepared by the low-temperature route has superior rate performance and cycle stability, implying promising application in lithium ion batteries. 4. Conclusions Nanostructured LTO with nanosheets morphology and excellent rate performance has successfully been prepared via a solution-synthesis process at a low temperature of 60 °C and following calcination. The as-prepared product exhibits a high crystallinity and purity. LTO–60 exhibited a high discharge capacity of 170.6 mAhg−1 at 1 C and a stable retention capacity of 148.9 mAhg−1 at 10 C after 100 cycles, indicating superior rate performance and cycle stability. In particular, the synthetic temperature of the solution-synthesis process is as low as 60 °C, which make the synthesis of pure nanostructured Li4Ti5O12 for large-scale applications in lithium ion batteries become possible.

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