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PTh as advanced anode for rechargeable lithium-ion batteries
Ceramics International 43 (2017) 7600–7606
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of core-shell Li4Ti5O12/PTh as advanced anode for rechargeable lithium-ion batteries
MARK
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Dong Xu , Peifeng Wang, Rui Yang College of Geomatics and Municipal Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium-ion batteries Li4Ti5O12 anode PTh layer Nanocomposite Electrochemical performance
A nanocomposite of Li4Ti5O12 particles coated with polythiophene (PTh) was fabricated as advanced anode for rechargeable lithium-ion batteries. The conducting PTh layer was successfully coated on the surface of Li4Ti5O12 through the in-situ oxidative polymerization method. Benefiting from the core-shell structure, specific capacities as high as 171.5, 168.2 and 151.1 mA h g−1 at 0.2, 1 and 10 C are obtained in the Li4Ti5O12/PTh composite. The electrochemical results also show that the Li4Ti5O12/PTh exhibits remarkably improved cycling performance as compared with the Li4Ti5O12 anode. Moreover, the charge-transfer resistance of Li4Ti5O12/PTh electrode is much lower than that of the bare Li4Ti5O12, revealing that the PTh coating can significantly increase the electron conductivity between the Li4Ti5O12 particles. The excellent electrochemical performance of the as-fabricated Li4Ti5O12/PTh composite can be ascribed to the PTh layer which can suppress the dissolution of active material into the LiPF6 electrolyte and enhance the electron conductivity of Li4Ti5O12 nanocrystals. Thus, the Li4Ti5O12/ PTh composite is an advanced anode for use in high performance lithium-ion batteries application.
1. Introduction Among the various devices for energy storage, rechargeable lithium-ion batteries have been regarded as the most promising device due to their good safety property, high-power density and excellent cycle-life [1–3]. It is known that anode is one of the most important part of the lithium-ion batteries. Nowadays, the commercial graphite with low cost has been widely used as anode in lithium-ion batteries. Nevertheless, the volume variation and low Li+-ion diffusion coefficient significantly inhibit its application in lithium-ion batteries [4,5]. Up to now, many efforts have been devoted to develop the alternative anode materials such as Si [6,7], SnO2 [8] and Co3O4 [9,10]. Among these anode materials, spinel Li4Ti5O12 [11–14] attracts extensive interest as the anode for long-life lithium-ion batteries due to its intrinsic characteristics. Li4Ti5O12 is a zero-stain insertion anode, which possesses superior reversibility and long cycle-life [15,16]. Moreover, it shows an extremely flat voltage plateaus at about 1.5 V [17], which can prevent the reduction of electrolyte between the particles of Li4Ti5O12. Nevertheless, the poor electron conductivity of the pure Li4Ti5O12 material (ca. 10–13 S cm−1) seriously limits its practical application [18]. Fortunately, the electron conductivity of Li4Ti5O12 has been improved by reducing the particle size [19,20], doping with metal ions (such as La3+ [21], Mg2+ [22], Al3+ [23], Ce3+ [24] and V5+ [25]) or
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coating with carbon materials [26–28] in recent years. However, it remains a challenge to fabricate the Li4Ti5O12-based composite with outstanding electrochemical performance. Over the past decade, it has been proved that conducting polymers (PANI [29], PEDOT [30]) are helpful to enhance the electron conductivity of Li4Ti5O12 material. For instance, the PANI-coated Li4Ti5O12 composite was successfully prepared using the in-situ polymerization method and it delivered an initial discharge capacity of about 191 mA h g−1 at 0.1 C [29]. Zhang et al. [30] synthesized the PEDOT decorated Li4Ti5O12 anode by a facile soft chemistry route. In the Li4Ti5O12/PEDOT material, the conducting PEDOT was uniformly coated on the surface of Li4Ti5O12 particles, which could significantly improve the conductivity of Li4Ti5O12 material. As an important conducting polymer, polythiophene (PTh) has many advantages in terms of good stability and high conductivity [31], and it has been used to improve the performances of Li3VO8 [32] and LiFePO4 [33] for lithium-ion batteries. The results reveal that the PTh coating can not only greatly enhance the electron conductivities of the electrode materials, but also suppress the dissolution of electrodes into the electrolyte. Therefore, among the different conducting coating materials, PTh layer is a promising coating to improve the electrochemical performance of Li4Ti5O12 anode. Meanwhile, there are few papers about the fabrication of Li4Ti5O12/PTh composite. Herein, we report on the fabrication of conducting PTh-coated Li4Ti5O12 anode material and the investigation of its electrochemical
Corresponding author. E-mail address: [email protected] (D. Xu).
http://dx.doi.org/10.1016/j.ceramint.2017.03.053 Received 21 February 2017; Received in revised form 7 March 2017; Accepted 8 March 2017 Available online 11 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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none solvent. The obtained mixtures were uniformly spread onto a Cu collector and heated at 110 °C for 12 h. Note that the weight loading of the anode material was about 3.9 mg/cm2. The cells were fabricated with Li foil as the reference electrode, Cellgard 2400 film as the separator and 1 M LiPF6 as the electrolyte. Galvanostatic charge/ discharge tests at various current rates between 1 and 2.5 V were carried out on the LAND tester. The charge/discharge capacities of the anode materials were calculated based on the Li4Ti5O12 material. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetery (CV) measurements of the Li4Ti5O12 and Li4Ti5O12/PTh anode materials were tested on an electrochemical workstation (CHI, Model 660a). 3. Results and discussion The XRD patterns of the obtained Li4Ti5O12 and Li4Ti5O12/PTh powders are illustrated in Fig. 1a. All the diffraction patterns of Li4Ti5O12 and Li4Ti5O12/PTh can be assigned to the Fd3m space of spinel Li4Ti5O12 [26]. No impurity peaks are detected in the XRD patterns, which reveal that the Li4Ti5O12 materials are highly crystallized. Besides, the PTh layer is presented as a relatively low degree of crystallinity and it does not destroy the structure of Li4Ti5O12. Fig. 1b shows the unit cell structure of spinel Li4Ti5O12 material [34]. Li4Ti5O12 can possess a negligible change in the unit cell volume during the cycling process, resulting in an excellent long cycle-life performance. During the discharge procedure, the Li+ insert into the structure of Li4Ti5O12 from tetrahedral 8a sites to the octahedral 16c sites with the formation of Li7Ti5O12 [35]. Fig. 2 shows the SEM images of Li4Ti5O12 and Li4Ti5O12/PTh materials. For the pure Li4Ti5O12 sample (Fig. 2a,b), it can be noted that the material is well crystallized with the particle size of 150– 200 nm. The small particles are believed to shorten the diffusion paths for the Li+-ions intercalation and deintercalation [19,20]. As shown in Fig. 2c,d, the particle size of Li4Ti5O12/PTh is about 200 nm which is bigger than that of Li4Ti5O12. This can be assigned to the coated PTh layer on the surface of Li4Ti5O12. What's more, after coating the PTh, the surface of Li4Ti5O12/PTh material is becoming smooth. This phenomenon is similar to that of PTh-coated LiV3O8 electrode [32]. The microstructures of Li4Ti5O12 and Li4Ti5O12/PTh samples are further investigated by TEM shown in Fig. 3. It is found that the pure Li4Ti5O12 (Fig. 3a,b) has a regular shape and the surface of Li4Ti5O12 is very smooth without any coated materials. As illustrated in Fig. 3c–e, the Li4Ti5O12/PTh nanoparticles present the core-shell structure with an average size of around 200 nm. The TEM image (Fig. 3f) further indicates that a thin PTh layer with a thickness of 10–15 nm has been coated on the surface of Li4Ti5O12 material. Therefore, the Li4Ti5O12 particles can be connected to each other through this PTh layer, leading to superior electron conductivity [33]. It should be pointed out that the amount of PTh in the Li4Ti5O12/PTh anode material is measured using the TGA analysis and made up about 10.5 wt% of the specimen. Fig. 4 gives the first charge/discharge profiles and cycling performances for Li4Ti5O12 and Li4Ti5O12/PTh electrodes at 0.2 C. Obviously, as presented in Fig. 4a,b, the Li4Ti5O12 and Li4Ti5O12/ PTh anode materials exhibit similar charge/discharge curves, revealing the PTh coating has no influence on the electrochemical reaction of Li4Ti5O12. There are flat voltage plateaus at about 1.5 V for both electrodes, which reveal the behavior of two-phase reaction [36]. As presented in Fig. 4b, the Li4Ti5O12/PTh delivers a capacity of 171.5 mA h g−1 at 0.2 C, which is higher than Li4Ti5O12 (163.3 mA h g−1). The detailed cycling performances for the anode materials are given in Fig. 4c,d. The specific capacity of Li4Ti5O12 drops to 156.5 mA h g−1 over 30 cycles with the capacity retention of only 95.8%. The bad electrochemical properties are assigned to the poor electron conductivity of Li4Ti5O12 [18]. On the contrary, the Li4Ti5O12/ PTh composite exhibits excellent cycling performance with no capacity fading over 30 cycles. To study the rate performances of Li4Ti5O12 and Li4Ti5O12/PTh, the
Fig. 1. (a) XRD patterns of Li4Ti5O12 and Li4Ti5O12/PTh powders; (b) Unit cell structure of the spinel Li4Ti5O12 material [34].
performances for lithium-ion batteries. The PTh layer was coated on the surface of Li4Ti5O12 through the in-situ oxidative polymerization method. Benefiting from the core-shell structure of Li4Ti5O12/PTh composite, the obtained electrode exhibited outstanding rate performance and cycling stability. Therefore, the Li4Ti5O12/PTh is an advanced anode for use in high performance lithium-ion batteries application. 2. Experimental 2.1. Materials preparation and characterization The Li4Ti5O12 and Li4Ti5O12/PTh anode materials were successfully prepared using the same method as described in our previously reported literature [13]. The morphology of Li4Ti5O12 and Li4Ti5O12/ PTh was observed through the scanning electron microscopy (JEOL JSM-7400F) and transmission electron microscopy (JEOL 200CX). The phase and purity of Li4Ti5O12 and Li4Ti5O12/PTh products were examined by the powder X-ray diffraction (XRD). 2.2. Electrochemical tests The battery performances of the electrode materials were evaluated by CR2032 cells, which were assembled in the glove box. The working electrode films were fabricated through mixing 85 wt% electrodes, 10 wt% Super p and 5 wt% PVDF dissolved in the N-methylpyrrolidi7601
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Fig. 2. SEM images of (a,b) Li4Ti5O12 and (c,d) Li4Ti5O12/PTh samples.
Fig. 3. TEM images of (a,b) Li4Ti5O12 and (c–f) Li4Ti5O12/PTh samples.
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Fig. 4. (a,b) Initial charge/discharge profiles and (c,d) cycling properties for Li4Ti5O12 and Li4Ti5O12/PTh at 0.2 C between 1.0 and 2.5 V.
anode is only 259 Ω which is much lower than that of Li4Ti5O12 (462 Ω). The low Rct could be helpful to overcome the kinetics restrictions and improve the depth of Li+-ions insertion and extraction [38]. Therefore, the Li4Ti5O12/PTh composite shows excellent electrochemical performance and it has a promising application as an advanced anode in lithium-ion batteries. The CV profiles for Li4Ti5O12 and Li4Ti5O12/PTh electrodes at a scan rate of 0.5 mV s−1 are illustrated in Fig. 7b. It is found that both electrodes have a couple of redox peaks which are consistent with the charge/discharge profiles. The reversible redox peaks are related to the process of Li+-ions insertion and deinsertion and the corresponding electrochemical reaction is as follows [39,40]:
anode materials were tested at different rates shown in Fig. 5. It can be noted that the discharge capacities of Li4Ti5O12/PTh anode (Fig. 5b) are much higher than that of Li4Ti5O12 (Fig. 5a) at each current rate. At 1 C, the Li4Ti5O12/PTh exhibits a flat charge/discharge plateaus and delivers a discharge capacity of 168.2 mA h g−1. Besides, it can still show the specific capacities of 160.5 and 151.1 mA h g−1 at 5 and 10 C, respectively. For the pure Li4Ti5O12, the discharge plateaus drops greatly with the increase of C-rate, revealing the big polarization. Fig. 6 compares the cycling properties of the anode materials at different current rates. As expected, the Li4Ti5O12/PTh composite also exhibits better cycling stability than the pure Li4Ti5O12 material. The electrochemical property of Li4Ti5O12/PTh is also compared with that of other conductive polymers modified Li4Ti5O12, and the detailed results are summarized in Table 1. The superior performances of Li4Ti5O12/PTh are assigned to the following reasons: (1) Nanosized Li4Ti5O12 particles can shorten the diffusion paths of Li+-ions intercalation and deintercalation [19]; (2) PTh coating can inhibit the dissolution of Li4Ti5O12 into the electrolyte solution; (3) PTh can significantly enhance the electron conductivity of Li4Ti5O12, which is helpful to facilitate the transport of electrons and Li+-ions during the cycling process. Fig. 7a illustrates the Nyquist plots of Li4Ti5O12 and Li4Ti5O12/PTh anode materials in the frequency of 1 MHz and 0.001 Hz. Obviously, both electrodes show the similar profiles with a depressed semicircle and a straight line. The inset of Fig. 7a is the equivalent circle. Here, the Rct is attributed to the reaction procedure of charge transfer between the electrode material and electrolyte solution [37]. According to Fig. 7a, it can be noted that the value of Rct for the Li4Ti5O12/PTh
Li4Ti5O12+3Li++3e-→Li7Ti5O12
(1)
Compared with the pure Li4Ti5O12 anode, the Li4Ti5O12/PTh exhibits more intense redox peaks and lower overpotential (190 mV vs. 245 mV). The above results indicate that the Li4Ti5O12/PTh composite has a higher Li+-ions diffusivity and a lower electrode polarization than the Li4Ti5O12 material. 4. Conclusions Li4Ti5O12/PTh composite has been chemically fabricated through an in-situ oxidative polymerization route. The results reveal that Li4Ti5O12/PTh shows a single phase in the XRD patterns and the PTh layer does not destroy the crystal structure of Li4Ti5O12. TEM images indicate that an uniform PTh layer with a thickness of 10– 7603
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Fig. 5. Charge/discharge profiles of Li4Ti5O12 and Li4Ti5O12/PTh electrodes at different rates between 1.0 and 2.5 V.
Fig. 6. Rate performances of Li4Ti5O12 and Li4Ti5O12/PTh at high rate of 1 C, 5 C and 10 C.
composite has a promising application as an advanced anode in lithium-ion batteries.
15 nm is coated on the surface of Li4Ti5O12 particles. Owing to the conducting PTh coating, the composite exhibits outstanding electrochemical properties including specific capacity, rate performance and cycling stability. All the results demonstrate that the Li4Ti5O12/PTh 7604
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Table 1 Rate performances for the obtained Li4Ti5O12 and Li4Ti5O12/PTh anode materials in this work and other conductive polymers modified Li4Ti5O12 in the previous papers. Samples
Li4Ti5O12 Li4Ti5O12/PTh Li4Ti5O12/ PANI [Ref. 29] Li4Ti5O12/ PEDOT [Ref. 30]
Initial discharge capacity (mA h g−1)
Capacity retention rate (%)
1C
5C
10C
1C/10
156.4 168.2 158 (1 C)
133.1 160.5 149 (2C)
111.5 151.1
98.0 92.5 99.6 97.8 51.2% at 2C over 80 cycles
168.7 (1C)
5C/30
10C/ 60 81.6 95.3
99.5% at 1C over 100 cycles
Fig. 7. (a) Nyquist plots of Li4Ti5O12 and Li4Ti5O12/PTh anode materials, the inset is the equivalent circuit; (b) CV curves of Li4Ti5O12 and Li4Ti5O12/PTh at a scan rate of 0.5 mV s−1 between 1.0 and 2.5 V. 389 (2016) 428–437. [13] D. Xu, P. Wang, R. Yang, Conducting polythiophene-wrapped Li4Ti5O12 spinel anode material for ultralong cycle-life lithium-ion batteries, Ceram. Int. 43 (2017) 4712–4715. [14] L. Peng, H.J. Zhang, L. Fang, Y. Zhang, Y. Wang, Novel peapoded Li4Ti5O12 nanoparticles for high-rate and ultralong-life rechargeable lithium ion batteries at room and lower temperatures, Nanoscale 8 (2016) 2030–2040. [15] Y. Yang, B. Qiao, X. Yang, L. Fang, C. Pan, W. Song, H. Hou, X. Ji, Lithium titanate tailored by cathodically induced graphene for an ultrafast lithium ion battery, Adv. Funct. Mater. 24 (2014) 4349–4356. [16] J.Y. Liao, V. Chabot, M. Gu, C. Wang, X. Xiao, Z. Chen, Dual phase Li4Ti5O12-TiO2 nanowire arrays as integrated anodes for high-rate lithium-ion batteries, Nano Energy 9 (2014) 383–391. [17] Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery, J. Am. Chem. Soc. 134 (2012) 7874–7879. [18] W. Fang, X.Q. Cheng, P.J. Zuo, Y.L. Ma, L.X. Liao, G.P. Yin, Hydrothermal-assisted sol-gel synthesis of Li4Ti5O12/C nano-composite for high-energy lithium-ion batteries, Solid State Ion. 244 (2013) 52–56. [19] G.Y. Liu, R.X. Zhang, K.Y. Bao, H.Q. Xie, G.Q. Liu, Synthesis of nano-Li4Ti5O12 anode material for lithium ion batteries by a biphasic interfacial reaction route, Ceram. Int. 42 (2016) 11468–11472. [20] C. Xu, L.H. Xue, W. Zhang, X. Fan, W.X. Zhang, Hydrothermal synthesis of Li4Ti5O12/TiO2 nano-composite as high performance anode material for lithiumion batteries, Electrochim. Acta 147 (2014) 506–512. [21] D. Wang, C.M. Zhang, Y.Y. Zhang, J. Wang, D.L. He, Synthesis and electrochemical properties of La-doped Li4Ti5O12 as anode material for Li-ion battery, Ceram. Int. 39 (2013) 5145–5149. [22] X. Bai, W. Li, A. Wei, X.H. Li, L.H. Zhang, Z.F. Liu, Preparation and electrochemical properties of Mg2+ and F- co-doped Li4Ti5O12 anode material for use in the lithium-ion batteries, Electrochim. Acta 222 (2016) 1045–1055. [23] H.L. Zhao, Y. Li, Z.M. Zhu, J. Lin, Z.H. Tian, R.L. Wang, Structural and electrochemical characteristics of Li4-xAlxTi5O12 as anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7079–7083. [24] J.P. Feng, Y.L. Wang, Ce-doped Li4Ti5O12/C nanoparticles embedded in multiwalled carbon nanotube network as a high-rate and long cycle-life anode for
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Enhanced electrochemical performance of core-shell Li4Ti5O12/PTh as advanced anode for rechargeable lithium-ion batteries
MARK
⁎
Dong Xu , Peifeng Wang, Rui Yang College of Geomatics and Municipal Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium-ion batteries Li4Ti5O12 anode PTh layer Nanocomposite Electrochemical performance
A nanocomposite of Li4Ti5O12 particles coated with polythiophene (PTh) was fabricated as advanced anode for rechargeable lithium-ion batteries. The conducting PTh layer was successfully coated on the surface of Li4Ti5O12 through the in-situ oxidative polymerization method. Benefiting from the core-shell structure, specific capacities as high as 171.5, 168.2 and 151.1 mA h g−1 at 0.2, 1 and 10 C are obtained in the Li4Ti5O12/PTh composite. The electrochemical results also show that the Li4Ti5O12/PTh exhibits remarkably improved cycling performance as compared with the Li4Ti5O12 anode. Moreover, the charge-transfer resistance of Li4Ti5O12/PTh electrode is much lower than that of the bare Li4Ti5O12, revealing that the PTh coating can significantly increase the electron conductivity between the Li4Ti5O12 particles. The excellent electrochemical performance of the as-fabricated Li4Ti5O12/PTh composite can be ascribed to the PTh layer which can suppress the dissolution of active material into the LiPF6 electrolyte and enhance the electron conductivity of Li4Ti5O12 nanocrystals. Thus, the Li4Ti5O12/ PTh composite is an advanced anode for use in high performance lithium-ion batteries application.
1. Introduction Among the various devices for energy storage, rechargeable lithium-ion batteries have been regarded as the most promising device due to their good safety property, high-power density and excellent cycle-life [1–3]. It is known that anode is one of the most important part of the lithium-ion batteries. Nowadays, the commercial graphite with low cost has been widely used as anode in lithium-ion batteries. Nevertheless, the volume variation and low Li+-ion diffusion coefficient significantly inhibit its application in lithium-ion batteries [4,5]. Up to now, many efforts have been devoted to develop the alternative anode materials such as Si [6,7], SnO2 [8] and Co3O4 [9,10]. Among these anode materials, spinel Li4Ti5O12 [11–14] attracts extensive interest as the anode for long-life lithium-ion batteries due to its intrinsic characteristics. Li4Ti5O12 is a zero-stain insertion anode, which possesses superior reversibility and long cycle-life [15,16]. Moreover, it shows an extremely flat voltage plateaus at about 1.5 V [17], which can prevent the reduction of electrolyte between the particles of Li4Ti5O12. Nevertheless, the poor electron conductivity of the pure Li4Ti5O12 material (ca. 10–13 S cm−1) seriously limits its practical application [18]. Fortunately, the electron conductivity of Li4Ti5O12 has been improved by reducing the particle size [19,20], doping with metal ions (such as La3+ [21], Mg2+ [22], Al3+ [23], Ce3+ [24] and V5+ [25]) or
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coating with carbon materials [26–28] in recent years. However, it remains a challenge to fabricate the Li4Ti5O12-based composite with outstanding electrochemical performance. Over the past decade, it has been proved that conducting polymers (PANI [29], PEDOT [30]) are helpful to enhance the electron conductivity of Li4Ti5O12 material. For instance, the PANI-coated Li4Ti5O12 composite was successfully prepared using the in-situ polymerization method and it delivered an initial discharge capacity of about 191 mA h g−1 at 0.1 C [29]. Zhang et al. [30] synthesized the PEDOT decorated Li4Ti5O12 anode by a facile soft chemistry route. In the Li4Ti5O12/PEDOT material, the conducting PEDOT was uniformly coated on the surface of Li4Ti5O12 particles, which could significantly improve the conductivity of Li4Ti5O12 material. As an important conducting polymer, polythiophene (PTh) has many advantages in terms of good stability and high conductivity [31], and it has been used to improve the performances of Li3VO8 [32] and LiFePO4 [33] for lithium-ion batteries. The results reveal that the PTh coating can not only greatly enhance the electron conductivities of the electrode materials, but also suppress the dissolution of electrodes into the electrolyte. Therefore, among the different conducting coating materials, PTh layer is a promising coating to improve the electrochemical performance of Li4Ti5O12 anode. Meanwhile, there are few papers about the fabrication of Li4Ti5O12/PTh composite. Herein, we report on the fabrication of conducting PTh-coated Li4Ti5O12 anode material and the investigation of its electrochemical
Corresponding author. E-mail address: [email protected] (D. Xu).
http://dx.doi.org/10.1016/j.ceramint.2017.03.053 Received 21 February 2017; Received in revised form 7 March 2017; Accepted 8 March 2017 Available online 11 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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none solvent. The obtained mixtures were uniformly spread onto a Cu collector and heated at 110 °C for 12 h. Note that the weight loading of the anode material was about 3.9 mg/cm2. The cells were fabricated with Li foil as the reference electrode, Cellgard 2400 film as the separator and 1 M LiPF6 as the electrolyte. Galvanostatic charge/ discharge tests at various current rates between 1 and 2.5 V were carried out on the LAND tester. The charge/discharge capacities of the anode materials were calculated based on the Li4Ti5O12 material. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetery (CV) measurements of the Li4Ti5O12 and Li4Ti5O12/PTh anode materials were tested on an electrochemical workstation (CHI, Model 660a). 3. Results and discussion The XRD patterns of the obtained Li4Ti5O12 and Li4Ti5O12/PTh powders are illustrated in Fig. 1a. All the diffraction patterns of Li4Ti5O12 and Li4Ti5O12/PTh can be assigned to the Fd3m space of spinel Li4Ti5O12 [26]. No impurity peaks are detected in the XRD patterns, which reveal that the Li4Ti5O12 materials are highly crystallized. Besides, the PTh layer is presented as a relatively low degree of crystallinity and it does not destroy the structure of Li4Ti5O12. Fig. 1b shows the unit cell structure of spinel Li4Ti5O12 material [34]. Li4Ti5O12 can possess a negligible change in the unit cell volume during the cycling process, resulting in an excellent long cycle-life performance. During the discharge procedure, the Li+ insert into the structure of Li4Ti5O12 from tetrahedral 8a sites to the octahedral 16c sites with the formation of Li7Ti5O12 [35]. Fig. 2 shows the SEM images of Li4Ti5O12 and Li4Ti5O12/PTh materials. For the pure Li4Ti5O12 sample (Fig. 2a,b), it can be noted that the material is well crystallized with the particle size of 150– 200 nm. The small particles are believed to shorten the diffusion paths for the Li+-ions intercalation and deintercalation [19,20]. As shown in Fig. 2c,d, the particle size of Li4Ti5O12/PTh is about 200 nm which is bigger than that of Li4Ti5O12. This can be assigned to the coated PTh layer on the surface of Li4Ti5O12. What's more, after coating the PTh, the surface of Li4Ti5O12/PTh material is becoming smooth. This phenomenon is similar to that of PTh-coated LiV3O8 electrode [32]. The microstructures of Li4Ti5O12 and Li4Ti5O12/PTh samples are further investigated by TEM shown in Fig. 3. It is found that the pure Li4Ti5O12 (Fig. 3a,b) has a regular shape and the surface of Li4Ti5O12 is very smooth without any coated materials. As illustrated in Fig. 3c–e, the Li4Ti5O12/PTh nanoparticles present the core-shell structure with an average size of around 200 nm. The TEM image (Fig. 3f) further indicates that a thin PTh layer with a thickness of 10–15 nm has been coated on the surface of Li4Ti5O12 material. Therefore, the Li4Ti5O12 particles can be connected to each other through this PTh layer, leading to superior electron conductivity [33]. It should be pointed out that the amount of PTh in the Li4Ti5O12/PTh anode material is measured using the TGA analysis and made up about 10.5 wt% of the specimen. Fig. 4 gives the first charge/discharge profiles and cycling performances for Li4Ti5O12 and Li4Ti5O12/PTh electrodes at 0.2 C. Obviously, as presented in Fig. 4a,b, the Li4Ti5O12 and Li4Ti5O12/ PTh anode materials exhibit similar charge/discharge curves, revealing the PTh coating has no influence on the electrochemical reaction of Li4Ti5O12. There are flat voltage plateaus at about 1.5 V for both electrodes, which reveal the behavior of two-phase reaction [36]. As presented in Fig. 4b, the Li4Ti5O12/PTh delivers a capacity of 171.5 mA h g−1 at 0.2 C, which is higher than Li4Ti5O12 (163.3 mA h g−1). The detailed cycling performances for the anode materials are given in Fig. 4c,d. The specific capacity of Li4Ti5O12 drops to 156.5 mA h g−1 over 30 cycles with the capacity retention of only 95.8%. The bad electrochemical properties are assigned to the poor electron conductivity of Li4Ti5O12 [18]. On the contrary, the Li4Ti5O12/ PTh composite exhibits excellent cycling performance with no capacity fading over 30 cycles. To study the rate performances of Li4Ti5O12 and Li4Ti5O12/PTh, the
Fig. 1. (a) XRD patterns of Li4Ti5O12 and Li4Ti5O12/PTh powders; (b) Unit cell structure of the spinel Li4Ti5O12 material [34].
performances for lithium-ion batteries. The PTh layer was coated on the surface of Li4Ti5O12 through the in-situ oxidative polymerization method. Benefiting from the core-shell structure of Li4Ti5O12/PTh composite, the obtained electrode exhibited outstanding rate performance and cycling stability. Therefore, the Li4Ti5O12/PTh is an advanced anode for use in high performance lithium-ion batteries application. 2. Experimental 2.1. Materials preparation and characterization The Li4Ti5O12 and Li4Ti5O12/PTh anode materials were successfully prepared using the same method as described in our previously reported literature [13]. The morphology of Li4Ti5O12 and Li4Ti5O12/ PTh was observed through the scanning electron microscopy (JEOL JSM-7400F) and transmission electron microscopy (JEOL 200CX). The phase and purity of Li4Ti5O12 and Li4Ti5O12/PTh products were examined by the powder X-ray diffraction (XRD). 2.2. Electrochemical tests The battery performances of the electrode materials were evaluated by CR2032 cells, which were assembled in the glove box. The working electrode films were fabricated through mixing 85 wt% electrodes, 10 wt% Super p and 5 wt% PVDF dissolved in the N-methylpyrrolidi7601
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Fig. 2. SEM images of (a,b) Li4Ti5O12 and (c,d) Li4Ti5O12/PTh samples.
Fig. 3. TEM images of (a,b) Li4Ti5O12 and (c–f) Li4Ti5O12/PTh samples.
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Fig. 4. (a,b) Initial charge/discharge profiles and (c,d) cycling properties for Li4Ti5O12 and Li4Ti5O12/PTh at 0.2 C between 1.0 and 2.5 V.
anode is only 259 Ω which is much lower than that of Li4Ti5O12 (462 Ω). The low Rct could be helpful to overcome the kinetics restrictions and improve the depth of Li+-ions insertion and extraction [38]. Therefore, the Li4Ti5O12/PTh composite shows excellent electrochemical performance and it has a promising application as an advanced anode in lithium-ion batteries. The CV profiles for Li4Ti5O12 and Li4Ti5O12/PTh electrodes at a scan rate of 0.5 mV s−1 are illustrated in Fig. 7b. It is found that both electrodes have a couple of redox peaks which are consistent with the charge/discharge profiles. The reversible redox peaks are related to the process of Li+-ions insertion and deinsertion and the corresponding electrochemical reaction is as follows [39,40]:
anode materials were tested at different rates shown in Fig. 5. It can be noted that the discharge capacities of Li4Ti5O12/PTh anode (Fig. 5b) are much higher than that of Li4Ti5O12 (Fig. 5a) at each current rate. At 1 C, the Li4Ti5O12/PTh exhibits a flat charge/discharge plateaus and delivers a discharge capacity of 168.2 mA h g−1. Besides, it can still show the specific capacities of 160.5 and 151.1 mA h g−1 at 5 and 10 C, respectively. For the pure Li4Ti5O12, the discharge plateaus drops greatly with the increase of C-rate, revealing the big polarization. Fig. 6 compares the cycling properties of the anode materials at different current rates. As expected, the Li4Ti5O12/PTh composite also exhibits better cycling stability than the pure Li4Ti5O12 material. The electrochemical property of Li4Ti5O12/PTh is also compared with that of other conductive polymers modified Li4Ti5O12, and the detailed results are summarized in Table 1. The superior performances of Li4Ti5O12/PTh are assigned to the following reasons: (1) Nanosized Li4Ti5O12 particles can shorten the diffusion paths of Li+-ions intercalation and deintercalation [19]; (2) PTh coating can inhibit the dissolution of Li4Ti5O12 into the electrolyte solution; (3) PTh can significantly enhance the electron conductivity of Li4Ti5O12, which is helpful to facilitate the transport of electrons and Li+-ions during the cycling process. Fig. 7a illustrates the Nyquist plots of Li4Ti5O12 and Li4Ti5O12/PTh anode materials in the frequency of 1 MHz and 0.001 Hz. Obviously, both electrodes show the similar profiles with a depressed semicircle and a straight line. The inset of Fig. 7a is the equivalent circle. Here, the Rct is attributed to the reaction procedure of charge transfer between the electrode material and electrolyte solution [37]. According to Fig. 7a, it can be noted that the value of Rct for the Li4Ti5O12/PTh
Li4Ti5O12+3Li++3e-→Li7Ti5O12
(1)
Compared with the pure Li4Ti5O12 anode, the Li4Ti5O12/PTh exhibits more intense redox peaks and lower overpotential (190 mV vs. 245 mV). The above results indicate that the Li4Ti5O12/PTh composite has a higher Li+-ions diffusivity and a lower electrode polarization than the Li4Ti5O12 material. 4. Conclusions Li4Ti5O12/PTh composite has been chemically fabricated through an in-situ oxidative polymerization route. The results reveal that Li4Ti5O12/PTh shows a single phase in the XRD patterns and the PTh layer does not destroy the crystal structure of Li4Ti5O12. TEM images indicate that an uniform PTh layer with a thickness of 10– 7603
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Fig. 5. Charge/discharge profiles of Li4Ti5O12 and Li4Ti5O12/PTh electrodes at different rates between 1.0 and 2.5 V.
Fig. 6. Rate performances of Li4Ti5O12 and Li4Ti5O12/PTh at high rate of 1 C, 5 C and 10 C.
composite has a promising application as an advanced anode in lithium-ion batteries.
15 nm is coated on the surface of Li4Ti5O12 particles. Owing to the conducting PTh coating, the composite exhibits outstanding electrochemical properties including specific capacity, rate performance and cycling stability. All the results demonstrate that the Li4Ti5O12/PTh 7604
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Table 1 Rate performances for the obtained Li4Ti5O12 and Li4Ti5O12/PTh anode materials in this work and other conductive polymers modified Li4Ti5O12 in the previous papers. Samples
Li4Ti5O12 Li4Ti5O12/PTh Li4Ti5O12/ PANI [Ref. 29] Li4Ti5O12/ PEDOT [Ref. 30]
Initial discharge capacity (mA h g−1)
Capacity retention rate (%)
1C
5C
10C
1C/10
156.4 168.2 158 (1 C)
133.1 160.5 149 (2C)
111.5 151.1
98.0 92.5 99.6 97.8 51.2% at 2C over 80 cycles
168.7 (1C)
5C/30
10C/ 60 81.6 95.3
99.5% at 1C over 100 cycles
Fig. 7. (a) Nyquist plots of Li4Ti5O12 and Li4Ti5O12/PTh anode materials, the inset is the equivalent circuit; (b) CV curves of Li4Ti5O12 and Li4Ti5O12/PTh at a scan rate of 0.5 mV s−1 between 1.0 and 2.5 V. 389 (2016) 428–437. [13] D. Xu, P. Wang, R. Yang, Conducting polythiophene-wrapped Li4Ti5O12 spinel anode material for ultralong cycle-life lithium-ion batteries, Ceram. Int. 43 (2017) 4712–4715. [14] L. Peng, H.J. Zhang, L. Fang, Y. Zhang, Y. Wang, Novel peapoded Li4Ti5O12 nanoparticles for high-rate and ultralong-life rechargeable lithium ion batteries at room and lower temperatures, Nanoscale 8 (2016) 2030–2040. [15] Y. Yang, B. Qiao, X. Yang, L. Fang, C. Pan, W. Song, H. Hou, X. Ji, Lithium titanate tailored by cathodically induced graphene for an ultrafast lithium ion battery, Adv. Funct. Mater. 24 (2014) 4349–4356. [16] J.Y. Liao, V. Chabot, M. Gu, C. Wang, X. Xiao, Z. Chen, Dual phase Li4Ti5O12-TiO2 nanowire arrays as integrated anodes for high-rate lithium-ion batteries, Nano Energy 9 (2014) 383–391. [17] Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery, J. Am. Chem. Soc. 134 (2012) 7874–7879. [18] W. Fang, X.Q. Cheng, P.J. Zuo, Y.L. Ma, L.X. Liao, G.P. Yin, Hydrothermal-assisted sol-gel synthesis of Li4Ti5O12/C nano-composite for high-energy lithium-ion batteries, Solid State Ion. 244 (2013) 52–56. [19] G.Y. Liu, R.X. Zhang, K.Y. Bao, H.Q. Xie, G.Q. Liu, Synthesis of nano-Li4Ti5O12 anode material for lithium ion batteries by a biphasic interfacial reaction route, Ceram. Int. 42 (2016) 11468–11472. [20] C. Xu, L.H. Xue, W. Zhang, X. Fan, W.X. Zhang, Hydrothermal synthesis of Li4Ti5O12/TiO2 nano-composite as high performance anode material for lithiumion batteries, Electrochim. Acta 147 (2014) 506–512. [21] D. Wang, C.M. Zhang, Y.Y. Zhang, J. Wang, D.L. He, Synthesis and electrochemical properties of La-doped Li4Ti5O12 as anode material for Li-ion battery, Ceram. Int. 39 (2013) 5145–5149. [22] X. Bai, W. Li, A. Wei, X.H. Li, L.H. Zhang, Z.F. Liu, Preparation and electrochemical properties of Mg2+ and F- co-doped Li4Ti5O12 anode material for use in the lithium-ion batteries, Electrochim. Acta 222 (2016) 1045–1055. [23] H.L. Zhao, Y. Li, Z.M. Zhu, J. Lin, Z.H. Tian, R.L. Wang, Structural and electrochemical characteristics of Li4-xAlxTi5O12 as anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7079–7083. [24] J.P. Feng, Y.L. Wang, Ce-doped Li4Ti5O12/C nanoparticles embedded in multiwalled carbon nanotube network as a high-rate and long cycle-life anode for
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