- Email: [email protected]
C with orthorhombic structure
Chemical Physics Letters 659 (2016) 88–92
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Synthesis and electrochemical properties of nonstoichiometric composition Li2.7Ti2(PO4)3/C with orthorhombic structure Xiang Yao, Zhi Su ⇑, Hui Pan College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, Xinjiang, China
a r t i c l e
i n f o
Article history: Received 4 June 2016 In final form 6 July 2016 Available online 7 July 2016 Keywords: Nonstoichiometric composition Li2.7Ti2(PO4)3/C Cathode materials Microwave-assisted synthesis Electrochemical properties
a b s t r a c t Carbon-coated Li2.7Ti2(PO4)3, a new mixed-valence titanium(III/IV) phosphate, is synthesized by the microwave-assisted sol-gel method using citric acid as both a chelating reagent and carbon source for the cathode material in lithium-ion batteries. The contents of Li, Ti and P are analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ion chromatography (IC). The microstructure, composition, and electrochemical performance of Li2.7Ti2(PO4)3/C samples are characterized by X-ray Diffraction (XRD), X-ray photoelectron spectroscopy(XPS), scanning electron microscope (SEM) and cyclic voltammetry (CV). The Li2.7Ti2(PO4)3/C sample exhibits a high initial discharge capacity of 123.6 mA h g 1 at 0.1C and outstanding cycling ability. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been considered to have great potential as power sources for the increasing energy demand. In previous reports, some positive electrode materials crystallized in an orthorhombic structure were prepared for LIBs, such as LiFeO2 [1] and o-LiMnO2 [2]. Recently, NASICON-type LiTi2(PO4)3 has aroused the attention of researchers, because of the highly reversible redox couple(Ti4+/Ti3+) and relatively low redox potential (around 0.5 V), which is used as an anode material for Li-ion rechargeable battery. However, the electrochemical performance of LiTi2(PO4)3 materials was restricted by inferior rate capability and fast capacity reduction [3]. There have been no reports on a new mixed-valence titanium (III/IV) phosphate, Li2.7Ti2(PO4)3, which was synthesized for use as a cathode material for LIBs. The preparation of a mixed valence of Ti3+ and Ti4+ is not easy, leading to difficulties during the synthesis process [4]. Li2.7Ti2(PO4)3 has many advantages including stable cycling ability, a low cost, and safety. Carbon coating is a powerful method used to effectively improve the electrical conductivity of materials [5]. Moreover, it also can reduce the particles size by suppressing the growth of particles to lower the lithium-ion diffusion length [6]. Thus, citric acid is employed as a carbon source and chelating reagent to prepare Li2.7Ti2(PO4)3 by the sol-gel method [7]. In order to decrease the reaction time originating from a traditional sol-gel process, a microwave synthesis method may be employed to aid the material synthesis. The final products can be quickly obtained because the ⇑ Corresponding author. E-mail address: [email protected] (Z. Su). http://dx.doi.org/10.1016/j.cplett.2016.07.014 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
microwave energy is directly absorbed by the samples [8]. In this study, Li2.7Ti2(PO4)3/C cathode material has been synthesized by the microwave-assisted sol-gel method. Furthermore, the preparation and electrochemical properties of Li2.7Ti2(PO4)3/C are described in detail. 2. Experimental 2.1. Preparation of Li2.7Ti2(PO4)3/C Appropriate amounts of the raw materials-LiCl, TiCl3 (16 wt%, purple solution), (NH4)3PO4, and carbon (citric acid as the carbon source)-were prepared to fabricate Li2.7Ti2(PO4)3 samples by the microwave-assisted sol-gel method. First, LiCl and (NH4)3PO4 were dissolved in deionized water with magnetic stirring to form solution A. Then, citric acid was dissolved in the TiCl3 purple solution with magnetic stirring to form solution B. Afterwards, solution A was added to solution B at a uniform speed while stirring for 1 h. The resulting solution was heated at 70 °C to form a gel and dried in a vacuum oven at 110 °C for 12 h to eliminate water. Finally, the resulting powder was pressed into pellets, placed into a crucible filled with activated carbon, and transferred to a 600-W microwave oven for 15 min. Li2.7Ti2(PO4)3/C materials can be successfully synthesized. 2.2. Characterization The structure and morphology of the Li2.7Ti2(PO4)3/C materials were characterized by X-ray diffraction (XRD, Bruker D2) with Cu
X. Yao et al. / Chemical Physics Letters 659 (2016) 88–92
Ka radiation from 10° to 70° with a step size of 0.02°, infrared (IR) spectroscopy (Shimadzu IRAffinity-1) in the range of 400– 4000 cm 1 with a resolution of 2 cm 1, scanning electron microscopy (SEM, Zeiss LEO 1430VP), and transmission electron microscopy (TEM, JEM-2010FEF). Elemental analyses for Li and Ti were carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Advantage), and the content of the P was measured by ion chromatography (IC, ICS-2100) to determine the chemical compositions of the samples. The valences of Ti in the Li2.7Ti2(PO4)3/C materials were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). For the electrochemical tests, the cathode was prepared by mixing the active material, acetylene black, and polytetrafluoroethylene (PTFE) with a weight ratio of 80:10:10 in deionized water. The resulting slurry was spread onto an aluminum foil, cut into small wafers with a diameter of 10 mm, and dried in a vacuum oven at 110 °C for 12 h. Electrochemical cells were fabricated in an argon-filled glove box using Li metal as the counter electrode and LiPF6 (1 mol L 1), including ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1, as the electrolyte. In addition, Celgard 2400 was used as the separator. The charge-discharge performance of the cells was characterized at room temperature by a battery testing system (LAND CT2001A) within the voltage range of 2.0–3.5 V. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) measurements were carried out at room temperature using an electrochemical workstation (Chenhua CHI 650D) in the potential range of 2.0–3.5 V at a scan rate of 0.1 mV s 1 and with frequencies ranging from 0.01 Hz to 0.1 MHz, respectively. 3. Results and discussion According to the analysis of chemical compositions of materials by ICP-AES and IC, the contents of Li, Ti, and P are 4.72, 24.01, and 71.27 wt%, respectively, demonstrating the chemical formula of the materials is nonstoichiometric Li2.7Ti2(PO4)3. Fig. 1a shows the XRD patterns of a Li2.7Ti2(PO4)3/C sample. All of the diffraction peaks can be indexed as orthorhombic Li2.7Ti2(PO4)3 (space group Pbcn) with the cell parameters a = 12.063(8) Å, b = 8.662(9) Å, and c = 8.710(8) Å (JCPDS: 80-2457), which coincides with previous reports [9]. There are no impurity phases detected from the XRD patterns, implying that single-phase Li2.7Ti2(PO4)3 can be prepared by this method and activated carbon plays an important role in the protective atmosphere. In addition, no obvious carbon peaks appeared in the XRD patterns, which suggests that the residual carbon has an amorphous structure and does not influence the crystal
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structure [10–12]. Furthermore, the carbon content in the final products was tested by an elemental analysis, and the result is merely 3.90 wt%, which contributes to the electrochemical performance. Fig. 1b shows the structure of NASICON-type Li2.7Ti2(PO4)3 observed along the c axis, and we can clearly see that each corner of the TiO6 octahedra is linked to a PO4 tetrahedron. Two Li atoms (Li1, Li2) appear at every asymmetric unit, and their sites are partially occupied in a channel structure that is approximately pentagonal, which is the main reason for the fast ionic conductivity [9]. To understand the chemical bonding and molecular structure of the Li2.7Ti2(PO4)3/C sample, the IR spectrum is shown in Fig. 2a. The absorption peak at 447 cm 1 is ascribed to TiAOATi bands, and the absorption peaks around 598 and 627 cm 1 could be connected with the stretching vibration of TiAO bands. Further, the absorption peak at 867 cm 1 could be attributed to symmetric PAO bands, and the absorption peaks near 999, 1037, and 1196 cm 1 may be related to PAOAP bridge vibrations, which are the characteristic absorptions of PAOAP linkers. Finally, the absorption peaks around 1437 and 1503 cm 1 are due to Fermi resonance peaks of PAOAP bridge vibrations. All of these assignments are consistent with those reported earlier [13–18]. To confirm the presence of the mixed Ti3+/Ti4+ valence in the Li2.7Ti2(PO4)3/C sample, the Ti 2p spectrum was recorded by XPS and is shown in Fig. 2b. The Ti 2p spectrum is generally made up of Ti 2p1/2 and Ti 2p3/2 peaks. Ti3+ for the Ti 2p1/2 and 2p3/2 peaks is located at 463.9 and 458.4 eV, respectively. Further, Ti4+ for the Ti 2p1/2 and 2p3/2 peaks is located at 464.3 and 459.9 eV, respectively, which is consistent with previous reports [19]. According to the careful peak separation of Ti3+ and Ti4+, it was found that Li2.7Ti2(PO4)3 consists of both Ti3+ (86%) and Ti4+ (14%), indicating that the average valence of Ti is 3.14. Additionally, we can clearly determine that the content of Li is deficient (2.7 mol) on the basis of the above analyses. Therefore, the chemical composition of the obtained materials is nonstoichiometric Li2.7Ti2(PO4)3 in nature. These results are similar with the above element analyses. The morphology of the Li2.7Ti2(PO4)3/C samples is shown in Fig. 3. The SEM image in Fig. 3a shows that the particles have a granular shape that is approximately 0.2–2 lm in diameter. To check the carbon coating on the Li2.7Ti2(PO4)3 particles, a TEM analysis of the sample was carried out. Fig. 3b shows a lattice spacing of 0.337 nm, corresponding to the (3 1 1) plane of orthorhombic Li2.7Ti2(PO4)3. It is clearly seen that about 9-nm-thick carbon layer is not uniform. The present carbon layer does prevent an increase in the particles size, which contributes to improving the electronic conductivity of Li2.7Ti2(PO4)3, thereby enhancing the electrochemical properties [20].
Fig. 1. (a) XRD patterns of a Li2.7Ti2(PO4)3/C powder; (b) structure of the projected unit cell of Li2.7Ti2(PO4)3 viewed along the c axis.
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Fig. 2. (a) IR spectrum of the Li2.7Ti2(PO4)3/C samples; (b) XPS spectra of Ti in Li2.7Ti2(PO4)3/C samples.
Fig. 3. (a) SEM and (b) TEM images of the Li2.7Ti2(PO4)3/C samples.
Fig. 4a and b shows the initial charge-discharge capabilities and cycling performance of Li2.7Ti2(PO4)3/C samples. In Fig. 4a, the initial charge-discharge curves exhibit three flat charging plateaus around 2.34, 2.46, and 2.86 V and three flat discharging plateaus around 2.26, 2.38, and 2.78 V. The charge-discharge curves of Li2.7Ti2(PO4)3 are similar to those of the previously reported mixedvalence orthorhombic Li1.5Ti2(PO4)3, which signifies smooth solid-solution reaction properties and adopts the same standard space group of Pbcn (#60) [21]. The initial charge and discharge capacities are 123.7 and 123.6 mA h g 1 at 0.1C, respectively, indicating that the coulombic efficiency of the first cycle is extremely high (99.92%). As shown in Fig. 4b, the Li2.7Ti2(PO4)3/C sample also exhibits an excellent cycling ability. After 200 cycles, the discharge capacity of the sample is 123.1 mA h g 1 at 0.1C, and the discharge capacity retention is 99.60%, implying better electrochemical reversibility [11]. When tested at 0.2, 0.5, 1, 2, and 5C, the discharge capacities of the first cycle are maintained at 115.8, 107.7, 100.6, 91.3, and 84.6 mA h g 1, respectively. Furthermore, the discharge capacity retentions after 200 cycles are still as high as 99.48, 98.98, 92.74, 84.99, and 78.84% at 0.2, 0.5, 1, 2, and 5C, respectively (as shown in Fig. 4b). The excellent electrochemical performance can be ascribed to the carbon coating and small particle size [10,22]. It can also be found from the figure, the first discharge capacity decreases with the increase of charge and discharge rate. This is due to the ratio increases, the polarization of materials becomes larger, leading to the capacity reduction. To further understand the reversibility and stability during the intercalation/de-intercalation reactions, the CV recorded for
Li2.7Ti2(PO4)3/C is shown in Fig. 4c [23]. The sample exhibits three redox couples, anodic peaks occur at 2.34, 2.46, and 2.86 V, and cathodic peaks occur at 2.26, 2.38, and 2.78 V, which are consistent with the flat charge-discharge plateaus. In fact, two pairs of redox peaks occur between 2.2 and 2.5 V, which respectively corresponds to Li+ insertion/extraction into/from Li2.7Ti2(PO4)3 according to the electrode reaction. The other redox peak at 2.78/2.86 V is attributed to redox process in association with Ti3+ [3]. The well-defined peaks and good symmetry of the CV plots confirm the excellent reversibility of the lithium-ion extraction and insertion reactions for the Li2.7Ti2(PO4)3/C composite materials [24,25]. Furthermore, the CVs of the three cycles almost overlap, suggesting less polarization [26]. The EIS curves and the equivalent circuit are shown in Fig. 4d, the small interrupt in the high-frequency domain is in connection with the solution resistance (Rs), and the first semicircle is involved in the lithium-ion migration resistance (Rf) caused by the SEI film formed on the cathode surface [27]. Another semicircle in the medium-to-low frequency region is related to the chargetransfer resistance (Rct), which is due to interface between the electrode and the electrolyte [28]. The sloping straight line in the low-frequency region is associated with lithium-ion diffusion in the bulk material, corresponding to the Warburg impedance (Zw) [29]. The EIS values are obtained by the equivalent circuit simulation, and the Rs, Rf, Rct, and Zw values are 2.58, 33.69, 54.93, and 93.41 X, respectively. The sample exhibits a low electrochemical impedance, demonstrating a small contact resistance (between the electrode and the particles) and charge-transfer polarization [30,31].
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Fig. 4. (a) The first charge-discharge curves and (b) cycling performance of Li2.7Ti2(PO4)3/C samples at various rates of 0.1C, 0.2C, 0.5C, 1C, 2C and 5C; (c) CVs and (d) EIS curves of Li2.7Ti2(PO4)3/C samples.
4. Conclusions Nonstoichiometric Li2.7Ti2(PO4)3/C composite material with orthorhombic structure can be successfully synthesized by the microwave-assisted sol-gel method. All of the diffraction peaks of the sample can be indexed as the orthorhombic structure. From the SEM and TEM results, particles with a carbon layer have a diameter of 0.2–2 lm. According to the charge-discharge tests, the initial charge and discharge capacities of sample are 123.7 and 123.6 mA h g 1 at 0.1C, respectively, indicating a high coulombic efficiency. After 200 cycles, the discharge capacity retention is 99.60% at 0.1C, suggesting good electrochemical reversibility. The excellent electrochemical performance can be ascribed to the carbon coating and small particle size. Additionally, the CV exhibits less polarization. These results suggest that Li2.7Ti2(PO4)3/C is a promising cathode material for LIBs. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21061015). References [1] Y.S. Lee, S. Sato, M. Tabuchi, C.S. Yoon, Y.K. Sun, K. Kobayakawa, Y. Sato, Structural change and capacity loss mechanism in orthorhombic Li/LiFeO2 system during cycling, Electrochem. Commun. 5 (2003) 549–554. [2] K.Y. Li, F.F. Shua, J.W. Zhang, K.F. Chen, D.F. Xue, X.W. Guo, S. Komarneni, Role of hydrothermal parameters on phase purity of orthorhombic LiMnO2 for use as cathode in Li ion battery, Ceram. Int. 41 (2015) 6729–6733. [3] J.X. Sun, Y.R. Sun, L.G. Gai, H.H. Jiang, Y. Tian, Carbon-coated mesoporous LiTi2(PO4)3 nanocrystals with superior performance for lithium-ion batteries, Electrochim. Acta 200 (2016) 66–74.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Synthesis and electrochemical properties of nonstoichiometric composition Li2.7Ti2(PO4)3/C with orthorhombic structure Xiang Yao, Zhi Su ⇑, Hui Pan College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, Xinjiang, China
a r t i c l e
i n f o
Article history: Received 4 June 2016 In final form 6 July 2016 Available online 7 July 2016 Keywords: Nonstoichiometric composition Li2.7Ti2(PO4)3/C Cathode materials Microwave-assisted synthesis Electrochemical properties
a b s t r a c t Carbon-coated Li2.7Ti2(PO4)3, a new mixed-valence titanium(III/IV) phosphate, is synthesized by the microwave-assisted sol-gel method using citric acid as both a chelating reagent and carbon source for the cathode material in lithium-ion batteries. The contents of Li, Ti and P are analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ion chromatography (IC). The microstructure, composition, and electrochemical performance of Li2.7Ti2(PO4)3/C samples are characterized by X-ray Diffraction (XRD), X-ray photoelectron spectroscopy(XPS), scanning electron microscope (SEM) and cyclic voltammetry (CV). The Li2.7Ti2(PO4)3/C sample exhibits a high initial discharge capacity of 123.6 mA h g 1 at 0.1C and outstanding cycling ability. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been considered to have great potential as power sources for the increasing energy demand. In previous reports, some positive electrode materials crystallized in an orthorhombic structure were prepared for LIBs, such as LiFeO2 [1] and o-LiMnO2 [2]. Recently, NASICON-type LiTi2(PO4)3 has aroused the attention of researchers, because of the highly reversible redox couple(Ti4+/Ti3+) and relatively low redox potential (around 0.5 V), which is used as an anode material for Li-ion rechargeable battery. However, the electrochemical performance of LiTi2(PO4)3 materials was restricted by inferior rate capability and fast capacity reduction [3]. There have been no reports on a new mixed-valence titanium (III/IV) phosphate, Li2.7Ti2(PO4)3, which was synthesized for use as a cathode material for LIBs. The preparation of a mixed valence of Ti3+ and Ti4+ is not easy, leading to difficulties during the synthesis process [4]. Li2.7Ti2(PO4)3 has many advantages including stable cycling ability, a low cost, and safety. Carbon coating is a powerful method used to effectively improve the electrical conductivity of materials [5]. Moreover, it also can reduce the particles size by suppressing the growth of particles to lower the lithium-ion diffusion length [6]. Thus, citric acid is employed as a carbon source and chelating reagent to prepare Li2.7Ti2(PO4)3 by the sol-gel method [7]. In order to decrease the reaction time originating from a traditional sol-gel process, a microwave synthesis method may be employed to aid the material synthesis. The final products can be quickly obtained because the ⇑ Corresponding author. E-mail address: [email protected] (Z. Su). http://dx.doi.org/10.1016/j.cplett.2016.07.014 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
microwave energy is directly absorbed by the samples [8]. In this study, Li2.7Ti2(PO4)3/C cathode material has been synthesized by the microwave-assisted sol-gel method. Furthermore, the preparation and electrochemical properties of Li2.7Ti2(PO4)3/C are described in detail. 2. Experimental 2.1. Preparation of Li2.7Ti2(PO4)3/C Appropriate amounts of the raw materials-LiCl, TiCl3 (16 wt%, purple solution), (NH4)3PO4, and carbon (citric acid as the carbon source)-were prepared to fabricate Li2.7Ti2(PO4)3 samples by the microwave-assisted sol-gel method. First, LiCl and (NH4)3PO4 were dissolved in deionized water with magnetic stirring to form solution A. Then, citric acid was dissolved in the TiCl3 purple solution with magnetic stirring to form solution B. Afterwards, solution A was added to solution B at a uniform speed while stirring for 1 h. The resulting solution was heated at 70 °C to form a gel and dried in a vacuum oven at 110 °C for 12 h to eliminate water. Finally, the resulting powder was pressed into pellets, placed into a crucible filled with activated carbon, and transferred to a 600-W microwave oven for 15 min. Li2.7Ti2(PO4)3/C materials can be successfully synthesized. 2.2. Characterization The structure and morphology of the Li2.7Ti2(PO4)3/C materials were characterized by X-ray diffraction (XRD, Bruker D2) with Cu
X. Yao et al. / Chemical Physics Letters 659 (2016) 88–92
Ka radiation from 10° to 70° with a step size of 0.02°, infrared (IR) spectroscopy (Shimadzu IRAffinity-1) in the range of 400– 4000 cm 1 with a resolution of 2 cm 1, scanning electron microscopy (SEM, Zeiss LEO 1430VP), and transmission electron microscopy (TEM, JEM-2010FEF). Elemental analyses for Li and Ti were carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Advantage), and the content of the P was measured by ion chromatography (IC, ICS-2100) to determine the chemical compositions of the samples. The valences of Ti in the Li2.7Ti2(PO4)3/C materials were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). For the electrochemical tests, the cathode was prepared by mixing the active material, acetylene black, and polytetrafluoroethylene (PTFE) with a weight ratio of 80:10:10 in deionized water. The resulting slurry was spread onto an aluminum foil, cut into small wafers with a diameter of 10 mm, and dried in a vacuum oven at 110 °C for 12 h. Electrochemical cells were fabricated in an argon-filled glove box using Li metal as the counter electrode and LiPF6 (1 mol L 1), including ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1, as the electrolyte. In addition, Celgard 2400 was used as the separator. The charge-discharge performance of the cells was characterized at room temperature by a battery testing system (LAND CT2001A) within the voltage range of 2.0–3.5 V. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) measurements were carried out at room temperature using an electrochemical workstation (Chenhua CHI 650D) in the potential range of 2.0–3.5 V at a scan rate of 0.1 mV s 1 and with frequencies ranging from 0.01 Hz to 0.1 MHz, respectively. 3. Results and discussion According to the analysis of chemical compositions of materials by ICP-AES and IC, the contents of Li, Ti, and P are 4.72, 24.01, and 71.27 wt%, respectively, demonstrating the chemical formula of the materials is nonstoichiometric Li2.7Ti2(PO4)3. Fig. 1a shows the XRD patterns of a Li2.7Ti2(PO4)3/C sample. All of the diffraction peaks can be indexed as orthorhombic Li2.7Ti2(PO4)3 (space group Pbcn) with the cell parameters a = 12.063(8) Å, b = 8.662(9) Å, and c = 8.710(8) Å (JCPDS: 80-2457), which coincides with previous reports [9]. There are no impurity phases detected from the XRD patterns, implying that single-phase Li2.7Ti2(PO4)3 can be prepared by this method and activated carbon plays an important role in the protective atmosphere. In addition, no obvious carbon peaks appeared in the XRD patterns, which suggests that the residual carbon has an amorphous structure and does not influence the crystal
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structure [10–12]. Furthermore, the carbon content in the final products was tested by an elemental analysis, and the result is merely 3.90 wt%, which contributes to the electrochemical performance. Fig. 1b shows the structure of NASICON-type Li2.7Ti2(PO4)3 observed along the c axis, and we can clearly see that each corner of the TiO6 octahedra is linked to a PO4 tetrahedron. Two Li atoms (Li1, Li2) appear at every asymmetric unit, and their sites are partially occupied in a channel structure that is approximately pentagonal, which is the main reason for the fast ionic conductivity [9]. To understand the chemical bonding and molecular structure of the Li2.7Ti2(PO4)3/C sample, the IR spectrum is shown in Fig. 2a. The absorption peak at 447 cm 1 is ascribed to TiAOATi bands, and the absorption peaks around 598 and 627 cm 1 could be connected with the stretching vibration of TiAO bands. Further, the absorption peak at 867 cm 1 could be attributed to symmetric PAO bands, and the absorption peaks near 999, 1037, and 1196 cm 1 may be related to PAOAP bridge vibrations, which are the characteristic absorptions of PAOAP linkers. Finally, the absorption peaks around 1437 and 1503 cm 1 are due to Fermi resonance peaks of PAOAP bridge vibrations. All of these assignments are consistent with those reported earlier [13–18]. To confirm the presence of the mixed Ti3+/Ti4+ valence in the Li2.7Ti2(PO4)3/C sample, the Ti 2p spectrum was recorded by XPS and is shown in Fig. 2b. The Ti 2p spectrum is generally made up of Ti 2p1/2 and Ti 2p3/2 peaks. Ti3+ for the Ti 2p1/2 and 2p3/2 peaks is located at 463.9 and 458.4 eV, respectively. Further, Ti4+ for the Ti 2p1/2 and 2p3/2 peaks is located at 464.3 and 459.9 eV, respectively, which is consistent with previous reports [19]. According to the careful peak separation of Ti3+ and Ti4+, it was found that Li2.7Ti2(PO4)3 consists of both Ti3+ (86%) and Ti4+ (14%), indicating that the average valence of Ti is 3.14. Additionally, we can clearly determine that the content of Li is deficient (2.7 mol) on the basis of the above analyses. Therefore, the chemical composition of the obtained materials is nonstoichiometric Li2.7Ti2(PO4)3 in nature. These results are similar with the above element analyses. The morphology of the Li2.7Ti2(PO4)3/C samples is shown in Fig. 3. The SEM image in Fig. 3a shows that the particles have a granular shape that is approximately 0.2–2 lm in diameter. To check the carbon coating on the Li2.7Ti2(PO4)3 particles, a TEM analysis of the sample was carried out. Fig. 3b shows a lattice spacing of 0.337 nm, corresponding to the (3 1 1) plane of orthorhombic Li2.7Ti2(PO4)3. It is clearly seen that about 9-nm-thick carbon layer is not uniform. The present carbon layer does prevent an increase in the particles size, which contributes to improving the electronic conductivity of Li2.7Ti2(PO4)3, thereby enhancing the electrochemical properties [20].
Fig. 1. (a) XRD patterns of a Li2.7Ti2(PO4)3/C powder; (b) structure of the projected unit cell of Li2.7Ti2(PO4)3 viewed along the c axis.
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Fig. 2. (a) IR spectrum of the Li2.7Ti2(PO4)3/C samples; (b) XPS spectra of Ti in Li2.7Ti2(PO4)3/C samples.
Fig. 3. (a) SEM and (b) TEM images of the Li2.7Ti2(PO4)3/C samples.
Fig. 4a and b shows the initial charge-discharge capabilities and cycling performance of Li2.7Ti2(PO4)3/C samples. In Fig. 4a, the initial charge-discharge curves exhibit three flat charging plateaus around 2.34, 2.46, and 2.86 V and three flat discharging plateaus around 2.26, 2.38, and 2.78 V. The charge-discharge curves of Li2.7Ti2(PO4)3 are similar to those of the previously reported mixedvalence orthorhombic Li1.5Ti2(PO4)3, which signifies smooth solid-solution reaction properties and adopts the same standard space group of Pbcn (#60) [21]. The initial charge and discharge capacities are 123.7 and 123.6 mA h g 1 at 0.1C, respectively, indicating that the coulombic efficiency of the first cycle is extremely high (99.92%). As shown in Fig. 4b, the Li2.7Ti2(PO4)3/C sample also exhibits an excellent cycling ability. After 200 cycles, the discharge capacity of the sample is 123.1 mA h g 1 at 0.1C, and the discharge capacity retention is 99.60%, implying better electrochemical reversibility [11]. When tested at 0.2, 0.5, 1, 2, and 5C, the discharge capacities of the first cycle are maintained at 115.8, 107.7, 100.6, 91.3, and 84.6 mA h g 1, respectively. Furthermore, the discharge capacity retentions after 200 cycles are still as high as 99.48, 98.98, 92.74, 84.99, and 78.84% at 0.2, 0.5, 1, 2, and 5C, respectively (as shown in Fig. 4b). The excellent electrochemical performance can be ascribed to the carbon coating and small particle size [10,22]. It can also be found from the figure, the first discharge capacity decreases with the increase of charge and discharge rate. This is due to the ratio increases, the polarization of materials becomes larger, leading to the capacity reduction. To further understand the reversibility and stability during the intercalation/de-intercalation reactions, the CV recorded for
Li2.7Ti2(PO4)3/C is shown in Fig. 4c [23]. The sample exhibits three redox couples, anodic peaks occur at 2.34, 2.46, and 2.86 V, and cathodic peaks occur at 2.26, 2.38, and 2.78 V, which are consistent with the flat charge-discharge plateaus. In fact, two pairs of redox peaks occur between 2.2 and 2.5 V, which respectively corresponds to Li+ insertion/extraction into/from Li2.7Ti2(PO4)3 according to the electrode reaction. The other redox peak at 2.78/2.86 V is attributed to redox process in association with Ti3+ [3]. The well-defined peaks and good symmetry of the CV plots confirm the excellent reversibility of the lithium-ion extraction and insertion reactions for the Li2.7Ti2(PO4)3/C composite materials [24,25]. Furthermore, the CVs of the three cycles almost overlap, suggesting less polarization [26]. The EIS curves and the equivalent circuit are shown in Fig. 4d, the small interrupt in the high-frequency domain is in connection with the solution resistance (Rs), and the first semicircle is involved in the lithium-ion migration resistance (Rf) caused by the SEI film formed on the cathode surface [27]. Another semicircle in the medium-to-low frequency region is related to the chargetransfer resistance (Rct), which is due to interface between the electrode and the electrolyte [28]. The sloping straight line in the low-frequency region is associated with lithium-ion diffusion in the bulk material, corresponding to the Warburg impedance (Zw) [29]. The EIS values are obtained by the equivalent circuit simulation, and the Rs, Rf, Rct, and Zw values are 2.58, 33.69, 54.93, and 93.41 X, respectively. The sample exhibits a low electrochemical impedance, demonstrating a small contact resistance (between the electrode and the particles) and charge-transfer polarization [30,31].
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Fig. 4. (a) The first charge-discharge curves and (b) cycling performance of Li2.7Ti2(PO4)3/C samples at various rates of 0.1C, 0.2C, 0.5C, 1C, 2C and 5C; (c) CVs and (d) EIS curves of Li2.7Ti2(PO4)3/C samples.
4. Conclusions Nonstoichiometric Li2.7Ti2(PO4)3/C composite material with orthorhombic structure can be successfully synthesized by the microwave-assisted sol-gel method. All of the diffraction peaks of the sample can be indexed as the orthorhombic structure. From the SEM and TEM results, particles with a carbon layer have a diameter of 0.2–2 lm. According to the charge-discharge tests, the initial charge and discharge capacities of sample are 123.7 and 123.6 mA h g 1 at 0.1C, respectively, indicating a high coulombic efficiency. After 200 cycles, the discharge capacity retention is 99.60% at 0.1C, suggesting good electrochemical reversibility. The excellent electrochemical performance can be ascribed to the carbon coating and small particle size. Additionally, the CV exhibits less polarization. These results suggest that Li2.7Ti2(PO4)3/C is a promising cathode material for LIBs. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21061015). References [1] Y.S. Lee, S. Sato, M. Tabuchi, C.S. Yoon, Y.K. Sun, K. Kobayakawa, Y. Sato, Structural change and capacity loss mechanism in orthorhombic Li/LiFeO2 system during cycling, Electrochem. Commun. 5 (2003) 549–554. [2] K.Y. Li, F.F. Shua, J.W. Zhang, K.F. Chen, D.F. Xue, X.W. Guo, S. Komarneni, Role of hydrothermal parameters on phase purity of orthorhombic LiMnO2 for use as cathode in Li ion battery, Ceram. Int. 41 (2015) 6729–6733. [3] J.X. Sun, Y.R. Sun, L.G. Gai, H.H. Jiang, Y. Tian, Carbon-coated mesoporous LiTi2(PO4)3 nanocrystals with superior performance for lithium-ion batteries, Electrochim. Acta 200 (2016) 66–74.
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