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C nanocomposite as cathode material for lithium ion battery
Electrochimica Acta 133 (2014) 564–569
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ generation of Li2 FeSiO4 /C nanocomposite as cathode material for lithium ion battery Jin Yi a , Meng-yan Hou a , Hong-liang Bao b , Cong-xiao Wang a , Jian-qiang Wang b , Yong-yao Xia a,∗ a Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, China b Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
a r t i c l e
i n f o
Article history: Received 27 February 2014 Received in revised form 24 March 2014 Accepted 29 March 2014 Available online 5 April 2014 Keywords: Li2 FeSiO4 Polyanionic Cathode material Lithium-ion battery Carbon coating
a b s t r a c t A Li2 FeSiO4 /C nanocomposite is prepared via solvothermal method in combination with carbon coating technology. The as-prepared Li2 FeSiO4 /C nanocomposite is a single phase Li2 FeSiO4 with nano-tickness coated carbon layer and connected by the mutual cross-linked carbon conductive matrix. As cathode material for lithium ion battery, the composite delivers a discharge capacity of 154 mAh g−1 at 1 C and exhibits good rate capability with a discharge capacity of 106 mAh g−1 at the rate of 10 C, which is ascribed to the small particle size and enhanced electronic conductivity using carbon coating technology. The asprepared Li2 FeSiO4 /C nanocomposite also behaves a good cycling stability with capacity retention of over 100 cycles. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction LiFePO4 has been demonstrated to be one of the most promising cathode material for lithium-ion batteries, especially for large-scale applications, such as plug-in hybrid vehicles and stationary powers, because of the inherent low cost, nontoxicity, and extremely high stability. Following the successful development of the olivinestructured LiFePO4 cathode, lithium transition metal silicates have been proposed as considerable cathode candidates, because of their high specific capacity (with a capacity of ∼333 mAh g−1 , according to insertion/extraction of two lithium ions per formula unit) [1–4]. Additionally, it is prepared from abundant and inexpensive raw materials, which demonstrates Li2 FeSiO4 to be a promising candidate for desirable cathode material of large-scale lithium ion batteries in the future [5,6]. However, as the intrinsic characteristics of polyanionic materials, Li2 FeSiO4 also suffers from the problems of small lithium ion diffusion coefficient and poor electronic conductivity, presenting major drawbacks to its practical implementation [7,8]. Over the
∗ Corresponding author. Tel.: +86 21 51630318, fax: +86 21 51630318. E-mail address: [email protected] (Y.-y. Xia). http://dx.doi.org/10.1016/j.electacta.2014.03.164 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
years, research efforts have been made to address these problems, including improving the purity, decreasing the particle size and increasing the electronic conductivity [9,10], such as adding sucrose as carbon source to increase the electronic conductivity, which unfortunately may lead to inhomogeneous distribution. Therefore, it is significant to develop a technology to cope with the above difficulties. Recently, benzoates are used as carbon source for in-situ generation carbon coating by thermal decomposition of the benzoates precursors, such as lead benzoate and manganese benzoate used in preparation of PbO@C and MnO/C, respectively [11,12]. In this study, we chose lithium benzoate (C6 H5 COOLi) as both lithium and carbon sources in preparation of Li2 FeSiO4. A Li2 FeSiO4 /C nanocomposite was prepared via solvothermal method in combination with carbon coating technology. Its structure was intensively studied by X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) and Fourier transform infrared spectroscopy (FTIR). The electrochemical performance of the asprepared Li2 FeSiO4 /C samples as cathode material of lithium ion battery were characterized and compared by galvanostatic charge/discharge tests. Furthermore, the effect of carbon coating technology on particle size and electronic conductivity was investigated.
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2. Experimental 2.1. Chemicals and preparation The preparation of Li2 FeSiO4 /C nanocomposite was performed via a mild solvothermal method followed by thermal vapor deposition (TVD) technology. In a typical preparation process: stoichiometric amounts of lithium benzoate (C6 H5 COOLi), ferrous oxalate dehydrate (FeC2 O4 ·2H2 O) and tetraethyl orthosilicate (TEOS) were dispersed in 45 mL ethylene glycol (EG) and magnetically stirred for 5 h. Then, the solution was transferred into a Teflon-lined stainless-steal autoclave, sealed, and maintained at 150 ◦ C for 3 days. The resulting wet gel precursor was treated at 400 ◦ C for 5 h under nitrogen gas (N2 ) flow. In order to decrease the aggregation of Li2 FeSiO4 /C, the precursor already treated at 400 ◦ C for 5 h was ball milled for 1 h, then the Li2 FeSiO4 /C nanocomposite (hereafter referred to as LFS@C)was obtained after calcination at 700 ◦ C for 12 h under N2 flow. For comparision, the Li2 FeSiO4 /C nanocomposite (hereafter referred to as LFS@C-2) was obtained under the same conditions in combination with the TVD technology, which a toluene vapor was carried by nitrogen gas (N2 ) via the reaction tube at a flow rate of 0.06 L min−1 to make a fluid-bed layer.
Fig. 1. Rietveld refined XRD pattern of the LFS@C (black line, experimental; red dots, calculation; green line, bragg positions; blue line, difference curve).
2.2. Characterization Particle size and morphology were characterized using a S4800 scanning electron microscope (SEM) and transmission electron microscope (TEM, JEM 2010F). X-ray powder diffraction (XRD) pattern was recorded on a Bruker D8 X-ray diffractometer using Cu K␣ radiation (= 0.15406 nm). The data were collected in the 10–90◦ range with a step size of 0.02◦ (2) and a constant counting time of 10 s per step. The structure was refined by the Rietveld method using the program TOPAS academic [13]. The X-ray absorption near edge structure (XANES) study was carried out on Beam Line BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF). A double Si (1 1 1) crystal monochromator, cooled by liquid nitrogen, was used to monochromatize the energy. The XANES data of the samples were obtained in transmission mode. The XANES data were analyzed using the Athena program of IFEFFIT for background removal, E0 selection and normalization. Energy calibration was obtained using Fe reference foil placed between the second and third ionization chamber [14]. Raman spectrum was obtained with a Dilor LabRam-1B microscopic Raman spectrometer, using Ar laser with an excitation wavelength of 514.5 nm. Thermogravimetric (TG) analyses were performed from 25 to 900 ◦ C with a heating rate of 5 ◦ C min−1 on a NETZSCH TG 209 F1 thermal analyzer (Germany). The TG curves were recorded in an oxygen flow of 50 mL min−1 . The baseline was subtracted in all cases. The Fourier transformed infrared spectroscopy (FTIR) was performed on a Nicolet 6700 FTIR (Thermo electro. USA). 2.3. Electrochemical tests The working electrodes were prepared by mixing 80% of the sample powders, 10% carbon black and 10% polyvinylidene fluoride (PVDF) dissolved in N-methyl-pyrrolidone (NMP), and the slurries were coated on aluminum foil using the doctor-blade technique. The electrode films were dried at 80 ◦ C for 2 h to remove the solvent before pressing. The electrode film was punched in the form of disks, typically with a diameter of 12 mm, and then dried at 80 ◦ C for 12 h under vacuum. The typical mass loading of the active material of the working electrodes was approximately 5 mg. Electrochemical tests were conducted using CR2016-type coin cells. The cells were assembled with the working electrode as prepared,
Fig. 2. XANES spectra of Fe, FeO, Fe2 O3 , Fe3 O4 and LFS@C.
lithium metal as anode, and Celgard 2300 film as separator in a glove-box filled with pure argon. The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1: 1: 1 by volume). Galvanostatic charge/discharge experiments were performed between 1.5 and 4.8 V at various constant current rates on a LAND CT2001A Battery Cycler (Wuhan, China). Lithium insertion into the working electrode was referred to as discharge, and the extraction as charge. The cell capacity was calculated based on the pure weight of active material by excluding the carbon content. 3. Results and Discussion Fig. 1 presents the Rietveld refined XRD pattern of LFS@C. It belongs to P21 /n structure. The Rietveld refinement shows the calculated cell parameters of a = 0.82198 nm, b = 0.50069 nm, c = 0.82266 nm and  = 99.1457◦ , which are consistent with the previous report [15]. The values of Rwp and Rp factors are 4.54 and 3.12, suggesting a satisfying accuracy of the fitting. No crystalline carbon was obviously detected in the XRD pattern since the in-situ as-formed carbon conductive matrix is amorphous. Further details on the structure of Li2 FeSiO4 /C were acquired via XANES analyses. The Fe K-edge XANES spectra of LFS@C nanocomposite and several reference compounds are demonstrated in Fig. 2. The shifts of Fe K-edge positions to higher energy with the increase of the oxidation state are observed. For LFS@C nanocomposite, the
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Fig. 3. FTIR spectrum of the LFS@C.
absorption edge around 7120 eV aligned with that of FeO. This illustrates that the Fe cation in Li2 FeSiO4 /C has a similar valence with FeO. The spectrum above 7122 eV are not aligned with that of the Fe2 O3 , demonstrating that the as-prepared Li2 FeSiO4 /C has high phase purity. The observed pre-edge peak in the Li2 FeSiO4 /C proves the tetrahedral coordination of Fe ions in the crystal, which is consistent with the reported results [4]. The phase composition of LFS@C nanocomposite was further analysed via FTIR technology and the result was presented in Fig. 3. The strong absorbance at 900–940 cm−1 is ascribed to the vibration of SiO4 4− [16,17]. The absence of Li2 SiO3 is confirmed from the absence of the characteristic absorption peaks of Si-O-Si bond of Li2 SiO3 at 1100 and 780 cm−1 [18]. All of the above findings suggest that the as-prepared is free from impurities. Representative SEM images of Li2 FeSiO4 /C precursor are illustrated in Fig. 4a. It is observed that the precursor is slightly aggregated with a diameter of about 30 nm. After being treated at 700 ◦ C, the LFS@C with a diameter of about 100-200 nm is obtained (shown in Fig. 4b). As a contrast, the particle size of LFS@C-2 nanocomposite is obviously decreased after treatment at 700 ◦ C accompanying with TVD treatment (shown in Fig. 4c). This significant difference indicates that TVD treatment is a contribution to suppress the growth of Li2 FeSiO4 crystal and then control the particle size of Li2 FeSiO4 . In order to further investigate the effect of TVD treatment on the particle size of Li2 FeSiO4 , the TEM measurements were performed, and the images of Li2 FeSiO4 /C precursor and nanocomposite are presented in Fig. 5. After being treated at 400 ◦ C and ball milled, the precursor with a diameter of about 30 nm are connected by carbon film, which is derived from the thermal decomposition of the benzoate precursor (shown in Fig. 5a and b), after following treatment at 700 ◦ C, the LFS@C is observed with a diameter of 150 nm and coated with a very thin carbon film of less than 1 nm (shown in Fig. 5c and d). However, after combining with TVD technology using toluene as carbon source, the particle size of LFS@C-2 is dramatically decreased to about 30 nm, similar to the precursor. These findings suggest that TVD treatment can effectively control the growth of crystal of Li2 FeSiO4 . Moreover, it can be clearly seen that the LFS@C-2 is surrounded by the mutual cross-linked carbon conductive matrix. And a uniform carbon layer with the thickness of 5 nm has also been successfully coated on the Li2 FeSiO4 surface, and confirmed by the result given in Fig. 5f. Compared with the LFS@C nanocomposite obtained without TVD, it is found that the particle size of LFS@C-2 is decreased after TVD treatment, providing the improved lithium-ion diffusion rate. On the other hand, the mutual
Fig. 4. SEM images of Li2 FeSiO4 /C precursor (a), LFS@C (b) and LFS@C-2 (c).
cross-linked carbon conductive matrix enables the interconnection of the LFS@C-2 nanocomposites, forming an efficient electrical network to mitigate the problem of poor electronic conductivity of Li2 FeSiO4 . Thermalgravimetric (TG) analyses are applied to determine the carbon content of Li2 FeSiO4 /C nanocomposite and the results are depicted in Fig. 6. It is found that both as-prepared Li2 FeSiO4 /C suffered from a weight increase from 100 wt% to 105 wt%, which is due to the oxidation of Fe2+ to Fe3+ . The carbon contents of Li2 FeSiO4 /C are obtained with the assumption that the second mass decrease in TG is caused by carbon burn-off [19]. Typically, around 2 wt% of carbon is found in the as-prepared LFS@C nanocomposite. Owing to the intrinsic poor electronic conductivity of Li2 FeSiO4 , more carbon content should be provided to improve the electronic conductivity [10,16,20]. After TVD treatment, the carbon content of LFS@C-2
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Fig. 5. TEM images of Li2 FeSiO4 /C precursor (a, b), LFS@C (c, d) and LFS@C-2 (e, f).
Fig. 7. Raman spectrum of LFS@C-2. Fig. 6. TG curves of LFS@C (a) and LFS@C-2 (b).
increases to 20%. In this case, Li2 FeSiO4 /C would offer high electronic conductivity to mitigate the conventional problems such as poor rate performance and cycling stability. In order to investigate the quality of the carbon conductive matrix after TVD treatment, the Raman spectroscopy was conducted and the result is displayed in Fig. 7. Base on the standard deconvolution of the obtained Raman spectrum using a fit resolved into four individual bands, four Raman signal at around 1180, 1350,
1490, and 1590 cm−1 are observed. The bands at around 1350 cm−1 (Peak 2) and 1590 cm−1 (Peak 4) are attributed to sp2 type carbon, while the others at around 1180 cm−1 (Peak 1) and 1510 cm−1 (Peak 3) are related to sp3 type carbon [21,22]. The integrated area ratio of sp3 and sp2 (Asp3 /Asp2 ) is used to roughly estimate the relatively amount of the graphite carbon. The low Asp3 /Asp2 ratio of 0.40 indicates that a large amount of the coated carbon in the nanocomposite exists in sp2 type. Additionally, the ID /IG ratio (intensity ratio of D and G bands) of Li2 FeSiO4 /C, are fitted to 1.08, which demonstrates
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Fig. 8. Typical charge/discharge curves of as-prepared Li2 FeSiO4 /C nanocomposite at different rates: LFS@C (A) and LFS@C-2 (C), and the corresponding dQ/dV vs. voltage plot of the charge curve at 1 C in the voltage range of 1.5–4.8 V (vs Li/Li+ ): LFS@C (B) and LFS@C-2 (D).
the graphitization degree of the composite. The small ID /IG ratio of Li2 FeSiO4 /C usually corresponds to high electronic conductivity, expecting the superior electrochemical performance [15]. Galvanostatic charge/discharge experiments were used to determine the electrochemical properties of the Li2 FeSiO4 /C nanocomposite. The typical charge/discharge curves for LFS@C at different current densities of 1 C, 2 C, 5 C, and 10 C are shown in Fig. 8(A), which exhibits the discharge capacity of 51, 40, 35 and 33 mAh g−1 at 1 C, 2 C, 5 C and 10 C (1 C = 166 mA g−1 ), respectively. Fig. 8B displays the dQ/dV vs. voltage plot of the charge curve at the rate of 1 C, from which we can only observe the oxidation peak located at 3.2 V in the charge process, corresponding to the conversion from Fe2+ to Fe3+ . The poor rate performance and absence of the
oxidation of Fe3+ to Fe4+ are attributed to the poor electronic conductivity. After TVD treatment, the electronic conductivity and rate performance have been obviously improved, with the discharge capacity of 154, 140, 121 and 106 mAh g−1 at 1 C, 2 C, 5 C and 10 C, and the occurrence of the oxidation peak of Fe3+ to Fe4+ located at 4.3 V for LFS@C-2 (shown in Fig. 8 C and D). It can be found that the LFS@C-2 presents good rate capability, which is comparable to the results reported in the literatures, such as 110 [10] and 132.1 [15] mAh g−1 at 1 C, 80 [7,9] and 100 [16] mAh g−1 at 10 C. It is well known that the transformation from Fe3+ to Fe4+ is rather difficult due to the low conductivity of Li2 FeSiO4 , thus limiting the capacity lower than 166 mAh g−1 . However, owing to the elaborate design of Li2 FeSiO4 /C nanocomposite connected by the mutual cross-linked
Fig. 9. Rate performance (A) and cycling stability at 1 C (B) of as-prepared Li2 FeSiO4 /C nanocomposite: LFS@C-2 (a) and LFS@C (b).
J. Yi et al. / Electrochimica Acta 133 (2014) 564–569
carbon conductive matrix with high conductivity, the conversion from Fe3+ to Fe4+ was achieved, which results in high capacities during charge/discharge cycles. Fig. 9A displays a pronounced difference on the rate performance of LFS@C and LFS@C-2. As can be seen, the LFS@C-2 nanocomposite presents noticeably superior rate performance than LFS@C. Furthermore, a capacity of 158 mAh g−1 can be retained when the current density reduced back to 1 C, indicating good cycling stability of the as-prepared nanocomposite after TVD treatment. The cycling performance for both samples at 1 C is displayed in Fig. 9B. After 100 cycles, the specific discharge capacity of the sample after TVD treatment is still stabilized around 158 mAh g−1 at 1 C, while 50 mAh g−1 for sample without TVD, definitely showing superior stability during cycling for both samples. It seems that the enhanced electronic conductivity is of central importance to explain the high-rate performance of Li2 FeSiO4 /C nanocomposite. The results presented here illustrate the benefits of the decreased particle size, coated carbon layer and mutual cross-linked carbon conductive matrix structure, which enables us to overcome the problem of poor electronic conductivity in the lithium metal silicate based cathode materials and achieve the aim of high rate capability and good cycling performance successfully. 4. Conclusions In summary, we report a novel Li2 FeSiO4 /C nanocomposite prepared via solvothermal method and in-situ carbon coating technology. The prepared Li2 FeSiO4 /C nanocomposite has a P21 /n structure without impurities. The particles are approximately 30 nm with a coating carbon layer and connected by the mutual cross-linked carbon conductive matrix. As cathode material, the Li2 FeSiO4 /C nanocomposite displays outstanding electrochemical performance with good rate performance and cycling stability. It delivers a discharge capacity of 106 mAh g−1 at the rate of 10 C, and with capacity retention of over 100 cycles. This is attributed to the short lithium ion diffusion path and enhanced electronic conductivity caused by the small particle size and the mutual cross-linked carbon conductive matrix. Therefore, we believe that electrode materials based on the mutual cross-linked carbon conductive matrix should be one of ideal candidates for expansion of Li ion battery technology in automobile, aerospace, and power grid applications which demand the development of light-weight, long-lasting batteries. Acknowledgements This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903), the National Natural Science Foundation of China (20925312), and Shanghai Science Technology Committee (13JC1407900).
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ generation of Li2 FeSiO4 /C nanocomposite as cathode material for lithium ion battery Jin Yi a , Meng-yan Hou a , Hong-liang Bao b , Cong-xiao Wang a , Jian-qiang Wang b , Yong-yao Xia a,∗ a Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, China b Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
a r t i c l e
i n f o
Article history: Received 27 February 2014 Received in revised form 24 March 2014 Accepted 29 March 2014 Available online 5 April 2014 Keywords: Li2 FeSiO4 Polyanionic Cathode material Lithium-ion battery Carbon coating
a b s t r a c t A Li2 FeSiO4 /C nanocomposite is prepared via solvothermal method in combination with carbon coating technology. The as-prepared Li2 FeSiO4 /C nanocomposite is a single phase Li2 FeSiO4 with nano-tickness coated carbon layer and connected by the mutual cross-linked carbon conductive matrix. As cathode material for lithium ion battery, the composite delivers a discharge capacity of 154 mAh g−1 at 1 C and exhibits good rate capability with a discharge capacity of 106 mAh g−1 at the rate of 10 C, which is ascribed to the small particle size and enhanced electronic conductivity using carbon coating technology. The asprepared Li2 FeSiO4 /C nanocomposite also behaves a good cycling stability with capacity retention of over 100 cycles. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction LiFePO4 has been demonstrated to be one of the most promising cathode material for lithium-ion batteries, especially for large-scale applications, such as plug-in hybrid vehicles and stationary powers, because of the inherent low cost, nontoxicity, and extremely high stability. Following the successful development of the olivinestructured LiFePO4 cathode, lithium transition metal silicates have been proposed as considerable cathode candidates, because of their high specific capacity (with a capacity of ∼333 mAh g−1 , according to insertion/extraction of two lithium ions per formula unit) [1–4]. Additionally, it is prepared from abundant and inexpensive raw materials, which demonstrates Li2 FeSiO4 to be a promising candidate for desirable cathode material of large-scale lithium ion batteries in the future [5,6]. However, as the intrinsic characteristics of polyanionic materials, Li2 FeSiO4 also suffers from the problems of small lithium ion diffusion coefficient and poor electronic conductivity, presenting major drawbacks to its practical implementation [7,8]. Over the
∗ Corresponding author. Tel.: +86 21 51630318, fax: +86 21 51630318. E-mail address: [email protected] (Y.-y. Xia). http://dx.doi.org/10.1016/j.electacta.2014.03.164 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
years, research efforts have been made to address these problems, including improving the purity, decreasing the particle size and increasing the electronic conductivity [9,10], such as adding sucrose as carbon source to increase the electronic conductivity, which unfortunately may lead to inhomogeneous distribution. Therefore, it is significant to develop a technology to cope with the above difficulties. Recently, benzoates are used as carbon source for in-situ generation carbon coating by thermal decomposition of the benzoates precursors, such as lead benzoate and manganese benzoate used in preparation of PbO@C and MnO/C, respectively [11,12]. In this study, we chose lithium benzoate (C6 H5 COOLi) as both lithium and carbon sources in preparation of Li2 FeSiO4. A Li2 FeSiO4 /C nanocomposite was prepared via solvothermal method in combination with carbon coating technology. Its structure was intensively studied by X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) and Fourier transform infrared spectroscopy (FTIR). The electrochemical performance of the asprepared Li2 FeSiO4 /C samples as cathode material of lithium ion battery were characterized and compared by galvanostatic charge/discharge tests. Furthermore, the effect of carbon coating technology on particle size and electronic conductivity was investigated.
J. Yi et al. / Electrochimica Acta 133 (2014) 564–569
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2. Experimental 2.1. Chemicals and preparation The preparation of Li2 FeSiO4 /C nanocomposite was performed via a mild solvothermal method followed by thermal vapor deposition (TVD) technology. In a typical preparation process: stoichiometric amounts of lithium benzoate (C6 H5 COOLi), ferrous oxalate dehydrate (FeC2 O4 ·2H2 O) and tetraethyl orthosilicate (TEOS) were dispersed in 45 mL ethylene glycol (EG) and magnetically stirred for 5 h. Then, the solution was transferred into a Teflon-lined stainless-steal autoclave, sealed, and maintained at 150 ◦ C for 3 days. The resulting wet gel precursor was treated at 400 ◦ C for 5 h under nitrogen gas (N2 ) flow. In order to decrease the aggregation of Li2 FeSiO4 /C, the precursor already treated at 400 ◦ C for 5 h was ball milled for 1 h, then the Li2 FeSiO4 /C nanocomposite (hereafter referred to as LFS@C)was obtained after calcination at 700 ◦ C for 12 h under N2 flow. For comparision, the Li2 FeSiO4 /C nanocomposite (hereafter referred to as LFS@C-2) was obtained under the same conditions in combination with the TVD technology, which a toluene vapor was carried by nitrogen gas (N2 ) via the reaction tube at a flow rate of 0.06 L min−1 to make a fluid-bed layer.
Fig. 1. Rietveld refined XRD pattern of the LFS@C (black line, experimental; red dots, calculation; green line, bragg positions; blue line, difference curve).
2.2. Characterization Particle size and morphology were characterized using a S4800 scanning electron microscope (SEM) and transmission electron microscope (TEM, JEM 2010F). X-ray powder diffraction (XRD) pattern was recorded on a Bruker D8 X-ray diffractometer using Cu K␣ radiation (= 0.15406 nm). The data were collected in the 10–90◦ range with a step size of 0.02◦ (2) and a constant counting time of 10 s per step. The structure was refined by the Rietveld method using the program TOPAS academic [13]. The X-ray absorption near edge structure (XANES) study was carried out on Beam Line BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF). A double Si (1 1 1) crystal monochromator, cooled by liquid nitrogen, was used to monochromatize the energy. The XANES data of the samples were obtained in transmission mode. The XANES data were analyzed using the Athena program of IFEFFIT for background removal, E0 selection and normalization. Energy calibration was obtained using Fe reference foil placed between the second and third ionization chamber [14]. Raman spectrum was obtained with a Dilor LabRam-1B microscopic Raman spectrometer, using Ar laser with an excitation wavelength of 514.5 nm. Thermogravimetric (TG) analyses were performed from 25 to 900 ◦ C with a heating rate of 5 ◦ C min−1 on a NETZSCH TG 209 F1 thermal analyzer (Germany). The TG curves were recorded in an oxygen flow of 50 mL min−1 . The baseline was subtracted in all cases. The Fourier transformed infrared spectroscopy (FTIR) was performed on a Nicolet 6700 FTIR (Thermo electro. USA). 2.3. Electrochemical tests The working electrodes were prepared by mixing 80% of the sample powders, 10% carbon black and 10% polyvinylidene fluoride (PVDF) dissolved in N-methyl-pyrrolidone (NMP), and the slurries were coated on aluminum foil using the doctor-blade technique. The electrode films were dried at 80 ◦ C for 2 h to remove the solvent before pressing. The electrode film was punched in the form of disks, typically with a diameter of 12 mm, and then dried at 80 ◦ C for 12 h under vacuum. The typical mass loading of the active material of the working electrodes was approximately 5 mg. Electrochemical tests were conducted using CR2016-type coin cells. The cells were assembled with the working electrode as prepared,
Fig. 2. XANES spectra of Fe, FeO, Fe2 O3 , Fe3 O4 and LFS@C.
lithium metal as anode, and Celgard 2300 film as separator in a glove-box filled with pure argon. The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1: 1: 1 by volume). Galvanostatic charge/discharge experiments were performed between 1.5 and 4.8 V at various constant current rates on a LAND CT2001A Battery Cycler (Wuhan, China). Lithium insertion into the working electrode was referred to as discharge, and the extraction as charge. The cell capacity was calculated based on the pure weight of active material by excluding the carbon content. 3. Results and Discussion Fig. 1 presents the Rietveld refined XRD pattern of LFS@C. It belongs to P21 /n structure. The Rietveld refinement shows the calculated cell parameters of a = 0.82198 nm, b = 0.50069 nm, c = 0.82266 nm and  = 99.1457◦ , which are consistent with the previous report [15]. The values of Rwp and Rp factors are 4.54 and 3.12, suggesting a satisfying accuracy of the fitting. No crystalline carbon was obviously detected in the XRD pattern since the in-situ as-formed carbon conductive matrix is amorphous. Further details on the structure of Li2 FeSiO4 /C were acquired via XANES analyses. The Fe K-edge XANES spectra of LFS@C nanocomposite and several reference compounds are demonstrated in Fig. 2. The shifts of Fe K-edge positions to higher energy with the increase of the oxidation state are observed. For LFS@C nanocomposite, the
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Fig. 3. FTIR spectrum of the LFS@C.
absorption edge around 7120 eV aligned with that of FeO. This illustrates that the Fe cation in Li2 FeSiO4 /C has a similar valence with FeO. The spectrum above 7122 eV are not aligned with that of the Fe2 O3 , demonstrating that the as-prepared Li2 FeSiO4 /C has high phase purity. The observed pre-edge peak in the Li2 FeSiO4 /C proves the tetrahedral coordination of Fe ions in the crystal, which is consistent with the reported results [4]. The phase composition of LFS@C nanocomposite was further analysed via FTIR technology and the result was presented in Fig. 3. The strong absorbance at 900–940 cm−1 is ascribed to the vibration of SiO4 4− [16,17]. The absence of Li2 SiO3 is confirmed from the absence of the characteristic absorption peaks of Si-O-Si bond of Li2 SiO3 at 1100 and 780 cm−1 [18]. All of the above findings suggest that the as-prepared is free from impurities. Representative SEM images of Li2 FeSiO4 /C precursor are illustrated in Fig. 4a. It is observed that the precursor is slightly aggregated with a diameter of about 30 nm. After being treated at 700 ◦ C, the LFS@C with a diameter of about 100-200 nm is obtained (shown in Fig. 4b). As a contrast, the particle size of LFS@C-2 nanocomposite is obviously decreased after treatment at 700 ◦ C accompanying with TVD treatment (shown in Fig. 4c). This significant difference indicates that TVD treatment is a contribution to suppress the growth of Li2 FeSiO4 crystal and then control the particle size of Li2 FeSiO4 . In order to further investigate the effect of TVD treatment on the particle size of Li2 FeSiO4 , the TEM measurements were performed, and the images of Li2 FeSiO4 /C precursor and nanocomposite are presented in Fig. 5. After being treated at 400 ◦ C and ball milled, the precursor with a diameter of about 30 nm are connected by carbon film, which is derived from the thermal decomposition of the benzoate precursor (shown in Fig. 5a and b), after following treatment at 700 ◦ C, the LFS@C is observed with a diameter of 150 nm and coated with a very thin carbon film of less than 1 nm (shown in Fig. 5c and d). However, after combining with TVD technology using toluene as carbon source, the particle size of LFS@C-2 is dramatically decreased to about 30 nm, similar to the precursor. These findings suggest that TVD treatment can effectively control the growth of crystal of Li2 FeSiO4 . Moreover, it can be clearly seen that the LFS@C-2 is surrounded by the mutual cross-linked carbon conductive matrix. And a uniform carbon layer with the thickness of 5 nm has also been successfully coated on the Li2 FeSiO4 surface, and confirmed by the result given in Fig. 5f. Compared with the LFS@C nanocomposite obtained without TVD, it is found that the particle size of LFS@C-2 is decreased after TVD treatment, providing the improved lithium-ion diffusion rate. On the other hand, the mutual
Fig. 4. SEM images of Li2 FeSiO4 /C precursor (a), LFS@C (b) and LFS@C-2 (c).
cross-linked carbon conductive matrix enables the interconnection of the LFS@C-2 nanocomposites, forming an efficient electrical network to mitigate the problem of poor electronic conductivity of Li2 FeSiO4 . Thermalgravimetric (TG) analyses are applied to determine the carbon content of Li2 FeSiO4 /C nanocomposite and the results are depicted in Fig. 6. It is found that both as-prepared Li2 FeSiO4 /C suffered from a weight increase from 100 wt% to 105 wt%, which is due to the oxidation of Fe2+ to Fe3+ . The carbon contents of Li2 FeSiO4 /C are obtained with the assumption that the second mass decrease in TG is caused by carbon burn-off [19]. Typically, around 2 wt% of carbon is found in the as-prepared LFS@C nanocomposite. Owing to the intrinsic poor electronic conductivity of Li2 FeSiO4 , more carbon content should be provided to improve the electronic conductivity [10,16,20]. After TVD treatment, the carbon content of LFS@C-2
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Fig. 5. TEM images of Li2 FeSiO4 /C precursor (a, b), LFS@C (c, d) and LFS@C-2 (e, f).
Fig. 7. Raman spectrum of LFS@C-2. Fig. 6. TG curves of LFS@C (a) and LFS@C-2 (b).
increases to 20%. In this case, Li2 FeSiO4 /C would offer high electronic conductivity to mitigate the conventional problems such as poor rate performance and cycling stability. In order to investigate the quality of the carbon conductive matrix after TVD treatment, the Raman spectroscopy was conducted and the result is displayed in Fig. 7. Base on the standard deconvolution of the obtained Raman spectrum using a fit resolved into four individual bands, four Raman signal at around 1180, 1350,
1490, and 1590 cm−1 are observed. The bands at around 1350 cm−1 (Peak 2) and 1590 cm−1 (Peak 4) are attributed to sp2 type carbon, while the others at around 1180 cm−1 (Peak 1) and 1510 cm−1 (Peak 3) are related to sp3 type carbon [21,22]. The integrated area ratio of sp3 and sp2 (Asp3 /Asp2 ) is used to roughly estimate the relatively amount of the graphite carbon. The low Asp3 /Asp2 ratio of 0.40 indicates that a large amount of the coated carbon in the nanocomposite exists in sp2 type. Additionally, the ID /IG ratio (intensity ratio of D and G bands) of Li2 FeSiO4 /C, are fitted to 1.08, which demonstrates
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Fig. 8. Typical charge/discharge curves of as-prepared Li2 FeSiO4 /C nanocomposite at different rates: LFS@C (A) and LFS@C-2 (C), and the corresponding dQ/dV vs. voltage plot of the charge curve at 1 C in the voltage range of 1.5–4.8 V (vs Li/Li+ ): LFS@C (B) and LFS@C-2 (D).
the graphitization degree of the composite. The small ID /IG ratio of Li2 FeSiO4 /C usually corresponds to high electronic conductivity, expecting the superior electrochemical performance [15]. Galvanostatic charge/discharge experiments were used to determine the electrochemical properties of the Li2 FeSiO4 /C nanocomposite. The typical charge/discharge curves for LFS@C at different current densities of 1 C, 2 C, 5 C, and 10 C are shown in Fig. 8(A), which exhibits the discharge capacity of 51, 40, 35 and 33 mAh g−1 at 1 C, 2 C, 5 C and 10 C (1 C = 166 mA g−1 ), respectively. Fig. 8B displays the dQ/dV vs. voltage plot of the charge curve at the rate of 1 C, from which we can only observe the oxidation peak located at 3.2 V in the charge process, corresponding to the conversion from Fe2+ to Fe3+ . The poor rate performance and absence of the
oxidation of Fe3+ to Fe4+ are attributed to the poor electronic conductivity. After TVD treatment, the electronic conductivity and rate performance have been obviously improved, with the discharge capacity of 154, 140, 121 and 106 mAh g−1 at 1 C, 2 C, 5 C and 10 C, and the occurrence of the oxidation peak of Fe3+ to Fe4+ located at 4.3 V for LFS@C-2 (shown in Fig. 8 C and D). It can be found that the LFS@C-2 presents good rate capability, which is comparable to the results reported in the literatures, such as 110 [10] and 132.1 [15] mAh g−1 at 1 C, 80 [7,9] and 100 [16] mAh g−1 at 10 C. It is well known that the transformation from Fe3+ to Fe4+ is rather difficult due to the low conductivity of Li2 FeSiO4 , thus limiting the capacity lower than 166 mAh g−1 . However, owing to the elaborate design of Li2 FeSiO4 /C nanocomposite connected by the mutual cross-linked
Fig. 9. Rate performance (A) and cycling stability at 1 C (B) of as-prepared Li2 FeSiO4 /C nanocomposite: LFS@C-2 (a) and LFS@C (b).
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carbon conductive matrix with high conductivity, the conversion from Fe3+ to Fe4+ was achieved, which results in high capacities during charge/discharge cycles. Fig. 9A displays a pronounced difference on the rate performance of LFS@C and LFS@C-2. As can be seen, the LFS@C-2 nanocomposite presents noticeably superior rate performance than LFS@C. Furthermore, a capacity of 158 mAh g−1 can be retained when the current density reduced back to 1 C, indicating good cycling stability of the as-prepared nanocomposite after TVD treatment. The cycling performance for both samples at 1 C is displayed in Fig. 9B. After 100 cycles, the specific discharge capacity of the sample after TVD treatment is still stabilized around 158 mAh g−1 at 1 C, while 50 mAh g−1 for sample without TVD, definitely showing superior stability during cycling for both samples. It seems that the enhanced electronic conductivity is of central importance to explain the high-rate performance of Li2 FeSiO4 /C nanocomposite. The results presented here illustrate the benefits of the decreased particle size, coated carbon layer and mutual cross-linked carbon conductive matrix structure, which enables us to overcome the problem of poor electronic conductivity in the lithium metal silicate based cathode materials and achieve the aim of high rate capability and good cycling performance successfully. 4. Conclusions In summary, we report a novel Li2 FeSiO4 /C nanocomposite prepared via solvothermal method and in-situ carbon coating technology. The prepared Li2 FeSiO4 /C nanocomposite has a P21 /n structure without impurities. The particles are approximately 30 nm with a coating carbon layer and connected by the mutual cross-linked carbon conductive matrix. As cathode material, the Li2 FeSiO4 /C nanocomposite displays outstanding electrochemical performance with good rate performance and cycling stability. It delivers a discharge capacity of 106 mAh g−1 at the rate of 10 C, and with capacity retention of over 100 cycles. This is attributed to the short lithium ion diffusion path and enhanced electronic conductivity caused by the small particle size and the mutual cross-linked carbon conductive matrix. Therefore, we believe that electrode materials based on the mutual cross-linked carbon conductive matrix should be one of ideal candidates for expansion of Li ion battery technology in automobile, aerospace, and power grid applications which demand the development of light-weight, long-lasting batteries. Acknowledgements This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903), the National Natural Science Foundation of China (20925312), and Shanghai Science Technology Committee (13JC1407900).
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