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C doped with MoO3 by a solution method
Solid State Ionics 177 (2006) 3309 – 3314 www.elsevier.com/locate/ssi
The preparation and characterization of olivine LiFePO4/C doped with MoO3 by a solution method Ming Zhang, Li-Fang Jiao, Hua-Tang Yuan ⁎, Yong-Mei Wang, Jian Guo, Ming Zhao, Wei Wang, Xing-Di Zhou Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China Received 1 May 2006; received in revised form 26 August 2006; accepted 8 September 2006
Abstract Composite Li0.99Mo0.01FePO4/C cathode materials were prepared by an easy solution method followed by heat-treating at various temperatures. XRD, SEM, TGA/DTA, EA, CV, XPS and charge–discharge cycles were used to evaluate the Li0.99Mo0.01FePO4/C composite powders. The results indicate that mix-doping method does not affect the olivine structure of the cathode but considerably improves its capacity delivery and cycle performance. Among the prepared cathode materials, the sample heat-treated at 700 °C for 12 h shows best electrochemical performances. It shows initial specific discharge capacities of 161 and 124 mAh g− 1 with C rates of 0.2C and 2C, respectively, which is ascribed to the enhancement of the electronic conductivity by ion doping and carbon coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium ion battery; Lithium iron phosphate; Electrochemical properties; Mix-doping; Cathode materials
1. Introduction The continuously increasing demand for higher performance and cheaper rechargeable batteries for different electronic devices during the last decade has forced researchers to study new classes of materials for replacement of the conventional Liion and NiMH materials. Among the various materials suitable for use as cathodes in Li-ion batteries, LiFePO4 (LFP) has recently attracted significant interest because of its low cost, low hygroscopicity and environmentally friendly components. However, some problems such as poor rate performance and poor conductivity limited its practical application in high power density batteries. In order to solve these disadvantages, much effort has been paid. Carbon coating is an efficient way to increase the electrochemical performance of these materials [1,2], as well as to avoid formation of the Fe3+ oxidation state [3–5]. However, the carbon coating method obviously helps nothing in the lattice electronic conductivity or chemical diffusion coefficient of lithium within the crystal [6]. In recent ⁎ Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H.-T. Yuan). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.09.009
years, researchers have found that substitution of Li+ or Fe2+ with supervalent cations such as Ti4+, Nb5+, and W6+ can result in increased intrinsic electronic conductivity of LFP [7–9]. Both methods greatly improve kinetics of materials in terms of capacity delivery, cycle life and rate capability. The traditional synthesis method of LiFePO4 is direct solidstate reaction of precursors at high temperature for a long time. But in recent years, wet methods that can offer many advantages such as better homogeneity, regular morphology, sub-micron sized particles, and large surface area are used [10–12]. Considerable improvement in the performance of cathode materials has been accomplished. On the basis of previous studies, Mo-doped LiFePO4/C composite powders were synthesized with an easy solution process in which formation of conductive carbon and metal doping were achieved simultaneously. The improved electrochemical performances will be demonstrated in this paper. 2. Experimental Li0.99Mo0.01FePO4/C samples were synthesized by a solution method. According to the stoichiometry, FeC2O4·2H2O
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M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314
(99.9%, BODI), LiNO3 (99.9%, BODI), MoO3 (99.9%, BODI), and (NH4)2HPO4 (99.9%, BODI) were mixed in an aqueous solution. After the starting materials were mixed, adequate amount of sucrose (99.9%, BODI) was added to the solution. The mixtures were heated gently with continuous stirring for several hours to evaporate water, and then mixed by ball-milling for 3 h. Thermal decomposition and crystallization temperatures of the resulting powder were investigated by TGA/DTA. The solid residue was calcined at 300 °C for 5 h, then sintered at 500 °C, 600 °C, 700 °C, and 800 °C for 12 h in flowing argon (99.9%), respectively. All the ultra powders obtained after the high temperature calcination was collected and subjected further to systematic characterization studies. For comparison, LiFePO4 was synthesized by the same way except sucrose and MoO3 were not added in the raw materials. A simultaneous thermogravimetric-differential thermal analysis (TG-DTA) apparatus SDT 2960 (TA instrument) was used for the thermal characterization. X-ray diffraction measurements of the as-prepared Li0.99Mo0.01FePO4 materials were carried out using X-ray diffraction (D/Max-2500) with Cu Kα radiation at room temperature. Particle morphology of the powders after calcination was observed using a scanning electron microscope (SEM, JSM 6400, JEOL, Japan). Power Xray photoelectron spectra (XPS)(AXIS ULTRA DLD produced by Kratos company and Shimadu company, with monochromatic MgKα radiation (hv = 1253.6 eV)) was done to determine the valence of Mo. Charge referencing was done against the binging energy (BE) of adventitious carbon (C 1s = 284.6 eV). The spectra were analyzed by the XPS Peak fit software. The composite positive electrodes were prepared by pressing a mixture of the active materials, conductive material (acetylene black) and binder (PTFE) in a weight ratio of 85/10/5. The Li metal was used as the counter and reference electrodes. The electrolyte was 1 M LiPF6 in a 6/3/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC). The cells were assembled in an argon-filled dry box. Charge–discharge tests were performed between 2.0 and 4.5 V. Cyclic voltammetry experiments were performed using a CHI660 Electrochemical Workstation at a
Fig. 2. X-ray diffraction patterns for Li0.99Mo0.01FePO4/ C sintered at 500, 600, 700 and 800 °C, respectively.
scan rate of 0.1 mV s− 1. All tests were performed at ambient temperature. 3. Results and discussion The DTA/TGA results of the precursor performed under flowing argon are shown as Fig. 1. The broaden endothermic peaks attributed to water evaporation and dehydration of sucrose are found at temperatures between 60 and 170 °C, the endothermic peaks at 260 ° caused by nitrate decomposition are also observed. Whereas the solely petite exothermic peak exhibited at 480 °C in the DTA curve under flowing argon is due to the crystallization of LiFePO4 [13,14]. There are no other impurity peaks such as Li3Fe2 (PO4)3 existing because of the argon atmosphere which can avoid oxidation of Fe2+. The weight loss at temperature between 60 and 140 °C is due to water vaporization and dehydration of sucrose, and the steep weight loss at temperatures between 150 and 230 °C is caused by the decomposition of nitrate. The organic materials that residual ammonia and hydrolysis-sugar were also decomposed and carbonized before 350 °C [15]. No more weight loss was found after temperature higher than 700 °C, whereas carbon was lost gradually by sublimation in the sample placed under flowing argon [16]. The results from the XRD analysis showed in Fig. 2 indicate all samples are in good agreement with standard LiFePO4 with an ordered olivine structure indexed by orthorhombic Pnmb. No impurity phases were detected by X-ray diffraction. It is found that olivine phase formed in the sample heat-treated at 500 °C that is in consistent with the DTA/TGA results showed in Fig. 1. As the calcination temperature increased, the diffraction peaks corresponding to the olivine structure become prominent, and an enhanced degree of crystallinity and better phase purity of samples have been realized, which are evident from the sharp and symmetric diffractograms of increased intensity. The crystalline intensity of samples prepared at 500 and 600 °C is
Table 1 Cell parameters of LiFePO4 and Li0.99Mo0.01FePO4/C sintered at 700 °C Fig. 1. The DTA/TGA curves for the precursor recorded over the temperature range from ambient to 800 °C at a heating rate of 5 °C min− 1 in Ar atmosphere at 20 ml min− 1 flow rate.
Sample
a/nm
b/nm
c/nm
V/nm3
LiFePO4 Li0.99Mo0.01FePO4/C
0.6006 0.6005
1.0321 1.0319
0.4685 0.4679
0.2904 0.2899
M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314
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Fig. 3. SEM photographs of Li0.99Mo0.01FePO4/C powders prepared at (a)500 °C, (b)600 °C, (c)700 °C, (d)800 °C.
less developed than that of the samples prepared at 700 and 800 °C, which means the samples are not well crystallized at low temperature. The carbon is in amorphous form, as there are no additional peaks on the XRD patterns belonging to its crystal modification, it is believed that the effect of carbon coating has restricted the undesirable conversion of iron and hence resulted in the phase pure formation of samples [17]. The results of element analysis (EA) for prepared samples also reveal that carbon content decreases from 15.4% to 7.3% with increasing heat-treatment temperature from 500 to 800 °C. The lattice parameters of the typical samples calcined at 700 °C are showed in Table 1. It is observed that the parameter a, b, c, and the crystal cell volume V decrease because the smaller Mo6+ occupies the Li+ or Fe2+ site in the crystal, inducing the shrinkage of the crystal cell. Although the site occupancy of the dopant Mo6+ has yet to be established, we believe that Mo6+ tends to occupy the Li+ site due to its smaller ionic radius in
octahedral coordination than Fe2+, and the dopants do not affect the olivine structure of samples [18]. The SEM microstructural observations (Fig. 3) reveal the microstructure of LFP powders with different calcination temperatures. Obviously, increasing the calcination temperature leads to crystal growth. The majority of materials is not well crystallized calcined at 500 °C. The powder prepared at 600 °C partially contains agglomerates of small particles due to the insufficient sintering. This result is similar to the finding by Takahashi and Hyung-Sun Kim [19,20]. By contrast, the particle size of the compound calcined at 700 °C ranges from 2 to 3 μm, and the morphologies have a well-shaped, smoother crystal with sharp edges morphology. When the temperature reaches 800 °C, the morphology of the sample is obviously particle-agglomerated, and the primary particle size ranges from 4 to 5 μm, nearly twice as large as that of sample at 700 °C. For LiFePO4, small particle size and well-shaped crystal are
Fig. 4. The results of the capacity retention studies which were performed with 0.2C rate and cutoff voltages of 2.0 and 4.5 V for the powders heat-treated at various temperatures.
Fig. 5. Results of the capacity retention studies performed with various C rates for the powder Li0.99Mo0.01FePO4/C prepared at 700 °C. The cells were cycled within the cutoff voltages of 2.0 and 4.5 V.
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Fig. 6. Results of the capacity retention studies performed with various C rates for LiFePO4 prepared at 700 °C. The cells were cycled within the cutoff voltages of 2.0 and 4.5V.
Fig. 8. First cyclic voltammetry profiles of Li0.99Mo0.01FePO4/C, Li0.99Mo0.01 FePO4 and LiFePO4 (700 °C).
important for enhancing the electrochemical properties [21]. Therefore, these results indicate that 700 °C could give more advantage in terms of crystallinity and suitably crystal size to materials. The results of the capacity retention study for the cells prepared with powders heat-treated at various temperatures are shown in Fig. 4. These experiments were accomplished by cycling the cells with 0.2C (1C rate taken to be 150 mA g − 1) rate between 2.0 and 4.5 V. The 700 °C prepared Li0.99Mo0.01FePO4/C sample manifests the best cycling performance among the 12 h heat-treated powders with initial specific capacity of 161 mAh g− 1, which is near the theoretical capacity of LiFePO4, and the capacity fading is neglectable. As for Li0.99Mo0.01FePO4/C composites calcined at 600 and 800 °C, the initial specific capacity is 136 and 146. This may be attributed to the fact that 700 °C prepared powder has higher crystallinity than those of the samples prepared at lower temperatures and has smaller particle size than that of 800 °C sample. This is consistent with the XRD results and analysis of SEM, and in agreement with the suggestion given by Yamada et al. that the LiFePO4 have to be heated at an appropriate temperature to prevent the undesirable particle growth and the presence of a noncrystalline phase for better cycling performance [21]. As pointed out by Franger et al. [22], the specific capacity is very dependant on the particle size. The reduction in grain size can be related to the fact that the carbon particles, uniformly distributed between the starting materials, can interfere with the coalescence of the grains [20].
From the results of the capacity retention study performed with various C rates, shown in Figs. 5 and 6, it is found that the initial specific discharge capacity decreases with increasing C rate. The 700 °C prepared Li0.99Mo0.01FePO4/C cathode exhibits a higher capacity of 161 mAh g− 1 for the first cycle at 0.2C (152, 138 and 124 mAh g− 1 for 0.5C, 1.0C and 2.0C, respectively). Although the specific capacity decreases with cycling more or less, it is easily found that the reversible capacity of the Li0.99Mo0.01FePO4/C composite is much higher than the LiFePO4 at high charge–discharge rates. For example, The discharge capacity of the Li0.99Mo0.01FePO4/C composite is 124 mAh g− 1 at 2C, and it reaches 107 mAh g− 1 after 50 cycles with a capacity loss of 13% while it decreases to 70 mAh g− 1 with a capacity loss as high as 23% for LiFePO4 whose first discharge capacity is 91 mAh g− 1at 2C. It is very clear that the mix-doping method not only increases the specific capacity but also enhances the stability of materials. Fig. 7 shows the initial voltage profiles of the pure and the mix-doped lithium iron phosphates calcined at 700 °C (0.2C). The two samples have the similar charge–discharge curves with flat plateaus corresponding to the lithium deintercalation and intercalation reactions but they vary in plateau voltages. The charge voltage plateau is 3.53–3.57 V for LiFePO4 and 3.48– 3.50 V for Li0.99Mo0.01FePO4/C composites, while the discharge voltage plateau is 3.31–3.33 V for LiFePO4 and 3.37–3.40 V for Li0.99Mo0.01FePO4/C composites. The lower electrochemical polarization of the doped sample suggests that
Fig. 7. The initial voltage profiles of LiFePO4 and Li0.99Mo0.01FePO4/C (700 °C).
Fig. 9. XPS spectra of Mo of discharged-Li0.99Mo0.01FePO4.
M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314 Table 2 Electronic conductivity of prepared samples (700 °C) Samples
K (S cm− 1)
LiFePO4 Li0.99Mo0.01FePO4 Li0.99Mo0.01FePO4/C
2.5 × 10− 9 3.4 × 10− 7 4.6 × 10− 5
the increased conductivity is induced by the doping method. Two possible conducting mechanisms may be involved. The first probable mechanism, as Chung et al. assumed [8], is p-type conduction by the holes generated at the top of the bulk valence Fe–O bands by the activation of the electrons to the empty impurity Mo states. The second probable mechanism is that the doped Mo6+, the vacancies on Li sites, and their neighboring Fe and O ion form a conducting cluster [23]. In addition, the residual carbon resulted from the decomposition of sucrose acts as nucleation site for the formation of Li0.99Mo0.01FePO4 crystals, helping in obtaining samples with uniform size. The dispersed carbon particles also promote the electrochemical reaction by enhancing the surface electronic conduction. Cyclic voltammetry were performed in order to investigate the effect of Li-site doping on the electrochemical properties of LiFePO4 by using a scanning rate of 0.1 mVs−1. The CV profiles of the typical doped samples LiFePO4, Li0.99Mo0.01FePO4 and Li0.99Mo0.01FePO4/C of 700 °C in the first cycle are shown in Fig. 8. They both exhibit a pair of redox peaks around 3.4 V vs. Li+/Li, but the peak profiles of Li0.99Mo0.01FePO4/C are more symmetric and spiculate. As shown in Fig. 8, Li0.99Mo0.01FePO4 exhibits an anodic peak at 3.53 V and a corresponding cathodic response at 3.31 V. The potential interval of Li0.99Mo0.01FePO4 was 0.22 V, whereas that of LFP was 0.28 V and 0.17 V for Li0.99Mo0.01FePO4/C. As for cyclic voltammogram, the potential interval between anodic peak and cathodic peak is an important parameter to value the electrochemical reaction reversibility [7]. The well-defined peaks and smaller value of potential interval show the enhancement of electrode reaction reversibility by mix-doped. Mo XPS core spectra for discharged-Li0.99Mo0.01FePO4 (700 °C) material is showed in Fig. 9. The Mo spectra appears complicated, which can be deconvoluted into four well-defined contributions. The four peaks, centered at 235.6 eV, 232.3 eV, 233.8 eV and 230.2 eV, correspond to Mo6+ (3d3/2), Mo6+ (3d5/2), Mo4+ (3d3/2) and Mo4+ (3d5/2) [24–27], respectively. It reveals that the Mo6+ ions are partly changed into Mo4+ ions after the material discharged. The data suggest that the doped samples clearly exhibit enhanced capacity during cycling, not only because the doped samples allow Li+ ions to be extracted more [28], but also because the reduction of Mo6+ to Mo4+ increases the resulting capacity. For Li0.99Mo0.01FePO4/C, in which both the lattice doping element (Mo6+) and non-lattice doping element (C) were used simultaneously, we cannot determine which aspect, i.e., the lattice doping or non-lattice doping is of the first magnitude for the improved capability, but we believe they both help to the better electrochemical performances of the doped composite. The electronic conductivities of all prepared samples calcined at 700 °C are measured with the four-electrode method, and the
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data are listed in Table 2. The conductivity of LiFePO4 is 2.5 × 10− 9 S cm− 1, at the same order of other researchers' result [3,18]. Li0.99Mo0.01FePO4/C shows the highest electronic conductivity among all samples, about a factor of ∼ 104 higher than the conductivity of LiFePO4. The electronic conductivity of Li0.99Mo0.01FePO4 is two orders of magnitude higher than that of undoped sample, reaching to about 10− 7 S cm− 1. It is obvious that doped LiFePO4 exhibits better electrochemical properties, which can be interpreted as the enhancement of electronic conductivity by carbon coating and ion doping. Since the individual LiFePO4 particles are connected by carbon, the active materials can be fully utilized for lithium extraction and insertion reactions. In addition, according to the mechanism proposed by Chung et al. and extensively discussed by Wang et al. [8,29], the ion dopant mainly takes the site of Li, leading the coexistence of Fe2+ and Fe3+ in single phase, and thus improving crystal electronic conductivity apparently. It must be pointed out that the conductive carbon and metal ions were used simultaneously in our work, they both contribute to the improvement of electrochemical performance. If the optimal amount of doped ions and carbon can be investigated deeply and the relationship between them can be known clearly, better electrochemical properties of LiFePO4 could be obtained. Further work is underway to understand the mechanism. 4. Conclusions The Olivine Li0.99Mo0.01FePO4/C composite powders were successfully synthesized by the solution method. Comparing to undoped LiFePO4, the sample heat-treated at 700 °C for 12 h shows better electrochemical properties in terms of capacity delivery and electrochemical reversibility. Both the lattice doping element (Mo6+) and the non-lattice doping element (C) help to the improvement of electrochemical performances. The excellent performance indicates that this mix-doped composite is a very promising cathode material for lithium ion batteries. Acknowledgements This work was supported by NSFC (20673062), TSTC (06YFJMJC04900) and Tianjin-Nankai Union funds. References [1] H. Huang, S. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) 170. [2] S. Myung, S. Komaba, R. Takagai, et al., Chem. Lett. 7 (2003) 566. [3] J. Baker, M.Y. Saidi, J.L. Swoyer, Electrochem. Solid-State Lett. 6 (2003) 53. [4] S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid-State Lett. 5 (2002) A231. [5] Z. Chen, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A1184. [6] H. Xie, Z.T. Zhou, Electrochim. Acta 51 (2006) 2063. [7] J.F. Ni, H.H. Zhou, J.T. Chen, X.X. Zhang, Mater. Lett. 59 (2005) 2361. [8] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Nat. Mater 1 (2002) 123. [9] S.Y. Chung, Y.M. Chiang, Electrochem. Solid-State Lett. 6 (12) (2003) A278. [10] A.D. Spong, G. Vitins, J.R. Owenz, J. Electrochem. Soc. 152 (12) (2005) A2376.
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[11] F. Croce, A.D. Epifanio, J. Hassoun, et al., Electrochem. Solid-State Lett. 5 (3) (2002) A47. [12] K. Striebel, A. Guerfi, J. Shim, et al., J. Power Sources 119–121 (2003) 951. [13] P.P. Prosini, M. Carewska, S. Scaccia, P. Wisniewski, S. Passerini, M. Pasquali, J. Electrochem. Soc. 149 (2002) 886. [14] S.H. Wu, K.M. Hsiao, W.R. Liu, J. Power Sources 146 (2005) 550. [15] M.R. Yang, W.H. Ke, S.H. Wu, J. Power Sources 146 (2005) 539. [16] J. Barker, M.Y. Saidi, J.L. Swoyer, Electrochem. Solid-State Lett. 6 (2003) 53. [17] H.S. Kim, B.W. Cho, W.I. Cho, J. Power Sources 132 (2004) 235. [18] D.Y. Wang, H. Li, S.Q. Shi, X.J. Huang, L.Q. Chen, Electrochim. Acta 50 (2005) 2955. [19] M. Takahashi, S. Tobishima, K. Takei, Y. Sakurai, Solid State Ion. 148 (2002) 283. [20] H.S. Kim, B.W. Cho, W.I. Cho, J. Power Sources 132 (2004) 235.
[21] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) 224. [22] S. Franger, F.L. Cras, C. Bourbon, H. Rouault, J. Power Sources 119–121 (2003) 252. [23] S.Q. Shi, L.J. Liu, C.Y. Yang, D.S. Wang, Z.X. Wang, L.Q. Chen, X.J. Huang, Phys. Rev., B 68 (2003) 195108. [24] T. Hirata, K. Yagisawa, M. Okochi, A. Yoshikawa, J. Mater. Sci. Lett. 8 (1989) 1089. [25] K.S. Kim, W.E. Baitinger, J.W. Amy, N.J. Winograd, J. Electron Spectrosc. Relat. Phenom. 5 (1974) 351. [26] B. Brox, I. Olefjord, Surf. Interface Anal. 13 (1988) 3. [27] W.E. Swartz, D.M. Hercules, Anal. Chem. 43 (1971) 1774. [28] S.H. Park, S.W. Oh, Y.K. Sun, J. Power Sources 146 (2005) 622. [29] G.X. Wang, S.L. Bewlay, K. Konstantinov, H.K. Liu, S.X. Dou, J.H. Ahn, Electrochim. Acta 50 (2004) 443.
The preparation and characterization of olivine LiFePO4/C doped with MoO3 by a solution method Ming Zhang, Li-Fang Jiao, Hua-Tang Yuan ⁎, Yong-Mei Wang, Jian Guo, Ming Zhao, Wei Wang, Xing-Di Zhou Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China Received 1 May 2006; received in revised form 26 August 2006; accepted 8 September 2006
Abstract Composite Li0.99Mo0.01FePO4/C cathode materials were prepared by an easy solution method followed by heat-treating at various temperatures. XRD, SEM, TGA/DTA, EA, CV, XPS and charge–discharge cycles were used to evaluate the Li0.99Mo0.01FePO4/C composite powders. The results indicate that mix-doping method does not affect the olivine structure of the cathode but considerably improves its capacity delivery and cycle performance. Among the prepared cathode materials, the sample heat-treated at 700 °C for 12 h shows best electrochemical performances. It shows initial specific discharge capacities of 161 and 124 mAh g− 1 with C rates of 0.2C and 2C, respectively, which is ascribed to the enhancement of the electronic conductivity by ion doping and carbon coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium ion battery; Lithium iron phosphate; Electrochemical properties; Mix-doping; Cathode materials
1. Introduction The continuously increasing demand for higher performance and cheaper rechargeable batteries for different electronic devices during the last decade has forced researchers to study new classes of materials for replacement of the conventional Liion and NiMH materials. Among the various materials suitable for use as cathodes in Li-ion batteries, LiFePO4 (LFP) has recently attracted significant interest because of its low cost, low hygroscopicity and environmentally friendly components. However, some problems such as poor rate performance and poor conductivity limited its practical application in high power density batteries. In order to solve these disadvantages, much effort has been paid. Carbon coating is an efficient way to increase the electrochemical performance of these materials [1,2], as well as to avoid formation of the Fe3+ oxidation state [3–5]. However, the carbon coating method obviously helps nothing in the lattice electronic conductivity or chemical diffusion coefficient of lithium within the crystal [6]. In recent ⁎ Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H.-T. Yuan). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.09.009
years, researchers have found that substitution of Li+ or Fe2+ with supervalent cations such as Ti4+, Nb5+, and W6+ can result in increased intrinsic electronic conductivity of LFP [7–9]. Both methods greatly improve kinetics of materials in terms of capacity delivery, cycle life and rate capability. The traditional synthesis method of LiFePO4 is direct solidstate reaction of precursors at high temperature for a long time. But in recent years, wet methods that can offer many advantages such as better homogeneity, regular morphology, sub-micron sized particles, and large surface area are used [10–12]. Considerable improvement in the performance of cathode materials has been accomplished. On the basis of previous studies, Mo-doped LiFePO4/C composite powders were synthesized with an easy solution process in which formation of conductive carbon and metal doping were achieved simultaneously. The improved electrochemical performances will be demonstrated in this paper. 2. Experimental Li0.99Mo0.01FePO4/C samples were synthesized by a solution method. According to the stoichiometry, FeC2O4·2H2O
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M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314
(99.9%, BODI), LiNO3 (99.9%, BODI), MoO3 (99.9%, BODI), and (NH4)2HPO4 (99.9%, BODI) were mixed in an aqueous solution. After the starting materials were mixed, adequate amount of sucrose (99.9%, BODI) was added to the solution. The mixtures were heated gently with continuous stirring for several hours to evaporate water, and then mixed by ball-milling for 3 h. Thermal decomposition and crystallization temperatures of the resulting powder were investigated by TGA/DTA. The solid residue was calcined at 300 °C for 5 h, then sintered at 500 °C, 600 °C, 700 °C, and 800 °C for 12 h in flowing argon (99.9%), respectively. All the ultra powders obtained after the high temperature calcination was collected and subjected further to systematic characterization studies. For comparison, LiFePO4 was synthesized by the same way except sucrose and MoO3 were not added in the raw materials. A simultaneous thermogravimetric-differential thermal analysis (TG-DTA) apparatus SDT 2960 (TA instrument) was used for the thermal characterization. X-ray diffraction measurements of the as-prepared Li0.99Mo0.01FePO4 materials were carried out using X-ray diffraction (D/Max-2500) with Cu Kα radiation at room temperature. Particle morphology of the powders after calcination was observed using a scanning electron microscope (SEM, JSM 6400, JEOL, Japan). Power Xray photoelectron spectra (XPS)(AXIS ULTRA DLD produced by Kratos company and Shimadu company, with monochromatic MgKα radiation (hv = 1253.6 eV)) was done to determine the valence of Mo. Charge referencing was done against the binging energy (BE) of adventitious carbon (C 1s = 284.6 eV). The spectra were analyzed by the XPS Peak fit software. The composite positive electrodes were prepared by pressing a mixture of the active materials, conductive material (acetylene black) and binder (PTFE) in a weight ratio of 85/10/5. The Li metal was used as the counter and reference electrodes. The electrolyte was 1 M LiPF6 in a 6/3/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC). The cells were assembled in an argon-filled dry box. Charge–discharge tests were performed between 2.0 and 4.5 V. Cyclic voltammetry experiments were performed using a CHI660 Electrochemical Workstation at a
Fig. 2. X-ray diffraction patterns for Li0.99Mo0.01FePO4/ C sintered at 500, 600, 700 and 800 °C, respectively.
scan rate of 0.1 mV s− 1. All tests were performed at ambient temperature. 3. Results and discussion The DTA/TGA results of the precursor performed under flowing argon are shown as Fig. 1. The broaden endothermic peaks attributed to water evaporation and dehydration of sucrose are found at temperatures between 60 and 170 °C, the endothermic peaks at 260 ° caused by nitrate decomposition are also observed. Whereas the solely petite exothermic peak exhibited at 480 °C in the DTA curve under flowing argon is due to the crystallization of LiFePO4 [13,14]. There are no other impurity peaks such as Li3Fe2 (PO4)3 existing because of the argon atmosphere which can avoid oxidation of Fe2+. The weight loss at temperature between 60 and 140 °C is due to water vaporization and dehydration of sucrose, and the steep weight loss at temperatures between 150 and 230 °C is caused by the decomposition of nitrate. The organic materials that residual ammonia and hydrolysis-sugar were also decomposed and carbonized before 350 °C [15]. No more weight loss was found after temperature higher than 700 °C, whereas carbon was lost gradually by sublimation in the sample placed under flowing argon [16]. The results from the XRD analysis showed in Fig. 2 indicate all samples are in good agreement with standard LiFePO4 with an ordered olivine structure indexed by orthorhombic Pnmb. No impurity phases were detected by X-ray diffraction. It is found that olivine phase formed in the sample heat-treated at 500 °C that is in consistent with the DTA/TGA results showed in Fig. 1. As the calcination temperature increased, the diffraction peaks corresponding to the olivine structure become prominent, and an enhanced degree of crystallinity and better phase purity of samples have been realized, which are evident from the sharp and symmetric diffractograms of increased intensity. The crystalline intensity of samples prepared at 500 and 600 °C is
Table 1 Cell parameters of LiFePO4 and Li0.99Mo0.01FePO4/C sintered at 700 °C Fig. 1. The DTA/TGA curves for the precursor recorded over the temperature range from ambient to 800 °C at a heating rate of 5 °C min− 1 in Ar atmosphere at 20 ml min− 1 flow rate.
Sample
a/nm
b/nm
c/nm
V/nm3
LiFePO4 Li0.99Mo0.01FePO4/C
0.6006 0.6005
1.0321 1.0319
0.4685 0.4679
0.2904 0.2899
M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314
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Fig. 3. SEM photographs of Li0.99Mo0.01FePO4/C powders prepared at (a)500 °C, (b)600 °C, (c)700 °C, (d)800 °C.
less developed than that of the samples prepared at 700 and 800 °C, which means the samples are not well crystallized at low temperature. The carbon is in amorphous form, as there are no additional peaks on the XRD patterns belonging to its crystal modification, it is believed that the effect of carbon coating has restricted the undesirable conversion of iron and hence resulted in the phase pure formation of samples [17]. The results of element analysis (EA) for prepared samples also reveal that carbon content decreases from 15.4% to 7.3% with increasing heat-treatment temperature from 500 to 800 °C. The lattice parameters of the typical samples calcined at 700 °C are showed in Table 1. It is observed that the parameter a, b, c, and the crystal cell volume V decrease because the smaller Mo6+ occupies the Li+ or Fe2+ site in the crystal, inducing the shrinkage of the crystal cell. Although the site occupancy of the dopant Mo6+ has yet to be established, we believe that Mo6+ tends to occupy the Li+ site due to its smaller ionic radius in
octahedral coordination than Fe2+, and the dopants do not affect the olivine structure of samples [18]. The SEM microstructural observations (Fig. 3) reveal the microstructure of LFP powders with different calcination temperatures. Obviously, increasing the calcination temperature leads to crystal growth. The majority of materials is not well crystallized calcined at 500 °C. The powder prepared at 600 °C partially contains agglomerates of small particles due to the insufficient sintering. This result is similar to the finding by Takahashi and Hyung-Sun Kim [19,20]. By contrast, the particle size of the compound calcined at 700 °C ranges from 2 to 3 μm, and the morphologies have a well-shaped, smoother crystal with sharp edges morphology. When the temperature reaches 800 °C, the morphology of the sample is obviously particle-agglomerated, and the primary particle size ranges from 4 to 5 μm, nearly twice as large as that of sample at 700 °C. For LiFePO4, small particle size and well-shaped crystal are
Fig. 4. The results of the capacity retention studies which were performed with 0.2C rate and cutoff voltages of 2.0 and 4.5 V for the powders heat-treated at various temperatures.
Fig. 5. Results of the capacity retention studies performed with various C rates for the powder Li0.99Mo0.01FePO4/C prepared at 700 °C. The cells were cycled within the cutoff voltages of 2.0 and 4.5 V.
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Fig. 6. Results of the capacity retention studies performed with various C rates for LiFePO4 prepared at 700 °C. The cells were cycled within the cutoff voltages of 2.0 and 4.5V.
Fig. 8. First cyclic voltammetry profiles of Li0.99Mo0.01FePO4/C, Li0.99Mo0.01 FePO4 and LiFePO4 (700 °C).
important for enhancing the electrochemical properties [21]. Therefore, these results indicate that 700 °C could give more advantage in terms of crystallinity and suitably crystal size to materials. The results of the capacity retention study for the cells prepared with powders heat-treated at various temperatures are shown in Fig. 4. These experiments were accomplished by cycling the cells with 0.2C (1C rate taken to be 150 mA g − 1) rate between 2.0 and 4.5 V. The 700 °C prepared Li0.99Mo0.01FePO4/C sample manifests the best cycling performance among the 12 h heat-treated powders with initial specific capacity of 161 mAh g− 1, which is near the theoretical capacity of LiFePO4, and the capacity fading is neglectable. As for Li0.99Mo0.01FePO4/C composites calcined at 600 and 800 °C, the initial specific capacity is 136 and 146. This may be attributed to the fact that 700 °C prepared powder has higher crystallinity than those of the samples prepared at lower temperatures and has smaller particle size than that of 800 °C sample. This is consistent with the XRD results and analysis of SEM, and in agreement with the suggestion given by Yamada et al. that the LiFePO4 have to be heated at an appropriate temperature to prevent the undesirable particle growth and the presence of a noncrystalline phase for better cycling performance [21]. As pointed out by Franger et al. [22], the specific capacity is very dependant on the particle size. The reduction in grain size can be related to the fact that the carbon particles, uniformly distributed between the starting materials, can interfere with the coalescence of the grains [20].
From the results of the capacity retention study performed with various C rates, shown in Figs. 5 and 6, it is found that the initial specific discharge capacity decreases with increasing C rate. The 700 °C prepared Li0.99Mo0.01FePO4/C cathode exhibits a higher capacity of 161 mAh g− 1 for the first cycle at 0.2C (152, 138 and 124 mAh g− 1 for 0.5C, 1.0C and 2.0C, respectively). Although the specific capacity decreases with cycling more or less, it is easily found that the reversible capacity of the Li0.99Mo0.01FePO4/C composite is much higher than the LiFePO4 at high charge–discharge rates. For example, The discharge capacity of the Li0.99Mo0.01FePO4/C composite is 124 mAh g− 1 at 2C, and it reaches 107 mAh g− 1 after 50 cycles with a capacity loss of 13% while it decreases to 70 mAh g− 1 with a capacity loss as high as 23% for LiFePO4 whose first discharge capacity is 91 mAh g− 1at 2C. It is very clear that the mix-doping method not only increases the specific capacity but also enhances the stability of materials. Fig. 7 shows the initial voltage profiles of the pure and the mix-doped lithium iron phosphates calcined at 700 °C (0.2C). The two samples have the similar charge–discharge curves with flat plateaus corresponding to the lithium deintercalation and intercalation reactions but they vary in plateau voltages. The charge voltage plateau is 3.53–3.57 V for LiFePO4 and 3.48– 3.50 V for Li0.99Mo0.01FePO4/C composites, while the discharge voltage plateau is 3.31–3.33 V for LiFePO4 and 3.37–3.40 V for Li0.99Mo0.01FePO4/C composites. The lower electrochemical polarization of the doped sample suggests that
Fig. 7. The initial voltage profiles of LiFePO4 and Li0.99Mo0.01FePO4/C (700 °C).
Fig. 9. XPS spectra of Mo of discharged-Li0.99Mo0.01FePO4.
M. Zhang et al. / Solid State Ionics 177 (2006) 3309–3314 Table 2 Electronic conductivity of prepared samples (700 °C) Samples
K (S cm− 1)
LiFePO4 Li0.99Mo0.01FePO4 Li0.99Mo0.01FePO4/C
2.5 × 10− 9 3.4 × 10− 7 4.6 × 10− 5
the increased conductivity is induced by the doping method. Two possible conducting mechanisms may be involved. The first probable mechanism, as Chung et al. assumed [8], is p-type conduction by the holes generated at the top of the bulk valence Fe–O bands by the activation of the electrons to the empty impurity Mo states. The second probable mechanism is that the doped Mo6+, the vacancies on Li sites, and their neighboring Fe and O ion form a conducting cluster [23]. In addition, the residual carbon resulted from the decomposition of sucrose acts as nucleation site for the formation of Li0.99Mo0.01FePO4 crystals, helping in obtaining samples with uniform size. The dispersed carbon particles also promote the electrochemical reaction by enhancing the surface electronic conduction. Cyclic voltammetry were performed in order to investigate the effect of Li-site doping on the electrochemical properties of LiFePO4 by using a scanning rate of 0.1 mVs−1. The CV profiles of the typical doped samples LiFePO4, Li0.99Mo0.01FePO4 and Li0.99Mo0.01FePO4/C of 700 °C in the first cycle are shown in Fig. 8. They both exhibit a pair of redox peaks around 3.4 V vs. Li+/Li, but the peak profiles of Li0.99Mo0.01FePO4/C are more symmetric and spiculate. As shown in Fig. 8, Li0.99Mo0.01FePO4 exhibits an anodic peak at 3.53 V and a corresponding cathodic response at 3.31 V. The potential interval of Li0.99Mo0.01FePO4 was 0.22 V, whereas that of LFP was 0.28 V and 0.17 V for Li0.99Mo0.01FePO4/C. As for cyclic voltammogram, the potential interval between anodic peak and cathodic peak is an important parameter to value the electrochemical reaction reversibility [7]. The well-defined peaks and smaller value of potential interval show the enhancement of electrode reaction reversibility by mix-doped. Mo XPS core spectra for discharged-Li0.99Mo0.01FePO4 (700 °C) material is showed in Fig. 9. The Mo spectra appears complicated, which can be deconvoluted into four well-defined contributions. The four peaks, centered at 235.6 eV, 232.3 eV, 233.8 eV and 230.2 eV, correspond to Mo6+ (3d3/2), Mo6+ (3d5/2), Mo4+ (3d3/2) and Mo4+ (3d5/2) [24–27], respectively. It reveals that the Mo6+ ions are partly changed into Mo4+ ions after the material discharged. The data suggest that the doped samples clearly exhibit enhanced capacity during cycling, not only because the doped samples allow Li+ ions to be extracted more [28], but also because the reduction of Mo6+ to Mo4+ increases the resulting capacity. For Li0.99Mo0.01FePO4/C, in which both the lattice doping element (Mo6+) and non-lattice doping element (C) were used simultaneously, we cannot determine which aspect, i.e., the lattice doping or non-lattice doping is of the first magnitude for the improved capability, but we believe they both help to the better electrochemical performances of the doped composite. The electronic conductivities of all prepared samples calcined at 700 °C are measured with the four-electrode method, and the
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data are listed in Table 2. The conductivity of LiFePO4 is 2.5 × 10− 9 S cm− 1, at the same order of other researchers' result [3,18]. Li0.99Mo0.01FePO4/C shows the highest electronic conductivity among all samples, about a factor of ∼ 104 higher than the conductivity of LiFePO4. The electronic conductivity of Li0.99Mo0.01FePO4 is two orders of magnitude higher than that of undoped sample, reaching to about 10− 7 S cm− 1. It is obvious that doped LiFePO4 exhibits better electrochemical properties, which can be interpreted as the enhancement of electronic conductivity by carbon coating and ion doping. Since the individual LiFePO4 particles are connected by carbon, the active materials can be fully utilized for lithium extraction and insertion reactions. In addition, according to the mechanism proposed by Chung et al. and extensively discussed by Wang et al. [8,29], the ion dopant mainly takes the site of Li, leading the coexistence of Fe2+ and Fe3+ in single phase, and thus improving crystal electronic conductivity apparently. It must be pointed out that the conductive carbon and metal ions were used simultaneously in our work, they both contribute to the improvement of electrochemical performance. If the optimal amount of doped ions and carbon can be investigated deeply and the relationship between them can be known clearly, better electrochemical properties of LiFePO4 could be obtained. Further work is underway to understand the mechanism. 4. Conclusions The Olivine Li0.99Mo0.01FePO4/C composite powders were successfully synthesized by the solution method. Comparing to undoped LiFePO4, the sample heat-treated at 700 °C for 12 h shows better electrochemical properties in terms of capacity delivery and electrochemical reversibility. Both the lattice doping element (Mo6+) and the non-lattice doping element (C) help to the improvement of electrochemical performances. The excellent performance indicates that this mix-doped composite is a very promising cathode material for lithium ion batteries. Acknowledgements This work was supported by NSFC (20673062), TSTC (06YFJMJC04900) and Tianjin-Nankai Union funds. References [1] H. Huang, S. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) 170. [2] S. Myung, S. Komaba, R. Takagai, et al., Chem. Lett. 7 (2003) 566. [3] J. Baker, M.Y. Saidi, J.L. Swoyer, Electrochem. Solid-State Lett. 6 (2003) 53. [4] S. Franger, F. Le Cras, C. Bourbon, H. Rouault, Electrochem. Solid-State Lett. 5 (2002) A231. [5] Z. Chen, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A1184. [6] H. Xie, Z.T. Zhou, Electrochim. Acta 51 (2006) 2063. [7] J.F. Ni, H.H. Zhou, J.T. Chen, X.X. Zhang, Mater. Lett. 59 (2005) 2361. [8] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Nat. Mater 1 (2002) 123. [9] S.Y. Chung, Y.M. Chiang, Electrochem. Solid-State Lett. 6 (12) (2003) A278. [10] A.D. Spong, G. Vitins, J.R. Owenz, J. Electrochem. Soc. 152 (12) (2005) A2376.
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