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nano-carbon webs composite cathode
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis and characterization of LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode Hui Liu a,b , Jingying Xie a,∗ a
Energy Science and Technology Laboratory, Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history:
a b s t r a c t LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode material was synthesized from
Received 4 August 2007
nanocrystalline Fe3 O4 through sol–gel process, followed by carbothermal reduction reac-
Received in revised form
tion in the presence of LiOH·H2 O and NH4 H2 PO4 . Sucrose molecule was introduced through
4 February 2008
liquid phase and used to construct carbon webs structure. FE-SEM and HR-TEM observa-
Accepted 11 February 2008
tions show that decomposed carbon can form typical carbon webs structure to connect and wrap LiFe0.9 Mg0.1 PO4 particles. Electrochemical tests indicated that the Mg-doping at 4c site of carbon-coated LiFePO4 did not affect the olivine structure of the lithium iron phosphate
Keywords:
but obviously improve its discharge capacity and rate capability, which would be ascribed to
LiFePO4
the increased electronic conductivity and/or the mobility of Li+ ion induced by the doping
Composite cathode
method. © 2008 Elsevier B.V. All rights reserved.
Doping Nano-carbon webs Li-ion batteries
1.
Introduction
Since the original work of Padhi et al. (1997), phospho-olivine LiMPO4 (M = Fe, Mn, Co, Ni) have appeared to be potential candidate for positive electrode materials for lithium-ion batteries. Yet in this family, LiFePO4 is currently the subject of many investigations because this material realizes the highest capacity (170 mAh g−1 ) at moderate current densities (Huang et al., 2001). In addition, it is inexpensive, nontoxic and high safety, three determinant advantages with respect to cobalt oxide-based materials for large-scaled applications such as hybrid electric vehicles. Nevertheless, the low intrinsic electronic conductivity and poor lithium ion diffusion result in losses of capacity during high-rate discharge. In order to solve these disadvantages, much effort has been paid by some groups, involving car-
∗
Corresponding author. Tel.: +86 21 62511070/8801. E-mail address: [email protected] (J. Xie). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.014
bon coating to improve the surface electronic conductivity of active particles (Dominko et al., 2003; Ravet et al., 2001; Bewlay et al., 2004; Wang et al., 2006a; Choi and Kumta, 2007) and supervalent cations doping in Li-site to enhance intrinsic electronic conductivity (Chung et al., 2002; Chung and Chiang, 2003). But the interpretation of the latter method was recently seriously questioned (Ravet et al., 2003) because carbon-containing precursors result in carbon-coated LiFePO4 and in the formation of conductive phases (iron phosphides and/or phosphocarbides) due to LiFePO4 reduction at the surface of the particles (Herle et al., 2004). In recent years, some researchers have also found that Fe-site doping may be an effective step to design robust industrial synthesis process (Barker et al., 2003; Wang et al., 2005; Islam et al., 2005; Hong et al., 2006; Mi et al., 2006; Wang et al., 2006b) in order to improve intrinsic electronic conductivity.
478
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
On the basis of above discussions, Mg-doped LiFePO4 /C composite powders were synthesized by a combination of sol–gel method and carbothermal reduction reaction, in which the formation of nano-carbon webs structure and metal doping were achieved simultaneously. Compared with LiFePO4 /C composite powders, the LiFe0.9 Mg0.1 PO4 /C sample exhibited improved electrochemical performances.
2.
Experimental
LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C samples were synthesized by a gel precursor preparation and carbothermal reduction reaction method. Stoichiometric Fe(NO3 )3 ·9H2 O (98%, A.R) and Mg(NO3 )2 ·9H2 O (98%, A.R) were added in 1.5 mol L−1 organic solution of 2-methoxyethanol (C3 H8 O2 , 98%, A.R), reacting at a constant magnetic stirring. Adequate amount of sucrose (98.5%, A.R) was dissolved in de-ionized water, and mixed with above solution with the C3 H8 O2 :H2 O volume ratio of 5:2. The solution was continuously heated and stirred for several hours to evaporate water and part of organic solvent, and then a rufous xerogel formed. The xerogel was calcined to form precursor powders at 400 ◦ C for 5 h in flowing argon (99.9%). The as-synthesized precursor powders were collected and mixed with stoichiometric amounts of LiOH·H2 O (98%, A.R) and NH4 H2 PO4 (98%, A.R), and subjected further to calcination at 725 ◦ C for 10 h. Cooled to room temperature, the LiFe0.9 Mg0.1 PO4 /C composite materials were obtained. For comparison, LiFePO4 /C composite powder was synthesized by the same way except that Mg(NO3 )2 ·9H2 O was not added in the raw materials. Phase structure of the powder was analyzed by X-ray diffraction (XRD) on a Rigaku-D/MAX-2200PC diffractometer using Cu K␣ radiation. The sample morphology was observed by a field-emission scanning electron microscope (FE-SEM) on a Hitachi S-4700 type of microscope and transmission electron microscope (TEM) on a JEOL high-resolution electron microscope (JEM-2010). Elemental carbon analysis in the obtained
LiFePO4 samples was measured by a C-S 800 Determinator (Eltar, Germany). Electrochemical characterization was performed by assembling CR2025 coin cells for galvanostatic charge/discharge. The electrodes were made by dispersing 80 wt.% active materials, 10 wt.% Super P, and 10 wt.% polyvinylidene fluoride (PVDF) in n-methyl pyrrolidone (NMP) to form a slurry. The slurry was then coated onto an Al foil. The coated electrodes were dried in a vacuum oven and then pressed at 1200 kg/cm2 . The coil cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) with a lithium foil as the counter electrode. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were galvanostatically charged and discharged between 2.5 and 4.2 V at ambient temperature on the electrochemical test instrument (CT2001A, Wuhang Land Electronic Co. Ltd., China). Cyclic voltammetry measurements were performed using electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) at a scan rate of 0.1 mV s−1 between 2.5 and 4.2 V. Electrochemical impedance spectroscopy (EIS) was measured with a frequency response analyzer (Solatron 1255B) interfaced with a potentiogalvanostat (Solatron 1287) controlled by a personal computer. The sinusoidal excitation voltage applied to the cells was 5 mV with a frequency range of between 0.01 Hz to 100 KHz. All potentials are cited in this paper with respect to the reference Li+ /Li. Currents and specific capacities were calculated based on the mass of LiFe0.9 Mg0.1 PO4 and LiFePO4 in the composite electrode.
3.
Results and discussion
When the nitrate, Fe(NO3 )3 ·9H2 O and Mg(NO3 )2 ·9H2 O, were mixed with 2-methoxyethanol, it would react to produce metal alkoxide (Mehrotra, 1988), respectively. The crystallization water in nitrate and strong electronegativity of alkoxy could make Fe(OCH2 CH2 OCH3 )3 hydrolyze and polymerize to form gel which was further dried to obtain the precursor xerogel. The XRD pattern of it with no peaks indicates completely amorphous state as seen in Fig. 1a.
Fig. 1 – X-ray diffraction patterns of (a) gel precursor, (b) Fe3 O4 , (c) LiFePO4 and (d) LiFe0.9 Mg0.1 PO4 samples; FE-SEM image of inset (a) nanocrystalline Fe3 O4 powders.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
After heating the amorphous xerogel at 400 ◦ C for 5 h in argon flow, the obtained powder was found to be a single phase Fe3 O4 as shown in Fig. 1b, indicating that there is no evidence of diffraction peaks of MgO due to its low content. Its average grain size estimated from the Scherrer law is about 36 nm. The morphology of Fe3 O4 is shown in inset (a) where individual particle size is much less than 50 nm that is close to the monocrystallite grain size of 30 nm, indicating the primary particles of Fe3 O4 are monocrystallites. Fig. 1c and d shows the XRD patterns of the pure LiFePO4 and doped LiFePO4 samples, respectively. The carbon is in amorphous form, as there are no obvious impurities such as Li3 PO4 and Fe2 P (Herle et al., 2004; Yamada et al., 2001; Franger et al., 2002), indexed by orthorhombic Pnma with an ordered olivine structure. The ˚ cell parameters calculated by the XRD results are a = 10.330 A, ˚ ˚ ˚ b = 6.007 A, and c = 4.689 A for LiFePO4 phase, and a = 10.312 A, ˚ and c = 4.686 A ˚ for LiFe0.9 Mg0.1 PO4 phase. The slight b = 5.998 A, change in the cell size indicates that the Fe2+ ions should be substituted by Mg2+ ions inducing the shrinkage of the crystal cell and the dopant does not affect the olivine structure of samples. This result is similar to the finding by Wang et al., 2005; Hong et al. (2006). The surface morphology of the LiFePO4 and LiFe0.9 Mg0.1 PO4 powders as well as the shape of the carbon coating has been investigated by FE-SEM and HR-TEM. Typical FE-SEM images for two samples in Fig. 2a and b display similar images. The powders are composed of well-dispersed secondary particles which are slightly agglomerated and show a small quantity
479
of fragments, where the residual carbon introduced by the decomposition of sucrose is distributed around the powders. The average particle sizes of two samples are less than 300 nm. The reduction of particle size can be related to the nanocrystalline precursor materials produced by sol–gel route and in situ introduced carbon which can interfere with the coalescence of the grains (Kim et al., 2004). The carbon content measured by C-S Determinator shows that the virginal sample and doped sample contain almost equal carbon amount of 3 wt.%. In order to further check carbon distribution in the powders, HR-TEM images were applied and two of them are shown in Fig. 2c and d. Some morphology like a nanometersized web was found around the particles. Fig. 2c shows that the LiFe0.9 Mg0.1 PO4 particles separated apart could be connected through the nano-sized carbon and the surface of the particle is wrapped with a carbon layer about 9 nm thick as shown in Fig. 2d. These typical nano-carbon webs can provide good electronic contact intra- and inter-particles so as to improve surface electrical conductivity. It was believed that the optimized surface morphology and the small size of the particles are favorable for the intercalation/de-intercalation process. For potential battery application, electrochemical performance of the LiFe0.9 Mg0.1 PO4 /C was examined comparing to the LiFePO4 /C sample. Fig. 3 shows the CV profiles of both of them. They both exhibit a pair of redox peaks around 3.4 V, but the peak profiles of LiFe0.9 Mg0.1 PO4 /C are more symmetric and speculate. The voltage separation of LiFe0.9 Mg0.1 PO4 /C
Fig. 2 – FE-SEM images of (a) LiFePO4 /C, (b) LiFe0.9 Mg0.1 PO4 /C samples; (c) and (d) HR-TEM images of the LiFe0.9 Mg0.1 PO4 /C sample.
480
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
Fig. 3 – Cyclic voltammograms of lithium cells with LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite, which were recorded in the second cycle at a scanning rate of 0.1 mV s−1 .
is 0.48 V, whereas that of LiFePO4 /C is 0.58 V. As for cyclic voltammogram, the potential interval between anodic peak and cathodic peak is an important parameter to value the electrochemical reaction reversibility. The well-defined peaks and smaller value of voltage separation show the improvement of electrode reaction reversibility by Mg-doping. The capacity and rate capability of LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C composite cathodes were determined by galvanostatic charge/discharge testing. Fig. 4 shows the voltage profiles of doped and undoped LiFePO4 electrode in the first cycle at a 0.1-C rate. Both electrodes exhibited very flat charge and discharge plateaus. While the plateau separation for LiFe0.9 Mg0.1 PO4 /C is lower than that of LiFePO4 /C composites, which corresponds to the lower electrochemical polarization of the doped sample suggesting the increased conductivity induced by the doping method. Doped LiFe0.9 Mg0.1 PO4 /C cathode and LiFePO4 /C cathode show a discharge capacity of 153 and 148 mAh g−1 respectively, approaching the theoretical capacity of 156 mAh g−1 of LiFe0.9 Mg0.1 PO4 (Barker et al.,
Fig. 4 – Initial charge and discharge curves of LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C samples.
Fig. 5 – Relationship between capacity and discharge rate of the LiFePO4 /C and the LiFe0.9 Mg0.1 PO4 /C sample; inset means the cycle performance at 2 C charge/discharge rate.
Fig. 6 – The Nyquist plots of LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite cathodes.
2003). Fig. 5 shows the relationship between capacity and discharge rate, and inset shows cycling performance of the doped and undoped LiFePO4 at a 2-C rate. We can see that the discharge capacities of LiFe0.9 Mg0.1 PO4 /C cathode under different rates are larger than that of the undoped LiFePO4 /C cathode. At a high rate of 2 C, 82% of the theoretical capacity is accessible for doped sample, but the reversible capacity of undoped sample is 73% of that. So it is very clear that the Mg-doped composite material not only increases the specific capacity, but also greatly increases the rate capability. According to the mechanism proposed by Wang et al. (2005), the increase of electrochemical properties should be probably related to the improved mobility of Li+ ion resulting from weakened Li–O interaction by doping, in addition to the enhanced intrinsic electronic conductivity in the doped sample. Beside, both LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite cathodes demonstrate good cycle life even at high rate of 2 C as seen in inset, which is in relation to the good olivine structures and low particle size of them.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
EIS was applied to further analyze the effect of Mgdoping on electrode impedance. Fig. 6 shows the Nyquist plots of LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C electrodes, respectively. It is composed of a depressed semicircle and a slopping line. The high-frequency semicircle is related to the charge transfer resistance, and the slopping line at the low frequency end corresponds to the Warburg impedance of longrange Li-ion diffusion (Lee et al., 2005). The charge-transfer impedance of LiFePO4 /C cathode is much bigger than that of LiFe0.9 Mg0.1 PO4 /C cathode, which further supports the above conclusion of the Mg-doped at 4c site improving the redox kinetics of Fe3+ /Fe2+ couple.
4.
Conclusions
The olivine LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite powders were successfully synthesized through the sol–gel method combined with carbothermal reduction reaction. In comparison with undoped LiFePO4 /C composite powders, the sample heat-treated at 725 ◦ C for 10 h shows better capacity delivery and rate capability. Both cyclic voltammetry and EIS tests showed that, the coexistence of Mg with Fe at 4c site would help to the improvement of redox kinetics of Fe3+ /Fe2+ in LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode. The improved electrochemical performances indicate that this composite with the conductive carbon and metal ions doping is a very promising cathode material for lithium-ion batteries.
references
Barker, J., Saidi, M.Y., Swoyer, J.L., 2003. Electrochem. Solid-state Lett. 6, A53.
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Bewlay, S.L., Konstantinov, K., Wang, G.X., Dou, S.X., Liu, H.K., 2004. Mater. Lett. 58, 1788. Choi, D., Kumta, P., 2007. J. Power Sources 163, 1064. Chung, S.Y., Chiang, Y.M., 2003. Electrochem. Solid-State Lett. A278, 6. Chung, S.Y., Bloking, J.T., Chiang, Y.M., 2002. Nat. Mater. 1, 123. Dominko, D., Gaberscek, M., Drofenik, J., Bele, M., Jamnik, 2003. J. Electrochim. Acta 48, 3709. Franger, S., Cras, F., Bourbon, C., Rouault, H., 2002. Electrochem. Solid-State Lett. 5, 231. Herle, P.S., Ellis, B., Coombs, N., Nazar, L.F., 2004. Nat. Mater. 3, 147. Hong, J., Wang, C., Kasavajjula, U., 2006. J. Power Sources 162, 1289. Huang, H., Yin, S.C., Nazar, L.F., 2001. Electrochem. Solid-State Lett. 4, A170. Islam, M.S., Driscoll, D.J., Fisher, C.A.J., Slater, P.R., 2005. Chem. Mater. 17, 5085. Kim, H.S., Cho, B.W., Cho, W.I., 2004. J. Power Sources 132, 235. Lee, C.Y., Tsai, H.M., Chuang, H.J., Li, S.Y., Lin, P., Tseng, T.Y., 2005. J. Electrochem. Soc. 152, A716. Mehrotra, R.C., 1988. J. Non-cryst. Solids 100, 1. Mi, C.H., Zhang, X.G., Zhao, X.B., Li, H.L., 2006. Mater. Sci. Eng. B 129, 8. Padhi, A.K., Nanjundaswamy, K.S., Goodenough, J.B., 1997. J. Electrochem. Soc. 144, 1188. Ravet, N., Chouinard, Y., Magnan, J.F., Besner, S., Simonean, M., Hovington, P., Armand, M., 2001. J. Power Sources 97, 503. Ravet, N., Abouimrane, A., Armand, M., 2003. Nat. Mater. 2, 702. Wang, D.Y., Li, H., Shi, S.Q., Huang, X.J., Chen, L.Q., 2005. Electrochim. Acta 50, 2955. Wang, Y.Q., Wang, J.L., Yang, J., Nuli, Y., 2006a. Adv. Funct. Mater. 16, 2135. Wang, G.X., Needham, S., Yao, J., Wang, J.Z., Liu, R.S., Liu, H.K., 2006b. J. Power Sources 159, 282. Yamada, A., Kudo, Y., Liu, K.Y., 2001. J. Electrochem. Soc. 148, A224.
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis and characterization of LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode Hui Liu a,b , Jingying Xie a,∗ a
Energy Science and Technology Laboratory, Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history:
a b s t r a c t LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode material was synthesized from
Received 4 August 2007
nanocrystalline Fe3 O4 through sol–gel process, followed by carbothermal reduction reac-
Received in revised form
tion in the presence of LiOH·H2 O and NH4 H2 PO4 . Sucrose molecule was introduced through
4 February 2008
liquid phase and used to construct carbon webs structure. FE-SEM and HR-TEM observa-
Accepted 11 February 2008
tions show that decomposed carbon can form typical carbon webs structure to connect and wrap LiFe0.9 Mg0.1 PO4 particles. Electrochemical tests indicated that the Mg-doping at 4c site of carbon-coated LiFePO4 did not affect the olivine structure of the lithium iron phosphate
Keywords:
but obviously improve its discharge capacity and rate capability, which would be ascribed to
LiFePO4
the increased electronic conductivity and/or the mobility of Li+ ion induced by the doping
Composite cathode
method. © 2008 Elsevier B.V. All rights reserved.
Doping Nano-carbon webs Li-ion batteries
1.
Introduction
Since the original work of Padhi et al. (1997), phospho-olivine LiMPO4 (M = Fe, Mn, Co, Ni) have appeared to be potential candidate for positive electrode materials for lithium-ion batteries. Yet in this family, LiFePO4 is currently the subject of many investigations because this material realizes the highest capacity (170 mAh g−1 ) at moderate current densities (Huang et al., 2001). In addition, it is inexpensive, nontoxic and high safety, three determinant advantages with respect to cobalt oxide-based materials for large-scaled applications such as hybrid electric vehicles. Nevertheless, the low intrinsic electronic conductivity and poor lithium ion diffusion result in losses of capacity during high-rate discharge. In order to solve these disadvantages, much effort has been paid by some groups, involving car-
∗
Corresponding author. Tel.: +86 21 62511070/8801. E-mail address: [email protected] (J. Xie). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.014
bon coating to improve the surface electronic conductivity of active particles (Dominko et al., 2003; Ravet et al., 2001; Bewlay et al., 2004; Wang et al., 2006a; Choi and Kumta, 2007) and supervalent cations doping in Li-site to enhance intrinsic electronic conductivity (Chung et al., 2002; Chung and Chiang, 2003). But the interpretation of the latter method was recently seriously questioned (Ravet et al., 2003) because carbon-containing precursors result in carbon-coated LiFePO4 and in the formation of conductive phases (iron phosphides and/or phosphocarbides) due to LiFePO4 reduction at the surface of the particles (Herle et al., 2004). In recent years, some researchers have also found that Fe-site doping may be an effective step to design robust industrial synthesis process (Barker et al., 2003; Wang et al., 2005; Islam et al., 2005; Hong et al., 2006; Mi et al., 2006; Wang et al., 2006b) in order to improve intrinsic electronic conductivity.
478
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
On the basis of above discussions, Mg-doped LiFePO4 /C composite powders were synthesized by a combination of sol–gel method and carbothermal reduction reaction, in which the formation of nano-carbon webs structure and metal doping were achieved simultaneously. Compared with LiFePO4 /C composite powders, the LiFe0.9 Mg0.1 PO4 /C sample exhibited improved electrochemical performances.
2.
Experimental
LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C samples were synthesized by a gel precursor preparation and carbothermal reduction reaction method. Stoichiometric Fe(NO3 )3 ·9H2 O (98%, A.R) and Mg(NO3 )2 ·9H2 O (98%, A.R) were added in 1.5 mol L−1 organic solution of 2-methoxyethanol (C3 H8 O2 , 98%, A.R), reacting at a constant magnetic stirring. Adequate amount of sucrose (98.5%, A.R) was dissolved in de-ionized water, and mixed with above solution with the C3 H8 O2 :H2 O volume ratio of 5:2. The solution was continuously heated and stirred for several hours to evaporate water and part of organic solvent, and then a rufous xerogel formed. The xerogel was calcined to form precursor powders at 400 ◦ C for 5 h in flowing argon (99.9%). The as-synthesized precursor powders were collected and mixed with stoichiometric amounts of LiOH·H2 O (98%, A.R) and NH4 H2 PO4 (98%, A.R), and subjected further to calcination at 725 ◦ C for 10 h. Cooled to room temperature, the LiFe0.9 Mg0.1 PO4 /C composite materials were obtained. For comparison, LiFePO4 /C composite powder was synthesized by the same way except that Mg(NO3 )2 ·9H2 O was not added in the raw materials. Phase structure of the powder was analyzed by X-ray diffraction (XRD) on a Rigaku-D/MAX-2200PC diffractometer using Cu K␣ radiation. The sample morphology was observed by a field-emission scanning electron microscope (FE-SEM) on a Hitachi S-4700 type of microscope and transmission electron microscope (TEM) on a JEOL high-resolution electron microscope (JEM-2010). Elemental carbon analysis in the obtained
LiFePO4 samples was measured by a C-S 800 Determinator (Eltar, Germany). Electrochemical characterization was performed by assembling CR2025 coin cells for galvanostatic charge/discharge. The electrodes were made by dispersing 80 wt.% active materials, 10 wt.% Super P, and 10 wt.% polyvinylidene fluoride (PVDF) in n-methyl pyrrolidone (NMP) to form a slurry. The slurry was then coated onto an Al foil. The coated electrodes were dried in a vacuum oven and then pressed at 1200 kg/cm2 . The coil cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) with a lithium foil as the counter electrode. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were galvanostatically charged and discharged between 2.5 and 4.2 V at ambient temperature on the electrochemical test instrument (CT2001A, Wuhang Land Electronic Co. Ltd., China). Cyclic voltammetry measurements were performed using electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) at a scan rate of 0.1 mV s−1 between 2.5 and 4.2 V. Electrochemical impedance spectroscopy (EIS) was measured with a frequency response analyzer (Solatron 1255B) interfaced with a potentiogalvanostat (Solatron 1287) controlled by a personal computer. The sinusoidal excitation voltage applied to the cells was 5 mV with a frequency range of between 0.01 Hz to 100 KHz. All potentials are cited in this paper with respect to the reference Li+ /Li. Currents and specific capacities were calculated based on the mass of LiFe0.9 Mg0.1 PO4 and LiFePO4 in the composite electrode.
3.
Results and discussion
When the nitrate, Fe(NO3 )3 ·9H2 O and Mg(NO3 )2 ·9H2 O, were mixed with 2-methoxyethanol, it would react to produce metal alkoxide (Mehrotra, 1988), respectively. The crystallization water in nitrate and strong electronegativity of alkoxy could make Fe(OCH2 CH2 OCH3 )3 hydrolyze and polymerize to form gel which was further dried to obtain the precursor xerogel. The XRD pattern of it with no peaks indicates completely amorphous state as seen in Fig. 1a.
Fig. 1 – X-ray diffraction patterns of (a) gel precursor, (b) Fe3 O4 , (c) LiFePO4 and (d) LiFe0.9 Mg0.1 PO4 samples; FE-SEM image of inset (a) nanocrystalline Fe3 O4 powders.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
After heating the amorphous xerogel at 400 ◦ C for 5 h in argon flow, the obtained powder was found to be a single phase Fe3 O4 as shown in Fig. 1b, indicating that there is no evidence of diffraction peaks of MgO due to its low content. Its average grain size estimated from the Scherrer law is about 36 nm. The morphology of Fe3 O4 is shown in inset (a) where individual particle size is much less than 50 nm that is close to the monocrystallite grain size of 30 nm, indicating the primary particles of Fe3 O4 are monocrystallites. Fig. 1c and d shows the XRD patterns of the pure LiFePO4 and doped LiFePO4 samples, respectively. The carbon is in amorphous form, as there are no obvious impurities such as Li3 PO4 and Fe2 P (Herle et al., 2004; Yamada et al., 2001; Franger et al., 2002), indexed by orthorhombic Pnma with an ordered olivine structure. The ˚ cell parameters calculated by the XRD results are a = 10.330 A, ˚ ˚ ˚ b = 6.007 A, and c = 4.689 A for LiFePO4 phase, and a = 10.312 A, ˚ and c = 4.686 A ˚ for LiFe0.9 Mg0.1 PO4 phase. The slight b = 5.998 A, change in the cell size indicates that the Fe2+ ions should be substituted by Mg2+ ions inducing the shrinkage of the crystal cell and the dopant does not affect the olivine structure of samples. This result is similar to the finding by Wang et al., 2005; Hong et al. (2006). The surface morphology of the LiFePO4 and LiFe0.9 Mg0.1 PO4 powders as well as the shape of the carbon coating has been investigated by FE-SEM and HR-TEM. Typical FE-SEM images for two samples in Fig. 2a and b display similar images. The powders are composed of well-dispersed secondary particles which are slightly agglomerated and show a small quantity
479
of fragments, where the residual carbon introduced by the decomposition of sucrose is distributed around the powders. The average particle sizes of two samples are less than 300 nm. The reduction of particle size can be related to the nanocrystalline precursor materials produced by sol–gel route and in situ introduced carbon which can interfere with the coalescence of the grains (Kim et al., 2004). The carbon content measured by C-S Determinator shows that the virginal sample and doped sample contain almost equal carbon amount of 3 wt.%. In order to further check carbon distribution in the powders, HR-TEM images were applied and two of them are shown in Fig. 2c and d. Some morphology like a nanometersized web was found around the particles. Fig. 2c shows that the LiFe0.9 Mg0.1 PO4 particles separated apart could be connected through the nano-sized carbon and the surface of the particle is wrapped with a carbon layer about 9 nm thick as shown in Fig. 2d. These typical nano-carbon webs can provide good electronic contact intra- and inter-particles so as to improve surface electrical conductivity. It was believed that the optimized surface morphology and the small size of the particles are favorable for the intercalation/de-intercalation process. For potential battery application, electrochemical performance of the LiFe0.9 Mg0.1 PO4 /C was examined comparing to the LiFePO4 /C sample. Fig. 3 shows the CV profiles of both of them. They both exhibit a pair of redox peaks around 3.4 V, but the peak profiles of LiFe0.9 Mg0.1 PO4 /C are more symmetric and speculate. The voltage separation of LiFe0.9 Mg0.1 PO4 /C
Fig. 2 – FE-SEM images of (a) LiFePO4 /C, (b) LiFe0.9 Mg0.1 PO4 /C samples; (c) and (d) HR-TEM images of the LiFe0.9 Mg0.1 PO4 /C sample.
480
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
Fig. 3 – Cyclic voltammograms of lithium cells with LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite, which were recorded in the second cycle at a scanning rate of 0.1 mV s−1 .
is 0.48 V, whereas that of LiFePO4 /C is 0.58 V. As for cyclic voltammogram, the potential interval between anodic peak and cathodic peak is an important parameter to value the electrochemical reaction reversibility. The well-defined peaks and smaller value of voltage separation show the improvement of electrode reaction reversibility by Mg-doping. The capacity and rate capability of LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C composite cathodes were determined by galvanostatic charge/discharge testing. Fig. 4 shows the voltage profiles of doped and undoped LiFePO4 electrode in the first cycle at a 0.1-C rate. Both electrodes exhibited very flat charge and discharge plateaus. While the plateau separation for LiFe0.9 Mg0.1 PO4 /C is lower than that of LiFePO4 /C composites, which corresponds to the lower electrochemical polarization of the doped sample suggesting the increased conductivity induced by the doping method. Doped LiFe0.9 Mg0.1 PO4 /C cathode and LiFePO4 /C cathode show a discharge capacity of 153 and 148 mAh g−1 respectively, approaching the theoretical capacity of 156 mAh g−1 of LiFe0.9 Mg0.1 PO4 (Barker et al.,
Fig. 4 – Initial charge and discharge curves of LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C samples.
Fig. 5 – Relationship between capacity and discharge rate of the LiFePO4 /C and the LiFe0.9 Mg0.1 PO4 /C sample; inset means the cycle performance at 2 C charge/discharge rate.
Fig. 6 – The Nyquist plots of LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite cathodes.
2003). Fig. 5 shows the relationship between capacity and discharge rate, and inset shows cycling performance of the doped and undoped LiFePO4 at a 2-C rate. We can see that the discharge capacities of LiFe0.9 Mg0.1 PO4 /C cathode under different rates are larger than that of the undoped LiFePO4 /C cathode. At a high rate of 2 C, 82% of the theoretical capacity is accessible for doped sample, but the reversible capacity of undoped sample is 73% of that. So it is very clear that the Mg-doped composite material not only increases the specific capacity, but also greatly increases the rate capability. According to the mechanism proposed by Wang et al. (2005), the increase of electrochemical properties should be probably related to the improved mobility of Li+ ion resulting from weakened Li–O interaction by doping, in addition to the enhanced intrinsic electronic conductivity in the doped sample. Beside, both LiFePO4 /C and LiFe0.9 Mg0.1 PO4 /C composite cathodes demonstrate good cycle life even at high rate of 2 C as seen in inset, which is in relation to the good olivine structures and low particle size of them.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 477–481
EIS was applied to further analyze the effect of Mgdoping on electrode impedance. Fig. 6 shows the Nyquist plots of LiFe0.9 Mg0.1 PO4 /C and LiFePO4 /C electrodes, respectively. It is composed of a depressed semicircle and a slopping line. The high-frequency semicircle is related to the charge transfer resistance, and the slopping line at the low frequency end corresponds to the Warburg impedance of longrange Li-ion diffusion (Lee et al., 2005). The charge-transfer impedance of LiFePO4 /C cathode is much bigger than that of LiFe0.9 Mg0.1 PO4 /C cathode, which further supports the above conclusion of the Mg-doped at 4c site improving the redox kinetics of Fe3+ /Fe2+ couple.
4.
Conclusions
The olivine LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite powders were successfully synthesized through the sol–gel method combined with carbothermal reduction reaction. In comparison with undoped LiFePO4 /C composite powders, the sample heat-treated at 725 ◦ C for 10 h shows better capacity delivery and rate capability. Both cyclic voltammetry and EIS tests showed that, the coexistence of Mg with Fe at 4c site would help to the improvement of redox kinetics of Fe3+ /Fe2+ in LiFe0.9 Mg0.1 PO4 /nano-carbon webs composite cathode. The improved electrochemical performances indicate that this composite with the conductive carbon and metal ions doping is a very promising cathode material for lithium-ion batteries.
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