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C doped with Mg2+ by reactive extrusion method
Advanced Powder Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method Liu Xu-heng, Zhao Zhong-wei ⇑ School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 5 March 2014 Accepted 17 March 2014 Available online xxxx Keywords: Lithium ion battery Reactive extrusion Lithium iron phosphate Doping with magnesium
a b s t r a c t Reactive extrusion method is used to synthesizing LiMgxFe1 xPO4/C, using LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO as raw materials and glucose as carbon source. Samples are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), TG–DTA analysis and electrochemical performance test. Results show that amorphous product can be achieved after the reactive extrusion process. The particle size increases with the increase of magnesium content. Appropriately Mg2+ doping can reduce the electrode polarization effectively without seriously effect on material structure and morphology. LiMg0.04Fe0.96PO4/C, showing the best electrochemical performances, has an initial discharge capacity of 155, 148, 140 and 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, respectively. The discharge capacities remain above 99% after 20 cycles. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Due to its well-known advantages, such as the high theoretical capacity, excellent thermal stability, inexpensive and environmentally benign and so on, LiFePO4 is considered as a promising cathode material for lithium ion battery, especially for large cell applications such as electric vehicles [1–6]. LiFePO4 can be synthesized by solid-state reaction with the advantage of operation simplicity and consequently low production cost [7–10], however the electrochemical performance of LiFePO4 produced by this method is usually not satisfied as the raw materials cannot be mixed homogeneously [11–14]. Solution methods of synthesizing LiFePO4 offset the disadvantages of solid-state method, but most of solution methods are difficult to expand to mass production scale due to expensive raw materials and hard-controlled technologic conditions [15–21]. Therefore, it is necessary to develop an economic and high-efficient synthesis method for high-performance LiFePO4 material. Recently, we have developed a novel synthesis method based on reactive extrusion. Reactive extrusion, which was firstly used in plastics industry, is a chemical reaction process that the monomer was polymerized or the polymers were modified in the extruder [22–25]. This process is a continuous application, which is different from the traditional mechanical milling. The strong mechanical force provided by twin screw extruder makes the raw materials grinded, crushed and sheared. The compounds are ⇑ Corresponding author. Tel.: +86 731 88830476; fax: +86 731 88830477. E-mail address: [email protected] (Z.-w. Zhao).
consequently easy to be mixed homogeneously. The new materials will also be obtained with the effect of shear heat and external heat [26,27]. Furthermore, extrusion method is a simple and easy process to be scaled up into industrial level. According to the advantage of reactive extrusion in the area of polymer production, it is expected that the reaction of synthesizing LiFePO4 may occur during the extrusion process. In this paper, reactive extrusion method is used to preparing lithium iron phosphate innovatively, which is different from the traditional synthesis methods, and the product shows excellent properties. The new method not only overcomes the disadvantage of conventional solid state method, which leads to the non-uniformity of raw material, but also overcomes the shortcoming of liquid method, which is difficult for the operation and for large-scale production. Reactive extrusion method can make the raw materials mixed homogeneously at molecular level. This method is very efficient just because the process of reactive extrusion is a continuous feeding and discharging process, and chemical reaction can be induced synchronously due to the mechanical force and external heat. These advantages described above can’t be possessed synchronously by other methods. 2. Experimental LiFePO4/C doped with Mg2+ was synthesized by the following procedure: (1) LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO powders were mixed based on the stoichiometric ratio and glucose as carbon source was then added in proportion of 3% with respect to the total weight, and then grinded for 10 min to form the precursor; (2) The precursor was added into the twin screw extruder with
http://dx.doi.org/10.1016/j.apt.2014.03.013 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
the feeding speed of 5 gs 1, controlling the rotation speed of screw at 250 r min 1 and the extrusion temperature at 100 °C; (3) The extruded product was heated in a tubular furnace at 550–720 °C for 8 h under argon atmosphere, and then cooled slowly to the ambient. The phase structure of the products was characterized by X-ray diffraction analysis (XRD, Rint-2000, Rigaku) with the Cu Ka radiation. The microstructure of the products was observed by scanning electron microscopy. Thermal analysis of the precursor was carried by SDT Q600 TG–DTA at the temperature between 10 and 900 °C with 10°C min 1 heating rate under argon flow. To make electrodes, a mixture of 90 wt.% sample, 5 wt.% carbon black and 5 wt.% of polyvinylidene fluoride (PVDF) were mixed together in N-methyl-2 pyrolidone. The mixture, which is slurry, was then deposited uniformly onto a thin Al foil. After heating the filmed Al foil for 12 h at 120 °C under vacuum, the dried filmed Al foil, which is known as cathode, was then cut into disks with diameter of 1.3 cm for assembling CR2025 coin-type cell. The electrolyte was 1 mol L 1 LiPF6 dissolved in ethyl carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a molar ratio of 1:1:1. The Li foil was used as an anode and the Celgard 2400 as the separator. CR2025 coin-type cell was then measured by LandCT2001A (Land, Wuhan) at different rate between 2.3 and 4.1 V. Cyclic voltammetry(CV) performance and electrochemical impedance spectroscopy (EIS) of the cell were measured and recorded via CHI660A (Cheng hua, Shang hai). The scan rate of CV was 0.2 mV s 1 in the voltage range of 2.8–4.2 V and the amplitude of AC signal was 5 mV over the frequency range between 100 kHz and 0.1 Hz.
weight (%)
90
-20 70
175
50
-30
211 0
200
-40 400
600
800
1000
Temperature (°C) Fig. 2. TG–DTA profiles for extruded product.
211 °C, another strong endothermic appear in the TG–DTA curves, which is 18% weight loss. This is attributed to the pyrolytic reaction of glucose. Along with the increase of temperature, an exothermic peak was shown obviously at 447 °C without appreciable weight loss, which can be related to the crystallization process of the compound. 3.2. Effects of temperature on the properties of samples Fig. 3 shows the X-ray diffraction patterns of LiMg0.02Fe0.98PO4/C synthesized at different temperatures for 8 h. Some impurity peaks are observed obviously when the temperature is below 600 °C. When the temperature is more than 600 °C, no impurities peaks can be observed. All diffraction peaks are indexed to pure LiFePO4 with a regular olivine structure. The peaks are more and more sharp and narrow, indicating that the crystallization of samples is improved with the increase of temperature. Fig. 4 presents the initial discharge curves of samples at different temperatures. The sample synthesized at 550 °C shows the lowest discharge capacity of 132 mA h g 1 at 0.2C rate, which is due to the existence of impurities. The one prepared at 680 °C shows the highest discharge capacity of 147 mA h g 1. The discharge capacity of the sample synthesized at 720 °C decreases to 143 mA h g 1. This is due to the growth of particles, which restricts the intercalation/ deintercalation of lithium ion.
Fig. 1 shows the X-ray diffraction pattern of the extruded product. It shows that there is no obvious diffraction peak, which means the compound is amorphous. The strong mechanical force provided by twin screw extruder may result in the change of crystal structure of raw materials, such as point defects, dislocation and deformation. These changes in crystal structure will lead to the increase of chemical reactivity of raw materials. Meantime, the shear heat, external heat and reaction heat provided by P2O5 due to its intense dehydration would enhance the activation energy of the system and hence enhance the reaction activity of raw materials. As a result, the reaction between the raw materials would be induced. The TG–DTA curves of the extruded product are shown in Fig. 2. The TG curve shows a weight loss between 135 and 420 °C. An endothermic peak was observed at 175 °C, which is related to elimination of crystallization water. When the temperature increases to
30
422
80
TG
3.1. structure analysis and thermogravimetric analysis of extruded product
20
-10
DTA
60
3. Results and discussion
10
0
447
100
Temperature difference
2
3.3. Effects of magnesium contents on the physical properties of samples Fig. 5 shows the XRD diffraction patterns of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium contents. All
40
50
60
70
2θ/ (°) Fig. 1. XRD pattern of extruded product.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
3
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
LiFePO4 450°C
550°C 600°C 650°C 700°C
750°C
10
20
30
40
50
60
70
2θ/ (°) Fig. 3. XRD pattern of LiMg0.02Fe0.98PO4/C synthesized at different temperatures.
4.4
Table 1 lattice parameters of LiMgxFe1 xPO4/C (x = 0.02, 0.04, 0.06, 0.08 and 0.1).
a bce d 4.0
Voltage/V
3.6 3.2
a---- 550 b---- 600 c---- 650 d---- 680 e---- 720
2.8 2.4
20
40
60
80
100
120
140
a (nm)
b (nm)
c (nm)
LiMg0.02Fe0.98PO4/C LiMg0.04Fe0.96PO4/C LiMg0.06Fe0.94PO4/C LiMg0.08Fe0.92PO4/C LiMg0.1Fe0.9PO4/C
1.0330 1.0328 1.0324 1.0321 1.0317
0.6010 0.6008 0.6005 0.6001 0.5998
0.4692 0.4691 0.4690 0.4688 0.4685
particles and are agglomerated. The particle size increases with the increase of magnesium content. The particle size of LiMg0.02Fe0.98PO4/C is near 500 nm. By contrast, the particle size of LiMg0.06Fe0.94PO4/C ranges from 1 to 2 lm, which is impeditive to the lithium ion diffusion.
a bce d 0
Samples
160
-1
Specific capacity/ mAhg
Fig. 4. Discharge curves of LiMg0.02Fe0.98PO4/C synthesized at different temperatures at 0.2 °C rate.
3.4. Electrochemical properties Fig. 7 is the CV profiles of LiMgxFe1 xPO4/C with different magnesium contents. It shows that the voltage interval increases initially and then decreases with the increase of magnesium content. The LiMg0.04Fe0.96PO4/C sample, which demonstrates the least voltage interval of 0.23 V, exhibits an anodic peak at 3.33 V and a corresponding cathodic response at 3.56 V, whereas LiMg0.1Fe0.9PO4/C exhibits an anodic peak at 3.23 V and a corresponding cathodic peak at 3.65 V, which is due to the longer path of Li+ diffusion as shown in Fig. 6. EIS test results in Fig. 8 show
peaks are indexed as LiFePO4 phase with an ordered olivine structure, which indicates that Mg2+ occupies Fe2+ positions and forms solid solutions without any impurity phase. The radius of Mg2+ is 0.066 nm, which is less than the one of Fe2+(0.074 nm). As a result, the unit cell of the crystal lattice is slightly smaller with the doping of Mg2+, which is listed in Table 1. Fig. 6 shows the surface morphology of LiMgxFe1 xPO4/C with different magnesium contents. It can be seen that the LiMgxFe1 xPO4/C has an irregular primary
LiFePO4
x=0.1
x=0.08
x=0.06
x=0.04
x=0.02
x=0
10
20
30
40
2θ (°)
50
60
70
Fig. 5. XRD pattern of LiMgxFe1 xPO4/C synthesized at 680 °C.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
4
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
Fig. 6. SEM images of of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium content. A—x = 0; B—x = 0.02; C—x = 0.04; D—x = 0.06; E—x = 0.08; F—x = 0.1.
0.6
scan rate: 0.2mV/s
LiFePO4/C
400
LiMg0.02Fe0.98PO4/C
0.4
LiMg0.04Fe0.96PO4/C LiMg0.08Fe0.92PO4/C
0.0
Z''/
Current (mA)
LiMg0.06Fe0.94PO4/C
300
0.2
LiMg0.1Fe0.9PO4/C
200
LiFePO4/C
-0.2
LiMg0.02Fe0.98PO4/C
-0.4
LiMg0.04Fe0.96PO4/C
-0.6
LiMg0.08Fe0.92PO4/C
100
LiMg0.06Fe0.94PO4/C
0
LiMg0.1Fe0.9PO4/C
-0.8 2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Voltage (V) Fig. 7. The cyclic voltammetry curves of LiMgxFe1 xPO4/C.
the similar trend as CV test results. The radius of the semicircle at high frequency region is related to the charge transfer resistance and the inclined line in the low frequency range represents the Warburg impedance, which are associated with Li+ diffusion. It is seen that LiMg0.04Fe0.96PO4/C exhibits a smaller charge-transfer resistance (about 200 X) as those of other samples. The smaller charge-transfer resistance indicates a lower electrochemical polarization, which is associated with the lower electronic and/or ionic resistance.
0
100
200
300
400
Z'/ Fig. 8. EIS spectra of LiMgxFe1 xPO4/C synthesized at 680 °C.
The discharge profiles of all samples at 0.2C rate are shown in Fig. 9. LiMg0.02Fe0.98PO4/C delivers a discharge capacity of 145 mA h g 1. LiMg0.04Fe0.96PO4/C exhibits the highest discharge capacity of 155 mA h g 1, which is about 91.4% of the theoretical capacity of LiFePO4. The discharge capacity of LiMg0.1Fe0.9PO4/C is only 142 mA h g 1. As mentioned before, the radius of Mg2+ (0.066 nm) is less than the one of Fe2+(0.074 nm), so the lithium ion is easier to diffuse in the material by the substitution of Mg for Fe. Mg2+ doping can improve the conductivity of material, but
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
4.4 16 25 43
4.0
Voltage/V
3.6
5
cycle performance of LiMg0.04Fe0.96PO4/C at different rates. LiMg0.04Fe0.96PO4/C shows excellent capacity retention. The discharge capacities at the 20th cycle reach 154, 147, 141, 137 mA h g 1 under 0.2C, 0.5C, 1C and 2C rate, respectively, corresponding to the capacity retention of 99%, 99.2%, 100.5% and 100.3%, compared with its initial discharge capacities. These results demonstrate an excellent stability of the material.
3.2 1 2 3 4 5 6
2.8 2.4
x=0 x=0.02 x=0.04 x=0.06 x=0.08 x=0.1
4. Conclusion
16 25 43
2.0 0
20
40
60
80
100
120
Specific capacity/mAhg
140
160
-1
Fig. 9. Discharge curves of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium content.
4.4
LiMgxFe1 xPO4/C was synthesized by reactive extrusion method using LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO as raw materials and glucose as additive. The XRD results indicated that amorphous product was achieved by reactive extrusion, and pure LiMgxFe1 xPO4/C can be synthesized above 600 °C. SEM revealed that the particle size of samples increased obviously with the increase of magnesium content. The LiMg0.04Fe0.96PO4/C sample exhibited the most impressive electrochemical performance with the initial capacities of 155, 148, 140, 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, and retains 99%, 99.2%, 100.5% and 100.3% of its initial discharge capacities after 20 cycles, respectively.
Voltage/V
43 2 1
4.0
References
3.6
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J, Phospho-olivines as positive electrode materials for rechargeable lithium batteries, Electrochem. Soc. 144 (1997) 1188–1194. [2] Y. Sundarayya, K.C. Kumara, C.S. Sunandana, Oxalate based non-aqueous sol– gel synthesis of phase pure sub-micron LiFePO4, Mater. Res. Bull. 42 (2007) 1942–1948. [3] M. Thuckeray, Lithium-ion batteries: an unexpected conductor, Nat. Mater. 1 (2002) 81–82. [4] Z.J. Jiang, Development and challenge of LiFePO4 cathode materials for Li-ion batteries, J. Funct. Mater. 41 (2010) 365–368. [5] Dragana Jugovic´, Dragan Uskokovic´, A review of recent developments in the synthesis procedures of lithium iron phosphate powders, J. Power Sources 190 (2009) 538–544. [6] Vanchiappan Aravindan, Joe Gnanaraj, Yun.-Sung Lee, Srinivasan Madhavi, LiMnPO4–A next generation cathode material for lithium-ion batteries, J. Mater. Chem. A 1 (2003) 3518–3539. [7] Christian Masquelier, Laurence Croguennec, Polyanionic (phosphates, silicates, sulfates) frameworks as electrode materials for rechargeable Li (or Na) batteries, Chem. Rev. 113 (2013) 6552–6591. [8] Dan Zhao, Yun.-long Feng, Yong.-gang Wang, Yong.-yao Xia, Electrochemical performance comparison of LiFePO4 supported by various carbon materials, Electrochim. Acta 88 (2013) 632–638. [9] J.K. Kim, J.W. Choi, G. Cheruvally, J.U. Kim, J.H. Ahn, G.B. Cho, A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries, Mater. Lett. 61 (2007) 3822–3825. [10] R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, S. Pejovnik, J. Jamnik, Impact of the carbon coating thickness on the electrochemical performance of LiFePO4/C compostites, J. Electrochem. Soc. 152 (2005) A607–A610. [11] Y.D. Cho, G.T.K. Fey, H.M. Kao, Physical and electrochemical properties of Ladoped LiFePO4/C composites as cathode materials for lithium-ion batteries, Solid State Electrochem. 12 (2008) 815–823. [12] Y.X. Wen, M.P. Zheng, Z.F. Tong, Preparation and performance of LiVxFe1 xPO4 as cathode material for lithium ion batteries, Chin. J. Power Sources 29 (2005) 713–715. [13] F.J. Yu, B. Yang, H.H. Yi, C.L. Hu, Progress in research on LiFePO4 cathode material in lithium ion batteries, YunNan Chem. Technol. 35 (2008) 45–49. [14] F. Yu, J.J. Zhang, Y.Y. Yang, G.Z. Song, Preparation and characterization of mesoporous LiFePO4/C microsphere by spray drying assisted template method, J. Power Sources 189 (2009) 794–797. [15] Muxina Konarova, Izumi Taniguchi, Synthesis of carbon-coated LiFePO4 nanoparticles with high rate performance in lithium secondary batteries, J. Power Sources 195 (2010) 3661–3667. [16] H.J. Kim, J.M. Kim, W.S. Kim, H.J. Koo, D.S. Bae, H.S. Kim, Synthesis of LiFePO4/C cathode materials through an ultrasonic-assisted rheological phase method, J. Alloys Compd. 509 (2011) 5662–5666. [17] Y.Y. Liu, C.B. Cao, Enhanced electrochemical performance of nano-sized LiFePO4/C synthesized by an ultrasonic-assisted co-precipitation method, Electrochim. Acta 55 (2010) 4694–4699. [18] Y.H. Yin, M.X. Gao, J.L. Ding, Y.F. Liu, L.K. Shen, H.G. Pan, A carbon-free LiFePO4 cathode material of high-rate capability prepared by a mechanical activation method, J. Alloys Compd. 509 (2011) 10161–10166. [19] X.Y. Wang, C. Miao, J. Zhou, C. Ma, H.F. Wang, S.Q. Sun, A novel synthesis of spherical LiFePO4 nanoparticles, Mater. Lett. 65 (2011) 2096–2099.
3.2 1 2 3 4
2.8 2.4
0.2C 0.5C 1C 2C 43 2 1
0
20
40
60
80
100
120
140
160
-1
Specific capacity/mAhg
Fig. 10. Discharge curves of LiMg0.04Fe0.96PO4/C at different rate.
Specific capacity/mAhg
-1
160
150
140
0.2C 0.5C 1C 2C
130
120 0
2
4
6
8
10
12
14
16
18
20
Cycle number Fig. 11. Cycle performance of LiMg0.04Fe0.96PO4/C at different rate.
the growth of particle size restricts the diffusion of lithium ion. Thus, it can be seen that appropriate magnesium content is indispensable for improving the electrochemical performance. The rate capacity of LiMg0.04Fe0.96PO4/C is shown in Fig. 10. As shown in figure, the voltages of discharge plateau decrease slightly along with the increase of discharge rate. The initial discharge capacities of LiMg0.04Fe0.96PO4/C reach 155, 148, 140, 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, respectively. Fig. 11 shows the
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
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[20] M.S. Bhuvaneswari, N.N. Bramnik, D. Ensling, H. Ehrenberg, W. Jaegermann, Synthesis and characterization of Carbon Nano Fiber/LiFePO4 composites for Li-ion batteries, J. Power Sources 180 (2008) 553–560. [21] J. Lee, A.S. Teja, Characteristics of lithium iron phosphate (LiFePO4) particles synthesized in subcritical and supercritical water, J. Supercrit. Fluids 35 (2005) 83–90. [22] R. Rached, R. Rahouadj, C. Fonteix, S. Hoppe, F. Pla, C. Cunat, Mechanical behaviour modelling of a nanostructured polyamide blends obtained by reactive extrusion process, Chem. Eng. Sci. 63 (2008) 3843–3857. [23] M.R. Badrossamay, G. Sun, Preparation of rechargeable biocidal polypropylene by reactive extrusion with diallylamino triazine, Euro. Polym. J. 44 (2008) 733–742.
[24] X.Z. Geng, Twin Screw Extrudes & its Application, first ed., China Light Industry Press, Beijing, 2003. [25] B. Mu, M.R. Thompson, Examining the mechanics of granulation with a hot melt binder in a twin-screw extruder, Chem. Eng. Sci. 81 (2012) 46– 56. [26] M.R. Thompson, B. Mu, P.J. Sheskey, Aspects of foam stability influencing foam granulation in a twin screw extruder, Powder Technol. 228 (2012) 339–348. [27] I. Amalia Kartika, P.Y. Pontalier, L. Rigal, Extraction of sunflower oil by twin screw extruder: Screw configuration and operating condition effects, Bioresource Technol. 97 (2006) 2302–2310.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method Liu Xu-heng, Zhao Zhong-wei ⇑ School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 5 March 2014 Accepted 17 March 2014 Available online xxxx Keywords: Lithium ion battery Reactive extrusion Lithium iron phosphate Doping with magnesium
a b s t r a c t Reactive extrusion method is used to synthesizing LiMgxFe1 xPO4/C, using LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO as raw materials and glucose as carbon source. Samples are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), TG–DTA analysis and electrochemical performance test. Results show that amorphous product can be achieved after the reactive extrusion process. The particle size increases with the increase of magnesium content. Appropriately Mg2+ doping can reduce the electrode polarization effectively without seriously effect on material structure and morphology. LiMg0.04Fe0.96PO4/C, showing the best electrochemical performances, has an initial discharge capacity of 155, 148, 140 and 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, respectively. The discharge capacities remain above 99% after 20 cycles. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Due to its well-known advantages, such as the high theoretical capacity, excellent thermal stability, inexpensive and environmentally benign and so on, LiFePO4 is considered as a promising cathode material for lithium ion battery, especially for large cell applications such as electric vehicles [1–6]. LiFePO4 can be synthesized by solid-state reaction with the advantage of operation simplicity and consequently low production cost [7–10], however the electrochemical performance of LiFePO4 produced by this method is usually not satisfied as the raw materials cannot be mixed homogeneously [11–14]. Solution methods of synthesizing LiFePO4 offset the disadvantages of solid-state method, but most of solution methods are difficult to expand to mass production scale due to expensive raw materials and hard-controlled technologic conditions [15–21]. Therefore, it is necessary to develop an economic and high-efficient synthesis method for high-performance LiFePO4 material. Recently, we have developed a novel synthesis method based on reactive extrusion. Reactive extrusion, which was firstly used in plastics industry, is a chemical reaction process that the monomer was polymerized or the polymers were modified in the extruder [22–25]. This process is a continuous application, which is different from the traditional mechanical milling. The strong mechanical force provided by twin screw extruder makes the raw materials grinded, crushed and sheared. The compounds are ⇑ Corresponding author. Tel.: +86 731 88830476; fax: +86 731 88830477. E-mail address: [email protected] (Z.-w. Zhao).
consequently easy to be mixed homogeneously. The new materials will also be obtained with the effect of shear heat and external heat [26,27]. Furthermore, extrusion method is a simple and easy process to be scaled up into industrial level. According to the advantage of reactive extrusion in the area of polymer production, it is expected that the reaction of synthesizing LiFePO4 may occur during the extrusion process. In this paper, reactive extrusion method is used to preparing lithium iron phosphate innovatively, which is different from the traditional synthesis methods, and the product shows excellent properties. The new method not only overcomes the disadvantage of conventional solid state method, which leads to the non-uniformity of raw material, but also overcomes the shortcoming of liquid method, which is difficult for the operation and for large-scale production. Reactive extrusion method can make the raw materials mixed homogeneously at molecular level. This method is very efficient just because the process of reactive extrusion is a continuous feeding and discharging process, and chemical reaction can be induced synchronously due to the mechanical force and external heat. These advantages described above can’t be possessed synchronously by other methods. 2. Experimental LiFePO4/C doped with Mg2+ was synthesized by the following procedure: (1) LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO powders were mixed based on the stoichiometric ratio and glucose as carbon source was then added in proportion of 3% with respect to the total weight, and then grinded for 10 min to form the precursor; (2) The precursor was added into the twin screw extruder with
http://dx.doi.org/10.1016/j.apt.2014.03.013 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
the feeding speed of 5 gs 1, controlling the rotation speed of screw at 250 r min 1 and the extrusion temperature at 100 °C; (3) The extruded product was heated in a tubular furnace at 550–720 °C for 8 h under argon atmosphere, and then cooled slowly to the ambient. The phase structure of the products was characterized by X-ray diffraction analysis (XRD, Rint-2000, Rigaku) with the Cu Ka radiation. The microstructure of the products was observed by scanning electron microscopy. Thermal analysis of the precursor was carried by SDT Q600 TG–DTA at the temperature between 10 and 900 °C with 10°C min 1 heating rate under argon flow. To make electrodes, a mixture of 90 wt.% sample, 5 wt.% carbon black and 5 wt.% of polyvinylidene fluoride (PVDF) were mixed together in N-methyl-2 pyrolidone. The mixture, which is slurry, was then deposited uniformly onto a thin Al foil. After heating the filmed Al foil for 12 h at 120 °C under vacuum, the dried filmed Al foil, which is known as cathode, was then cut into disks with diameter of 1.3 cm for assembling CR2025 coin-type cell. The electrolyte was 1 mol L 1 LiPF6 dissolved in ethyl carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a molar ratio of 1:1:1. The Li foil was used as an anode and the Celgard 2400 as the separator. CR2025 coin-type cell was then measured by LandCT2001A (Land, Wuhan) at different rate between 2.3 and 4.1 V. Cyclic voltammetry(CV) performance and electrochemical impedance spectroscopy (EIS) of the cell were measured and recorded via CHI660A (Cheng hua, Shang hai). The scan rate of CV was 0.2 mV s 1 in the voltage range of 2.8–4.2 V and the amplitude of AC signal was 5 mV over the frequency range between 100 kHz and 0.1 Hz.
weight (%)
90
-20 70
175
50
-30
211 0
200
-40 400
600
800
1000
Temperature (°C) Fig. 2. TG–DTA profiles for extruded product.
211 °C, another strong endothermic appear in the TG–DTA curves, which is 18% weight loss. This is attributed to the pyrolytic reaction of glucose. Along with the increase of temperature, an exothermic peak was shown obviously at 447 °C without appreciable weight loss, which can be related to the crystallization process of the compound. 3.2. Effects of temperature on the properties of samples Fig. 3 shows the X-ray diffraction patterns of LiMg0.02Fe0.98PO4/C synthesized at different temperatures for 8 h. Some impurity peaks are observed obviously when the temperature is below 600 °C. When the temperature is more than 600 °C, no impurities peaks can be observed. All diffraction peaks are indexed to pure LiFePO4 with a regular olivine structure. The peaks are more and more sharp and narrow, indicating that the crystallization of samples is improved with the increase of temperature. Fig. 4 presents the initial discharge curves of samples at different temperatures. The sample synthesized at 550 °C shows the lowest discharge capacity of 132 mA h g 1 at 0.2C rate, which is due to the existence of impurities. The one prepared at 680 °C shows the highest discharge capacity of 147 mA h g 1. The discharge capacity of the sample synthesized at 720 °C decreases to 143 mA h g 1. This is due to the growth of particles, which restricts the intercalation/ deintercalation of lithium ion.
Fig. 1 shows the X-ray diffraction pattern of the extruded product. It shows that there is no obvious diffraction peak, which means the compound is amorphous. The strong mechanical force provided by twin screw extruder may result in the change of crystal structure of raw materials, such as point defects, dislocation and deformation. These changes in crystal structure will lead to the increase of chemical reactivity of raw materials. Meantime, the shear heat, external heat and reaction heat provided by P2O5 due to its intense dehydration would enhance the activation energy of the system and hence enhance the reaction activity of raw materials. As a result, the reaction between the raw materials would be induced. The TG–DTA curves of the extruded product are shown in Fig. 2. The TG curve shows a weight loss between 135 and 420 °C. An endothermic peak was observed at 175 °C, which is related to elimination of crystallization water. When the temperature increases to
30
422
80
TG
3.1. structure analysis and thermogravimetric analysis of extruded product
20
-10
DTA
60
3. Results and discussion
10
0
447
100
Temperature difference
2
3.3. Effects of magnesium contents on the physical properties of samples Fig. 5 shows the XRD diffraction patterns of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium contents. All
40
50
60
70
2θ/ (°) Fig. 1. XRD pattern of extruded product.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
3
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
LiFePO4 450°C
550°C 600°C 650°C 700°C
750°C
10
20
30
40
50
60
70
2θ/ (°) Fig. 3. XRD pattern of LiMg0.02Fe0.98PO4/C synthesized at different temperatures.
4.4
Table 1 lattice parameters of LiMgxFe1 xPO4/C (x = 0.02, 0.04, 0.06, 0.08 and 0.1).
a bce d 4.0
Voltage/V
3.6 3.2
a---- 550 b---- 600 c---- 650 d---- 680 e---- 720
2.8 2.4
20
40
60
80
100
120
140
a (nm)
b (nm)
c (nm)
LiMg0.02Fe0.98PO4/C LiMg0.04Fe0.96PO4/C LiMg0.06Fe0.94PO4/C LiMg0.08Fe0.92PO4/C LiMg0.1Fe0.9PO4/C
1.0330 1.0328 1.0324 1.0321 1.0317
0.6010 0.6008 0.6005 0.6001 0.5998
0.4692 0.4691 0.4690 0.4688 0.4685
particles and are agglomerated. The particle size increases with the increase of magnesium content. The particle size of LiMg0.02Fe0.98PO4/C is near 500 nm. By contrast, the particle size of LiMg0.06Fe0.94PO4/C ranges from 1 to 2 lm, which is impeditive to the lithium ion diffusion.
a bce d 0
Samples
160
-1
Specific capacity/ mAhg
Fig. 4. Discharge curves of LiMg0.02Fe0.98PO4/C synthesized at different temperatures at 0.2 °C rate.
3.4. Electrochemical properties Fig. 7 is the CV profiles of LiMgxFe1 xPO4/C with different magnesium contents. It shows that the voltage interval increases initially and then decreases with the increase of magnesium content. The LiMg0.04Fe0.96PO4/C sample, which demonstrates the least voltage interval of 0.23 V, exhibits an anodic peak at 3.33 V and a corresponding cathodic response at 3.56 V, whereas LiMg0.1Fe0.9PO4/C exhibits an anodic peak at 3.23 V and a corresponding cathodic peak at 3.65 V, which is due to the longer path of Li+ diffusion as shown in Fig. 6. EIS test results in Fig. 8 show
peaks are indexed as LiFePO4 phase with an ordered olivine structure, which indicates that Mg2+ occupies Fe2+ positions and forms solid solutions without any impurity phase. The radius of Mg2+ is 0.066 nm, which is less than the one of Fe2+(0.074 nm). As a result, the unit cell of the crystal lattice is slightly smaller with the doping of Mg2+, which is listed in Table 1. Fig. 6 shows the surface morphology of LiMgxFe1 xPO4/C with different magnesium contents. It can be seen that the LiMgxFe1 xPO4/C has an irregular primary
LiFePO4
x=0.1
x=0.08
x=0.06
x=0.04
x=0.02
x=0
10
20
30
40
2θ (°)
50
60
70
Fig. 5. XRD pattern of LiMgxFe1 xPO4/C synthesized at 680 °C.
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
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X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
Fig. 6. SEM images of of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium content. A—x = 0; B—x = 0.02; C—x = 0.04; D—x = 0.06; E—x = 0.08; F—x = 0.1.
0.6
scan rate: 0.2mV/s
LiFePO4/C
400
LiMg0.02Fe0.98PO4/C
0.4
LiMg0.04Fe0.96PO4/C LiMg0.08Fe0.92PO4/C
0.0
Z''/
Current (mA)
LiMg0.06Fe0.94PO4/C
300
0.2
LiMg0.1Fe0.9PO4/C
200
LiFePO4/C
-0.2
LiMg0.02Fe0.98PO4/C
-0.4
LiMg0.04Fe0.96PO4/C
-0.6
LiMg0.08Fe0.92PO4/C
100
LiMg0.06Fe0.94PO4/C
0
LiMg0.1Fe0.9PO4/C
-0.8 2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Voltage (V) Fig. 7. The cyclic voltammetry curves of LiMgxFe1 xPO4/C.
the similar trend as CV test results. The radius of the semicircle at high frequency region is related to the charge transfer resistance and the inclined line in the low frequency range represents the Warburg impedance, which are associated with Li+ diffusion. It is seen that LiMg0.04Fe0.96PO4/C exhibits a smaller charge-transfer resistance (about 200 X) as those of other samples. The smaller charge-transfer resistance indicates a lower electrochemical polarization, which is associated with the lower electronic and/or ionic resistance.
0
100
200
300
400
Z'/ Fig. 8. EIS spectra of LiMgxFe1 xPO4/C synthesized at 680 °C.
The discharge profiles of all samples at 0.2C rate are shown in Fig. 9. LiMg0.02Fe0.98PO4/C delivers a discharge capacity of 145 mA h g 1. LiMg0.04Fe0.96PO4/C exhibits the highest discharge capacity of 155 mA h g 1, which is about 91.4% of the theoretical capacity of LiFePO4. The discharge capacity of LiMg0.1Fe0.9PO4/C is only 142 mA h g 1. As mentioned before, the radius of Mg2+ (0.066 nm) is less than the one of Fe2+(0.074 nm), so the lithium ion is easier to diffuse in the material by the substitution of Mg for Fe. Mg2+ doping can improve the conductivity of material, but
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
X.-h. Liu, Z.-w. Zhao / Advanced Powder Technology xxx (2014) xxx–xxx
4.4 16 25 43
4.0
Voltage/V
3.6
5
cycle performance of LiMg0.04Fe0.96PO4/C at different rates. LiMg0.04Fe0.96PO4/C shows excellent capacity retention. The discharge capacities at the 20th cycle reach 154, 147, 141, 137 mA h g 1 under 0.2C, 0.5C, 1C and 2C rate, respectively, corresponding to the capacity retention of 99%, 99.2%, 100.5% and 100.3%, compared with its initial discharge capacities. These results demonstrate an excellent stability of the material.
3.2 1 2 3 4 5 6
2.8 2.4
x=0 x=0.02 x=0.04 x=0.06 x=0.08 x=0.1
4. Conclusion
16 25 43
2.0 0
20
40
60
80
100
120
Specific capacity/mAhg
140
160
-1
Fig. 9. Discharge curves of LiMgxFe1 xPO4/C synthesized at 680 °C with different magnesium content.
4.4
LiMgxFe1 xPO4/C was synthesized by reactive extrusion method using LiOHH2O, FeC2O42H2O, P2O5 and nano-MgO as raw materials and glucose as additive. The XRD results indicated that amorphous product was achieved by reactive extrusion, and pure LiMgxFe1 xPO4/C can be synthesized above 600 °C. SEM revealed that the particle size of samples increased obviously with the increase of magnesium content. The LiMg0.04Fe0.96PO4/C sample exhibited the most impressive electrochemical performance with the initial capacities of 155, 148, 140, 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, and retains 99%, 99.2%, 100.5% and 100.3% of its initial discharge capacities after 20 cycles, respectively.
Voltage/V
43 2 1
4.0
References
3.6
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3.2 1 2 3 4
2.8 2.4
0.2C 0.5C 1C 2C 43 2 1
0
20
40
60
80
100
120
140
160
-1
Specific capacity/mAhg
Fig. 10. Discharge curves of LiMg0.04Fe0.96PO4/C at different rate.
Specific capacity/mAhg
-1
160
150
140
0.2C 0.5C 1C 2C
130
120 0
2
4
6
8
10
12
14
16
18
20
Cycle number Fig. 11. Cycle performance of LiMg0.04Fe0.96PO4/C at different rate.
the growth of particle size restricts the diffusion of lithium ion. Thus, it can be seen that appropriate magnesium content is indispensable for improving the electrochemical performance. The rate capacity of LiMg0.04Fe0.96PO4/C is shown in Fig. 10. As shown in figure, the voltages of discharge plateau decrease slightly along with the increase of discharge rate. The initial discharge capacities of LiMg0.04Fe0.96PO4/C reach 155, 148, 140, 137 mA h g 1 at 0.2 C, 0.5 C, 1 C and 2 C rate, respectively. Fig. 11 shows the
Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013
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Please cite this article in press as: X.-h. Liu, Z.-w. Zhao, Synthesis of LiFePO4/C doped with Mg2+ by reactive extrusion method, Advanced Powder Technology (2014), http://dx.doi.org/10.1016/j.apt.2014.03.013