Communication pubs.acs.org/crystal
New Method of Postmodifying the Particle Size and Morphology of LiFePO4 via Supercritical Carbon Dioxide Ming Xie,†,‡ Xiaoxue Zhang,*,† Jarmo Laakso,† Hao Wang,‡ and Erkki Levan̈ en† †
Department of Materials Science, Tampere University of Technology, P.O. Box 589, FI-33101 Finland The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100024, China
‡
ABSTRACT: Supercritical carbon dioxide (scCO2) was applied for the first time to post-treat two hydrothermally synthesized LiFePO4 of H1 and H2 to entirely alter their particle size and morphology. H1 had the morphology of platelets aggregated in one direction in size of about 20−30 μm, together with impurity of Fe2PO5. After the scCO2 treatment, the impurity phase was removed, and the aggregated platelets were broken into separate rhombus plates in size of about 3−5 μm, along with the formation of flowerlike balls of about 10−20 μm. On the other hand, H2 had the main morphology of platelets agglomerated in random directions in size of about 5 μm and covered by materials of glassy state. After the scCO2 measurement, the morphology was greatly changed and cubic particles of 1 and 2 μm were formed, together with the better-shaped and separately distributed platelets of 1 and 2 μm. Therefore, by optimizing the experimental parameters in both the hydrothermal synthesis and supercritical carbon dioxide processing, it is possible to precisely control the particle size and morphology of LiFePO4. This work also presents the possibility to control the particle size and morphology of other materials with a post-treatment by supercritical carbon dioxide.
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environmentally benign solvent and has a number of positive impacts on material processing such as solvent replacement, energy efficiency, better separation, and a possibility of an entirely new innovative processing method for producing new functional breakthrough materials.13 To the authors’ knowledge, scCO2 has not been reported to modify the particle size and morphology of LiFePO4. This study shows that scCO2 can remove impurities as well as change the particle size and morphology of the hydrothermally synthesized LiFePO4. This communication also presents the possibility of controlling the particle size and morphology of other materials via scCO2 treatment. Two lithium iron phosphate samples, named as H1 and H2, were prepared by a hydrothermal method with a similar process reported elsewhere.14 The starting materials were iron(II) sulfate heptahydrate (FeSO4·7H2O, >99.5%, Merck), lithium hydroxide (LiOH, >98.0%, Merck), diammonium hydrogenphosphate ((NH4)2HPO4, >99.5%, Tianjin Fuchen Chemical Reagents Factory), and ortophosphoric acid (H3PO4, 85%, VWR). The stoichiometric ratio of FeSO4·7H2O, LiOH, H3PO4 (in synthesis of H1) or (NH4)2HPO4 (in synthesis of H2) was 1:3:1, respectively. FeSO4·7H2O and a water solution of H3PO4 (or (NH4)2HPO4) were mixed first and then quickly introduced to LiOH. The mixture was vigorously stirred for 1 min and then quickly transferred to a Parr PTFE-lined stainless steel autoclave (Parr 4748, Parr Instrument Company, Illinois) and heated at 180 °C for 8 h. The autoclave was then cooled to room temperature and the resulting green precipitate was
he lithium-ion battery is of great importance in many applications such as electronic devices and plug-in hybrid electronic vehicles. The cost, safety, environmental friendliness, capacity, stability, and lifetime of the cathode material have been major concerns to researchers searching to develop new active materials. Current commercial lithium-ion batteries use mixed cobalt−manganese oxide based materials (LiCoxMn1−xO2) for cathodes. However, the cobalt oxide-based materials are unsafe and environmentally hazardous, while manganese-based materials suffer from capacity fading during cycling, especially at high temperatures.1 Lithium iron phosphate (LiFePO4) has been recognized to be one of the most promising candidate cathode materials in lithium-ion batteries. However, LiFePO4 suffers from its poor electronic conductivity,2 which can typically be solved by coating the LiFePO4 particle with carbon3 and by minimizing the particle size.4 Smaller LiFePO4 particles with less aggregation/ agglomeration can be coated with more carbon and have larger lithium ion diffusion rates to the electrolyte because of increased surface area, therefore resulting in better electrode performance. Meanwhile, particle morphology has been shown to be crucial to its electrode performance.5−7 For example, spherical particles are reported to have better performance than irregular particles.8 Tremendous efforts have been done to improve the performance of LiFePO4, and one strategy is to synthesize LiFePO4 with different particle size and morphology.9−11 In this communication, a new method by using supercritical carbon dioxide (scCO2) processing is reported to modify the particle size and morphology of LiFePO 4 synthesized by a hydrothermal method. Supercritical carbon dioxide (scCO2) is carbon dioxide that is held beyond supercritical conditions (Tc of 31.1 °C and Pc of 73.8 bar).12 Supercritical carbon dioxide is a nontoxic and © 2012 American Chemical Society
Received: March 5, 2012 Revised: April 5, 2012 Published: April 11, 2012 2166
dx.doi.org/10.1021/cg3003146 | Cryst. Growth Des. 2012, 12, 2166−2168
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washed, filtered, and dried at 60 °C overnight. The supercritical carbon dioxide treatments were conducted using a supercritical fluid reactor (SFE R250, Thar Technologies Inc., Pittsburgh, PA), which is a static pressure system. The hydrothermally synthesized H1 (or H2) was put into a high pressure reactor, and CO2 was further introduced. While the temperature and pressure were raised, the CO2 (99.8%, L00120, AGA) reached the supercritical state. HC1 was prepared by treating H1 with scCO2 with a pressure of 100 bar at 100 °C for 24 h, and HC2 was obtained by treating H2 with scCO2 with a pressure of 200 bar at 100 °C for 24 h. The crystalline phase was determined by X-ray diffraction (Siemens D-500; Siemens AG, Karlsruhe, Germany) using Cu Kα radiation from 2θ of 20° to 70° with a step size of 0.02° and a count time of 1.2 s per step. The particle size and morphology were examined by field emission scanning electron microscopy (FESEM, Zeiss ULTRAplus FEG-SEM; Carl Zeiss NTS GmbH, Oberkochen, Germany). The samples were coated with a carbon layer to avoid charging. Particle size distribution was measured by laser diffraction using a Sympatec HELOS system (H9093, SUCELL, Sympatec GmbH, ClausthalZellerfeld, Germany) installed with WINDOX software. The sample and a small amount of Dispex as dispersing agent were dispersed in water and sonicated during the measurement. A 100 mm lens with a measuring range of 0.7−175 μm was used, and X50 was obtained from the measurement to describe a particle diameter corresponding to 50% of the cumulative distribution. The XRD patterns of the hydrothermally synthesized H1, H2 from different starting materials are shown in Figure 1, together
Figure 2. (a) SEM image of H1; (b, c) SEM images of HC1; (d) TEM image of a part of a rhombus plate in HC1 with its corresponding SAED pattern; (e) SEM image of H2; (f) SEM image of HC2.
μm and the morphology of platelets aggregated mainly in one direction in Figure 2a. However, after the scCO2 treatment, the aggregated platelets were broken into separate rhombus plates in size of about 3−5 μm, along with the formation of flowerlike balls of about 10−20 μm in Figure 2b. The clear look of separate rhombus plates is shown in Figure 2c. The lines in the marked area could be seen as the sign of the plate split from another. This morphology change could be explained by the high diffusivity and near zero surface tension of scCO2,15 allowing scCO2 penetrating into the holes between the plates shown in Figure 2a and then further splitting the aggregated platelets into separate ones. Furthermore, the formation mechanism of the flowerlike balls after the scCO2 treatment is still not clear and additional research is currently being conducted. The morphology change possibly leads to the intensity difference in the diffraction peaks around 2θ of 30° in the XRD patterns of H1 and HC1. The TEM image of part of a rhombus plate is shown in Figure 2(d), where its corresponding selective area electron diffraction (SAED) pattern is inserted and indexed according to LiFePO4 with a beam direction of [011]̅ . On the other hand, the hydrothermally synthesized H2 has the main morphology of platelets agglomerated in random directions and covered by materials of glassy state (see Figure 2e). However, after the scCO2 treatment, the morphology was entirely changed. Cubic particles of 1 and 2 μm were grown, together with the better-shaped and separately distributed platelets of 1 and 2 μm, as shown in Figure 2f. The particle size alteration during the scCO2 treatment is also revealed in Figure 3, together with the change in particle size distribution. X50 describes a particle diameter corresponding to 50% of the cumulative distribution and can be seen as an indicator of average particle size. After the scCO2 treatment of
Figure 1. XRD patterns of H1, HC1, H2, and HC2. The marked peaks are from Fe2PO5, and all the other peaks are from LiFePO4.
with the XRD patterns of the HC1 and HC2 obtained by the scCO2 treatment of H1 and H2, respectively. In the XRD pattern of H1, there are three peaks from Fe2PO5 (JCPDS card 36-84), marked by arrows in Figure 1. However, after the scCO2 treatment with a pressure of 100 bar at 100 °C, the peaks of Fe2PO5 disappear in the XRD pattern of HC1 and all the diffraction peaks in HC1 are from LiFePO4 according to the JCPDS card 40-1499. On the other hand, all the diffraction peaks in H2 and HC2 are corresponding entirely to LiFePO4. The size and morphology of the hydrothermally synthesized LiFePO4 were changed by supercritical carbon dioxide treatment, as shown in Figure 2. The hydrothermally synthesized H1 has an average particle size of about 20−30 2167
dx.doi.org/10.1021/cg3003146 | Cryst. Growth Des. 2012, 12, 2166−2168
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Communication
the Laboratory of Ceramic Materials at TUT are also appreciated for providing such a collegial working atmosphere.
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(1) Xie, H. M.; Wang, R. S.; Ying, J. R.; Zhang, L. Y.; Jalbout, A. F.; Yu, H. Y.; Yang, G. L.; Pan, X. M.; Su, Z. M. Adv. Mater. 2006, 18, 2609−2613. (2) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188−1194. (3) Wang, G. X.; Yang, L.; Chen, Y.; Wang, J. Z.; Bewlay, S.; Liu, H. K. Electrochim. Acta 2005, 50, 4649−4654. (4) Delacourt, C.; Poizot, P.; Levasseur, S.; Masquelier, C. Electrochem. Solid-State Lett. 2006, 9, A352−A355. (5) Gaberscek, M.; Dominko, R.; Jamnik, J. Electrochem. Commun. 2007, 9, 2778−2783. (6) Dokko, K.; Koizumi, S.; Nakano, H.; Kanamura, K. J. Mater. Chem. 2007, 17, 4803−4810. (7) Xie, M.; Zhang, X.; Wang, H. Ionics 2011, 17, 299−305. (8) Ying, J. R.; Jiang, C. Y.; Wan, C. R. J. Power Sources 2004, 129, 264−269. (9) Ellis, B.; Kan, W. H.; Makahnouk, W. R. M.; Nazar, L. F. J. Mater. Chem. 2007, 17, 3248−3254. (10) Franger, S.; Cras, F. L.; Bourbon, C.; Rouault, H. J. Power Sources 2003, 119−121, 252−257. (11) Hsu, K. F.; Tsay, S. Y.; Hwang, B. J. J. Mater. Chem. 2004, 14, 2690−2695. (12) Leitner, W. Nature 2000, 405, 129−130. (13) DeSimone, J. M.; Tumas, W. Green Chemistry Using Liquid and Supercritical Carbon Dioxide; Oxford University Press: 2003. (14) Yang, S.; Zavalij, P. Y.; Whittingham, M. S. Electrochem. Commun. 2001, 3, 505−508. (15) Zhang, J.; Davis, T. A.; Matthews, M. A.; Drews, M. J.; LaBerge, M.; An, Y. H. J. Supercrit. Fluids 2006, 38, 354−372.
Figure 3. Particle size distributions of H1, HC1, H2, and HC2. X50 values from the measurement are also inserted.
the H1 sample, the X50 was lowered greatly from 23.5 to 6.4 μm due to the breaking of platelets aggregates, as shown in the SEM images in Figure 2. The aggregated platelets in Figure 2a have size of about 20−30 μm. After the scCO2 treatment, the flowerlike ball has size of about 10−20 μm and the separate rhombus plates are a few micrometers, resulting in an X50 of 6.4 μm and a wider particle size distribution than that of H1. Furthermore, the particle size change in H2 and HC2 is not very distinct and the X50 of HC2 (2.3 μm) is slightly larger than that of H2 (1.8 μm), as a result of a morphology change. In summary, supercritical carbon dioxide was used for the first time to modify the size and morphology of two hydrothermally synthesized LiFePO4 from different starting materials, and the impurity phase in H1 was also removed by a scCO2 treatment process. After the scCO 2 treatment, aggregation was largely reduced and different morphologies were obtained. By optimizing the experimental parameters (time, pressure, temperature) in the hydrothermal synthesis and supercritical carbon dioxide processing, it is possible to precisely control the particle size and morphology of LiFePO4. This work contributes to promotion of “green” processing by environmentally benign scCO2 and exhibits the possibility to control the particle size and morphology of other materials with a post-treatment by supercritical carbon dioxide.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone: +358 40 849 0197. Fax: +358 03 3115 2330. E-mail: xiaoxue.zhang@tut.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from China Scholarship Council is greatly acknowledged for Ming’s visiting research at Tampere University of Technology (TUT), Finland. The scientific guidance and support from Tapio Mäntylä, emeritus professor, is also warmly appreciated. Dr. Mari Honkanen is acknowledged for collecting the SAED pattern, Leo Hyvärinen and Saara Heinonen are thanked for their assistance in particle size measurements, and Juha-Pekka Nikkanen is also appreciated for the help in planning the experiments at the beginning of Ming’s visit to TUT. Other members of staff from the hosting group of 2168
dx.doi.org/10.1021/cg3003146 | Cryst. Growth Des. 2012, 12, 2166−2168
