Hydrothermal synthesis of lithium iron phosphate cathodes

Electrochemistry Communications 3 (2001) 505±508

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Hydrothermal synthesis of lithium iron phosphate cathodes Shoufeng Yang, Peter Y. Zavalij, M. Stanley Whittingham

*

Department of Chemistry, Institute for Materials Research, Binghamton University, Binghamton, NY 13902-1600, USA Received 8 May 2001; received in revised form 24 May 2001

Abstract Hydrothermal methods have been successfully applied to the synthesis of lithium iron phosphates. Li3 Fe2 …PO4 †3 was synthesized by heating at 700°C LiFePO4 …OH†, formed hydrothermally in an oxidizing environment. Crystalline LiFePO4 was formed in a direct hydrothermal reaction in just a few hours, and no impurities were detected. This result leads to the possibility of an easy scaleup to a commercial process. The samples were characterized by X-ray di€raction, thermogravimetric analysis and scanning electron microscopy. Both phosphates were tested as the cathode in lithium batteries and showed results comparable to those formed by conventional high-temperature synthesis. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal synthesis; Lithium iron phosphate; Redox couple; Cathode; Olivine

1. Introduction Since the chalcogenide material TiS2 was discovered [1] in the early 1970s to be able to intercalate lithium ions into its structure, other materials such as MoS2 [2], LiCoO2 [3], and LiMn2 O4 [4] have been investigated and commercialized. But these materials either provide rather low discharge capacity, high cost or poor cyclability, so other compounds have been investigated extensively, especially Fe2‡ =Fe3‡ redox couple-based materials such as NASICON (sodium super ionic conductor) and olivine compounds [5±8]. For the NASICON compounds A3 Fe2 …XO4 †3 (A ˆ Li, Na; X ˆ P, As, S), its 3D framework is made up of the XO4 tetrahedra and the FeO6 octahedra. Each FeO6 octahedron shares its corners with six tetrahedra and each XO4 tetrahedron shares its corners with four octahedra. This structure allows lithium intercalation and extraction, making it a promising cathode material for lithium batteries [5]. Since the FeO6 octahedron is separated from other octahedra by XO4 tetrahedra, it reduces the conductivity for this material, so the compound has a large polarization e€ect during the cycling, as shown later during the discussion. Of these NASICON compounds, Li3 Fe2 …PO4 †3 was reported by *

Corresponding author. E-mail address: [email protected] (M. Stanley Whittingham).

Goodenough's group [6] to generate about 2.8 V vs lithium while maintaining excellent capacity retention. Due to the Fe3‡ =Fe2‡ redox couple, 2 mol of lithium can be removed and reversibly intercalated into Li3 Fe2 …PO4 †3 , delivering a capacity of 128 Ah/kg, which is comparable to that of the LiCoO2 cathode, where only half the lithium can be cycled in practical cells, and it is much lower in cost. LiFePO4 is of the olivine formula MNXO4 , where M and N are cations with di€erent sizes. Its structure is composed of PO4 tetrahedra and FeO6 octahedra. Each FeO6 octahedron shares edge with one tetrahedron along c-axis and two corners in the ab-plane. This compound is environmentally benign and cheap, making it ideal for battery applications. Padhi et al. [7] reported that 0.8 mol of lithium can be reversibly extracted at 0:05 mA=cm2 . The polyanion group stabilizes the structure and lowers the Fermi level of this redox couple through the Fe±O±X inductive e€ect, thus providing a higher voltage. Its discharge voltage is 3.5 V, almost 0.7 V higher than Li3 Fe2 …PO4 †3 . Since its formula has 1 mol of lithium, it should be charged ®rst and its theoretical capacity is 170 Ah/kg. During the recharge, its volume shrinks only about 6.8% [8], comparable to that of LiTiS2 , thus minimizing mechanical decrepitation and the resulting loss of electrical contact during cycling. In this paper, we exploited the advantages of hydrothermal synthesis, quick easy synthesis at low energy

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cost and readily scalable. We report the rapid formation of several lithium iron phosphates including crystalline LiFePO4 , where no impurities were observed, making it an excellent alternative to high-temperature synthesis. Previously, our group synthesized a series of manganese and vanadium compounds such as Kx MnO2
yH2 O [9], Li0:6 V2 d O4 d
H2 O [10], MnV2 O5 [11], etc. This method involves reaction at low temperature (100±200°C) for a short period (1 h to 3 days), which is very good for commercialization of these compounds.

2.3. Analysis of LiFePO4 and LiFePO4 …OH † X-ray di€raction patterns were collected for LiFePO4 …OH†, LiFePO4 and Li3 Fe2 …PO4 †3 on a Scintag XDS 2000 di€ractometer with CuKa radiation and Ge(Li) solid-state detector. Thermogravimetric analysis was performed on a Perkin Elmer model TGA7 to determine the nature of the products formed. Fig. 1 shows the TGA for LiFePO4 …OH† in oxygen, indicating a weight decrease of 4.4% consistent with the expected weight loss of 5.1% for the reaction 6LiFePO4 …OH† ! 2Li3 Fe2 …PO4 †3 ‡ Fe2 O3 ‡ 3H2 O

2.1. Synthesis of LiFePO4 …OH † and Li3 Fe2 …PO4 †3 FeCl2
4H2 O (Aldrich) and LiOH (Aldrich) with molar ratio 1:2 were dissolved in deionized water separately and mixed to get an Fe…OH†2 precipitate. The slurry was ®ltered and washed, then transferred to a 100 ml beaker; 10 ml 15% H2 O2 (J.T. Baker) was added slowly to the slurry while stirring for 3 min. P2 O5 (Fisher) and LiOH (molar ratio 1:3) were dissolved in another beaker with warm water (50°C) to get a lithium phosphate solution. Then the two samples were mixed together and stirred for 5 min. The solution was transferred to the Te¯on-lined Parr reactor and reacted at 170°C for 3 days. The ®nal solution was cooled and ®ltered, the bright yellow precipitate was air-dried at 40°C overnight. Then the sample was transferred into a crucible and heated to 700°C in a furnace in air for 12 h. 2.2. Synthesis of LiFePO4 LiFePO4 was prepared by direct hydrothermal synthesis of FeSO4 (98%, Fisher), H3 PO4 (Fisher) and LiOH (Aldrich) in the stoichiometric ratio 1.0:1.0:3.0. FeSO4 and H3 PO4 solution were mixed ®rst to avoid Fe…OH†2 because it is easily oxidized to Fe(III), then LiOH solution was added to the mixture with stirring for 1 min. pH was 7.56 and the solution was quickly transferred to Parr reactor for up to 5 h at 120°C. After the sample was cooled, the pH was 6.91. The light green precipitate was ®ltered and air-dried at 40°C for 2 h. For several similar syntheses, pH was slightly di€erent and it had no e€ect on the ®nal product. Several attempts had been tried with Fe(II) salt and Li3 PO4 , all of them had some impurities. When FeCl2 (Aldrich) and …NH4 †2 Fe…SO4 †2 (Fisher) were used instead of FeSO4 , the same compound was synthesized. Due to the easy oxidation of Fe(II) salt, sometimes reddish impurities will be produced, especially when the iron salt is not new, but they can be easily separated from LiFePO4 in a centrifuge.

XRD analysis con®rmed the formation of Li3 Fe2 …PO4 †3 and Fe2 O3 . In order to determine the purity of LiFePO4 , standard K2 Cr2 O7 solution was used to titrate the amount of Fe2‡ in the product. Each time, about 100 mg LiFePO4 was used and dissolved in the mixture of 40 ml 1 M H2 SO4 and 20 ml 5 M H3 PO4 , and phenylamine sulfonic acid was used as the indicator. For each titration, we calculated the amount of Fe2‡ based on the titration and on the assumption of 100% LiFePO4 , respectively. The ratio of these two will be denoted as g. If no impurities exist in the sample, g should be 1.000. 2.4. Electrochemical testing for LiFePO4 and Li3 Fe2 …PO4 †3 To test these materials as cathodes in lithium cells, the lithium iron phosphate, carbon black, Te¯on were mixed in a mortar for 20 min. For Li3 Fe2 …PO4 †3 , the weight ratio was 75:15:10, and for LiFePO4 , the material was coated with carbon by dissolving it in sucrose solution, weight ratio for LiFePO4 and sucrose was 9:1. It was ®red at 700°C for 3 h [12]. The ®nal brown material was used as the cathode. The weight ratio was 70:20:10. The sample was ground and crushed into a thin sheet and put on an Exmet stainless steel grid and hot pressed at 100°C for 30 min. A lithium sheet was used as the anode and the electrolyte was 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (EC: 101 100 Weight, %

2. Experiment

99 98 97 96 95

100

200

300 400 500 Temperature,˚C

Fig. 1. TGA of LiFePO4 …OH† in oxygen.

600

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DMC ˆ 1:1, EM Industries) and the separator was Celgard 2400 (Hoechst Celenese Corp). The electrodes were submerged in the electrolyte and put in a plastic bag. The assembling of this battery was done in a glove box ®lled with pure helium. For the Li3 Fe2 …PO4 †3 , the current density was about 0:5 mA=cm2 and the cut-o€ voltage was 1.9 and 4.1 V. For the LiFePO4 , the current density was about 0:14 mA=cm2 and cut-o€ voltage was 2.0 and 4.5 V. The battery performance was collected on a MacPile system galvanostatically at 25°C (MacPile A3.10, Biologic, Claix, France). Fig. 4. Electrochemical performance of Li3 Fe2 …PO4 †3 .

3. Results and discussion LiFePO4 …OH† was synthesized as the precursor for Li3 Fe2 …PO4 †3 , it was indexed in the triclinic system with  space group P 1 and the cell parameters: a ˆ 5:367…1† A,   b ˆ 7:305…1† A, c ˆ 5:131…1† A. a ˆ 109:31…1†°, b ˆ 97:84…1†°, c ˆ 106:37…5†°. Li3 Fe2 …PO4 †3 was indexed as

 follows: space group is P2l =n with a ˆ 8:579…2† A,   b ˆ 8:622…2† A, c ˆ 12:041…3† A, b ˆ 90:46…2†°. Fig. 2 shows the X-ray powder di€raction patterns for LiFePO4 …OH† (a) and Li3 Fe2 …PO4 †3 (b). For LiFePO4 …OH†, the arrows point to the peaks of an unknown impurity. In the case of LiFePO4 , we performed a Rietveld re®nement using the WinCSD crystallographic software. It was indexed in the orthorhombic system with space  group Pnma and cell parameters: a ˆ 10:381…7† A,   b ˆ 6:013…5† A, c ˆ 4:716…3† A. Fig. 3 shows the high crystallinity and purity of the material formed. Several attempts were made to test the cathodic behavior of LiFePO4 …OH†, but no capacity was observed between 1.9 and 4.1 V. However, its thermal product Li3 Fe2 …PO4 †3 showed highly reversible capacity as indicated in Fig. 4. This capacity at 0:5 mA=cm2 is shown per formula weight of Li3 Fe2 …PO4 †3 after backing out the weight of the ferric oxide. The capacity is about 91 Ah/kg, consistent with that reported by Masquelier [6]. The morphology for LiFePO4 was determined on a JEOL 8900 SEM (see Fig. 5). The average particle size is about 3 lm, and this is smaller than the 20 lm average

Fig. 2. XRD pattern for LiFePO4 …OH† (a), and Li3 Fe2 …PO4 †3 (b).

Fig. 3. Experimental (dotted), calculated (line) and the di€erence (bottom) for LiFePO4 , the vertical lines show the re¯ections.

Fig. 5. Morphology of LiFePO4 .

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hydrothermal reaction. The crystal size was smaller and the Fe2‡ purity was higher compared to high-temperature synthesis. After a rough carbon coating of this material, a capacity of 100 A h/kg was obtained at 0:14 mA=cm2 . We will optimize the carbon coating process for these hydrothermally synthesized materials, and are presently exploring microwave assisted hydrothermal as an even more rapid synthetic approach. Acknowledgements Fig. 6. Electrochemical performance of LiFePO4 .

size of LiFePO4 reported by Yamada et al. [8]. The Fe(II) purity was determined by titration, which indicated that the g value, was always between 0.99 and 1.00, demonstrating that the purity for this material was almost 100%. Fig. 6 shows the cycling pro®le for LiFePO4 with a small amount of unoptimized carbon coating. About 0.6 mol lithium can be inserted reversibly at 0:14 mA=cm2 . Raising the temperature to 60°C allowed the rate to be increased to over 0:5 mA=cm2 whilst maintaining a similar capacity. Our earlier results without carbon coating showed only 0.4 mol of lithium intercalation/ deintercalation at 0:05 mA=cm2 with the same cell con®guration. Optimized carbon coatings on high-temperature LiFePO4 permit capacities of 0:8 Li=LiFePO4 to be attained [13]. 4. Conclusions We have synthesized Li3 Fe2 …PO4 †3 by a combination of the hydrothermal method and high-temperature process. The initial discharge capacity was about 91 Ah/ kg. This method has to be improved due to the impurities. LiFePO4 was very readily synthesized by a direct

We thank the US Department of Energy, Oce of Transportation Technologies, for support of this work through the BATT program at Lawrence Berkley National Laboratory of this work. Initial results of this work have been reported in the cognizant DOE reports. References [1] M.S. Whittingham, Science 192 (1976) 1126. [2] R.R. Chianelli, M.B. Dines, Inorg. Chem 17 (1978) 2758. [3] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783. [4] M.M. Thackery, W.I.F. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18 (1983) 461. [5] A.K. Padhi, K.S. Nanjundaswamy, C. Masquelier, S. Okada, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1609. [6] C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Solid State Chem. 135 (1998) 228. [7] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [8] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224. [9] R. Chen, P. Zavalij, M.S. Whittingham, Chem. Mater. 8 (1996) 1275. [10] T. Chirayil, P.Y. Zavalij, M.S. Whittingham, J. Electrochem. Soc. 143 (1996) L193. [11] F. Zhang, P.Y. Zavalij, M.S. Whittingham, Electrochem. Commun. 1 (1999) 564. [12] N. Ravet, S. Besner, et al., European Patent: EP 1 049 182 A2. [13] M.B. Armand, IBA Meeting, Kwe Maritane, South Africa, March 2001.