Enhanced Electrochemical Properties of LiFePO₄ as Positive Electrode of Li-Ion Batteries for HEV Application

Christian M. Julien, Karim Zaghib, Alain Mauger, Henri Groult

Institut de Minéralogie et Physique de la Matière Condensée, Université Pierre et Marie Curie, Paris, France.
Institut de Recherche d’Hydro-Québec, Varennes, Canada.
Physicochimie des Electrolytes, Collo?des et Systèmes Analytiques, Université Pierre et Marie Curie, Paris, France.

DOI: 10.4236/aces.2012.23037                

Abstract

LiFePO₄ materials synthesized using FePO₄(H₂O)₂ and Li₂CO₃ blend were optimized in view of their use as positive electrodes in Li-ion batteries for hybrid electric vehicles. A strict control of the structural properties was made by the combination of X-ray diffraction, FT-infrared spectroscopy and magnetometry. The impact of the ferromagnetic clus-ters (γ-Fe₂O₃ or Fe₂P) on the electrochemical response was examined. The electrochemical performances of the opti-mized LiFePO₄ powders investigated at 60℃ are excellent in terms of capacity retention (153 mAh·g^-1 at 2C) as well as in terms of cycling life. No iron dissolution was observed after 200 charge-discharge cycles at 60℃ for cells containing Li foil, Li₄Ti₅O₁₂, or graphite as negative electrodes.

Keywords

Olivine; Nanostructured Compounds; Local Structure; Magnetometry; Lithium-Ion Batteries

1. Introduction

Since the introduction of lithium-ion batteries based on lithium cobaltate (LiCoO₂) by Sony in 1991, great efforts have been addressed to find an alternative material with both sides of the battery. However, the expansion of their applications from the portable market to the electric (EVs) and hybrid vehicles (HEVs) requests lower cost and better safety characteristic electrode material. Among the well-known Li-insertion compounds, the olivine LiFePO₄ (LFP) compound is being extensively investigated as a positive electrode material for Li-ion batteries because of its low cost, low toxicity, and relatively high theoretical specific capacity of 170 mAh·g^–1 [1,2]. The current debate for the utilization of LiFePO₄ in large-size batteries (for HEV, for instance), is mainly focused on the perceived poor rate capability because of a low electronic conductivity. Another aspect concerns the material purity and the non-migration of iron ions through the electrolyte. The high-temperature performance is also a critical issue because batteries may be operated at elevated temperatures (around 60˚C). The early drawback of highly resistive LiFePO₄ has been resolved by painting the particle surface with carbon [3-6].

Recently, significant effort has been underway to improve LiFePO₄ by developing a new synthesis route via carbon coating [7]. The 1D Li channels make the olivine performance sensitive not only to particle size, but also to impurities and stacking faults that block the channels. Various types of iron-based impurities have been identified in the olivine framework: for examples γ-Fe₂O₃, Fe₃O₄, Li₃Fe₂(PO₄)₃, Fe₂P₂O₇, Fe₂P, Fe₃P, Fe₇₅P₁₅C₁₀, etc. Critical quality control of the product is necessary to obtain a complete understanding of synthesis conditions using combination of experiments such as Raman spectroscopy and magnetic measurements [8-12].

In this paper, we report the results obtained on several samples of LiFePO₄ (LFP) with special attention to the new generation of phospho-olivine materials used in lithium cells operating at 60˚C. The magnetic properties are correlated with the electrochemical performance of the positive electrode materials. Magnetization and susceptibility measurements appear to be a powerful probes for impurity detection at very low concentration of trivalent iron (<1 ppm). Electrochemical performances of Liion cells with Li₄Ti₅O₁₂ (LTO) negative electrode are reported with a strict control of iron dissolution by postmortem analysis.

2. Experimental

The optimized LiFePO₄ material was synthesized by solidstate reaction. Samples were prepared from FePO₄(H₂O)₂ and Li₂CO₃. A stoichiometric amount of precursors was thoroughly mixed together in isopanol. After drying, the blend was heated at 500˚C - 800˚C for 8 h under reducing atmosphere. Four samples have been considered heated at carbon-coated LiFePO₄ (C-LFP) was prepared with sucrose and cellulose acetate as the carbon precursors in acetone solution according to the following procedure. The carbon-free powder was mixed with the carbon precursors. The dry additive corresponded to 5 wt% carbon in LiFePO₄. After drying, the blend was heated at 700˚C for 4 h under argon atmosphere. The quantity of carbon coat represents about 1 wt% of the material (C-detector, LECO Co., CS 444). It should be noted that the choice of this moderate sintering temperature minimizes the amount of Fe³⁺ ions present in the powder since the presence of Fe³⁺ has been detected by Mössbauer experiments at sintering temperatures below 500˚C, and both trivalent Fe₂O₃ and Li₃Fe₂(PO₄)₃ are formed in such large quantities that they are detected by X-rays by sintering above 800˚C [13]. Nevertheless, we know from our prior work [10,11] that LiFePO₄, even with an intermediate sintering temperature in the range 500˚C - 800˚C, does contain Fe₂O₃ nanoparticles, although in such small quantities that they can be detected only by investigation of magnetic properties.

X-ray diffractometry (XRD) was carried out with a Philips X’Pert apparatus equipped with a CuKα X-ray source (λ = 1.5406 Å). Slice views were examined with a scanning electron microscope (SEM, Philips XL30). Fourier transform infrared (FTIR) absorption spectra were recorded with a Fourier transform interferometer (model Bruker IFS113v) in the wavenumber range 150 - 1400 cm^–1 at a spectral resolution of 2 cm^–1. Magnetic measurements (susceptibility and magnetization) were carried out with a fully automated magnetometer (MPMS-5S from Quantum Design) using an ultra-sensitive Superconducting Quantum Interference Device (SQUID) in the temperature range 4 - 300 K. The experimental details are given elsewhere [11]. The electrochemical properties of LiFePO₄ were measured at 60˚C in cells with metallic lithium as the negative electrode. The electrolyte was 1 M LiPF₆ in EC/DEC (1/1) solvent. The measurements were carried out following the experimental procedure previously described [14] using the coffee-bag technology developed at Hydro-Québec. Coffee-bag or laminated battery technology was described by Zaghib and Armand [15].

3. Results and Discussion

3.1. Structure and Morphology of LiFePO4

Figure 1 shows the typical XRD patterns of the LiFePO₄ electrode material. The XRD pattern of sample synthesized from the mixture FePO₄(H₂O)₂ + Li₂CO₃ agrees very well with that of phospho-olivine LiFePO₄ [16] and no impurity was detected. The XRD diagram of the new

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, “Phospho-Olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries,” Journal of the Electrochemical Society, Vol. 144, No. 4, 1997, pp. 1188-1194. doi:10.1149/1.1837571
[2]	N. Ravet, Y. Chouinard, J. F. Magnan, S. Besner, M. Gauthier and M. Armand, “Electroactivity of Natural and Synthetic Triphylite,” Journal of Power Sources, Vol. 97-98, 2001, pp. 503-507. doi:10.1016/S0378-7753(01)00727-3
[3]	N. Ravet, A. Abouimrane and M. Armand, “From Our Readers: On the Electronic Conductivity of Phospho-Olivines as Lithium Storage Electrodes,” Nature Materials, Vol. 2, 2003, pp. 702-703. doi:10.1038/nmat1009b
[4]	N. Ravet, S. Besner, M. Simoneau, A. Vallee, M. Armand and J. F. Magnan, “Electrode materials with high surface conductivity,” US Patent No. 6962666, 2005.
[5]	Y. Hu, M. M. Doeff, R. Kostecki and R. Finones, “Electrochemical Performance of Sol-Gel Synthesized LiFePO4 in Lithium Batteries,” Journal of the Electrochemical Society, Vol. 151, No. 8, 2004, pp. A1279-A1285. doi:10.1149/1.1768546
[6]	S. L. Bewlay, K. Konstantinov, G. X. Wang, S. X. Dou and H. K. Liu, “Conductivity Improvements to Spray-Produced LiFePO4 by Addition of a Carbon Source,” Materials Letters, Vol. 58, No. 11, 2004, pp. 1788-1791. doi:10.1016/j.matlet.2003.11.008
[7]	K. Zaghib, V. Battaglia, P. Charest, V. Srinivasan, A. Guerfi and R. Kostecki, Extended Abstract of the International Battery Association & Hawaii Battery Conference, Wailoloa, 9-12 January 2006.
[8]	A. A. Salah, P. Jozwiak, J. Garbarczyk, F. Gendron, A. Mauger and C. M. Julien, “FTIR Features of Lithium Iron Phosphates Used as Positive Electrodes in Rechargeable Lithium Batteries,” 207th ESC Meeting, Quebec City, 15-20 May 2005.
[9]	M. M. Doeff, Y. Hu, F. McLarnon and R. Kostecki, “Effect of Surface Carbon Structure on the Electrochemical Performance of LiFePO4,” Electrochemical and Solid-State Letters, Vol. 6, No. 10, 2003, pp. A207-A209. doi:10.1149/1.1601372
[10]	A. Ait-Salah, A. Mauger, F. Gendron and C. M. Julien, “Magnetic Studies of the Carbothermal Effect on LiFePO4,” Physica Status Solidi (a), Vol. 203, No. 1, 2006, pp. R1-R3.
[11]	A. Ait-Salah, A. Mauger, C. M. Julien and F. Gendron, “Nano-Sized Impurity Phases in Relation to the Mode of Preparation of LiFePO4,” Materials Science and Engineering: B, Vol. 129, No. 1-3, 2006, pp. 232-244. doi:10.1016/j.mseb.2006.01.022
[12]	A. Ait-Salah, A. Mauger, K. Zaghib, J. B. Goodenough, N. Ravet, M. Gauthier, F. Gendron and C. M. Julien, “Reduction Fe3+ of Impurities in LiFePO4 from Pyrolysis of Organic Precursor Used for Carbon Deposition,” Journal of the Electrochemical Society, Vol. 153, No. 9, 2006, pp. A1692-A1701. doi:10.1149/1.2213527
[13]	A. Yamada, S. C. Chung and K. Hinokuma, “Optimized LiFePO4 for Lithium Battery Cathodes,” Journal of the Electrochemical Society, Vol. 148, No. 3, 2001, pp. A224-A229. doi:10.1149/1.1348257
[14]	K. Zaghib, J. Shim, A. Guerfi, P. Charest and K. A. Striebel, “Effect of Carbon Source as Additives in LiFePO4 as Positive Electrode for Lithium-Ion Batteries,” Electrochemical and Solid-State Letters, Vol. 8, No. 4, 2005, pp. A207-A210. doi:10.1149/1.1865652
[15]	K. Zaghib and M. Armand, “Electrode Covered with a Film Obtained from an Aqueous Solution Containing a Water Soluble Inder, Manufacturing Process and Usesthereof,” Canadian Patent No. CA 2411695, 2002.
[16]	K. Striebel, J. Shim, V. Srinivasan and J. Newman, “Comparison of LiFePO4 from Different Sources,” Journal of the Electrochemical Society, Vol. 152, No. 4, 2005, pp. A664-A670. doi:10.1149/1.1862477
[17]	A. Ait-Salah, P. Jozwiak, J. Garbarczyk, K. Benkhouja, K. Zaghib, F. Gendron and C. M. Julien, “Local Structure and Redox Energies of Lithium Phosphates with Olivine- and Nasicon-Like Structures,” Journal of Power Sources, Vol. 140, No. 2, 2005, pp. 370-375. doi:10.1016/j.jpowsour.2004.08.029
[18]	R. Bacewicz, P. Woroniecki and J. Garbarczyk, “Raman Scattering in AgI-Ag4O-P4O4 Glasses,” Physics and Chemistry of Glasses, Vol. 40, No. 3, 1999, pp. 175-176.
[19]	A. Adamczyk, M. Handke and W. Mozgawa, “FTIR Studies of BPO42SiO2, BPO4SiO2 and 2BPO4SiO2 Joints in Amorphous and Crystalline Forms,” Journal of Molecular Structure, Vol. 511-512, 1999, pp. 141-144. doi:10.1016/S0022-2860(99)00152-0
[20]	R. P. Santoro and R. E. Newnham, “Antiferromagnetism in LiFePO4,” Acta Crystallographica, Vol. 22, 1967, pp. 344-347. doi:10.1107/S0365110X67000672
[21]	G. Arnold, J. Garche, R. Hemmer, S. Strobele, C. Vogler and M. Wohlfahrt-Mehrens, “Fine-Particle Lithium Iron Phosphate LiFePO4 Synthesized by a New Low-Cost Aqueous Precipitation Technique,” Journal of Power Sources, Vol. 119-121, 2003, pp. 247-251. doi:10.1016/S0378-7753(03)00241-6
[22]	C. M. Julien, K. Zaghib, A. Mauger, M. Massot, A. Ait-Salah, M. Selmane and F. Gendron, “Characterization of the Carbon Coating onto LiFePO4 Particles Used in Lithium Batteries,” Journal of Applied Physics, Vol. 100, No. 6, 2006, Article ID: 063511. doi:10.1063/1.2337556
[23]	S. H. Yu, C. K. Park, H. Jang, C. B. Shin and W. II Cho, “Prediction of Lithium Diffusion Coefficient and Rate Performance by Using the Discharge Curves of LiFePO4 Materials,” Bulletin of the Korean Chemical Society, Vol. 32, No. 3, 2011, pp. 852-856.
[24]	K. Zaghib, P. Charest, M. Dontigny, A. Guerfi, M. Petitclerc and M. Duchesne, “Olivines: 10 Years R & D at Hydro-Québec in Li-Ion Batteries,” Rechargeable Lithium and Lithium Ion Batteries Battery/Energy Technology, Washington DC, 7-12 October 2007, Abstract No. 637.