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.

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].