Determination of the Diffusion Coefficient of LiFePO4

Abstract

The galvanostatic intermittent titration technique (GITT) and the impedance spectroscopy (IS) have been extensively used to calculate the lithium chemical diffusion coefficient in intercalation materials. The classical application of these techniques is related to systems in which the concentration of the intercalant changes monotonically as intercalation proceeds. Such a situation is valid only for topotactic solid-state intercalation reactions that lead to the formation of solid-solution phases. When the intercalation/de-intercalation of lithium is accompanied by strong electron–ion interactions, the intercalation proceeds following one or several reaction fronts, and leads to the co- existence of two phases (Andersson et al., Electrochem. Solid State Lett. 3, 66–68, 2000; Conway Electrochim. Acta. 38, 1249–1258, 1993). The co-existence of two phases makes the meaning of the chemical diffusion coefficient as a function of the composition unclear. In such a case, the chemical diffusion coefficient as obtained from GITT and IS measurements, may be taken as an effective measure which reflects the intensity of long- and short-range interactions between the intercalated species. The intercalation of lithium into electrode materials can be described and treated similarly to an adsorption process at the metal/solution interface (Vorotyntsev and Badiali, Electrochim. Acta. 39, 289–306, 1994) or to the charging of electronically conductive polymers (McKinnon and Haering, Modern Aspect in Electrochemistry, Plenum Press, New York, 1987). In the case of strong interactions between the intercalated species, a Frumkin-type sorption isotherm was used to describe the intercalation process and derive fundamental thermodynamic properties (Vorotyntsev and Badiali, Electrochim. Acta. 39, 289–306, 1994; Levi et al., J. Electrochem. Soc. 146, 1279–1289, 1999; Sato et al., J Power Sour 68, 674–679, 1997; Nishizawa et al., Electrochem. Solid State 1, 10–12, 1998; Barker et al., J. Power Sour 52, 185–192, 1994). In this chapter the chemical diffusion coefficient of lithium in 10 wt% carbon added LiFePO4 was calculated by using GITT and IS. The lithium insertion in LiFePO4 was treated as an insertion process with a Frumkin-type sorption isotherm (Prosini et al., Solid State Ionics 148, 45–51, 2002). A minimum in the lithium chemical diffusion coefficient (DLi) was predicted by the model for strong attractive interactions between the intercalation species and the host matrix. The DLi was measured as a function of the lithium content by using the galvanostatic intermittent titration technique (GITT). The diffusion coefficient was found to be 1.8 × 10?14 and 2.2 × 10?16 cm2 s?1 for LiFePO4 and FePO4, respectively with a minimum in correspondence of the peak of the differential capacity. The DLi has also been measured by impedance spectroscopy for various lithium contents. The calculated values are in very good agreement with the previously calculated ones.