Lithium iron phosphate (LiFePO₄) is known to undergo a two-phase coexistence reaction during charging and discharging, and this phase transition is reported to be the rate-determining step of the cell reaction (1). Such a phase transition involves not only the rearrangement of the LiFePO₄/FePO₄ border but also the charge transfer process at the electrode/electrolyte interface, but the detail has been so far unknown. If the rearrangement of the solid border is truly the rate-determining step, it is expected that the phase transition behavior is not affected by the type of electrolyte used for the cell. In this study, we compare the activation energy of the phase transition reaction of LiFePO₄ using the potential step measurements in non-aqueous and aqueous electrolyte cells to clarify the effect of the charge transfer at the electrode/electrolyte interface.

The LiFePO₄ powder (Hosen) was mixed with carbon black (Denka) and polyvinylidene fluoride (Kureha) in a weight ratio of 85:10:5 with 1-methyl-2-pyrrolidone (KANTO CHEMICAL, 99%). The slurry was coated onto a titanium current collector and dried at 80°C. A non-aqueous-type three-electrode cell composed of the LiFePO₄ as a working electrode, lithium foils as a counter and reference electrodes, and a 1 mol dm^–3 solution of LiPF₆ in ethylene carbonate and diethyl carbonate (1:1 v/v) as an electrolyte solution was constructed in an Ar-filled glove box. An aqueous-type three-electrode cell was composed of the LiFePO₄ as a working electrode, a Ni wire as a counter electrode, Ag/AgCl/sat’d KClaq as a reference electrode, and a 0.5 mol dm^–3 Li₂SO₄ aqueous solution as an electrolyte solution. The charging–discharging behavior was firstly measured to confirm the cell characteristics and then the potential step measurement was employed. The Avrami plots based on the potential step results (2) were used to obtain the reaction rate constant k. These electrochemical measurements were performed at the chosen temperature. Then we calculated the activation energy of kusing the Arrhenius plot. The electrochemical measurements were performed in a constant temperature oven.

Figure 1 (a) shows the transient current behavior for the cell with the aqueous electrolyte measured during the potential step from open circuit potential to 0.4 V, where LiFePO₄ is completely converted into FePO₄. Figure 1 (b) shows the Avlami plot (2), where f corresponds to the volume fraction of the converted FePO₄, calculated from the accumulated current values. The slope was nearly unity, indicating that the system follows the first-order reaction kinetics f = 1-exp(-kt), as well as the case for the non-aqueous electrolytes. Then, we calculated the activation energy using the Arrhenius plot in the aqueous electrolyte and the non-aqueous electrolyte. The activation energy of the aqueous electrolyte was smaller than that of the non-aqueous electrolyte, suggesting the effect of the electrode/electrolyte interface on the phase transition behavior.

References

(1) C. Fongy, A.-C. Gaillot, S. Jouanneau, D.Guyomard, and B. Lestriez, J. Electrochem. Soc., 157, A885-A891 (2010).

(2) J. Allen, T.R. Jow, and J. Wolfenstine, Chem. Mater., 19, 2108-2011 (2007).

(3) Y. Orikasa, T. Maeda, Y. Koyama, T. Minato, H. Murayama, K. Fukuda, H. Tanida, H. Arai, E. Matsubara, Y. Uchimoto, and Z. Ogumi, J. Electrochem. Soc. 160, A3061-A3065 (2013).