Phase Transition Behavior of Biphasic Systems Affected By the Charge-Transfer Process
Authors: Yamamoto Chihiro , Ikezawa Atsunori , Arai Hajime 
Published: February 2024

Lithium-ion batteries have attracted attention with the development of electric vehicles and the introduction of renewable energy. For use in these applications, lithium-ion batteries with fast charge and discharge acceptability are particularly important. The charge and discharge reactions of lithium-ion batteries are limited by several factors such as ion and electron transfer in the electrode and electrolyte, charge transfer at the electrode-electrolyte interface, and reaction rates during phase transitions.
It has been reported that the rate limiting factor of the biphasic materials is the phase boundary rearrangement, and the activation energy of such phase transition has been investigated in previous studies. However, there are variations in the reported activation energy values even for the same electrode material. Since there are many unexplored factors that affect the phase transition rate, we have examined the effect of these factors on the phase transition rate using LiFePO4as model electrodes. In our previous presentation, we showed that the phase transition behavior of LiFePO4 electrode changes depending on the electrolyte, suggesting the influence of the electrode/electrolyte interface. (1) In this presentation, we report the relationship between the activation energy of the biphasic reaction and that of the solvation/desolvation of the electrolyte measured using a four-probe cell (2), and the effect on the phase transition behavior when the transferred ion species is changed from Li+ to Na+.
A non-aqueous-type three-electrode cell was 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 and 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 KCl aq 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 (3) were used to obtain the reaction rate constant k. Further experiments at different temperatures were performed and the Arrhenius plots were drawn to calculate the activation energy of the phase transition. (3) The activation energy for solvation/desolvation of the electrolyte was calculated by making a four-probe cell with the counter electrode and reference electrode symmetrically arranged across the solid electrolyte Li2OAl2O3SiO2P2O5TiO2 (OHARA) and measuring AC impedance spectra at different temperatures. For the Na+ system, the LiFePO4 electrode was electrochemically delithiated to produce FePO4, and then Na+ was inserted into FePO4 to obtain NaFePO4 in the aqueous electrolyte. Electrochemical measurements were performed with the obtained electrode.
The potential step measurements were performed with the aqueous and non-aqueous electrolyte cells. Their activation energies were obtained from the Arrhenius plots with the k values and compared. The activation energy measured in the aqueous electrolytes was smaller than that measured in the nonaqueous electrolyte. The activation energy for the solvation/desolvation processes was also smaller for the aqueous electrolytes system, suggesting that the charge transfer process between electrode and electrolyte is involved in the rate-determining process of the phase transition. Next, we performed the same experiment by changing the mobile ion species from Li+ to Na+. The activation energy for Na+extraction was nearly zero, suggesting very fast phase transition. If the volume change corresponding to the phase transition limits the rate, large Na+ extraction would lead to high activation energy than Li+ extraction. The tendency was the opposite, suggesting that the weak interaction of Na+ and water affects the whole phase transition.
References
(1) C. Yamamoto, A. Ikezawa and H. Arai, The 242nd Electrochemical Society Meeting, A02-0130 (2022).
(2) Z. Ogumi., Electrochemistry, 78, 319 (2010).
(3) J. Allen, T.R. Jow, and J. Wolfenstine, Chem. Mater., 19, 2108-2011 (2007).
(4) 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).

Figure 1