Analysis of Phase Transition Dynamics Under Temperature Controlled Conditions
Authors: Arai Hajime , Takahashi Ikuma , Naka Takahiro , Komatsu Hideyuki , Uyama Takeshi , Fukuda Katsutoshi , Murayama Haruno , Orikasa Yuki , Koyama Yukinori , Matsubara Eiichiro , Uchimoto Yoshiharu , Ogumi Zempachi 
Published: February 2020

1. Introduction

Analyzing dynamic phenomena occurring in rechargeable batteries is indispensable to elucidate the charge-discharge mechanism of electrode materials and to improve battery performances such as rate capability and durability. For electric vehicle applications, batteries as power sources can be used at various temperature environments and thus the dynamic behavior at various temperatures should also be clarified. To capture such non-equilibrium behavior at various temperatures, analytical methods that have high time resolutions should be employed under temperature controlled conditions. In this study we select LiNi0.5Mn1.5O4 and LiFePO4 electrodes as examples and report their dynamic behavior measured in situ using temperature controlled X-ray diffraction (XRD) analysis with a high time resolution (0.5 s / pattern).

2. Experiment
s

Pouch-type cells containing composite working electrodes and metallic lithium as counter and reference electrodes were used for the in situmeasurements. The XRD measurements (wavelength =1 Å) were conducted in transmission modes at BL28XU and BL46XU at SPring-8, Japan.

3. Results and discussion

Figure 1 shows the behavior of LiNi0.5Mn1.5O4 during potential step charging and discharging at 40 and -10 ºC. The cell potential was immediately changed to force a complete phase transition from LiNi0.5Mn1.5O4 (Li1) to Ni0.5Mn1.5O4 (Li0) on charging and vice versa on discharging. At 40 ºC the current reached zero and the reaction completed in ca. 10 min. The phase transition occurred on charging from Li1 to Li0 via Li0.5Ni0.5Mn1.5O4 (Li0.5) as reported [1]. On discharging, the reverse reaction proceeded but the XRD intensity of Li0.5 was weak. This can be explained by the kinetics of Li0.5 ↔ Li0 being slower than that of Li1 ↔ Li0.5 [2]. Namely, the Li0.5 phase can have XRD coherent domains before it is oxidized to Li0 on charging while it is immediately reduced to Li1 on discharging, due to the close potential values of the two redox couples. The capacity at -10 ºC was half of that at 40 ºC after 30 min for both charging and discharging. Figure 1 indicates that at -10 ºC all the Li1 phase changes to Li0.5 on charging, while a half of the Li0 phase changes to Li1 and the rest remains as Li0 on discharging. This result again demonstrates that the phase transition Li0.5 ↔ Li0 is kinetically slower than Li0.5 ↔ Li0, which is more pronounced at lower operating temperatures. It is also noted that the solid-state phase transition is the rate determining step in this case, rather than the ion transportation in liquid electrolytes that is often considered as a slow process at low temperatures. We also show that a metastable Li
x
FePO4 phase [3] is well stabilized at low temperatures. These results show the importance of in situ techniques that directly capture the reacting species.

Acknowledgement

This work was supported by RISING project of NEDO.

References

[1] S. Mukerjee et al., Electrochim. Acta, 49 3373 (2004).
[2] H. Arai et al., J. Mater. Chem. A, 1, 10442 (2013).
[3] Y. Orikasa et al., J. Amer. Chem. Soc., 135, 5497 (2013).