Analysis of Phase Transition Dynamics Under Temperature Controlled Conditions

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 LiNi_0.5Mn_1.5O₄ 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. Experiments

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 LiNi_0.5Mn_1.5O₄ during potential step charging and discharging at 40 and -10 ºC. The cell potential was immediately changed to force a complete phase transition from LiNi_0.5Mn_1.5O₄ (Li₁) to Ni_0.5Mn_1.5O₄ (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 Li_0.5Ni_0.5Mn_1.5O₄ (Li_0.5) as reported [1]. On discharging, the reverse reaction proceeded but the XRD intensity of Li_0.5 was weak. This can be explained by the kinetics of Li_0.5 ↔ Li₀ being slower than that of Li₁ ↔ Li_0.5 [2]. Namely, the Li_0.5 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 Li_0.5 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 Li_0.5 ↔ Li₀ is kinetically slower than Li_0.5 ↔ 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_xFePO₄ 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).