Investigation of Mechanism of Reaction Distribution Formation in Composite Cathodes for Li Ion Batteries By Using Operando 2D-XAS
Authors: Chiba Kazuki , Kimura Yuta , Watanabe Toshiki , Nakamura Takashi , Amezawa Koji , Tanida Hajime , Uchimoto Yoshiharu , Ogumi Zempachi 
Published: February 2020

Lithium-ion batteries (LIBs) are currently growing in popularity for high rate/output applications. However, it is known that, under high rate charging/discharging, the present composite cathodes for LIBs often show lower capacity than expected. One of the reasons of such a capacity loss is reaction distribution formation in the composite cathode. The reaction in the composite cathodes is considered to be strongly influenced by the sluggish ion transport [1]. Thus, it is important to understand the mechanism of the reaction distribution formation, especially to understand the influence of the ion transport in composite cathodes on the reaction distribution formation. From the above backgrounds, we have proposed to use a model composite cathode and applied two-dimensional X-ray absorption spectroscopy (2D-XAS) to this model cathode to observe the reaction distribution [2]. The ion transport in the model composite cathode is restricted to only along the in-plane direction. If the ion transport is a rate-controlling process, the reaction distribution is formed in the in-plane direction. Therefore, the reaction distribution formed due to the slow ion transport can be easily evaluated by 2D-XAS. In this study, we performed operandoobservation of the reaction distribution formation in the model composite cathode, while changing parameters which may influence the ion transport, such as temperature, charging rate and salt concentration. Taking the obtained results into account, we discussed the mechanism of the reaction distribution formation in a model composite cathode.
LiCoO2 (LCO) powders, acetylene black, and organic binder PVDF were mixed with the weight ratio of 80:10:10 to fabricate the composite cathode slurry. The slurry was uniformly spread on an aluminum foil, and covered by a kapton film. After the slurry with the kapton film was dried, the electrode sheet was cut into 10 × 10 mm and the model composite cathode was obtained. Electrochemical cells were fabricated with the model composite cathode as a cathode, Li metal as an anode, EC-EMC (3:7 v/v%, LiPF6) as electrolyte and separator. The electrochemical cells were charged up to 4.2 V at various temperatures (287, 300 and 313 K) and with different charging rate (4.5, 6 and 8 mA·cm-2) and different salt concentration (0.3, 1 and 2 mol·l
-1). The reaction distribution formed during charging in the model composite cathode was evaluated by using operando 2D-XAS measurements, which were carried out at the beam line BL37XU in the SPring-8, Japan. In our measurements, the state of charge (SOC) of LCO was evaluated from the peak top energy of Co-Kedge XANES spectra. The time resolution and position resolution were 1.5 min. and 6.5 × 6.5 µm respectively.
Figure 1 (a), (b), and (c) show the reaction distribution in the model composite cathode at the cut off voltage at 287, 300, and 313 K, respectively. The black area means charged area while the white area means uncharged area. Regardless of temperature, the edge of the model composite cathode was deeply charged while the inner part was little charged. This indicates that formation of the reaction distribution is caused by slow ion transport. Comparing the three conditions, the reacted area was narrower when the temperature was lower.
It is considered that resistance of the ion transport and that of the charge transfer change when the temperature changes. It is known that ion conductivity is larger at higher temperature [3]. Therefore, the resistance of the ion transport becomes smaller at higher temperature. Also, it is known that the resistance of the charge transfer becomes smaller at higher temperature [4]. Taking these into account, the reacted area become wider. The above discussion can qualitatively explain the experimental results shown in Fig. 1. In the presentation, we will discuss the formation mechanism of the reaction distribution in more detail.
[1] J. Liu, et al., J. Phys. Chem. Lett.,
1(2010) 2120-2123.
[2] T. Nakamura, et al., Solid State Ionics, 262(2014) 66-69.
[3] Z. Zhang, et al., Vehicle Technologies Program Annual Merit Review and Peer Evaluation Meeting (2012) 1-22.
[4] Xiang-Yun Qiu, et al., phys. Chem. Chem. Phys., 14 (2012) 2617-2630.

Figure 1