Evaluation of the Reaction Distribution in LiCoO2 Composite Electrode Studied By In Situ X-ray Absorption Spectroscopy

1. Introduction

Conventional LiCoO₂ composite cathodes often show insufficient performance and stability under high rate charging/discharging [1]. On the other hand, it was reported that a single particle of LiCoO₂could show sufficient rate characteristics even under high rate discharging [2]. These results indicated that ionic transportation thorough the liquid electrolyte filled in the particle interspaces controls the reaction progress in the composite electrode. The slow ionic transportation in the electrolyte may make a part of the composite electrode electrochemically inactive, resulting in the inhomogeneous reaction distribution and the degradation of the practical charge/discharge capacity.

In this work, the reaction distributions in an LiCoO₂composite electrode were directly evaluated by using the two-dimensional in-situ X-ray absorption spectroscopy (2D-XAS). A composite electrode laminated by a polyimide film was applied so that the influence of the ionic transportation in the liquid electrolyte in the composite electrode was clearly observed.

2. Experimental

To prepare a composite electrode, LiCoO₂, acetylene black and organic binder were mixed with the weight ratio of 75:15:10. The slurry added 1-methyl-2-pyrolidone was uniformly spread on an aluminum foil by the screen printing technique. A polyimide film was put on the spread slurry as an insulating layer against the liquid electrolyte. The electrode was dried in an oven at 353 K. The schematic illustration of the laminated electrode is shown in Fig. 1.

The cell was fabricated by an Li metal, LiPF₆-dissolved EC-DMC (1:1 in volume), and the laminated electrode as an anode, an electrolyte, and a cathode, respectively. The laminated electrode was charged up to 4.75 V vs. Li/Li⁺ with the current density of 90 mAcm^-2for the cross-section of the electrode. The applied current density corresponded to 150 C.

During the charging, 2D-XAS measurements [3] were simultaneously carried out at BL28XU, SPring8, JASRI, Japan. In the measurements, the intensity of the transmission X-ray was recorded by a CCD camera placed behind a fluorescent plate. It took about 80 sec for one measurement. The brightness, e.g. the intensity of the transmission X-ray, in the selected area of the obtained images was first quantified, and the XAS spectrum was obtained by plotting the brightness as a function of the X-ray energy. For LiCoO₂, the absorption peak in Co K-edge XAS spectra is known to shift to higher energy with charging. Thus, in this work, the peak top energy of the Co K-edge XAS spectrum was evaluated as an index showing the reaction progress. By repeating the above procedures for other selected areas, a reaction distribution map was constructed. As references of the charged and discharged active materials, the composite electrodes charged to 0, 25, 50, 75, and 100 % relative to the whole capacity were prepared and their Co K-edge XAS spectra were measured.

3. Result and Discussion

During charging the laminated electrode, the extraction of the Li from active materials is considered to take place preferentially from the electrode edge, if the ionic transport in the electrolyte is slow enough relative to the charging rate. Figure 2 shows two-dimensional reaction maps of the laminated cathode at 0, 152, 228, 304, and 380 sec after starting the charging of the laminated electrode. In these figures, fully charged/discharged area was displayed as black/white color, respectively. These figures clearly show that the charging reaction started from the edge part of the laminated electrode and progressed inhomogeneously in the composite electrode. Active materials far from the edge were not charged even when those near the edge were fully charged. This result demonstrated that the slow ionic transportation in the liquid electrolyte causes inhomogeneous reaction distribution in the composite electrode.

4. References

[1] J. Fergus, J. Power Sources, 195(2010) 939.

[2] K. Dokko, N. Nakata, K. Kanamura, J. Power Sources, 189(2009) 783-785.

[3] T. Nakamura, T. Watanabe, K.Amezawa, H. Tanida, K. Ohara, Y. Uchimoto, Z. Ogumi, Solid State Ionics, in press, DOI:10.1016/j.ssi.2013.10.013 (2013) .