Characterization of Electrode Materials during Degradation Process of Li-ion Batteries

Noboru SATO*, Koichi HIRAO, Eiji HAYASHI and Gen KATAGIRI

Tochigi R&D Center, Honda R&D Co., Ltd.; 4630 Shimotakanezawa, Haga-machi, Haga-gun, Tochigi 321-3393 Japan
Toray Research Center Inc.; 3-3-7 Sonoyama, Otsu-shi 520-8567 Japan

Lithium ion batteries using LiMn2O4 for a positive electrode and graphite for a negative electrode are expected as power sources for electric vehicles (EV) or hybrid electric vehicles (HEV) due to superior high specific energy and power. However, cycle degradation is a serious problem, in particular, at high temperature region because batteries in EV and HEV are used in a wide range of temperature from −30 to 55 °C.
In this work, the cycle degradation of battery at 45 °C and the degradation mechanism have been investigated. The cycle degradation with lowering the capacity and the rise in the internal resistance of the battery was observed. In order to examine the mechanism of this degradation, the structural and compositional change of each electrode were investigated by various analytical methods.
The large dispersion of Mn (II) at electrode surface was observed by means of ESR for the degraded electrode (Figs. 3 and 4). A broad 7Li-NMR peak was observed at 400--600 ppm in the initial sample, while it disappeared and another broad peak was observed at 0 ppm in the degraded sample (Fig. 5). This fact suggests that the Jahn-Teller-distortion induced a phase transition from cubic LiMn2O4 to tetragonal Li2Mn2O4 phase and the disorder of the structure. The phase transition is considered to be one of the origins of cycle degradation, since the lithium ion mobility is smaller in tetragonal than in cubic phase.
The thick film was observed on the surface of degraded negative electrode. The phosphoric acid component which is considered to be a decomposition product of LiPF6 was observed in the film (Fig. 7). It is suggested that the formation of film on the negative electrode will be one of the reasons why the increase of internal resistance of Li-ion batteries is observed in cycle degradation.

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