Safety Characteristics of Chemically Delithiated Cathode Active Materials

1. Introduction

Lithium ion batteries are utilized for various applications, from small size battery for mobile phones and computers, to large scale energy source for automobile and stationary energy supply. It is necessary for lithium ion battery to be charged and discharged safely under any circumstance and usage, and various kinds of active materials have been studied for this purpose. Especially for large scale energy storage, thermal and structural stability of active materials is quite important. In this study, cathode active materials in their charged state are obtained by chemical extraction of lithium using NO₂BF₄ oxidizer, and by using those samples, we can evaluate the thermal and structural stability of cathode active materials alone without any interference of other electrode components such as carbon and binder additives.

  2. Experimental

Lithium has been chemically extracted from lithium nickel oxide (LiMeO₂, Me = Ni_0.80Co_0.15Al_0.05, NCA) with NO₂BF₄ in acetonitrile ^{(1, 2)}. Chemical extraction of lithium has been carried out in a dry nitrogen-filled glove box in which the atmosphere is kept low levels of oxygen(～10 ppm) and water (dew point lower than – 55℃), respectively.

LiMeO₂ + NO₂BF₄ → Li_(1-x)MeO₂ + x LiBF₄ + x NO₂↑

Li_(1-x)MeO₂ with various values of lithium content can be obtained by controlling the reaction time with NCA and NO₂BF₄. The products after the reaction were washed by dry acetonitrile under same dry nitrogen atmosphere to remove LiBF₄, and dried under vacuum at ambient temperature. The lithium concentration in the Li_(1-x)MeO₂ oxides were determined by atomic absorption. Thermal decomposition of the obtained samples is evaluated by monitoring oxygen generated during raising sample temperature by TPD-MS(temperature programmed desorption MS), and structural characterizations were also carried out with in situ X-ray diffraction and Raman spectroscopy.

  3. Results anddiscussions

Figure 1 shows the X-ray diffraction patterns of Li_(1-x)MeO₂ with different lithium content ( x= 0.23, 0.60, 0.63, 0.67, 0.70). The observed diffraction patterns can be indexed based on the R-3m space group and are consistent with the layered rock salt structure of a-NaFeO2. Diffraction peak of (003) shifts lower when the lithium extraction proceeds from x= 0.23 to 0.63, and shifts higher for the sample x = 0.67 and 0.70, which is the same tendency as electrochemically charged NCA ⁽³⁾.

Oxygen generation behaviors of Li_(1-x)MeO₂ are shown in Figure 2. From all the samples, oxygen generation is detected around 270 ℃ and 400 ℃, and oxygen generation temperature decreases with increasing amount of lithium extraction from NCA. Oxygen generation at 270 ℃ is due to the structural change from the layered rock salt structure to spinel structure, and 400 ℃ is due to the structural change from spinel to rock salt structure. Addition to it, new peak of oxygen generation around 300 ℃ is observed for the sample x = 0.63 and 0.70. Large amount of lithium extraction possibly generates unstable structure, and the origin will be discussed in the presentation.

References

(1) I. Belharouak, W. Lu, D. Vissers, K. Amine., Electrochemistry Communications 8 (2006) 329–335　　　　　　

(2) S. Venkatraman, A. Manthiram., Solid State Ionics 176 (2005) 291–298　　　　　　　　　

(3) Won-Sub Yoon a, Kyung Yoon Chung b, James McBreen a, Xiao-Qing Yang, Electrochemistry Communications 8 (2006) 1257–1262