Elsevier

Journal of Power Sources

Volume 196, Issue 3, 1 February 2011, Pages 1365-1370
Journal of Power Sources

7Li nuclear magnetic resonance studies of hard carbon and graphite/hard carbon hybrid anode for Li ion battery

https://doi.org/10.1016/j.jpowsour.2010.09.026Get rights and content

Abstract

The hard carbon is attractive for the Li ion battery because of its higher capacity than the theoretical value of 372 Ah kg−1 based on the composition of stage 1 Li-intercalated graphite, LiC6. However, since the Li-doping reaction occurs at the potential of around 0 V versus Li/Li+ reference electrode, it is often pointed out the possibility of Li metal deposition on the surface of anode. From the viewpoint of the safety, it may be a moot point. In the present study, 7Li NMR measurement was performed to estimate the degree of Li metal deposition on the surface of graphite and hard carbon anode. As a result, it is clarified that the Li metal deposition does not occur up to 110% over-discharge of the reversible capacity of hard carbon, whereas in the case of graphite anode, Li metal deposition occurred above 105% over-discharge of the capacity. From the 7Li NMR spectroscopy, the safety limit of hard carbon is rather superior to that of graphite.

Introduction

About twenty years have passed since the Li ion battery using a carbon as an anode was commercialized. During the first few years, the electrochemical performance of most carbons was investigated. Nowadays, two types of carbons are commercially available. The first one consists of what are called “artificial graphites”, which are graphitized above ca. 2500 °C. Since such carbons have high crystallinities, natural graphite should also be included in this group. In general, these graphites show excellent cycle performance, high capacities of 350–370 Ah kg−1, and coulombic efficiencies higher than 90%. Therefore, graphite is commercially used as the anode of most Li ion batteries that are used in many electronic devices such as mobile phones, computers, and digital cameras. However, as the reversible capacity of graphite is limited to 372 Ah kg−1 base on the composition of the 1st stage Li-intercalated graphite, LiC6, newly anode material with higher capacity such as graphite hybridized with silicon has been developed [1]. On the other hand, the hard carbon is the second candidate for the anode of Li ion battery. It was, for example, prepared by the heat treatment of isotropic pitch at 1000–1100 °C, and shows a higher capacity than 372 Ah kg−1. It was firstly reported by Takahashi et al. and Sonobe et al. [2], [3]. After that, several groups also reported the carbons with similar structure and electrochemical performances [4], [5], [6], [7], [8], [9], [10]. The hard carbon materials have been recently received much attention as an anode for the large scale Li ion battery for hybrid electric vehicles because of the excellent cyclability and high input/output performances. It is considered that such carbons accommodate the lithium species in the micro-pores as well as in the interlayer. Tatsumi et al. reported that the 7Li NMR signal observed at room temperature of fully lithiated hard carbon split into two peaks, high chemical shift signal above 100 ppm and low chemical shift at ca. 20 ppm at low temperature [4], [6]. The former signal was explained by quasimetallic Li forming a lithium cluster in the pore of hard carbon structure and the latter was attributed to be the Li ion intercalated into graphene layers. After that, almost the same results were reported by several groups [7], [8]. However, the 7Li NMR studies ever reported were only concerned for the hard carbon with the reversible capacity less than 500 Ah kg−1. Dahn et al. reported that the hard carbons derived from sugar shows higher capacity than 500 Ah kg−1 [9]. We also succeeded the hard carbons with reversible capacity of 520 Ah kg−1 at the pilot plant level [10]. As a matter of fact, all hard carbons are not identical in composition, texture and structure. They may contain variable amounts of heteroatoms and have different open and closed pores. As a result, the sites available to lithium accommodation may differ from each other, which can lead to different 7Li NMR results. Hence, it is important to elucidate the pore structures in detail. Moreover, since the Li-doping reaction occurs at the potential of around 0 V vs. Li/Li+ reference electrode, it is often pointed out the possibility of Li metal deposition on the surface of anode. It may be a moot point from the viewpoint of the safety. In the present study, two kinds of hard carbons with high and low reversible capacities and the hybrid anode with graphite and hard carbon were investigated by means of 7Li NMR measurement in order to compare the state of Li species among them and to estimate the safety limit for the Li metal deposition by the over-charging of the Li-ion battery.

Section snippets

Preparation of carbons

As a starting material for the preparation of hard carbon, two kinds of pitches with different oxygen contents (9.4 wt% and 15.2 wt%, Osaka Gas Chemical Co., Ltd.) were used. The materials were milled and classified, so that the median diameter, D50 was adjusted to ca. 5–7 μm. Thus obtained pitches were carbonized at 1100 °C. In the following, the hard carbons prepared from the pitches with 9.4 wt% and 15.2 wt% oxygen contents will be abbreviated as “HC-9” and “HC-15”, respectively. In order to

Charge/discharge profiles of hard carbons

Fig. 1 shows the charge/discharge profiles of NBP-Graphite and two kinds of hard carbons, HC-9 and HC-15. The electrochemical characteristics are summarized in Table 1. NBP-Graphite showed a typical electrochemical performance of graphite, and its initial coulombic efficiency and reversible capacity were 84.6% and 345 Ah kg−1, respectively. HC-9 and HC-15 also showed a typical profile of hard carbon. That is, the plateau below 0.2 V and turning point at ca. 0.2 V were observed in the discharge

Conclusion

As discussed above, the 7Li NMR measurement revealed that the graphite can discharged up to only 105% without Li metal deposition, whereas the hard carbon can be discharged up to 120–130%. It is concluded that the latter is superior to the former from the viewpoint of battery safety. Secondary, the hard carbon has multiple sites for Li storage, that is, reversible and irreversible sites for the Li-doping/dedoping reaction. The ratio of the irreversible sites is estimated to be 20–30% of

Acknowledgements

This study was conducted as a part of national project, “Dispersed type Battery Energy Storage Technology Research and Development” under the contract with New Energy and Industrial Technology Development Organization (NEDO) for the “New Sunshine Program” by Agency of Industrial Science and Technology (AIST), Ministry of International Trade and Industry (MITI). One of us (C. Natarajan) sincerely acknowledges NEDO for the award of research fellowship.

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