Stability of Hard Carbon Coated Low Purity Natural Graphite As a Lithium-Ion Battery Anode Material

Abstract

Natural graphite is widely used in commercialized batteries by virtue of low cost, high capacity and stable charge/discharge performance [1]. However, multi-level purification process for battery-grade natural graphite (>99.9%) accompanies high manufacturing cost, although the raw material price is low enough. Accordingly, if we can avoid some purification steps such as acid-based purification process without loss of battery performance, we can reduce the manufacturing costs as well as environmental budget. In order to use low-purity graphite as anode material, one needs a way to prevent potential side reactions. In this study, we adopted carbon coating method which has been reported to effectively reduce the irreversible capacity and extend the cycle life of Li-ion battery [2]. The hard carbon coating on the low-purity graphite was expected to improve the reliability of Li-ion battery by blocking the impurity diffusion. For hard carbon coating, spherical graphite was blended with thermosetting polymer and then sintered in a nitrogen atmosphere at 1300℃. For electrochemical measurements, we assembled 2032 type coin half-cells. Characteristics of the half-cell were evaluated as a function of cycle number and with different C-rates. Structural and thermal stability of hard carbon coating layer was also evaluated. To examine the structural stability, microstructure of hard carbon layer before and after the repeated charge-discharge cycles was observed using TEM. Thermal stability was evaluated by measuring capacity fading of cells after the elevated temperature storage. Based on the measured results, we discussed the validity of the hard carbon coating for the practical use of low-purity graphite. ♦ Reference [1] Y.P. Wu, C. Jiang, C. Wan, R. Holze, Solid State Ionics 156 (2003) 283. [2] J.J. Li, X.M. He, C.Y. Jiang, C.R. Wan, J. New Mater. Electrochem. Syst. 9 (2006) 21. ♦ Acknowlgement This research was financially supported by the Ministry of Trade, Industry and Energy(MOTIE) and Korea Institute for Advancement of Technology(KIAT) through the Promoting Regional specialized Industry Figure 1