Abstract:
Graphite has been used as an anode material in state-of-the art Li-ion batteries due to high capacity (337 mAh/g) and low potential (0.1 — 0.3 V vs. Li+/LI) of its lithium intercalation compound (Li,C6, X=1) (Kurzweil and Brandt, et al., 2009). Reversible intercalation and deintercalation of Li+ ions with graphite are attributed to successful formation of a stable and protecting solid electrolyte interface (SE1) on the graphite surface, which is known to complete in initially few cycles. Previous studies have shown that formation of the SE1 is greatly affected by electrolyte composition, morphology and surface chemistry of graphite (Fu et al., 2006). For this reason, only a limited number of graphite has been found to be suitable for the anode of Li-ion batteries. To use natural graphite that is inexpensive and abundant, many researchers have currently focused on the surface modification of natural graphite (Fu et al., 2006). The graphite surface can be modified by mild oxidation in air and using solution of strong oxidant (Balasooriya, et al., 2006,2007). Mild oxidation induce acidic group on the graphite surface, which act as surface film to produce SEI resulting high reversible capacity. However, in gas-solid inter-phase oxidation reaction, control of the homogeneity of the product is difficult to maintain. Consequently, a liquid-solid interface oxidation reaction has been introduced as the use of chemical oxidant. Nitric acid is well known as a strong oxidant and its standard potential is > I .5V (Wu et al., 2003).
Sri Lanka is well known for high quality vein graphite, containing 95-99% of pure carbon (Herath, 1995). They were categorized into four structural types, namely, coarse flakes of radial (CFR) graphite, coarse striated-flaky (CSF) graphite, needle-platy (NPG) graphite and shiny-slippery-fibrous (SSF) graphite, based on the structural and physical characteristics (Touzain, et al., 2010). Recent investigation pointed out that vein graphite from Bogala mine have sufficient electrical conductivity to be used as potential candidate for Lithium ion rechargeable batteries (Geethika, et al., 2010) and chemical oxidation in (NH4)2 5208 increase the reversible capacity (Balasooriya, et al., 2006,2007). The present study aims to chemical oxidation of natural vein graphite in nitric acid in order to upgrade the quality of vein graphite as anode material for lithium ion rechargeable batteries.
