The surface properties of several well-characterized commercial carbon materials were modified by thermal and chemical treatments. The reversible capacities for lithium intercalation of a sponge green coke and a fuel green coke for lithium intercalation increased by as much as 25% after heat treatment in both reducing (5%H 2 /Ar) and oxidizing (CO 2 ) environments. The irreversible capacity loss increased significantly with CO 2 treatment at 800°C. The trend of larger capacity losses with CO 2 treatment is also observed with a synthetic graphite (SFG6) which was produced by heat treatment at about 3000°C. Carbon fibers that were first impregnated with LiOH solution followed by reaction with CO 2 to form Li 2 CO 3 tended to show lower irreversible capacity losses.
INTRODUCTIONThe development of carbonaceous materials for lithium intercalation covers a wide range of carbons and graphites with different properties. The carbon microstructure, physical properties and chemical composition can be tailored to a large extent to optimize its electrochemical performance such as the reversible capacity, the irreversible capacity loss and cycleability. Factors which affect these and other behavior are being studied extensively in this project. The carbon crystallographic parameters and its chemical composition (i.e., surface functional groups and additives) are generally regarded as critical for obtaining a high lithium storage capacity. Many extensive works and reviews have been reported in the literature (1-6).The irreversible capacity loss during the first cycle is attributed primarily to the formation of the solid-electrolyte interphase (SEI) (7) and associated side reactions. The SEI directly affects the carbon anode (and the cell) performance parameters such as the lithium intercalation/deintercalation capacity, cycling efficiency, charge/discharge rate, cycle life, shelf life and safety. While this phenomena at the lithium/organic electrolyte interphase have been studied extensively (7-13), the actual morphology of the SEI is regarded as complex. It is believed to change with the stage of charge and in different electrolytes (8,14). Peled and co-workers (8-10, 15) have discussed the SEI processes in great details. Aside from the surface area (16), factors which influence the formation of the solid-electrolyte interface (SEI) are much less understood. In widely-used EC, DEC, DMC and PC-based electrolytes, an SEI layer containing primarily Li 2 CO 3 is postulated (8, 15). A number of practical means to reduce the irreversible capacity have been considered. A partial oxidation in an oxidizing atmospheres such as in the presence of acetylene black or flowing oxygen gas at elevated temperature (12) was found to improve both intercalation capacity and cycleability. The increase in performance was attributed to the increased surface area and exposed inner fiber layer but the effects on the irreversible capacity loss was not further elucidated. The removal of adsorbed water and surface hydroxyls during heat treatment in vacuu...