Purely hexagonal graphite and the influence of surface modifications on its electrochemical lithium insertion properties

High-temperature treatment of the highly crystalline synthetic graphite TIMREX(R) SLX50 under inert gas atmosphere led to an increased crystallinity with no evidence of rhombohedral stacking defects in the hexagonal graphite crystal structure as well as a significantly lower specific BET (Brunauer-Emmett-Teller) surface area. The first electrochemical Li+ insertion in this purely hexagonal graphite indicated coinsertion of solvated lithium ions which caused significant exfoliation of the graphite structure and an increased irreversible capacity compared to the untreated graphite. A progressive oxidation treatment of the heat-treated TIMREX(R) SLX50 in air preserved its purely hexagonal crystal structure. However, the exfoliation effects during the first electrochemical Li+ insertion disappeared gradually with the oxidation temperature and finally vanished at oxidation temperatures above 800degreesC. Surface analysis investigations on TIMREX(R) SLX50 before and after heat-treatment indicated a surface curing effect. The amounts of prismatic surfaces (polar edges), low-energy defects located on the graphite basal planes, disordered carbon on the graphite particle surface, as well as the superficial oxygen concentration decreased as a result of the heat-treatment. A progressive oxidation of the heat-treated hexagonal graphite tends to increase the amount of disordered carbon atoms, the oxygen atom concentration, as well as the amount of prismatic surfaces, but keeps the number of low-energy defects unchanged. These results indicated that not the graphite crystal structure but the surface properties are the responsible parameters for the exfoliation of the graphite structure and the irreversible capacity observed during the first electrochemical Li+ insertion. (C) 2002 The Electrochemical Society.