On the reduction of lithium insertion capacity in hard-carbon anode materials with increasing heat-treatment temperature

Non-graphitizable'' or hard" carbon anode materials have almost twice the capacity (per unit mass) of graphitic materials that currently represent the industrial standard for Li-ion batteries. One problem with hard carbon is the small hysteresis in the voltage profile between charge and discharge. This small hysteresis has been correlated to residual hydrogen content after pyrolysis (<0.5% by mass) and can almost be eliminated by increasing the heat-treatment temperature (HTT) above 1100 degrees C. However at this temperature hard carbon begins to show a reduction in reversible capacity. This capacity reduction is correlated to a shift in chemical potential of lithium inserted into the hard-carbon structure and to the closure of micropores in the sample. Hard carbons were prepared by pyrolysis of sucrose between HTTs of 900 and 1400 degrees C. The structure of these materials was:determined by wide angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). WAXS results show that the number of stacked layers increases with HTT, and SAXS measurements show that the micropore size also increases with HTT. N-2 Brunauer-Emmett-Teller (BET) surface area and CO2 gas adsorption measurements show a dramatic decrease in surface area and open micropore volume for HTTs greater than 1100 degrees C. These results indicate that the micropores in the samples begin to close and produce what we call "embedded fullerenes." Based on.recent results of intercalation in C-60, we believe that embedded fullerenes are impenetrable by lithium, and as a result the number of available sites for lithium insertion decreases. This, we propose, is the mechanism for the observed capacity loss in hard carbon at HTTs greater than 1100 degrees C.