The electrochemical reaction of lithium with high-capacity dense sputtered carbon

Carbon films were prepared by DC magnetron sputtering at various argon pressures ranging from 1 to 30 mTorr. The film sputtered at the lowest pressure (1 mTorr) was fully amorphous, showing no evidence of the (0 0 2) Bragg diffraction peak, and a density of 1.9 ± 0.3 g/cc, indicating little, if any, porosity. The film sputtered at the highest pressure (30 mTorr) showed a weak and broad (0 0 2) Bragg peak and had a density of 1.35 ± 0.15 g/cc, indicating significant porosity. The low pressure sputtered films were almost pure carbon (<1% O by weight) while the high pressure sputtered carbon films contained between 13% and 21% O by weight (depending on the measurement method) which was presumably incorporated when the porous films were exposed to air after sputtering. Electrochemical testing in Li/carbon film cells showed that the low pressure sputtered carbon had a reversible specific capacity of about 800 mAh/g between 5 mV and 2 V, an irreversible capacity of 200 mAh/g and an average delithiation potential of about 1 V vs. Li/Li+. Heating the low pressure sputtered carbon to 900 °C in argon decreased the reversible capacity, the irreversible capacity and the average voltage to 600 mAh/g, 100 mAh/g and 0.75 V, respectively. Since the density of these films is almost as large as graphite, these unheated materials have volumetric capacities near 1600 mAh/cm3 while the heated materials are near 1200 mAh/cm3.