Effect of carbon black materials on the electrochemical properties of sulfur-based composite cathode for lithium-sulfur cells.

To investigate the effect of the electrode materials on the electrochemical performance of Li-S cells, sulfur cathodes were constructed using four types of carbon blacks: Ketjenblack EC-600JD (KB-600), Printex XE-2, Cabot BP-2000, and Super-P. It was found that the electrochemical performance of sulfur cathode was strongly dependent on the type of carbon black used. In the first discharge, the sulfur cathodes containing carbon blacks with a high surface area, KB-600 (SBET = 1270 m2/g), Printex XE-2 (SBET = 950 m2/g), or Cabot BP-2000 (SBET = 1487 m2/g), showed much higher discharge capacities (>1200 mA h/g) than the sulfur cathode (710 mA h/g) with Super-P (SBET = 62 m2/g). It was observed that the sulfur cathodes with KB-600, Printex XE-2, or Cabot BP-2000, which showed very similar discharge capacities one another at a low rate of 0.2 C, exhibited significantly different electrochemical behavior (the discharge capacity and midvoltage) at a high rate of 1.0 C. In particular, the sulfur cathode with KB-600 showed an extremely high capacity (831 mA h/g) with a midvoltage of 2.07 V at a 1.0 C rate, and excellent capacity retention (79%) after 50 cycles.

Synthesis of amorphous carbon materials for lithium secondary batteries.

A new and effective approach to enhance electrochemical properties of amorphous carbons is presented. Phosphorus-doped amorphous carbons have been prepared by incorporating a phosphorus compound into petroleum cokes and carbonizing them at 850 degrees C for 1 h. It was observed that reversible capacity of amorphous carbons was greatly improved by incorporating a very small amount of phosphorus (around 1%), implying that extra lithium-storage-sites were created by phosphorus doping. In addition, the phosphorus-doped amorphous carbons showed outstanding rate capability (205 mA h/g at 5 C) and excellent capacity retention of about 90% after 50 cycles, comparable to that of undoped carbons. Very interestingly, a trade-off relation between capacity and cycle property, which is very common in electrode materials, was not found in the phosphorus-doped amorphous carbons.