Anodic performances of cokes derived from a series of coals with variable ranks were examined in a Li-ion secondary battery by carbonizing at 700 and 1000°C under normal, pressurized and co-carbonized conditions. The coke carbonized at 700°C exhibited discharge at 0.12-0.8 V and 0.8-1.25 V vs Li/Li+ which were reversible too and much higher than the charge potentials, respectively. The capacity of the coke which was different principally in the discharge at 0.8-1.25 V, increased with the rank of the parent coal, reflecting the extent of optical anisotropy. The particular capacity decreased markedly with the charge/discharge cycle. The coke carbonized at 1000°C exhibited discharge at 0.12 V and 0.12-1.25 V. The coke from lower rank caused a larger capacity except for the coke from Yallourn coal of the lowest rank. The pressurized carbonization increased the capacity of both cokes prepared at 700 and 1000°C from Tanitoharum coal of lower rank, while the co-carbonization increased the capacity of the coke only carbonized at 700°C, inducing some anisotropy. The three kinds of sites in the coke to accommodate the reduced lithium ion are discussed based on the performances. Charge-transfer site for 0.12 to 0.8 V discharge, edge of hexagonal plane for 0.8 V discharge and some voids among hexagonal carbon clusters are assumed for both cokes at 700 and at 1000°C, respectively. The second and third sites are present particularly in the anisotropic-graphitizable and isotropic glass-like cokes, respectively. Improved efficiency in the first cycle was observed by the pressurized carbonization, being apparently related to the surface of coke coated with carbon derived from volatile matters.
All Science Journal Classification (ASJC) codes
- Materials Science(all)