Anodic performances of coke from coals

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.