Abstract:
In order to explore the potential application of low rank coal as negative electrode material in lithium-ion batteries, low rank coal from the Shendong Bulianta and Jinjie mining areas were subjected to processes such as crushing, purification, oxidation, and carbonization. The chemical composition and structure of raw coal, intermediate products in each process, and the final hard carbon were systematically analyzed through industrial analysis, X-ray spectroscopy, CHNSO element analysis, and physical adsorption/desorption. And the performance of coal based hard carbon as a negative electrode for lithium batteries was studied. X-ray diffraction shows that both low rank coal raw materials in the two mining areas contain a small amount of graphite-like domains, while the graphite-like domains in the Bulianta mining area are more abundant. After air peroxidation, the oxygen content increased significantly, but the size of the amorphous microcrystalline layer inside the low rank coal was not significantly changed, indicating that air oxidation is a mild and efficient oxidation method. After high-temperature carbonization treatment of the oxidized low rank coal at 1 200 ℃, coal based hard carbon was obtained. Compared to the one-step carbonization process that directly increases from room temperature to 1 200 ℃, the two-step carbonization process that involves low-temperature carbonization at 800 ℃ followed by treatment at 1 200 ℃, the treatment at 800 ℃ is beneficial for the growth of microcrystalline structures and the repair of defects. However, due to differences in the aromaticity, crystallinity, and volatile content of raw coal, the coal based hard carbon derived from the two mining areas showed significant differences in structure and performance under the same process. In terms of pore structure, the specific surface area of Bulianta coal after carbonization at 1 000 ℃ reaches 390 m
2/g, while the specific surface area of Jinjie coal after carbonization at
1000 ℃ has been reduced to 18.9 m
2/g. After treatment at 1 200 ℃, the specific surface area of the hard carbon of the two raw materials is between 3−4 m
2/g. In terms of electrochemical performance, the specific capacity and initial coulombic efficiency (ICE) of the hard carbon obtained from the treatment of Bulianta low rank coal at
1000 ℃ are only 235.6 mAh/g and 37.59%, respectively. When the treatment temperature is increased from 1 000 ℃ to 1 200 ℃, the specific capacity and ICE of the hard carbon are significantly increased to over 341.7 mAh/g and about 80.37%; While the specific capacity of Jinjie low rank coal hard carbon treated at 1 000 ℃ reaches to 345 mAh/g. When the treatment temperature raised to 1 200 ℃, the capacity slightly decreased and the ICE increased from 66% to 80%.