Low-Temperature Synthesis of Highly Crystalline Graphite from Lignite, Sub-Bituminous, Bituminous, and Anthracite Coal Ranks Using Metallic Fe Catalysts

The efficient and low-temperature production of crystalline graphite from coal would open a range of opportunities to reduce production costs and diversify supply chains for commercial graphite production. This study evaluates the synthesis of highly crystalline graphite from lignite, sub-bituminous, bituminous, and anthracite coal ranks using metallic Fe catalysts at process temperatures of 1200–1500 °C. The effects of carbon feedstock particle size, Fe catalyst particle size, Fe:Coal-char ratio, and process time on graphite crystallinity were also evaluated. The results show this approach produces highly crystalline graphite powders with a degree of graphitization (DG%) ranging from 81–96%, d-spacing (d002) of 0.33712–0.33577 nm, crystallite size in the c-axis (Lc) of 39–48 nm, crystallite size in the a-axis (La) of 34–124 nm, number of graphite layers (Nc) of 114–143, hexagonal to rhombohedral ratio (2H/3R) of 51/49–82/18, BET surface area of 3.40–24.14 m2/g, and ash yield of <4.11 wt %. When graphite made from sub-bituminous feedstock is used as a model coal-based material for fabricating lithium-ion battery anodes, the material delivers a first discharge capacity of 301 mAh/g with an initial Coulombic efficiency of 88% at 0.1C, and a fully reversible capacity of 322 mAh/g with ∼100% Coulombic efficiency over 100 cycles at 0.5C. This study establishes that all coal ranks can be efficiently graphitized using metallic Fe catalysts and can be used to produce highly crystalline graphite that can be directly incorporated into the current graphite supply chain.