Bituminous Coal Char-Derived Hard Carbon As a Low-Cost Anode Material for Sodium-Ion Batteries

Increasing demands in the global energy markets for large-scale electric energy storage systems have made it essential to improve current rechargeable battery technologies. Due to the high price and low safety of current commercial lithium-ion batteries, as well as potential resource limitations of lithium, there is a need to explore alternative ion intercalation battery technologies [1]. Sodium-ion batteries (SIBs), having a theoretical energy density comparable with lithium-ion, are a promising next-generation energy storage technology to replace lithium-ion batteries in large-scale, stationary energy storage applications. A current challenge in achieving high performance SIBs is improving capacity and reducing cost of anode materials. While carbons derived from coal tar pitch have been widely studied as intercalation materials, hard carbons derived from coal char (the solid component of bituminous and sub-bituminous coals) are only recently gaining attention as SIB anode materials with surprisingly high sodium storage capacity, good rate performance, and long cycle life [2]. Bituminous coal char is used primarily as a fuel source for power production, making it an extremely low-cost electrode material. The high mineral ash content of with bituminous coal char, however, is a significant challenge to achieving high anode capacities with this material. This work investigates the effect of pyrolysis conditions and deashing parameters on the material properties and sodium-ion intercalation capacity of hard carbons derived from bituminous, SUFCO mine coal char. SUFCO coal, known as steam coal, is a local Utah coal used for power generation. Pyrolysis temperature and atmosphere, as well as acid/base washing conditions, are varied to produce anode materials with varying carbon structure and ash content. Deashing conditions explored in this work include treating raw coal char with various basic and dilute acid (10%) solutions, as well as a high concentration mixture of H 2 SO 4 :HNO 3 (30:70), at different temperatures (22°C and 80 °C) and for varying times. The structure and composition of the resulting carbons are characterized via scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and pore-size distribution measurements. Electrochemical behavior is investigated by assembling coin cells (half-cells using Na metal and NaClO 4 electrolyte), in a nitrogen glove box (LC Technology Solutions Inc, <0.1 ppm H 2 O, <0.1 ppm O 2 ) using an MTI CR20XX coin cell press. All coin cell testing is performed using a Gamry Interface 1000E potentiostat and a Gamry four-terminal coin cell holder. The results provide insight into process-structure-performance relationships for coal char-derived hard carbons as SIB anode materials, including charge-discharge capacity, cycle life, rate performance, and electrochemical impedance. This work is supported in-part by NSF Award #1742696. [1] P. Bai, Y. He, X. Zou, X. Zhao, P. Xiong, Y. Xu, Elucidation of the Sodium-Storage Mechanism in Hard Carbons, Adv. Energy Mater. 8 (2018) 1–9. [2] Yunming Li, Yong-Sheng Hu, Xingguo Qi, Xiaohui Rong, Hong Li, Xuejie Huang, Liquan Chen, Advanced sodium-ion batteries using superior low cost pyrolyzed anthracite anode: towards practical applications, Energy Storage Materials 5 (2016) 191–197. Figure 1