Structural Design of Coal-Based Solid Waste-Derived Silicon/Carbon Composites for Electrochemical Energy Storage

The large-scale mining and processing of coal resources inevitably result in the continuous generation and accumulation of solid wastes such as fly ash and coal gasification slag. However, their utilization rate remains low, with primary dependence on open-air stockpiling, leading to substantial resource depletion and ecological hazards. The existing resource utilization of fly ash and coal gasification slag is still confined to low-value-added fields like construction materials and agriculture. It is critically necessary to explore their inherent resource properties and develop high-value utilization pathways, thereby addressing environmental challenges, while enhancing economic benefits. Some researches reveal that fly ash and coal gasification slag both contain abundant silicon components in their phase compositions. This characteristic endows them with significant potential as precursors for synthesizing silicon anode materials. This study was to fabricate silicon/carbon composites with fly ash and coal gasification slag as precursors, provide novel insights into the high-value application of coal-based solid waste resources and expand the methodology for low-cost preparation of silicon-based materials for lithium-ion batteries.

In the fabrication of distinct silicon-carbon composites, a fly ash-derived silicon material was prepared via uniformly grinding a mixture with 1 g of fly ash, 1 g of magnesium powder and 2 g of sodium chloride. Afterwards, the ground mixture was heated in argon atmosphere by magnesiothermic reduction at 750 ℃ for 8 h. A purified silicon material was obtained after sequential washing with hydrochloric acid and hydrofluoric acid. For the fly ash-derived silicon-carbon composite, this silicon material was dispersed with phenolic resin at an equal mass ratio, modified via cetyltrimethylammonium bromide assistance, and carbonized at 800 ℃ for 2 h to form a carbon-coated structure. In contrast, the coal gasification slag-derived composite was synthesized in a single step via subjecting 1 g of coal gasification slag, 0.6 g of magnesium powder, and 2 g of sodium chloride to identical magnesiothermic reduction conditions, thereby retaining inherent carbon to generate an interconnected silicon-carbon architecture.

The morphologies and microcrystalline structures of all the obtained samples were analyzed by X-ray diffraction, Raman spectroscopy, X-ray energy dispersive spectroscopy, scanning electron microscopy, and transmission electron microscopy. The thermal behavior was evaluated by thermogravimetric analysis, and the microstructures were characterized via BET specific surface area and pore size distribution measurements. The two composite materials were assembled into lithium-ion half-cells for charge–discharge cycling, cyclic voltammetry, and impedance testing to evaluate their lithium storage performance.

Silicon materials obtained via the magnesiothermic reduction process with fly ash as a precursor exhibit a high crystallinity, primarily composed of densely packed crystalline particles. The corresponding fly ash-based silicon-carbon composite fabricated via a carbon-coating strategy exhibits a “core-shell structure” with a carbon content of 28.1%. This carbon layer predominantly consists of amorphous carbon enriched with abundant sp³-hybridized structural defects and edge disorders, significantly affecting the interfacial charge transfer dynamics. In contrast, the coal gasification slag-derived composite synthesized via one-step magnesiothermic reduction demonstrates a distinctive mosaic architecture incorporating 36.9% carbon with substantially enhanced sp²-hybridized carbon domains, thus facilitating a superior electrical conductivity.

Fly ash-based silicon-carbon composite and coal gasification slag-derived composite both have high specific surface areas (i.e., 215.7 and 274.8 m²·g^–1) and rich hierarchical pore structures (i.e., synergistic distribution of microporous and mesoporous pores). As lithium-ion battery anodes, fly ash-derived silicon-carbon composite and coal gasification slag-derived silicon-carbon composite exhibit reversible capacities of 800 and 420 mA·h·g^–1 at 50 mA·g^–1, respectively. These composites maintain reversible capacities of 348 and 334 mA·h·g^–1 after 300 cycles at 500 mA·g^–1, respectively, showing superior rate capability and cycling stability. This study demonstrates that silicon-carbon composites with high-performance lithium storage characteristics can be prepared by using low-cost coal-based solid waste–fly ash and coal gasification slag as precursors, which is helpful for promoting the development and application of low-cost, high-performance energy storage materials.

A silicon-carbon composite with a “core–shell structure” was synthesized by using fly ash as a precursor through a magnesia-based reduction process combined with a carbon coating strategy. Also, a silicon-carbon intergrown composite exhibiting an “mosaic architecture” was prepared by using coal gasification slag as a precursor by a one-step magnesia-based reduction process. The abundant sp³ structural defects and edge defects in fly ash-based silicon-carbon composite could enhance its lithium storage capacity, while the significant sp² carbon in the coal gasification slag-based composite could improve its structural stability. Consequently, the composites both exhibited excellent rate performance and cycling stability when used as anodes in lithium-ion batteries. This study could provide some insights for the high-value utilization of coal-based solid waste resources and expand avenues for low-cost preparation of silicon-based materials for lithium-ion batteries.