Green and efficient graphitization of biomass waste empowered by molten salt electrolysis: mechanistic exploration and energy storage applications dual-driven by experiments and simulations†

Abstract

The efficient conversion of biomass waste into graphite materials with high crystallinity and graphitization degree is one of the key ways to recycle biomass waste and obtain high value-added carbon materials sustainably. We propose a low-temperature electrochemical conversion strategy-a molten salt electrolysis graphitization process – that is simple, efficient, catalyst-free, clean, environmentally friendly, and sustainable. The method involves a direct one-step, template-free conversion of biomass waste into graphitic materials with tunable microstructures in the form of petal-like nanosheets by cathodic polarization of biomass waste in molten CaCl₂ at 950 °C. The graphitization transformation mechanism was analyzed and further investigation of the potential removal mechanism of heteroatoms (e.g., oxygen, nitrogen, and sulfur) during the conversion process was carried out by experimentation and simulation. The graphitic material's unique microstructure improved lithium-ion diffusion kinetics. When applied as a negative electrode of lithium-ion batteries it delivered a specific capacity of 335.69 mA h g⁻¹ (1C), and the reversible capacity was maintained at 340.02 mA h g⁻¹ after 500 cycles (0.2C), with a coulombic efficiency of 99.96%. The process proposed in this paper is a coupled process integrating deoxygenation-impurity removal, defect elimination, graphitization, micro- and nano-structure construction, and self-purification, which is conducive to the establishment of a stable, closed-loop carbon cycle in the production, application, recycling and reuse of biomass waste.