Cothermal Conversion of Lignite and Microplastics Enabling Microcrystalline Regulation of Hard Carbon for Improved Sodium-Ion Storage

Coal, particularly low-rank coal, is a cost-effective and abundant carbon resource, making it a key candidate for developing hard carbon electrodes for sodium-ion batteries (SIBs). However, the formation of long-range carbon microcrystals during high-temperature thermal conversion limits the sodium storage capacity and rate performance of the resulting carbon material. To overcome this bottleneck, we have developed a copyrolysis strategy by incorporating microplastics into the thermal conversion process of low-rank lignite, aiming to regulate the carbon microcrystalline structure. We observed that typical thermoplastic microplastics, such as polyethylene (PE) and polyethylene terephthalate (PET), interact significantly with lignite during copyrolysis. This interaction creates a liquid-like phase microenvironment that helps inhibit the condensation of coal macromolecular radicals and manage the content of heteroatomic functional groups, particularly oxygen groups, in the coal-based carbon framework. The cross-linking effect is closely associated with the type of used microplastics. Specifically, during the low-temperature pyrolysis process, PE primarily acts as a hydrogen supplier, while preserving more C–O groups without altering the oxygen content of the material. Conversely, PET significantly increases the oxygen content of the material by retaining a great number of its inherent C═O structures in the resulting carbon. By fine-tuning the copyrolysis conditions and microplastic types, an optimized hard carbon microcrystalline structure with shorter microcrystalline lengths and increased interlayer spacing can be produced. When evaluated as a negative electrode for SIBs, the obtained carbon electrode shows comprehensive improvement in sodium-ion storage properties, including a large reversible capacity of up to 350 mA h g–1, excellent rate capability, and good cycling stability (maintaining 94% after 1000 cycles at 2C). This study not only presents a practical approach for enhancing sodium-ion storage properties by optimizing the carbon microcrystalline structure through cothermal interactions between coal and microplastics but also provides a low-carbon and high-value utilization pathway for both coal and waste microplastics.