The recovery of graphene and carbon nanotubes (CNTs) from spent energy storage devices like lithium-ion batteries and supercapacitors has gained significant attention due to the growing demand for sustainable material recycling. These carbon nanomaterials are valuable components in electrodes, and their reclamation can reduce environmental impact while lowering production costs for new devices. Several methods, including mechanical, chemical, and thermal processes, have been explored to extract and purify these materials for reuse. Each approach has distinct advantages and challenges, particularly concerning purity, structural integrity, and performance in subsequent applications.

Mechanical recycling methods involve physical separation techniques to recover graphene and CNTs from electrode materials. A common approach includes shredding or crushing spent batteries and supercapacitors to liberate the active materials from current collectors. Ball milling is frequently employed to break down composite structures, followed by sieving or air classification to separate carbon nanomaterials from metal oxides and binders. While mechanical methods are energy-efficient and scalable, they often yield products with lower purity due to residual contaminants. For instance, electrode compositions typically include conductive additives, polymeric binders, and metal particles that are difficult to remove entirely through physical means. Despite this, recovered graphene and CNTs can still be suitable for less demanding applications, such as conductive fillers in composites.

Chemical recycling offers a more selective approach by dissolving or degrading non-carbon components while preserving the graphene and CNT structures. Acid leaching is widely used to dissolve transition metals like cobalt, nickel, and manganese from battery cathodes. Hydrochloric acid, nitric acid, or mixtures thereof effectively extract metals, leaving behind a carbon-rich residue. Alkaline treatments can also remove polymeric binders such as polyvinylidene fluoride (PVDF). Another chemical method involves solvent extraction, where organic solvents dissolve binders without damaging the carbon nanostructures. However, aggressive chemical treatments may introduce defects or functional groups that alter the electrical and mechanical properties of graphene and CNTs. Post-treatment steps, such as annealing or reduction processes, are often necessary to restore their original conductivity and structural integrity.

Thermal recycling leverages high-temperature processes to decompose organic components and recover carbon nanomaterials. Pyrolysis is a common technique where spent electrodes are heated in an inert atmosphere to temperatures exceeding 500 degrees Celsius, causing binder decomposition and metal oxide reduction. The organic materials volatilize, leaving behind purified carbon structures. In some cases, controlled oxidation is applied to remove amorphous carbon while preserving crystalline graphene and CNTs. Thermal methods are effective in achieving high purity but require precise temperature control to prevent excessive degradation of the desired nanomaterials. Additionally, the energy consumption of these processes can be substantial, impacting their overall sustainability.

Purity remains a critical challenge in recycling graphene and CNTs from spent devices. Even after extensive processing, trace metals, residual binders, or oxidation byproducts may persist, affecting performance in high-sensitivity applications. Advanced purification techniques, such as density gradient centrifugation or electrochemical refining, have been explored to address these issues. The presence of defects, such as vacancies or functional groups, can also influence the electrical and mechanical properties of recycled materials. For example, reduced graphene oxide obtained from recycled sources may exhibit lower conductivity than pristine graphene due to residual oxygen groups.

The reusability of reclaimed graphene and CNTs in new energy storage devices depends on their structural quality and purity. Studies have demonstrated that recycled carbon nanomaterials can be reintegrated into battery anodes or supercapacitor electrodes with comparable performance to virgin materials. In lithium-ion batteries, recycled graphene has been shown to enhance electrode conductivity and cycling stability. Similarly, CNTs recovered from supercapacitors retain their high surface area and electrochemical activity, making them suitable for reuse. However, performance consistency remains a concern, as variations in recycling conditions can lead to batch-to-batch differences in material properties.

Efforts to optimize recycling processes focus on improving yield, reducing energy consumption, and minimizing environmental impact. Hybrid approaches combining mechanical, chemical, and thermal steps have shown promise in balancing efficiency and purity. For instance, initial mechanical separation can reduce the volume of material requiring chemical treatment, while subsequent thermal processing ensures high-purity output. Life cycle assessments indicate that recycling graphene and CNTs can significantly lower the carbon footprint compared to synthesizing new materials, provided the processes are energy-efficient and generate minimal waste.

Future advancements in recycling technologies may involve more selective separation techniques, such as electrochemical methods or bioleaching, to enhance recovery rates and purity. Additionally, standardization of recycling protocols will be crucial to ensure consistent quality across industrial applications. As the adoption of graphene and CNTs in energy storage continues to grow, developing sustainable recycling pathways will be essential to closing the material lifecycle and supporting a circular economy.

In summary, reclaiming graphene and CNTs from spent batteries and supercapacitors presents a viable strategy for sustainable material reuse. Mechanical, chemical, and thermal methods each offer distinct advantages, though challenges related to purity and performance must be addressed. With continued refinement of recycling techniques, recovered carbon nanomaterials can play a significant role in the next generation of energy storage devices while reducing reliance on virgin resources.