Carbon-based materials, particularly graphene, hard carbon, and carbon nanotubes (CNTs), have emerged as promising candidates for anodes and cathodes in sodium-ion (Na+) and potassium-ion (K+) batteries. These materials offer unique structural and electrochemical properties that address the challenges of accommodating larger Na+ and K+ ions compared to lithium ions. Their tunable architectures, high conductivity, and robust mechanical stability make them suitable for reversible ion storage, enabling high-performance batteries.

The ion storage mechanisms in these carbon materials primarily involve intercalation and adsorption. Intercalation occurs when Na+ or K+ ions insert between the graphitic layers of carbon, while adsorption involves ion binding to surface sites, defects, or porous structures. The larger ionic radii of Na+ (1.02 Å) and K+ (1.38 Å) compared to Li+ (0.76 Å) pose challenges for intercalation, necessitating tailored structural designs to prevent sluggish kinetics and structural degradation.

Graphene, with its single-atom-thick layered structure, exhibits high surface area and excellent electrical conductivity. However, its dense stacking limits ion accessibility. To enhance Na+/K+ storage, graphene is often modified into porous or expanded structures. For example, holey graphene frameworks with in-plane pores facilitate faster ion diffusion, achieving capacities of ~300 mAh/g for Na+ and ~250 mAh/g for K+. Heteroatom doping (e.g., nitrogen or sulfur) introduces defects and active sites, improving adsorption and pseudocapacitive contributions.

Hard carbon, a non-graphitic carbon with disordered microstructures, is particularly effective for Na+ storage. Its randomly oriented graphene layers and nanopores provide ample intercalation and adsorption sites. Hard carbon anodes typically deliver capacities of 250–350 mAh/g for Na+ batteries, with sloping voltage profiles attributed to pore filling and plateau regions from interlayer insertion. The pore size distribution is critical; micropores (<2 nm) enhance adsorption, while larger mesopores improve ion transport. For K+ storage, hard carbon shows lower but still appreciable capacities (~200 mAh/g) due to the larger ion size.

Carbon nanotubes offer one-dimensional conductive pathways and interstitial channels for ion storage. Single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) exhibit different behaviors. SWCNT bundles allow intercalation between tubes, while MWCNTs utilize inter-wall spacing. Functionalized CNTs with oxygen or nitrogen groups enhance wettability and ion adsorption. CNT-based electrodes achieve ~200–300 mAh/g for Na+ and ~150–250 mAh/g for K+, with rate capability benefiting from the conductive network.

Structural design plays a pivotal role in optimizing performance. For graphene, creating 3D architectures with vertical alignment or spacers (e.g., carbon nanoparticles) prevents restacking and increases accessible sites. Hard carbon benefits from controlled pyrolysis of organic precursors to tailor porosity and interlayer spacing. CNTs are often integrated into hybrid structures with graphene or porous carbon to combine conductivity with high surface area.

Performance benchmarks highlight the trade-offs between capacity, rate capability, and cycling stability. Graphene-based anodes exhibit high rate performance (>500 mA/g) but may suffer from initial coulombic inefficiency due to solid-electrolyte interface (SEI) formation. Hard carbon excels in cycling stability (>1000 cycles with >90% retention) but requires careful electrolyte optimization to minimize irreversible capacity loss. CNTs demonstrate exceptional rate capability but need densification to improve volumetric energy density.

Comparative metrics for Na+ storage:
Material Capacity (mAh/g) Rate Performance Cycling Stability
Graphene 250–350 Excellent Moderate
Hard Carbon 300–350 Good Excellent
CNTs 200–300 Excellent Good

For K+ storage, capacities are generally lower due to kinetic limitations, but recent advances in defect engineering have narrowed the gap.

Challenges remain in scaling production, reducing costs, and improving electrode formulations. Future research may focus on in-situ characterization to elucidate ion storage mechanisms and machine learning-guided material design. Carbon-based materials continue to offer a versatile platform for advancing Na+/K+ battery technologies, balancing performance and sustainability.

The development of these anodes and cathodes underscores the importance of material science in enabling next-generation energy storage. By refining synthesis techniques and understanding structure-property relationships, carbon nanomaterials can meet the demands of emerging battery systems.