Here are some novel techniques to improve the sodium storage capability of hard carbon (HC) materials for sodium-ion batteries (SIBs):

1. Structural Optimization of Hard Carbon

• Pore Engineering: Introduce micropores and mesopores to enhance sodium ion accessibility and storage. This can be achieved through templating methods or chemical activation using KOH or ZnCl₂.
• Graphitization: Partial graphitization of HC can increase conductivity while retaining disordered regions for sodium storage. Controlled pyrolysis at higher temperatures (≥1500°C) can achieve this.
• Heteroatom Doping: Incorporate heteroatoms like nitrogen, sulfur, or phosphorus to modify the electronic structure and improve conductivity and sodium storage sites.

2. Surface Modification

• Functional Groups: Add oxygen- or nitrogen-containing functional groups to the HC surface to increase active sites for sodium adsorption.
• Coating: Apply thin layers of conductive materials (e.g., graphene, carbon nanotubes, or metallic coatings) to reduce interfacial resistance.

3. Composite Design

• Binary or Ternary Composites: Combine HC with other materials such as TiO₂, Sn, or transition metal oxides to enhance capacity and cycling stability.
• Hard Carbon/Soft Carbon Blends: Mix HC with soft carbon to improve both the capacity and initial coulombic efficiency (ICE).

4. Electrolyte Engineering

• Electrolyte Additives: Use additives like fluoroethylene carbonate (FEC) to stabilize the solid electrolyte interphase (SEI), which can significantly enhance the performance of HC anodes.
• Localized High-Concentration Electrolytes: Optimize electrolyte composition to reduce side reactions and improve compatibility with HC.

5. Defect Engineering

• Create defects in the carbon structure to provide more sites for sodium storage. This can be done via controlled chemical or plasma treatments.

6. 3D Frameworks

• Develop 3D interconnected HC frameworks to enhance electron transport and mitigate volume changes during cycling.

7. Advanced Synthesis Techniques

• Biomass-Derived HC: Use precursors like cellulose, lignin, or fruit peels to produce cost-effective and tunable HC materials.
• Hydrothermal Carbonization: Pre-treat precursors using hydrothermal methods to produce more uniform HC structures.
• Plasma Treatment: Use plasma to modify surface properties and introduce desired functional groups.

8. In Situ/Operando Studies

• Use advanced techniques like in situ TEM or operando XRD to study sodium storage mechanisms and refine material design based on real-time observations.