Short answer: Hard carbon is a non-graphitizable, disordered carbon with curved graphene layers and closed nanopores. Unlike graphite, it can store sodium ions reversibly, which makes it the standard anode for sodium-ion batteries, typically delivering around 250-350 mAh/g.
Why can’t sodium-ion batteries use graphite?
Sodium ions do not form stable intercalation compounds with graphite in standard carbonate electrolytes, so graphite gives very low capacity. Hard carbon stores sodium in three ways: adsorption at defects, insertion between wider-spaced layers, and filling of closed nanopores at low voltage.
Key properties to check
- Reversible capacity (mAh/g) and the share delivered on the low-voltage plateau.
- Initial Coulombic efficiency (ICE): higher is better; low surface area helps.
- Specific surface area (BET): usually kept low to limit SEI losses.
- Particle size (D50) and tap density for electrode loading.
- Ash and impurity content, which depend on the precursor.
Hard carbon vs graphite
| Hard carbon | Graphite | |
|---|---|---|
| Structure | Disordered, closed pores | Ordered, layered |
| Main use | Sodium-ion anodes; also high-rate Li-ion | Lithium-ion anodes |
| Na storage | Good (about 250-350 mAh/g) | Very poor in carbonates |
| Li capacity | Sloping, lower density | 372 mAh/g theoretical |
How is hard carbon made?
By carbonizing precursors that do not graphitize, such as biomass (coconut shell, wood, starch), sugars, phenolic resins or pitch blends, typically at 1000-1600 °C under inert gas. Precursor and temperature control pore structure, capacity and ICE.
Building a sodium-ion test cell
Hard carbon electrodes are usually made with a water-based CMC/SBR binder or PVDF, a conductive additive such as Super P, and aluminum foil (sodium does not alloy with aluminum). Test in coin cells against sodium metal; see our coin cell supplies.
Frequently asked questions
What is the capacity of hard carbon in sodium-ion cells?
Commercial and research hard carbons typically deliver about 250-350 mAh/g, depending on precursor and processing.
Can sodium-ion anodes use aluminum foil?
Yes. Because sodium does not alloy with aluminum, both electrodes can use aluminum foil, which saves cost and weight compared with copper.
Can I buy hard carbon from NanoLithium?
We supply carbon materials, binders, foils and cell parts for sodium-ion research. Contact us to ask about hard carbon availability and grades.
