Sodium-ion battery materials: hard carbon, Prussian blue and layered oxides

Lithium makes up only about 0.0065% of the Earth’s crust and is unevenly distributed; sodium makes up about 2.65% (Li P. et al. 2026), roughly 400 times more (calculated). Sodium-ion batteries (SIBs) use the same “rocking-chair” principle as lithium-ion batteries, so much of the manufacturing know-how carries over (Li P. et al. 2026; Shen Q. et al. 2026).

Because aluminium is compatible with sodium electrochemistry, both electrodes can sit on aluminium foil, avoiding the cost and weight of the copper anode foil of lithium-ion cells (Li P. et al. 2026). Although Na⁺ is larger than Li⁺, solvated sodium ions have a smaller Stokes radius and a lower desolvation barrier, which helps at low temperature (Li P. et al. 2026).

The trade-off is energy: low energy density is the main challenge of SIBs (Zhou & Shen 2025), so they target large-scale energy storage, low-speed vehicles and two-wheelers, base stations and entry-level electric vehicles (Zhou & Shen 2025). See the battery materials hub for the lithium-ion materials they compete with.

Graphite’s 0.335 nm layer spacing suits Li⁺ (0.076 nm) and gives 372 mAh g⁻¹, but Na⁺ (0.102 nm) gives less than 35 mAh g⁻¹ (Zhou & Shen 2025). Calculations show that sodium–graphite intercalation compounds have a positive formation energy: unlike lithium, and unlike the larger potassium, sodium does not form a stable compound with graphite (Zhou & Shen 2025).

Hard carbon, a disordered carbon made by carbonising precursors such as biomass or anthracite, is the standard SIB anode at 300–400 mAh g⁻¹ (Zhou & Shen 2025). Its voltage curve has two parts:

• a sloping region (2.0–0.1 V vs Na/Na⁺) from sodium adsorbed at defects and surfaces;
• a plateau below 0.1 V from sodium filling closed pores (best below 1 nm) and intercalating into pseudo-graphitic domains with layer spacings of 0.36–0.40 nm (Zhou & Shen 2025).

More defects add sloping capacity but trap sodium irreversibly and lower the initial Coulombic efficiency (Zhou & Shen 2025).

• Layered transition-metal oxides (NaₓTMO₂) stand out for cost and electrochemical performance and give the highest energy density among SIB cathodes, but they are sensitive to air and humidity, crack along microcracks during cycling and are less safe at high voltage (Ma Q. et al. 2026; Shen Q. et al. 2026).
• Prussian blue analogues (PBAs) have an open framework, low cost and a scalable synthesis (Jang et al. 2026); their performance is limited by crystal (lattice) water, structural defects and a relatively low volumetric energy density (Shen Q. et al. 2026).
• Polyanion compounds are safe and stable. NASICON-type frameworks give three-dimensional Na⁺ diffusion, a strong inductive effect and excellent thermal stability, and vanadium fluorophosphates such as Na₃V₂(PO₄)₂F₃ (NVPF) work at high voltage (Cai et al. 2026). Iron-based Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) delivers about 3.2 V and 129 mAh g⁻¹ with stability beyond 10,000 cycles and contains no V, Co, Ni or Cr (Shen Q. et al. 2026). The energy density of polyanions is limited by their heavy formula units (Li P. et al. 2026).

Lithium iron phosphate (LiFePO₄) cells are the main competitor of sodium-ion cells; SIBs serve as a complement and substitute, and SIBs with iron-based NFPP cathodes are more cost-effective than LiFePO₄ (Shen Q. et al. 2026). Sodium-ion cells store less energy per kilogram than lithium-ion cells (Zhou & Shen 2025), but remain usable at low temperature: an Ah-level pouch cell with a hard carbon anode gave 163 Wh kg⁻¹ (stack level) at 25 °C and still 107 Wh kg⁻¹ at −50 °C (Li P. et al. 2026).

NFPP is at a critical stage of industrialisation, with many companies developing it, and is expected to follow the manufacturing route of LiFePO₄ (Shen Q. et al. 2026). PBA and layered oxide cathodes are also being moved toward practical deployment (Jang et al. 2026; Ma Q. et al. 2026).

For a paper, slide or poster, draw a rocking-chair cell with Na⁺ shuttling between a cathode (layered oxide sheets, a Prussian blue cage or a polyanion framework) and a hard carbon anode, both on aluminium foil. For the anode, show hard carbon as curved, stacked graphene fragments (pseudo-graphitic domains) enclosing small closed pores, with sodium adsorbed at defects (sloping region) and clustered in pores (plateau). In LabCanvas, the figure editor's Library has battery objects, including hard carbon, that you can edit and label, and the 3D Studio has battery models for a cell-level view; export the figure as a high-resolution image.