Sodium’s Spotlight: Na-ion Basics

In this edition of Battery Burn Book, we turn our attention to sodium-ion (Na-ion) batteries. With growing demand for cost-effective, scalable, and sustainable energy storage, Na-ion systems are emerging as a promising alternative to lithium-ion (Li-ion) technology. While sodium and lithium share similar electrochemical behavior, key differences in material selection and cell design are critical to understanding how Na-ion batteries function, and where they might offer advantages.

Cathode Materials

Na-ion cathodes often borrow design principles from Li-ion systems, but with sodium-compatible chemistries. Layered transition metal oxides, which are structurally similar to nickel manganese cobalt oxides (NMC), are a common choice due to their relatively high energy density. One drawback of this material is structural degradation when charged to high voltages (> 4.0V) during cycling. Another widely studied material is Na₃V₂(PO₄)₃ (NVP). Although NVP has a lower capacity compared to layered oxides, it offers excellent thermal and structural stability, making it well-suited for long cycle life and high-rate capability applications. Another candidate, Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP), provides excellent cycling stability and safety advantages, similar to an LFP cathode in Li-ion systems. Lastly, Prussian White, which has the general structure of NaₓMn[Fe(CN)₆] · yH₂O, shows promise due to its low-cost synthesis and relatively high capacity. On the other hand, Prussian White is extremely moisture sensitive, which becomes a significant challenge for processing and long-term stability. Overall, there are a wide variety of sodium cathode materials, and the best choice will depend greatly on the application.

Anode Materials

In contrast to Li-ion, Na-ion systems cannot use graphite as an anode material due to the larger ionic radius of sodium, which prevents its intercalation into graphite. Hard carbon has emerged as the most widely adopted anode material for Na-ion batteries. Hard carbon features a disordered structure that can accommodate sodium ions through a combination of intercalation and adsorption mechanisms. Hard carbon is derived from biomass-derived carbons such as coconut shells. The performance of hard carbons is strongly influenced by the choice of precursor material, leaving significant opportunities for enhancement. One of the main challenges associated with hard carbon is high initial capacity loss (ICL).  A significant portion of sodium is irreversibly consumed in forming the solid electrolyte interphase (SEI) during the first charge.

Electrolyte Formulations

The electrolyte systems used in Na-ion batteries are largely similar to those in Li-ion technology. Typical formulations consist of organic carbonate solvents such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), combined with sodium salts like NaPF₆. While the general solvent-salt chemistry is comparable, the solvation behavior of Na⁺ and its interaction with electrolyte components can differ significantly from lithium, especially during SEI formation on the hard carbon anode. Additives tailored to enhance SEI stability and reduce ICL are an important area of ongoing research.

Cell Design Considerations

Na-ion cells benefit from a few design advantages that differentiate them from Li-ion. Notably, aluminum can be used as the current collector for both cathode and anode. This is possible because sodium does not alloy with aluminum at typical anode potentials, in contrast to lithium. This eliminates the need for copper current collectors, reducing both cost and cell weight. Additionally, the absence of copper dissolution issues enables Na-ion batteries to be discharged to 0 V, which simplifies storage and transport after formation.

Why Consider Sodium-Ion?

Sodium is far more abundant than lithium and widely distributed globally, reducing dependence on geographically concentrated or geopolitically sensitive supply chains. Its compatibility with the existing Li-ion manufacturing infrastructure allows for streamlined integration. Several battery developers are now targeting low-range or urban electric vehicles, such as scooters, e-bikes, and small passenger cars, as viable applications for Na-ion. While Na-ion is unlikely to replace Li-ion in long-range or high-performance EVs in the near term, they are well-positioned for grid storage, backup power, and other applications where cost, longevity, and material availability are key drivers.

Some Resources We Love:

• You can read more about NFPP cathode material here

• Read this is Prussian White is your Na-ion cathode material of choice