How Sodium-Ion Batteries Work

The Basic Principle

All rechargeable batteries work the same way: ions move from one electrode to another through an electrolyte, and electrons flow through an external circuit to balance the charge. When you charge the battery, ions move one way; when you discharge it, they move back. A sodium-ion battery does exactly this — but with sodium ions (Na+) instead of lithium ions (Li+).

The Three Main Components

The Cathode (Positive Electrode)

The cathode is where sodium ions are stored when the battery is discharged. Three main cathode chemistries are used in commercial sodium-ion batteries:

• Layered transition metal oxides (NaₓMO₂, where M = Fe, Mn, Ni) — used by CATL and HiNa. High energy density, good cycle life.
• Prussian blue analogues (NaₓM[Fe(CN)₆]) — used by Natron Energy. Excellent power density, long cycle life, simpler manufacturing.
• Polyanion compounds (Na₃V₂(PO₄)₂F₃ and similar) — high voltage and stability, but lower energy density.

The cathode is the most expensive part of the battery, and the area where most R&D is focused. The goal is higher energy density without using expensive materials like vanadium.

The Anode (Negative Electrode)

Lithium-ion batteries use graphite anodes. Sodium ions are too large to insert efficiently into graphite's structure, so sodium-ion uses hard carbon instead. Hard carbon is a disordered form of carbon that can be made from:

• Biomass — coconut shells, sugar cane, apple pomace
• Petroleum pitch — a refinery byproduct
• Resins and polymers — synthetic routes

Hard carbon has a capacity of ~300–350 mAh/g — lower than graphite's 372 mAh/g — but it's cheap, sustainable and abundant. Researchers are also exploring phosphorus anodes (much higher capacity, but with volume expansion issues) and alloy anodes (tin, antimony).

The Electrolyte

The electrolyte is the medium that allows sodium ions to travel between the electrodes. Most sodium-ion batteries use a liquid electrolyte — sodium hexafluorophosphate (NaPF₆) dissolved in organic carbonate solvents, similar to lithium-ion electrolytes. The key difference is the salt: NaPF₆ instead of LiPF₆.

Some companies are developing solid-state sodium-ion batteries using ceramic or polymer electrolytes. These could improve safety further, but are still in the research phase.

What Happens During Charging?

• Sodium ions leave the cathode and travel through the electrolyte to the anode
• At the anode, sodium ions insert into the hard carbon structure
• Electrons flow from the cathode to the anode through the external circuit
• The battery stores energy as chemical potential

What Happens During Discharging?

• Sodium ions leave the anode and travel back through the electrolyte to the cathode
• Electrons flow from the anode to the cathode through the external circuit — powering your device
• The battery releases stored energy

Why Sodium-Ion Charges Faster

Sodium ions move through the electrolyte faster than lithium ions in some conditions. Combined with the open structure of hard carbon (which has larger interlayer spacing than graphite), sodium-ion batteries can accept high charge currents. CATL has demonstrated 80% charge in 15 minutes — faster than most commercial lithium-ion cells.

Why Sodium-Ion Handles Cold Better

At low temperatures, ion mobility in the electrolyte decreases — that's why lithium-ion batteries lose capacity in the cold. Sodium ions have a lower solvation energy than lithium ions, meaning they shed their solvent shell more easily when entering an electrode. This makes the charge-transfer step faster even at low temperatures, which is why sodium-ion retains ~90% capacity at -20°C.

The Separator

Like lithium-ion, sodium-ion batteries use a porous polymer separator (typically polyethylene or polypropylene) between the anode and cathode. This prevents electrical shorting while allowing ions to pass through. The separator is essentially the same as in lithium-ion — no new manufacturing needed here.

The Bottom Line

Sodium-ion batteries work on the same fundamental principle as lithium-ion — ions shuttling between electrodes. The differences are in the materials: hard carbon instead of graphite, sodium-based cathodes instead of lithium-based ones, and NaPF₆ electrolyte salt. These material differences are what give sodium-ion its unique advantages: lower cost, better cold-weather performance, faster charging and improved safety.

Learn More

• What Is a Sodium-Ion Battery? — the basics
• Sodium-Ion vs Lithium-Ion — full comparison
• The Companies Building Sodium-Ion Batteries — who's doing what

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