Sodium-ion Battery Production Process: From Raw Materials to Finished Cells

Sodium-ion batteries (Na-ion batteries) have attracted significant attention as a promising alternative to lithium-ion batteries due to the abundance and low cost of sodium resources. The production process of sodium-ion batteries shares many similarities with lithium-ion batteries, but there are also some key differences due to the unique properties of sodium-based materials. This article outlines the key steps in the sodium-ion battery manufacturing process.

Cathode Materials

Common cathode materials for sodium-ion batteries include layered oxides (NaxTMO2, where TM = transition metal), polyanionic compounds (such as Na3V2(PO4)3), and Prussian blue analogs. These materials are synthesized through solid-state reaction, sol-gel processes, or co-precipitation methods.

Anode Materials

Hard carbon derived from biomass or pitch is the most widely used anode material for sodium-ion batteries. The hard carbon precursors are carbonized at high temperatures (typically 1000-1300°C) to create a disordered carbon structure suitable for sodium-ion storage.

Electrolyte

The electrolyte usually consists of sodium salts (such as NaClO4, NaPF6, or NaTFSI) dissolved in carbonate-based solvents (EC, DMC, PC). Solid-state electrolytes, including NASICON and sulfide-based materials, are also under development.

Separator

Polyethylene (PE) and polypropylene (PP) separators, commonly used in lithium-ion batteries, can also be applied to sodium-ion batteries, though the compatibility with Na-ion electrolytes is carefully evaluated.

Slurry Preparation

The active materials (cathode and anode), conductive additives (carbon black), and binders (such as PVDF, CMC, or SBR) are mixed with solvents (NMP for cathode, water for anode) to create a uniform slurry.

Coating

The slurry is evenly coated onto aluminum foil (cathode) and copper foil (anode). For some sodium-ion batteries, both electrodes may use aluminum foil, depending on the voltage window and material properties.

Drying

The coated electrodes are dried in ovens to remove residual solvents. The drying temperature and duration are carefully controlled to prevent material degradation.

After drying, the electrodes pass through a pair of precision rollers to achieve uniform thickness, improve density, and ensure good contact between the active materials and current collectors.

The electrodes are cut into the desired shapes (usually rectangular for pouch cells or cylindrical for cylindrical cells). The positive electrode, separator, and negative electrode are stacked or wound into the final cell format.

Pouch Cells

The stacked electrode-separator layers are enclosed in an aluminum-plastic pouch. Electrolyte is injected into the pouch, and the pouch is heat-sealed to prevent leakage.

Cylindrical and Prismatic Cells

The wound electrode assembly is inserted into a metal can. Electrolyte is added, followed by sealing with a cap.

The assembled cells undergo an initial charging process, known as formation. This step allows the solid electrolyte interface (SEI) layer to form on the anode surface, which is critical for battery stability. Formation protocols for sodium-ion batteries may differ slightly from lithium-ion cells due to different SEI chemistries.

After formation, the cells are left to age for several days to stabilize their internal chemistry. Each cell undergoes quality control tests, including capacity checks, internal resistance measurements, leakage detection, and safety tests.

Tested cells are assembled into modules and battery packs. Battery management systems (BMS) are integrated to monitor voltage, temperature, and current to ensure safe operation.

The sodium-ion battery production process leverages much of the existing lithium-ion battery infrastructure, making it relatively easy for manufacturers to adopt. However, sodium-ion materials exhibit different electrochemical and physical properties, requiring some adjustments in slurry formulation, electrolyte selection, and formation protocols. As sodium-ion technology continues to mature, its cost advantage and raw material abundance could make it a strong competitor in large-scale energy storage applications.