A sodium-ion battery (SIB) is a rechargeable energy storage device that uses sodium ions (\(\text{Na}^+\)) as charge carriers moving between two electrodes. This technology is gaining attention as a potential alternative to lithium-ion batteries due to the vast abundance of sodium, sourced easily from materials like common salt and seawater. Sodium’s ubiquity helps mitigate geopolitical supply chain risks and offers a path toward lower manufacturing costs once production scales up. Furthermore, SIBs exhibit favorable safety characteristics, including a lower risk of thermal runaway, making them appealing for large-scale applications like grid storage and certain electric vehicles.
Essential Structure of a Sodium Ion Cell
The cell requires two current collectors, which are thin metal foils that transport electrons to and from the external circuit. A major distinction from lithium-ion cells is that SIBs can use aluminum foil for both the positive (cathode) and negative (anode) current collectors. This is possible because sodium does not alloy with aluminum at room temperature, which is a key factor in reducing material costs and cell weight.
Between these collectors lie the electrodes—the anode and the cathode—which hold the active materials responsible for storing and releasing the sodium ions. A porous polymer separator physically separates the two electrodes to prevent an internal short circuit. The separator acts as an electronic insulator while allowing the sodium ions to pass freely through its pores.
Finally, the entire system is soaked in an electrolyte, the medium for ion transport within the cell. This liquid is composed of a sodium salt dissolved in an organic solvent mixture, providing the necessary ionic conductivity for the battery to function.
Choosing the Active Materials
The performance of a sodium-ion battery is determined by the specific chemical materials coated onto the current collectors. For the negative electrode, or anode, the material of choice is typically hard carbon, a disordered form of carbon. Hard carbon is favored because its structure contains interlayer spacing and nanopores large enough to accommodate the larger sodium ions, unlike the graphite used in most lithium-ion batteries. During charging, sodium ions are stored through a combination of intercalation between the layers and adsorption onto the pore surfaces.
The positive electrode, or cathode, must contain a sodium-based compound that can reversibly release and reabsorb ions. Three main chemical families are currently used: layered transition metal oxides, polyanionic compounds, and Prussian Blue Analogues (PBAs). Layered oxides offer high energy density but can be susceptible to structural changes. Polyanionic materials, such as sodium vanadium phosphate, often provide excellent stability and long cycle life. PBAs are attractive due to their low cost and open framework structure, which facilitates rapid sodium ion movement.
The electrolyte is formulated by dissolving a sodium salt, such as sodium hexafluorophosphate (\(\text{NaPF}_6\)), into a blend of organic solvents. This sodium salt provides the \(\text{Na}^+\) ions that shuttle back and forth, while the solvent ensures the ions can move quickly between the electrodes. The specific combination of these materials dictates the cell’s voltage, energy storage capacity, and overall lifespan.
Assembling the Battery Cell
The fabrication of a sodium-ion cell begins with the preparation of electrode slurries. This involves combining the chosen active material powders with a conductive additive, such as carbon black, and a polymeric binder in a solvent. The binder holds the active materials and conductive agents together and adheres them to the current collector foil.
The slurry is coated uniformly onto the aluminum current collector foils in a continuous process, followed by a drying stage to remove the solvent. The dried electrodes are then subjected to calendering, a high-pressure mechanical process that compresses the material layer. Calendering increases the density of the electrode and improves electrical contact between the active particles and the current collector, which boosts the battery’s capacity and performance.
The prepared cathode and anode sheets are cut into the required shapes and assembled into the cell structure, either by stacking alternating layers of electrode and separator or by winding them into a jelly roll. This assembly is placed into a casing, which is then filled with the liquid electrolyte under a vacuum to ensure complete wetting. The final stage is formation, an initial charging process where a stable solid-electrolyte interphase (SEI) layer forms on the anode surface. This layer, created by the controlled decomposition of the electrolyte, prevents continuous side reactions, ensuring the battery’s long-term cycle stability.
Sodium Ion Movement During Charging and Use
When the cell is being charged, an external power source forces sodium ions to exit the cathode structure. These positively charged ions move through the liquid electrolyte and across the porous separator toward the anode. Simultaneously, electrons travel through the external circuit from the cathode to the anode, maintaining electrical neutrality. Upon reaching the anode, the sodium ions are stored in the hard carbon structure, completing the charge cycle. The process reverses during discharge: the sodium ions spontaneously exit the anode and shuttle back to the cathode, releasing electrons through the external circuit to power a device.