The Promise of Sodium-Ion Batteries

Lithium-ion (Li-ion) technology dominates the current battery market, offering high energy density, long life and continually decreasing costs. But lithium isn’t perfect, and it isn’t the only game in town.

Sodium-ion battery technology is a promising Li-ion alternative. In this article, we’ll explore how sodium-ion batteries work and how they compare to Li-ion.

Intro to Sodium-ion Batteries

A sodium-ion (Na-ion) battery is a type of rechargeable battery that uses sodium ions as charge carriers. Na-ion batteries are similar in design and construction to Li-ion batteries, but they use sodium compounds in place of lithium.

Sodium-ion batteries contain sodium-based electrodes and (typically) liquid electrolytes with dissociated sodium salts in solvents. When these batteries are charging, sodium ions travel from the cathode into the anode, and the electrons travel through the external circuit. Discharging reverses the process, with sodium ions traveling from the anode and reintegrating in the cathode, while the electrons travel through the external circuit. The typical cell voltage of a sodium-ion battery is 2.3–2.5V.

The operating principle of sodium-ion batteries. (Source: CIC Energigune.)

The operating principle of sodium-ion batteries. (Source: CIC Energigune.)

Sodium-ion Battery Cathodes

Generally, there are three variations of sodium-ion battery cathodes: polyanion, Prussian blue analogs (PBAs), and layered oxides. Polyanion and PBA cathodes have low atomic packing density, which results in low volumetric energy density. This allows them to be used in tools and starter applications. In contrast, layered oxide cathodes have both higher volumetric and gravimetric energy densities, which is better suited for grid energy storage systems (ESSs).

Rare elements such as nickel and cobalt can be used to increase the energy density of Na-ion cathodes. These elements enable higher reversible capacity and nominal voltage. However, these elements have a high cost and pose safety and environmental concerns in manufacturing and battery end of life.

Layered oxide cathodes are the most studied type of Na-ion battery cathode. Today, lithium, nickel and cobalt are typically used in these cathodes. However, replacing these rare materials with sodium, iron and magnesium could potentially maintain their high performance while lowering costs. According to a 2020 essay in Advanced Energy Materials by Hayley S. Hirsh et al., the material cost of a particular rock salt (Na2/3Fe1/3Mn2/3O2) is less than one-fifth that of cathodes containing lithium and nickel. The cathode is the most expensive part of a Na-ion battery, according to the essay, accounting for 44 percent of the total battery cost.

Cost estimate of Na-ion batteries with different cathode materials. (Source: Hirsh et al.)

Cost estimate of Na-ion batteries with different cathode materials. (Source: Hirsh et al.)

Sodium-ion Battery Anodes

The graphite-based anodes common to Li-ion batteries cannot be used in Na-ion batteries, though several alternatives have been analyzed. Hard carbon (carbon that cannot be converted to graphite) has a chemical potential that is similar to sodium metal and a high sodium capacity. It can be produced from different biomass materials and as such is considered environmentally friendly.

However, hard carbon consumes a significant amount of sodium and forms a solid electrolyte interface, which decreases the coulombic efficiency and reversible capacity of Na-ion batteries. This can be partially mitigated by using sacrificial salts.

Currently, the most efficient solution is using anodes created from hard carbon and metallic sodium or metallic sodium alloys. High-capacity metallic anodes have high storage capacity, mass density and chemical potential, but they are not stable.

Challenges of Sodium-ion Batteries

Although sodium-ion batteries show high potential, even when used in grid-scale applications, they have several challenges that must be solved before they can be suitable for commercial use. Na-ion batteries are sensitive to air and impurities that could form on the surface of cathodes containing iron and magnesium. This presents the risk of water penetration that can weaken battery performance. Solving this problem with a so-called dry environment manufacturing process would increase the cost and final price of these batteries.

To ensure a long service life of Na-ion batteries, it is important to have a stable interface between the electrodes and electrolytes. Interfacial degradation increases cell impedance, which decreases coulombic efficiency and shortens battery life. The electrolyte should be thermodynamically stable against electrodes, which should be coated to enable high stability.

Another challenge for Na-ion batteries is that the electrolyte should be suitably robust for grid-scale applications. The batteries should be resistant to operation in a wide range of temperatures to decrease the cost of thermal management systems. Those applications require a long life cycle, which implies minimal electrolyte leakage and gas generation.

A bigger battery system means bigger safety risks, and the fire and explosion risk of grid-scale Na-ion battery systems must be minimized. Sodium-ion batteries are much more thermally stable than Li-ion batteries and can operate in a wide range of temperatures. Yet using conventional flammable organic liquid electrolytes creates the risk of electrolyte leakage and gas generation. Improvements have already been made in ionic liquids, as well as in solid-state electrolytes. Replacing liquid electrolytes with nonflammable solid-state electrolytes could be an enabler of commercial Na-ion batteries.

Environmental Impact of Na-ion Batteries

Lithium-ion batteries contain toxic materials that are limited in availability, necessitating a concerted effort for battery recycling (see our series on Battery Recycling Technologies for more information).

Na-ion batteries are built from widely available, low-cost and environmentally friendly materials. Additionally, since Na-ion technology is in its early stages, it presents the opportunity to prepare recycling-friendly manufacturing processes from the very beginning, potentially increasing recycling efficiency and decreasing energy demand.

The Potential of Sodium-ion Batteries

The cost of Na-ion batteries is significantly less than that of Li-ion batteries—from around $40 per kWh for Na-ion to around $137 per kWh for Li-ion (based on average 2020 prices). The limited geographical availability of materials used in Li-ion batteries causes price instability and it is expected that the price of these batteries will increase as the demand for them grows. In contrast, the wide availability of sodium in the mature and stable mining industry will probably keep the prices of Na-ion batteries stable. This is important for grid-scale applications where market stability is essential for making long-term profitability planning.

Sodium-ion batteries are already available from several companies worldwide. However, mass production has not yet occurred because of remaining challenges.

Na-ion batteries show great promise in all critical battery parameters, including energy density, price, and life span. However, for this technology to be produced on a mass scale, it will be necessary to shift the focus away from Li-ion technology and to increase industry and government support for this alternative.