Sodium batteries: The technology of the future?

The battery sector is bustling with innovation. Research into increasingly efficient and higher performance technologies that can bring added value to the market never stops.

The last few years has seen a renewed interest in sodium-ion batteries, largely because of the economic benefits they yield.

Our electrification experts Marco Righi, Alan Pastorelli and Daniele Invernizzi discussed it during episode 20 of Battery Weekly 2023, our weekly programme on the world of batteries, delving deep into the significant upward trend that sodium batteries are witnessing and what’s holding back their deployment on a large scale.

Sodium-ion batteries are definitely growing in popularity in the fields of energy storage and electric mobility. However, these batteries still suffer from a number limitations that need to be resolved before they can be marketed for a large range of applications.

Let’s find out together what sodium batteries are and their characteristics.

What are sodium batteries and how do they work: similarities and differences vs. lithium batteries

Like lithium, sodium is an alkali metal found in Group 1 of the periodic table.

The two metals are placed precisely one under the other in the first column of the periodic table, meaning they share a number of physical and chemical properties.

These similar properties led researchers to carry out the first studies on sodium batteries between 1970 and 1990, about the same time as the studies on lithium batteries. The latter, however, ended up enjoying greater success and went on to be commercialised, putting the sodium battery on the back burner.

What do sodium batteries and lithium batteries have in common?

The working principle underlying sodium-ion batteries and lithium-ion batteries is practically the same and many electrode materials used in sodium-ion technology were borrowed from lithium-ion technology.

Both technologies, in fact, use ions to carry and store energy. Sodium ions move from the cathode (positive electrode) to the anode (negative electrode) through the electrolyte and separator, carried by the electrical current during the charging phase. During discharge, the ions return towards the cathode and a stream of electrons, i.e. electric current, flows in the external circuit in the opposite direction with respect to the charge.

The cathode is the positive pole of the battery and is made up of cathode material (e.g. LFP, NMC) and current collector. The anode, the negative pole of the battery, is made up of anode material (e.g. carbon or graphite) and the current collector.

A sodium cell is basically made up of a cathode consisting of a material capable of containing sodium, an anode generally made of carbon, and a liquid electrolyte containing sodium atoms in ionic form. The electrolyte is an organic liquid that fills the cell’s internal volume, acting as a connecting link between the cathode and anode that enables the ions to move.

Image taken from the article Daniel, C.; Besenhard, J.O. Handbook of Battery Materials; John Wiley & Sons: Weinheim, Germany, 2012

How do sodium batteries and lithium batteries differ?

There are substantial differences between the two elements from a purely chemical point of view. The atomic radius of a sodium cation is 0.3 Å larger than the lithium counterpart. This means that its atomic weight and mass is over 3 times larger than that of lithium.

This alone brings significant technical problems in need of a solution: in the movement between the anode and cathode, the mass of the sodium atom, being 3 times larger than that of lithium, creates greater mechanical stress that translates into high deterioration of the cell.

As a consequence, sodium batteries have a short cycle-life and do not perform as well as lithium batteries because graphite, which is the anode material most commonly used in lithium batteries, suffers irreversible exfoliation reactions in the interaction with the sodium ion and self-destructs after a few life cycles.

Therefore, one of the most complex aspects is identifying a suitable negative electrode that can be used in place of graphite and capable of increasing the life cycle of sodium batteries.

Moreover, the standard reduction potential of sodium ions is lower than lithium, in other words, their tendency to gain electrons is lower. This means that compared to a lithium cell, the sodium battery will be able to supply a lower maximum voltage: the nominal voltage of the sodium cell is 2.3 – 2.5V vs. lithium’s 3.2 – 3.7V. Sodium and lithium both carry the same charge if we take into account that the electrochemical processes taking place in sodium-ion batteries and lithium-ion batteries are the same. However, ounce-for-ounce, a sodium battery will carry less charge than a lithium one, in other words, it will have a lower energy density.

Due to these two characteristics combined, a sodium battery can store 40% less energy than a lithium battery.

Pros and cons of sodium batteries

Sodium batteries are receiving renewed attention mostly because there is a need for concrete alternatives to lithium in applications where part of the production can be differentiated.

While lithium exists in nature within many rocks and in some brine, the amount in the earth’s crust is not inexhaustible. On top of that, extracting lithium requires energy.

The high demand for this raw material along with its limited availability in nature has driven its price through the roof, earning it the name “white gold”. Going forward, lithium batteries are bound to increase in demand and this has raised questions about the availability of the raw materials and the sustainability of an economy solely based on this chemistry.

Needless to say, achieving the highest performance for the specific application of the technology is a not-insignificant aspect in the search for alternative chemistries.

Can sodium batteries be a viable alternative to lithium? Let’s delve deeper into the pros and cons of sodium batteries.

Benefits of sodium batteries

• Readily available
• Low-cost
• Safety
• Low-temperature resistance
• Low impact on the environment

One of the most interesting aspects of this technology is the wide availability in nature of its constituent raw materials. Sodium, in fact, is the sixth most abundant element in the earth’s crust. This feature makes sodium batteries economically competitive, which is an important aspect for manufacturers.

Sodium batteries also ensure high standards of safety because cells based on this chemical element are neither flammable nor susceptible to explosions or short circuits. What is more, these batteries can withstand extreme high and low temperatures, having the possibility to operate in a range between −20°C and 60°C, whereas the optimal operation temperature range for lithium cells is between 0 °C and 50°C .

The raw materials are readily available in nature and can be extracted at low costs and with low energy use, making sodium a material with a low impact on the environment. 

Limitations of sodium batteries

• Low energy density
• Short cycle-life

A major disadvantage of sodium batteries is their energy density, in other words, the amount of energy stored with respect to the battery’s volume. The density of sodium batteries is still relatively low, between 140 Wh/Kg and 160 Wh/kg, compared to lithium-ion battery’s 180 Wh/Kg–250 Wh/Kg.

Another great deterrent to the practical use of sodium batteries is their short life cycles. The fast degradation is due to the larger mass of sodium ions, which is 3 times greater than that of lithium ions. Sodium ions produce greater mechanical stress in the movement between anode and cathode, causing the destruction of the graphite—the anode material—after a few cycles.

Primary applications: where is the use of sodium batteries beneficial?

Sodium batteries might prove to be an alternative to lithium batteries in applications where the economic factor is more important than performance.

More specifically, low costs and low energy density make sodium-ion batteries especially suitable for stationary applications and energy storage systems. These include photovoltaic and wind power systems with an intermittent production profile. In fact, Na-ion devices have a high level of safety that makes them suitable for applications such as these, requiring frequent hourly and daily charge and discharge cycles.

Sodium-ion technology is not very widespread in this field as of yet due to low cycle-life, still unable to meet the high number of charge and discharge cycles these applications require. If sodium cells were to become competitive in terms of life cycle duration with new research developments, they could definitely be a good technology for stationary applications.

 What is the outlook for sodium-ion technology?

According to forecasts, the sodium-ion battery market is expected to grow at a rate of 27% per year over the next decade. Annual production will presumably go from 10 GWh in 2025 to approximately 70 GWh in 2033, an increase of nearly 600%.

Sodium-ion technology could become even more widespread thanks to the fact that largely the same technologies are used for sodium-cell and lithium-cell production, providing the possibility to convert the production lines and making it even more cost-effective.

Although sodium-ion batteries still exhibit some problems to solve, interest in these accumulators is growing in the world of electrification, so much so that major international players in the field of battery manufacturing are turning their attention to this technology.

Sodium batteries have particularly sparked the curiosity of the automotive sector.

CATL, the world’s largest manufacturer of lithium-ion batteries for electric vehicles and of energy storage systems, brought sodium-ion chemistry under the spotlight in 2021, presenting it as one of the emerging technologies on which it would be investing to differentiate its production.

The Chinese giant is doing this on the insight that replacing a slice of the market now held by lithium-ion batteries with sodium-ion batteries would substantially bring down the price of lithium batteries.

CATL has come up with an innovative idea to overcome the drawbacks of sodium batteries: that of developing a hybrid battery pack. This involves mixing and matching sodium-ion batteries and lithium-ion batteries in a certain proportion, integrating them into one battery system and using a smart BMS to control the different battery systems. Depending on needs, the vehicle could exploit the low-temperature performance of the sodium-ion battery or the high energy density. The project is still at the experimental stage, but the eyes of the entire industry are already on the Chinese company.

Despite some critical issues needing resolution, sodium-ion technology is definitely carving out an increasingly bigger slice of the market for itself. Research in this field is buzzing now—as it is for other emerging technologies, such as, for example, solid-state batteries, —and large resources are being invested to overcome the barriers that are currently hindering deployment on a large scale. Introducing this technology in the market could bring tangible advantages to sectors where energy density is secondary to the economic factor and that currently rely on lithium batteries alone.