As a sustainable and cost-effective alternative to traditional lithium-ion technology, sodium-ion batteries utilise abundant sodium to deliver high-performance energy storage solutions. They have the potential to offer excellent safety, long cycle life, and robust performance, and are being considered in applications ranging from stationary energy storage to electric vehicles. Explore the science behind them below.
Sodium-ion batteries
How abundant are battery raw materials?
Scaling up a battery technology requires minerals, where the relative abundance of the base elements has a strong influence on the cost of the resulting battery.
Lithium is essential for lithium-ion batteries. Considering the relative abundance of different elements in the earth’s crust, there is approximately 20 parts per million of lithium which is relatively low compared to other elements. Of course this varies depending on geographical location but serves as a broad indicator.
An alternative to the lithium-ion battery is the sodium-ion battery. Here you can see that there is approximately 23,000 parts per million of sodium in the earth’s crust, which is over a thousand times more than lithium. Of course, there are other metals to consider in a battery, including nickel, but this analysis gives a first indication of what materials we should be considering if we want to use earth abundant materials.
Adapted from: Recycling of sodium-ion batteries. Zhao et al. Nature Reviews Materials
Mineral production
The abundance of elements varies with different locations. For lithium, one of the main sources is from brines, with Chile one of the main producers. Here, there might be approximately 0.04 wt% lithium in these brines, compared with sodium which has a concentration of around 9 wt%, which is over 200 times greater.
For lithium-ion batteries, other key metals include nickel, manganese, cobalt, aluminium and copper. For primary production, iron is by far the most mined metal, followed by aluminum. Copper is used as an anode current collector and is produced in a reasonable amount, but demand will likely be high as these are needed in electric motors, cables and other applications. In the case of nickel and cobalt which are used in high energy density batteries, there are some supply concerns.
Metals are also produced by co-production, but the amount of material here isn’t as significant.
Therefore, not only do we need to find alternatives to lithium, but also for metals such as nickel and cobalt.
Adapted from: A cost and resource analysis of sodium-ion batteries. Vaalma et al. Nature Reviews Materials. 2018
Note: The area of the iron circle should be multiplied by ~3.6x to be scaled accurately but has been represented smaller here for practicalities
Sodium-ion battery characteristics
Sodium-ion batteries are being developed due to their potential cost, safety, sustainability and performance advantages over traditional lithium-ion batteries.
These batteries can be made with widely available and inexpensive materials with sodium being significantly more abundant than lithium. Furthermore, sodium-ion batteries can use aluminium for the anode current collector instead of copper used in lithium-ion cells. This ultimately reduces the supply chain risks.
Furthermore, they can be safer than lithium-ion batteries, as they can be stored at 0 V posing less risk during transport. Here, traditional lithium-ion batteries are generally stored at approximately 30% state-of-charge. Also, the electrolytes used in sodium-ion systems generally have a higher flash point than lithium-ion battery systems, reducing flammability risks.
Another advantage, is that the process for making sodium-ion batteries is very similar to that of lithium-ion, meaning that the scale-up of the technology can benefit from the work already done in lithium-ion batteries.
Currently, the energy density for sodium-ion batteries is still lower than high-energy lithium-ion cells, which use nickel, but they are approaching the energy density of high-power lithium iron phosphate cells. The cycle life of cells is reasonable in some configurations but one of the interesting elements is that sodium-ion batteries can have quite high-power characteristics.
Adapted from: Sodium-ion batteries: Inexpensive and sustainable energy storage. Lilley. Faraday Institution. 2021
Lithium-ion vs
sodium-ion batteries
When comparing lithium-ion batteries with sodium-ion batteries, the following are some key considerations. Firstly, whilst atomically sodium is 3.3x heavier than lithium (sodium 23 g/mol vs lithium 6.9 g/mol), it's important to note that lithium or sodium in a battery only accounts for a small amount of the cell mass and that the energy density is mostly defined by the electrode materials and other components in the cell.
Thus, while current sodium-ion batteries have relatively low energy densities, there is the potential for this to increase in the coming years.
Another advantage is that a sodium-ion battery uses aluminum for the anode current collector rather than copper, which is about 75% less dense and is also cheaper (copper 8.96 g/cm3 vs aluminum 2.7 g/cm3)
Example lithium-ion battery composition
At the cell level, the table to the left compares current and future sodium-ion battery designs against current lithium-ion cell technologies. Note that current lithium-ion cell technology will continue to improve and the values provided are estimates and dependent on exact configuration.
For specific energy, high nickel lithium-ion batteries have a specific energy of ~270 Wh/kg, whereas for high power lithium-iron phosphate cells, this is ~170 Wh/kg. For a sodium-ion battery, with a layered transition metal oxide cathode and hard-carbon anode, there have been reports of specific energies of ~160 Wh/kg which is competitive against lithium-iron phosphate cells.
However, when comparing energy density (Wh/L) values, it can be seen that improvements are still needed, in part due to the low density of hard carbon compared to graphite. This is not too far from lithium iron phosphate, however is quite far from the NMC lithium-ion cells.
For specific power and power density, sodium-ion batteries have some clear advantages due to the high sodium-ion mobility in the electrodes and the nature of the charge storage mechanism. Furthermore, there are some potential lifetime advantages, however this depends heavily on many factors and this table should not be taken as universal.
* 80% depth of discharge at 1C
Lifetime numbers can vary considerably depending on cycling condition and cell configuration
LIB performance is also expected to increase
LIB – Lithium-ion battery, NIB – Sodium-ion battery, NMC - Lithium nickel manganese cobalt oxide, LFP - Lithium iron phosphate
Adapted from: 2021 Roadmap for sodium-ion batteries. Tapia-Ruiz et al. (Industrial targets. Rudola et al). Journal of Physics. 2021
Cost
One of the key arguments for the use of sodium-ion batteries is that they’re lower cost. It’s been estimated that at scale, a sodium-ion battery with a layered metal oxide cathode and hard carbon anode will have ~25-30% lower material costs than a lithium iron phosphate battery (Commercialisation of high energy density sodium-ion batteries: Faradion’s journey and outlook. Rudola et al. Journal of Materials Chemistry A. 2021).
Here, 2 of the main differences in a sodium-ion cell vs a lithium-ion cell is that they replace lithium and copper with cheaper sodium and aluminium, which gives ~12% cost reduction with most of this being due to the aluminium current collector (~3% for sodium swap and ~9% for current collector).
However, most of the cost of a cell is defined by the electrode materials. For sodium-ion cells, hard carbon is one of the most used anode materials. However, because hard carbon has a lower density and is more porous than the graphite used in lithium-ion batteries, for the same amount of active material, a cell will need to use more electrolyte which adds cost and mass (Graphite 2.24 g/cm3 vs hard carbon 1.5 g/cm3).
Furthermore, hard carbon is generally more expensive than natural graphite with some hard carbons currently having lower performance than graphite (Natural graphite 10 $/kg vs hard carbon 15 $/kg).
Comparing the costs of different lithium-ion and sodium-ion battery configurations, there are potential cost advantages, but the exact number depends heavily on the chemistry being used. The plot on the left compared 3 popular lithium-ion battery chemistries; LFP, NCA and NMC, with different sodium-ion battery configurations. In the near term, sodium-ion cells are likely to use a sodium layered metal oxide cathode with a hard carbon anode, but in the future the cathode may well improve with some viewing future anodes having a blend of phosphorous or other material with a higher specific capacity.
However, when interpreting these numbers, it's important to note that material prices do vary and there is uncertainty around whether the theoretical performance of future materials can be practically achieved.
LFP – Lithium iron phosphate, nG – Natural graphite, NCA – Lithium nickel cobalt aluminium oxide, NMC – Lithium nickel manganese cobalt oxide, NMO – Sodium metal oxide (NaMnO2), sHC – Standard hard carbon, ASC – Advanced sodium-ion cathode,
PHC – Phosphorus-hard carbon composite, aPHC – Advanced PHC
Adapted from: A cost and resource analysis of sodium-ion batteries. Vaalma et al. Nature Reviews Materials. 2018
Potential applications
Currently, the energy density and lifetime of sodium-ion cells still need to improve, alongside scaling to achieve the potential cost benefits. However, there are likely to be some early pilot applications and potentially early adoption in stationary energy storage units. Subsequently, applications in power tools and back-up power applications might appear, before finding more e-mobility applications such as e-scooters, ebikes and light electric vehicles.
Here, it’s important to note that innovations and scale-up in materials takes time. The plot on the right highlights the battery technology readiness levels coined by the US Joint Centre for Energy Storage Research, where it can take in-excess of 10 years to go from scientific breakthrough to full commercialisation.
Adapted from: The joint centre for energy storage research: A new paradigm for battery research and development. Crabtree. AIP Conference Proceedings. 2015.
Adapted from: Sodium-ion batteries: Inexpensive and sustainable energy storage. Lilley. Faraday Institution. 2021
How does a sodium-ion battery work?
Sodium-ion batteries have a similar design and operating principle to a lithium-ion battery, however there are a few notable material and mechanistic differences between the two. Notably sodium is used as the charge carrier as opposed to lithium, and aluminium is used on the anode current collect instead of copper.
Therefore, since sodium has replaced lithium, different materials in the anode and cathode are used to accommodate the different ions, with sodium-ions having a larger atomic radii than lithium-ions. This larger ion can affect the stability of the electrode crystal structures and also impacts the transport properties which is how fast the ions move through the electrode and the subsequent power of the battery.
In terms of how a sodium-ion battery works, this is essentially the same as a lithium-ion cell. When fully charged, sodium-ions sit in the anode and when discharged they move to the cathode, with the electron moving around an external circuit. This process is then reversed when charging.
Comparing lithium-ion and sodium-ion battery materials
The main components of interest in a battery are the cathode, anode, electrolyte and current collectors. Combined, they will define the performance, cost, durability, safety and sustainability of the battery.
Lithium-ion battery cathodes commonly use materials such as lithium iron phosphate, lithium nickel manganese cobalt oxide and lithium cobalt oxide. For sodium-ion batteries, the most common cathodes include layered transition metal oxides, Prussian blue analogues and polyanion compounds. Here, both lithium-ion and sodium-ion cathodes use the same aluminium current collector, PVDF binder to provide mechanical robustness and conductive carbons to facilitate electron flow.
For lithium-ion battery anodes, materials such as graphite, silicon and lithium-metal are used, often with a CMC and SBR binder and copper current collectors. For a sodium-ion battery, potential anode materials include hard and soft carbon alongside sodium-metal. Here PVDF binders and an aluminium current collector is used, with conductive carbon being used in both systems.
For the electrolyte, lithium-ion batteries generally use a lithium hexafluorophosphate salt whereas sodium-ion batteries generally use a sodium hexafluorophosphate salt. Here, the solvent which dissolves the salt can come from the same or similar carbonate family.
Adapted from: Recycling of sodium-ion batteries. Zhao et al. Nature Reviews Materials. Note: Materials listed are not exhaustive.
PVDF - Polyvinylidene fluoride, CMC with SBR - Sodium carboxyl methyl cellulose with styrene butadiene rubber, EC – Ethylene carbonate, DMC – Dimethyl carbonate, DEC – Diethyl carbonate, PC- Propylene carbonate, FEC – Fluoroethylene carbonate, VC – Vinylene carbonate, EMC – Ethyl methyl carbonate
Many different sodium-ion battery cathode materials have been proposed in recent years, however, here we focus on 3 materials which have shown emergent commercial uptake.
Layered transition metal oxides, similar to the NMC lithium-ion battery cathodes, generally have the best specific energy with reasonable cycle life. Their conductivity is good giving them good power capabilities, but the materials can be air sensitive and improvements to the cycling stability are still needed.
Prussian blue analogues can have potentially good cycle life due to the robust crystal structure which also enables high power capability. However, these materials generally have low specific capacity, resulting in lower specific energy/energy density.
Polyanion compounds can also have good cycle life due to a robust crystal structure but again low specific capacity. In addition to this, these materials also generally have low electrical conductivity, however they can have higher voltages and good thermal stability, giving rise to potentially better safety characteristics.
Here, it’s important to note that cathode materials are still under development and improvements are likely to continue, and also different materials are likely to be suitable for different applications.
Sodium-ion battery cathode materials
Note: Not all combinations of possible materials are listed
Note: The above categorisations are generalisations with exceptions likely in each category
Adapted from: Design of cathode materials for sustainable sodium-ion batteries. Sayahpour et al. MRS Energy & Sustainability. 2022
Layered transition metal oxides
Layered transition metal oxides generally have relatively high specific capacities and good power characteristics due to good electronic and sodium-ion conductivity. However, sodium-ions are inserted and removed, a reasonable amount of volume change can occur, which can result in poor lifetime if not managed.
Here, there are 2 main types of layered transition metal oxides which are classed based on whether sodium resides in a prismatic or octahedral crystallographic site. These are therefore termed P- and O-type materials followed by an index which indicates the number of metal oxide layers needed to make a repeating unit, with P2 and O3 type cathodes the most common.
Whilst exact properties vary, P2-type cathodes generally have better power and stability and O3-type cathodes usually have higher specific capacity. However, a challenge with this class of material is that they tend to be air and moisture sensitive, with water inserting into the sodium layers; reducing performance.
Adapted from: 2021 roadmap for sodium-ion batteries. Tapia-Ruiz et al. (Layered transition-metal oxides: P2 | O3 materials. Brittain et al). Journal of Physics. 2021
Prussian blue analogues
Prussian blue analogues have a similar crystal structure to the Prussian blue synthetic pigment. They have a robust crystal structure which potentially enables long lifetime, with this open structure enabling good sodium-ion conductivity, but the poor electronic conductivity needs to be overcome for high power capability.
A drawback of this open structure however is that it reduces the energy density of the material, meaning that the resulting specific energy/energy density of the cell is generally lower than transition metal oxides. Furthermore, there can be problems in that some synthesis routes can result in water existing in the structure of the material which can react with the electrolyte in a full cell; reducing the lifetime.
However, many Prussian blue analogues use earth abundant metals such as manganese and iron making them potentially quite low cost.
Polyanion compounds
The polyanion family of materials generally have higher operating voltages than the other materials. Their robust crystal structure can provide good lifetime, and the strong bonding with the oxygen in the structure improves their potential safety characteristics.
However, the specific capacity is generally lower than metal oxides and their low electronic conductivity usually means they need to be carbon coated. Here, there are a number of different types of materials each with their pros and cons, including: phosphates, pyrophosphates, fluorophosphates, mixed phosphates, sulfates and silicates.
Adapted from: Polyanion-type cathode materials for sodium-ion batteries. Jin et al. Chem Soc Rev. 2020
Note: Within a category, the characteristics can change based on the exact composition
Cathode comparison
Visualising how each class of materials compares to each other, this plot shows the typical nominal voltage of each cathode with their general specific capacity. Note that the exact numbers can vary depending on a range of factors.
For the metal oxides, which are broken down into P2 and O3 type materials, these generally have the highest specific capacity.
Prussian blue analogues have somewhat lower specific capacities but have a more theoretically stable crystal structure which can give better lifetime not shown on this plot.
The polyanion compounds generally have the lowest specific capacity, but highest voltage and improved safety, again not shown on this plot. Here, it’s important to note that not all possible materials are shown.
Adapted from: Sodium-ion batteries: Present and future. Hwang et al. Chem Soc Rev. 2017
Note: Shown materials are not exhaustive and locations are approximate
Sodium-ion battery anode materials
For sodium-ion battery anodes, there are generally 3 main types of materials being considered.
Carbonaceous materials, where hard carbon is perhaps the most popular, has reasonable specific capacity and good lifetime.
Metal oxides and sulfides generally have much higher specific capacities, but their lifetime can be poor.
Alloying materials such as tin and phosphorus have high specific capacities, but they experience significant volume change during cycling leading to mechanical pulverisation of the electrode and poor lifetime.
Here, insertion, conversion and alloying are the 3 main mechanisms for sodium storage in these materials.
Note: Within a category, the characteristics can change based on the exact composition
Anode comparison
This chart shows the nominal voltage vs specific capacity of different sodium-ion battery anode materials, allowing for ease of comparison. Here, ideal properties for anodes includes low voltage and high specific capacity.
Alloying materials generally have the highest specific capacity, though they have poor lifetime.
Metal oxides and sulfides can also have high specific capacities, but generally low lifetimes.
Carbonaceous anodes such as hard carbon have a reasonable specific capacity and lifetime which has resulted in their likely first commercialisation.
Adapted from: Sodium-ion batteries: Present and future. Hwang et al. Chem Soc Rev. 2017
Note: Shown materials are not exhaustive and locations are approximate
Hard carbon
Hard carbon is the most likely candidate anode material for commercially viable sodium-ion batteries. The materials has randomly orientated graphitic domains with a higher interlayer spacing than graphite; facilitating the sodiation of the material. Here, the graphite interlayer spacing is ~3.3 Å whereas hard carbon is ~3.7-4.0 Å
Hard carbon can be made from different carbonaceous precursors such as biomass and certain polymers through a high temperature (~1000 °C) heat treatment process which results in non-graphitizable structures, even at high temperatures.
These materials, often have a high surface area and porosity, with lots of defects which results in multiple different ways sodium-ions can be stored in their structure. However, because of this high porosity, the density of hard carbon is generally low, resulting in poor volumetric specific capacity.
Adapted from: Hard carbon as sodium-ion battery anodes: Progress and Challenges. Xiao et al. ChemSusChem. 2018
2021 Roadmap for sodium-ion batteries. Tapia-Ruiz et al. (Hard carbon. Au et al). Journal of Physics. 2021
Summary
Sodium-ion batteries have the potential to offer a low cost, sustainable and safer alternative to lithium-ion batteries
Material abundance = lower supply chain issues
Uses cheaper aluminium anode current collector
Potentially safer; can be stored at 0 V, higher flash point
Can have potentially high-power capabilities
Main cathodes include: layered metal oxides,
Prussian blue analogues and polyanion compoundsLayered metal oxides generally have higher capacity
but lower lifetime
Main anodes include: carbonaceous, metal oxides/sulfides and alloys
Hard carbon is the main anode being considered
Energy density low but improving (approaching LFP)
Likely to be complimentary to lithium-ion batteries