A salt ion battery

Faradion was formed three years ago to develop new batteries, and quickly decided to focus on sodium. Chairman Chris Wright believes that in time their batteries will be able to match the performance of current lithium ion technology. There are many misconceptions about the relative performance of sodium ion batteries, he says. ‘People told themselves that once lithium worked, lets explore other lithium materials, people didn’t ask themselves whether or not they should change from lithium to sodium.’

Sodium being the heavier alkali metal, there is an assumption that sodium batteries will have lower energy densities than lithium batteries. ‘That’s not the case…because in a complex salt like lithium cobalt oxide, the alkali metal ion makes hardly any difference to the mass,’ Wright explains. He says that people also assume sodium would lead to lower power rate performance, but Faradion has found with the right cathode material, this is also not true. This has been backed up by a recent study from Gerbrand Ceder at Massachusetts Institute of Technology (MIT), who showed that in some cases energy barriers to ion migration in cathodes are lower for sodium compared with lithium.²

According to University of Bath materials chemist Saiful Islam, sodium ions have similar intercalation chemistry to lithium ions and so, not surprisingly, a lot of cathode materials tested for sodium batteries are analogous to those used for lithium. But he thinks there are exciting opportunities for discovering new types of sodium cathode materials with many structures yet to be investigated. Faradion has tested several thousand materials and found a number that came close to lithium battery cathode performance, including the sodium iron phosphate frameworks, Na₄Fe₃(PO₄)₂P₂O₇, Na₇Fe₄(P₂O₇)₄PO₄ and the layered sodium oxo-metallate Na₃Ni₂SbO₆.

A problem with sodium ion batteries is finding the right anode material as graphite cannot be used. Faradion chief technology officer Jerry Barker explains: ‘The interlayer space is just too narrow to accommodate sodium.’ One alternative is to use a metal anode such as tin or antimony, which forms an alloy with the sodium ion, but this approach can lead to large volume changes in the anode causing mechanical damage and decreases in energy capacity over multiple charging cycles. Faradion solved the problem by designing a ‘hard carbon’ anode, a disordered carbon with a larger interlayer spacing able to accommodate the bigger sodium ion. Hard carbon is made by heating carbohydrates, including simple sugars, to over 1000˚C under nitrogen before milling into a fine powder.

Researchers are also turning to nanostructured composite materials. Nanotechnologist, Gurpreet Singh at Kansas State University, US, has designed a 10-20μm composite sheet made from interleaved molybdenum disulfide and graphene layers that forms a flexible electrode.³ The insulating disulphide will intercalate sodium ions but the one atom thick carbon layers provide a conducting support material. Singh’s anode undergoes a chemical reaction, where molybdenum reacts with intercalated sodium ions, forming sodium sulphite and molybdenum, and this provides greater charge storage than simple intercalation.

Using a composite structure also reduces destructive volume changes by preventing the sulphide regions agglomerating. Singh says his material changes volume by up to 40% during the process, compared with using metal anodes, which can change volume by up to 500%. Since publication of their results in January 2014, Singh’s team has successfully increased the number of charging cycles the anode can efficiently withstand to over 1000, which is comparable to industry standards.

So far, Faradion’s demonstrator sodium ion batteries have a specific energy of around 140Wh/kg, which is significantly better than a lithium iron phosphate (LiFePO₄) battery at around 100Wh/kg, but just short of the lithium cobalt oxide cell at up to 180Wh/kg. They are encouraged to have got this far in three years. For the moment, they are concentrating on developing batteries for large stationary storage applications, for example, grid storage of energy from wind turbines and solar power. These are the most cost sensitive applications were a move to sodium makes the greatest sense. Faradion predicts its sodium batteries will be up to 30% less expensive than lithium equivalents. The cost savings are not just in the cathode. ‘There is also cost saving because you are replacing a lithium based electrolyte with a sodium based electrolyte. Sodium electrolytes are more ionically conductive and therefore you can use lower concentrations of the salts’ says Barker.

A different approach to sodium ion batteries is being taken by Aquion Energy, which started production of its Aqeuous Hybrid Ion battery in 2014. The US company, spun out of Carneige Mellon University, Pittsburgh, by Jay Whitacre, has developed possibly the lowest cost sodium batteries using a water-based sodium sulphate electrolyte. The system uses a lithium manganese oxide cathode, but the lithium ions are extracted on the first discharge, leaving a high capacity manganese oxide cathode, which can intercalate both sodium and lithium ions.

The technology is large; each cell, designed for stacking, has the footprint of a standard pallet (33cm x 31cm) and weighs approx 10kg. The batteries have a specific energy of only 40Wh/kg but a lifetime of 3000 cycles, making them ideal for the growing large-scale stationary storage market but unsuitable for mobile uses.

To claim that sodium is new to the market is a little misleading – in fact, sodium batteries have been in use for at least a decade, but the hitch is that these are high temperature batteries made from liquid sodium. Ford Motor pioneered the sodium-sulphur battery in the 1960s and sold the technology to Japanese company NGK where production continues for large-scale grid energy storage. The batteries operate at 350°C to allow sodium ions to be conducted through a solid sodium β-alumina (NaAl₁₁O₁₇) electrolyte membrane to molten sulphur. Their energy capacities are comparable to the best lithium ion batteries, but the big problem is safety, clearly illustrated in 2011 when a leaking battery installed at Mitsubishi Materials’ Tsukuba plant in Japan caught fire. Another high temperature molten sodium battery was invented in 1985 in South Africa. Known as the ZEBRA battery (Zeolite Battery Research Africa project), it uses molten sodium aluminium chloride as the electrolyte with a nickel cathode and molten sodium anode. It operates at a slightly lower 245°C and is being marketed by several companies for electric cars and even submarines.

The search is still on for ambient temperature sodium sulphur batteries because of their high theoretical energy capacities. This had seemed difficult, but in a paper in February 2014, Guo-Yu Guo, from the Chinese Academy of Sciences, Beijing, showed that it could be done with a sulphur-carbon composite cathode containing a metastable small sulphur molecule (S_2-4) within carbon micropores.⁴ In this form, sulphur is more reactive and the reversible formation of sodium sulphite (Na₂S) drives the cell.

Guo’s battery had a massive specific energy of 750 Wh/kg with a lifespan of 200 cycles. While analogous lithium-sulphur batteries may offer even higher energy capacities, its excellent performance and low cost has got to make sodium-sulphur a technology to watch. Guo believes such batteries for stationary storage might be commercially available within five years.

Another experimental battery type is the metal-air, or more correctly metal-oxygen, battery for which sodium and lithium are both candidates. The cell is powered by the oxidation of a metal anode, introducing oxygen into the battery through a carbon fibre membrane. Using oxygen as the cathode reduces the battery weight, so the theoretical energy capacities are very high – double that of lithium ion batteries. Philipp Adelhelm from the Justus-Liebig-University Giessen, Germany, has worked on both lithium and sodium-air batteries. For lithium, after a single cycle, the formation of unstable lithium peroxide (Li₂O₂) severely limits cell reversibility, but for sodium the situation was ‘surprisingly different,’ says Adelhelm. His work has shown that in sodium-air cells, a stable sodium superoxide (NaO₂) forms in a reversible reaction, which so far works efficiently for at least 100 charging cycles.⁵ This number is still low – commercial batteries would be expected to have lives of 500 to several thousand cycles, but these early result show promise. Adelhelm adds that the other advantage over lithium ion batteries is that they do not require expensive transition metal cathode materials. ‘The real key is not only to exchange lithium ions with sodium but to find electrode materials made from abundant elements,’ he says.

With the first ambient temperature sodium batteries arriving soon, will sodium have an impact on the battery market? Faradion thinks it will take a ‘significant share’, starting with the most cost sensitive stationary storage sector. Ultimately, Chris Wright says, ‘we don’t see any obvious limits to the application of sodium ion batteries to those markets that are already served by lithium’. But where size rather than cost is the issue, as with mobile phones or laptops, most experts are still convinced that lithium has the advantage.

For now, perhaps lithium’s biggest advantage is the maturity of its technology. With 30 years of research behind it, there remains a push to maximise lithium ion battery potential, which still has some way to go. In 2012, the US Department of Energy launched a technology roadmap for electric vehicles, which sets a course to double the specific energy of lithium ion batteries from 100Wh/kg to 250Wh/kg by 2017.

The potential for new types of sodium batteries is exciting, but other contenders for lithium’s crown are out there too, for example, magnesium. The doubly charged magnesium ion could theoretically provide higher energy storage capacities than sodium or lithium and MIT spin-out Pellion Technologies has been developing magnesium batteries since 2009. Further into the future, Saiful Islam says ‘beyond-lithium, technology could include organic-based batteries using greener chemistry. But whichever direction the future takes, major breakthroughs will depend on new materials and a greater understanding of battery operation’.

1 K. Mizushima et al, Materials Research Bulletin, 1980, 15(6), 783.
2 S. Ping Ong et al, Energy Environ. Sci., 2011, 4, 3680.
3 L. David, R. Bhandavat and G. Singh, ACS Nano, 2014, doi: 10.1021/nn406156b.
4 S. Xin et al, Advanced Materials, 2014, 26(8), 1308.
5 P. Hartmann et al, Nature Materials, 2013, 12, 228.