They power Small phones and two-ton electric cars. They also dare to build a growing number of grid-storage systems that smooth the flow of electricity from wind and solar power plants. Without these, the electrification needed to avoid the ill effects of global warming would be unimaginable. And in 2019 they won three of their pioneers Nobel Prizes.
But lithium-ion (Li-ion) batteries have downsides. Lithium is scarce for one. And the best Li-ion batteries, which have layered-oxide cathodes, also need cobalt and nickel. These metals are also scarce—and cobalt is problematic because much of it is mined in the Democratic Republic of Congo, where working conditions leave much to be desired. A second type of Li-ion battery, a so-called polyanionic design, uses lithium iron phosphate (LFP), does not require nickel or cobalt. But such batteries cannot store as much energy per kilogram as layered-oxide.
A clutch of companies, however, think they have an alternative: making batteries with sodium instead. Unlike lithium, sodium is abundant: it makes up most of the salt in the sea. And chemists have found that layered-oxide cathodes that use sodium instead of lithium can get by without cobalt or nickel to jazz them up. The idea of making sodium-ion (or non-ion) batteries at scale is therefore gaining traction. Engineers are tweaking the design. Factories, especially in China, are rising. For the first time since the Li-ion revolution began, lithium’s place on the electrochemical pedestal is being challenged.
salt of the earth
Lithium and sodium, members of a group called the alkali metals, sit just below hydrogen in the first column of the periodic table. Alkali metals are famously reactive. (If you drop some into water, a lot will fizzle. Others will explode.) This is because the outermost shell of electrons surrounding the nucleus of an alkali-metal atom has only one. These “valence” electrons are easily shed, creating positive ions (cations) that can bond with negative counterparts (anions), such as hydroxyl ions derived from water. The result is compounds like lithium hydroxide and sodium chloride, better known as table salt.
However, if the missing electrons are sent to their destination through a wire rather than directly to a neighboring atom or group of atoms, while the cations travel separately through a medium called an electrolyte, the result is an electrochemical cell. . As electrons travel through the wire, energy can be drawn from it (see figure). Conversely, if the entire process is reversed by applying a current, the cell can be recharged.
All this applies to sodium as well as lithium. Given sodium’s cost advantages, non-chemists may wonder why it wasn’t preferred over lithium in the first place. The answer is that the sodium atom, which has 11 protons, 12 neutrons and an extra electron shell, is larger and heavier than lithium (three protons and three neutrons). A sodium battery will be larger and heavier than a lithium one of the same capacity.
Small size and a low weight are crucial for phones, and at least desirable in cars. But they are not important everywhere. Sodium batteries can serve grid-scale storage, home storage, and heavy-duty transportation such as lorries and ships.
China’s interest stems in part from the government’s current five-year economic plan, which begins in 2021, and which aims, among other things, to explore different battery chemistries. Benchmark Mineral Intelligence, a London firm, lists 36 Chinese companies that are either developing or investigating sodium batteries. These companies mostly play their cards close to their chest – in four cases the benchmark researchers couldn’t determine exactly which battery chemistry was involved. The leader of the pack, however, usually agrees to be CATLBased in Fujian.
CATL Already the world’s largest maker of Li-ion car batteries. In 2021 it announced the world’s first sodium battery for electric vehicles. Chery, a Chinese carmaker, will use catl’s Sodium batteries, along with some lithium ones, have iCAR brand, will be launched soon.
BYD, CATL’Its main rival, and a carmaker in its own right, is similarly active. Its Seagull hatchback, which was unveiled at the Shanghai Auto Show in April, will soon sport a Na-ion battery as well. Farasis Energy, another established battery-maker, teamed up with Jiangling Motors; HiNa Battery Technology, a firm specialized in manufacturing non-ion batteries, is collaborating with it JAC Group, yet another car maker; And Svolt, a subsidiary of Great Wall Motors, has a ready automotive partner in its parent company.
According to Benchmark, these five firms, along with 22 others, are using layered-oxide cathodes (besides the four unknowns, the rest are working with either a polyanionic design or a third method involving an iron-containing material called Prussian blue). And this is where cobalt and nickel come in. Experience has shown that oxide layers containing cobalt and nickel ions (along with manganese, which is cheaper and less controversial to mine) make the best lithium battery cathodes.
Cobalt and nickel (and also manganese and iron) are so-called transition metals, which have more than one valence electron. Whereas lithium and sodium ions always have a single positive charge, cobalt, for example, can form ions with a +2 or +3 charge. When an electron reaches the cathode of a layered-oxide battery, it reacts with a transition-metal ion, reducing its positive charge by one and creating a net negative charge. An alkali-metal ion (which is positively charged) moves into the crystal structure to balance the charge.
In sodium batteries, layered-oxide cathodes can be made of only manganese and iron (although they can be spiced with metals such as copper and titanium to improve performance). Why is not entirely clear. Dominik Bresser of the Karlsruhe Institute of Technology in Germany thinks it’s because sodium atoms’ large size and slightly different electronic properties allow them to fit into a wide range of crystals. Whatever the answer, the practical result is a major reduction in materials costs. This flexibility also allows engineering into the characteristics of non-ion batteries, such as high power output, that are difficult to achieve with Li-ion.
Starting the grid
Among them, according to Rory McNulty, a research analyst at Benchmark, Chinese firms have 34 non-ion battery factories, built or announced domestically, and one planned in Malaysia. In contrast, established battery-makers elsewhere are not yet showing much interest. Even without a five-year plan to guide them, though, some non-Chinese startups are looking to steal a march by developing alternatives to layered oxide in hopes of improving the technology, lowering its cost, or both.
Natron Energy of Santa Clara, California is the most interesting of these neophytes. It is adopting the Prussian blue method. Prussian blue, a common dye, is cheap. But Natron hopes it can prolong a battery’s service life. At least at this point, Na-ion layered-oxide cathodes are less durable than their Li-ion counterparts. Natron claims its cells can withstand 50,000 charging and discharging cycles—ten to 100 times longer than commercial Li-ion batteries. The firm has built a factory in Michigan, which it says will begin production later this year.
Other non-Chinese companies are less advanced, but full of hope. Altris, in Sweden, which is also building a factory, employs a material called Prussian white that replaces some of the iron in Prussian blue with sodium. Tiamat, France, uses a polyanionic design with vanadium. And Faradion, in Britain (now owned by Reliance, an Indian company), wants to stick with a layered-metal-oxide system.
It remains to be seen how everything will turn out. Dr. McNulty urged caution, at least in the short term. Battery technology takes time to mature (the first research on lithium batteries dates back to the 1960s). Benchmark predicts that sodium battery production capacity in 2030 will be about 140 gigawatt-hours of storage per year. However, the company thinks that just over half of this capacity will actually churn out cells. That’s 2% of its projection for lithium-cell production that year.
Sodium batteries, however, look attractive. For grid storage, they seem like serious competitors LFPs-Although they also have to compete with other novel approaches, such as vanadium flow-batteries. Their main competitor in the lorry and shipping market is probably hydrogen fuel cells, but these are an untested technology that relies on as-yet-unbuilt infrastructure to deliver hydrogen.
For weight-sensitive, high-cost applications such as electric cars or even airplanes, their future is less certain. The important factor will be the price of materials. If the prospect for lithium, cobalt and nickel produces enough new mines to keep them down, the incentive to pay scientists and engineers to increase the amount of energy per kilogram that sodium batteries can store could evaporate. But if those metals are priced higher, a sunny plateau for sodium may be indicated. ■