Sodium-ion batteries 'set to challenge' dominant lithium-ion technology

Sodium-ion battery technology could approach the performance of lithium-ion at much lower cost and with better safety, claims UK firm

A British start-up has developed a battery technology which, it claims, could lead to batteries for domestic energy use with properties comparable to Tesla’s recently-unveiled ‘Powerwall’ but at around a third of the cost. Faradion’s batteries, which could also be used in vehicles, also have considerable safety advantages over conventional lithium-ion technologies, the company claims.

They key to the technology is that rather than using lithion-ion cells, Faradion’s batteries are based on sodium-ion chemistry; it has released results showing results from a sodium-nickel-zinc-tin compound with a layered structure. Sodium and lithium occupy the same group on the Periodic Table — alkaline earth or Group I metals, and therefore have very similar chemistries. However, as sodium is many times more abundant on Earth than lithium, mostly because of its presence in seawater, sodium ion cells are cheaper than lithium ion — sodium salts are some 90 per cent cheaper than those of the lighter metal — and also more sustainable, explained Faradion chief technical officer Jerry Barker. ‘This means that the total cost of the battery would be around 30-35 per cent cheaper than a comparable lithium-ion battery,’ he claimed.

Faradion's sodium-ion pouch cell, with the battery made by Williams Advanced Engineering containing two banks of 12 cells.

Founded in Sheffield in 2010, Faradion is a leader in developing sodium-cell technology with some 18 patent applications over the past five years. This development has been part-funded by the technology agency InnovateUK in collaboration with Williams Advanced Engineering and Oxford University, and the fruit of the project, the world’s first sodium-ion powered vehicle, an electrically-assisted bicycle, was unveiled yesterday at Williams’ Oxfordshire headquarters.

Among sodium’s advantages over lithium is that it does not form alloys with aluminium, Barker said. This means that in a sodium-ion cell, the current collectors at both the cathode and anode can be made from aluminium foil. This is significant, because in lihium-ion cells, lithium’s tendency to alloy with aluminium means that the negative electrode current collector has to be made of copper; and it’s this which is responsible for one of the main drawbacks of lithium-ion cells: their poor characteristics if they are discharged completely. When the cell is at zero charge, the copper starts to dissolve, reducing the cell’s performance. Because of this, lithium-ion cells have to be stored and transported at in a 20-40 per cent state of charge, which leads to safety concerns, Barker said.

The chemical reactions inside batteries generate heat, and as the temperature inside lithium-ion cells climb, they enter a state known as self-heating. The charscteristics vary depending on cell chemistry, but LiCoO2 (LCO) cells become self-heating at 90°C, and the temperature can climb at a rate of 4000°C per minute, sometimes leading to fire and explosion. LiFePO4 (LFP) cells, sometimes marketed as ‘safe’ Li-ion cells, become self-heating at 100°C and heat at a rate of up to 150°C per minute. Sodium-ion cells also have this property, but Barker claims that Faradion cells become self-heating at 150°C and heat at a maximum rate of 52°C. ‘This indicates that they are much safer than LCO or LFP technologies,’ Barker said.

Moreover, they can be transported in a short-circuited state with no energy stored, making them much easier to transport than Li-ion, which are characterised as a hazardous cargo and subject top strict controls by the International Civil Aviation Authority in terms of the size and number of cells that are allowed in consignments. ‘There are incidents of charged Li-ion batteries producing smoke, extreme heat, catching fire or exploding,’ Barker said. ‘Faradion has patented a method for transporting and storing cells that avoids those risks.’

In terms of performance, Barker said that Faradion’s cells approach those of li-ion technologies. In terms of cathode specific energy Faradion’s cells come in at around 500Whr/kg; lower than LCO but higher than LFP. ‘We’re still testing our second-generation cells on a discharge-recharge cycle, but we’ve extrapolated to a thousand cycles and that indicates capacity should be at 93 per cent of the original,’ he said.

Barker said that sodium technology is a direct drop-in replacement for lithium, in terms of usage and manufacturing; the manufacturing process has the same steps for both types of cell and a production line could be changed from one to the other easily, he said. Faradion hopes to license its technology to manufacturers rather than undertake large-scale production itself.

The company believes its biggest potential market is in static urban energy storage. ‘You could have individual batteries for homes with photovoltaic panels, or for a street or bocks of flats,’ chairman Chris Wright told The Engineer. ‘But the potential there is huge. However, recently we’ve had a lot of enquiries from the automotive sector.’

The e-bike project has seen Faradion develop its cells, while Williams built the battery and designed the battety management system. Faradion already has an improvements programme in its sights, including reducing the size of the cell pouches from about 20x10cm to 10x5cm, optimising the hard-carbon electrodes for use with sodium.