Sodium batteries’ durability and performance boosted by scandium additive

Adding manganese oxide, nickel and small amount of scandium can significantly improve both the durability and performance of sodium-ion batteries. The early stage finding shows that the batteries’ lifespan and charging performance can increase more than three times when scandium is used as a coating or structural additive.

Concerns about the cost and environmental impact of lithium mining has increased interest in finding alternative battery systems for use in electric vehicles and energy storage.

‘Sodium-ion batteries seems to be the most advanced of these because they are so similar conceptually to lithium-ion batteries,’ says Marca Doeff, a recently retired energy-storage expert who previously worked at Lawrence Berkeley National Laboratory in the US.

Sodium is more abundant than lithium, making it cheaper and reducing the risk of material shortages. Sodium-ion batteries are also safer to transport than lithium batteries, explains Doeff. ‘You can take sodium-ion batteries and discharge them all the way to zero volts and ship them that way and they’re perfectly safe,’ she says.

Despite these potential advantages, sodium-ion batteries have struggled to meet performance goals. ‘The general problem is the lifetime or durability. That’s still challenging compared with the existing lithium-ion battery technology,’ says Shinichi Kumakura, a battery scientist who co-led the new research alongside Shinichi Komaba at Tokyo University of Science in Japan. ‘We also have to improve energy density or charging speed.’

One way of improving sodium-ion batteries is to ‘dope’ the cathode with a small amount of other material. Another is to add a coating to the surface that acts as a protective layer.

Sodium-ion batteries containing a combination of manganese oxide and nickel have been tested before but have some stability issues. ‘Many sodium-layered oxides … suffer from issues like phase changes, lattice strain, and surface instability,’ says Xiaowen Zhan, a materials scientist at Brown University in the US.

Improved performance

In the new study, Kumakura’s team added a small percentage of scandium, a rare, electrochemically inactive metal, to the cathode structure and also tested a surface coating with a similar makeup to see if it could improve the battery’s durability and performance.

In single-battery test cells, the researchers found that untreated cells had only 19% capacity left after 100 cycles. However, this went up to 68% when scandium was added to the cathode structure and 75% when a coating containing scandium was used.

The team also tested full batteries over 300 cycles, observing up to 92% retention of capacity with scandium doping or coating, versus almost nothing when no scandium was included. Charging speed also increased more than threefold after scandium was added to the structure or coating.

‘I think the main strength is not only that scandium improves the cycling stability of this cathode, but that [Kumakura’s team] compared two different ways of using the scandium’ says Zhan.

The research is ‘a good start’, comments Doeff, although she notes that much more work would be needed to make the material a commercial reality.

Kumakura acknowledges that while the addition of scandium worked well, it is not cheap or abundant to source. ‘I think it’s not the best element from a commercial or practical point of view,’ he notes, adding that the team is already investigating other options with similar properties. ‘Cerium could be an option – it’s a rare-earth element, but it’s not too expensive compared with scandium,’ he says.