Solid-state technology – the quest for the ‘Holy Grail’

Innovation has powered Saft's success throughout our history. One of the people leading the way is Kamen Nechev, our Chief Technology Officer for over a decade. We asked him to tell us more about Saft's approach to innovation and to explain how solid-state technology might shape the future of the battery industry.

How does Saft approach innovation?

Our innovation activities fall into two main categories. In the near to middle term, around two to five years ahead, we focus directly on product development. This includes new chemistries such as lithium titanate oxide (LTO) that can extend the cycle life and fast-charging capability of lithium-ion (Li-ion) batteries. We are also looking at the whole battery system to develop the advanced models, algorithms and associated control electronics that will get the best out of the new chemistries. 

Looking five to eight years ahead and beyond, we are developing new battery technologies that will be critical to our long term future. Perhaps the most exciting is our work on solid-state batteries.

Why is Saft so excited about solid-state batteries?

Currently, any kind of rechargeable battery – nickel-based, lead-acid or Li-ion – relies on two electrodes that exchange ions through a liquid electrolyte. This works very well. However, there is a drawback, because some liquid electrolytes are flammable. If a Li-ion battery is subjected to any abuse or damage it could cause the electrolyte and electrodes to overheat. That’s why we pay so much attention to designing and manufacturing our Li-ion batteries to mitigate any risk of fire. 

Replacing the liquid with a solid electrolyte gives a Li-ion battery that is inherently much safer. But it is not quite that simple. Solid electrolytes are far worse at conducting ions. The concept was therefore a non-starter until very recently. But a new generation of solid electrolytes now offers the levels of conductivity we need.

There is a second benefit to the solid-state design. It puts us on the path to what has long been considered the ‘Holy Grail’ of Li-ion battery design – using metallic lithium.

What difference does metallic lithium make?

Using metallic lithium directly, instead of carbon electrodes that host lithium ions, would theoretically almost double capacity of a Li-ion cell since lithium alone has than 10 times more capacity than a carbon electrode. We could get significantly more performance from the same size of battery – or the same performance from a much smaller and lighter battery. But putting metallic lithium into a liquid electrolyte presents two major technical challenges.

First, to use metallic lithium we need to get it to ‘passivate’ by forming a microscopically thin surface oxide layer called SEI (solid electrolyte interface) that keeps it chemically stable in the electrolyte. Re-forming this layer at each cycle consumes lithium and reduces capacity by around one percent - so in less than 100 cycles the battery becomes unusable.

The second challenge is to prevent the growth of tiny whiskers (dendrites) on the metallic electrode as it cycles between discharge and charge. Eventually, the dendrites cause internal short circuits and thermal runaway.

The situation changes when we use a solid electrolyte as it protects the metallic lithium and restricts the growth of dendrites. The result is a battery with excellent energy density that can cycle efficiently and offers enhanced safety, low self-discharge and improved life at high temperatures.

Where is Saft in the race to develop solid-state batteries? 

Saft is well positioned in the top five companies working on solid-state batteries. A crucial factor is that Total, our parent company, views investment in innovation as central to its long term vision. Solid-state technology is also a core element of the European Battery Alliance in which we have joined forces with other industrial champions such as Manz, Siemens, Solvay and Umicore. We have also developed an excellent global network of academic and industrial partners as well as investing in Ionic Materials, a US leader in solid-state electrolytes.

When will solid-state batteries become commercially available?

In the next three to four years, solid-state batteries should be in the hands of early adopters for specialist low-volume applications such as defense, space and aviation. The next stage, four to five years from now, will see the technology taken up in high-volume consumer markets, mainly for mobile phones and tablets.

Solid-state batteries will emerge as a mature technology in about eight to ten years or so, when their combination of low cost, high energy density/low weight and long life will be ideal for electric vehicle and energy storage applications.

What is particularly interesting is that solid-state technology is ideal for a pouch cell format. This offers the flexibility to move away from the traditional cell/ module/system format. Instead we might be able to create batteries as a single unit that forms an integral structural element in a vehicle or aircraft.

How important might solid-state batteries become?

If the good progress made so far continues then we will see a slow but substantial shift to solid-state batteries. And one day they might replace Li-ion batteries everywhere, with the exception of special high-power applications.

Kamen.nechev@saftbatteries.com