Silicon boost for lithium-ion batteries

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Electric cars are built around lithium-ion batteries. They are too expensive, too big and don’t keep cars moving long enough. However, amazing little changes to the electromobility power sources that are being made in Germany could lead to a breakthrough.

Nothing works without electricity. In mobile devices, it usually comes from a lithium-ion battery. But only for a short time, as everyone knows. Actually, due to the material used for its negative electrode (also known as the anode when it is discharging, this kind of power supply is limited.

Until now, the material that proved itself most for this purpose has been graphite, one of the natural forms of the chemical element carbon. It absorbs lithium ions, which bind the electrons when the battery is charging. However, it stores only one lithium ion for every six atoms of carbon. In mathematical terms, that results in a maximum storage capacity of 370 mAh (milliampere hours) per gram of carbon.

Other anode materials promise considerably higher values. The lithium-ion storage capacity of silicon, for example, is about ten times higher. In addition, it is available in almost unlimited quantities, is non-toxic and extremely inexpensive. Unfortunately, it swells up to three times its size when the lithium ions are stored (charged) and shrinks back to its original size when it discharges. The mechanical stresses that occur destroy the battery after a few charging cycles. Global research efforts in this field are focusing on preventing that from happening.

Lithium-ion batteries with tubes instead of crystals

Amprius Lithium-Ionen-AkkusIn 2008, researchers at Stanford University developed electrodes from silicon nanotubes that survive the mechanical loads of charging and discharging considerably better than silicon crystals. The patents were used to found Amprius Inc., one of the leading companies in this field. Now the Chinese are producing batteries for smartphones that are 10% better than conventional lithium-ion batteries. They are aiming for a 40-percent increase next year. For the first time ever, a roll-to-roll technique was introduced that can also be used to produce silicon nanotube anodes for electric automobiles.

Besides start-ups like Amprius, the industry’s major corporations such as Samsung and Panasonic are doing their own silicon-anode thing. Not to mention Panasonic, which is building the “Gigafactory” for lithium-ion batteries in Nevada with Tesla. Elon Musk even referred to “a little” silicon in his batteries already at the end of last year. It will be interesting to see what happens.

Research yes—Production no

Naturally, companies here in Germany are also working to further develop lithium-ion batteries. And that is despite the fact that producing the cells probably will not be economical during the next ten years, as Daimler’s Development Officer Thomas Weber said in a recent interview with “Autogazette”.

Lithium-ion batteries (Image: HZB)
Lithium ions migrate through the electrolyte (yellow) into the layer of crystalline silicon (c-Si). During the charging cycle, a 20-nm layer (red) develops on the silicon electrode adsorbing extreme quantities of lithium atoms. (Image: HZB).

A week ago, a team from the HZB Institute for Soft Matter and Functional Materials announced that, for the first time ever, it had observed lithium-ions live in silicon. According to them, an approximately 20-nanometer layer with an extremely high lithium content formed near the interface to the electrolytes: There were 25 lithium atoms for every ten atoms of silicon. In the next layer, the same number of silicon atoms accommodate only one lithium atom. Both layers together are 100 nanometers thick.

In other words, one extremely thin layer of silicon is all it takes to absorb a sufficient quantity of lithium. That not only significantly reduces the tension forces on the crystal structure, it also reduces the amount of material and, above all, energy needed during manufacturing.

Mathematically, the maximum capacity of this type of silicon-lithium battery is approximately 2300 mAh per gram, which is six times higher than that of a lithium-ion battery with a graphite electrode (370 mAh/g).

It’s anyone’s guess what those numbers will be in actual practice. After all, capacity is certainly not determined by a single electrode. It depends on the chemistry of the entire cell. Which is why a complex system like a battery is optimized to meet a number of different requirements—some of which are conflicting. Including to ensure efficient production. But even if the six-fold increase turns into just a doubling of capacity in the long run, that would still mean a breakthrough for electromobility, even in Germany.

 

logo electronica eYou can find out all about the latest developments in power supplies in Halls A2/A3 at electronica 2016 in Munich. You will find suppliers who can provide the right component for any application.

 

Lithium-Ionen-Akku (Bild: pixabay).

Silizium im Akku verlängert die „Tankintervalle“. (Bild: pixabay).