Why lithium metal batteries fail

Breaking lithium pieces reduce the charge capacity and prevent recharging

Lithium metal batteries are very powerful, but so far they have not recharged. © iarti / iStock
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Blocking pieces of metal: lithium metal batteries have up to ten times the energy density of lithium-ion batteries. But so far they are not rechargeable. The reason why researchers have specified with a new method - with a surprising result. Because lithium is not the problem at the anode, but the lithium pieces that break off there and then form a blockade in the electrolyte, as the researchers report in the journal "Nature".

Whether in a notebook, cell phone or in electric cars - today they usually receive their electricity from lithium-ion batteries. However, these can lose their charge capacity over time and even explode in heat. For energy-intensive applications such as electromobility, batteries with a higher energy density would be better - then the batteries could be smaller and lighter. Researchers are therefore looking for alternatives, including sodium or silicon instead of lithium ions.

Powerful, but not rechargeable

But there is another alternative: lithium-metal batteries. In these, the anode is not made of graphite, but of metallic lithium. This makes these batteries much better power storage with a six to ten times higher energy density than lithium-ion batteries. Lithium metal batteries have been used as button cells in watches, pacemakers and other medical devices. In larger form, the range of electric cars could increase dramatically.

But there is a catch: So far, lithium-metal batteries are not rechargeable. Their capacity drops so much already after a charging cycle that they become virtually useless afterwards. The cause of this effect so far was the formation of ramified lithium deposits on the anode, combined with an accumulation of lithium compounds in the electrolyte. Both together block the passage of active lithium through the battery. In extreme cases, the deposits can also lead to explosions.

Look inside

However, what exactly happens when discharging in lithium-metal batteries has hitherto been difficult to observe in detail. Chengcheng Fang from the University of California San Diego and his colleagues have now developed a method to more accurately determine which lithium form deposits where and in what quantities. They used, among other things, the fact that metallic lithium reacts with hydrogen under release of hydrogen the gas thus reduces the amount of reactive lithium. display

The surprising result: Contrary to expectations, the amount of lithium compounds in the electrolyte did not change during discharge. It remained about the same, even if the capacity of the test batteries dropped. This was different with the metallic lithium: "To our surprise, however, we found a linear relationship between the amount of inactive metallic lithium and the charge capacity, " Fang and his colleagues report.

Blockage of lithium fragments

Further analysis revealed that the problem is apparently not the lithium deposited as such at the anode, but the fragments breaking off from these deposits. These metal pieces accumulate in the electrolyte. Because they are separated from the anode, they can no longer participate in the electrochemical cycle. As a result, these fragments form inactive blockages, as the researchers report.

"This is an important discovery because it shows that the main cause of failure of the lithium-metal batteries is this non-reactive metallic lithium and not the lithium compounds in the electrolyte, " says Fang.

Cross section through columnar deposits of metallic lithium at the anode of a lithium metal battery. Meng lab / Nature

It depends on the electrolyte

The bottom line: knowing this process makes it possible to develop solutions to the problem. Fang and his team have already taken a first step in this direction. In tests, they found that the fragmentation of lithium is closely related to the nature of the electrolyte. Thus, most common electrolytes produce stale, fragile outgrowths at the anode, which are easily broken off, as the experiments showed.

In contrast, an electrolyte that was specially optimized by the researchers made it possible to grow compact, sturdy columns of metallic lithium at the anode. These pillars break off less easily and as a result the charge capacity is retained. In the test, these batteries achieved 96 Coulomb efficiency in the first charge cycle, Fang and his team report. This corresponds to a relatively high capacity.

"Controlling the microstructure and nanostructure is obviously the key, " says Fang's colleague Shirley Meng. "We hope that our findings will stimulate further research in this direction so that lithium-metal batteries can be taken to the next level." The goal is to produce a long-lasting and rechargeable lithium-metal battery. (Nature, 2019; doi: 10.1038 / s41586-019-1481-z)

Source: University of California - San Diego

- Nadja Podbregar