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Added chemicals inhibit dendrite formation in lithium batteries

18 June 2015

Encouraging work by US researchers indicates that lithium based batteries can be made safer by the addition of chemicals that suppress the formation of dendrites.

Deposits forming on the anode of a lithium metal battery. When lithium nitrate is added to the electrolyte, destructive lithium metal dendrites grow on the surface (image courtesy of W. Li et al, Nature Communications)

In a study that could improve the safety of next-generation batteries, researchers have discovered that by adding two chemicals to the electrolyte of a lithium metal battery, the formation of dendrites – 'fingers' of lithium that pierce the barrier between the battery’s halves, causing it to short-circuit, overheat and sometimes combust - can be avoided.

The findings, published June 17 in Nature Communications, could help remove a major barrier to developing lithium-sulphur and lithium-air batteries, promising future technologies that could store up to ten times more energy per weight than batteries now used in consumer electronics and electric cars.

“Because these batteries would be much lighter than today’s rechargeable batteries, they have a lot of potential for extended-range electric vehicles,” says Yi Cui, an associate professor at Stanford University and the US Department of Energy’s SLAC National Accelerator Laboratory. “But one of the things that’s been holding them back is their tendency to form dendrites, which are also the culprit behind overheating and occasional fires in today’s lithium-ion batteries.”

Dendrites form when a battery electrode degrades, and metal ions become deposited on the electrode’s surface. When those finger-like deposits elongate until they penetrate the barrier between the two halves of the battery, they can cause electrical short-circuits, overheating and fires.

In a previous study published last October, Cui and his colleagues reported that they had developed a 'smart' lithium-ion battery that senses when dendrites start to puncture the barrier, enabling the battery to be replaced before the situation becomes dangerous. 

The new research addresses battery technologies that have yet to reach the market, and it takes a different approach: adding chemicals to the electrolyte to prevent dendrite formation. One compound, lithium nitrate, has been under investigation for a long time as an additive to improve battery performance. The other, lithium polysulphide, has been considered a nuisance: formed when a sulphur electrode degrades, it travels to the lithium metal electrode and damages it.

Deposits forming on the anode of a lithium metal battery. When lithium polysulphide is added as well as lithium nitrate, harmless pancake-like deposits form instead of dendrites (image courtesy of W. Li et al, Nature Communications)

The research team realised their combined effect had not been studied before; together the chemicals could potentially react with lithium metal to form a stable, solid interface between the electrode and the electrolyte.

The team assembled coin cell batteries and added various concentrations of the two chemicals to the ether-based electrolyte. Following many charge/discharge cycles, the batteries were disassembled and the electrodes examined using an electron microscope and an X-ray technique that revealed their morphology and chemical composition.

It was discovered that by adding both chemicals in just the right amounts, lithium dendrite formation ceased and harmless pancake-like deposits grew instead. The lithium metal acquired a stable coating that helped protect it from further degradation and actually improved the battery’s performance.

In tests, batteries with both chemicals added operated at 99 percent efficiency after more than 300 charge-discharge cycles, compared with significantly decreased efficiency after 150 cycles for batteries treated with lithium nitrate alone.

“This is a really exciting observation,” says Fiona Li, a postdoctoral researcher in Cui’s lab and first author of the Nature Communications paper. “We had been doing experiments all along with these two chemicals in there, but this was the first time we looked at the synergistic effect. This does not completely solve all the problems associated with lithium metal batteries, but it’s an important step.”

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