Salt has long been used to preserve meat, and now researchers at Cornell University have found that adding certain salts to the anodes of lithium-based batteries can also increase their useful life by a very large factor, solving long-standing problems associated with cell degradation. The advance can be adapted to other metal-based chemistries, including the lighter and more energy dense lithium-sulfur cells and, according to the researchers, might see commercial applications in as little as three years.

When a metal-based battery is being built, it is crucial that the anode material be deposited as evenly as possible. This is because, as the battery goes through several charge and discharge cycles, even the tiniest of imperfections will give rise to harmful microscopic crystals known as dendrites. The dendrites can produce internal short circuits on the surface of the electrode that cause the battery to overheat. And if the heating gets out of control, this can lead to potentially dangerous thermal runaway. Moreover, if the heat melts the crystal itself, this creates regions of "orphaned" or electrically disconnected metal that result in a slow but steady decrease in cell capacity over time.

To many researchers, the problem of dendritic growth – a big issue in many metal-based batteries, including lithium-ions – can be somewhat managed, but never completely eradicated. No matter how evenly you try to deposit the anode material, a certain amount of small-scale defects is inevitable and so, in their view, the phenomenon can only be slowed down to a limited extent, by controlling the operating conditions of the battery and designing the electrolyte with care so it doesn't accelerate the growth of defects.

Now, a research team at Cornell led by Prof. Lynden Archer has taken a new approach to the problem that could prevent the growth of these harmful crystals. After examining the chemical stability of the deposition process, the scientists decided to add halide salts to the electrolyte. As expected, this created a nanostructured coating on the anode of the lithium battery which was able to very effectively prevent the formation of dendrites. According to the researchers, this advance produced a very substantial increase in the battery's cycle life.

"Our results show that if untreated, dendritic growth will cause a lithium battery to fail after only 65 hours of continuous charge/discharge cycling," Prof. Archer told Gizmag. "In contrast, addition of the halide salt additives extends the lifetime of the cells from 1,800 hours (i.e. an increase by a factor of around 25) to indefinitely, particularly when the additives are used in conjunction with a nanoporous separator."

The test was performed at room temperature and at substantially higher current densities than those normally used to evaluate polymer or ceramic electrolytes, which makes the result even more impressive.

According to the researchers, the method can also be adapted to other electrode chemistries. For instance, the team has found that a similar approach can be used to create dendrite-free electrodeposition in sodium, and they believe they will soon be able to apply the same technique to other metal electrodes including aluminum, zinc and lead with relative ease, once they have worked out exactly which salt additives to employ in each case.

"Because the halide additives can be used to reinforce existing electrolytes, we do not anticipate any significant hurdles to their optimization and commercial implementation in rechargeable lithium-polymer and lithium ion batteries in current use," says Archer. "We expect these additives to be ready for use in existing battery systems within one year." Archer tells us that the next step for his team will be to demonstrate the technology in full cells that employ thick intercalating cathodes, such as nickel-manganese-cobalt (NMC) and lithium iron phosphate (LFP).

Beyond that, the team will look into applying their advance to so-called "conversion" cathodes, such as sulfur and oxygen, which have the potential for much higher specific capacities (lithium-sulfur batteries being one of the most promising battery chemistries).

"Application of our discovery to enable high-energy lithium-sulfur batteries will require additional research and development," says Archer. "With robust funding, we expect that these systems will require an additional two years of commercial development before becoming suitable for large-scale manufacturing."

The advance was published in a recent edition of the journal Nature Materials.

Source: Cornell University

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