Carbon nanotube anode reduces risk in lithium batteries

This is the claim of researchers at Texas A&M University whose results are published in Nano Letters.

"We have designed the next generation of anodes for lithium batteries that are efficient at producing large and sustained currents needed to quickly charge devices," said Juran Noh, a material sciences graduate student at Texas A&M. "Also, this new architecture prevents lithium from accumulating outside the anode, which over time can cause unintended contact between the contents of the battery's two compartments, which is one of the major causes of device explosions."

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A commonly used anode material is graphite. In these anodes, lithium ions are inserted between layers of graphite. However, Noh said this design limits the amount of lithium ions that can be stored within the anode and can require more energy to remove the ions from the graphite during charging.

Also, lithium ions can accumulate on the anode's surface to form dendrites. Over time, the dendrites grow and eventually pierce through the material that separates the battery's two compartments. This breach causes the battery to short circuit and can cause fire. Dendrites also affect the battery's performance by consuming lithium ions, rendering them unavailable for generating a current.

Noh said another anode design involves using pure lithium metal instead of graphite. Compared to graphite anodes, those with lithium metal have a much higher energy content per unit mass or energy density, but they can fail in the same catastrophic way due to dendrites.

Noh and her colleagues designed anodes using highly conductive, lightweight carbon nanotubes. These carbon nanotube scaffolds contain pores for lithium ions to enter and deposit, but structures do not bind to lithium ions favourably.

The team made two other carbon nanotube anodes with slightly different surface chemistry: one laced with an abundance of molecular groups that can bind to lithium ions, and another that had the same molecular groups but in a smaller quantity. With these anodes, they built batteries to test the propensity to form dendrites.

Unsurprisingly, the researchers found that scaffolds made with just carbon nanotubes did not bind to lithium ions well; there was almost no dendrite formation, but the battery's ability to produce large currents was compromised. Scaffolds with an excess of binding molecules formed many dendrites, shortening the battery's lifetime.

However, the carbon nanotube anodes with an optimum quantity of the binding molecules prevented the formation of dendrites. In addition, a vast quantity of lithium ions could bind and spread along the scaffold's surface, thereby boosting the battery's ability to produce large, sustained currents.

"When the binding molecular groups are abundant, lithium metal clusters made from lithium ions end up just clogging the pores on the scaffolds," Noh said in a statement. "But when we had just the right amount of these binding molecules, we could 'unzip' the carbon nanotube scaffolds at just certain places, allowing lithium ions to come through and bind on to the entire surface of the scaffolds rather than accumulate on the outer surface of the anode and form dendrites."

Noh said that their top-performing anodes handle currents five times more than commercially available lithium batteries. She noted this feature is particularly useful for large-scale batteries, such as those used in electric cars, that require quick charging.

"Building lithium metal anodes that are safe and have long lifetimes has been a scientific challenge for many decades," said Noh. "The anodes we have developed overcome these hurdles and are an important, initial step toward commercial applications of lithium metal batteries."