New Li-ion anode achieves 70 percent charge in just two minutes

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A proof of concept nanotube-based anode for lithium-ion batteries has been developed by researchers at the Nanyang Technological University (Photo: NTU)

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Researchers at the Nanyang Technological University (NTU) in Singapore have developed a new, proof-of-concept anode for lithium-ion batteries that can charge to 70 percent of its capacity in only two minutes and has a very long lifespan of ten thousand charge/discharge cycles. The advance could lead to the production of high-rate lithium-ion batteries, with interesting implications for personal electronics and, perhaps, even electric vehicles.

Lithium-ion batteries owe their popularity to their ability to store large amounts of energy into a relatively small and light package; however, they can take a fairly long time to charge. This is largely due to the limitations of the battery anode, which is usually made of graphite. Namely, the lithium ions inside the battery need to travel a longer distance than strictly needed to reach the anode, taking more time than necessary. Secondly, the limited surface area of the electrode also slows down the rate at which the charging/discharging electrochemical reactions can take place.

A team of scientists led by Professor Xiaodong Chen has developed a proof-of-concept battery anode that addresses both these problems at the same time. The researchers replaced the standard graphite anode with a gel material containing long nanotubes made out of titanium dioxide, resulting in a much faster-charging battery which is also significantly more long-lived.

In a standard li-ion battery, the graphite electrode repeatedly expands and contracts, causing mechanical stresses that can lead to battery failure. By contrast, materials like titanium dioxide are very strong candidates for building long-lasting batteries because they don't significantly expand or contract during charge cycles.

The long TiO2 nanotubes were manufactured with a simple mixing and stirring process (bottom row), vastly improving the length of the structure compared to previous methods (above) (Image: NTU)

Chen and colleagues were able to produce titanium dioxide nanotubes which were up to 40 micrometers in size – two orders of magnitude longer than previously achieved – by a simple process of mixing it with sodium hydroxide and stirring. Crucially, this made the nanostructures long enough to be useful for building a battery anode.

Shaping the material into intercalating nanotubes inside the anode allowed the researchers to greatly reduce the distance that lithium ions needed to travel in order to transfer their charge. Moreover, the nanotube structures also have a very large surface area of 130 square meters per gram (40,000 sq ft/oz), which significantly speeds up the chemical reactions that drive charging and discharging.

As a result, their proof-of-concept battery was able to reach 70 percent of its capacity in only two minutes at a current of 8.5 A (about four times greater than an iPad charger), though the researchers tell us it would take another hour for the battery to fully charge. The prototype was also tested for an impressive 10,000 charge/discharge cycles, which is about ten times more than current lithium-ions.

The battery can charge at a very high rate of 25C for 10,000 cycles, compared to the typical 0.8C rate and 1,000 cycles for standard li-ion batteries (Image: NTU)

The technology is being licensed for mass-production, which Prof. Chen expects to take place within the next two years. The researchers tell us that scaling up the size of the battery is not an issue and will be done over the next year.

This development could potentially lead to much faster-charging and longer-lived batteries for our personal electronics. As far as the much larger batteries in electric cars, however, this pushes against the issue of supplying a high enough current to charge the battery that quickly (i.e., you'd need a very big cable, generate a lot of waste heat, and put a very strong stress on the electric grid).

A paper published in the journal Advanced Materials describes the advance.

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