N-type polymer for fast organic battery

Researchers at the University of Houston have developed an efficient, conductive electron-transporting polymer, a long-missing puzzle piece that will allow ultrafast battery applications.

The discovery relies on a ‘conjugated redox polymer’ design with a naphthalene-bithiophene polymer that has traditionally been used for applications including transistors and solar cells. With the use of lithium ions as dopant, researchers found it offered significant electronic conductivity and remained stable and reversible through thousands of cycles of charging and discharging energy.

The breakthrough addresses a decades-long challenge for electron transport conducting polymers, said Yan Yao, assistant professor of electrical and computer engineering at the UH Cullen College of Engineering and lead author of the paper.

Researchers have long recognised the promise of functional organic polymers but until now have not been successful in developing an efficient electron-transport conducting polymer to pair with the established hole-transporting polymers. The lithium-doped naphthalene-bithiophene polymer proved both to exhibit significant electronic conductivity and to be stable through 3000 cycles of charging and discharging energy, Yao said.

The discovery could lead to a cheaper alternative to traditional inorganic-based energy devices, including lithium batteries. Ultimately, it could translate into less expensive consumer devices and even less expensive electric cars, said Yao.

Yanliang Liang, a research associate at UH and first author on the paper, said conventional inorganic metal-based batteries and energy storage devices are expensive partly because the materials used to make them, including cobalt and silicon-based compounds, require huge energy expenditures to process. Organic polymers can be processed at relatively low temperatures, lowering the cost.

They also produce less CO₂, he said, adding to their environmental advantage. In addition, while conventional materials are finite, organic polymers could potentially be synthesised from biomass.

The basic polymer used in the work was discovered in 2009 and was provided by members of the research team from Polyera Corporation, a technology company based in Illinois, said Yao. Although naphthalene-bithiophene has been used for transistors and other applications since its discovery, this is the first time it has been converted for use in energy storage. That was done through the addition of lithium and raised the polymer’s doping level from a previously reported 0.1 to 2.0.

The results are record-setting. The polymer exhibits the fastest charge-discharge performance for an organic material under practical measurement conditions, allowing a battery to be 80% charged within 6 seconds and fully charged in another 18 seconds, Liang said.

Conventional inorganic batteries still are capable of holding more energy than the organic battery, and Yao said work will continue to improve the storage capacity of the material. His group also will continue to do basic scientific research on the polymer to learn more about it, he said.