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University of Houston researchers discover n-type polymer for fast organic battery

The discovery could lead to a cheaper alternative to traditional inorganic-based energy devices, including lithium batteries

6 Apr 2015 | Editor

Researchers at the University of Houston have reported developing an efficient conductive electron-transporting polymer that will allow ultrafast battery applications.

The discovery relies upon a "conjugated redox polymer" design with a naphthalene-bithiophene polymer, which 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, described in the Journal of the American Chemical Society and featured as ACS Editors’ Choice for open access, 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.

According to the announcement the 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 3,000 cycles of charging and discharging energy.

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.

The researchers 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, adding to their environmental advantage. And 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 Corp.

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 percent charged within 6 seconds and fully charged in another 18 seconds,concluded the researchers.

Conventional inorganic batteries still are capable of holding more energy than the organic battery, more researcher will be undertaken to improve the storage capacity of the material.

The researchers said, "Organic -conjugated polymers are emerging as a materials class for energy-related applications, enabling a path to a more sustainable energy landscape without the need of energy-intensive, expensive and sometimes toxic metal-based compounds." The researchers added, "a model polymer, P(NDI2OD-T2), was stably and reversibly n-doped to a high doping level of 2.0, a significant progress for electron-transporting π-conjugated polymers. … With rational molecular design, π-conjugated redox polymers will establish new design space in polymer chemistry and see wide-spread applications, especially in energy-related ones such as batteries, supercapacitors and thermoelectrics."

Heavily n-Dopable π-Conjugated Redox Polymers with Ultrafast Energy Storage Capability

Yanliang Liang | Zhihua Chen | Yan Jing | Yaoguang Rong | Antonio Facchetti | Yan Yao

J. Am. Chem. Soc. | DOI: 10.1021/jacs.5b02290 | Publication Date (Web): March 31, 2015

Abstract

We report here the first successful demonstration of a “π-conjugated redox polymer” simultaneously featuring a π-conjugated backbone and integrated redox sites, which can be stably and reversibly n-doped to a high doping level of 2.0 with significantly enhanced electronic conductivity. The properties of such a heavily n-dopable polymer, poly{[N,N’-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} (P(NDI2OD-T2)), were compared vis-a-vis to those of the corresponding backbone-insulated poly{[N,N’-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5’-[2,2’-(1,2-ethanediyl)bithiophene]} (P(NDI2OD-TET)). When evaluated as a charge storage material for rechargeable Li batteries, P(NDI2OD-T2) delivers 95% of its theoretical capacity at a high rate of 100C (72 s per charge−discharge cycle) under practical measurement conditions, as well as 96% capacity retention after 3000 cycles of deep discharge−charge. Electrochemical, impedance, and charge transport measurements unambiguously demonstrate that the ultrafast electrode kinetics of P(NDI2OD-T2) are attributed to the high electronic conductivity of the polymer in the heavily n-doped state.

pdf of article

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