Controlling thermal conductivities can improve energy storage.

Controlling the flow of heat through materials is important for many technologies. While materials with high and low thermal conductivities are available, materials with variable and reversible thermal conductivities are rare, and other than high pressure experiments, only small reversible modulations in thermal conductivities have been reported.

For the first time, researchers at the University of Illinois at Urbana-Champaign have experimentally shown that the thermal conductivity of lithium cobalt oxide (LixCoO 2 ), an important material for electrochemical energy storage, can be reversibly electrochemically modulated over a considerable range.

"This work is the first experimental demonstration of the electrochemical modulation of the thermal conductivity of a material, and, in fact, the only demonstration of large variable and reversible thermal conductivities in any material by any approach, other than very high pressure experiments," explained Paul Braun, a professor of materials science and engineering (MatSE) at Illinois. The results of research have been reported in the article, "Electrochemically Tunable Thermal Conductivity of Lithium Cobalt Oxide," appearing in Nature Communications .

One technology that may be directly impacted by this work is the field of electrochemical energy storage. Understanding and controlling heat evolution and dissipation in rechargeable batteries is critical. Yet prior to this work, it was not even known that the thermal conductivity of materials commonly used as cathodes changed significantly as a function of the state of charge.

"Our work opens up opportunities for dynamic control of thermal conductivity and additionally, may be important for thermal management in electrochemical energy storage devices which use cathodes based on transition metals oxides such as lithium cobalt oxide," added MatSE professor David Cahill, one of the paper's co-authors.

A better understanding of the thermal properties of battery electrodes may help in the design of batteries that can be charged more rapidly, deliver more power, and operate with a greater margin of safety, since the heat generated during fast cycling and temperature variations in general are very detrimental to lithium-ion batteries.

Flexible plastics turn mechanical vibrations into electrical energy

The shrinking dimensions and decreased power consumption of modern electronic gadgets have created opportunities for energy harvesting processes that tap into free, green energy from the environment. Vibration harvesters, for example, produce small amounts of electricity from everyday mechanical disturbances such as wind currents, traffic noise or footsteps.

Now, Kui Yao and co-workers from the A STAR Institute of Materials Research and Engineering in Singapore have discovered a way to give lightweight polymer vibration harvesters a hundredfold boost in energy output —a finding that may help to eliminate manual battery recharging in microsensors and mobile devices.

Many vibration harvesters contain piezoelectric substances that create an electric voltage when mechanically bent. By fabricating piezoelectric materials into cantilevers that resemble a diving board, these devices can oscillate from ambient vibrations and generate electricity. Researchers often use piezoelectric ceramics because they impart large amounts of electrical charges; however, the brittleness of ceramics makes them unsuitable for prolonged and large vibrational movements.

Yao and co-workers investigated a plastic-based piezoelectric material , polyvinylidene fluoride (PVDF), which is low cost and readily undergoes mechanical strain. To make efficient vibration harvesters from PVDF, researchers must stack the polymer in multiple layers, improving the output current and reducing the electrical impedance that is inherent to piezoelectric materials. But when too many thin piezoelectric layers are stacked, the cantilever can become too stiff for bending-mode vibrational harvesting.

To optimize piezoelectric harvesting with plastic films, the team deployed an analytical approach. Developing a mathematical model of a multilayered polymer cantilever coated with metal electrodes, the researchers systematically calculated how different material parameters affected the energy output.