Power/Performance Bits: Sept. 12

Water-based li-ion battery; waste heat into electricity; supercapacitors from trees.


Water-based li-ion battery
Researchers at the University of Maryland and the U.S. Army Research Laboratory developed a lithium-ion battery that uses a water-salt solution as its electrolyte and reaches the 4.0 volt mark desired for household electronics, without the fire and explosive risks associated with some commercially available non-aqueous lithium-ion batteries.

The battery provides identical energy density as standard li-ion batteries, said Kang Xu, an ARL fellow who specializes in electrochemistry and materials science. “In the past, if you wanted high energy, you would choose a non-aqueous lithium-ion battery, but you would have to compromise on safety. If you preferred safety, you could use an aqueous battery such as nickel/metal hydride, but you would have to settle for lower energy. Now, we are showing that you can simultaneously have access to both high energy and high safety.”

Previous research produced a similar 3.0 volt battery with an aqueous electrolyte but was stymied from achieving higher voltages by the so-called “cathodic challenge,” in which one end of the battery, made from either graphite or lithium metal, is degraded by the aqueous electrolyte. To solve this problem, researchers designed a new gel polymer electrolyte coating that can be applied to the graphite or lithium anode.

Four V Li-ion batteries assembled with water-in-salt gel electrolyte. (Source: Jhi Scott, ARL photographer)

This hydrophobic coating expels water molecules from the vicinity of the electrode surface and then, upon charging for the first time, decomposes and forms a stable interphase–a thin mixture of breakdown products that separates the solid anode from the liquid electrolyte. This interphase protects the anode from debilitating side reactions, allowing the battery to use desirable anode materials, such as graphite or lithium metal, and achieve better energy density and cycling ability.

The gel coating also boots the safety of the battery. Even when the interphase layer is damaged (if the battery casing were punctured, for instance), it reacts slowly with the lithium or lithiated graphite anode, preventing the smoking, fire, or explosion that could otherwise occur if a damaged battery brought the metal into direct contact with the electrolyte.

The team is working on increasing the number of full-performance cycles that the battery can complete and to reduce material expenses where possible. “Right now, we are talking about 50-100 cycles, but to compare with organic electrolyte batteries, we want to get to 500 or more,” Wang said.

Xu said the interphase chemistry needs to be perfected before it can be commercialized. He also said more work needs to be done on scaling up the technology in big cells for testing. With enough funding, the 4-volt chemistry could be ready for commercializing in about five years.

Waste heat into electricity
Physicists at Washington State University developed a multicomponent, multilayered composite material capable of turning waste heat into electricity.

Instead of combining a common metal like aluminum or copper with a conventional semiconductor material like silicon, the team’s device, called a van der Waals Schottky diode, is made from a multilayer of microscopic, crystalline indium selenide. They used a simple heating process to modify one layer of the indium selenide to act as a metal and another layer to act as a semiconductor.

“The ability of our diode to convert heat into electricity is very large compared to other bulk materials currently used in electronics,” said Yi Gu, an associate professor of physics at WSU. “In the future, one layer could be attached to something hot like a car exhaust or a computer motor and another to a surface at room temperature. The diode would then use the heat differential between the two surfaces to create an electric current that could be stored in a battery and used when needed.”

The left panel shows the schematic lattice structures of the alpha-beta In2Se3 van der Waals metal-semiconductor junction, and the right panel shows an optical micrograph of a junction device. (Source: Yi Gu/WSU)

Unlike its conventional counterparts, the diode has no impurities or defects at the interface where the metal and semiconductor materials are joined together. The smooth connection between the metal and semiconductor enables electricity to travel through the multilayered device with almost 100 percent efficiency.

“When you attach a metal to a semiconductor material like silicon to form a Schottky diode, there are always some defects that form at the interface,” said Matthew McCluskey, a professor of physics at WSU. “These imperfections trap electrons, impeding the flow of electricity. Gu’s diode is unique in that its surface does not appear to have any of these defects. This lowers resistance to the flow of electricity, making the device much more energy efficient.”

The team is currently investigating new methods to increase the efficiency of their indium selenide crystals. They are also exploring ways to synthesize larger quantities of the material so that it can be developed into useful devices.

Supercapacitors from trees
Researchers at the Qilu University of Technology and Shandong Jianzhu University found that fallen leaves can be converted into a porous carbon material that makes an excellent supercapacitor.

The investigators used a multistep process to convert leaves from the phoenix tree, common in northern China, into a form that could be incorporated into electrodes as active materials. The dried leaves were first ground into a powder, then heated to 220 degrees Celsius for 12 hours. This produced a powder composed of tiny carbon microspheres. These microspheres were then treated with a solution of potassium hydroxide and heated by increasing the temperature in a series of jumps from 450 to 800 C.

The chemical treatment corrodes the surface of the carbon microspheres, making them extremely porous. The final product, a black carbon powder, has a very high surface area that improves the final product’s electrical properties.

Scanning Electron Microscopy (SEM) image of porous carbon microspheres. (Source: Hongfang Ma/Qilu University of Technology)

The investigators ran a series of standard electrochemical tests on the porous microspheres to quantify their potential for use in electronic devices. The current-voltage curves for these materials indicate that the substance could make an excellent capacitor. Further tests showed that the materials are, in fact, supercapacitors, with specific capacitances of 367 Farads/gram, over three times higher than values seen in some graphene supercapacitors.

The researchers focus on ways to convert waste biomass into porous carbon materials that can be used in energy storage technology. In addition to tree leaves, the team and others have successfully converted potato waste, corn straw, pine wood, rice straw and other agricultural wastes into carbon electrode materials.

The supercapacitive properties of the porous carbon microspheres made from phoenix tree leaves are higher than those reported for carbon powders derived from other biowaste materials. The fine scale porous structure seems to be key to this property, since it facilitates contact between electrolyte ions and the surface of the carbon spheres, as well as enhancing ion transfer and diffusion on the carbon surface. The investigators hope to improve even further on these electrochemical properties by optimizing their process and allowing for doping or modification of the raw materials.