Power/Performance Bits: Nov. 1

New approach to switches

According to the National Resource Defense Council, Americans waste up to $19 billion annually in electricity costs due to always-on digital devices in the home that suck power even when they are turned off.

With that in mind, a team from University of Utah devised a new kind of switch for electronic circuits that uses solid electrolytes such as copper sulfide to grow a wire between two electrodes when an electrical current passes through them, turning the switch on. When the polarity of the electrical current is reversed, the metallic wire between the electrodes breaks down–leaving a gap between them–and the switch is turned off. A third electrode is used to control this process of growing and breaking down the wire.

“The distance between the two electrodes where the wire is grown can be as little as a nanometer long,” said Massood Tabib-Azar, electrical and computer engineering professor at the University of Utah.

Consequently, billions of these switches could be built onto a computer processor or in solid-state memory chips such as the RAM in a laptop computer. In a smartphone, this technology could be employed in the communications circuitry of the phone, which typically wastes battery power while it is in a state waiting to be used.

Besides better power efficiency, another advantage of this technology is it would produce less heat in the appliance or device because less electrical current is constantly running though its circuitry. Heat buildup has especially been a problem with laptops and phones and can affect the reliability of components over time.

Right now, the only disadvantage to this process is that it is slower than typical switches in regular silicon-based electronics because of the time it takes to grow and break down the wires. But the team expects that to improve as he and his researchers continue to optimize the process. They also said this technology could be used for devices where speed isn’t a priority but battery power is.

“In lots of applications you really don’t utilize the full speed of the silicon anyway,” said Tabib-Azar. “Right now, the biggest problem to solve is reducing the power leakage and addressing the energy-efficiency issues.”

Self-healing battery

Scientists from Fudan University (Shanghai, China), the Samsung Advanced Institute of Technology, and the Samsung R&D Institute China introduced a thin, flexible, lithium ion battery with self-healing properties that can be safely worn on the body. Even after completely breaking apart, the battery can grow back together without significant impact on its electrochemical properties.

The electrodes in these batteries consist of layers of parallel carbon nanotubes. Between the layers, the scientists embedded the necessary lithium compounds in nanoparticle form (LiMn(2)O(4) for one electrode, LiTi(2)(PO(4))(3) for the other). In contrast to conventional lithium ion batteries, the lithium compounds cannot leak out of the electrodes, either while in use or after a break. The thin layer electrodes are each fixed on a substrate of self-healing polymer. Between the electrodes is a novel, solvent-free electrolyte made from a cellulose-based gel with an aqueous lithium sulfate solution embedded in it. This gel electrolyte also serves as a separation layer between the electrodes.

The battery as an armband on a doll. (Source: “A Self-Healing Aqueous Lithium-Ion Battery”/Wiley-VCH 2016)

After a break, it is only necessary to press the broken ends together for a few seconds for them to grow back together. Both the self-healing polymer and the carbon nanotubes “stick” back together perfectly. The parallel arrangement of the nanotubes allows them to come together much better than layers of disordered carbon nanotubes. The electrolyte also poses no problems. Whereas conventional electrolytes decompose immediately upon exposure to air, the new gel is stable. Free of organic solvents, it is neither flammable nor toxic, making it safe for this application.

As a test, the team placed their battery around the elbow of a jointed doll and found that the capacity and charging/discharging properties were maintained, even after repeated break/self-healing cycles.

Batteries built with fossilized algae

Researchers at the University of California, Riverside developed an inexpensive, energy-efficient way to create silicon-based anodes for lithium-ion batteries from the fossilized remains of single-celled algae called diatoms.

Lithium-ion batteries are composed of an anode, a cathode, and an electrolyte made of lithium salt dissolved in an organic solvent. While graphite is the material of choice for most anodes, its performance is a limiting factor in making better batteries and expanding their applications. Silicon, which can store about 10 times more energy, is being developed as an alternative anode material, but its production through the traditional method is expensive and energy-intensive.

So, the team turned to a cheap source of silicon — diatomaceous earth — and a more efficient chemical process. Diatomaceous earth is an abundant, silicon-rich sedimentary rock that is composed of the fossilized remains of diatoms deposited over millions of years. Using a process called magnesiothermic reduction, the group converted this low-cost source of silicon dioxide (SiO2) to pure silicon nano-particles.

Electron microscopy showing one of the unique geometries observed in the nano-silicon power derived from diatomaceous earth. (Source: UC Riverside)

“A significant finding in our research was the preservation of the diatom cell walls — structures known as frustules — creating a highly porous anode that allows easy access for the electrolyte,” said Cengiz Ozkan, professor of mechanical engineering at UCR.

This research is the latest in a series of projects by the team to create lithium-ion battery anodes from environmentally friendly materials. Previous research focused on developing and testing anodes from portabella mushrooms and beach sand.