Power/Performance Bits: March 3

Black phosphorus photodetectors

Phosphorus, a highly reactive element commonly found in match heads, tracer bullets, and fertilizers, can be turned into a stable crystalline form known as black phosphorus. In a new study, researchers from the University of Minnesota used an ultrathin black phosphorus film 20 atoms thick to demonstrate high-speed data communication on nanoscale optical circuits.

In the team’s study, the black phosphorus photodetectors showed improvement in efficiency over comparable devices using graphene and rivaled that of comparable devices made of germanium. As a bonus, black phosphorus and other two-dimensional materials can be grown separately and transferred onto any material, making them more versatile than germanium.

The high performance photodetector uses a few layers of black phosphorus (red atoms) to sense light in the waveguide (green material). Graphene (gray atoms) is also used to tune the performance. (Source: University of Minnesota College of Science and Engineering)

Black phosphorus also has a widely-tunable band gap that varies depending on how many layers are stacked together, and can absorb light in the visible range as well as in the infrared. This large degree of tunability makes black phosphorus a unique material that can be used for a wide range of applications from chemical sensing to optical communication.

Additionally, black phosphorus is a direct-band semiconductor, so it has the potential to efficiently convert electrical signals back into light. Combined with its high performance photodetection abilities, black phosphorus could also be used to generate light in an optical circuit, making it a one-stop solution for on-chip optical communication.

Silicon nanofiber electrodes

Researchers at the University of California developed a novel paper-like material for lithium-ion batteries. The material, composed of sponge-like silicon nanofibers more than 100 times thinner than human hair, has potential to boost by several times the amount of energy that can be delivered per unit weight of the battery.

Conventionally produced lithium-ion battery anodes are made using copper foil coated with a mixture of graphite, a conductive additive, and a polymer binder. The researchers, suspecting the performance of graphite has been nearly tapped out, started experimenting with other materials. They focused on silicon, which has a specific capacity, or electrical charge per unit weight of the battery, nearly 10 times higher than graphite.

The problem with silicon is that is suffers from significant volume expansion, which can quickly degrade the battery. The silicon nanofiber structure created by the team circumvents this issue and allows the battery to be cycled hundreds of times without significant degradation.

Scanning electron microscope images of (a) SiO2 nanofibers after drying, (b) SiO2 nanofibers under high magnification (c) silicon nanofibers after etching, and (d) silicon nanofibers under high magnification. (Source: UC Riverside)

The nanofibers were produced using a technique known as electrospinning, whereby 20,000 to 40,000 volts are applied between a rotating drum and a nozzle, which emits a solution composed mainly of tetraethyl orthosilicate. They were then exposed to magnesium vapor to produce the sponge-like silicon fiber structure.

The researchers’ future work involves implementing the silicon nanofibers into a pouch cell format lithium-ion battery.

Dendrite-erasing electrolyte

Researchers from the Department of Energy’s Pacific Northwest National Laboratory discovered a new electrolyte for lithium batteries that eliminates dendrites – the microscopic, pin-like fibers that cause rechargeable batteries to short circuit – while also enabling batteries to be highly efficient and carry a large amount of electric current.

Anticipating graphite anodes could be near their peak energy capacity, the team at PNNL took another look at older rechargeable battery designs. Seeking to develop an electrolyte that worked well in batteries with a high-capacity lithium anode, they noted others had some success with electrolytes with high salt concentrations and decided to use large amounts of the lithium bis(fluorosulfonyl)imide salt they were considering. To make the electrolyte, they added the salt to the solvent dimethoxyethane.

The researchers built a circular test cell that was slightly smaller than a quarter. The cell used the new electrolyte and a lithium anode. Instead of growing dendrites, the anode developed a thin, relatively smooth layer of lithium nodules that didn’t short-circuit the battery. After 1,000 repeated charge and discharge cycles, the test cell retained 98.4 percent of its initial energy while carrying 4 milliAmps of electrical current per square centimeter of area.

Two scanning electron microscope images that illustrate how a traditional electrolyte can cause dendrite growth (a, left), while PNNL’s new electrolyte instead causes the growth of smooth nodules that don’t short-circuit batteries (b, right). (Source: PNNL)

The new electrolyte’s high efficiency could also open the door for an anode-free battery. An electrolyte with more than 99 percent efficiency means there’s potential to create a battery that only has a negative current collector, without an active material coating, on the anode side.

The electrolyte needs to be refined before it’s ready for mainstream use, however. The team is evaluating various additives to further enhance their electrolyte so a lithium battery using it could achieve more than 99.9 percent efficiency, the level needed for commercial adoption.  They are also examining which cathode materials would work best in combination with their new electrolyte.