Power/Performance Bits: Jan. 23

Atomristors; lithium-sulfur batteries; imperfect perovskites.


Atomristors for thin memory
Engineers at The University of Texas at Austin and Peking University developed a thin memory storage device with dense memory capacity. Dubbed “atomristors,” the device enables 3-D integration of nanoscale memory with nanoscale transistors on the same chip.

“For a long time, the consensus was that it wasn’t possible to make memory devices from materials that were only one atomic layer thick,” said Deji Akinwande, associate professor in the Department of Electrical and Computer Engineering at UT Austin. “With our new ‘atomristors,’ we have shown it is indeed possible.”

By using metallic atomic sheets (graphene) as electrodes and semiconducting atomic sheets (molybdenum sulfide) as the active layer, the entire memory cell is a sandwich about 1.5 nanometers thick, which makes it possible to densely pack atomristors layer by layer in a plane. This is a substantial advantage over conventional flash memory, which occupies far larger space. In addition, the thinness allows for faster and more efficient electric current flow.

Illustration of a voltage-induced memory effect in monolayer nanomaterials, which layer to create “atomristors.” (Source: Cockrell School of Engineering, UT Austin)

Akinwande sees the technology potentially enabling a memory architecture with 3-D connections akin to those found in the human brain. “The sheer density of memory storage that can be made possible by layering these synthetic atomic sheets onto each other, coupled with integrated transistor design, means we can potentially make computers that learn and remember the same way our brains do,” Akinwande said.

The team also discovered another application for the technology. In devices such as smartphones and tablets, radio frequency switches are used to connect incoming signals from the antenna to one of the many wireless communication bands in order for different parts of a device to communicate with one another.

The atomristors are the smallest radio frequency memory switches to be demonstrated with no DC battery consumption, which can ultimately lead to longer battery life.

“Overall, we feel that this discovery has real commercialization value as it won’t disrupt existing technologies,” Akinwande said. “Rather, it has been designed to complement and integrate with the silicon chips already in use in modern tech devices.”

Improving lithium-sulfur batteries
Researchers at Lawrence Berkeley National Laboratory, Argonne National Laboratory, MIT, and UC Berkeley developed a new lithium-sulfur battery component that allows a doubling in capacity compared to a conventional lithium-sulfur battery, even after more than 100 charge cycles at high current densities.

Lithium-sulfur batteries are cheaper and weigh less than typical lithium-ion batteries, but become unstable over time as their electrodes deteriorate. To improve this, the team developed a new polymer binder that actively regulates key ion transport processes within a lithium-sulfur battery.

“The new polymer acts as a wall,” said Brett Helms, a staff scientist at Berkeley Lab’s Molecular Foundry. “The sulfur is loaded into the pores of a carbon host, which are then sealed by our polymer. As sulfur participates in the battery’s chemical reactions, the polymer prevents the negatively charged sulfur compounds from wandering out. The battery has great promise for enabling the next generation of EVs.”

When a lithium-sulfur battery stores and releases energy, the chemical reaction produces mobile molecules of sulfur that become disconnected from the electrode, causing it to degrade and ultimately lowering the battery’s capacity over time.

Research on making lithium-sulfur batteries more stable has focused on protective coatings for their electrodes, and developing new polymer binders that act as the glue holding battery components together. These binders are intended to control or mitigate the electrode’s swelling and cracking.

This illustration shows the formation of complex ion clusters during the cycling of a lithium-sulfur battery cell. The clusters consist of cationic polymer binders, battery electrolyte, and anionic sulfur-active materials. (Source: Berkeley Lab)

The new binder keeps the sulfur in close proximity to the electrode by selectively binding the sulfur molecules, counteracting its migratory tendencies.

Molecular simulation confirmed that the polymer has an affinity for binding the mobile sulfur molecules, and also predicted that the polymer would likely show a preference for binding different sulfur species at different states of charge for the battery.

The team also examined the performance of lithium-sulfur cells made with the new polymer binder. Through a set of experiments, they were able to analyze and quantify how the polymer affects the chemical reaction rate in the sulfur cathode, which is key to achieving high current density and high power with these cells.

The team found the polymer nearly doubled the battery’s electrical capacity over long-term cycling.

Imperfect perovskites
Scientists at Helmholtz-Zentrum Berlin and the University of Oxford explained an unusual characteristic of perovskite solar cells:  the numerous holes created during the manufacturing process don’t lead to significant short circuits between the front and back contact.

Metal-organic perovskite layers for solar cells are frequently fabricated through spin coating and subsequently baking, where the solvent evaporates and the material crystallizes. The thin perovskite film that results from spin coating on compact substrates is generally not perfect, but instead exhibits many holes, which could lead to short circuits in the solar cell by the adjacent layers of the solar cell coming into contact. This should reduce the efficiency level considerably, but efficiencies now exceed 22%.

Simplified cross-section of a perovskite solar cell: the perovskite layer does not cover the entire surface, but instead exhibits holes. The scientists could show that a protective layer is being built up which prevents short circuits. (Source: HZB)

Using scanning electron microscopy, the team mapped the perovskite’s surface morphology. They subsequently analyzed the sample areas exhibiting holes for their chemical composition using spectromicrographic methods at BESSY II.

“We were able to show that the substrate was not really exposed even in the holes, but instead a thin layer is being built up essentially as a result of the deposition and crystallization processes there that apparently prevents short circuits,” said doctoral student Claudia Hartmann.

They also found that the energy barrier the charge carriers had to overcome in order to recombine with one another in the event of a direct encounter of the contact layers is relatively high.

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