Power/Performance Bits: May 5

Single material batteries

Engineers at the University of Maryland created a battery made entirely out of a single material that, by incorporating the properties of both the electrodes and electrolyte, can both move electricity and store it.

The reason the new battery is revolutionary is because it solves the problem of what happens at the interface between the electrolyte and the electrode. A prolonged interaction between the two can result in a wall of useless material that keeps the batteries from working well. The wall increases the resistance at this solid-electrolyte interface. This in turn increases the heat in the battery, rendering the battery even less useful.

“Our battery is 600 microns thick, about the size of a dime, whereas conventional solid state batteries are thin films — forty times thinner. This means that more energy can be stored in our battery,” said Fudong Han, the first author of the paper and a graduate student at UMD.

UMD Engineers made a battery of all one material simply by sprinkling carbon (red) into each side of a new material (blue) that forms the electrolyte and both electrodes at the ends of the battery. (Source: Maryland NanoCenter)

The new material consists of a mix of sulfur, germanium, phosphorus and lithium. This compound is used as the ion-moving electrolyte. At each end, the scientists added carbon to this electrolyte to form electrodes that push the ions back and forth through the electrolyte as the battery charges and discharges.

Though the battery is extremely easy to make – a powder compressed in a plastic and steel cylinder – it is still at the proof-of-concept stage, Han said. “We are still testing how many times it can change and discharge electricity to see if it is a real candidate for manufacturing.”

Sulfide-based compounds are not particularly environmentally friendly materials, Han said. “So next we will try to use oxides, which do not degrade into a poisonous gas, instead.” The battery’s solid powder is, however, safer than the current liquid-based batteries.

Seeing perovskite’s flaws

A new study from the University of Washington and Oxford University suggests that perovskite materials, generally believed to be uniform in composition, actually contain flaws that can be engineered to further improve the material’s prospects in solar devices.

The research team used a technique called confocal optical microscopy. Often used in biology, the team applied it to semiconductor technology to find defects in the perovskite films that limit the movement of charges and limit the efficiency of the devices. Perovskite solar cells have so far have achieved efficiencies of roughly 20%, compared to about 25% for silicon-based solar cells.

By correlating fluorescent images with electron microscopy images, the team identified ‘dark’ or poorly performing regions of the perovskite material at intersections of the crystals. This surprising result held true even for samples corresponding to the state-of-the-art solar cell efficiencies. In addition, they discovered that they could ‘turn on’ some of the dark areas by using a simple chemical treatment.

The quality of the perovskite materials for electronic device applications improved after chemical treatment, remediating the “dark” areas. (Source: University of Washington)

“Surprisingly, this result shows that even what are being called good, or highly-efficient perovskite films today still are ‘bad’ compared to what they could be. This provides a clear target for future researchers seeking to improve and grow the materials,” said David Ginger, professor of chemistry at UW.

The imaging technique also offers an easy way to identify previously undiscovered flaws in perovskite materials and to pinpoint areas where their composition can be chemically altered to boost performance.

Dr Sam Stranks, now based at MIT, said: “We’ve seen device efficiencies increase by roughly 25% with this chemical treatment and the challenge is now to find other treatments and tricks to further improve these materials. The end goal would be to make the entire film uniformly ‘bright’ enabling us to have a solar cell operating at the thermodynamic efficiency limits.”

Overheating lithium-ion X-rayed

What happens when lithium-ion batteries overheat and explode has been tracked inside and out for the first time. Using high energy synchrotron X-rays and thermal imaging, a team led by researchers from University College London got a look at how internal structural damage to batteries evolves in real-time, and an indication of how this can spread to neighboring batteries.

The team looked at the effects of gas pockets forming, venting and increasing temperatures on the layers inside two distinct commercial Li-ion batteries as they exposed the battery shells to temperatures in excess of 250 degrees C.

The battery with an internal support remained largely intact up until the initiation of thermal runaway, at which point the copper material inside the cell melted indicating temperatures up to ~1000 degrees C. This heat spread from the inside to the outside of the battery causing thermal runaway.

In contrast, the battery without an internal support exploded, causing the entire cap of the battery to detach and its contents to eject. Prior to thermal runaway, the tightly packed core collapsed, increasing the risk of severe internal short circuits and damage to neighboring objects.

Battery showing thermal runaway during thermal abuse tests. (Source: Donal Finegan/UCL)

Dr Paul Shearing of UCL said: “Although we only studied two commercial batteries, our results show how useful our method is in tracking battery damage in 3D and in real-time. The destruction we saw is very unlikely to happen under normal conditions as we pushed the batteries a long way to make them fail by exposing them to conditions well outside the recommended safe operating window. This was crucial for us to better understand how battery failure initiates and spreads. Hopefully from using our method, the design of safety features of batteries can be evaluated and improved.”