Power/Performance Bits: March 21

Tiny redox flow batteries for chips

Researchers at ETH Zurich and IBM Research Zurich built a tiny redox flow battery capable of both powering and cooling stacks of chips.

In a flow battery, an electrochemical reaction is used to produce electricity out of two liquid electrolytes, which are pumped to the battery cell from outside via a closed electrolyte loop. Such batteries are usually used in large scale stationary energy storage applications.

“The chips are effectively operated with a liquid fuel and produce their own electricity,” said Dimos Poulikakos, professor of thermodynamics at ETH Zurich. Since the scientists use two liquids that are known to be suitable both as flow-battery electrolytes and as a medium to also effect cooling, excess heat can also be dissipated from the chip stack via the same circuit.

The battery built by the scientists is around 1.5 millimeters thick. The idea would be to assemble chip stacks layer by layer: a chip, then a thin battery micro-cell that supplies the chip with electricity and cools it, followed by the next chip and so on.

The output of the new micro-battery reaches a record-high in terms of its size: 1.4 watts per square centimeter of battery surface. Subtracting the power required to pump the liquid electrolytes to the battery, the resulting net power density is 1 watt per square centimeter.

Channel networks ensure that the liquid electrolytes fully penetrate the porous electrodes and react electrochemically. (Source: Marschewski et al. Energy and Environmental Science 2017, adapted)

The research also showed that the electrolyte liquids are capable of cooling a chip, and are able to dissipate heat amounts many times over what the battery generates as electrical energy.

Although the power density of the new micro-flow battery is very high, the electricity produced is still not entirely sufficient to operate a computer chip. In order for the flow battery to be used in a chip stack, it must be further optimized.

As the scientists point out, the new approach is also interesting for other applications: in lasers, for example, which have to be supplied with energy and cooled; or for solar cells, where the electricity produced could be stored directly in the battery cell and used later when needed. The system could also keep the operating temperature of the solar cell at the ideal level.

Storing data on DNA

Researchers at Columbia University and the New York Genome Center encoded 2 megabytes of data on DNA using a more efficient error-correcting reading and writing process that resulted in error-free retrieval.

The team chose six files to encode into DNA: the Kolibri operating system, an 1895 French film, “Arrival of a train at La Ciotat,” a $50 Amazon gift card, a computer virus, the Pioneer plaque and a 1948 study by information theorist Claude Shannon.

They compressed the files into a master file, and then split the data into short strings of binary code made up of ones and zeros. Using an erasure-correcting algorithm called fountain codes, they randomly packaged the strings into so-called droplets, and mapped the ones and zeros in each droplet to the four nucleotide bases in DNA: A, G, C and T. The algorithm deleted letter combinations known to create errors, and added a barcode to each droplet to help reassemble the files later.

In all, they generated a digital list of 72,000 DNA strands, each 200 bases long, and sent it in a text file to a San Francisco DNA-synthesis startup, Twist Bioscience, that specializes in turning digital data into biological data. Two weeks later, they received a vial holding a speck of DNA molecules.

To retrieve their files, they used modern sequencing technology to read the DNA strands, followed by software to translate the genetic code back into binary. They recovered their files with zero errors, the study reports.

They also demonstrated that a virtually unlimited number of copies of the files could be created with their coding technique by multiplying their DNA sample through polymerase chain reaction (PCR), and that those copies, and even copies of their copies, and so on, could be recovered error-free.

A single gram of DNA is capable of holding 215 petabytes of data using the team’s coding strategy. The capacity of DNA data-storage is theoretically limited to two binary digits for each nucleotide, but the biological constraints of DNA itself and the need to include redundant information to reassemble and read the fragments later reduces its capacity to 1.8 binary digits per nucleotide base.

The team applied fountain codes to make the reading and writing process more efficient. With their DNA Fountain technique, the team was able to encode an average of 1.6 bits into each base nucleotide – at least 60% more data than previously published methods, and close to the 1.8-bit limit.

Cost still remains a barrier. The researchers spent $7,000 to synthesize the DNA they used to archive their 2 megabytes of data, and another $2,000 to read it. Though the price of DNA sequencing has fallen exponentially, there may not be the same demand for DNA synthesis, says Sri Kosuri, a biochemistry professor at UCLA who was not involved in the study. “Investors may not be willing to risk tons of money to bring costs down,” he said.

But the price of DNA synthesis can be vastly reduced if lower-quality molecules are produced, and coding strategies like DNA Fountain are used to fix molecular errors, according to Yaniv Erlich, a computer science professor at Columbia. “We can do more of the heavy lifting on the computer to take the burden off time-intensive molecular coding,” he said.

Glass electrolyte

Researchers at the University of Texas at Austin developed an all-solid-state battery that is both low-cost and noncombustible and has a long cycle life with a high volumetric energy density and fast rates of charge and discharge.

The team, led by John Goodenough, co-inventor of the lithium-ion battery, demonstrated that their new battery cells have at least three times as much energy density as today’s lithium-ion batteries. The battery formulation also allows for a greater number of charging and discharging cycles, which equates to longer-lasting batteries, as well as a faster rate of recharge (minutes rather than hours).

Instead of the fire-prone liquid electrolytes in lithium-ion batteries, the researchers rely on glass electrolytes that enable the use of an alkali-metal anode without the formation of dendrites, the “metal whiskers” that can form a bridge across liquid electrolytes and lead to a short circuit. The use of an alkali-metal anode (lithium, sodium or potassium) also increases the energy density of a cathode and delivers a long cycle life. In experiments, the researchers’ cells have demonstrated more than 1,200 cycles with low cell resistance.

Additionally, because the solid-glass electrolytes can operate, or have high conductivity, at -20 degrees Celsius, this type of battery in a car could perform well in subzero degree weather. According to the team, this is the first all-solid-state battery cell that can operate under 60 degree Celsius.

Another advantage is that the battery cells can be made from earth-friendly materials. “The glass electrolytes allow for the substitution of low-cost sodium for lithium. Sodium is extracted from seawater that is widely available,” said Maria Helena Braga, a senior research fellow at UT Austin.

The researchers are working on several patents. In the short term, they hope to work with battery makers to develop and test their new materials in electric vehicles and energy storage devices.