Power/Performance Bits: July 15

Liquefied gas electrolyte; inductive charging temperature; capturing solar heat.


Liquefied gas electrolyte
Researchers at UC San Diego, U.S. Army Research Laboratory, and South 8 Technologies developed an electrolyte that could enable the replacement of the graphite anode in lithium-ion batteries with lithium-metal. Such a change would increase energy density 50% at the cell level, making for lighter batteries with more capacity. However, lithium-metal anodes are not compatible with conventional electrolytes used in lithium-ion batteries, often resulting in low cycling efficiency and dendrite growth.

Instead, the team has been working on a liquefied gas electrolyte. Plus, as they are made from gases liquefied under moderate pressures, they are more resistant to freezing than standard liquid electrolytes, allowing them to function both at room temperature and extremely low temperatures down to minus 60 C.

In tests with lithium-metal half-cells, the anode’s cycling efficiency was 99.6% for 500 charge cycles at room temperature, an improvement over the team’s prior efforts (97.5% in 2017) as well as for lithium metal anodes with a conventional liquid electrolyte, which is 85%.

At minus 60 C, the lithium-metal anode cycling efficiency was 98.4%; most conventional electrolytes fail to work below minus 20 C.

Additionally, the liquefied gas electrolyte promotes smooth, compact deposition of lithium particles on the lithium-metal anodes, lowering the risk of dendrite formation, which can result in lower efficiency, short circuits, or fires.

The university spinout South 8 Technologies is commercializing the technology.

Inductive charging temperature
Researchers at the University of Warwick warn that inductive charging of mobile devices generates more heat that wired charging, shortening the lifespan of the device’s battery.

While convenient, inductive charging systems generate heat both in the device and charger itself. Plus, the close contact means heat transfers between the two, and a smartphone with an inductive coil has little means to shield the phone and its battery from heat generated by the charger.

Lithium-ion batteries age more quickly under high temperatures, which accelerate the growth of passivating films on the cell’s electrodes and make the surface underneath unreactive. A lithium ion battery dwelling above 30 C is typically considered to be at elevated temperature, exposing the battery to risk of a shortened useful life.

Additionally, when a phone is misaligned on an inductive charging base, the charging system typically increases the transmitter power or adjusts operating frequency to compensate, with the side effect of increasing heat generation.

To test whether inductive charging was generating battery-harmful heat, the researchers tested wired, inductive, and misaligned inductive with simultaneous charging and thermal imaging over time to generate temperature maps. While the team used an iPhone 8 Plus in their tests, they said that the issues raised apply to all phones or devices using inductive charging.

The three modes of charging, based on (a) AC mains charging (cable charging) and inductive charging when coils are (b) aligned and (c) misaligned. (Credit: WMG, University of Warwick)

When charged with conventional mains power, the maximum average temperature reached within 3 hours of charging did not exceed 27 °C. When the phone was properly aligned for inductive charging, the temperature peaked at 30.5 °C but gradually reduced for the latter half of the charging period.

When the phone was misaligned, the temperature also peaked at 30.5 °C, but it reached that temperature sooner and stayed hot longer, 125 minutes versus 55 minutes when properly aligned. Plus, the maximum input power to the charging base was greater in the test where the phone was misaligned (11W) than the well-aligned phone (9.5 W), and the maximum average temperature of the charging base while charging under misalignment reached 35.3 °C, two degrees higher than the temperature detected when the phone was aligned, which reached 33 °C.

The researchers note that improvements to inductive charging design, such as using ultrathin coils, higher frequencies, and optimized drive electronics, could be more efficient and reduce heating.

Capturing solar heat
Researchers at MIT created a transparent aerogel that could be used to capture much more of the sun’s heat for high-temperature industrial processes or home heating. The aerogel uses a silica structure that allows light to pass through while preventing solar heat from escaping.

Typically, solar heat collectors use a vacuum between a layer of glass and dark solar-absorbing material; while frequently used in concentrating solar collectors, it’s relatively expensive to install and maintain.

While aerogels themselves are not new, they typically have only around 70% transparency. The new silica-based aerogel lets in 95% of incoming sunlight while remaining highly insulating. To make it, the team mixed a catalyst with a silicon-containing compound in a liquid solution to form a wet gel, then dried it to get all the liquid out. This left a matrix that is mostly air but retained the original mixture’s structure.

In tests on a rooftop on the MIT campus, a passive device consisting of a solar-absorbing dark material covered with a layer of the new aerogel was able to reach and maintain a temperature of 220 C (392 F), in the middle of winter when the outside air was below 0 C. Such high temperatures were previously only generated with concentrating systems, which use mirrors to focus sunlight on the collector. Rooftop collectors used for home hot water systems usually produce temperatures around 80 C.

The aerogel works “like a greenhouse effect. The material we use to increase the temperature acts like the Earth’s atmosphere does to provide insulation, but this is an extreme example of it,” said Lin Zhao, an MIT graduate student.

The aerogel collectors could directly replace vacuum-based collectors, as both work by heating circulating liquid or by using heat pipes. The materials used to make the aerogel are all abundant and inexpensive; the only costly part of the process is the drying, which requires a critical point dryer, a specialized device that allows for a very precise drying process to extracts the solvents from the gel while preserving its nanoscale structure.

Given that making the aerogel is a batch process, rather than roll-to-roll, the production rate could be limited if scaled up. “The key to scaleup is how we can reduce the cost of that process,” said Evelyn Wang, professor and head of the Department of Mechanical Engineering at MIT. Still, the team sees an economic advantage to this system for some uses, particularly when compared to vacuum-based systems.

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