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BATTERY TECHNOLOGY Innovative batteries could enable eVTOL flying cars

From Luke James

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Penn State researchers have found a way to improve lithium-ion batteries for faster charging, higher energy density, and longer life cycles with the hope of preparing for next-gen technologies like eVTOL flying cars and robotic assistants.

In a LiB, lithium ions flow from the cathode to the anode, resulting in a positive energy charge.
In a LiB, lithium ions flow from the cathode to the anode, resulting in a positive energy charge.
(Source: Chao-Yang Wang Lab, Penn State)

"I think flying cars have the potential to eliminate a lot of time and increase productivity and open the sky corridors to transportation," said Chao-Yang Wang of a Penn State research team that reckons it has found a way to improve lithium-ion batteries (LiBs) to such a dramatic extent that may be key to unlocking flying cars in the not-too-distant future.

LiBs are used in many modern applications, from laptops to electric vehicles (EVs) due to their useful properties like high energy density. At the same time, there are several issues plaguing LiBs—general safety, discharge issues, limited life cycles just to name a few—that are inhibiting next-gen applications. “…electric vertical takeoff and landing vehicles (eVTOLs) are very challenging for the [lithium-ion] batteries,” says Wang.
It is these and other practical limitations that have propelled a flurry of research into ways to make LiBs more capable of supporting next-gen applications like eVTOLs.

Improving charge speed

For large-scale next-gen applications like eVTOLs (and, indeed, regular EVs) batteries must have a high energy density and fast charging rate. To address energy density, engineers must turn to electrode engineering, and research efforts the world over have identified many advanced metals that, in theory, significantly increase density. To address charging rate, engineers must delve into the LiB charging process.

In LiBs, lithium ions move from the negative electrode (the cathode) to the positive electrode (the anode) through an electrolyte during discharging and in reverse during charging. During the charging process, lithium ions move through the battery’s electrolyte and the mobility of lithium ions through the electrolyte solution dictate how quickly a battery can charge.

Now, researchers at Penn State University have reportedly found a way to effectively improve the charging speed and energy density of LiBs while exploring the technical requirements for eVTOLs in tandem.

"Batteries for flying cars need very high energy density so that you can stay in the air," says Wang. "And they also need very high power during take-off and landing. It requires a lot of power to go vertically up and down." Weight is also an obvious consideration; the vehicles will have to be able to lift the batteries up and sacrifice as little power as possible to do so.

Heat is a critical aspect

The researchers say that heat is a critical to allow the battery to charge quickly without the formation of dendrites—‘spike’ structures that can form and effectively kill the battery. They also reportedly found that heating up the battery also enabled a rapid discharge of energy. With this approach, the Penn State researchers presented two energy-dense LiBs that can recharge with enough energy for a 50-mile eTOL trip in around 10 minutes. The batteries demonstrated by the researchers can also handle more than 2,000 fast charges before noticeable degradation comes into play.

This is notable because, usually, high energy density slows down the battery charging process. When a battery is empty, internal resistance is low, but as the battery charge increases it becomes more difficult to get more energy into the battery. In addition, fast charging can reduce battery life because layers of solid electrolyte interphase (SEI) form in the electrode due to the high currents. This consumes lithium ions and reduces electrode charge/discharge efficiency.

The researchers say that they were able to achieve fast charging, high energy density, and longer life cycle in a single battery simply by heating it up to 140 °F with a nickel foil. This is because at higher temperatures, li-ion mobility increases without having an impact on charge/discharge efficiency.

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