The iPhone Air’s thin design might be eye catching, and the amount of engineering that went into miniaturizing its logic board is impressive. But according to Gene Berdichevsky, co-founder and CEO of battery materials manufacturer Sila, the real breakthrough might lurk elsewhere inside the aluminum and glass enclosure.

“The battery in the new iPhone is pretty remarkable,” Berdichevsky told TechCrunch. “The completely arbitrary, two-dimensional shape — you look at the shape, and it’s pretty amazing.”

“I just got back from Asia and, and I got a chance to see some of these cells,” he added. “It’s a revolutionary piece of battery tech.”

Berdichevsky knows a thing or two about batteries. As Tesla’s seventh employee, he led engineering on the original Roadster’s battery, which became the template for subsequent Teslas. Today, he leads Sila, which is producing silicon anode materials for consumer electronics and, soon, electric vehicles.

The iPhone Air’s notched design is made possible by a technology Apple has patented called a metal can battery. The key detail is in the name: a metal casing that surrounds the entire cell, adding strength and physical durability. Most batteries used in consumer electronics are pouch cells, which have a soft plastic casing that’s cheap to manufacture and allows for some degree of swelling.

Apple has used L-shaped batteries in iPhones for years. All lithium ion batteries swell to some degree, and the interior corner of the L becomes a pinch points when that swelling occurs.

“Those are very finicky, and this basically makes it bulletproof. You can now build batteries in any two-dimensional shape you want,” Berdichevsky said.

Metal can batteries allow Apple to make the most of the small space within the iPhone Air. “They’re able get very close to the edges,” he said. It allows the battery to worm its way into whatever space is free after the various circuit boards have been positioned.

Ultimately, Berdichevsky thinks that most phones will adopt metal can batteries despite their added expense. The extra energy storage will be worth it.

It will also be “very key” for smaller devices like AR and VR glasses, he said, adding that he saw a few prototypes while he was in China. “It’s even more of an energy density improvement because it lets you fit into weird shapes,” Berdichevsky said.

The switch to such a complex battery is likely why Apple hasn’t swapped carbon anodes in its lithium ion batteries for silicon-heavy versions, also known as silicon-carbon. 

“If you’re bringing [a new battery design] online, you kind of go, ‘You know what? Let’s use yesterday’s chemistry,’” Berdichevsky said.

But the transition to metal can construction could facilitate the move to silicon anodes in the near future. Pure silicon anodes can store around 50% more energy than traditional graphite anodes, but the material is prone to swelling. Companies like Sila have developed proprietary ways to manage that swelling within the material, but it still needs to be accounted for at the cell level.

“It definitely will help introduce silicon in these kind of devices,” Berdichevsky said. “It lets us push the performance limits more. We’ve always had these trade offs, and we have to manage the swell. You still have to do that, but you can push it a little bit more. It’s pretty revolutionary.”