Historically, making a thin phone meant trading the usual battery for one that’s slimmer and less robust. And what good is a thin phone with poor battery life? Up-and-coming silicon-carbon battery technology may be the answer.

The technology has been creeping into view. It has been in research for decades, and it’s already in use with notable products. Whoop fitness trackers have had silicon-carbon batteries since 2021. Chinese phone brands Xiaomi and Honor have more recently used the tech in their phones; OnePlus employs it in the OnePlus 13, and even Nothing embraced it in its new Phone (3). So far, the technology’s application has been split by form factor, with folding phones enjoying thinner designs and candy bar phones earning bigger battery capacities

Apple is expected to debut the iPhone 17 Air this week, and it’s rumored to be a mere 5.5 millimeters thick, making it the slimmest iPhone ever. The company may be using a silicon-carbon battery to ensure it can get close to or match the battery life you’d expect from a modern-day iPhone.

Thin Skin

Most phones are somewhere between 8 to 9 millimeters thick. The Samsung Galaxy S25 Ultra is 8.2 mm, the iPhone 16 Pro Max is 8.25 mm, and the Google Pixel 10 Pro XL is 8.5 mm. But phone makers have been shedding millimeters like it’s nobody’s business, and 2025 has seen a blossoming of super-slim devices.

This past spring, Samsung introduced the Galaxy S25 Edge, merely 5.8 mm. The even more recent Galaxy Z Fold7 is impressively 4.2 mm when unfolded, but Huawei’s tri-folding phone, the Mate XT Ultimate, beats it at 3.6 mm unfolded. Smaller brands like Tecno are getting in on the action, teasing a super-thin phone at MWC 2025, and subsequently announcing a 5.93-mm handset.

It’s not the first time that phone makers have chased thinness as a feature. The early 2010s saw a similar race from the same players; at Apple’s 2012 September event, the first words about the iPhone 5 were that it was “the thinnest phone we have ever made,” and at 7.6 mm thick, Apple went so far as to call it the world’s thinnest smartphone at the time. We’ve even seen thinner since: the Oppo R5 from 2014 was 4.85 mm.

With the new thinness wars, only a select few of these companies are utilizing silicon-carbon batteries so far, like Huawei and Honor. Samsung seems to be more cautious, opting to use a standard lithium-ion battery and finding ways to make its display more power-efficient to make up for the smaller battery capacity. (Given the company’s history with batteries, it’s understandable.) But these tweaks weren’t enough—the Edge’s battery life is still lackluster.

So what exactly is a silicon-carbon battery, and how could it impact the iPhone 17 Air?

What Is a Silicon-Carbon Battery?

In a lithium-ion battery, lithium ions travel from the anode to the cathode as your phone discharges during use. When you charge your device, those ions travel from the cathode back to the anode, and rinse and repeat. The anode is usually made of graphite.

A “silicon-carbon battery” is actually a misnomer. These batteries are still lithium-ion batteries; however, the graphite anode has been replaced with a silicon-carbon anode. Silicon can store approximately 10 times the number of lithium ions by weight than graphite, according to Rick Luebbe. He’s the CEO of Group14, one of the top companies in the silicon battery space. Other players include Enovix, Sila Nanotechnologies, and Nexeon.

The graphite anode takes up a large amount of space in a typical lithium-ion battery—around 60 percent, Luebbe says, depending on the battery design. A silicon anode takes up less space in the battery, resulting in the ability for a higher energy density by expanding the cathode.

It’s worth noting that a company like Group14 or Sila isn’t actually making the battery; these silicon anodes are what is known as a drop-in product. “We are powder makers—magic black powder,” Luebbe says. The powdered silicon-carbon anode is sent to battery manufacturers like ATL, which produces batteries for smartphones and laptops. A company like ATL can simply replace its graphite powder with the silicon-carbon mixture without disruption.

A phone maker can take one of two approaches. You can make your smartphone roughly the same thickness as before, but now increase the energy capacity of the battery, like the OnePlus 13. That could ensure longer battery run time. Or, since the battery doesn’t need to be as large anymore, you can keep roughly the same energy capacity as prior models and take advantage of the space savings to slim the phone down.

Manufacturers like Honor, OnePlus, and Nothing use silicon-carbon in their latest candy bar phones. These devices maintain their standard thickness for the most part and feature increased battery capacities. For example, the OnePlus 13 has a bigger 6,000-mAh battery and is thinner than its predecessor, but its 8.5 mm thickness is on par with most traditional phones.

Apple has taken the second approach with caveats. Rumors suggest the iPhone 17 Air’s battery capacity will sit around 2,900 mAh, a steep drop from prior iPhone models, especially at the 6.6-inch screen size. But the company is supposedly making up for it with power-saving tricks to make sure battery life remains similar to other iPhones, including Apple’s more efficient C1 modem that debuted on the iPhone 16e earlier this year.

Luebbe declined to comment on whether Group14’s silicon-carbon composite is being used in the iPhone 17 Air’s batteries; Sila Nanotechnologies and Enovix did not immediately respond to a request for comment.

What’s the Catch?

The problem with silicon batteries is that they expand. When you lithiate raw silicon, Luebbe says, it can expand up to three times its initial volume. Lithium-ion batteries also swell; you’ve probably heard of or maybe even experienced this, as it can happen for a myriad of reasons. It means something has gone wrong, and the battery is now a safety risk.

It’s this problem that researchers and companies have spent decades trying to solve, and the solution lies in the carbon part of the name. It starts to get a bit technical here—and each silicon anode company has its own proprietary process—but Luebbe says Group14’s approach is to start with a porous carbon material.

“Imagine a carbon sponge, but the pores of that sponge are on the single-digit molecule wide. We’re talking less than 10 nanometers wide,” he says. These pores are filled with silane gas (the silicon), but only about halfway. The particle you’re left with is made up of silicon, carbon, and void space. When the lithium ions head over from the cathode to the anode and the silicon lithiates, it expands to fill the void spaces of the particle.

“It mitigates the expansion at the particle level, so the battery doesn’t see the expansion, so it stabilizes the battery, and you get excellent cycle life,” Luebbe says. “That’s the critical insight in the invention: really learning how to internalize that expansion, so that it’s insulated from the battery chemistry and mechanical operations.”

Vincent Chevrier has been a researcher in the silicon field for 15 years and is a partner at battery consulting firm Cyclikal. He says while silicon is here to stay as a material to be used in lithium-ion batteries, there are still a few challenges for broader adoption, including cost.

Companies like Group14 use silane gas instead of solid silicon, which yields better battery performance, but could be 10 times the cost. That could make it harder to sell their composite to battery makers, and it could drive up the prices of consumer electronics. The iPhone 17 Air is rumored to cost around $1,099, a potential $200 bump from the iPhone 16 Plus it’s expected to replace, though there could be other factors affecting its price, like tariffs.

Chevrier also says he often sees silicon-carbon makers inflate the energy density claims. Group14, for example, says on its website that its silicon batteries can deliver up to 50 percent more energy density than conventional lithium-ion batteries. But if the material is just dropped in to replace graphite and not much else is changed with the battery, you’re more likely going to see a 10 percent boost in energy density with a switch to a silicon-carbon anode. Redesign the battery cell, and then it’d be possible to see an increase of up to 30 percent.

Silicon-carbon batteries also have worse cycle life than graphite. That’s how many full charge-to-discharge cycles (0 to 100 percent) a battery goes through before its capacity degrades below 80 percent. In terms of the energy efficiency of silicon—how much of the energy put in the battery ends up as heat and how much is stored as energy—Chevrier says “more energy is lost to heat than graphite.”

Graphite could have the potential to achieve 5,000 charge cycles, whereas Group14’s composite sits at 1,000 cycles, or about three years, depending on how often you fully charge the phone. However, Chevrier points out that current batteries with graphite anodes do not hit 5,000 charge cycles, because companies like Apple cram higher energy density, which stresses the battery but keeps the phone operating throughout the day. That’s why the iPhone’s battery doesn’t last as long as it did after two years of use. iPhone 16 batteries can retain 80 percent of their capacity at 1,000 charge cycles. Switching to silicon-carbon may not necessarily make much of a difference in the lifecycle of the battery, despite claims otherwise.

So It Goes

The crucial thing to remember is that despite the advances in batteries over the years, including silicon-carbon anodes, new technologies arrive at the same time to claim a slice of that excess power. The growing list of artificial intelligence features running locally on phones is increasing, and they’ll happily sip up more power if they can get it. That means you may not enjoy longer battery life even if a device maker stuffs in a silicon-carbon anode and expands battery capacity.

In the case of the iPhone 17 Air, Apple is taking the space-saving advantage of silicon-carbon anodes and experimenting with a thin and lightweight design to see if consumers will bite, without resorting to a smaller screen. It could also prove to be a testing ground for a future folding iPhone—Apple would ensure it won’t be dramatically thicker than a traditional iPhone.

Whether or not the company will succeed in delivering just the right amount of battery life in this design will have to wait until we can finally take the Air out for a spin.