What is a silicon carbon battery and is it the answer to smartphone battery life issues?

Igor Bonifacic for Engadget

In the last few years, you may have noticed an increasing number of smartphones shipping with silicon carbon batteries. These are increasingly common after major Chinese manufacturers like Honor, Oppo and Xiaomi embraced the technology. Now, Samsung has joined in, using silicon-carbon technology in the batteries of the Galaxy Z Fold 8 and 8 Ultra. 

Silicon carbon batteries can be far more capacious than traditional lithium ion, with some phones using the tech boasting of true two-day battery life. It's more than likely that, as these cells become the standard in Asian phones, we'll see more and more of them in Western handsets. So let's find out what's so special about silicon carbon batteries, why they offer dramatically bigger battery life, and why it might come at the cost of your phone's lifespan.

Batteries are essentially made up of three parts: anode, cathode and the electrolyte which sits between them. When a battery is fully charged, all of the electrons are held in the anode, which then slowly releases them to the cathode. The flow of electrons through the electrolyte causes a series of electrical and chemical reactions that release energy. In the cells commonly found in your phone, you can add more electrons to the anode many times over. But the quantity of power you can store degrades over time as the electrolyte runs out of the materials necessary to react with.

Think about a battery as a water slide at an aqua park on a very hot day. There are hundreds of screaming kids (electrons) all lined up at the top (anode) and they all want to get down to the bottom (cathode) as quickly as possible. The slide has water flowing down it (electrolyte) which facilitates this movement of kids from the top to the bottom. Naturally, those kids will then race back up to the top for another go (recharging), and will continue to do so time and again. But, eventually, the aqua park's water reserves run out, and when it gets too dry to slide upon, it can no longer be used.

It's likely you're reading this article on a device equipped with a lithium ion battery, which have become ubiquitous in consumer electronics over the last two and a half decades. They are so popular because, compared to other options, they're light, energy dense and can be charged and discharged many times over before they start to degrade. That's not to say they're perfect; they require a lot of effort to become as reliably useful as they are today for use in our phones, tablets, laptops, EVs and homes. In general, a lithium ion battery is made up of a graphite powder anode, a lithium ion phosphate powder cathode and an ethylene carbonate electrolyte.

Dr. Ruth Sayers is an expert in battery chemistry who had stints at Imperial College and the University of Liverpool, and recently became CEO of AmpliSI, a company making silicon for use inside silicon carbon batteries. She explained that the world has spent much of the last two decades refining the cathode and electrolyte inside a lithium ion cell. "Everything else has been optimized and developed as much as it can," she said, "we've been using the same graphite anode in lithium ion batteries for decades." At this point, those graphite anodes are the "element that is limiting the battery performance," she said, adding "silicon is considered the challenger to graphite."

Calling these cells silicon carbon batteries is a bit of a misnomer, given it'll still use a lithium based material for the cathode. In fact, it won't even use silicon carbon for all of the anode, because right now the physics won't allow for it. Instead, it'll just use a small percentage of silicon carbon, which is mixed in with the graphite you currently find in these cells. 

This is basically what Samsung and other phone makers are currently doing. In fact, even in the product page for the Z Fold 8 and Fold 8 Ultra, Samsung still lists the battery type as "Lithium-ion." The company did explain that it introduced silicon-carbon material to improve energy density, noting that the material offered clear advantages in that regard. And according to the definition everyone else in the industry is using, that's a silicon carbon battery.

"Adding silicon is nothing new," explained Dr. Juho Heiska, head of R&D at Finland's Seinäjoki University of Applied Sciences. "Tesla has already used something like a 3-to-5-percent mix in some of its NCA [nickel cobalt aluminum battery] models," he added. (Tesla has been adding silicon carbon to its batteries since at least 2016, according to the Los Angeles Times.) What has changed is that manufacturing, engineering and chemical processes have improved to enable even greater mixes of silicon carbon.

Dr. Heiska said that many manufacturers are working on "drop-in" materials which can be added to existing production processes. "You take 75 percent of the graphite, 25 percent silicon powder, mix them together and make the slurry in the same way," he explained. But, despite this, Heiska said these cells are far more expensive to manufacture, which is why it makes sense for them to debut in smartphones. After all, "the unit price for the battery is not [excessively] high," he said, especially compared to the sort of costs required to build a cell for an EV.

Samsung is arguably the largest smartphone manufacturer who has embraced silicon carbon cells. The Korean giant is being cautious about its approach, saying it isn't looking to make the sorts of super-capacious cells found in some Chinese devices. The new lineup of Z Fold phones uses small quantities of silicon in order to shrink the size of its batteries and therefore hardware. But Samsung also appears to believe the batteries will degrade far faster than their lithium predecessors. In regulatory filings made with the EU, the company said the cells will only last 1,200 charge cycles — a substantial drop from the 2,000 pledged for the previous generation.

The addition of a silicon carbon and graphite anode into these batteries have a number of big upsides. "Graphite has a capacity of about 330 milliamp hours per gram," explained Dr. Sayers, "silicon has a tenfold increase in capacity compared to graphite." The electrochemical reaction that occurs when a silicon carbon battery is used is very different from a lithium ion battery, too. "It's actually a chemical reaction, called alloying," said Dr. Sayers, "your lithium reacts with your silicon and forms different forms of lithium silicon." She added "that then means you can store more and more lithium into it, which gives it that really big capacity."

The greater capacity by weight means batteries can be much bigger, in terms of their charge, but smaller in terms of volume. It's why we're seeing smartphone batteries suddenly gain a huge amount of extra capacity without becoming as thick as they were a decade or more ago. The OnePlus 15, for instance, is only a millimeter or two thicker than an iPhone 17, but its battery has a rated capacity of 7,300mAh and survived for a staggering 38.5 hours in our rundown test.

Greater capacity goes hand-in-hand with the ability for silicon carbon batteries to charge much faster than their counterparts, too. "One of the reasons you cannot charge graphite batteries quickly is because the lithium ions cannot get between the graphite layers," said Dr. Heiska. "Silicon is faster to charge and lithiates first [...] so it really does help with the fast charging."

The other plus is that silicon is cheap, abundant and easily available, eliminating some of the risk inherent in other battery chemistries. Dr. Sayers explained that, as with many industries, the focus is on sourcing low cost and abundant raw materials free from broader geopolitical issues.

As with all new technologies, there are plenty of downsides which have held back widespread adoption until now. For instance, silicon carbon batteries offer "slightly better fast charging and slightly better capacity," said Dr. Heiska, "but you will get a slightly shorter cycle life out of the cell." Put simply, when you add an electric charge to a material, you cause it to deform in some way, often growing in size. That's not ideal when you're trying to package up substances into a fixed shape, such as a smartphone battery.

All batteries are prone to this deformation, the extent of which is determined by the materials the cells are made of. Dr. Heiska explained graphite will expand by about 10 percent during a cycle while silicon expands by a whopping 400 percent. This is one of the reasons we don't see a battery made purely out of silicon, because its "life cycle would be so poor [...] nobody would buy those products," he said.

"At a chemistry level, you're inserting lithium into your silicon material, and therefore making alloys of lithium and silicon," explained Dr. Sayers. "You can control how much lithium you put in, how much of the silicon you utilize," she added. At that point, manufacturers just need to find a balance between capacity and longevity that consumers will tolerate. Sayers added the hope will be to find a ratio of silicon and graphite that offers a "bigger boost in energy and still maintain your [battery] lifetime."

Dr. Heiska noted that there are various projects under way to improve the quality of silicon carbon cells. This includes coming up with different manufacturing processes and mixing in extra materials, such as flurophosphates, which may improve stability.

The trade-off between battery capacity and life does make smartphones the ideal place to grow the technology. After all, there's still an expectation people will upgrade their devices every few years or so, even in these more straitened times. Plus, if your phone has enough juice to last two days before needing a recharge, your habits may shift to less regular charging anyway.