Hardwired
In Hardwired, AC Senior Editor Harish Jonnalagadda delves into all things hardware, including phones, audio products, storage servers, and networking gear.
Samsung didn’t change too many things with the Galaxy Z Fold 8 Ultra, but a key area of differentiation is silicon-carbon battery tech. This is one of the reasons how Samsung managed to slot in a bigger 5,000mAh battery in the foldable while retaining a similar svelte design as the Z Fold 7. In fact, all of Samsung’s latest foldables use silicon batteries, but it’s the Z Fold 8 and Fold 8 Ultra where the difference is immediately noticeable. I asked Samsung a list of questions around its implementation of silicon-carbon tech, and it is interesting to see how the brand is going about integrating the tech into its latest foldables.
As a refresher, a silicon battery has a small mixture of a silicon-carbon material in the anode, leading to an increased energy density. The amount of silicon content in the anode varies by brand, and Samsung didn’t share how much it is using in its foldables. To give you some context, most brands use around a mixture of 10% silicon by weight in the anode, with some — like Xiaomi’s Redmi Note 17 Pro Max that has a 9,210mAh battery — going with a slightly higher 16% silicon ratio to provide greater energy density.
Why don’t brands just use an anode that’s entirely made out of silicon-carbon? That’s because while silicon has much better energy density — ten times as much as graphite — it undergoes expansion and contraction during charging, something you don’t want in a phone battery. As there is a greater risk of long-term stability or battery swelling with extended use, it is mixed with carbon to deliver a mechanically stable structure while offering better electrical conductivity.
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Brands are being (justifiably) cautious of the tech, and a Samsung spokesperson said the brand “reengineering the battery system” to facilitate the use of silicon-carbon in the anode, and to ensure that the higher-density batteries it is using this year are just as safe to use as traditional lithium-ion batteries:
“On the anode side, a thin carbon coating helps manage the reactivity of silicon-carbon particles, helping mitigate sudden volume changes during charge and discharge and supporting long-term stability,” said Samsung. “On the cathode side, coating and doping help maintain surface stability and structural integrity, even under higher voltage conditions.”
Another thing to consider is long-term durability, and Samsung says it added “additives” to the electrolyte to “help form a protective layer on the silicon surface, reducing silicon reactivity and supporting long-term cell stability.”
Samsung also made tweaks to the separator — a polymer sheet that sits between the cathode and anode — which has a “high-porosity structure” to “improve ion movement while maintaining strength and thermal stability.”
What’s interesting is that Samsung is using different battery architectures for its foldables. The Galaxy Z Fold 8 and Fold 8 Ultra use a “dual-path charging architecture that distributes power more efficiently across the battery system, enabling faster, more stable and reliable charging while improving heat management.” This system enables 45W charging on both devices, but with the Flip 8, Samsung “separately optimized for the unique spatial and thermal requirements” of the compact foldable, so it has a battery size and charging tech that’s unchanged to last year.
With Samsung just getting started with silicon-carbon batteries, it’s understandable that it is playing it safe. While foldables like the Razr Fold, Vivo X Fold 5, and the Find N6 come with 6,000mAh silicon batteries, the Z Fold 8 Ultra has a much more modest 5,000mAh unit. It is still considerable progress from last year, and I’m glad to see Samsung leveraging this tech — I can’t wait to see how it evolves with the Galaxy S27 next year.
The unique size of the Galaxy Z Fold 8 makes it exciting in its own right, and it is a delightful foldable.