Solid-state batteries are the future. They’re more powerful, compact, safe, and sustainable than Lithium-ion alternatives, but true all-solid state batteries can’t be mass-produced cost-effectively. That’s why Kuxiu has gone semi-solid state for what it calls the “world’s safest” power bank, while calling dibs on it being a world first.

Kuxiu’s $79.99 S2 Qi2 5000mAh MagSafe Solid-State Power Bank supports Qi2 for 15W wireless charging. And with a 5,000mAh (19Wh) capacity, it holds enough energy to easily charge the latest iPhone 16 models — with batteries ranging from 3,561mAh to 4,685mAh — from zero to full at least once. And despite using the truncated “solid state” in the S2 name and product page, the company confirmed to me that it’s built around a semi-solid state battery and that distinction matters.

I’ve been carrying the Kuxiu S2 for the past few weeks. While I didn’t hammer, pierce, or tear the battery apart or expose it to fire like Kuxiu did, I can at least confirm that it otherwise works as expected.

“Solid state” ambiguity

It’s not just you, the battery industry itself can’t seem to agree on what qualifies as a solid-state battery. There’s all-solid state, semi-solid state, and quasi-solid state, to name just a few of the more popular terms. All-solid state is where the hype is and it’s the liquid-free battery of the future. But getting there at scale will likely require a few go-between steps.

Solid-state batteries use a solid electrolyte, but differ in terms of the amount of liquid present. If the amount of the liquid electrolyte is between 5 and 10 percent of the weight of the active part of the battery then it’s typically classified as semi-solid state. Less than 5 percent and it’s quasi-solid state. 0 percent, you guessed it: all-solid state. The battery inside the Kuxiu S2 contains a solid-liquid mixed electrolyte that forms a gel-like consistency that’s visible in the video above. It therefore fits the definition of semi-solid state, according to the company.

So why is “Battery Type: Li Polymer Battery” listed in the fine print on the label stamped onto Kuxiu’s S2 power bank? LiPo batteries have been around in power banks for years, and suffer from relatively short lifespans and low energy densities — exactly the opposite of what Kuxiu claims for the new S2. Turns out that the solid electrolyte inside solid-state batteries can be made from various materials like ceramics and yes, polymers. So Kuxiu’s claim is likely true, the S2 is a Li Polymer battery that’s also a solid-state battery, or semi-solid state to be precise. Therefore, it may very well be the world’s first power bank to use a solid-state battery as the company says.

For comparison, the Lithium-ion battery found in the power bank you likely have laying around at home contains over 25wt percent liquid, making it more prone to leakage, thermal runaway, and fire than the s2. But Kuxiu’s power bank doesn’t benefit from every advantage of going solid state — like ever higher energy densities and greater safety achieved with all-solid state batteries — but it’s a step in the right direction. Notably, Kuxiu can mass produce the S2 today using many existing manufacturing processes to help keep costs down.

Companies like Anker will tell you that a typical 5,000mAh power bank usually starts to degrade after 300–500 charging cycles. At that point, it’ll only hold about 60 to 80 percent of the original capacity. Kuxiu tells me that the S2 is good for 1,000 charging cycles before capacity drops to 80 percent. So, while it costs about 60 percent more than a comparable liquid-based power bank from companies like Anker or EcoFlow, it should easily last twice as long before you see any noticeable degradation.

The energy density is another advantage of the semi-solid state battery inside the S2. Kuxiu claims 360Wh/kg compared to 300Wh/kg for liquid cells. The S2 is certainly compact and lightweight at 10.4 x 6.7 x 1.0cm / 145g, which makes it more portable than EcoFlow’s direct competitor which measures 10.8 x 7.0 x 1.44cm / 180g. But EcoFlow’s 5,000mAh power bank also includes a kickstand and 30W of bidirectional wired charging. Anker, however, edges out the S2 in size and weight with its 5K power bank measuring 10.2 x 7.0 x 0.9cm / 120g. But that likely says more about the materials used to decorate and protect the batteries than their respective energy densities.

Performance

As a MagSafe-compatible Qi2 power bank, the Kuxiu S2 automatically starts charging as soon as its 16 N52 magnets grab hold of my iPhone 15 Pro. Those magnets are strong and maintain the bond even as I slip the combo into a loose pocket or bag. Still, they’re not immune to a well-placed bump that could send the battery to the floor. That could spell trouble for the aluminum S2 sandwiched between two sheets of Corning Gorilla Glass, each protected by an “explosion-proof film.” After three weeks of careless handling, my S2 review unit has a small but noticeable nick on the shell, but otherwise looks new.

Sadly, the bottom of the battery extends beyond my iPhone 15 Pro to upset the symmetry I’d likely enjoy when paired with one of Apple’s larger Qi2-compatible iPhones. At least the cameras aren’t obscured. I could also do without all the specs listed on the outside of the unit. The power bank does work with or without a MagSafe-compatible case on my iPhone, but the case does cause the S2 to heat up slightly (making it less efficient) to help overcome the gap.

The USB-C port on the bottom of the S2 features passthrough, so you can charge the power bank and a magnetically attached iPhone or AirPods at the same time. Charging the S2 solo from my MacBook’s wall charger took the power bank from zero to full in a rather slow 90 minutes.

The power bank can also charge non-Qi2 devices thanks to its bidirectional USB-C port. That’s useful to juice up devices like over-the-ear headphones and Android phones at up to 20W. The S2 also proved capable of topping up my beefy 14-inch MacBook Pro, albeit very, very slowly.

A double-tap on the button along the bottom edge puts the power bank into a low-current mode. That way, the S2 can charge small devices like smart rings and wireless earbuds while bypassing automatic shutoff mechanisms designed for more power-hungry devices. Smart, but not unique to the S2.

I do wish that Kuxiu put a kickstand on the S2 like so many of its competitors. At least it’s available in both black and titanium colors to better match your phone.

Misleading battery marketing aside, I’m very impressed by the Kuxiu S2 Qi2 5000mAh MagSafe “Solid-State” Power Bank in terms of what it is and what it promises. Yes, it’s more expensive than similarly specced power banks using traditional liquid electrolytes, but the S2’s semi-solid state battery should prove to be safer and cheaper than competitors over its much longer lifetime.

Photos by Thomas Ricker / The Verge