This article is from The Spark, MIT Technology Review's weekly climate newsletter. To receive it in your inbox every Wednesday, sign up here.

I’d be willing to bet that you probably haven’t spent much time thinking about the liquid that sloshes around inside batteries. 

But this liquid—called the electrolyte—is one of their key ingredients, and it dictates a lot about how they work, as well as how safe they are. And I’ve seen a growing number of alternative battery makers talk about using an interesting ingredient in their electrolyte: water. 

Lithium-ion batteries that power EVs and laptops today have to use organic solvents like ethylene carbonate to shuttle charge around (we’ll get into the details on why later). But chemistries that make it possible to rely on water instead could mean even safer batteries. And as we put more batteries to use in large storage systems on the grid, that could be a major benefit. 

I recently wrote about one alternative battery maker called Eos, which got a huge loan from the US Department of Energy last week.  So for this edition of the newsletter, let’s dive into how Eos and other battery makers are looking to change up battery chemistries with water. 

A hot topic: battery safety

It can be tricky to talk about lithium-ion batteries and safety, because there’s a lot of misinformation out there and emotions can run hot. But it’s worth digging in here on why so many alternative battery makers emphasize safety when they talk about their technology. 

Lithium-ion batteries can and sometimes do catch fire, usually when they’re damaged or when they get too hot, kicking off chemical reactions in a process called thermal runaway. Devices that use lithium-ion batteries typically have safety systems in place to manage this risk: electric vehicles have cooling systems installed around battery packs, for example. 

But sometimes things can go wrong. Manufacturing defects happen (remember those Chevy Bolt fires?). It’s not totally clear how often EVs catch fire in general, though some data suggests it’s much less often than gas-powered vehicles. But on the other hand, EV fires can burn hotter than fires in conventional cars, and they are harder to put out. 

The question of safety could become an even more important one as we start to use batteries in a new way: on the power grid. As we install more renewable energy on the grid, there’s a growing need for large-scale energy storage installations that can save solar power for use at night, for instance. 

These storage systems are great news for cutting emissions, but things can go wrong with them as well. As Canary Media reported, New York has seen a few battery fires this summer in large-scale stationary storage installations on the grid. No injuries were reported in any of the fires, and damage was mostly limited to the batteries themselves. But a string of fires doesn’t look too great. 

There’s also growing concern about fires started by e-bikes in New York City. These fires, which can be deadly, have mostly been caused by bikes that aren’t repaired correctly or use substandard batteries, highlighting the need for regulation and tight quality control of batteries. 

What it comes down to is this: lithium-ion batteries can catch fire. It doesn’t happen often, and there are many, many safety controls that can be put in place to manage the risk effectively. But some battery makers want to build alternatives that can’t catch fire in the first place. 

Watering it down

Lithium-ion chemistry has been optimized over decades to pack a lot of energy into a small, lightweight device and deliver a lot of power. 

Part of that optimization is in the liquid electrolyte: standard lithium-based batteries use organic solvents mixed with salts to shuttle charge around. Theoretically, batteries can use water as the solvent, but they usually don’t. That’s for a pretty good reason: the high voltage common in lithium-ion batteries, which is needed to deliver high power, can pull water apart into hydrogen and oxygen. 

But when it comes to huge storage installations on the grid, there’s a different balance to strike. Rather than focusing on packing lots of energy into a small battery, researchers and companies want above all to lower the batteries’ cost.  

So batteries destined for storage on the grid can make some compromises. They might not need to charge and discharge so quickly, and it’s less crucial to get them as small and light as possible. 

That opens up the possibility of using heavier materials, like iron and zinc. And with lower power and lower voltages needed, companies can use water with salts mixed in as an electrolyte. That could help save on costs, make the batteries easier to manufacture, and also help with safety. You’d probably have a hard time setting water-based batteries on fire, even if you tried. 

Some companies are leaning into the benefits of using water in their alternative batteries as they start to make progress toward commercialization. 

Form Energy is one of the leaders in building alternative batteries for the grid. The company’s batteries are sometimes called “rust batteries” because they use iron and water, and the reactions are similar to the ones that happen metal rusts with exposure to moisture. Form’s website touts its batteries’ safety, saying the systems have “no risk of thermal runaway.” The company broke ground on a factory in West Virginia earlier this year. 

Eos Energy is also building batteries with a water-based electrolyte, using zinc as a primary cathode ingredient. When I asked Francis Richey, Eos’s VP of research and development, what the benefits of the company’s chemistry were, the first thing he brought up was safety: “Number one, it’s safe. It’s a non-flammable technology.”

There are plenty of challenges ahead for alternative batteries, like the difficulty of competing on price: lithium-ion cells have been around for decades, and costs have plummeted in that time. But there are potential upsides to having more options—including workhorse systems that could ease safety concerns about large battery installations. 

Related reading

Read more about the Department of Energy’s loan to Eos Energy, and how the company’s zinc batteries work, in my latest story. 

Iron batteries on the grid were on our 2022 list of Breakthrough Technologies—see why in this blurb from last year.

Sodium-ion batteries can be built with electrolytes based on either water or organic solvents. And many battery experts are intrigued by this alternative chemistry for both EVs and stationary storage systems. 

It’s not easy bringing a battery startup into the world, though. Read more about the rise, fall, and rebirth of sodium-ion battery maker Aquion in my colleague James Temple’s story from 2017. 

Another thing

If you haven’t registered for ClimateTech yet, I’ve got another great reason for you to change that: we’re revealing a special project at the event this year. 

The Tech Review climate crew has been hard at work putting together a list of 15 companies to watch in climate tech. These companies span all sorts of sectors, and they come from around the world. I can’t give anything else away just yet, but you might recognize a few names if you’re a longtime reader of this newsletter …

Join us on MIT’s campus or virtually on October 4-5 for the first look at the list, along with a ton of exciting conversations about the technology, policy, and business of climate. Hope to see you there! 

Keeping up with climate  

New factories for EVs and their batteries are popping up across the US South. Some of them could transform small towns—like this Ford factory under construction near a 400-person town in Tennessee. (Wall Street Journal)

→ My colleague David Rotman took a close look at how $100 billion investment in computer chips could affect Syracuse, New York. (MIT Technology Review)

The US Environmental Protection Agency was publishing warnings about climate change 40 years ago. It’s pretty wild how accurate they were on everything from how much warming was expected to how much various technologies might help. (The Messenger) 

Two virtual power plants just came online in Texas. Clusters of small batteries can be used to meet demand on the grid, though the program is pretty small right now. (Canary Media) 

If you include plug-in hybrids, the title of world’s biggest electric-vehicle maker goes to a Chinese company called BYD. And now, the company is trying to expand beyond China’s borders. (Rest of World) 

→ Here’s how China came to dominate the world of EVs. (MIT Technology Review)

A group of researchers signed a letter calling for more research on ocean-based carbon removal efforts. While these methods could help suck carbon out of the atmosphere, there are questions about both how well they might work and how they’d affect ecosystems. (The Verge)

Fertilizer is a huge climate problem. Some startups want to change that, though there’s some skepticism about how well replacements might perform. (Canary Media)