In 2019, Tom Sisto was working as a researcher in the laboratories of Columbia University, developing synthetic molecules for solar cells, when his team stumbled upon a “serendipitous” discovery.
“Very much by accident, we found a set of chemistries that never degraded when they were charged or discharged,” Sisto, who is the co-founder and CEO of XL Batteries, told Latitude Media. “We were using them for solar cell applications…and we looked at it and said, ‘Well, that’s a battery molecule, not a solar cell molecule.’”
That molecule, originally contained in a layer of bright red paint that absorbed light in a solar cell, became the basis for the long-duration energy storage systems developed by his company, XL Batteries, which today announced a $7.5 million seed investment from Merrin Investors, Latitude Media has exclusively learned. This latest round brings the startup’s total funding to date to $28 million.
XL Batteries develops flow batteries, which are rechargeable energy storage systems that generate electricity by circulating liquid electrolytes through a cell stack. Sisto compares them to a car, which functions thanks to an engine, a gas tank, and the fuel in it. In XL Batteries’ flow batteries, cell stacks assembled in an industry-standard shipping container are the equivalent of the engine; a liquid solution containing the startup’s proprietary molecule is the gas; and the industrial storage tanks where the solution is stored are the equivalent of the car’s gas tank.
Electricity goes into the liquid, and then the system charges and discharges as the liquid flows through the cells. Unlike a car, however, it doesn’t need to be refueled. “It’s a closed loop circulating system — you’re able to charge and discharge the tanks again and again and again,” Sisto said.
The system can last for more than 20 years, and the hours of storage it can discharge depends on the amount of liquid in the tanks and the number of stackable containers a company installs. “The duration is whatever size tank the customer may want,” Sisto said. “Let’s pretend the electrodes are lithium: you can add as much lithium as you want, and that’s how much duration you get. If you have a 10-hour battery and you want to make it a 20-hour one, you just double the size of the tank.”
The company recently finished a pilot project with bulk liquid services provider Stolthaven Terminals.
In May, it announced an agreement with data center developer Prometheus Hyperscale. Per the agreement, XL Batteries will deliver and commission a 333-kilowatt demonstration-scale system at Prometheus’ facility in 2027; Prometheus has committed to buy one 125 megawatt-hour commercial-scale system in 2028 and another one in 2029.
New chemistry, old tech
XL Batteries’ new molecule application is part of the energy storage industry’s hunt for new chemistries to make long-duration energy storage economical at scale — a milestone that has remained out of reach for the industry so far. But while the molecule is new, Sisto stresses that the technology is not.
The first vanadium flow battery patent was filed in the late 1980s, and the first large-scale implementation occurred in Japan in the 1990s, when Mitsubishi Electric Industries and Kashima-Kita Electric Power Corporation used it for load-levelling at a power station. The industry started piloting them for grid applications in the 2000s.
But they have traditionally relied on vanadium, a metallic element that gets dissolved in sulfuric acid, and are considerably more expensive than their lithium counterparts. (Vanadium is what the UK-based Invinity, one of the biggest companies in the space, uses for its flow batteries, which have a discharge duration of up to 12 hours.)
“Vanadium is too expensive, 90% of it is controlled by China and Russia, and it only dissolves in sulfuric acid, so the liquid is highly corrosive, which drives a lot of cost within the system,” Sisto said. “What we are using instead is [a molecule from] organic petrochemical feedstocks into pH-neutral water, which changes the price structure of the existing platform.”
Petrochemical feedstocks, which come from oil and gas, are plentiful and are the basis of most modern plastics, synthetic fibers, rubbers, fertilizers, and many chemicals. Replacing vanadium with XL Batteries’ proprietary liquid makes flow battery systems three to four times cheaper than they would otherwise be, according to Sisto: “Burning oil and gas once for energy is unsustainable. Transforming it into something that can store energy for 20 years is not a problem.”
Compared to lithium-ion batteries, Sisto says that XL Batteries’ systems start to make sense economically for energy storage of at least six hours. While flow batteries’ target efficiency for commercial systems is slightly lower than lithium systems — with a 70-75% target efficiency versus an 85-90% for lithium, according to Sisto — they have similar response times. Plus, they don’t have a flammability problem, which makes them well-suited for industries with high safety needs, such as petrochemicals.
