XL Batteries ($10M for low-cost redox flow battery based on saltwater organic electrolytes for renewable energy storage)

XL Batteries, an American energy storage startup founded in 2019, develops organic redox flow battery technology based on cost-effective, non-corrosive, and non-flammable aqueous electrolytes. The aqueous electrolytes contain innovative organic redox active materials which are stable under charging and discharging conditions. The XL Batteries redox flow battery is suitable for grid-scale renewable energy storage.

Challenges: Store renewable energy at a low cost

By 2022, the world had about 1,185 GW of installed solar capacity and about 906 GW of installed wind capacity. Solar power has grown at a 24% annual rate over the last decade. The US solar industry installed 6.1 GW of capacity in the first quarter of 2023, 47% more than in the first quarter of 2022. On the other hand, the average power of new wind turbines installed in the US in 2022 was 3.2 MW, 7% higher than in 2021.

As more and more solar cells and wind turbines are installed, the ability to store this clean electricity would significantly increase the efficiency and reliability of these intermittent renewable energy sources.

Flow batteries store and release electrical energy through the movement of electrolytes. A typical flow battery consists of two half-cells separated by a membrane. Each half-cell contains an electrode and an electrolyte. The anolyte and catholyte contain redox-active species dissolved in an aqueous or non-aqueous solvent. They are stored outside of the battery. When the battery is charged or discharged, the electrolytes flow through the half-cells in contact with the electrodes via external pumps. This is very different from lithium-ion batteries.

Most flow battery systems are based on vanadium, Fe—Cr, and Zn—Br. They are simple systems and easy to scale up. They can have a long lifespan of more than 20 years. This makes them good candidates for storing renewable energy. However, the capital cost of flow batteries is around $800 per kilowatt-hour, which is more than twice as much as lithium-ion batteries. Therefore, it is necessary to develop cost-effective redox flow batteries.

XL Batteries Technology

XL Batteries develops organic redox flow batteries for grid-scale renewable energy storage based on cost-effective aqueous electrolytes. The core technology of XL Batteries is innovative organic redox active materials for the aqueous electrolytes. The aqueous electrolytes are non-corrosive, non-flammable, and are stable under charging and discharging conditions.

XL Batteries redox flow battery

The diagram below depicts the XL Batteries organic redox flow battery system.

XL Batteries organic redox flow battery system (ref. US20220255106A1).
XL Batteries organic redox flow battery system (ref. US20220255106A1).

The flow battery system has two half-cells with the negative and the positive electrode separated by an ion-conducting membrane separator. The two half-cells are fed from the anolyte and catholyte tank by the electrolyte circulation pumps.

The negative and positive electrode can be made of carbon felt or a carbon paper.

The electrolytes are saltwater containing dissolved organic redox active materials.

The membrane can be a Daramic 175 or a cellulose membrane. It separates the anolyte and catholyte and prevents the organic redox active materials from crossing over. Ion exchange occurs between anolyte and catholyte through the membrane.

XL Batteries redox flow battery electrolytes

  • Anolyte

The anolyte comprises aromatic imide compound dissolved in an aqueous solution of 0.5 M NaCl.

The aromatic imide can be perylene diimide salt ([PDI][TFSI]₂). Its molecular structure is presented by the following formula:

Perylene diimide in anolyte of the XL Batteries redox flow battery.
Perylene diimide in anolyte of the XL Batteries redox flow battery.

The aromatic imide can also be spiro-ethylphthalimide. Its molecular structure is presented by the following formula:

Spiro-ethylphthalimide in catholyte of the XL Batteries redox flow battery.
Spiro-ethylphthalimide in anolyte of the XL Batteries redox flow battery.

R represents alkyl, ether, or ammonium salt.

The spiro-fused compound can sterically block deleterious reactivity in order to prolong the lifetimes of organic radicals. The use of spiro-fusion enhances the solubility of the molecule due to the inability of molecular packing.

  • Catholyte

The catholyte comprises ferrocene compounds dissolved in an aqueous solution of 0.5 M NaCl.

The ferrocene compound can be a dendrimer-like tetraferrocene (Fc4). Its molecular structure is presented by the following formula:

Dendrimer-like tetraferrocene in anolyte of the XL Batteries redox flow battery.
Dendrimer-like tetraferrocene in catholyte of the XL Batteries redox flow battery.

The dendrimer-like structure of Fc4 reduces its ability to crossover the membrane.

How XL Batteries redox flow battery works

During charging and discharging, the surface of the negative electrode (anode) undergoes the following redox reactions:

Redox reactions of perylene diimide during charging and discharging of the flow battery.
Redox reactions of perylene diimide during charging and discharging of the flow battery.

The following redox reactions occur on the surface of positive electrode (cathode):

Redox reactions of tetraferrocene during charging and discharging of the flow battery.
Redox reactions of tetraferrocene during charging and discharging of the flow battery.

Redox reactions of tetraferrocene during charging and discharging of the flow battery.

The water flow battery has a capacity retention of over 99.99% and a coulombic efficiency of over 99.99%. The battery is stable for 1,000 cycles. The energy density of this water flow battery is not disclosed, but it is most likely lower than that of the majority of redox flow batteries based on vanadium, Fe—Cr, and Zn—Br.

XL Batteries Patent

  • US20220255106A1 Redox flow batteries and compounds for battery application

XL Batteries Redox Flow Battery Applications

XL Batteries redox flow batteries facilitate the widespread adoption of solar and wind energy by providing a stable, efficient, and sustainable grid-scale energy storage solution.

XL Batteries Products

XL Batteries has signed a memorandum of understanding with Stolthaven Terminals for the development of an industrial-scale flow battery.

XL Batteries Funding

XL Batteries has raised a total of $10.2M in funding over 2 rounds:

Their latest funding was raised on Sep 7, 2022 from a Venture – Series Unknown round.

The funding types of XL Batteries.
The funding types of XL Batteries.
The cumulative raised funding of XL Batteries.
The cumulative raised funding of XL Batteries.

XL Batteries Investors

XL Batteries is funded by 7 investors:

SIP Global Partners and Jeffrey Schwartz are the most recent investors.

The funding rounds by investors of XL Batteries.
The funding rounds by investors of XL Batteries.

XL Batteries Founder

Thomas Sisto and Anouck Champsaur are Co-Founder.

XL Batteries CEO

Thomas Sisto is CEO.

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