Sinergy Flow ($2M for cheap, environmentally friendly, and high-energy zinc-aqueous polysulfide flow battery for renewable energy storage)

Sinergy Flow, an Italian energy storage startup founded in 2022, develops zinc-polysulfide rechargeable flow batteries that are environmentally friendly, cheap, and high-energy for stationary energy storage. The Sinergy Flow battery uses electrolytes made from zinc and sulfur, which are earth abundant and low-cost. This means that it can reuse sulfur waste from the oil and gas industry, which is in line with the Circular Economy principles. Sinergy Flow’s battery system is modular and scalable, so it can be used for both on-grid and off-grid energy storage on a large scale.

Challenges: Flow battery for renewable energy storage

To reduce global carbon emissions, more and more renewable energy sources of solar cells and wind turbines are being installed. In the meantime, the need for energy storage systems that are easy to use, scalable, and compact is growing. The ability to store this clean electricity would significantly increase the efficiency and reliability of these intermittent renewable energy sources.

Flow batteries use the movement of electrolytes to store and release electrical energy. A typical flow battery has two half-cells that are separated by a membrane. Each half-cell has an electrode and an electrolyte. The electrolytes, called anolyte and catholyte, contain the redox-active species dissolved in an aqueous or non-aqueous solvent. They are stored outside the battery. When the battery is charged or discharged, they flow through the half-cells in contact with the electrodes, thanks to 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 over 20 years. This makes them good candidates for storing renewable energy.

However, the high cost of vanadium salts, as well as the capacity decay and toxicity of the last two systems, has limited their application worldwide. The capital cost of flow batteries is around $800 per kilowatt-hour, which is more than twice that of lithium-ion batteries. Therefore, we need flow battery systems that are cheap, high-energy, and environmentally-friendly.

This book Redox Flow Batteries provides a full understanding of flow batteries from fundamentals to commercial applications. (see on Amazon)

Sinergy Flow Technology

Sinergy Flow (Sinergy) has developed a safe, cost-effective, and high-energy rechargeable flow battery system for long-term (> 10 h) storage of renewable energy. The flow battery uses acid-free aqueous electrolytes that are made from zinc (Zn) and sulfur (S), which are earth-abundant and environmentally-friendly. Sulfur can be obtained from the oil and gas industry. This low-cost Zn-polysulfide flow battery can provide a power source with high energy efficiency of 70-85%.

Sinergy battery

The diagram below depicts the Sinergy battery system.

Sinergy Flow’s zinc-polysulfide rechargeable flow battery system (ref. US20230187675A1).
Sinergy Flow’s zinc-polysulfide rechargeable flow battery system (ref. US20230187675A1).

The Sinergy flow battery consists of electrolyte storage tanks for anolyte and catholyte, as well as two half-cells separated by a porous membrane. The system has components of pipes, pumps, and controllers (not shown).

  • Anolyte

The aqueous anolyte contains zinc ions (Zn²⁺), hydroxide ions (OH⁻), conductive particles, and additives.

Hydroxide ions are provided by dissolving  lithium hydroxide (LiOH), sodium hydroxide (NaOH), and/or potassium hydroxide (KOH).

Zinc ions are provided by dissolving zinc oxide (ZnO) or zinc zinc acetate (Zn(CH₃COO)₂).

Conductive particles are suspended in the electrolyte. They are zinc and carbon particles.

Additives include hydrogen evolution inhibitors (silicates), Zn complexing agents (Rochelle salts), leveling agents (polyethylene glycol, polyethylenimine, etc.), and corrosion inhibitors. These additives increase and stabilize the battery performance.

The hydrogen evolution suppressor improves the coulombic efficiency of the battery and decreases side reactions during the charging phase. This additive may also be effective in preventing pH fluctuations in the electrolyte.

Leveling agent reduces the dendritic growth of electrodeposited zinc, which would negatively impact the battery’s long-time performance.

  • Catholyte

The aqueous catholyte contains hydroxide ions, polysulfide ions (Sₘ²⁻), and catalyst particles.

Hydroxide ions are provided by dissolving lithium hydroxide (LiOH), sodium hydroxide (NaOH), and/or potassium hydroxide (KOH).

Polysulfide ions are provided by dissolving sulfur (S), sodium sulfide (Na₂S), and/or potassium sulfide (K₂S). Polysulfide ions with a chain length between 2 and 5 (Sₘ²⁻, m = 2–5) exist in large quantities within an alkaline polysulfide solution.

The catalyst particle is cobalt sulfide (Co₃S₄).

  • Electrodes

The porous negative electrode is composed of a zinc metallic foam, a zinc coated metallic foam, or a percolated network of dispersed zinc particles. In comparison to a flat electrode, the porous electrode has a higher surface area to minimize the over potential for zinc plating/dissolution, resulting in a higher cycle life .

The porous positive electrode is composed of carbon felt, a carbon fabric, or a percolated network of dispersed carbon particles. The electrode is modified with cobalt sulfide (Co₃S₄) to promote the electrochemical reactions of polysulfide ions during charge and discharge process of the battery.

  • Multilayer membrane separator

The multilayer membrane separator is composed of a homogeneous separator, such as Celgard® and Nafion™, coupled to a second layer containing carbon particles decorated with catalyst and non-catalytic particles. The multilayer membrane reduces polysulfide ion permeability through the membrane, thereby preventing self-discharge and rapid capacity decay.

How Sinergy battery works?

The process of charging and discharging the Sinergy flow battery is explained below. During charging and discharging, the anolyte and catholyte from their respective storage tanks are pumped through the battery’s half-cell chamber. Redox reactions occur in the chambers of the half-cells and the electrolytes are recirculated to their storage tank and fresh electrolytes are introduced to the electrolytic cells. This flow battery generally has a cell potential between 0.7 V and 1.4 V. It provides efficiency as high as 70-85%.

  • Charging
The charging process of the Sinergy flow battery.
The charging process of the Sinergy flow battery.

A voltage from an external power source is applied to the battery during charging. On the surface of the negative electrode, zinc ions in the flowing anolyte are reduced to zinc metal through a chemical reaction:

Zn²⁺ + 2e⁻ → Zn↓

On the surface of the positive electrode, polysulfide ions are oxidized via the following reaction:

Sₘ²⁻ → Sₙ²⁻ + 2(m-n)e⁻

  • Discharge
The discharging process of the Sinergy flow battery.
The discharging process of the Sinergy flow battery.

During discharging, the battery outputs power. Metallic zinc on the negative electrode is oxidized to zinc ions, which are returned to the anolyte:

Zn → Zn²⁺ + 2e⁻

Polysulfide ions in the catholyte are reduced on the surface of positive electrode via the following reaction:

Sₙ²⁻ + 2(m-n)e⁻ → Sₘ²⁻

Sinergy battery cost

The Sinergy Zn-polysulfide flow battery is significantly cheaper than most common redox flow batteries. This battery costs less than $200 per kWh for the battery components and less than $60 per kWh for electrolyte and tanks.

Sinergy Flow Patent

  • US20230187675A1 Environmentally friendly zinc/aqueous polysulfide rechargeable flow battery with high energy efficiency

Sinergy Flow Battery Applications

Sinergy’s rechargeable flow battery can be used for renewable energy storage solutions. It is suitable for long-term energy storage (>10 h).

Sinergy Flow Products

Sinergy is developing rechargeable flow battery systems based on low-cost, environmentally-friendly, and earth-abundant materials of zinc and sulfur.

Sinergy Flow Funding

Sinergy Flow has raised a total of €1.8M in funding over a Seed round on Nov 16, 2022.

Sinergy Flow Investors

Sinergy Flow is funded by 3 investors:

PoliHub and 360 Capital are the most recent investors.

Sinergy Flow Founder

Alessandra Accogli, Gabriele Panzeri, and Matteo Salerno are Co-Founder.

Sinergy Flow CEO

Alessandra Accogli is CEO.

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