Science Explained: What Are Flow Batteries?

Flow batteries, an electrochemical energy storage technology proposed by Thaller in 1974, represent a new type of rechargeable battery. Comprising stack cells, electrolytes, electrolyte storage and supply units, and management control units, they operate by separating positive and negative electrolytes into distinct circulation loops. These high-performance batteries feature high capacity, broad application environments, and extended cycle life, positioning them as a new energy product.

Components of Flow Batteries

1. Stack Unit: Composed of multiple single cells connected in series, this is where electrochemical reactions occur. Each single cell includes a cathode, anode, and separator. Electrolytes for the cathode and anode circulate separately, separated by an ion-exchange membrane, enabling the conversion of electrical energy into chemical energy and vice versa through redox reactions.

2. Electrolyte: Active materials are stored within the electrolyte. Anode and cathode electrolytes typically consist of solutions containing metal ions in different oxidation states. For example, in a vanadium flow battery, the cathode electrolyte comprises solutions of V(V) and V(IV) ions, while the anode electrolyte contains solutions of V(III) and V(II) ions.

3. Electrolyte Storage and Supply Unit: Includes electrolyte tanks and pumps. Tanks store positive and negative electrolytes, while pumps circulate electrolytes from tanks to the stack for reactions, maintaining closed-loop flow.

4. Management and Control Unit: Responsible for monitoring and regulating the battery's operational status, such as charging/discharging processes, electrolyte flow rate, temperature, etc., to ensure safe and efficient battery operation.

Working Principle of Flow Batteries

Flow battery technology pushes anode and cathode electrolytes into the stack through separate pipelines. Through changes in the valence state of active elements, it achieves charging and discharging by converting between electrical and chemical energy. Unlike traditional ion storage batteries, flow batteries are a type of secondary energy storage battery technology that uses active chemical substances stored in liquid electrolytes. Not only do they differ structurally, but energy is stored within the anode and cathode electrolytes themselves.

Classification of Flow Batteries

1. Vanadium Redox Flow Battery

Principle: Utilizes the conversion between different oxidation states of vanadium ions to achieve energy storage and release.

Features: High technological maturity, excellent safety, long cycle life, but relatively high cost.

Applications: Suitable for large-scale energy storage, such as grid peak shaving and renewable energy storage.

2. Zinc-Iron Flow Battery

Principle: Achieves charging and discharging through redox reactions between zinc and iron.

Features: Low cost, high energy density, excellent safety, and environmental friendliness.

Applications: Suitable for large-scale energy storage, particularly in commercial and industrial sectors.

3. Iron-Chromium Flow Battery

Principle: Stores and releases electrical energy through changes in the valence states of iron and chromium ions.

Features: Wide operating temperature range; abundant and stable electrolyte raw materials.

Applications: Suitable for long-duration energy storage scenarios, such as grid storage and renewable energy grid integration.

4. Zinc-Bromine Flow Battery

Principle: Converts electrical energy through redox reactions between zinc and bromine.

Features: Relatively high energy density and lower cost, but bromine exhibits strong corrosivity.

Applications: Suitable for distributed energy storage systems and grid peak shaving.

5. Aqueous Organic Flow Battery

Principle: Uses aqueous solutions of redox-active organic molecules as electrolytes.

Features: High reaction activity, strong molecular design flexibility, safety and environmental friendliness, high overall energy efficiency.

Applications: Suitable for large-scale energy storage with significant development potential.

Advantages of Flow Batteries

1. High Safety: Electrolytes are stored in external containers outside the battery stack, eliminating combustion and explosion risks and meeting energy storage safety requirements.

2. Extended Cycle Life: Typically exceeds 15,000 charge-discharge cycles, far surpassing lithium batteries' 3,000-6,000 cycles, with a service life exceeding 20 years.

3. Scalability: Capacity can be easily expanded by adding electrolyte or enlarging storage tanks without complex disassembly procedures.

4. Environmentally Friendly: Flow batteries primarily utilize recyclable materials, minimizing environmental impact.

5. Flexible Site Selection: Fully automated and enclosed operation allows for greater site flexibility, simplified maintenance, and low operational costs.

6. Abundant Raw Materials: Raw materials for flow batteries are plentiful and cost-effective domestically, eliminating import dependency and offering exceptional cost-effectiveness.