Research Pushes Vanadium Flow Battery Boundaries

Energy storage is a pressing challenge in renewable energy. Without the ability to reliably store large amounts of energy for extended periods, the dream of a fully renewable grid may never realized.

 

Flow battery storage in Dalian, China. Image used courtesy of DICP

 

Vanadium flow batteries (VFBs) are a promising technology that offers scalable and highly efficient energy storage. A group from the Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, has made a significant breakthrough in VFB technology.

 

A Primer on Vanadium Flow Batteries

Vanadium flow batteries are rechargeable flow batteries using vanadium ions in various oxidation states to store chemical potential energy. The vanadium flow battery stack, a critical component of these batteries, plays a central role in their operation and efficiency.

A vanadium flow battery stack is composed of multiple individual cells. Each cell consists of two compartments separated by an ion exchange membrane. In these compartments, vanadium-based electrolyte solutions flow in separate circuits. The two solutions are typically a mix of vanadium ions in different oxidation states (V2+/V3+ in the negative and V4+/V5+ in the positive half-cell). The ion exchange membrane allows for ion transfer while preventing the mixing of the two electrolyte solutions.

 

A vanadium redox flow battery. Image used courtesy of Lerdwithayaprasit et al.

 

During operation, an electrochemical reaction transfers electrons and ions. When the battery is discharging, vanadium ions in one solution give up electrons (oxidation), and ions in the other solution gain electrons (reduction). During charging, an external energy source reverses this process, restoring the ions to their original oxidation states.

 

VFBs have the advantages of

• Long life cycle: Vanadium flow batteries can last thousands of cycles with minimal degradation, making them suitable for long-term applications.
• Scalability: Size determines the power capacity of a vanadium flow battery, whereas the amount of electrolyte determines the energy capacity. This distinct separation allows for greater scalability and flexibility in design.
• Safety and stability: VFBs are relatively safe as they use a single element (vanadium) in different states, reducing the risk of cross-contamination and the battery's stability over time.
• High efficiency: These batteries can achieve high energy efficiency, especially in applications where they are frequently charged and discharged.

For these reasons, they have garnered much attention from engineers in renewables and energy storage.

 

A 70 kW Vanadium Flow Battery Stack

A group from DICP has developed a vanadium flow battery stack with a power density of 70 kW, substantially surpassing the traditional 30 kW-level stacks. The research focused on enhancing the stack's design to increase power density without a corresponding increase in size. This was achieved through a series of sophisticated improvements.

They developed highly selective weldable porous composite membranes, enhancing the battery's efficiency and durability. These membranes are integral to the battery's electrochemical process, facilitating improved ion transfer and reducing energy loss. Additionally, the team introduced weldable, highly conductive bipolar plates into the stack, which improved the stack’s conductivity and enhanced overall performance.

 

The 70 kW-level vanadium flow battery stack. Image used courtesy of DICP

 

The team also optimized the design by implementing a short flow path and an ultra-thin battery structure. This approach minimized internal resistance and maximized energy efficiency, leading to a more compact and effective stack. Furthermore, the group refined the flow channels to have low flow resistance and high distribution uniformity, ensuring a more consistent electrolyte distribution and boosting the battery's operational stability and efficiency.

The result is a stack with a power density of 120 kW/m^3, an 81% energy efficiency, and stable operation after 1,2000 cycles. The stack is modular, allowing it to be upgraded up to 500 kW without greatly increasing the size of the power units. 

 

Enabling Renewables

A fully renewable grid needs reliable and scalable energy storage systems. Through their new research, the DICP team has made a significant step toward this future with a more scalable and efficient long-term energy storage solution. The team’s 70 kW VFB stack could make renewable energy more feasible and affordable on a large scale.