The Stack Structure Of Vanadium Flow Battery

The Stack Structure Of Vanadium Flow Battery

The vanadium liquid flow battery energy storage system is mainly composed of a battery stack, an electrolyte storage and supply unit, a battery management system, a power conversion system, an energy management system, etc. The battery stack is the most critical component of a vanadium liquid flow battery (VRFB) and determines the power of the VRFB.

1. Basic structure

The VRFB stack is usually assembled from several or dozens of single cells in the form of a filter press. Its main components include: end plates, guide plates, current collectors, bipolar plates, electrode frames, electrodes, ion conduction membranes and sealing materials. Generally, the single cells are connected in series, with the positive and negative electrodes between two adjacent cells connected by bipolar plates, and the current collectors output voltage at both ends of the stack, thus forming a VRFB stack with a certain voltage level. The working current of the stack is determined by the actual operating current density and electrode area, the number of single cells in series in the stack determines the output voltage and power of the stack, and the rated power density of the stack is determined by the rated working current density and the voltage of a single cell.

2. Distribution of electrolyte
For VRFB, the flow distribution of electrolyte inside the battery is a key factor affecting the performance of the battery stack. The electrolyte flows into the inlet pipeline of the battery stack, enters the common pipeline, and flows into the branch flow channels in the electrode frame of each single cell in parallel one by one, then flows through the electrode to participate in the electrochemical reaction, and then flows out of the battery stack through the outlet branch flow channel and the common pipeline. Among them, the factor that has the greatest impact on the performance of the battery stack is the flow of electrolyte in the branch pipeline in the electrode frame and the electrode. If the  electrolyte in the electrode is unevenly distributed, it will produce a large concentration polarization, reducing the working current density of the battery stack.

The common pipeline is responsible for connecting each battery in the battery stack and plays the role of evenly distributing the electrolyte to each battery. Therefore, the selection of its flow form and the design of its structural parameters directly affect the uniformity of the electrolyte distribution in the electrode, thereby affecting the voltage uniformity of the battery stack, and further affecting the performance, stability and service life of the battery stack.

3. Sealing materials and structures
VRFB uses an ion-conducting membrane to separate the electrolytes on the positive and negative sides. Sealing technology is required in the battery stack to prevent the electrolytes on the two sides from penetrating each other, reduce the coulombic efficiency and energy storage capacity of the battery stack, and improve operational safety. At the same time, sealing technology is also required to prevent the electrolyte from leaking to the outside of the battery stack. The commonly used sealing material for VRFB is rubber material, which is required to have excellent corrosion resistance, chemical stability and elasticity.

4. Battery stack integration
Bipolar plates, seals, electrode frames, electrodes, ion-conducting membranes, electrodes, electrode frames, seals, etc. are stacked together to form a single cell of VRFB. Several or dozens of single cells are stacked together in the manner of a filter press and current collectors and end plates are installed on both sides to assemble a VRFB battery stack. The battery stack assembly process is mainly divided into two steps:

① Positioning. The battery stack components increase significantly with the increase in the number of single cells. A 30 kW battery stack is usually composed of about 50 single cells, and there are hundreds of components. Assembling these components one by one according to the positioning structure can avoid misalignment to ensure uniform distribution of the electrolyte and prevent leakage.

② Assembly pressure uniformity. When the press is pressurized, the parallelism of the pressure surface and the end plate and the pressurization speed are extremely important. Poor parallelism or too fast running speed will cause deformation of the battery stack and even component ejection.