(1) Working principle of vanadium battery
Flow storage systems are often referred to as redox flow energy storage systems (Redox-Flow Cell or Redox-flow Cell for Energy Storage Systems, flow storage power stations or flow batteries), developed by Thaller LH (NASA Lewis Research Center, Cleveland, United States) proposed an electrochemical energy storage principle in 1974.
As shown in Figure 1, the flow storage system consists of a stack, an electrolyte solution, and an electrolyte solution storage and supply system, a system control system, and a charge-discharge system. The core of the flow storage system is the electric stack, which consists of dozens or even hundreds of cells for oxidation and reduction reactions. The single cells that realize the charging and discharging process are formed in series or in parallel according to specific requirements, and the structure is similar to that of the fuel cell stack. Different from the active material in the battery that is usually contained in the positive and negative electrodes of the battery, the positive and negative redox active materials in the flow battery are respectively dissolved in the electrolyte solution installed in the two liquid storage tanks, each with a pump. Pass the solution through the flow battery. The electrolyte circulates in the stack, and reduction and oxidation reactions take place on the porous electrodes on both sides of the ion-exchange membrane, respectively. During the reaction, in order to form a closed loop inside the battery and maintain the charge balance of the solution on both sides of the membrane, there must be an ion synchronously migrated from one side of the membrane to the solution on the other side through the ion exchange membrane. Electrochemical systems composed of different active substances have their own specific reaction processes, reaction products and migrating ions.
The vanadium battery uses vanadium ions of different valences dissolved in a certain concentration of sulfuric acid solution as the active material for the positive and negative electrodes. The positive and negative electrodes of the battery are separated into two independent chambers by an ion exchange membrane. Usually, the positive active pair of vanadium battery is VO2+/VO+, and the negative electrode is V2+/V3+. The reactions that take place on the electrodes are as follows:
Positive: VO2++ H2O-e→VO+2 +2H+
Overall battery reaction: VO2+ +H2O+ V3+→VO+2 +V2+ +2H+
(2) Charge and discharge characteristics of vanadium batteries
1) Typical charge-discharge curve. A single battery is charged and discharged at a constant current, and its charge-discharge curve is shown in Figure 2. The horizontal axis is the charge and discharge time related to the capacity, and the vertical axis is the port voltage of the battery. From the figure, we can see that the charge-discharge curves are relatively flat, indicating that the charge-discharge characteristics are relatively good.
2) The effect of charge-discharge current density on the performance of vanadium batteries. Figure 3 shows the relationship between the coulombic efficiency and energy efficiency of vanadium batteries and the charge-discharge current density. From the figure we can see that:
With the increase of the charge and discharge current density, the coulombic efficiency of the battery increases. This is because the vanadium battery has a certain self-discharge reaction during the charge and discharge process. The smaller the losses, the higher the coulombic efficiency of the battery.
With the further increase of the charging current density, serious side reactions occur in the battery, resulting in a decrease in the energy efficiency of the battery. This is mainly because the voltage efficiency of the vanadium battery increases with the increase of the charge and discharge current density, and the polarization of the battery increases, resulting in a decrease in the voltage efficiency of the battery, thereby reducing the energy efficiency of the battery.
3) The effect of charge-discharge current density on the capacity of vanadium batteries. The relationship between the charge-discharge capacity and the current density of the vanadium battery is shown in Figure 4. From the figure, we can know that:
The charging capacity of vanadium batteries decreases with the increase of current density, which is mainly due to the increase of charging current density, which shortens the charging time and reduces the capacity loss caused by battery self-discharge; after the charging current density reaches a certain value, the charging capacity The amount of reduction is getting smaller and smaller. This is due to the intensified side reactions of the electrodes, which make the battery easily reach the upper limit of the charging voltage.
The discharge capacity decreases with the increase of the current density, because the electrochemical polarization increases with the increase of the discharge current density, which causes the discharge voltage to reach the lower discharge limit faster, and therefore the discharged capacity of the battery is lower.
(3) Characteristics of vanadium batteries
Vanadium batteries are an excellent energy storage system with many unique advantages:
1) The power and capacity are large, and the rated power and rated capacity are independent: the power depends on the battery stack, and the capacity depends on the electrolyte. The purpose of increasing the battery capacity can be achieved by increasing the amount of electrolyte or increasing the concentration of the electrolyte; by increasing the number of monolithic batteries and the electrode area, the power of vanadium batteries can be increased. Currently, the power of vanadium batteries in commercial demonstration operation in the United States Has reached 6MW.
2) High conversion efficiency: can reach 70%~80%.
3) During charging and discharging, the battery only has a liquid phase reaction, and the complex solid phase change of ordinary batteries does not occur, so the electrochemical polarization is small.
4) Long service life: Since the positive and negative active materials of vanadium batteries only exist in the positive and negative electrolytes, respectively, there is no phase change common to other batteries during charging and discharging, and 100% deep discharge can be performed without damaging the battery. Long service life; the electrolyte is sealed and stored in two different storage tanks when the battery is not in use, and there is no problem of self-discharge and electrolyte deterioration that often exist in ordinary batteries. In the 4MW vanadium battery project supporting a 32MW wind farm in Hokkaido, Japan, the vanadium battery energy storage system has been charged and discharged more than 270,000 times, and it is still running normally, far higher than the lead-acid battery’s more than 1,000 times.
5) Fast response speed: The vanadium battery stack is filled with electrolyte and can be started in an instant. It only takes 0.02s to switch between charge and discharge states during operation, and the response speed is 1ms.
6) The theoretical charge-discharge time ratio is 1:1 (actual operation is 1.5~1.7), which supports frequent high-current charge and discharge. In-depth charge and discharge have little effect on battery life. The positive and negative active materials of the battery are in liquid phase in the charge and discharge state. , there is no risk that the separator will be punctured by the growth of dendrites on the electrodes of batteries such as nickel-metal hydride batteries and lithium-ion batteries, resulting in a short circuit of the battery.
7) The self-discharge rate is low and can be stored for a long time.
8) High safety and reliability: Vanadium battery has no potential explosion or fire danger, even if the positive and negative electrolytes are mixed, there is no danger, but the temperature of the electrolyte increases slightly.
9) Low cost: the battery has a simple structure, other materials except the ion membrane are cheap, and the replacement and maintenance costs are low.
10) The electrolyte can be recycled, no harmful gas is generated during the electrode reaction process, and no pollution to the environment. It is a new type of environmentally friendly battery.