1. Vanadium redox flow battery and its main components
① Flow battery
Flow battery, also known as redox battery, belongs to a secondary energy storage battery processing technology in which active chemical substances are stored in a liquid electrolyte. Different from solid state batteries, the cathode and anode of flow batteries are stored in a storage tank outside the battery in the form of an electrolyte solution. According to the different active materials, flow batteries can be divided into vanadium redox flow battery, iron-chromium flow batteries, and zinc-bromine flow batteries. Among them, the vanadium redox flow battery technology is currently the most mature and most industrialized flow battery technology, and is already in the early stage of industrialization.
② The main components of flow batteries
From the functional division, flow batteries can be divided into energy units (electrolyte, storage tanks), power units (electric pile), auxiliary units (transportation, temperature control, power electronic equipment, etc.). Among them, the electrolyte and electric pile are the components with the highest technical content and the highest cost in the flow battery system.
Electrolyte is the storage medium for electrical energy in flow batteries, and its volume and concentration determine the maximum energy that a flow battery energy storage system can store. Therefore, the electrolyte is the core material in the flow battery, which directly affects the performance and cost of the energy unit. The electrolyte consists of three parts: active material, matrix and additives. In an vanadium redox flow battery, the active material of the electrolyte is vanadium sulfate, the matrix is generally an aqueous solution of sulfuric acid, and the additives are various organic and inorganic complexing agents.
● Electric pile
Similar to a hydrogen fuel cell, the stack of an vanadium redox flow battery is a place for electrochemical reactions, and its performance will directly affect the overall performance of the system. Vanadium redox flow battery is generally composed of multiple electric piles in parallel. The main components of the electric pile of the vanadium redox flow battery include ion exchange membranes, electrodes, bipolar plates and other key components.
2. Advantages and disadvantages of vanadium redox flow battery
① Main advantages
● Higher security
The electrolyte of the vanadium redox flow battery is an acidic aqueous solution of vanadium ions, which operates at normal temperature and pressure without the risk of thermal runaway. At the same time, it is not flammable and does not generate heat accumulation. The cathode and anode active materials react mildly and have intrinsic safety. The current mainstream lithium-ion batteries use highly flammable organic electrolytes, and there is a risk of deflagration in the event of thermal runaway, which can only reduce the probability but cannot be absolutely avoided.
● Long life and low attenuation
Compared to other batteries, vanadium flow batteries theoretically have extremely long cycle life:
- The reaction process is only the valence state change of vanadium ions, does not involve liquid-solid phase transition, and there is no problem of electrolyte cross-contamination;
- The electrode material does not participate in the electrochemical reaction during the charging and discharging process, and is an inert electrode. The materials such as electrodes and bipolar plates have good stability and do not involve replacement;
- The capacity fading caused by the valence imbalance of vanadium ions caused by the migration of the electrolyte between the cathode and anode and side reactions can be recovered by low-cost physical and chemical means.
Therefore, compared with about 5000 cycles of lithium iron phosphate battery, the vanadium redox flow battery has obvious characteristics of long life and low attenuation. At the same time, after the battery life expires, the vanadium electrolyte solution can be recycled and reused. The cost of the electrolyte solution accounts for about 40% of the total cost of the energy storage system. After the energy storage system is scrapped, the residual value is relatively high. To sum up, the characteristics of long life and high recyclable value make the full life cycle cost of vanadium redox flow batteries lower than the cost of lithium batteries.
● The upper limit of energy storage is high and controllable
The power cell stack and the energy cell electrolyte of the vanadium redox flow battery are two independent systems. The battery capacity depends on the amount of external active solution. By increasing the volume of the electrolyte, the energy storage capacity of the vanadium battery can be arbitrarily increased. Compared with the lithium battery, its scalability is very large and the adjustment is easy.
● Vanadium resources are relatively abundant
China is a major producer and consumer of vanadium metal, and its resources are independent and controllable. According to statistical data, China's vanadium reserves are estimated to be about 9.5 million tons by the end of 2021, accounting for 40% of the global reserves. In 2021, China's vanadium production will reach 73,000 tons, accounting for 2/3 of global production. Therefore, China has a strong ability to control vanadium resources in terms of reserves and production capacity. In addition, the vanadium electrolyte can be recycled or recycled for a long time. Therefore, China's vanadium resources can fully support the large-scale development of all-vanadium redox flow batteries.
● Fast response time
During the operation of the vanadium redox flow battery, the charge-discharge state switching takes only 0.02 seconds, and the response speed is 1 millisecond.
● Environmentally friendly and resource recyclable
The vanadium redox flow battery does not involve pollution and emissions during operation, and the electrolyte can be recycled. It is a green and environmentally friendly form of energy storage. In vanadium redox flow batteries, vanadium exists in the acidic aqueous solution in the form of ions. It does not exist in the form of vanadium oxide, which is corrosive to a certain extent but non-toxic. At the same time, it is closed and operated during the working process, which will basically not cause harm to the environment and human body.
In addition, from the perspective of the whole life cycle, the recycling and processing of various materials of the lithium battery energy storage system after the expiration of its life is relatively difficult. The vanadium electrolyte of the vanadium redox flow battery can be recycled for a long time in the battery field or extracted into other markets such as steel and alloys. The processing of key components of the stack, as well as pipelines, valves and pumps, is also simpler and has no environmental burden, so it is superior to lithium batteries in terms of recovery costs and pollution emissions.
② Main disadvantage
● High cost of initial installation
The biggest disadvantage of vanadium redox flow battery is that the initial installation cost is relatively high. According to the calculation of the vanadium redox flow battery project that has disclosed the specific investment amount, the total investment cost of the project is 3.8-6.0 RMB/Wh. Even if the price of lithium carbonate has risen sharply since the beginning of this year, the cost of lithium-ion batteries has increased, and the cost of energy storage lithium battery companies projects is only 1.5-1.8 RMB/Wh, less than half of that of vanadium redox flow battery.
Higher initial installation cost limits the promotion of vanadium redox flow battery. Although the initial installation cost of vanadium redox flow battery is relatively high, due to its long life and low attenuation characteristics, if it is calculated based on the number of cycles in the full life of 20,000, its full life cost of electricity is lower than that of lithium batteries.
● Low energy density
The mass and volume of vanadium redox flow batteries are huge, so their specific energy and specific power are much lower than other battery systems. At present, the energy density of vanadium redox flow batteries is less than 50Wh/kg, which is quite different from the energy density of lithium iron phosphate of 160Wh/kg.
● Environmental requirements are relatively high
Because the vanadium in the positive electrolyte of the vanadium redox flow battery is easy to precipitate and block the flow channel at high temperature (above 45°C), and the electrolyte solidifies at low temperature (below the freezing point of the electrolyte). Therefore, the general operating temperature of vanadium redox flow battery is required to be between 0 and 45 °C. The above-mentioned higher environmental requirements also limit the use of vanadium redox flow battery.
3. Vanadium redox flow battery cost reduction path
In the cost structure of vanadium redox flow battery, the electrolyte and stack are the most important parts. Among them, the electrolyte accounts for about 40% of the cost, the stack accounts for 35%, and other components account for 25%. With the development of vanadium redox flow battery industrialization, referring to the path of rapid cost reduction of lithium battery cost after commercial application, it can be predicted that the cost of vanadium redox flow battery will still have a large room for reduction after mass production.
● Technological progress drives down costs
Vanadium redox flow batteries are still in the early stage of commercialization, and there is a large room for cost reduction brought about by technological progress. For example, in the field of electrolyte, the theoretical storage of 1kWh of electrical energy requires 5.6kg of V2O5, while the current utilization rate of the electrolyte is about 70%, and about 8kg of V2O5 is needed to store 1kWh of electrical energy. If the utilization rate of the electrolyte is increased to 80%, in fact, only 7kg of V2O5 is needed to store 1kWh of electrical energy. At the same time, advances in stack design and membrane materials will also drive down the cost of stacks.
● Large-scale production will bring cost reductions
At present, the key components of vanadium redox flow batteries are mainly produced or customized by manufacturers. Subsequent technological progress and large-scale mass production will drive the cost to continue to decline.
On the one hand, technological progress in key components and system design will drive the continuous cost reduction and efficiency improvement of vanadium redox flow battery stacks. On the other hand, the overall scale of the vanadium redox flow battery industry is limited at this stage, and the key components are mainly produced by vanadium redox flow battery manufacturers or customized in small batches. With the subsequent increase in production scale, the overall manufacturing cost is expected to drop significantly.
Localization in China will bring cost down
At present, some key components of vanadium redox flow batteries, such as ion exchange membranes, are still imported from abroad and are expensive. In the future, with the improvement of localization and scale of key components, the cost will also be greatly reduced.
4. Application of vanadium redox flow battery
Due to the above characteristics of the vanadium redox flow battery, it is not suitable for new energy vehicles, nor is it currently suitable for energy storage on the side of wind and solar power. The characteristics of large capacity, high safety and long life make it suitable for long-term energy storage fields such as grid side and independent energy storage power stations.
As new energy sources such as wind and solar gradually become the main body of the power system, its volatility and intermittent impact on the power grid will become more and more obvious. The future energy storage system needs to consider the fluctuation of new energy output during the day and even between seasons, so long-term energy storage will become an indispensable part of the future power system.
At present, the vanadium redox flow battery is still in the early stage of industrialization, and its proportion in the energy storage field is relatively low. With the launch of several large-scale vanadium redox flow battery energy storage projects and the maturity of China's energy storage business model, it is expected that the industrialization of vanadium redox flow batteries will accelerate. By 2025, the annual installed capacity is expected to reach more than 10GW.