1. Definition and principles of flow batteries
Flow battery is a new type of storage battery, which is an electrochemical conversion device that uses the energy difference in the oxidation state of certain elements (usually metals) to store or release energy. Different classes of flow batteries have different chemistries, including vanadium, which is most commonly used, and zinc-bromine, polysulfide-bromine, iron-chromium, and iron-iron, which are less commonly used.
According to the different active substances in the electrochemical reaction, aqueous/hybrid flow batteries are further divided into iron-chromium flow batteries, vanadium redox flow batteries, zinc-based flow batteries, iron-based flow batteries, etc.
2. Different types of flow batteries
① Iron chrome flow battery
The iron-chromium flow battery is the earliest proposed flow battery technology. At present, iron-chromium flow batteries still have some technical problems, such as:
- The hydrogen evolution problem of the anode reduces the energy efficiency of the battery;
- The cross-contamination of the cathode and anode will reduce the battery capacity and efficiency, resulting in the need for high selectivity of the ion-conducting membrane used, and the current cost of importing perfluorosulfonic acid membranes is relatively high;
- The redox property of chromium is poor, and the optimal working temperature of the battery is relatively high.
② Vanadium redox flow battery
Vanadium redox flow battery is currently the most commercialized and technologically mature flow battery technology. It has the characteristics of high energy efficiency, long cycle life, and high power density, and is suitable for large and medium-sized energy storage scenarios. However, for vanadium redox flow batteries, the cost of vanadium electrolyte accounts for about 60% of the battery cost, which greatly increases the initial investment threshold.
③ Zinc bromine flow batteries
The cathode of the zinc-bromine flow batteries adopts Br-/Br2 electric pair, and the anode adopts Zn2+/Zn electric pair. When the cathode is charged, Br- is oxidized to Br2 simple substance, and the Br2 simple substance will combine with related substances in the solution and settle at the bottom of the electrolyte solution. Therefore, the zinc-bromine flow batteries are single deposition flow batteries. Zinc-bromine flow batteries are a more successful commercialized flow battery technology besides all-vanadium flow battery.
In terms of application, due to its excellent modular design, low cost, and high safety features, early zinc-bromine flow batteries were more used in user-side arbitrage and improving power supply stability, and the scale of use was small. In recent years, the rapid development of renewable energy has led to the large-scale application of zinc-bromine flow batteries on the power generation side and the grid side.
④ Zinc-iron flow battery
Alkaline zinc-iron flow batteries were proposed in 1981, followed by neutral and acidic zinc-iron flow batteries, but the latter two have not reached the level of engineering applications. Alkaline zinc-iron flow batteries have a high open circuit voltage, and can be cycled at high current densities for a long time when combined with porous membranes and porous electrodes. Acidic zinc-iron flow batteries make full use of the advantages of high solubility and stable electrochemical performance of iron ions in acidic media, but the anode side is greatly affected by pH.
Neutral zinc-iron flow batteries have gradually attracted attention due to their non-toxic and harmless environment and mild environment. Combining with porous membranes can effectively reduce battery costs. No matter what kind of zinc-iron flow battery, there are zinc dendrites and limited surface capacity on the anode side, which has become a problem that must be considered in the industrialization of zinc-iron flow batteries.
Technically, zinc-iron flow batteries, like other deposition batteries and zinc batteries, face the problem that zinc dendrites cannot be completely decoupled from power and capacity, and their negative electrode surface capacity is low. At the same time, as a relatively new type of flow battery, zinc-iron flow batteries have an immature industrial chain of ion-conducting membranes and other related components, which greatly restricts their commercial promotion and application.
⑤ Zinc-nickel single flow battery
Zinc-nickel single flow batteries combine the advantages of zinc-nickel secondary battery and flow battery. Similar to the structure of the zinc-bromine single-flow battery, cathode and anode of the zinc-nickel single-flow battery use the same electrolyte, no ion exchange membrane is required, and the structure is simple.
In terms of application, zinc-nickel flow batteries are still in the commercial demonstration stage. The overall performance of the zinc-nickel flow batteries in the laboratory stage is better, and preliminary application demonstrations have also been carried out. However, due to the rapid rise in nickel prices, the price competitiveness of zinc-nickel single-flow batteries has rapidly weakened, and the development and deployment of technologies are at a relatively stagnant stage.
At the technical level, further research is needed on the short circuit of the battery and the reduction of battery life caused by zinc dendrites and accumulation. Zinc-nickel single flow batteries have low area capacity of the cathode and anode and cannot completely decouple power and capacity, and the problems that the cathode of the battery needs high-cost sintered nickel to ensure a long life have yet to be resolved.
⑥ All iron flow battery
Compared with vanadium, iron has higher utility and lower cost. All-iron flow batteries are divided into acidic and alkaline systems, and acidic all-iron flow batteries are relatively mature in commercial development.
The technical problems of the all-iron flow battery mainly lie in the anode hydrogen evolution reaction similar to the iron-chromium flow battery and the need to suppress the formation of iron hydroxide precipitates.
These problems can greatly reduce the operating efficiency of the battery, reduce the battery capacity, and risk clogging the ion-conducting membrane. There are few reports on the research and commercial development of this system of flow batteries in China.
⑦ High-performance zinc-based flow batteries
In addition, in 2022, based on a deep understanding of the redox reaction mechanism of iodine, researchers proposed an iodine cathode solution based on polyiodine complexes. The capacity of the iodine cathode is effectively unlocked, and the high-energy and long-term cycle operation of the zinc-iodine flow battery is realized.
The discharge capacity of the improved zinc-iodine flow battery has been significantly increased by 58%, and it can cycle stably for 600 cycles at 70% energy efficiency, which provides a new way for the development of high-performance zinc-iodine flow battery.
⑧ Zinc-air flow battery
During the charging process of zinc-air flow batteries, oxygen evolution reaction occurs at the cathode, and zinc ions will be deposited as metal zinc on the metal anode. During the discharge process, an oxygen reduction reaction occurs at the cathode, and the zinc on the anode dissolves and is stored in the electrolyte in the state of zinc ions.
Technically, zinc-air flow batteries, like most other zinc flow batteries, also face the problem of zinc dendrites. At the same time, it also faces the problems of low current density and incomplete development of oxygen evolution and oxygen reduction double-effect catalysts.
In the long run, vanadium redox flow batteries in vanadium battery companies in China will be a substitute for lithium batteries in the direction of energy storage. Vanadium redox flow batteries are currently the most widely used flow battery technology, which has the advantages of being suitable for large-scale energy storage, high energy conversion efficiency, long cycle life, and convenient charging.
Moreover, the power and capacity of the battery system are independent of each other, which is suitable for large-scale energy storage scenarios. At the same time, the vanadium redox flow battery has good charge and discharge performance and high energy conversion efficiency. From the perspective of downstream application scenarios, the value of liquid flow batteries is mainly used in power grid peak regulation, emergency power generation devices, electric vehicle power supplies and other fields.
At the policy level, with the establishment of China's dual carbon targets, the adjustment of energy structure is accelerating, the installed capacity of new energy power generation is increasing, and the demand for energy storage is also increasing accordingly. At the same time, the Chinese government has also issued a series of policies to encourage the development of new energy storage technologies.