Research progress and industrialization direction of zinc bromide flow batteries
Classification：Industrial News
- Author：ZH Energy
- Release time：May-09-2024
【 Summary 】In 2023, the global renewable energy installed capacity increased by 50% compared to the previous year, reaching 510 gigawatts, with solar photovoltaics accounting for about three-quarters. By early 2
The International Energy Agency recently released a report showing that in 2023, the global renewable energy installed capacity increased by 50% compared to the previous year, with an additional installed capacity of 510 gigawatts, and solar photovoltaics accounting for about three-quarters. By early 2025, renewable energy, including photovoltaics, will become the world's primary source of electricity. According to the official website of the National Energy Administration, as of the end of November 2023, the cumulative installed capacity of solar power generation in China was about 560 million kilowatts, a year-on-year increase of 49.9%; The main power generation enterprises completed an investment of 320.9 billion yuan in solar power generation, a year-on-year increase of 60.5%. With the development of green energy such as photovoltaics, the development of supporting energy storage technologies has also become a top priority. Among them, flow batteries have received widespread attention due to their high safety, adjustable power and capacity design, etc. In the introduction of liquid flow battery technology, some development routes have been popularized, and this time we will focus on zinc bromine liquid flow batteries (ZBFB). 参考资料 [11]Wang C, Li X, Xi X, et al. Bimodal highly ordered mesostructure carbon with high activity for Br2/Br redox couple in bromine based batteries[J]. Nano Energy, 2016, 21(3): 217-227.产品系列：
1. Basic Principles
The basic principle of a zinc bromine flow battery is as follows: during charging, the zinc ions in the left anode liquid are reduced to two electrons and adsorbed onto the anode plate; The bromine ions in the cathode solution on the right lose electrons and are oxidized, becoming elemental bromine. The elemental bromine is immediately captured and fixed by quaternary ammonium salts in the electrolyte, forming a complex precipitate and collected in the storage tank. During discharge, the zinc on the negative electrode surface dissolves, and at the same time, the complex bromine at the bottom of the storage tank is re pumped into the circulation circuit by the ion pump on the cathode side and dispersed, transforming into bromine ions. The electrolyte returns to the initial state of zinc bromide. The basic principle is shown in the following figure:
The main active component of the electrolyte is zinc bromide aqueous solution, and unlike all vanadium flow batteries, the ratio of zinc bromide aqueous solution used for both positive and negative electrodes of the electrolyte is completely consistent, so there will be no cross contamination of the electrolyte in zinc bromide flow batteries. The positive and negative electrolytes are independent of each other and circulate independently. At present, the electrolyte technology of zinc bromide flow batteries is relatively mature and has high safety performance, which is due to the difficulty of heat transfer and high safety characteristics of zinc bromide itself.
As an important component of zinc bromide flow batteries, battery separator materials can improve the mechanical strength of the membrane and prevent zinc dendrite perforation by selecting appropriate membrane materials and optimizing factors such as pore size and thickness. In zinc/bromine flow battery systems, perfluorosulfonic acid cation exchange membranes produced by DuPont in the United States are generally used, with Nafion membranes as a representative. In addition to the conventional Nafion membrane, the currently studied microporous membrane (porous membrane) materials mainly serve to separate the electrolyte of the positive and negative electrodes of the flow battery, and allow some ions to pass through the membrane while blocking macromolecular substances such as bromine complexes, thereby improving battery self discharge and ensuring sustained high Coulombic efficiency of the battery. And compared to traditional Nafion membranes, microporous membranes have significant cost advantages.
2. Research direction
At present, research on zinc bromine flow batteries mainly focuses on increasing the reaction contact area while minimizing the concentration of bromine ions passing through the separator, reducing zinc dendrites, and reducing self discharge. This is also the main problem that zinc bromine flow batteries need to solve. In addition, the improvement of the internal structure of batteries is also an important direction, mainly including improving electrode materials and structures, designing flow channel structures, and improving membrane materials and structures.
Zinc dendrites are a major problem in zinc bromide batteries, caused by the non-uniformity of the zinc ion charging and deposition process. Due to the electrochemical polarization of the electrode surface, uneven deposition can cause the growth of zinc dendrites, ultimately piercing the separator and causing internal short circuits in the battery. The main methods to inhibit the formation of zinc dendrites include adding zinc dendrite inhibitors to the electrolyte, selecting appropriate battery separators, preparing new electrode materials, and optimizing the electrolyte circulation rate.
3. Research progress
From the perspective of additives, research by Professor Wang Jianming's team at Zhejiang University has shown that the simultaneous use of Bi3+and tetrabutylammonium bromide as electrolyte additives has a significant inhibitory effect on the dendritic growth of the zinc side electrode, and has almost no effect on the electrochemical reaction of the zinc side electrode [2]. The main improvement principle is that Bi3+is reduced as an electroplating layer substrate before zinc deposition, and this metal substrate effect improves the conductivity of the electrode, thereby promoting the uniform deposition of zinc on the electrode current collector, thereby achieving the effect of suppressing dendrites by improving the current distribution [3]. Studies have shown that adding polysorbitol ester (P20) to ZnBr2 solution can improve the uniformity of zinc deposition and de coating on the entire electrode surface, thereby increasing the current efficiency of the battery for multiple cycles. This is mainly because P20 promotes the uniform mixing of the aqueous phase and polybrominated compound phase, allowing the Br2/2Br redox reaction to occur uniformly on the electrode surface, thereby promoting the uniform deposition of zinc on the electrode.
The stability and durability of zinc bromide electrolytes have also received attention from some researchers. Donghyeon Kim et al. investigated the effects of zinc chloride, potassium chloride, lithium perchlorate, and sodium perchlorate as supporting electrolytes on the electrochemical behavior of the system. And it was found that adding a small amount of lithium perchlorate to 2 M ZnBr2 effectively improved the conductivity of the solution and enhanced the electrochemical stability of the zinc bromide electrolyte [4].
The optimization of electrode materials is also an important means. Zhang Huamin et al. designed and developed highly ordered mesoporous carbon electrode materials, providing more active sites for the electrochemical reaction of 2Br -/Br2. Due to the matching reaction rate between the bromine side electrode and the zinc side electrode, the formation of zinc dendrites is effectively suppressed [5]. In addition, the situation of zinc dendrites can also be improved by optimizing the electrolyte circulation rate. Yang's experiment shows that when the electrolyte circulation rate is 100 mL min-1, the grain size of zinc crystals is significantly smaller than that of the electrolyte circulation rate of 50 mL min-1, and the Coulombic efficiency of zinc bromide batteries is as high as 95.8% [6]. In addition, the charging method can also suppress the growth of zinc dendrites. During battery charging and reverse charging, the diffusion and mass transfer direction of Zn2+in the solution is opposite. The uneven deposition of zinc caused by electrode polarization is greatly improved during reverse charging, thereby inhibiting the formation of zinc dendrites; In addition, during reverse charging, due to the reverse effect of the current, zinc dendrites dissolve first, which inhibits the formation of dendrites [7].
There are also many studies on positive electrode materials. For several common commercial carbon materials, Zhang Huamin et al. studied four common carbon materials: acetylene black, expanded graphite, carbon nanotubes, and BP2000. The experiment found that the materials have a large specific surface area and high degree of graphitization, which have a positive impact on the bromine oxidation-reduction catalytic performance and electron transfer resistance of the materials. They also found that BP2000 has the best energy efficiency among the four carbon materials due to its large specific surface area and other factors. At a current density of 20 mA cm-2, the energy efficiency reaches 84.4% [8].
According to relevant literature, ZBB Company in the United States is at the forefront of developing zinc bromine flow battery systems in the world. Its system has been successfully tested by power plant users in the United States, and its technical research is mature, but it has not been publicly disclosed. Its product can provide 17 kW of power at maximum charging rate and 25 kW of power at maximum discharge rate [9]. At present, some parts of zinc bromide flow batteries can be localized, and their cost is gradually approaching that of traditional lead-acid batteries. However, the energy density of zinc bromide flow batteries can reach 3 to 5 times that of lead-acid batteries [10]. Beijing Baineng Huitong is a battery company that started with the research and development of zinc bromine flow battery components. Currently, it has taken the lead in localizing key components and materials such as microporous ion separators and complexing agents for zinc bromine flow batteries nationwide, and has achieved the production of some domestically produced battery cells. In 2014, the zinc bromine flow battery product produced by Anhui Meineng Energy Storage successfully passed the inspection of State Grid and was qualified for the national grid.
In the future, zinc/bromine flow batteries will have unique competitive advantages due to their low cost and easy availability of raw materials, relatively high content of zinc and bromine elements on Earth, and ease of extraction. After solving problems such as zinc dendrites, their low battery cost will be further highlighted.
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