As the name suggests, flow batteries use liquid electrolytes to store energy by dissolving electrolytes that can undergo redox reactions. And for lithium-ion batteries, their energy carrier is solid electrode materials, and the lithium ions rich in the liquid electrolyte shuttle between the positive and negative electrodes as conductive ions, combining with the positive and negative electrode materials to achieve the effect of releasing and storing energy.



The positive and negative electrolyte components of a flow battery are different, so a semi permeable membrane is needed to separate them and avoid mutual contamination. On the one hand, this membrane needs to separate the redox ions in the electrolytes on both sides and prevent them from passing through, which is called the blocking effect; On the other hand, it is necessary to allow conductive particles (such as protons or sodium ions) to pass through to form an electric current, which is called ion conductivity. This is a hedging mechanism, if we want to improve the barrier effect, the ion conductivity will correspondingly decrease; If we want to increase conductivity, the barrier effect will become weaker. So, when preparing membranes, people usually need to find a point to achieve the optimal combination of these two properties, in order to achieve the optimization of the circulation efficiency of liquid flow batteries.



However, if the redox ions contained in the positive and negative electrode liquids are the same element, even if they permeate each other, they will not cause pollution, which eliminates the need for barrier effect? This is the symmetrical flow battery, among which the most famous representative is the all vanadium flow battery, which is also why it is the most mature in the development of flow battery technology.





The all vanadium flow battery was studied by NASA in the 1970s and achieved success in the 1980s. Over the past 40 years, it has mainly focused on optimizing the performance of various components, improving the overall efficiency of the battery, and reducing system costs. In the past two years, with the increasing demand for long-term energy storage (discharge time>4 hours), we can see that various regions have begun to build all vanadium flow battery industry bases, and many commercial projects with a capacity of hundreds of megawatts are also being tendered or constructed. However, the most concerning aspect of this technology is that the reserve of vanadium metal, the core material of its electrolyte, is very limited and will definitely not be able to meet global energy storage needs in the future. Therefore, people have begun to explore other symmetric flow batteries, including all chromium, all lead, all copper, all iron, etc.



After more than a decade of exploration and development, all iron is the first to stand out among these options, as evidenced by the listing of ESS Tech (stock code: GWH), an all iron flow battery company based in Oregon, USA. This company had previously received financing from Bill Gates' investment fund, which attracted a lot of attention at the beginning of its listing. The stock price quickly climbed from the issue price of $10 to the highest point of $28 within a few days. However, two months later today, its stock price has fallen back to nearly the issue price of $11. Is it because the market has calmed down, or is it because people have carefully studied and found that its products and technology are not as excellent as they imagined? With this question in mind, we have studied the patent of ESS company, hoping to use it to understand this all iron liquid flow technology.



When the flow battery of ESS is in a fully discharged state, the ions in both sides of the electrolyte are ferrous ions. When charging, the iron divalent ion on the negative electrode will be reduced to iron metal, while the iron divalent ion on the positive electrode will be oxidized to iron trivalent ion. So this flow battery can also be understood as a solid-state metal flow composite battery. Do you feel familiar with the term "solid metal" when you see it? By the way, this is a similar concept to the popular lithium-metal solid-state batteries nowadays - the negative electrode energy storage material uses solid metal. The biggest challenge for solid-state metal electrodes is the generation of sharp dendrites during metal growth, which can puncture the objects they encounter. So in lithium metal solid-state batteries, in order to prevent dendrites from piercing the polymer separator, people can only replace traditional polymer separators and liquid electrolytes with solid electrolytes, in order to suppress the growth of dendrites and ensure the lifespan of lithium metal batteries.





So returning to the all iron flow battery, from the reactor structure shown in the figure below, it can be seen that the electrodes for the growth of iron metal are adjacent to the polymer separator, which poses the biggest risk for all iron flow batteries - the dendrites of iron metal may puncture the polymer separator. To solve this problem, people can also emulate lithium metal solid-state batteries by using solid-state separators. However, the resistivity of solid-state separators is very high, which is an unbearable loss for water-based batteries with already limited voltage, so this solution is not practical.





In order to reduce this risk, ESS has adopted some other methods, such as studying electrolyte additives to reduce the growth rate of dendrites; Also, reducing the current density during charging can make the current more evenly distributed on the electrode and reduce the rate of dendrite growth. From the patent diagram below, it can be seen that all iron flow batteries require a current density of less than 20mA/cm2 at room temperature. Compared to all vanadium flow batteries with a current density between 100-150mA/cm2, all vanadium is 5-8 times the current density of all iron. What does this mean? This means that to achieve the same power, the area of an all iron reactor needs to be 5-8 times that of an all vanadium reactor, and the cost will correspondingly increase by so many times. The cost saved by the all iron system from electrolyte materials has been squandered on the reactor.





There is another issue with all iron flow batteries, which is that the electrolyte needs to maintain an acidic environment with a pH value not higher than 4, otherwise iron ions can easily form and precipitate iron hydroxide. However, in acidic environments, the negative electrode undergoes a side reaction during charging to generate hydrogen gas, which promotes a continuous increase in pH. In order to control the pH value of the electrolyte, ESS added two acid tanks (106 and 108 in the figure below) to the system and connected them to the reactor through pipelines. When the system detects that the pH of the electrolyte is above 4, it will pump in an acidic solution to adjust the pH back. The two possible side effects of this solution are: 1. Further increase in system costs; 2. With the continuous addition of acid, the concentration of electrolyte becomes thinner and thinner, and it is necessary to replace all electrolytes at regular intervals or find ways to evaporate the water.





In summary, compared to the all vanadium liquid flow system, there are still many technical difficulties that need to be solved in the all iron liquid flow system. Except for all iron, other all chromium, all lead, and all copper systems also have similar solid metal problems, so they cannot compete with all vanadium. Why is it that nature only produces vanadium, an element that can exist in a fully ionic state in liquid flow batteries, but cruelly limits its storage to a smaller amount? What a trick!



ZH Energy Storage Company will hold a course on long-term energy storage market and technology analysis in the near future. Interested parties can scan the code to add the company's responsible person's WeChat account for consultation.





Reference:

US Patent US20170179516, Electrolytes for iron flow battery。



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The Global Long Term Energy Storage Council releases a White Paper on Long Term Energy Storage (Abstract and Documents)



Author and Company Profile:

Dr. Xie Wei, co-founder and chief scientist of ZH Energy Storage, holds a Bachelor's and Master's degree in Materials from Tsinghua University, and a Ph.D. in Chemical Engineering from the University of Texas at Austin. He has served as a Senior R&D Engineer at United Technologies and Chief R&D Scientist at Form Energy, a startup in Boston. With over ten years of experience in liquid flow battery product development, he was responsible for the all vanadium liquid flow battery project, which won the 2013 US 100 R&D Award.



Relying on the industry experience and outstanding research and development capabilities accumulated by its founder Dr. Xie Wei in the energy storage field, ZH Energy Storage Company will launch mature liquid flow battery products to meet the market demand for large-scale long-term energy storage (discharge time greater than 4 hours) and achieve economic applicability. The company's long-term goal is to build a world leading liquid flow battery technology platform, communicate upstream and downstream industrial chains, reduce the cost of liquid flow battery systems through large-scale production, and become a leading enterprise in the field of long-term energy storage.