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Highly concentrated essence, become a flow battery technology expert in five minutes

Classification：Industrial News

- Author：ZH Energy

- Release time：Jul-31-2023

【 Summary 】So far, numerous new liquid flow battery systems have been developed, among which the technology of vanadium flow battery (VFB) is the most mature, and the energy storage technology of all vanadium fl

ZH Energy Storage will debut at the August World Battery Industry Expo with new products such as 32kW battery stacks and non fluorine ion exchange membranes

Introduction to the basics of liquid flow batteries

So far, numerous new liquid flow battery systems have been developed, among which the technology of vanadium flow battery (VFB) is the most mature, and the energy storage technology of all vanadium flow battery is mainly used in large-scale engineering level energy storage power stations above megawatts.

Compared with traditional secondary batteries, the biggest difference of all vanadium flow batteries lies in their unique structural design: the positive and negative active material electrolyte of all vanadium flow batteries is stored in the external storage tank of the battery, and is transported to the internal electrodes of the stack through pipelines by a circulating pump for oxidation-reduction reactions (different vanadium ion valence state transitions) to achieve charging and discharging. Thanks to this unique structure, by changing the size of the battery stack or the amount of electrolyte used, the output power and energy storage capacity of the battery can be independently designed, which is difficult for traditional secondary batteries such as lithium batteries to achieve.

The structure and working principle of all vanadium flow batteries

In addition, high safety and ultra long service life have become the advantages of all vanadium flow batteries. In a fully liquid electrolyte, only the valence state changes and does not involve phase changes, avoiding the problem of membrane puncture caused by phase transition and dendritic growth. At the same time, aqueous electrolytes reduce the risk of battery ignition and explosion. As is well known, power production accidents caused by battery short circuits and self ignition are pain points in the development process of energy storage power stations. With the continuous expansion of the construction scale of energy storage power stations, the requirements for their safety performance have become more stringent. On July 1, 2022, the National Energy Administration issued the "25 Key Requirements for Preventing Electricity Production Accidents (2022 Edition) (Draft for Comments)", which clearly stated that medium and large electrochemical energy storage power stations should not use ternary lithium batteries, sodium sulfur batteries, and should not use cascading power batteries. This policy direction indirectly chose liquid flow batteries, which directly led to the daily limit up of multiple stocks. In fact, as early as March 21, 2022, the implementation plan for the development of new energy storage during the 14th Five Year Plan included 100 megawatt level liquid flow battery technology as one of the key directions for tackling new energy storage core technology equipment during the 14th Five Year Plan. At present, flow batteries, especially all vanadium flow batteries, are undoubtedly standing at the forefront of the country's development of energy storage technology.

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Key materials for flow batteries

A flow battery mainly consists of two parts: electrolyte and stack, which includes electrodes, ion exchange membranes, bipolar plates, collector plates, end plates, and other parts. Electrolyte is not only the active substance in battery reactions, but also the carrier for ion transport. The ion exchange membrane separates the positive and negative electrolytes and serves as a current carrier for both the positive and negative electrolytes. The electrode provides a place for the electrochemical reaction of the battery. At present, the research hotspots of all vanadium flow batteries mainly focus on finding high-performance and low-cost electrode materials, developing ion exchange membranes with low cost, high selectivity, and long lifespan, as well as preparing electrolytes with high concentration, strong ion transport capacity, and good stability.

Structural diagram of liquid flow battery stack

Vanadium ion electrolyte is the energy storage medium for all vanadium flow batteries. Its preparation methods are mainly divided into chemical preparation method and electrolytic preparation method. Currently, the publicly available preparation methods are mainly pure electrolysis method or a combination of chemical method and electrolysis method.

The physical and chemical properties of the electrolyte (conductivity, electrolyte density, viscosity, vanadium ion concentration, pH value), impurity content and type, operating temperature, etc. directly affect the reaction activity, efficiency, and lifespan of the battery. Specifically, the conductivity of the electrolyte directly affects the ion transfer rate and internal resistance of all vanadium flow batteries. The higher the viscosity of the electrolyte, the lower the flow rate of the electrolyte in all vanadium flow batteries under the same operating conditions, and the higher the power consumption of the electrolyte circulation pump. When the concentration of sulfate ions is constant, the higher the concentration of vanadium ions, the lower the conductivity and viscosity of the electrolyte. In addition, the conductivity of the electrolyte increases with the increase of temperature, while the viscosity decreases with the increase of temperature.

At present, the cost of vanadium electrolyte is too high, exceeding more than half of the cost of the entire vanadium flow battery energy storage system. Developing low-cost scale production technology is the key path to solve the comprehensive commercialization of all vanadium flow batteries. Meanwhile, divalent vanadium is easily oxidized by air, and there is a risk of precipitation of pentavalent vanadium in the positive electrode electrolyte at high temperatures. Ensuring the air stability of divalent vanadium (V2+) and the high-temperature stability of pentavalent vanadium (V5+) is also a key factor in the long-term stable operation of the energy storage system of all vanadium flow batteries. In this regard, selecting appropriate electrolyte additives can play a stabilizing role. In addition, optimizing the composition and concentration of the electrolyte itself is also an important method to improve the overall performance of the electrolyte.

Cost composition of all vanadium flow battery energy storage system

Ion exchange membranes are mainly used to separate positive and negative electrolytes and maintain ion concentration balance, and are the core components of fuel cells. According to the ion charge state of exchange conduction within the membrane, it can be divided into cation exchange membranes and anion exchange membranes. According to the degree of fluorination, it can also be divided into perfluorosulfonic acid ion exchange membranes, partially fluorinated ion exchange membranes, and non fluorinated ion exchange membranes.

Ion exchange membranes should not only be able to isolate active substances and conduct ions, but also be able to operate stably under extremely harsh conditions (strong acid, strong oxidation, high potential, high current). Therefore, a high-performance ion exchange membrane should have excellent ion selectivity (blocking vanadium and preventing active substances from crossing), high conductive ion transport capacity, excellent electrochemical stability and mechanical strength, and low cost.

At present, perfluorosulfonic acid ion exchange membranes, represented by DuPont's Nafion membranes, are widely used in the commercial field. Taking Nafion115 membrane as an example, it exhibits excellent ion conductivity and chemical stability in all vanadium flow batteries, but its ion selectivity is poor and its price is expensive, seriously limiting the commercialization of all vanadium flow batteries. Therefore, the focus of research and development is to develop ion exchange membranes with high ion selectivity, high durability, and low cost.

The microstructure and properties of Nafion and SPEEK membranes

The electrode is the site where electrochemical reactions occur in all vanadium flow batteries and is another core component of the stack. Its performance directly affects the electrochemical reaction rate, battery internal resistance, electrolyte diffusion, and ultimately affects the overall performance of the battery, such as energy efficiency. Electrode materials can be divided into two categories by type: metal electrodes and carbon electrodes.

Metal materials were first studied for their excellent conductivity and mechanical strength, mainly including gold (Au), titanium (Ti), platinum (Pt), iridium oxide (IrO2), and so on. However, the high cost limits its large-scale production, so researchers have shifted their focus to carbon materials with lower costs. Carbon electrode materials mainly include graphite, graphite felt, glassy carbon, carbon cloth, etc. Especially, carbon felt has a relatively low price and good electrochemical performance, which can meet the practical requirements of electrode materials for all vanadium flow batteries.

Development history of carbon based materials for all vanadium flow batteries

The influencing factors of carbon felt on battery performance are related to the types and quantities of functional groups on the surface of electrode materials, the surface morphology of carbon fibers, the porosity of electrode materials, the distribution status of carbon fibers in the warp and weft directions, and the conductivity of electrode materials.

Carbon felt materials used in all vanadium flow batteries
In addition to the electrolyte and battery stack mentioned above, matching supporting systems such as pump valves, pipelines, battery management systems (BMS), etc. also directly affect the normal operation of the entire energy storage system.

summary

With the continuous updating and iteration of all vanadium flow battery technology, the localization of raw materials, the maturity of all vanadium flow battery technology, and the continuous reduction of costs, it is widely believed in the energy storage industry that all vanadium flow batteries have reached the conditions for commercial development. Considering that all vanadium flow batteries are currently the most mature technological route, and with the encouragement and support of national policies for the development of flow batteries, all vanadium flow batteries are an ideal technology to meet the requirements of large-scale energy storage industrialization. At the same time, their technical performance and cost have also reached the level of scalable development. As of now, many large-scale all vanadium flow battery energy storage projects have been completed and connected to the grid, which will become a trend. With the deepening of research and investment, the performance of all vanadium flow battery products will gradually improve. At the same time, the upstream and downstream industrial chain of all vanadium flow battery products is gradually improving, which is conducive to the quality control and cost reduction of all vanadium flow battery energy storage technology products. According to a report by IDTechEx in the UK, vanadium flow batteries targeting the energy storage market may surpass lithium-ion batteries in installed capacity by around 2031.

In summary, with the accumulation of technology, continuous improvement of the industrial chain, policy support, and excellent performance, all vanadium flow batteries have entered the fast lane of energy storage development, and the industrialization process has significantly accelerated compared to before. They will be widely promoted and applied in the next five to ten years.

Reference:

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