Sectoral dynamics
Special Report on the All Vanadium Liquid Flow Battery Industry: Tracing the Source Along the Flow and Devulgarizing the Ultra "Vanadium"
(Report Producer/Author: Guotai Jun'an Securities, Pang Junwen, Shi Yan)
1. Technical Explanation: The Past and Present Lives of All Vanadium Flow Batteries
With the proposal of the "carbon peak, carbon neutrality" goal, China's energy structure adjustment is accelerating, and the gradual replacement of traditional fossil fuels by new energy will be a historical trend. China has a vast territory and abundant solar and wind energy resources, but these natural energy sources have intermittent and fluctuating characteristics. Directly integrating them into the power grid will encounter great difficulties, and smoothing must be carried out first. At the same time, there is often a mismatch between power supply and demand in time and space, exhibiting phenomena such as peak valley bands and regional imbalances. An important way to solve the above problems is energy storage technology, especially electrochemical energy storage, which has advantages such as high efficiency, fast response speed, and is not limited by geographical environment. It is suitable for smooth processing of wind and solar power generation on the supply side and also for energy management on the demand side. Compared to other electrochemical energy storage technologies, liquid flow batteries have inherent safety and ultra-long cycle life, making them particularly suitable for large-scale energy storage power plants.
1.1. Basic concepts and historical background
A liquid flow battery is a liquid phase electrochemical energy storage device, in which the active substance is completely dissolved in the electrolyte and energy is stored and released through the oxidation valence state changes of the active elements. It belongs to an oxidation-reduction battery. Generally speaking, a liquid flow battery requires two sets of redox pairs to form a positive and negative charge. As the battery charges and discharges, the oxidation valence (potential) of the positive and negative active elements changes accordingly. Taking the early, classic, and widely studied Fe Cr dual flow battery as an example, its working pair is Fe2+/3+/Cr2+/3+, with FeCl2 as the active material and CrCl3 as the negative active material. The electrolyte matrix is hydrochloric acid, and the positive and negative ions are isolated using a proton conduction membrane (to avoid direct contact between the positive and negative active materials and the occurrence of self oxidation reduction reactions). When the battery is discharged in a fully charged state, the positive * active substance undergoes a reduction reaction: Cr3++e → Cr2+, while the negative * active substance undergoes an oxidation reaction: Fe2+→ Fe3++e, which can be merged into: Cr3++Fe2+→ Cr2++Fe3+, that is, the oxidation of divalent Fe ions by trivalent Cr ions and their own reduction process. The electrons start from the negative * and pass through the external circuit to reach the positive *. The process of charging and energy storage is the opposite.
The origin of liquid flow battery technology is very long, spanning over a century* As early as 1884, French engineer Charles Renard invented the zinc chlorine liquid battery, which was used as the power source for military airship propellers. The battery has a range of 23 minutes, a round-trip flight distance of 8 kilometers, and an overall weight of 435kg. It uses chromium trioxide and concentrated hydrochloric acid as the chlorine source. This battery is similar to today's zinc bromide flow battery (but without an additional fluid drive system), and was used as a primary battery at that time without significant competitiveness, but later disappeared. In 1949, more than half a century later, German scientist Walter Kango invented the "liquid storage battery" and applied for an official patent. The battery uses chromium sulfate and ferrous chloride as the working substance and is stored in a separate container, with sulfuric acid as the matrix and graphite as inert electricity. This patent is considered a patent for liquid flow batteries in history. Afterwards, Kango further screened 6 sets of pairs that can be used to construct liquid batteries, using transition metal salts such as titanium chloride, iron chloride, and chromium sulfate as active substances. The device structure of this type of liquid storage battery has taken on the embryonic form of modern liquid flow batteries, but the design is simple and the cycling performance is poor. The main reason is that the self discharge caused by cross contamination of positive and negative metal ions is severe, leading to voltage instability and rapid capacity decay. Moreover, the raw materials used are often highly corrosive or toxic, and do not have significant advantages in cost, so they have little commercial value.
The progress of modern liquid flow battery technology is closely related to the development of ion exchange membrane technology. Around 1950, membrane technology made a breakthrough and people obtained ion exchange membranes with selective permeability, laying the foundation for the development of modern liquid flow battery technology. In 1955, General Electric Company sulfonated polystyrene to obtain the first Proton Exchange Membrane (PEM) and used it as the stack separator for fuel cells. PEM only allows protons to pass through, blocking the passage of other ions, so this technology was quickly transplanted into liquid flow batteries as a positive and negative separator to suppress internal self discharge. In the 1980s, General Motors collaborated with DuPont to develop Nafion proton exchange membranes based on their patented perfluorinated sulfonic acid resin technology, which was applied by Ballard in Canada to fuel cells, greatly improving their performance. Due to its excellent proton conductivity, strong oxidation resistance, and acid corrosion resistance, perfluorosulfonic acid membranes were quickly introduced into liquid flow batteries and remain the mainstream separator material for liquid flow batteries to this day.
The embryonic stage of technology (1971-1985): In 1971, Japanese scientists Ashimura and Miyake first proposed the modern concept of a liquid flow battery. By dissolving positive and negative active substances in the electrolyte, reversible redox reactions occur on inert electrodes to achieve mutual conversion of electrical and chemical energy. Since 1973, NASA has been conducting research on liquid flow batteries for solar energy storage systems on lunar bases, primarily considering their safety, efficiency, and operational life, while cost is a secondary factor. A year later, NASA scientist L. H. Thaller first proposed a practical detailed model of a liquid flow battery, using FeCl2 and CrCl3 as positive and negative active substances and stored in two external storage tanks, hydrochloric acid as the matrix, anion exchange membrane as the separator, and a circulating pump as the driving force for the liquid flow, forming the first Fe Cr dual flow battery. Afterwards, there was a wave of research on Fe-Cr flow cells worldwide, with the United States and Japan successfully developing prototype Fe-Cr flow cells of kW scale with a capacity of over 10 kWh as supporting facilities for photovoltaic arrays. However, due to the poor reversibility of the Cr3+/Cr2+semi reaction and the cross contamination caused by some Fe and Cr ions passing through the separator, the working voltage is unstable and the capacity decreases, greatly reducing the actual service life of the battery. These issues involve the physical and chemical nature of the Fe Cr system. At that time, ion exchange membrane technology was limited and difficult to properly solve, resulting in the gradual elimination of the Fe Cr system. At present, the research and development of Fe Cr flow batteries abroad has almost stagnated, and the only demonstration experimental project of EnerVault in the United States was also discontinued in June 2015; The main focus in China is still on the research and development of the National Electric Power Investment Group, and its 31.25kW level Fe Cr liquid flow reactor "Ronghe No.1" has started mass production.
Research and development demonstration period (1986-2000): After more than ten years of exploration, the vast majority of candidate material systems for liquid flow batteries have been phased out due to various insurmountable defects. The main ones that have finally entered the practical demonstration stage are zinc bromide liquid flow batteries and all vanadium liquid flow batteries. Among them, zinc bromine liquid flow battery is a unilateral deposition type liquid flow battery, which has the advantages of high energy density and low raw material cost. However, the volatility, high toxicity, strong corrosion and easy permeability of liquid bromine, as well as the precipitation of zinc dendrites, greatly reduce the actual capacity, cycle life, and safety of the battery. In contrast, although the energy density of all vanadium flow batteries is not as high as that of zinc bromide flow batteries, their performance in other aspects has more potential for rapid commercialization. Since 1984, Maria Skyllas Kazacos and others from the University of New South Wales (UNSW) in Australia have been conducting systematic research on all vanadium flow batteries, including the kinetic mechanism of the electrical process, the production and modification of electrical materials, the optimization of ion exchange membranes, and the formulation of electrolytes. The active material of the all vanadium flow battery they designed is sulfate with different valence vanadium ions, and the substrate is sulfuric acid solution. The team first applied for a patent for all vanadium flow batteries in 1986, officially authorized it in 1988, and began constructing a 1kW level experimental stack with an energy efficiency of 72~88%. Subsequently, UNSW resold the technology to Pinnacle, a mining company based in Melbourne, Australia. In 1993, UNSW collaborated with Thai Gypsum Products to attempt the application of vanadium batteries in solar houses. In 1994, all vanadium flow batteries were used as backup power sources for golf carts and submarines. The research results of UNSW are a milestone in the history of all vanadium flow batteries, marking the beginning of the technology's transition from laboratory to industrialization.
Early commercialization (2001 to present): After entering the 21st century, all vanadium flow batteries began to truly move towards commercialization, mainly represented by American and Japanese companies in the early stages. In 2001, Vanteck acquired 59% of the shares of Pinnacle Company and obtained core patent rights. The following year, it was renamed as Vanadium Battery Energy Storage System Technology Development Company (VRB Power System). In 2004, the company further acquired Reliable Power Company to control the all vanadium flow battery market in the entire North American region, mainly engaged in technology development and authorization transfer of vanadium batteries, becoming the largest all vanadium flow battery company in the world at that time. Meanwhile, between 2000 and 2002, SEI Corporation of Japan built multiple all vanadium flow battery energy storage systems and used them for office buildings, factory power supply, as well as supporting facilities for wind farms and golf course photovoltaic arrays. In 2005, SEI Company established a 4MW/6MWh all vanadium liquid flow battery energy storage system in Taiqian cho, Hokkaido, as a frequency modulation and amplitude modulation supporting facility for 36MW wind power plants. This was the world's largest demonstration system for all vanadium liquid flow energy storage battery engineering at that time. Subsequently, the 2008 financial crisis erupted, causing a certain degree of impact on the all vanadium flow battery industry. SEI Company temporarily suspended the development of the liquid flow battery project until 2011 when it resumed commercial operation.
1.2. Working principle and core materials
All vanadium flow battery, also known as "vanadium battery" in commercial terms, refers to the use of vanadium compounds as the active material for both positive and negative electrolytes in the flow battery. The positive and negative redox pairs of all vanadium flow batteries are VO2+/VO2+- V3+/V2+, and the active material is sulfate with different valence vanadium ions. The electrolyte substrate is sulfuric acid aqueous solution. When the battery is discharged at full charge, the positive active substance undergoes a reduction reaction: VO2++e → VO2+, with a standard potential of+1.004 V; The negative * active substance undergoes an oxidation reaction: V2+→ V3++e, with a standard potential of -0.255 V. The whole battery reaction can be combined as follows: VO2++V2+→ VO2++V3+, with an open circuit voltage of 1.259 V, that is, the oxidation of divalent hydrated vanadium ions by pentavalent vanadate ions to trivalent hydrated vanadium ions, and the process of being reduced to tetravalent vanadate ions. The electrons start from the negative * and pass through the external circuit to reach the positive *. The process of charging and energy storage is the opposite. In actual operation, due to complex factors such as overpotential, the open circuit voltage of all vanadium flow batteries is generally 1.5-1.6 V.
At present, the all vanadium system is a mature solution in dual flow batteries. All dual flow batteries have similar stack structures, with the main difference being the differences in active substances, which are the core factors determining the theoretical energy density. From the perspective of electrochemical theory, as long as there are two sets of electric pairs with different potential differences, their valence compounds can be used as positive and negative active substances to form a liquid flow battery. However, in actual battery production, more factors need to be considered, such as the stability of active substances, solubility, reversibility of electrical reactions, matching of electrochemical windows, and so on. When truly entering the commercialization stage, it also involves constraints such as safety, cost, efficiency, lifespan, and environmental protection, which is a multidisciplinary and complex system engineering. Over the years, researchers have conducted extensive research on liquid flow batteries based on these complex factors, accumulating rich experimental data. After undergoing extensive screening, all vanadium flow batteries have become the first liquid flow battery solution that may achieve large-scale commercial application at this stage. The entire system of an all vanadium flow battery consists of energy units, power units, transportation systems, control systems, additional facilities, and other components, among which the energy unit and power unit are the core modules.
1.2.1. Electrolyte materials: core elements of energy units
The positive and negative * electrolyte of all vanadium flow batteries is their true energy storage medium, which is the core of the energy unit and generally consists of three parts: active substance, matrix, and additive. The concentration of active substances in the electrolyte and the total amount (volume) of the solution fundamentally determine the energy density and upper limit of the energy storage capacity of the entire battery system; The thermal stability of the electrolyte determines the working temperature range and stability of the battery.
Active substance: The electrolyte active substance of vanadium sulfate all vanadium flow battery is vanadium sulfate, among which vanadium element is the active element. The reason why vanadium is chosen as the core working element is because its ground state electronic configuration is [Ar] 3d24S2, which has a rich and varied oxidation valence state. The+2,+3,+4, and+5 valences can stably exist in acidic aqueous solutions, and the reduction potential of positive and negative * is precisely suitable for the electrochemical window of water. In addition, the characteristic spectra of hydrated vanadium ions with different valence states are very different, making it easy to identify: divalent vanadium is purple, trivalent vanadium is dark green, tetravalent vanadium is blue, and pentavalent vanadium is yellow. Concentration quantitative analysis can be conducted using UV Vis spectroscopy to monitor the state of charge (SOC) of the electrolyte in real-time. Sulfates of vanadium with different valence states serve as active substances, with positive and negative redox pairs: VO2+/VO2+- V3+/V2+, positive * reactions: VO2++e ⇌ VO2+, negative * reactions: V2+⇌ V3++e, and full cell reactions: VO2++V2+⇌ VO2++V3+. In an ideal scenario, the positive and negative * active ions in the uncharged original electrolyte are VO2+and V3+, respectively, with a ratio of 1:1 to meet the stoichiometric requirements and fully utilize the active substance.
Additives: Organic and inorganic complexing agents generally require the addition of a small amount of additives to increase the solubility and stability of vanadium ions in the electrolyte to inhibit solid precipitation. There are various types of electrolyte additives, which can be divided into two categories: organic and inorganic. Organic additives are generally multi dentate ligands with coordination functional groups such as hydroxyl, thiol, and amino groups. They can form stable complexes with vanadium ions, inhibit the nucleation and growth of V2O5 solids, and also act as dispersants, reducing the surface energy of particles and inhibiting the agglomeration of colloidal particles. Common organic additives include amino acids, polyols, amino sulfonic acids, as well as some surfactants and water-soluble polymers. Inorganic additives are generally salts, where anions or cations can form coordination bonds with vanadium ions, such as phosphates, ammonium salts, etc. Their mechanism of action is also to inhibit the nucleation and growth of V2O5 solids, thereby stabilizing the electrolyte. The dosage of additives depends on the specific type and electrolyte concentration, generally ranging from 1% to 3%. Excessive use can hinder ion transport mechanisms, increase the ohmic effect of the electrolyte, and reduce system energy efficiency. (Report source: Future Think Tank)
1.2.2. Electric stack materials: core elements of power units
The stack is the place where all vanadium flow batteries undergo electrochemical reactions, which determines the power characteristics of the system. The performance of the stack directly affects the overall performance of the system. An all vanadium flow battery stack is essentially composed of multiple single cells stacked and connected in series, usually fastened by a filter press. It has one or more sets of electrolyte circulation systems inside, and the current inlet and outlet ports are a unified set. The main components of all vanadium liquid flow single cell include: battery, double plate, diaphragm, end plate, seal, and other fasteners.
Electricity: The electricity of all vanadium flow batteries does not participate in electrochemical reactions, but serves as a reaction site where active substances gain or lose electrons on the surface of the electricity, undergo reduction or oxidation, and achieve mutual conversion between electrical and chemical energy. The physical and chemical properties of electrical materials have a significant impact on all vanadium flow batteries: firstly, the conductivity and catalytic performance of electrical materials directly affect the chemical state and current density of the battery, thereby affecting energy efficiency; Secondly, the physical and chemical stability of electrical materials directly affects the overall working stability and actual lifespan of batteries. Therefore, electrical materials must have high chemical inertness, mechanical strength, conductivity, and a larger surface area. Early use of metal electricity included elemental metals such as gold, lead, and titanium, as well as alloy materials such as titanium based platinum and titanium based iridium oxide. However, metal electrical materials have many defects, some with poor electrochemical reversibility and some with high costs, making them difficult to use on a large scale and for a long time. Afterwards, people switched to carbon based electrical materials, such as graphite, glassy carbon, carbon felt, graphite felt, carbon cloth, and carbon fiber. These carbon materials have good chemical stability, good conductivity, easy preparation, and low cost. Research has found that the electrical reversibility of glassy carbon is poor; Graphite and carbon cloth batteries are prone to erosion and loss during the charging and discharging process, and the specific surface area of these materials is small, resulting in high internal resistance of the battery and difficulty in high current charging and discharging; Although carbon paper has a large specific surface area and good stability, it has poor hydrophilicity and low electrochemical activity. At present, the widely used electrical materials are carbon felt or graphite felt, both of which belong to carbon fiber textile materials.
Double plate: In all vanadium flow batteries, the double plate is a conductive separator that is tightly attached to the battery, used to separate the positive and negative electrolytes of two adjacent single cells, collect current, and support the battery, thereby achieving series connection of multiple single cells inside the stack. The ideal double panel material has good gas and liquid resistance, conductivity, chemical inertness, and mechanical strength. The purpose of gas and liquid blocking is to prevent cross contamination of positive and negative electrolyte on both sides of the plate, which is a basic requirement for double plates. High conductivity not only includes the low impedance of the dual board itself, but also requires a low contact resistance between the dual board and the battery, in order to reduce the internal resistance of the battery. Due to the presence of strong oxidizing and reducing electrolytes on both sides of the double plate, in order to operate in this harsh environment for a long time, the double plate material must have high chemical inertness* After that, the double * board must have good mechanical strength and processability as a supporting device* The initial use is metal double * plate or pure graphite double * plate. The former has good mechanical strength but poor corrosion resistance (expensive for precious metals such as gold and platinum), while the latter has good corrosion resistance but high brittleness and high processing cost. At present, one solution is to modify graphite double * plates to improve mechanical strength and processability; Another solution is to use carbon plastic composite double * plates, which are mixed with conductive fillers and polymer resin to form. They have good mechanical strength and corrosion resistance, but their conductivity is reduced (the resistivity is 1-2 orders of magnitude higher than that of metal and graphite double * plates). Currently, electrical materials are also prone to wear and tear, and their actual service life under normal operating conditions is about two years. They need to be replaced after expiration. At present, researchers can obtain integrated electrical double plates with good electrochemical performance and low etching resistance by bonding them together through hot pressing or molding.
Diaphragm: Selective ion permeation, key to long lifespan. The separator in all vanadium flow batteries is an ion conducting membrane located in the center of each single cell, used to separate the positive and negative electrolytes inside the single cell, preventing active substances from mixing with each other and causing self discharge due to "liquid jumping". At the same time, it allows for selective transfer of specific ions, and the internal circuit of the battery is conductive. The performance of the separator directly affects the efficiency and lifespan of the battery, and generally requires high ion selectivity, ion conductivity, chemical stability, and mechanical strength. In theory, options include cation exchange membrane, anion exchange membrane, and porous separation membrane. Among them, the cation/anion exchange membrane has negative/positive charge groups that can allow specific types of cations or anions to pass through; The porous separation membrane has no charged groups and is screened and retained by ion radius. At present, the most widely used in all vanadium flow batteries is proton conduction membrane, which belongs to cation exchange membrane and has mature technology. The typical representative is the Nafion membrane produced by DuPont Company. This is a type of perfluorinated sulfonic acid resin with good chemical stability and ion conductivity, but poor ion selectivity and high cost ($500-800/square meter). Afterwards, people attempted to modify the benzene sulfonyl plasma selective group onto the partially fluorinated polymer carbon chain, resulting in a partially fluorinated membrane. The ion selectivity was significantly improved, but the chemical stability was reduced, and radiation technology was required. Considering the high cost of fluorinated resins, people have turned to developing non fluorinated hydrocarbon membranes, one type is non fluorinated ion exchange membranes without pores, and the other type is porous non fluorinated separation membranes. Poreless non fluorinated ion exchange membranes introduce ion selective groups on non fluorinated polymers, such as sulfonated poly (aryl ether ketone), which have good ion selectivity and conductivity, but lower chemical stability and are severely damaged after several hundred cycles. The typical representative of porous non fluorine separation membranes is nanofiltration membranes, which have no charged groups on the surface, but are distributed with a large number of nanoscale micropores, allowing the passage of hydrated protons with smaller radii, and not allowing the passage of hydrated vanadium ions with larger radii. At present, perfluorinated sulfonic acid resin membranes have begun to be replaced domestically, while the application of non fluorinated membranes is in the ascendant, which is of great significance for reducing the cost of battery systems.
Seals: Sealing is an important guarantee for the performance of vanadium batteries. The system operates in a fully sealed manner, strictly avoiding external and internal leakage of the electrolyte. If leakage occurs, divalent hydrated vanadium ions are prone to oxidation in the air, resulting in capacity loss, and highly corrosive electrolytes may damage other components of the stack. If internal leakage occurs, the positive and negative electrolytes may mix with each other, which will directly affect the performance and lifespan of the stack, and leakage is not easily detected from outside the stack. Due to the strong oxidizing and reducing properties of the positive and negative electrolytes in all vanadium flow batteries, and the electrolyte matrix being sulfuric acid, ordinary rubber sealing materials cannot withstand this environment and special fluororubber must be used as the sealing element. In addition, the fluororubber material used for sealing components should have appropriate hardness, tensile strength, elongation at break, and tear strength, and the compression plastic deformation should be as small as possible, and additional self tightening devices are required. However, the price of fluororubber is very expensive, about 300000 to 400000 yuan/ton, and it still faces problems such as aging and plastic deformation during long-term operation. The scientific research team of the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences simplified the sealing process through integrated laser welding technology, achieving the integration of diaphragm electric double plate, saving fluororubber components, which is of great significance for reducing the cost of the stack.
1.3. Manufacturing processes and technical barriers
1.3.1. Electrolyte material manufacturing: formula and process are key
The vanadium battery electrolyte is made by reducing vanadium pentoxide in sulfuric acid and can be produced on a large scale using processes such as chemical or electrolytic methods. Early vanadium battery electrolytes were directly prepared by dissolving vanadium oxysulfate (VOSO4) in sulfuric acid solution, with the advantage of simple operation. However, vanadium oxysulfate was expensive and economically poor, making it unsuitable for large-scale production. At present, the methods for mass production of vanadium battery electrolytes are divided into chemical reduction method and electrolysis method, both of which essentially reduce pentavalent vanadium to low prices. The chemical reduction method involves mixing pentavalent vanadium raw materials (such as vanadium pentoxide, ammonium metavanadate, etc.) with sulfuric acid solution, adding reducing agents (such as oxalic acid, sulfur dioxide, etc.), and heating to obtain a low valent vanadium salt solution through the reaction. The electrolysis method involves the anionic reduction of pentavalent vanadium raw materials in an electrolytic cell to obtain a low valent vanadium salt solution. The advantage of chemical method is that the process and equipment are simple, but the disadvantage is that the reaction is slow and requires high-temperature treatment. The advantage of electrolysis method is that it can be produced in large quantities at room temperature, with high production efficiency, but the disadvantage is that it requires a lot of electrical energy consumption. The oxidation valence state of vanadium ions in the initial state of the electrolyte is between 3-4. After input into the stack, pre charging begins. The positive vanadium ions are uniformly oxidized to+5 valence, and the negative vanadium ions are uniformly reduced to+2 valence. At this point, the adjustment of the positive and negative vanadium ions in the electrolyte valence state is completed, and work can begin.
Electrolyte accounts for a significant portion of the total cost of all vanadium flow battery systems (typically 30% to 50%). Although the basic raw material for electrolytes is vanadium pentoxide, which belongs to homogeneous products, the performance and cost of the electrolytes produced vary greatly due to the different production routes and additives used by different manufacturers. In terms of performance, the main reason is that the electrolyte formula has unique characteristics, especially concentration, acidity, and additives, which are protected by enterprises in the form of patents. At the same time, the differences in technology among different enterprises can lead to differences in the impurity content of the electrolyte, which can also be reflected in battery performance. In addition, the processing costs vary among different production processes. At present, the market price of electrolyte is about 1500 yuan/kW · h, and storing 1kW · h of electricity requires about 10kg of vanadium pentoxide. Therefore, the price of vanadium pentoxide in the form of electrolyte is about 150000 yuan/ton. At present, the spot price of vanadium pentoxide on the market is about 100000 yuan/ton, so the unit cost of processing vanadium pentoxide into electrolyte is about 50000 yuan/ton. In other words, two-thirds of the cost of the electrolyte comes from vanadium pentoxide, and one-third comes from processing costs. Due to the fact that vanadium pentoxide itself is extracted from vanadium slag and stone coal, if the starting point of the electrolyte process is directly from raw materials such as vanadium slag and stone coal, skipping the vanadium pentoxide process, the entire manufacturing process can be shortened, thereby significantly reducing the cost of the electrolyte. This requires the enterprise to have a considerable production capacity and strong control over the upstream.
1.3.2. Electric stack materials and assembly: complex materials and precise assembly
1.3.2.1. Core materials of the stack: electric *, double * plate, diaphragm
Electricity: Carbon based textile material. Carbon felt or graphite felt are commonly used as electrical materials. Carbon felt is made from organic polymer fiber blankets through heat treatment processes such as pre oxidation and inert atmosphere carbonization, while graphite felt is made by further graphitizing the carbon felt at high temperatures above 2000 ℃. This type of carbon fiber has a large specific surface area, good chemical stability and conductivity, but it is prone to oxidation and detachment during long-term use. Therefore, it needs to be modified, including material intrinsic treatment, metallization treatment, and oxidation treatment, or co made with inert polymer matrix into composite materials (but with reduced conductivity).
Double panel: carbon plastic composite material. Carbon plastic composite double plates are widely used in current all vanadium flow stacks. The processing performance and structural strength of carbon plastic composite double * plate are significantly better than those of non porous hard graphite plate; The liquid resistance performance is significantly better than that of flexible graphite plates; The corrosion resistance is much stronger than that of ordinary metal double * plates, and the manufacturing process is simple and the cost is low. The raw materials for carbon plastic composite double panel include polymer matrix and conductive filler. The polymer matrix is generally inert plastics such as PE, PP, PVC, or epoxy resin; The conductive filler is divided into two parts. The main conductive filler is graphite powder, and the secondary conductive filler can be amorphous carbon such as carbon black and carbon fiber. After mixing, it is processed and formed through molding, injection molding, and other methods. Conductive fillers form a three-dimensional conductive network within the polymer matrix, while also improving mechanical strength to a certain extent.
Diaphragm: perfluorosulfonic acid resin. Perfluorinated sulfonic acid resin membrane is currently widely used as a separator in all vanadium flow stacks. From the perspective of molecular structure, the main skeleton of perfluorinated sulfonic acid resin is a polytetrafluoroethylene structure, and the branched end group is a perfluorinated vinyl ether structure with sulfonic acid groups. The synthesis route is: tetrafluoroethylene and perfluorinated ether sulfonyl fluoride are copolymerized under the action of an initiator, then hydrolyzed and acidified. The synthesis of perfluorinated sulfonic acid resin is relatively difficult, and the greater difficulty lies in the subsequent processing and film forming process. The key is to reduce processing losses and manufacture films with uniform thickness and excellent performance. The core melt extrusion and rolling forming technology has long been monopolized by DuPont in the United States, and domestically produced films are prone to defects such as "pinholes" and difficult to meet usage requirements, so they can only rely on imports, This is an important reason for the high price of perfluorinated sulfonic acid resin membranes. At present, the processing and molding technology of perfluorinated sulfonate resin can be divided into: melt extrusion, gel extrusion, solution pouring, tape casting, etc. In recent years, China has gradually begun to promote the domestic substitution of perfluorinated sulfonic acid resin membranes and has achieved significant results. Representative enterprises include Dongyue Group and Jiangsu Kerun.
1.3.2.2. Stack Assembly: Stacking and fastening, requirements
The assembly design of all vanadium flow stack requires high technical requirements. From the overall layout and single cell structure of the stack, there are many similarities between all vanadium flow cells and proton exchange membrane fuel cells. However, the vanadium battery system does not require the use of precious metal catalysts and there is no problem of difficult medium storage. In fact, many liquid flow battery research and development teams have rich experience in hydrogen fuel cell stack research, such as the Dalian Institute of Chemical Physics, Chinese Academy of Sciences. The assembly of the all vanadium liquid flow stack is completely consistent with that of the hydrogen fuel stack, both of which are assembled and fastened using a filter press. This assembly method seems simple, but in reality, it has high technical requirements. Firstly, overlapping and tightening will compress the electrical hole structure, which greatly tests the withstand voltage performance of the dual plate; Secondly, there is a hard contact between the battery and the double panel, which relies on a certain amount of compression force to reduce the interface contact resistance. If the bonding is poor, it will reduce the voltage efficiency of the battery stack; At the same time, the leakage prevention requirements of the stack are very high, and leakage of liquid and gas not only causes capacity attenuation, but also may cause safety accidents.
2. Horizontal comparison: vanadium battery vs. lithium battery, sodium battery, sodium sulfur battery
The construction of liquid flow batteries is completely different from ordinary secondary batteries such as lithium-ion batteries. Firstly, the electricity of the liquid flow battery is made of inert materials, and the positive and negative electricity itself does not participate in electrochemical reactions. The active substances that actually participate in the reaction have independent energy storage units, which form a closed loop between the internal and external storage tanks of the battery along the mass transfer line under the action of a circulating pump. The active substances are supplied to the electricity in a timely manner, and the reaction products are quickly extracted, thereby avoiding concentration difference and thermal accumulation effects. In other words, the liquid flow stack unit is just a place where electrochemical reactions occur, and the active substances are separated from it in spatial distribution, which means two meanings: firstly, the power characteristics and capacity size of the battery are relatively independent, so it can have great flexibility in design and application; Secondly, the active substances are stored separately in external storage tanks, which facilitates operation, maintenance, and safety management. This is precisely the root cause of the safety, flexibility, and other advantages of liquid flow batteries compared to other secondary battery technologies. In addition, the active substances of liquid flow batteries are generally completely dissolved in the electrolyte to form a homogeneous system, unlike lithium-ion batteries that adhere to the current collector. Therefore, there is no complex solid-state phase transition or mechanical strain or other destructive factors, which is the root cause of the long cycle life of liquid flow batteries compared to other secondary battery technologies. (Report source: Future Think Tank)
2.1. Vanadium battery vs. lithium battery: The performance characteristics are completely opposite, and the application scenarios are completely different
The performance characteristics of all vanadium flow batteries and lithium-ion batteries are completely opposite, and the application scenarios of the two are far different, but they are actually not on the same track. Firstly, in terms of (mass) energy density, all vanadium flow batteries are much lower than lithium-ion batteries. At present, the low value of lithium iron phosphate batteries in lithium-ion battery systems is more than three times that of all vanadium flow batteries. If additional facilities are included, the volume of the entire vanadium flow battery system when storing the same level of energy is about 3-5 times that of lithium-ion batteries. Therefore, the current water-based all vanadium flow batteries are almost impossible to be used in the field of automotive power batteries or small consumer electronics. Large scale static energy storage has low requirements for energy density and high tolerance for spatial factors such as floor area, making it the main application scenario for all vanadium flow batteries.
2.2. Vanadium vs. sodium electricity: Their advantages and disadvantages are highly complementary, or they may coexist in the energy storage market
All vanadium flow batteries and sodium ion batteries have strong complementarity. The former is suitable for large-scale energy storage, while the latter is suitable for small-scale flexible energy storage. Liquid flow battery is an electrochemical energy storage device in the liquid phase (mainly aqueous phase system), characterized by the active working substance dissolved in the electrolyte, and energy storage and release are achieved by changing the oxidation valence state of the active substance. Typical representatives include all vanadium flow battery, iron chromium flow battery, zinc bromine flow battery, etc. The major advantages of liquid flow batteries lie in the inherent safety of their aqueous phase system and their ultra long cycle life, making them particularly suitable for medium to large electrochemical energy storage facilities. However, their disadvantages include low energy density and narrow operating temperature range, making them difficult to miniaturize or apply in high-cold areas. In contrast, the energy density of sodium ion batteries is about three times higher than that of liquid flow batteries, and they can withstand low temperatures of -40 ℃. However, due to their identical basic principles and structure with lithium-ion batteries (both belong to ion intercalation type secondary batteries and use flammable organic electrolytes), their intrinsic safety and cycle life are not as good as that of liquid flow batteries. In the future, sodium ion batteries and liquid flow batteries are expected to achieve hierarchical complementary advantages in the energy storage field. For example, household and mobile small energy storage devices have high energy density requirements and are suitable for using sodium ion batteries; Large and medium-sized electrochemical energy storage power plants have high safety requirements and are suitable for using liquid flow batteries.
2.3. Vanadium vs. sodium sulfur: Energy density is not fundamental, safety issues are vetoed with one vote
Although the energy density of sodium sulfur batteries is much higher than that of liquid flow batteries, their safety is lacking and has been rejected by multiple countries at home and abroad. Sodium sulfur batteries were born in the 1960s, and industrialization exploration abroad has lasted for more than half a century. Their advantages include high energy density, high power density, high Coulombic efficiency, long lifespan, and low cost. They once dominated the mainstream of electrochemical energy storage technology abroad, with a typical representative being Japan's NGK company. However, the fatal disadvantage of sodium sulfur batteries is their poor safety performance. They use elemental sulfur and metallic sodium separately for positive and negative reactions, and only use brittle ceramic separators for separation. The operating temperature is above 300 ℃, and once the separator is damaged, explosive reactions may occur. Since 2011, the research and application of sodium sulfur batteries have been stagnant abroad, and domestic policies have explicitly rejected the application of sodium sulfur batteries in the field of medium to large-scale energy storage. In contrast, although all vanadium flow batteries cannot compete with sodium sulfur batteries in terms of energy density, their significant safety advantages as medium to large-scale energy storage devices are sufficient to make up for their shortcomings.
3. Industry status: Complete technology, only lacking in Dongfeng
At present, the technology of all vanadium flow batteries is complete, but the industrial chain is not yet sound, demand has not yet opened up, production capacity is developing rapidly, and the scale effect has not yet been shown. We believe that the all vanadium flow battery industry may still be in a transitional stage from an introduction period to a growth period in the next three years, and is expected to experience explosive growth at the end of the 14th Five Year Plan period.
3.1. Industrial structure: Long chain and high complexity
The entire vanadium flow battery industry chain also includes three parts: upstream, midstream, and downstream, but it is more complex than lithium-ion batteries and involves multiple industries. Upstream: raw material supply, electrolyte preparation, and stack material processing. The main raw materials include vanadium pentoxide, sulfuric acid, carbon materials, polymer materials, as well as various auxiliary materials, involving industries such as basic chemical engineering, steel smelting, and non-ferrous metals. Among them, vanadium ore and its processing industry are at the core and are the source of vanadium pentoxide, the raw material for electrolyte. Mid stream: stack assembly, control system, other equipment and accessories, among which the technical barriers for stack assembly and control system are high, involving various consumables and electronic components. Downstream: Terminal application market, mainly for various types of energy storage users, including power generation side, grid side, and electricity consumption side.
3.1.1. Upstream: Vanadium ore and vanadium processing, manufacturing of stack materials
(1) Vanadium ore and vanadium processing: strongly related to the steel industry
Due to the close proximity of the ion radius of vanadium to the valence states of iron, titanium, aluminum, phosphorus, etc., it is easy to undergo isomorphic doping substitution. Therefore, vanadium in nature is generally associated with these elements. In addition, due to the variability of valence states, vanadium ions can also replace elements such as molybdenum, chromium, tungsten, niobium, manganese, and copper in the lattice of compounds. Therefore, vanadium is a lithophilic element that generally exists in a dispersed state in ores. Its natural distribution characteristics are: large reserves, wide distribution, and low content. In nature, there are few high-grade vanadium ores. Among the more than 70 known vanadium containing minerals, only a few have higher vanadium content, mainly including green sulfur vanadium ore, vanadium lead zinc ore, vanadium copper lead ore, vanadium lead ore, potassium vanadium uranium ore, and vanadium mica. These vanadium rich mining areas are mostly located in Africa, the Americas, and other regions, and their reserves are also very limited. For example, Peru's green sulfur vanadium mine, which was once the main source of vanadium raw materials, has been exhausted. After World War II, one of the main sources of vanadium was the byproduct of uranium extraction from vanadium uranium ore. This was thanks to the rapid development of the atomic energy industry. Its main raw material was potassium vanadium uranium ore (K2 (UO2) 2 (VO4) 2-3H2O), which is a potassium uranyl vanadate salt water complex mainly produced in the United States, Australia, and other places. After the 1970s, the enrichment process of vanadium was further improved, and people could enrich and extract vanadium from some low-grade ores, mostly vanadium bearing iron ore or vanadium bearing carbonaceous shale, which greatly expanded the channels for obtaining vanadium.
Vanadium bearing carbonaceous shale is the main component of vanadium resources in China, accounting for 87% of the total vanadium reserves in China. Carbonaceous shale, also known as "stone coal", belongs to sedimentary minerals and is formed by the sedimentation and metamorphism of ancient vanadium rich fungi, algae, and plankton. Although stone coal is also known as "coal", its calorific value is generally 4184 kJ/kg (only 1/5 of the calorific value of ordinary coal), with a carbon content of only 10-15% and an ash content of up to 70-88%, hence the name "stone coal". Stone coal is rich in chemical elements. In addition to carbon, the ash contains more than 20 elements such as calcium, silicon, aluminum, vanadium, molybdenum, silver, gallium, cesium, potassium, chromium, arsenic, mercury, lead, cadmium, etc. Vanadium exists in stone coal as phases such as vanadium mica, vanadium containing kaolin, vanadium containing tourmaline, and vanadium containing garnet. The overall vanadium content (calculated as vanadium pentoxide) is generally 0.13-1.2%, with a lower grade. Most stone coal needs to undergo oxidation roasting before extracting vanadium, mainly to decarbonize and oxidize the low valent vanadium to pentavalent. The heat generated in this step can be used for power generation or heating. Then, it is mixed with sodium or calcium salts and subjected to secondary roasting to convert vanadium into vanadate, or the product of the first roasting can be directly pickled. The vanadium extraction industry from stone coal in China started in the late 1970s and has played an important role in the vanadium industry after more than 30 years of development. However, traditional processes have low conversion rates and severe pollution. Provinces such as Henan, Hubei, Chongqing, Shaanxi, Xinjiang, and Guizhou that have stone coal vanadium extraction industries mostly adopt industrial policies prohibiting the use of salt (including low salt) roasting vanadium extraction technology for new enterprises. At present, China urgently needs to develop a new and green vanadium extraction process from stone coal to fully utilize this resource.
Vanadium containing solid waste is an important vanadium resource besides ore vanadium, especially in certain areas where fuel ash and industrial waste vanadium catalysts can be used to extract vanadium. For example, crude oil from Venezuela, Russia, and other regions has a relatively high vanadium content, with fuel ash containing 4.4-19.2% vanadium, up to 40%, and a large total base with important recycling value. Due to the fact that smoke ash is a combustion product, all vanadium elements in it have been oxidized to pentavalent, and there is no need for further roasting and pre oxidation. Generally, after crushing, vanadium is directly heated and soaked in sodium hydroxide aqueous solution, which can convert vanadium into sodium vanadate and enter the aqueous phase. The residual vanadium in the filter residue can be further extracted and enriched with salt acid, and then extracted and separated. This wet leaching process can extract over 90% vanadium from cigarette ash with high purity. In addition, abandoned vanadium catalysts are also important vanadium resources, which can avoid pollution and save resources after recycling. The main vanadium catalysts include: contact method sulfuric acid catalysts, flue gas denitrification catalysts, and various vanadium catalysts in petrochemical synthesis. They can be converted into vanadate by sodium hydroxide for recovery, or converted into vanadium oxysulfate for recovery after reduction and acidification. Due to the fact that vanadium ore is a non renewable resource, the recovery and recycling of vanadium containing solid waste have far-reaching significance.
(2) Manufacturing of Electric Stack Materials: Large Development Space and High Technical Barriers
The stack materials for all vanadium flow batteries include several key materials such as batteries, double plates, separators, and seals. The raw materials are mostly carbon materials and polymer materials, which are closely related to the organic chemical industry. Due to differences in material selection and processes among manufacturers, there are also differences in the cost and performance of stack materials. In addition, there is still significant room for improvement in the technology and process of existing stack materials, and related research and development work is still ongoing, gradually achieving domestic substitution.
Double plates are made of modified graphite or carbon plastic composite double plates using graphite as the main raw material. Among them, graphite double plates are made by mixing graphite powder with resin or asphalt, and then undergoing high-temperature integrated graphitization treatment at 2500~2700 ℃ in a graphitization furnace. They are then cut and polished. Graphite double plates have the advantages of high density and low resistivity, but the manufacturing process is time-consuming, costly, and the material is brittle, making it prone to fragmentation during pressing and fastening. Therefore, they are not suitable for high-power, large-scale stacks, and are only suitable for small stacks. At present, carbon plastic composite double * plates are mostly used in high-power electric stacks. Generally, conductive carbon powder (such as graphite powder, carbon black, carbon fiber, etc.) is mixed with thermoplastic hydrocarbon polymers (such as PE, PP, PVC, etc.), and then blockers and release agents are added. Then, they are processed and formed by injection molding or compression molding.
The membrane material * initially used perfluorinated proton exchange membranes, but may later switch to non fluorinated ion conducting membranes. Perfluorinated proton exchange membranes were first used in the chlor alkali industry, successfully achieving industrialization, and later widely used in hydrogen fuel cells. Compared to fuel cell separators, all vanadium flow cell separators not only require high chemical stability and mechanical strength, but also good ion selective permeability. At present, domestic enterprises such as Dongyue Group have the ability to independently produce perfluorinated sulfonic acid resin membranes, but high-quality Nafion membranes for liquid flow batteries still need to be imported, which is very expensive. For example, the price of Nafion 115 used to be 700 US dollars per square meter, because the forming technology of sulfonic fluorohydrocarbon polymers has long been monopolized by DuPont in the United States. In addition, the ion selectivity of Nafion membrane is not satisfactory. The other route is to use non fluoride ion conduction membrane, that is, non ion exchange membrane. This technology is the direction of independent development in China. The scientific research team represented by the Chinese Academy of Sciences Dalian Institute of Chemical Physics has made important achievements and has core intellectual property rights.
3.1.2. Mid stream: Assembly and control system of the entire stack
Electric stack assembly: high technical barriers and long research and development cycle
The assembly technology barrier of all vanadium flow stack is relatively high. The assembly of the all vanadium liquid flow stack is completely consistent with that of the hydrogen fuel stack, both of which are laminated and fastened using a filter press. Many liquid flow battery research and development teams in the industry have years of experience in the development of hydrogen fuel cell stacks, such as the Dalian Institute of Chemistry, Chinese Academy of Sciences. The lamination and tightening of the filter press will generate compressive stress on the electric and double plates, and excessive pressure may cause plastic deformation or even fracture of the plates; Insufficient pressure can lead to poor adhesion between the battery and the double panel, increasing contact resistance, reducing the voltage efficiency of the stack, and may also lead to liquid and gas leakage, resulting in system capacity attenuation and even inability to work. Generally speaking, the larger the power scale of the stack, the larger the working area of the internal materials, and the greater the difficulty of its assembly process. Under the current technological framework, there are not many enterprises with large-scale production capacity for all vanadium liquid flow electric stacks. However, after years of development, it is difficult to make significant changes to the main structure of the stack. Therefore, these leading enterprises have a first mover advantage and maintain this advantage through continuous optimization and upgrading.
Control system: High maturity, self developed or outsourced
The control system includes PCS, BMS, EMS, etc. The required hardware devices are the basic components of the power electronics industry, and the related industries are relatively mature. Customized production can be achieved through cooperation with relevant enterprises. The electrolyte transportation system consists of components such as pipelines, circulating pumps, frequency converters, control valves, sensors, heat exchangers, etc. These devices are common in the field of chemical production and can be directly purchased for self processing or outsourced for design. Other facilities include fire protection equipment, building materials, etc., which account for a relatively low proportion of the total cost of the vanadium flow battery system and have relatively small profit margins.
3.1.3. Downstream: Energy storage - generation side, grid side, and consumer side
Downstream of the industrial chain are various types of energy storage users, which can be divided from top to bottom according to the main structure of the power industry chain: generation side, grid side, and electricity consumption side. Under different access methods, the electrical capacity, construction specifications, acceptance standards, and operation modes of energy storage equipment also vary. At present, industrialized energy storage technologies mainly include pumped energy storage and electrochemical energy storage, which mainly include lithium-ion batteries, lead-acid batteries, flow batteries, and other battery technologies. According to data statistics from China Energy Storage Network, as of the end of 2020, pumped storage accounted for 89% of the cumulative installed capacity of domestic energy storage, a decrease of four percentage points from 2019; Electrochemical energy storage accounts for 11%, of which 89% are lithium-ion batteries, about 10% are lead-acid batteries, and only about 0.7% are liquid flow batteries. In the newly added electrochemical energy storage in 2020, lithium-ion batteries accounted for 97%. However, considering safety and other factors, the increment of lithium-ion battery energy storage in the future may decrease, and the entire market's increment may gradually shift towards liquid flow batteries, especially the mature technology of all vanadium liquid flow batteries. (Report source: Future Think Tank)
3.2. Main enterprises: High market concentration, with Chinese enterprises leading the way
The world of research and industrialization of all vanadium flow batteries in China is divided into two categories of domestic vanadium battery enterprises: one is a start-up enterprise created through self research and technology transformation by research institutes, represented by Dalian Rongke; Another type is enterprises that absorb and merge foreign technologies, and then optimize and upgrade them, represented by Beijing Puneng. Most of the foreign enterprises related to all vanadium flow batteries are small-scale, mainly distributed in Japan, North America, and Europe. The complexity of the entire vanadium flow battery industry chain is high, with the core links being the material end and the equipment end. The material end mainly includes electrolyte materials and stack materials, while the equipment end mainly includes the overall assembly and control system.
3.2.1. Material end: electrolyte material, stack material
(1) Electrolyte material
The core electrolyte material of all vanadium flow batteries is vanadium compounds. Electrolyte manufacturing is divided into two steps. Firstly, the production of core precursors, namely vanadium chemicals (vanadium pentoxide, ammonium metavanadate, etc.). Currently, large vanadium refining and processing enterprises have the relevant technology and production capacity; Afterwards, the precursor was converted into electrolyte. Currently, Dalian Borong New Materials Co., Ltd., a global leading enterprise in the production of vanadium electrolyte, has a global market share of over 80%.
(2) Stack material
The stack materials of all vanadium flow batteries are highly similar to hydrogen fuel stacks. Currently, all vanadium flow batteries have not been widely applied, so several representative enterprises mainly rely on self-developed or outsourced processing to produce electrical materials for their own use. Once market demand increases in the future, there is a high probability that supply will exceed demand. Due to the fact that materials such as batteries, double plates, and separators in all vanadium liquid flow stack materials are almost identical to the corresponding components of hydrogen fuel cells, enterprises currently engaged in the research and production of hydrogen fuel stack materials are more likely to transform into stack material suppliers for vanadium batteries in the future and should pay attention to them.
3.2.2. Equipment side: Manufacturing of the entire stack and control system
Vanadium battery enterprises in China can be roughly divided into two categories: one is a start-up enterprise created through self research and technology transformation by research institutes, mostly carried out through school enterprise cooperation mode, represented by Dalian Rongke; The other type is enterprises that acquire corresponding technologies to participate in competition through mergers or control, represented by Beijing Puneng. The main enterprises include Wuhan Nanrui, Shanghai Electric, Sichuan Weilide, Shanghai Shenli Technology, etc. Each has its own core technology and has a general research and development time of over 10 years.
4. Future development: improving performance and reducing costs
All vanadium flow batteries have inherent safety and long lifespan that cannot be replaced by other electrochemical energy storage technologies. The main reason for hindering large-scale commercial use is: limited application scenarios due to single performance, and insufficient economic efficiency due to high initial costs. The future development direction of all vanadium flow batteries mainly lies in improving battery performance to expand application scenarios and reducing the initial investment cost of the system.
4.1. Current pain points: low energy density, narrow working temperature range, and high initial cost
(1) Low energy density of all vanadium flow batteries: limited application scenarios
At present, the energy density of water-based sulfuric acid based all vanadium flow batteries is only 20-50W · h/kg, which is less than one-third of that of lithium iron phosphate batteries. Relatively low energy density means storing the same level of energy. All vanadium flow batteries require a larger weight and volume than lithium-ion batteries, resulting in limited practical application scenarios and can only be used for static energy storage devices, making it difficult to apply to vehicle power systems or portable electronic products. In fact, the important application of lithium-ion batteries for rapid industrialization and cost reduction is their rich application scenarios, which can be used for both consumer electronics and automotive power batteries. Therefore, once the technology is basically mature, the demand side can quickly increase production and scale effects can be demonstrated.
(2) The working temperature range of all vanadium flow battery is narrow: additional temperature control system is required
At present, the ideal working temperature range for water-based sulfuric acid based all vanadium flow batteries is 5-45 ℃, and temperature control adjustment is required outside this temperature range. Due to the thermodynamic instability of aqueous sulfate based vanadium salt solutions, the current temperature requirements for all vanadium flow battery electrolytes are relatively strict. Direct exposure to both high and low temperatures can cause the system to malfunction. Negative low valent vanadium ions are prone to crystallization and precipitation at low temperatures, while positive pentavalent vanadium ions are prone to aggregation and solid precipitation of vanadium pentoxide at high temperatures. Both of these situations can lead to a decrease in electrolyte capacity and increase the resistance of the stack flow path, exacerbating concentration differences, and even damaging the stack in severe cases. Therefore, it is generally necessary to control the temperature and adjust the feedback of the electrolyte in all vanadium flow batteries, which will consume an average of about 5% of internal energy and add additional equipment, resulting in a lower energy density and larger volume of the entire system.
(3) High initial cost of all vanadium flow battery: insufficient initial economy
At present, the initial investment cost of all vanadium flow batteries is approximately 3000 yuan/kW · h, which is significantly higher than other mature energy storage technologies and lacks initial economic efficiency. At present, the core cost of all vanadium flow batteries lies in the electrolyte and stack materials, which together account for about 70% of the system cost and are difficult to significantly reduce in the short term. Although all vanadium flow batteries have high residual value and long lifespan, resulting in low average costs throughout their entire life cycle, the prerequisite is for the industrial chain to be connected and a closed loop of "production use recycling" to be formed. The promotion of industrialization also requires reducing initial costs and increasing downstream demand acceptance of this technology, which creates a paradox. Due to the insufficient economic efficiency of initial investment and the need for large funds for long-term maintenance of technology research and development, enterprises lack the ability to continue research and development and promotion. Eventually, they can only give up research and development and package and sell the technology. This is the main reason why foreign vanadium battery research has been lukewarm for many years.
4.2. Technical Outlook: Material Improvement, System Optimization
4.2.1. Improvement of electrolyte system: higher energy density and lower usage cost
(1) Improving electrolyte concentration and stability: hydrochloric acid based all vanadium flow batteries
Improving the concentration and stability of the electrolyte is the key to increasing the energy density of the system and expanding the working temperature range. The traditional sulfuric acid based all vanadium flow battery has low energy density and narrow operating temperature range, which is essentially due to the difficulty in improving the solubility of sulfate and the poor thermal stability of the solution. To improve the performance of vanadium sulfate based electrolytes, a common solution is to add a complex stabilizer, but so far, there is no additive that can simultaneously consider the high temperature stability of positive and negative electrolytes. Vanadium ions have vacant 3D electron orbitals, while chloride ions, as a weak field ligand, can effectively chelate with vanadium ions, thereby improving the solubility and stability of vanadium salts. Based on this idea, the research team of Pacific Northwest National Laboratory (PNNL) in the United States took the lead in developing a mixed acid system vanadium electrolyte of "sulfuric acid+hydrochloric acid" in 2011, and then developed a pure hydrochloric acid based vanadium electrolyte. Among them, the vanadium ion concentration in the mixed acid system reaches 2.5 mol/L, the energy density is increased by 70% compared to the original sulfuric acid based vanadium battery, and the working temperature range is -5~50 ℃; The vanadium ion concentration of the full hydrochloric acid based vanadium electrolyte reaches 5mol/L, and the energy density is twice that of the sulfuric acid based vanadium battery. The working temperature range is -20~60 ℃. Due to the high vapor pressure of the hydrochloric acid system and the easy release of chlorine gas under overcharging conditions, and the difficulty in meeting the requirements of the stack system materials at that time, this technology did not enter large-scale commercialization.
(2) Expanding the Electrochemical Window of Electrolytes: Organic Non aqueous Flow Batteries
Organic non aqueous electrolytes can significantly increase the working voltage, thereby increasing the energy density of vanadium batteries. Whether traditional sulfuric acid based vanadium batteries or improved hydrochloric acid based vanadium batteries, their electrolytes belong to aqueous solutions. The selection of the types of positive and negative active substances is subject to the electrochemical window of water. The potential of positive and negative substances cannot be higher than the oxygen evolution potential of water, and the potential of negative substances cannot be lower than the hydrogen evolution potential of water. Therefore, the positive and negative working voltage of water based all vanadium flow batteries is lower, and the energy density of the battery is directly proportional to the working voltage. Therefore, only by breaking through the water environment can energy density jump. Non aqueous organic solvent electrolytes have two huge advantages: 1) a significant increase in the variety of active substances available, allowing for the selection of lightweight, inexpensive metal ions or even compounds other than vanadium, thereby increasing specific capacity and reducing costs; 2) The significant increase in working voltage is of great help in improving the energy density of liquid flow batteries. However, organic non aqueous flow batteries also have many drawbacks: short cycle life, low energy efficiency, and the toxicity and flammability of organic solvents. Overall, organic non-aqueous electrolytes are an important direction for achieving miniaturization of liquid flow batteries, but there may be some discounts in terms of safety. Currently, there is still a long way to go from practical applications.
(3) Using cheap metals as active elements: all iron flow batteries
Replacing vanadium with cheap metals such as iron as the active element can reduce the cost of electrolytes from the source. The electrolyte cost of all vanadium flow batteries accounts for 30-50% of the total system cost, and the fundamental reason is the high price of vanadium. If some cheap metals are used to replace vanadium as the active element, the cost of the electrolyte can be fundamentally reduced. A typical example is an aqueous all iron flow battery, where the positive and negative redox pairs are Fe2+/Fe3+- Fe0/Fe2+, the active material is ferrous chloride, and the substrate is hydrochloric acid aqueous solution. When the battery is discharged at full charge, the positive active substance undergoes a reduction reaction: Fe3++e → Fe2+, with a standard potential of+0.77 V; The negative active substance undergoes an oxidation reaction: Fe0 → Fe2++e, with a standard potential of -0.44 V. The overall reaction of the entire battery can be merged into: Fe3++1/2Fe0 → 3/2Fe2+, with an open circuit voltage of 1.21 V, which is the process of trivalent iron ions reacting with zero valent elemental iron to become divalent ferrous ions. The electrons start from the negative * and pass through the external circuit to reach the positive *. The process of charging and energy storage is the opposite. Due to the strong acidic environment of the electrolyte, and the reduction potential of ferrous ions being lower than that of hydrogen ions, the negative charge of all iron flow batteries often accompanies severe hydrogen evolution reactions during charging, leading to an increase in solution pH, causing ferrous ions to undergo hydrolysis and precipitation of paste like hydroxides. At the same time, all iron liquid flow batteries belong to the "liquid deposition" type of liquid flow batteries, where negative ions deposit solid elemental metal iron during charging. When the deposition is uneven, sharp iron dendrites are formed, which can easily puncture the diaphragm and cause internal short circuits.
4.2.2. Improvement of ion conduction membrane: Non fluorinated porous filter membrane, replacing perfluorinated resin
Perfluorinated sulfonic acid resin membranes have high costs and poor ion selectivity, making non fluorinated porous membranes an important alternative in the future. At present, perfluorosulfonic acid resin represented by Nafion 117 is still a commonly used vanadium battery separator, with good stability and conductivity, but high cost and poor ion selectivity. The main development direction in the future is to develop new types of separators to replace perfluorinated sulfonic acid resin membranes. One promising solution is to develop non fluorinated porous filter membranes, which are currently a research hotspot in membrane materials for liquid flow batteries. China's research in this field is at the forefront of the world. Zhang Huamin's team of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, originally proposed the ion screening conduction mechanism of "no ion exchange group", and developed a non fluoride porous membrane with good stability and ion selectivity. However, currently, the conductivity of most non fluorinated porous membranes is relatively low compared to perfluorinated sulfonic acid resin membranes, resulting in a larger internal resistance, which requires further research and improvement. (Report source: Future Think Tank)
4.2.3. Whole system structure optimization: small vanadium electrical module, oriented towards household energy storage
By integrating various modules and manufacturing miniaturized vanadium electrical modules with high integration, it is expected to serve as a flexible energy storage device for household use. Due to the inherent safety of all vanadium flow batteries, they can be used in densely populated residential areas, such as household energy storage systems. However, the large volume of typical all vanadium flow battery systems limits their application in household energy storage. Therefore, some domestic and foreign enterprises have begun to explore small vanadium battery modules, mainly by increasing the power density of the battery stack to achieve miniaturization, and then integrating various modules to improve integration, thereby reducing the total volume of the system. Currently, miniaturized all vanadium flow batteries can achieve the same size as household refrigerators
