Vanadium Battery | Energy Storage Sub-Segment – Flow Battery

 

Basic Concepts

All-vanadium flow battery, full name is all-vanadium redox battery (VRB), also known as vanadium battery, is a type of flow battery, a liquid redox renewable battery with metal vanadium ions as active substances. All-vanadium flow battery uses +4 and +5 valence vanadium ion solution as the active substance of the positive electrode, and +2 and +3 valence vanadium ion solution as the active substance of the negative electrode, which are stored in their respective electrolyte storage tanks. When the battery is charged and discharged, the positive and negative electrolytes undergo redox reactions on both sides of the ion exchange membrane. At the same time, through the action of the external pump of the battery stack, the electrolyte in the storage tank is continuously sent into the positive and negative chambers to maintain the concentration of ions and realize the charging and discharging of the battery.

 

 

Key Components

 

Electrolyte

The positive and negative electrolytes of the all-vanadium flow battery are its real energy storage medium and the core of the energy unit. They are generally composed of three parts: active substances, matrix, and additives. 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 reliability of the battery.

 

Active Substance: vanadium sulfate

The active substance of the electrolyte of the all-vanadium flow battery is vanadium sulfate, in which vanadium is the active element. The reason why vanadium is chosen as the core working element is that the ground state electronic configuration of vanadium is [Ar]3d24S2, which has a rich and varied oxidation valence state. +2, +3, +4, and +5 valences can all exist stably in an acidic aqueous solution environment, and the reduction potential of the positive and negative electrodes just fits the electrochemical window of water.

 

In addition, the characteristic spectra of hydrated vanadium ions of different valence states are very different and easy to identify: divalent vanadium is purple, trivalent vanadium is dark green, tetravalent vanadium is blue, and pentavalent vanadium is yellow. UV-Vis spectroscopy can be used for quantitative concentration analysis to monitor the state of charge (SOC) of the electrolyte in real time. Sulfates of vanadium of different valence states are used as active substances. The positive and negative electrode redox pairs are: VO2 + /VO2+ -V3+/V2+, positive electrode reaction: VO2 + + e ⇌ VO2+, negative electrode reaction: V2+ ⇌ V3+ + e, full battery reaction: VO2 + + V2+ ⇌ VO2+ + V3+.

 

Ideally, the positive and negative active ions of the uncharged original electrolyte are VO2+ and V3+, respectively, and the ratio of the two should be 1:1 to meet the stoichiometric ratio requirements and make full use of the active substances.

 

 

Matrix: Sulfuric Acid Aqueous Solution

The electrolyte matrix of all-vanadium flow battery is generally sulfuric acid aqueous solution, which is used to maintain the low pH of the electrolyte, inhibit the hydrolysis of vanadium ions, increase the conductivity of the electrolyte, and reduce ohmic polarization.

 

The main reason for using sulfuric acid aqueous solution is that sulfate ions are chemically inert and not easily oxidized or reduced, so there are relatively few side reactions. At the same time, sulfuric acid is not volatile, and the vapor pressure of its aqueous solution is low, so the internal pressure of the system generally does not change much. Although sulfuric acid, as a matrix supporting the electrolyte, does not contribute directly to energy storage, its content will directly affect the discharge capacity and energy efficiency of the electrolyte.

 

As the concentration of sulfuric acid increases, the viscosity of the electrolyte increases, resulting in increased flow resistance and intensified concentration polarization effects, resulting in a sudden voltage drop at the end of discharge and a small total discharge capacity; the overall conductivity of the electrolyte increases, the ohmic polarization effect is reduced, and the energy efficiency is improved. Taking all factors into consideration, the concentration of sulfuric acid in the electrolyte is generally controlled at 2~3mol/L.

 

Additives:

Organic and inorganic complexing agents In order to increase the solubility and stability of vanadium ions in the electrolyte, a small amount of additives are generally added to inhibit the precipitation of solids. There are many types of electrolyte additives, which are divided into two categories: organic and inorganic. Organic additives are generally multidentate ligands with coordination functional groups such as hydroxyl, thiol, and amino groups. They can form relatively stable complexes with vanadium ions, inhibit the nucleation and growth of V2O5 solids, and also act as dispersants to reduce the surface energy of particles and inhibit the coagulation of colloid particles.

 

Common organic additives include: amino acids, polyols, aminosulfonic acid, and some surfactants and water-soluble polymers. Inorganic additives are generally salts, in which 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 1~3%. Excessive use will hinder the ion transport mechanism, increase the ohmic polarization effect of the electrolyte, and reduce the energy efficiency of the system.

 

Manufacturing Process

The electrolyte of vanadium batteries is made by reducing vanadium pentoxide in sulfuric acid, and can be mass-produced by chemical methods or electrolysis. The early vanadium battery electrolyte was directly prepared by dissolving vanadium oxysulfate (VOSO4) in sulfuric acid solution. The advantage is that the operation is simple, but the price of vanadium oxysulfate is expensive, the economy is poor, and it is not suitable for large-scale production. At present, the methods for mass production of vanadium battery electrolytes are divided into chemical reduction method and electrolysis method, which are essentially to reduce pentavalent vanadium to low-valent.

 

• The chemical reduction method is to mix pentavalent vanadium raw materials (such as vanadium pentoxide, ammonium metavanadate, etc.) with sulfuric acid solution, put in a reducing agent (such as oxalic acid, sulfur dioxide, etc.) and heat it to react to obtain a low-valent vanadium salt solution. The advantage of the chemical method is that the process and equipment are simple, and the disadvantage is that the reaction is slow and requires high-temperature treatment.

 

• The electrolysis method is to reduce the pentavalent vanadium raw material by cathode in the electrolytic cell, and also obtain a low-valent vanadium salt solution. The advantage of the electrolysis method is that it can be mass-produced at room temperature and has high production efficiency. The disadvantage is that it consumes more electricity.

 

The oxidation valence of vanadium ions in the electrolyte in the initial state is between 3 and 4. After entering the battery stack, pre-charging begins. The vanadium ions at the anode are uniformly oxidized to +5 valence, and the vanadium ions at the cathode are uniformly reduced to +2 valence. At this point, the valence adjustment of the positive and negative electrolytes is completed and it can start working.

 

 

 

The electrolyte accounts for the largest proportion of the total cost of the all-vanadium flow battery system (generally 30%~50%). Although the basic raw materials of the electrolyte are all vanadium pentoxide, which is a homogeneous product, the performance and cost of the electrolyte produced are also quite different due to the different electrolyte production routes and additives adopted by different manufacturers.

 

• In terms of performance, the electrolyte formula is unique, especially the concentration, acidity and additives, which are protected by companies in the form of patents.
• At the same time, the differences in technology between different companies will cause differences in the impurity content of the electrolyte, which will also be reflected in the battery performance.
• In addition, the processing costs of different production processes are different.

 

The current market price of electrolyte is about 1,500 yuan/kW·h. It takes about 10kg of vanadium pentoxide to store 1kW·h of electricity. Therefore, the price of vanadium pentoxide in the form of electrolyte is about 150,000 yuan/ton. The current spot price of vanadium pentoxide on the market is about 100,000 yuan/ton, so the unit cost of processing vanadium pentoxide into electrolyte is about 50,000 yuan/ton.

 

In other words, 2/3 of the cost of electrolyte comes from vanadium pentoxide, and 1/3 comes from processing costs. Since vanadium pentoxide itself is extracted from vanadium slag and stone coal, if the process starting point of the electrolyte is directly from raw materials such as vanadium slag and stone coal, skipping the vanadium pentoxide link, then the entire manufacturing process can be shortened, thereby greatly reducing the cost of electrolyte, which requires the company to have a considerable production capacity and a strong control over the upstream.

 

Stack

The stack is the place where the all-vanadium liquid flow battery performs electrochemical reactions, which determines the power characteristics of the system, and the performance of the stack will directly affect the overall performance of the system. An all-vanadium liquid flow battery stack is essentially composed of multiple single cells stacked in series, generally stacked and tightened in the form of a filter press, with one or more electrolyte circulation systems inside, and a unified set of current inlet and outlet ports. The main components of the all-vanadium liquid flow cell include: electrodes, bipolar plates, diaphragms, end plates, seals and other fasteners.

 

 

The structure and principle of all-vanadium liquid flow battery are similar to those of hydrogen fuel cells. The stack is the core component of the system and is the place where electrochemical reactions occur and electricity is generated. The assembly of the all-vanadium liquid flow stack is exactly the same as that of the hydrogen fuel stack. Both are stacked and fastened in the manner of a filter press. This assembly method seems simple, but it actually has high technical requirements.

First, stacking and fastening will compress the electrode and change the electrode pore structure, which is a great test of the pressure resistance of the bipolar plate; secondly, there is hard contact between the electrode and the bipolar plate, that is, a certain pressing force is relied on to reduce the interface contact resistance. If the fit is poor, the voltage efficiency of the stack will be reduced; at the same time, the stack has very high requirements for leakage prevention. Leakage of liquid and gas will not only cause capacity attenuation, but may also cause safety accidents.

 

 

Among the common materials of vanadium and hydrogen, graphite bipolar plates are currently basically domestically produced, while proton exchange membranes and gas diffusion layers are still mainly imported. Metal plates are basically not considered in vanadium flow batteries. Even metal plates after coating are difficult to work stably for a long time in an acidic liquid environment.

 

The machining process of machined graphite bipolar plates is complicated and costly. Vanadium flow batteries mainly use carbon-plastic composite plates because their thermoplastic or molding processes are relatively simple to machine, but the increase in resistivity caused by mixed polymer resins is still a problem that needs to be solved.

 

Electrodes

The electrodes of all-vanadium flow batteries do not participate in electrochemical reactions, but only serve as the site of the reaction. The active substances gain or lose electrons on the electrode surface, undergo reduction or oxidation, and realize the mutual conversion between electrical energy and chemical energy.

 

The physicochemical properties of electrode materials have an important impact on all-vanadium liquid flow batteries:

 

First, the conductivity and catalytic performance of the electrode directly affect the polarization state and current density of the battery, and thus affect the energy efficiency;

 

Second, the physicochemical stability of the electrode material directly affects the overall working stability and actual life of the battery. Therefore, the electrode material must have high chemical inertness, mechanical strength, conductivity, and preferably a large specific surface area.

 

In the early days, metal electrodes were used, including 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 electrode materials have many defects, some of which have poor electrochemical reversibility, and some are too expensive to be used on a large scale and for a long time.

 

Later, people switched to carbon electrode 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.

 

The study found that glassy carbon electrodes have poor reversibility; graphite and carbon cloth electrodes are easily etched and lost during the charge and discharge process, and these materials have a small specific surface area, resulting in a large internal resistance of the battery, making it difficult to charge and discharge with a large current; although carbon paper electrodes have a large specific surface area and good stability, they have poor hydrophilicity and low electrochemical activity.

 

At present, the most widely used electrode materials are carbon felt or graphite felt, both of which are carbon fiber textile materials.

 

 

Carbon Felt is made of organic polymer fiber felt through heat treatment processes such as pre-oxidation and inert atmosphere carbonization, while graphite felt is made by further graphitizing carbon felt at a high temperature above 2000℃. This type of carbon fiber electrode has a large specific surface area, good chemical stability and conductivity, but is prone to oxidation and shedding during long-term use. Therefore, it is also necessary to modify it, including intrinsic material treatment, metallization treatment and oxidation treatment, or to make a composite material with an inert polymer matrix (but the conductivity will be reduced).

 

 

Bipolar Plate

The bipolar plate in the all-vanadium liquid flow battery is a conductive separator, which is closely attached to the electrode, used to separate the positive and negative electrolytes of two adjacent cells, collect current, and support the electrodes, so as to realize the series connection of multiple cells inside the battery stack. The ideal bipolar plate material has: good gas and liquid resistance, conductivity, chemical inertness, and mechanical strength.

 

The purpose of gas and liquid resistance is to prevent the positive and negative electrolytes on both sides of the plate from penetrating and cross-contamination, which is the most basic requirement for bipolar plates. High conductivity includes both the low impedance of the bipolar plate itself and the low contact resistance between the bipolar plate and the electrode, which is to reduce the internal resistance of the battery.

 

Since the two sides of the bipolar plate are strong oxidizing and strong reducing electrolytes, respectively, the bipolar plate material must have high chemical inertness to operate for a long time in such a harsh environment. Finally, as a supporting electrode, the bipolar plate must have good mechanical strength and machinability.

 

Initially, metal bipolar plates or pure graphite bipolar plates were used. The former has good mechanical strength but poor corrosion resistance (precious metals such as gold and platinum are too expensive), while the latter has good corrosion resistance but is brittle and has high processing costs.

 

At present, one solution is to modify the graphite bipolar plate to improve mechanical strength and processability; another solution is to use carbon-plastic composite bipolar plates, which are mixed with conductive fillers and polymer resins to form a mold, which has good mechanical strength and corrosion resistance, but the conductivity is reduced (the resistivity is 1~2 orders of magnitude higher than that of metal and graphite bipolar plates).

 

At present, the electrode material is also a consumable material. The actual service life under normal working conditions is about two years, and it needs to be replaced after expiration. At present, researchers have bonded the electrode and the bipolar plate into one by hot pressing or molding, which can obtain an integrated electrode-bipolar plate with good electrochemical performance and not easy to etch.

 

 

 

 

Since the carbon-plastic composite bipolar plate contains a large amount of insulating polymer matrix, its overall resistivity is 1~2 orders of magnitude higher than that of the graphite bipolar plate. Increasing the content of conductive filler can improve the conductivity, but excessive use will reduce the mechanical properties of the bipolar plate, especially the bending strength. Therefore, there are high technical barriers for carbon-plastic composite bipolar plates with both high conductivity and mechanical strength.

 

Diaphragm: ion selective permeation, the key to long life

The diaphragm in the all-vanadium liquid flow battery is an ion conduction membrane located in the center of each single cell. It is used to separate the positive and negative electrolytes inside the single cell to prevent the active substances from mixing with each other and self-discharging due to “liquid jumping”. At the same time, it allows the selective transfer of specific ions to ensure the internal circuit of the battery is conductive.

 

The performance of the diaphragm directly affects the efficiency and life of the battery. Generally, it requires: high ion selectivity, ion conductivity, chemical stability, and mechanical strength. Theoretically, the following can be selected: cation exchange membrane, anion exchange membrane, and porous separation membrane.

 

Among them, cation/anion exchange membranes have negative/positive charge groups that allow specific types of cations or anions to pass through; porous separation membranes have no charged groups and are screened and intercepted by ion radius.

 

At present, the most widely used proton conduction membrane in all-vanadium flow batteries is a cation exchange membrane with mature technology. The typical representative is the Nafion membrane produced by DuPont, which is a type of perfluorosulfonic acid resin with good chemical stability and ion conductivity, but poor ion selectivity and high cost (500~800 US dollars/square meter).

 

Later, people tried to modify ion-selective groups such as benzene sulfonyl on partially fluorinated polymer carbon chains to make partially fluorinated membranes, which significantly improved ion selectivity, but reduced chemical stability, and required radiation technology.

 

Considering the high cost of fluorinated resins, people turned to the development of non-fluorocarbon membranes, one type is a non-porous non-fluorinated ion exchange membrane, and the other is a porous non-fluorinated separation membrane. The non-porous non-fluorine ion exchange membrane is a membrane that introduces ion selective groups on non-fluorine polymers, such as sulfonated polyaryletherketone. It has good ion selectivity and conductivity, but its chemical stability is reduced and it is severely damaged after a few hundred cycles.

 

The typical representative of the porous non-fluorine separation membrane is the nanofiltration membrane, which has no charged groups on the surface but has a large number of nano-scale micropores, allowing hydrated protons with a smaller radius to pass through, but not allowing hydrated vanadium ions with a larger radius to pass through. At present, perfluorosulfonic acid resin membranes have begun to be replaced by domestic products, while the application of non-fluorine membranes is in the ascendant, which is of great significance for reducing the cost of battery systems.

 

Perfluorosulfonic acid resin membrane is the most widely used diaphragm in all-vanadium liquid flow battery.

From the molecular structure, the main skeleton of perfluorosulfonic acid resin is polytetrafluoroethylene structure, and the branched end group is perfluorovinyl ether structure of sulfonic acid group. The synthesis route is: tetrafluoroethylene and perfluoroether sulfonyl fluoride are copolymerized under the action of initiator, and then hydrolyzed and acidified.

The synthesis difficulty of perfluorosulfonic acid resin is acceptable, and the greater difficulty lies in the subsequent processing and membrane forming link. The key is to reduce processing loss and produce a membrane with uniform thickness and excellent performance. The core melt extrusion calendering molding technology has long been monopolized by DuPont in the United States. Domestic membranes are prone to defects such as “pinholes” and are difficult to meet the use requirements, so they can only rely on imports. This is an important reason for the high price of perfluorosulfonic acid resin membranes.

At present, the processing and molding technologies of perfluorosulfonic acid resins are divided into: melt extrusion method, gel extrusion method, solution casting method, cast film method, etc. In recent years, China has gradually begun to promote domestic substitution of perfluorosulfonic acid resin membranes and has achieved remarkable results. Representative companies include Dongyue Group and Jiangsu Kerun.

 

Sealing Parts

Sealing is an important guarantee for the performance of vanadium batteries. The system is fully sealed to strictly avoid external and internal leakage of electrolyte.

 

If external leakage occurs, divalent hydrated vanadium ions are easily oxidized in the air and capacity loss occurs, and the highly corrosive electrolyte may damage other components of the battery stack.

 

If internal leakage occurs, the positive and negative electrolytes may mix with each other, which will directly affect the performance and life of the battery stack, and it is not easy to find leakage from the outside of the battery stack.

 

Because the positive and negative electrolytes of the all-vanadium liquid flow battery are highly oxidizing and reducing, and the electrolyte matrix is ​​sulfuric acid, ordinary rubber sealing materials cannot withstand this environment at all, and special fluororubber must be used as seals.

 

In addition, the fluororubber material used for seals should have appropriate hardness, tensile strength, elongation at break and tear strength, and the compressive plastic deformation should be as small as possible, and additional self-tightening devices are required. However, the price of fluororubber is very expensive, about 300,000 to 400,000 yuan/ton, and it still faces problems such as aging and plastic deformation in long-term operation.

 

The research team of Dalian Institute of Chemical Physics of Chinese Academy of Sciences simplified the sealing process through integrated laser welding technology, realized the integration of diaphragm-electrode-bipolar plate, and saved fluororubber components, which is of great significance for reducing the cost of battery stack.

 

Technology Origin

In 1985, Professor Marria of the University of New South Wales (UNSW) in Australia began to study vanadium sulfate as positive and negative electrolytes and proposed the all-vanadium flow battery. The company’s main contribution is to find that high-concentration vanadium (V) solution can be stably present in sulfuric acid medium by oxidizing vanadium (IV) solution, thus making the all-vanadium flow battery have practical value.

 

At the same time, the process developed by the company to prepare vanadium battery solution from vanadium oxide has low cost and good performance, which is also an important reason for the promotion of vanadium batteries.

 

In 1995, the Institute of Electronic Engineering of China Academy of Engineering Physics first carried out research on vanadium batteries in China, successfully developed 500W and 1kW prototypes, and has many patents such as electrolyte solution preparation.

 

In 2006, the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences successfully developed a 10kW test battery stack and passed the acceptance of the Ministry of Science and Technology, marking the phased success of China’s all-vanadium flow battery system. Central South University, Tsinghua University and others have also successfully developed KW and above battery packs.

 

Since 2010, my country’s megawatt-level all-vanadium flow battery demonstration projects have begun to be carried out one after another. my country’s vanadium flow batteries have been applied in smart grids, communication base stations, power supply in remote areas, renewable energy and peak shaving and valley filling projects.

 

 

Since 2019, my country’s liquid flow battery energy storage demonstration projects have been accelerating construction. In February 2022, the first phase of the “200MW/800MWh Dalian Liquid Flow Battery Energy Storage Peaking Power Station National Demonstration Project”, a 100MW/400MWh all-vanadium liquid flow battery energy storage power station, completed the main construction and entered the single module commissioning stage. It is expected to be completed and connected to the grid by the end of the year. It is the world’s largest vanadium liquid flow energy storage project.

 

 

Advantages

The electrodes of flow batteries are made of inert materials. The positive and negative electrodes themselves do not participate in the electrochemical reaction. The active substances that actually participate in the reaction have independent energy storage units. Under the action of the circulating pump, a closed loop is formed between the internal and external storage tanks of the battery stack along the mass transfer line, and the active substances are supplied to the electrodes in time, and the reaction products are quickly extracted, thereby avoiding concentration polarization and heat accumulation effects.

 

In other words, the flow battery stack unit is only a place where electrochemical reactions occur, and the active substances are separated from it in spatial distribution, which means two meanings:

 

First, the power characteristics of the battery are relatively independent of the capacity, so there can be great flexibility in design and application;

 

Second, the active substances are stored separately in external storage tanks, which is convenient for operation, maintenance and safety management. This is the source of the safety and flexibility of flow batteries compared to other secondary battery technologies.

 

In addition, the active materials of flow batteries are generally completely dissolved in the electrolyte to form a homogeneous system, and are not attached to the current collector like lithium-ion batteries. Therefore, there is no complex solid-state phase change, no destructive factors such as mechanical strain, which is the root of the much longer cycle life of flow batteries than other secondary battery technologies.

 

High Safety.

The all-vanadium flow battery is a water circulation system, which is non-flammable and does not accumulate heat. The positive and negative active materials react mildly, so it has intrinsic safety. At the same time, the liquid homogeneous system of the all-vanadium flow battery avoids the “barrel effect” and is easy to manage and control.

 

Unlike lithium batteries, the electrolyte and the battery stack of the flow battery are phase-separated, which fundamentally overcomes the self-discharge phenomenon of traditional batteries. The active material is dissolved in the electrolyte, and the battery stack only provides a place for the electrochemical reaction. There is no phase change in the electrode reaction process. The risk of dendrite growth piercing the diaphragm is greatly reduced in the flow battery. Thermal runaway, overheating, combustion and explosion will not occur. Overcharging and overdischarging will not cause explosions and battery capacity reduction. It supports frequent charging and discharging, and can be charged and discharged hundreds of times a day.

 

The working principle of the flow battery determines that it is a safer technical route among the current electrochemical energy storage technology routes.

 

Strong Scalability.

The power and capacity of the all-vanadium liquid flow battery are independent of each other. The power is determined by the specifications and number of the battery stack, and the capacity is determined by the concentration and reserve of the electrolyte.

When the power is constant, to increase the energy storage capacity, it is only necessary to increase the volume of the electrolyte storage tank or increase the volume or concentration of the electrolyte without changing the size of the battery stack.

The power is increased by increasing the power of the battery stack and increasing the number of battery stacks, and the storage capacity is increased by increasing the electrolyte, which is convenient for the expansion of the battery scale and can be used to build kilowatt-level to 100 megawatt-level energy storage power stations. The power of vanadium batteries in commercial demonstration operation in the United States has reached 6 megawatts.

 

Long Life.

Compared with other electrochemical energy storage technologies, the most prominent feature of flow batteries is their long cycle life, which can be as low as 10,000 times, and some technical routes can even reach more than 20,000 times, with an overall service life of 20 years or more.

 

Long service life: Since the positive and negative active substances of vanadium batteries are only present in the positive and negative electrolytes respectively, the phases of other batteries do not change during the charging and discharging process, and deep discharge can be performed without damaging the battery, and the battery has a long service life.

 

The longest-running vanadium battery component in the Canadian VRB power system commercial demonstration has been operating normally for more than 9 years, with a charge and discharge cycle life of more than 18,000 times, far higher than the 1,000 times of fixed lead-acid batteries.

 

Low Cost Over The Entire Life Cycle.

The positive and negative active materials of vanadium batteries exist in the positive and negative electrolytes respectively. There is no phase change common in other batteries during charging and discharging, and deep discharge can be performed without damaging the battery. During the charging and discharging process, the vanadium ions as active materials only undergo valence changes in the electrolyte, do not react with the electrode materials, and do not produce other substances. After long-term use, they still maintain good activity.

 

Therefore, vanadium batteries have a long service life. The number of charge and discharge cycles of all-vanadium liquid flow batteries is more than 10,000 times, and some can reach more than 20,000 times. Calculated over the entire life cycle, the cost of vanadium batteries is 0.3-0.4 yuan/Wh, which is lower than the cost of lithium batteries (about 0.5 yuan/Wh).

 

The Electrolyte Can Be Recycled And Reused.

 

In the all-vanadium liquid flow battery, the vanadium element exists in the form of ions in the acidic aqueous solution, rather than in the form of vanadium oxide. It is corrosive but non-toxic, and it operates in a closed manner during operation, which basically does not cause harm to the environment and human body.

 

The cost of the electrolyte solution accounts for 40% of the total cost of the energy storage system. After the energy storage system is scrapped, the vanadium electrolyte solution can be recycled and reused, with a high residual value and will not cause pollution to the environment; in addition, the vanadium electrolyte of the all-vanadium liquid flow battery can be recycled for a long time in the battery field or vanadium can be extracted to enter other market fields such as steel and alloys. The electrodes in the battery stack material use carbon/graphite felt, and the bipolar plates mostly use graphite or carbon materials, which will not cause pollution to the environment after scrapping.

 

Good Electrochemical Performance:

Due to the high catalytic activity of vanadium battery electrodes, positive and negative active substances are stored in positive and negative electrolyte storage tanks respectively, avoiding the self-discharge consumption of positive and negative active substances. The efficiency of charge and discharge energy conversion of vanadium batteries reaches 75%, which is much higher than the 45% of lead-acid batteries; the response speed is fast. During operation, the charge and discharge state switching takes only 0.02 seconds, and the response speed is 1 millisecond; the vanadium battery can be instantly charged by changing the electrolyte.

 

Raw Materials Are Self-Controlled.

Unlike lithium batteries, China’s dependence on foreign lithium raw materials is relatively high. The vanadium ore reserves are about 9.5 million tons, accounting for 39% of the world’s vanadium resource reserves, ranking first in the world. The resources needed to develop vanadium batteries can be self-controlled.

 

Disadvantages

High initial installation cost.

The total investment cost of vanadium battery projects that have disclosed specific investment amounts is concentrated in 3.8-6.0 yuan/Wh; among them, the cost of four-hour energy storage systems is concentrated in 3.8-4.8 yuan/Wh, and the cost of 2-3 hour energy storage systems is slightly higher, at 4.65-6 yuan/Wh, which is still higher than lithium batteries overall. The initial investment amount of lithium energy storage projects in 2021 is close to 2 yuan/Wh, and the initial installation cost of vanadium batteries is more than twice that of lithium-ion batteries.

 

Low Conversion Efficiency.

During operation, all-vanadium liquid flow batteries have high requirements for ambient temperature, and pumps are also required to maintain the flow of electrolytes. Therefore, their losses are large, and the energy conversion efficiency is 75%, which is lower than that of lithium batteries.

 

Low Energy Density.

Limited by the solubility of vanadium ions and the design of the battery stack, compared with other batteries, all-vanadium liquid flow batteries have a lower energy density of only 12-40Wh/kg. The volume is huge. The mass specific energy of vanadium batteries is 1/3~1/2 of that of lithium batteries or sodium sulfur batteries, which will make the battery heavy and large. This also directly leads to the fact that vanadium batteries are not suitable for electric vehicles and can only be used in static energy storage devices.

 

Limited by the solubility of different vanadium ions in the range of 10℃~40℃, the total vanadium concentration of all-vanadium liquid flow batteries is limited to less than 2M, which restricts the improvement of the specific energy of all-vanadium liquid flow energy storage systems.

 

Consumables Need to be maintained in time.

The graphite plate will be etched by the cathode liquid. If the user operates properly, the graphite plate can be used for two years. If the user does not operate properly, a single charge can completely etch the graphite plate and the battery stack can only be scrapped. Under normal use, maintenance must be performed by professionals every two months. This high-frequency maintenance is expensive and laborious.

 

The Volume is Too Large.

Due to the upper limit of ion solubility in the electrolyte, the specific energy density of vanadium batteries is low, and the technology is difficult to break through. The volume of vanadium batteries with the same energy can reach 3-5 times that of lithium batteries, and the mass can reach 2-3 times.

 

It has Strict Requirements On The Ambient Temperature.

The pentavalent vanadium in the cathode liquid of vanadium batteries is easy to precipitate vanadium pentoxide when it is left still or the temperature is higher than 45℃. The precipitated precipitate blocks the flow channel, covers the carbon felt fiber, deteriorates the performance of the battery stack, and even scraps the battery stack. The electrolyte temperature of the battery stack can easily exceed 45℃ during long-term operation.

 

On the other hand, the temperature cannot be lower than the freezing point of the electrolyte, otherwise the electrolyte will solidify and the battery will not be able to operate. Therefore, the general operating temperature is required to be between 0~45℃

 

 

The performance characteristics of all-vanadium flow batteries and lithium-ion batteries are completely opposite, and their application scenarios are very different. In fact, they are not on the same track.

 

At present, it is almost impossible for aqueous all-vanadium flow batteries to be used in vehicle power batteries or small consumer electronics.

 

Large-scale static energy storage does not require high energy density and has a high tolerance for space factors such as floor space, so it has become the main application scenario of all-vanadium flow batteries.

 

After the industrial chain is improved, the average cost of all-vanadium flow batteries will be much lower than that of lithium-ion batteries, and it is expected to become the mainstream in the field of medium and large-scale energy storage.

 

All-vanadium flow batteries and sodium-ion batteries are highly complementary. The former is suitable for large and medium-sized energy storage, while the latter is suitable for small and flexible energy storage.

In the future, sodium-ion batteries and flow batteries will be expected to achieve hierarchical complementary advantages in the field of energy storage.

For example, household and mobile small energy storage devices have high energy density requirements and are suitable for sodium-ion batteries; large and medium-sized electrochemical energy storage power stations have high safety requirements and are suitable for flow batteries.