Can liquid-flow batteries replace lithium-ion batteries?

A new type of liquid-flow battery has been developed by a team at Harvard University. The team said the liquid-flow batteries could be used not only in smartphones but also in new energy applications, including renewable energy. In the mobile era, battery technology has become a top priority, and even there is no mobile era without batteries. However, problems such as weak endurance exist in the batteries of mobile devices. The breakthrough of battery technology has always been a cutting-edge problem, which has constrained the further development of the mobile era. So researchers have been exploring more efficient energy sources for power generation in the hope of improving endurance.

In fact, liquid-flow batteries are not new technologies and have been around since the 1960s. Compared with lithium batteries, liquid-flow batteries do have some advantages. However, this technology has been in the research and development stage, and it has not been put into practical application. The reason is its own limitations. In spite of the obstacles, the exploration must continue. While humans use relatively sound new battery technologies, they are also constantly looking for cleaner energy to upgrade battery technologies.

First, the characteristics of liquid-flow batteries determine the advantages, and in some respects better than lithium batteries

The Harvard team was led by Michael Aziz, a professor of materials and energy technology, and Roy Gordon, a professor of chemistry and materials science. The new liquid-flow battery they studied is based on an organic molecule in a neutral pH aqueous solution for power generation, and its safety and life are better than current battery products.

In fact, the liquid-flow battery field is not considered a "wasteland." In the 1960s, the Redox battery of the iron-chromium system had already appeared and could be regarded as the predecessor of the all-vanadium liquid-flow battery. After years of research and development, the technology has made great progress and is expected to be put into commercial use. Compared with lithium-ion batteries, this liquid-flow battery does have an advantage.

First, its size can be large or small, and its design is flexible.

For energy storage systems, the most important factors are electricity and power. In general, vanadium liquid-flow batteries can withstand power depending on the size of the reactor, and the amount of electricity is proportional to the size of the energy storage tank. Regardless of the requirements of the project for the energy storage system, the designer can flexibly make the corresponding design and can adjust at any time.

On the other hand, lithium-ion batteries are coated with energy storage materials on the surface of the collector to form electrodes. Their processes and properties are fixed and it is difficult to adjust according to specific projects. Under the two relative ratios, the advantages of liquid-flow batteries are obvious.

More importantly, liquid-flow batteries are extensible. The liquid-flow battery has almost the same structure and control method regardless of the storage amount. As long as the energy storage electrolyte is mixed evenly, the SOC (charge and discharge depth) can be guaranteed to be consistent.

If you want to make lithium batteries of the same size, you need to stack the number of batteries, and use an extremely complex BMS (battery management system) to manage the temperature and SOC of each battery. With a little carelessness, overcharging, over-discharging, and overheating can lead to battery scrapping and even danger, which is why smart phone batteries sometimes explode.

Second, the long life of liquid-flow batteries.

At present, the life of lithium batteries on the market is about 1000~5000 times. The main energy storage principle is the embedding and deintercalation on solid-state electrodes, which is prone to cracks and ends the battery life.

The charging and discharging mechanism of liquid-flow batteries is based on changes in valence, rather than physical changes in ordinary batteries, so the service life is extremely long. Moreover, due to the separation of ion exchange membranes between positive and negative poles, all-vanadium liquid-flow batteries avoid the possibility of cross-infection of positive and negative electrolytes due to mixing, which is longer than other liquid-flow batteries.

Third, the safety of liquid-flow batteries is extremely high.

As mentioned in the first point, the characteristics of liquid-flow batteries guarantee their safety. There are no fire or explosion hazards, and there will be no safety problems even if there is a large current.

In addition, the energy efficiency of liquid-flow batteries is as high as 75 % to 80 %, the start-up speed is only 0.02 S, and the battery components are mostly cheap carbon materials without the need for precious metals as catalysts.

At present, companies producing all-vanadium liquid-flow batteries worldwide mainly include UniEnergyTechnologies of the United States, Gildemeister of Austria, Sumitomo Electric Co., Ltd. of Japan, and Dalian Rongke Energy Storage Technology Development Co., Ltd. of China.

Among them, Rongke Energy Storage Co., Ltd. has a total installed capacity of more than 12 MW of all-vanadium liquid-flow batteries, accounting for 40 % of the world's total installed capacity, and it also has the world's first 5MW large-scale industrial energy storage device that is actually connected to the network. This means that China's indicators are at the world's leading level.

Although liquid-flow batteries have so many advantages and have a certain scale of production and application, they have not yet seen large-scale commercial inputs and entered the consumer market because of the limitations of liquid-flow batteries themselves.

Second, liquid-flow batteries have been unable to be commercialized, and they have many limitations

As an energy storage system, liquid-flow batteries are still in the experimental stage in the field of wind power storage, and commercial use is even more difficult. The new liquid-flow batteries studied by Harvard University above are also in the development stage. We can first explore the limitations of the main vanadium batteries in liquid-flow batteries.

Theoretically, vanadium compounds can be used as additives in existing lithium batteries, which is similar to the use of graphene.

However, the pentavalent vanadium ions in the positive electrode of the vanadium battery will precipitate a highly toxic substance called vanadium pentoxide at temperatures above 45 degrees. The precipitation of this substance will block the flow path, cover the carbon felt fiber, deteriorate the performance of the electric reactor, and eventually cause the battery to be scrapped. More importantly, vanadium pentoxide, a highly toxic substance, may have serious consequences.

In addition, all-vanadium liquid-flow batteries need to be invested in extremely high costs. For example, a 5-kilowatt liquid-flow battery requires a total cost of 406,000 main materials, plus additional input of secondary materials and manpower costs.

Finally, liquid-flow batteries have a very low energy density, about 40 Wh/kg, and these batteries are liquid, so they cover a large area.

Based on the above limitations, liquid-flow batteries are difficult to apply on a large scale and are more difficult to commercialize.

The exploration of liquid-flow batteries represents the determination of mankind to continuously search for new energy sources, but this technology is not yet mature enough. Graphene battery technology, by contrast, has been used in smart devices, and humans are constantly looking for cleaner energy for power generation.

Third. Secure battery technology is commercially available and there are more possibilities for the future

In today's emerging battery technology, graphene battery technology is relatively safe. At the end of last year, Huawei launched its first lithium-ion battery using graphene technology at the 57th Japan battery conference. This type of battery uses new high-temperature technology to increase the upper temperature of lithium-ion batteries by 10 degrees, and its service life is also twice that of ordinary lithium-ion batteries.

Graphene seems to be more reliable than the new flow battery that is still under development. Of course, graphene itself has some limitations, but it has been applied in smart devices.

Therefore, in the current situation, graphene will be used more in the next phase of upgrading battery technology. In the development of battery technology, it is impossible to do it overnight, and a gradual transition through sound and mature technology should achieve better results.

Of course, this is not to say that the battery technology field can be stagnant for stability. On the contrary, in order to make battery technology no longer a drag on the development of the mobile era, it should be bolder to use all possible energy to power the advancement of battery technology. There have been related studies and progress has been made.

For example, the research team at the University of Pennsylvania recently developed a new power generation method that uses the concentration difference between the carbon dioxide emitted by the fossil fuel power plant and the carbon dioxide in the air to generate electricity. This device, called "flow cell", produces an average power density of 0.82 watts per square meter, which is about 200 times higher than the value obtained by previous approximation methods. The research results have been published in the latest issue of Environmental Science and Technology.

Similarly, Finnish scientists have also made some progress in studying how to use kinetic energy, heat energy and solar energy to power equipment. The researchers developed a ferroelectric material called KBNNO, which converts heat and pressure into electricity. Researchers at UniversityofOulu (University of Loo) in Finland used perovskite crystal structures to extract energy from multiple energy sources and hope to collect more energy through research.

The process of manufacturing such equipment is not complex, and once the best materials are found, it will be possible to commercialize the technology in the following years. If that were to happen, we might not have to plug mobile devices into sockets to charge them, but instead get a steady stream of electricity from natural energy to make it truly clean.

From the above results, we can make optimistic predictions. In the future, more new technologies will appear in the battery field. They can increase the utilization rate and endurance of batteries. In the development of battery technology and even any kind of technology, it is necessary to advance steadily and mature, but also need bold and avant-garde innovation. The combination of the two can better promote the further development of the mobile era.

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