Technical analysis of all-vanadium liquid flow batteries

Due to global warming, the world is beginning to transition to low carbon. Energy storage, as an indispensable part of the low-carbon process, has been developing rapidly in the past two years. At present, the main energy storage battery is lithium-ion battery, but due to the lithium battery raw material prices gradually outrageous, the capital will turn its attention to the excellent nature of the liquid flow battery.

Vanadium battery development history

Liquid current battery has a very rich history of development, the earliest is the United States NASA in 1973 began to study, when the research system is mainly Fe (3 +)/Cr (2 +) electric pairs.

In 1976. research scholars found that vanadium can be used as the active substance of the liquid current battery; in 1958. scholars theoretically proved the feasibility of vanadium batteries, and in the following year, the all-vanadium ion redox liquid current battery was formally introduced and patented.

As research scholars continue to deepen the study, articles about vanadium battery electrodes, electrolyte, diaphragm and so on are endless. The advantages and disadvantages of vanadium batteries are gradually recognised.

Vanadium batteries have the following advantages over other energy storage batteries. First of all, the battery capacity and output power is relatively independent, the battery capacity depends only on the electrolyte concentration and the amount of electrolyte, the output power depends on the size of the power pile; Secondly, vanadium battery charging and discharging only the vanadium ion valence state of the change, can be achieved in depth discharge; At the same time, only one kind of ion reaction occurs, there is no other interference, the theoretical life of the infinite; When the vanadium battery system is closed, the battery has no self-discharge phenomenon, the response speed When the vanadium battery system is switched off, the battery has no self-discharge phenomenon, fast response speed, instantaneous charging can be realised, and there is no safety hazard; lastly, the recycling treatment of the battery is simpler and more convenient than other batteries, and it is not harmful to the environment.

Disadvantages are also very obvious, vanadium battery energy density is low, can only reach 40Wh/kg, with a lithium-ion battery difference of more than ten times; vanadium battery cost compared to other liquid current batteries, such as iron and zinc, is much higher, and covers a large area, the working temperature range is narrow, limiting the application of vanadium batteries.

Vanadium battery principle and materials

Vanadium batteries are mainly composed of electrolyte, electrodes, selective proton exchange membranes, bipolar plates and fluid collectors.

Among them, the electrolyte accounts for the highest proportion of the cost, which can reach 50%. The main component of positive and negative electrolyte is vanadium ion sulphate solution with different valence, in the industrial field, the most critical raw material of electrolyte is vanadium pentoxide, and the source of vanadium pentoxide, the current mainstream is vanadium pentoxide extracted from vanadium slag produced in the process of vanadium and titanium magnetite iron ore steelmaking.

At present, there are three main methods of vanadium electrolyte preparation: physical dissolution method, chemical reduction method, electrolysis method.

The physical dissolution method is to take sulphuric acid to dissolve the high concentration of vanadium oxide sulphate solid worth;; chemical reduction method is to take the single vanadium oxide solid worth.

Chemical reduction method is the use of monomer sulphur, sulphite and other reducing agents in certain conditions will be pentavalent vanadium reduced to tetravalent or trivalent vanadium;; electrolysis is currently the mainstream production of vanadium.

Electrolysis is currently the mainstream preparation method, using vanadium pentoxide as the raw material, produced in sulphuric acid, the operating temperature is generally minus 5 ℃ to 50 ℃.

In addition, adding some additives to the electrolyte will also affect the electrolyte performance. For example, adding additives such as methanesulfonic acid, glycerol, n-propanol, etc. can enhance the energy efficiency or energy density of the electrolyte.

In the field of electrodes, vanadium battery electrodes mainly include carbon electrodes, metal electrodes and composite electrodes. The carbon electrode has the characteristics of strong stability, corrosion resistance, good electrical conductivity, low cost, etc., but the electrochemical activity is poor, to be further improved.

Metal electrodes have the advantages of good conductivity, high mechanical properties, high electrochemical activity, etc., but the discharge process is easy to form a passivation film, which seriously affects the reaction.

Composite electrodes are generally made of polymer and carbon materials mixed with processing and moulding, with low internal resistance, good electrochemical stability, high conductivity and other advantages.

In the diaphragm, the most widely used cation exchange membrane for the Nafion membrane, the membrane voltage efficiency of more than 90%, good conductivity, increase the thickness of the membrane, can effectively improve battery efficiency, but the cost will increase.

Anion exchange membrane in vanadium batteries through the membrane electrolysis anions and electrolysis in the anionic cross, such as sulfate ions, to complete the transmission of carriers.

In addition, amphoteric ion exchange membrane is one of the current research hotspots, which has the characteristics of cation and anion exchange membrane.