“Zinc “partner, ”zinc“ future!
Zinc-based liquid current battery energy storage technology has the advantages of low cost, high safety, high energy density, etc. It is a typical representative of hybrid liquid current batteries, and is suitable for use as a stationary energy storage system on the user side, which helps to promote the transformation of the energy structure and realize the goal of “dual carbon”.
Since the introduction of zinc-bromine flow battery in 1977, it has developed into the most diverse type of flow battery in energy storage system by pairing zinc negative electrode with different positive electrode redox electrodes.
In the last push, the development history and industrialization status of zinc-based liquid current batteries were sorted out, and the main problems and solution strategies of zinc negative electrodes were summarized
Energy storage is in the ascendant, and zinc is in the ascendant.
Next, let’s learn about the advantages, existing problems and solution strategies of various redox pairs that can be applied to the positive electrode of zinc-based liquid current batteries.
Zinc-bromine flow battery
Bromine has the advantages of abundant reserves, low cost, high electrode potential, high solubility, high theoretical energy density, etc. Zinc-bromine flow batteries using zinc negative electrode to match with them have been developed to be more mature, and are gradually being recognized by the market.
Bromine ions are oxidized into polybrominated ions, mainly bromotriions and bromopentaions, during the charging process. The discharge process is the opposite, and the overall charging and discharging process has good reversibility.
Bromine positive electrode has problems such as high corrosiveness, high volatility, high diffusivity and low reactivity.
Strong corrosiveness
The strong corrosiveness of bromine in the charging state requires good corrosion resistance in the components of the battery that come into contact with it, making the battery system more costly.
High volatility
The strong volatility of the charging state of bromine will cause the loss of active substances, if not operated properly, it is very easy to spread into the air and cause pollution to the environment.
High diffusivity
The bromine monomers and polybromides generated after charging will easily diffuse through the diaphragm to the negative electrode and react with zinc, which will reduce the charging capacity of the battery.
Low reactivity
In order to inhibit the diffusion and volatilization of bromine, the existing battery system often introduces the necessary complexing agent, but this will reduce the bromine electro-pair reaction activity is low, which further leads to the reduction of battery power density.
Solution strategy
Battery structure design: By designing a single-fluid flow battery (only the zinc negative side is connected to the electrolyte tank, pumps, piping, etc.), the damage caused by the strong corrosive nature of bromine to the battery devices can be mitigated.
Electrolyte regulation: Adding a complexing agent to the electrolyte to immobilize the bromine in the electrolyte can effectively inhibit its volatilization, and the size of the polybrominated ions complexed with the complexing agent increases, so that they are screened by the diaphragm to inhibit their diffusion. Further development of new complexing agents is essential due to the consideration of the effect of complexing agents on the electro-pairing reaction activity of bromine.
Electrode material modification: Improving the catalytic activity of the electrode by introducing active groups into the electrode material can effectively reduce the polarization and improve the reaction kinetics of bromoelectric pair.
Electrode structure design: By constructing different sizes of holes in the electrode, the diffusion of polybrominated compounds can be restricted.
Diaphragm modification: By modifying the diaphragm, the diffusion of bromine species through the diaphragm to the negative electrode can be prevented.
Zinc-Manganese Flow Battery
The common zinc-manganese batteries on the market utilize a solid-solid phase transition reaction from manganese dioxide to manganese hydroxide, which can only be used as a primary battery due to its poor reversibility.
Zinc-manganese flow batteries as secondary batteries utilize the deposition and dissolution reaction between soluble divalent manganese ions and manganese dioxide, and their reversibility is relatively better.
Manganese is inexpensive and environmentally friendly. The divalent manganese ion and manganese dioxide electrode pair has high electrode potential, with zinc negative electrode assembled into liquid flow battery has high open circuit voltage and theoretical energy density.
Zinc-manganese flow batteries have problems such as poor cycle stability and limited surface capacity.
Poor cycle stability
The conversion process from Mn2+ to MnO2 involves the disproportionation reaction of trivalent manganese, which is relatively poorly reversible and leads to the efficiency and capacity degradation of the battery.
Limited surface capacity
Since it involves the deposition of MnO2, its energy density is limited by the amount deposited per unit area. However, the conductivity of MnO2 is relatively poor, and an increase in surface capacity will lead to an increase in battery polarization.
Solution strategy
Adjustment of pH: By introducing a small amount of hydrogen ions, the deposition process of MnO2 can be improved and the cycling stability can be enhanced.
Introducing redox pairs: By introducing other pairs with better reversibility, such as iodine (I2/I-), the reversibility of the reaction can be improved and the face capacity can be increased .
Adding conductive agent: Adding a small amount of conductive agent to form a conductive network can be beneficial to the electronic conduction between MnO2 and electrode, increasing the surface capacity of its deposition.
At present, manganese-based flow battery is still in the relatively primary research stage, we can hope that more research strategies to improve the problems of manganese-based flow battery, so as to realize the commercialization of the application.
Zinc-iodine flow battery
The zinc-iodine flow battery system was first proposed in 2014, for the iodine positive electrode, during the charging process, I- will be oxidized to I2, and due to the coordination effect of I-, I2 will be dissolved in the form of I3-, which avoids its precipitation, and the discharge reaction occurs in reverse, and the I3- is reduced to I- again.
Zinc-iodine system has high solubility of both positive and negative active substances, so it has high theoretical energy density; the positive electrodes have good electro-pair dynamics, which is expected to realize high power density; compared with zinc-bromine system, its volatility, oxidizability, and corrosiveness are reduced, which makes it less demanding on the battery system materials and more friendly to the environment.
However, there are still many problems in the application of zinc-iodine liquid current system.
Low utilization rate of active substance
In the charging process of positive electrode, if completely reacted to generate solid I2 will lead to the blockage of pumps and pipelines in the battery system, so it can only be reacted to soluble I3-, the utilization rate of electrolyte is low.
Serious membrane contamination
The Nafion 115 membrane used in the traditional zinc-iodine system will adsorb the oxidized I3- in the solution, causing serious membrane contamination.
Higher cost
The price of iodine is expensive (>150,000 RMB/ton), and the low electrolyte utilization will further increase the cost of the zinc-iodine system.
Solution Strategies
In general, the current zinc-iodine flow battery system is still in the preliminary stage of development, and the solution strategy for the anode side is mainly aimed at improving the utilization rate of the electrolyte in order to reduce the cost, and at the same time, improving the ionic conductivity of the electrolyte and the ionic transport performance of the membrane material to improve the operating current density.
(1) Utilize the “size sieving” effect to design porous membranes to ensure high ionic transport rate while effectively avoiding membrane contamination.
(2) Adjust the electrolyte components to improve the electrolyte conductivity and increase the working current density.
(3) Effectively improve the energy density of the battery and reduce the cost by realizing the multi-electron transfer of iodine.
(4) Through the design of battery structure (single liquid flow), avoiding the clogging problem of pipeline and pump, and at the same time, improving the utilization rate of electrolyte.
Zinc-iron flow battery
Iron-based positive electrode pairs have good electrochemical activity and reversibility, and iron salts are inexpensive, so researchers have formed a zinc-iron flow battery system with a zinc negative electrode. Depending on the pH of the electrolyte, zinc-iron flow batteries can be further divided into different active pairs that participate in electrochemical reactions in different environments.
For neutral and alkaline zinc-iron flow batteries, due to the different reaction mechanisms and pairs of active substances, the problems they face are not the same.
Alkaline iron positive electrode
In alkaline zinc-iron flow batteries, the main problems faced by iron positive electrodes are low solubility and rapid capacity degradation.
Currently, the performance of alkaline zinc-iron flow batteries is improved by increasing the solubility of the active substance by adding a complexing agent to water to complex the active pairs, or by using KOH as a supporting electrolyte based on the homoionization effect.
Neutral iron positive electrode
The stability of active pairs in neutral zinc-iron flow battery is poor, easy to hydrolyze and precipitate, resulting in battery capacity decline, while the unhydrolyzed iron ions are also easy to pollute the membrane, affecting the life of the battery.
To address this problem, the stability of the iron positive electrode can be improved by adding a complexing agent to the electrolyte to achieve stable cycling.
Acidic iron positive electrode
Divalent/trivalent iron ion active electric pairs have high solubility, high potential, good electrochemical activity and good reversibility in strong acidic electrolyte. However, in the zinc-iron flow battery system, the zinc deposited on the negative side reacts with acid to form hydrogen, resulting in a low Coulombic efficiency of the battery.
An amphoteric zinc-iron battery was constructed by constructing a buffer (NaCl solution) for charge balancing between the positive and negative electrodes, and using a cation-exchange membrane and an anion-exchange membrane to separate the buffer from the negative and positive electrodes, respectively, to improve the feasibility of an acidic iron positive electrode.
Zinc-nickel flow battery
Nickel electrode has the advantages of high cell potential, low cost, etc., and its charging and discharging is a solid-solid phase reaction, so the zinc-nickel flow battery system can use only one set of pumps and piping, and does not need expensive ion exchange membrane, which greatly reduces the cost of the battery system.
The use of zinc-nickel single-fluid batteries is limited by the following issues.
Differences in electrical conductivity of charge and discharge products
Due to the poor conductivity and electrochemical activity of the nickel electrode to the conductivity and electrochemical activity, so that the battery operation of the working current density is low.
Serious side reactions
Oxygen precipitation side reaction of nickel electrode leads to mismatch of Coulombic efficiency of positive and negative electrodes, zinc accumulation at negative electrode, and thus short circuit of the battery.
Poor cycle stability
Nickel ball surface in the charging process, the local area overcharging, the generation of γ-NiOOH, nickel ball expansion and fragmentation, which in turn leads to the loss of active substances.
Solution strategy
(1) Use three-dimensional porous nickel foam negative electrode instead of two-dimensional nickel sheet negative electrode to reduce the polarization of the battery and improve the current density and power density of the battery.
(2) Improve the conductivity of nickel hydroxide by doping with Co, coating with CoO/CoOOH, or constructing a NiS conductive layer in situ.
(3) Introducing a second redox pair to consume the accumulated zinc and avoid short-circuiting the battery.
(4) Increasing the thickness of nickel foam increases the negative electrode hydrogen precipitation side reaction and the self-dissolution reaction of zinc, which makes the positive and negative electrodes Coulombic efficiency equal and solves the problem of zinc accumulation.
Prospect and Outlook
In the framework of new power systems, high energy density zinc-based liquid current batteries are emerging on the user side by virtue of their small size and low cost. As a type of liquid current battery in the energy storage system, its advantage of being applicable to different application scenarios is further emphasized. Under the strategic demand of building a new type of energy storage system, zinc-based flow batteries, together with all-vanadium flow batteries, will serve the transformation and upgrading of the national as well as the world’s energy structure in a more powerful way.
