In order to "access" freely, they become energy "porters"

Author | Buye

Energy is everywhere, but energy supply is still urgent. The vast northwest region contains a lot of renewable energy sources such as "wind" and "light", but these energy sources are often too far away from cities in energy centers.

At present, the realization of long-distance transmission of energy seems to be the best choice to solve the contradiction.

For a long time, transmission lines and power stations have acted as energy "porters", but there are always ditches and bumps that are difficult to cross.

In order to achieve a more flexible power supply, researchers hope to build a bridge between energy collection and utilization, and innovate energy storage methods.

Li Xianfeng, a researcher at the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences (hereinafter referred to as the Dalian Institute of Chemical Physics), has been deeply involved in the field of flow batteries for more than 20 years.

Recently, the "New Generation Of Liquid Battery Energy Storage Technology and Industrialization Team" led by the team won the 2021 Chinese Academy of Sciences Science and Technology for Development Award.

"Different from traditional batteries, flow batteries store liquid electrolytes externally, the energy storage medium is an aqueous solution, there is no risk of fire and explosion, high safety, long life, and can also be combined on demand to achieve autonomous regulation of power and capacity, which has broad application prospects in the field of energy storage in power systems." Li Xianfeng told China Science Daily.

Break through from inside the battery

The battery looks like a black box, but the actual interior has a complex structure, mainly composed of key materials and core components such as diaphragms, bipolar plates, electrodes and so on.

"Improving the performance of energy storage batteries needs to take into account both local and overall, and the research and development process needs to consider the interaction and influence of various factors." Li Xianfeng said.

Membrane materials mainly play a role in blocking the positive and negative electrodes and transporting ions.

"The higher the ion conductivity of the membrane, the faster the protons pass, but the ion selectivity will deteriorate, resulting in ions on both sides of the membrane 'shuttling' between each other, increasing the battery side reaction, affecting the battery performance."

Shi Dingqin, a senior engineer at the Dalian Institute of Chemicals, said. Therefore, how to balance the ion conductivity and ion selectivity of the membrane.

The development of a new diaphragm material that takes both into account is a key technical problem for researchers to solve.

In order to prevent ions from shuttling from one side of the membrane to the other, the researchers are committed to the study of the membrane ion transport mechanism, improving membrane ion selectivity by cutting off the transport pathway of ions within the diaphragm.

However, this method still cannot completely solve the balance between the two.

Li Pioneer's team found that the radius of different ions in the electrolyte of the flow battery is different, "can the selective transmission of ions be achieved by regulating the pore size of the membrane?" With this idea in mind, the team embarked on a new journey to develop porous ionic conductive membrane materials.

The microscopic world is complex and changeable, regulation is difficult, and research has not been able to make a breakthrough for a considerable period of time, but the team's belief has never wavered.

In the end, starting from the innovative molecular structure, they broke through the balance between membrane ion conductivity and ion selectivity, prepared highly selective, highly conductive, low-cost porous ion conductive membrane materials, and successfully realized mass production.

Bipolar plates are another key material for batteries, playing the role of connecting each single cell into a stack and collector.

Similar to membrane materials, bipolar plate materials also face the "double high" contradiction.

"The higher the carbon content, the stronger the conductivity, but the toughness will become worse, which is not conducive to the compact assembly of the stack." Dr. Liu Tao of Dalian Institute of Chemicals said that the development of new bipolar plate materials with high conductivity and high toughness is the main goal of the team.

In recent years, the team has focused on the innovation and structural design of bipolar plate materials, and successfully built a more developed conductive network by integrating multi-scale and multi-dimensional toner particles into polymer toughening networks.

However, there are countless toner and polymer materials on the market at present, and how to find the best material and ratio from many materials is an urgent problem to be solved. The team has experimented with thousands of formulations for this purpose.

"Even if the formula is determined, it is still not enough to meet the actual application, but also to ensure the needs of preparation and large-scale production, improve the production efficiency of materials, and reduce costs." Liu Tao said.

To this end, the team continued to explore and try in the scale up and mass production process, and finally developed a new type of high conductivity, high toughness, weldable carbon-plastic composite bipolar plate, and realized mass production.

Give power an account

The key to whether the stack is good or not depends on the discharge power and efficiency, and the high efficiency can ensure that the same storage power can be released more power.

However, there are losses during the charging and discharging of all batteries.

"Put in 1 kWh of electricity, release 0.8 kWh of electricity, and the amount of electricity that disappears out of thin air is a loss." Zheng Qiong, an associate researcher at dalian institute of chemicals, gave an example.

After successfully developing high-performance diaphragm and bipolar plate materials, the next step was to optimize the stack structure and process to minimize losses and increase power.

Power is determined by current and voltage. Theoretically, the voltage is constant, and the power will increase by increasing the current, but all batteries have polarization problems.

Xing Feng, an associate researcher at the Dalian Institute of Chemicals, said that the existence of "polarization" has made the battery power increase while its efficiency has decreased.

Therefore, researchers are eager to increase the current and increase the power at the same time, the stack can always be in a high-efficiency operating state.

In scientific research, Xing Feng added an "occupational disease" - love to settle accounts. "To increase power, improve efficiency, and reduce costs, the core is control polarization, and it is necessary to constantly simulate and calculate the details." Xing Feng said.

In practical work, battery polarization does not equal zero, and can only be continuously reduced through technological improvements. "Only by thinking deeply about the reaction in the battery and every detail in the transmission process can the stack be more efficient." Xing Feng said.

The stack is to the battery system what the heart is to the human body, with important functions, complex structure, and cumbersome debugging and optimization work, which involves the whole body.

Often this indicator is improved, but other indicators are low, and it is difficult to pursue a comprehensive improvement of indicators.

In the beginning, the team could only make a stack with less than 1 kilowatt of power, and the cost was high, far from meeting the market access threshold.

Today, the team has developed a new generation of 30 kilowatts of high-power density stacks, with a power density increase of nearly 1 times, a cost reduction of nearly 40%, and is already in the early stages of commercialization and industrialization.

Nowadays, industrialization is in full swing.

The team used laser welding technology to achieve a direct seal between the porous ion conductive film and the electrode frame, eliminating the dependence on the gasket between the internal components of the stack, and improving the reliability and assembly level of the stack.

"The new generation of all-vanadium flow battery technology has begun to take shape, continue to develop, and continue to advance." Li Xianfeng said.

Serve the construction of new power systems

The electrolyte is the "blood" of the battery operation, and vanadium ion is the type of electrolyte that the team is very optimistic about.

There are 4 valence states of this ion, and reversible conversion can be carried out between different valence vanadium ions to complete the cycle of charging, discharging and recharging.

At the same time, the all-vanadium flow battery composed of vanadium ions can greatly reduce the risk of fire and explosion, and the safety is extremely high.

The study found that the full vanadium flow battery can be freely combined into a battery pack, its output power can reach hundreds of megawatts, and the energy storage capacity reaches hundreds of megawatts, which can store 100,000 kWh of electricity.

Dalian Rongke Energy Storage Technology Development Co., Ltd. (hereinafter referred to as Dalian Rongke) is an enterprise focusing on the industrialization of energy storage technology for all-vanadium flow batteries established by dalian chemical institute technology. The two sides have been cooperating deeply to jointly promote the commercialization demonstration project of all-vanadium flow batteries and verify the feasibility of laboratory results.

In 2016, based on the technical support of the team, Dalian Rongke undertook the construction task of the national demonstration project of the world's largest power all-vanadium flow battery energy storage and peak shaving power station.

In 2020, the team launched a new generation of all-vanadium flow battery technology 30-kilowatt stack, which received many praises and a number of intended cooperation units came to visit.

At present, the team has established cooperation intention relationships with many domestic and foreign enterprises, and the team's all-vanadium flow battery technology has occupied more than 60% of the market.

In the process of industrialization, the team found that enterprises have strong research and development needs. To this end, the team expands the dimension of cooperation, sets up laboratories in enterprises, and builds a joint research and development platform.

"The market needs to grow on petri dishes for scientific research from the start." Wang Xiaoli, deputy general manager of Dalian Rongke, said.

Industrial development, standards first. The team also led the formulation of more than 20 standards, including the first international standard for flow batteries, which improved the international discourse power of continental flow battery technology.

"In the final analysis, the full vanadium flow battery solves the problem of storage of surplus electric energy on the power generation side, and in the long run, it is also necessary to solve the energy storage problem on the user side." Yuan Zhizhang, a researcher at the Dalian Institute of Chemicals, said.

Using zinc-based flow batteries to solve user-side needs is another industrialization direction for the team.

Zinc reserves are large, low cost, low potential, high energy density, but zinc-based flow batteries in the long cycle process, the reactant ions in the electrolyte are more likely to diffuse to the protrusion of the electrode surface, the formation of zinc dendrites, the continuous growth of the latter will puncture the diaphragm, and eventually lead to battery short circuit failure.

Therefore, zinc dendrite formation and growth problems must be solved before zinc-based flow batteries can achieve real applications.

After years of research, the team developed a new type of electrode material, which realized the effective regulation of zinc deposition morphology, solved the formation and growth problems of zinc dendrites well, and made zinc-based liquid flow batteries go further to large-scale applications.

Li Xianfeng said that the carbon peak, carbon neutrality target has brought greater challenges to the existing power system, the development of large-scale energy storage technology, so that the power generated by new energy coupled with the existing power system, will make power transmission more efficient, power more accessible, so as to help the construction of a new power system with new energy as the main body.