Separators in Battery Are More Than Just “Plastic Films”

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The separators in battery, a thin battery membranes that physically separates the anode and cathode, is one of the critical cell components to ensure cell safety. Its primary function is to prevent physical contact between the anode and cathode while facilitating ion transport in the cell. Therefore, the quality of the battery separators dramatically influences the battery’s performance.

What is the development status of lithium battery separators in China and worldwide? What problems should we face in developing high-performance battery separators for the new energy industry?

Never easy to produce high-performance lithium battery separators

Briefly, the separators in battery is a thin layer of battery membranes, but it is more than just an ordinary plastic film.

Professor Li Xue from Kunming University of Science and Technology Lithium-ion battery material Engineering Laboratory told the Science and Technology Daily reporter that polyethylene separators are mainly made in the wet process, and polypropylene separators are primarily made in the dry process.

Commonly, an electrochemical cell consists of an anode and cathode separated by an ion-permeable or ion conductive membrane –the separator – one of the main components. On the one hand, separators should preferably enable good ion conductivity to ensure high energy and power densities. On the other hand, the separators in battery should exhibit an insignificant electronic conductivity to prevent self-discharge.

Thus, the separators in battery plays a vital role in improving the battery’s overall performance, such as capacity, cycle, and safe operation. As the li-ion battery electrolyte is an organic solvent, separators should also have excellent resistance to organic solvents. Due to their excellent mechanical properties, chemical stability, and low cost, polyethylene and polypropylene are the two primary materials for lithium battery separators on the market.

Currently, the mass production of high-end lithium battery separators is still tricky for most manufacturers in China.

In general, the high-performance separators should generally possess the following properties:

  • Excellent electronic insulation to ensure the mechanical isolation of positive and negative electrodes
  • Specific pore size and porosity to provide a low resistance
  • High ionic conductivity
  • Good permeability to lithium ions
  • Good mechanical properties, including puncture strength, tensile strength, etc.
  • The thickness is as small as possible
  • Good dimensional stability and flatness
  • Thermal stability and automatic shutdown protection performance

When short-circuit occurs, the Li-ion batteries can reach temperature values up to 170°C within seconds. Suppose the battery gets too hot during operation. In that case, the polymer with the lower melting point softens and closes the pores of the polymer membrane, thus shutting down the ion conduction and battery operation to prevent fire or explosion.

Today, most cell manufacturers prefer separators with a combination of different polymers or multilayer stacks. Those battery membranes can prevent thermal runaways by improving the separator’s shutdown ability while maintaining mechanical integrity.

Among them, the ceramic-coated separators have become attractive as the ceramic coating layer can enhance the mechanical stability and thus ensure the isolation of the electrodes at elevated temperatures.

PP (polypropylene) separators VS PE (polyethene) separators

  • The PP separator has a higher melting point. However, the PE separator has a lower melting point but is more sensitive to environmental stress affecting the separator properties;
  • The PP separator has a lower density;
  • The pore’s closing temperature of PP separators is higher than PE separators;
  • The tensile strength of PE separators is better than PP separators.

Professor Li Xue also introduced that the typical industrial battery membranes on the market mainly include:

  • PP base film + ceramic coating;
  • PE base film + ceramic coating;
  • PE double layer or Multilayer Films.

Until now, some new separator materials have been developed and applied, such as coated polyester film, cellulose film, polyimide film, polyamide film, spandex or aramid film, etc. However, they were invented for power lithium-ion batteries and are not entirely optimized for other applications.

Therefore, they are currently in a small output and high price. As reported, these separators have the advantages of high-temperature resistance, low-temperature output, long charging cycle life, and sufficient mechanical strength.

“Overall, the separators in battery shows a clear trend of diversification.” Li Xue said.

Key technological problems of separators in battery

Solving breakthrough scientific challenges for battery technology is critical in research projects for new energy vehicles.

In November 2020, the “New Energy Vehicle Industry Development Plan (2021-2035)” issued by the State Council will help guide investment in the R&D of core materials for lithium-battery manufacturing, including positive and negative materials, electrolytes, separators, membrane electrodes, etc. The plan focused on tackling key technological problems of developing a high-safety, low-cost, long-cycle life power Li-ion battery and fuel cell system and accelerating power battery technology’s R&D and industrialization.

As the new energy vehicle market switches from policy-oriented to market-driven, vehicle manufacturers have continuously improved requirements for Lithium-ion batteries, such as battery safety, battery life, and service life.

Significantly, the leading battery manufacturer brings out a much higher material quality requirement. Among them, the lithium battery separator is one of the core materials with high technical barriers. Its performance affects the lithium battery’s discharge capacity, cycle life, and safety.

Therefore, battery manufacturing has incredibly high requirements for the separators in battery, such as pore size, uniformity, and distribution consistency.

In addition to ensuring battery safety, the separators in battery tends to be thinner and lightweight today, which can effectively increase the energy density of the lithium battery. Therefore, improve the capacity of electrode materials in a single Li-ion battery and thus ultimately improves the lithium battery life.

The reporter also learned from the interview that SEMCORP is now one of the world-leading lithium battery separator enterprises, with global competitiveness in production capacity, product quality, cost-efficiency, and technology research and development.

In 2021, SEMCORP developed rapidly with its production capacity, and revenue increased significantly. SEMCORP has become the largest battery separator manufacturer globally regarding production capacity and shipment volume.

“The separators in battery is a key component with the most technical barriers in lithium-ion battery manufacturing. Although it is the last material to be localized among the four main materials in China, the localization proportion of separators has exceeded 92% in 2019. And the domestic separators manufacturer are now also supplying all leading battery manufacturers worldwide.” Pang Qizhi, an assistant to the chairman of SEMCORP, told reporters.

2022 top 1 lithium ion battery separator manufacturer

In 2021, SEMCORP developed rapidly with its production capacity, and revenue increased significantly. SEMCORP has become the largest battery separator manufacturer globally regarding production capacity and shipment volume.

“The separators in battery is a key component with the most technical barriers in lithium-ion battery manufacturing. Although it is the last material to be localized among the four main materials in China, the localization proportion of separators has exceeded 92% in 2019. And the domestic separators manufacturer are now also supplying all leading battery manufacturers worldwide.” Pang Qizhi, an assistant to the chairman of SEMCORP, told reporters.

From the production scale point of view, SEMCORP has become the world-leading wet-process lithium battery separator producer, and its diversified lithium battery separator product can meet the evolving needs of society.

“Through years of development, we now possess a well-established R&D team. And our development scope includes base films, coating equipment, membrane preparation process, improvement of raw materials and auxiliary materials, coating process, slurry formulation, recycling, and energy-saving technology, as well as prospective technological studies on battery separators.” Pang Qizhi introduced that SEMCORP has recently achieved breakthroughs in core and key technologies and made a great success in product quality and developing new products.

Until now, SEMCORP has won 280 valid patents, including 13 international patents. Besides, another 236 patents are pending, including 56 international patents.

“As the world’s largest supplier of wet-process lithium battery separators, we now can supply leading battery manufacturers such as LGES, Samsung SDI, Panasonic, CATL, and CALB.” Pang Qizhi said confidently.

Solve the technical challenges of the battery separator industry.

Driven by the growing demand for energy storage devices, the total output of the battery separator industry snowballed in 2021.

According to Huajing Industrial Research Institute statistics, the total lithium-ion battery market in 2021 comprised 7.6 billion square meters of separators, a year-over-year increase of 150%, of which the production in December is 840 million square meters, a year-over-year increase of 109.3% and a month-over-month increase of 10.3%.

The dry-processed separator is the most commonly used method in lithium-ion battery separator manufacturing. In contrast to the dry process, the wet process has better control over the pore size, distribution, and porosity, so it is generally used to manufacture high-performance membranes.

“Compared to other countries, the domestic membrane industry started relatively late, and thus the domestic technology on battery separators is immature. In terms of wet-precess separators, many separator producers still face the “Stuck neck” problems such as processing methods and production equipment.

Professor Li Xue told the reporter, “We have made significant progress in the research and development capabilities of separator materials. However, the need to improve consistency is a common problem in the domestic separator material industry, mainly including irregular defects, thickness, porosity, pore distribution, and pore size.

Today, the dry-process technology in domestic is very mature. However, our high-performance wet-process technology is lagging compared with foreign manufacturers. Especially the over-reliance on imports of raw material polyethylene and production equipment.

To this end, professor Li Xue suggested that improving domestic battery membranes enterprises’ research and development capability is the priority.

On the one hand, we encourage separators manufacturers to recruit more highly educated talent to join the R&D team.

On the other hand, we should promote co-developing technologies in the domestic battery separator industry to solve the problems of high-performance separator manufacturing.

Furthermore, we must integrate the domestic separator supply chain as soon as possible to avoid vicious competition between enterprises and ensure a stable separator supply.

Moreover, we must adjust production capacity and prevent overproduction or insufficient production.

Lastly, we should strengthen financial and policy support for well-developed companies to help those companies go further.

Pang Qizhi believes that to solve the difficulties of high-performance separator materials in our country, we must have a long-term and rational planning of industrial structure.

At the same time, we must strengthen the industry supervision and improve professional industry standards and thus establish industry admittance from the technical level, production scale, resource utilization, environment protection, safety requirements, etc.

In addition, enterprises must increase investment in R&D to enhance their core competitiveness. After all, for separator enterprises, the most critical thing is to conquer core technologies, improve process level and product quality, establish high-efficiency production lines and increase productivity.

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Hi, I'm the author of this post, and I have been in this field for more than 5 years. If you want to wholesale battery separator or separator-related products, feel free to ask me any questions.

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