6 Basic Properties You Should Know About Lithium Battery Separators

Table of Contents

What is Battery Separator?

The battery separators is the part of the electrochemical cell that is placed between the electrodes of opposite polarity.

The term “separator” emphasizes the functionality of isolating the anode from the cathode spatially. Its function is to prevent electric contact of the electrodes, which would cause self-discharge, but simultaneously allows ion flow as a basis for energy delivery. 

The separator in lithium ion battery can be either ion conductive (solid electrolytes) or ion-permeable (pervious membranes). However, polymer-based porous membranes are the most commonly used separators for lithium-ion batteries.

battery separators

Types of battery separators

The separator is one of the most critical materials in the structure of the lithium-ion battery.

Based on the differences in physical and chemical properties, generally, we categorize lithium-ion battery separators as woven separators, non-woven separators (non-woven fabrics), microporous membranes, composite separators, separator paper, etc.

Although there are various types of battery separators on the market, the commercialized Li-ion battery separators are mainly polyolefin membranes.

According to the separator structure and composition of battery separators, the market’s most popular li-ion battery separators are porous polymer membranes, non-woven separators and inorganic composite films.

Porous polymer membranes

Refers to the separator with uniform pore distribution produced through mechanical methods, thermal phase separation, immersion precipitation, etc.

Non-woven separators

Composed of directional or random fibers, usually organics or ceramic gel composites, the non-woven separator has better physical and chemical properties.

Inorganic composite separators

A composite separator mainly compounded from inorganic nanoparticles and high polymer.

Battery separator material

The industrial battery separators can be fabricated from various battery separator material.

However, polyolefin materials are vastly used at the beginning of the research and development of lithium-ion battery separators. Mainly because of its excellent mechanical properties, chemical stability, and cost-effectiveness.

Recently, there have been studies on preparing lithium battery separators with other materials.

For example, using the phase conversion method to produce polyvinylidene fluoride (PVDF) based copolymer membranes and developing battery separators based on cellulose composite films.

Overall, polyethylene (PE) and polypropylene(PP) based polyolefin microporous membranes still significantly dominate the lithium battery separator market.

Temperature Resistant PP base films have a better temperature resistance.
Density PE base films have a higher density than PP base films.
Melting Temperature PP membranes have a higher melting temperature (165°C )
Shutdown PE base films have a lower melt temperature (130°C) that provides high-speed shutdown capability.
Tensile strength PE base films have better tensile strength.
Chemical Resistant PP base films are more chemically resistant than PE base films.

Battery Separators' Application

Consumer electronics
  • Monolayer PP Base Films
  • Monolayer PE Base Films
Electric Vehicle
  • PP/PE base film+ceramic coating
  • Bilayer PP/PE Separators
  • Bilayer PP/PE Separators
  • Trilayer PP/PE/PP Separators

Small Li-ion Battery for Consumer Electronic Devices

Power Batteries / Large Power Storage Systems Batteries

Lithium-ion battery separator manufacturing process

Technology differentiation has become an important strategy for battery separator companies, especially lithium-ion batteries. It is because of the highly competitive environment in the energy storage market.

Little information is available on industrial producers’ detailed processing methods of novel ceramic composite separators. Generally, industrial producers and research institutions have applied for patents for synthesis routes and processing steps.

Below drawing is the battery separator manufacturing process provided by YINGDA Securities.

Wet process

Wet process

( Manufacturing process summarized by YINGDA Securities)

Dry process

Dry process

( Manufacturing process summarized by YINGDA Securities)

Properties of Lithium battery separators

A battery separator is a polymeric membrane between the positively charged anode and negatively charged cathode to prevent an electrical short circuit. In general, the types of batteries are different, and the separators used are also different.

Since the electrolyte is an organic solvent system for lithium-ion batteries, the separator should generally possess the following properties:

Pore size and Pore structure

The pore size and structure are usually influenced by polymer compensation and stretching conditions. The pore size affects the transit of the anode and cathode of the li-ion battery.

Moreover, non-uniform pore structure will cause non-uniform internal current densities. This feature will be crucial as battery manufacturers increase cell capacity with thinner separators.

Gurely

The battery separator supplier uses Gurley to evaluate separator permeability, which is usually characterized by air permeability. The Gurley number expresses the time required for a specific amount of air to pass through a specific separator area under specific pressure.

Overall, the Gurley number is proportional to the battery’s internal resistance. Thus, the larger the value, the larger the battery’s internal resistance.

Shutdown

The lithium-ion battery separators protect short circuits and overcharge in lithium-ion cells. Separators exhibit a significant increase in impedance at a temperature of about 130℃, effectively stopping ionic transport between the electrodes.

Therefore, the lithium battery separator should be able to shut down at a temperature slightly lower than the temperature where thermal runaway occurs.

Porosity

Porosity is significant for battery separators. The battery separator should have a suitable pore density that can hold the electrolyte while allowing ionic transport between the electrodes.

It will be hard to close the pores if the porosity is too large when a battery shutdown is needed. Typically, lithium-ion battery separators have a porosity of 40%.

Thermal Stability

The lithium-ion battery can get poisoned by the water. Materials going into the cell must typically stay at about 80 ℃ and in a vacuum condition.

Therefore, the battery separator should be stable for various temperatures without curling or puckering. Generally, the shrinkage should be less than 5% after 60 mins at 90℃(in vacuum)in both MD and TD directions.

Tensile Strength

The battery separator should have a sufficient tensile strength to prevent stretching during the winding process and be thin enough to increase the cell capacity.

Development Trends of Battery Separators

The development of the lithium-ion battery separator industry mainly shows two trends. One is the consumer lithium-ion battery separator, which tends to be thinner and lighter.

This kind of battery separator is suitable for distributed application architecture systems such as mobile phones, notebook computers, and Internet of Things applications that have a higher requirement on the capacity and portability of lithium-ion batteries.

The other is a power battery separator fabricated with thick film or a multi-layer composite separator. 

This separator is suitable for power batteries for electric vehicles and large-scale energy storage power systems, which require excellent power output, large capacity, and higher safety performance.

As one of the four critical components of lithium-ion batteries, lithium battery separators play a vital role in improving the all-around performance of batteries.

Overall, lithium battery separators’ development mainly revolves around improving the battery’s capacity, circulation, safety, and power performance.

YOUME is a China battery separator manufacturer committed to supporting all lithium battery manufacturers with higher quality, safer and lower-cost battery separators.

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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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