1. Property Concerns of Lithium Ion Battery Separators
Business Challenge
Lithium-ion batteries have one of the best energy-to-weight ratios, no memory effect, and a slow
loss of charge when not in use. This makes them one of the most popular types of batteries for
portable consumer electronics in today’s marketplace. Lithium-ion batteries can be found in
cameras, laptops, power tools, toys and more. However, as recent well-publicized recalls
demonstrate, they can pose a risk of fire and overheating. In addition, they can suffer from a
reduced lifespan if not properly treated. As a result, new and stricter requirements are being
implemented globally to address potential hazards, leaving manufacturers and purchasers of
products containing lithium-ion batteries to meet increasing requirements and overcome safety
concerns.
Lithium Ion Battery Separators
A Lithium Ion battery Separator is a permeable
membrane placed between the anode and
cathode of a l-ion battery. The main function of
a separator is to keep the positive and negative
electrodes, the cathode and anode respectively,
apart to prevent electrical short circuits while
also allowing the transport of ionic charge
carriers which are needed to complete the
circuit during the passage of current in an
electrochemical cell.
As mentioned above, separators are
power-driven spacers. They can be produced
with fiberglass cloth or flexible plastic films made from nylon, polyethylene or polypropylene. It
must be absorbent and slim to allow the charged lithium ions to pass without obstruction and it
should take up the minimum of space allowed, leaving available space for the cathode active
elements. The separators must be able to withstand penetration and branching moss-like
crystalline minerals in order to prevent the electrodes from contamination. If the separator breaks
down or is infiltrated, this will weaken high power cells end results.
Properties of Lithium Ion Battery Separators
 Chemical Stability
 Thickness of Separator
 Porosity of Separator
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2.  Pore Size
 Permeability
 Mechanical Strength
 Wet ability
 Stability
 Thermal Capabilities
Thickness of Separator
A battery separator must be relatively thin in order to facilitate the high energy and power densities
of the battery. However, if the separator is too thin, it can decrease the mechanical strength and
safety of the battery. Additionally, a separator should have uniform thickness in order to support
the long like cycle of a battery. In current technologies, 25.4μm is generally accepted as the
standard width. The thickness of a polymer
separator can be measured using the T411 om-83
method developed under the auspices of the
Technical Association of the Pulp and Paper
Industry
Permeability
The separator should not limit the electrical
performance of the battery. Usually the presence of
a polymer separator will increase the resistance of
the electrolyte by a factor of four to five. The ratio of
the resistance of the separator filled with electrolyte
divided by the resistance of the electrolyte alone is
called the MacMullin number. Air permeability can
be used indirectly to estimate the MacMullin number. Air permeability is expressed in terms of the
Gurley value, which is defined as the time required for a specific amount of air to pass through a
specific area of the separator under a specific pressure. The Gurley value reflects the tortuosity of
the pores, when the porosity and thickness of the separators are fixed. A separator with uniform
porousness is vital to the long life cycle of a battery. Deviations from uniform permeability will
result in uneven current density distribution, which causes the formation of Li crystals on the
graphite anode.
Mechanical Strength
The separator must be strong enough to withstand the tension of the winding operation during
battery assembly. The mechanical strength of the polymer separator is also very important.
Mechanical strength is typically defined in terms of the tensile strength in two directions, the
machine direction and the transverse direction, and terms of the tear resistance and puncture
strength. All of these parameters are defined in terms of Young’s modulus.
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3. Solutions
In order to ensure the properties of Lithium Ion Battery separators, some related tests are
becoming necessary. Labthink instruments Co., Ltd is the only manufacturer of testing equipment
professionally for thin film filed. Everything equipment they have is strictly applying to international
standards and considered the manufacturers and users benefits of batteries
Thickness Measurement
A Gauge used to measure a separator thickness must be with high precision and resolution.
Labthink Thickness Tester is professional designed for thin film and paper, with 0 ~ 2mm test range
and 0.1 μm resolution. This can totally meet separator requirements.
Mechanical Strength
Labthink Tensile Tester, which is different from the common UTM, is specially designed for flexible
packaging with little force value and high precision. Labthink Tensile Strength Tester XLW(PC) with
0.5% of reading value can determinate a separator’s mechanical strength accurately
Permeability
There is an instrument specially designed for battery separator air permeation testing in Labthink
–BTY-B2P Gas Permeability Tester. It is is based on the combination of differential pressure method
and can reach a test range 1~10,000cm3/m2·24h·0.1Mpa with resolution 0.01Pa, according to
international standard ASTM D1434. All the test units and precision can meet the test
requirements of battery separator.
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