Quality Requirements 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 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 and 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 Polymer Separators

• Chemical Stability
• Thickness of Separator
• Porosity of Separator
• 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.

Solutions 
To meet the requirements of battery separators and ensure the safety of manufacturers and end-user, we must control the quality of separators used in Lithium Ion battery. Therefore, the related test should be conducted for battery separators, such thickness measurement, permeability analysis and mechanical tensile strength testing. Each testing needs a high precise testing machine in order to get an accurate test results. 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

More detail infomation please contact us via labthinkchina@gmail.com.