Application Spotlight – Polyester Separator Films for EV Lithium-Ion Battery Cells

Lithium-ion battery technology is at the forefront of the growing electric vehicle movement that has taken the American automotive industry by storm in recent years. It’s estimated that 40% or more of U.S. consumer automotive sales could be electric vehicles by 2030. Many of the improvements seen in electric vehicles over the course of the past fifteen years can be directly linked to innovation in lithium-ion battery technology.

Lithium-ion batteries are both lightweight and powerful, allowing manufacturers the design flexibility to pack more battery cells into a confined space for higher charge capacity and improved distance range. However, there are also some safety concerns surrounding lithium-ion batteries due to their thermal instability under certain conditions. As a result, improving safety has been a point of emphasis for manufacturers and government funding toward the electric vehicle market in recent years.

One such safety measure is improving the design of separator films, which serve as a key component within individual battery cells. Separators function as a thin insulating barrier that prevent oppositely charged electrodes from touching each other. They are critical for battery safety and maintaining the functional integrity of individual cells, as malfunctions can lead to short circuits, fires, or even explosions. This has given rise to demand for biaxially-oriented polyester film separators which offer superior mechanical and thermal properties compared to lower cost alternatives used in previous generations of battery cells, reducing safety risks significantly. This article will aim to explain the function of separator films and why polyester film is preferred for optimizing safety in cell design.

The Role of Separator Films Within Lithium-Ion Battery Cells

Each individual cell within a lithium-ion battery is made up of two electrodes – a positively charged cathode and a negatively charged anode – on opposite sides, a liquid electrolyte that carries lithium ions between the two, and a dielectric separator film (see Figure 1). The separator plays a key role in preventing direct contact between the anode and cathode, isolating electrons within the cell, and allowing the free passage of lithium ions carried by the electrolyte through small pores in the film.

Figure 1. Key components of a lithium-ion battery cell.

During charging and discharging, the separator acts as a permeable membrane for lithium ions. When these positively charged ions move from one electrode to the other, it creates an electrical potential difference – also known as voltage – due to the surplus of negatively charged electrons left behind. This voltage forces these remaining electrons through the external circuit to rejoin the lithium ions within the opposite electrode. Since the separator film is a dielectric barrier, it ensures that electrons travel via the external circuit rather than through the separator and opposite charges never come into contact with one another.

During the charging process, lithium ions residing in the cathode travel to the anode, creating voltage that forces the surplus electrons through the external circuit into the anode to be stored as energy. When the battery is in use, lithium ions move back to the cathode, triggering these stored electrons to discharge from the anode and power the vehicle as they travel back through the external circuit before returning to the cathode. See Figure 2.

Figure 2. Charging and discharging processes shown within a lithium-ion battery cell.

Key Properties Required for Separator Films

With these processes and cell construction in mind, these six properties of separator films are critical for optimal performance, longevity, and safety:

  • Dielectric – poor conductor of electric current, keeping electrons isolated from opposing electrodes within the cell
  • Chemical stability – chemically inert, meaning it does not react with the liquid electrolyte or electrodes
  • Hydrolysis resistance – can soak with the liquid electrolyte without deteriorating over time
  • Thermal stability – high temperature resistance so that it will not soften, distort, or fuse with other components within the cell
  • Mechanical strength – good tensile strength to ensure dimensional stability in manufacturing
  • Porosity – allows lithium ions to pass through the separator freely like a permeable membrane

Polyester PET and PEN films are excellent at all of these critical properties due to a combination of both their inherent characteristics and the results of the biaxial orientation process they undergo during manufacturing. This makes them ideal for next generation battery separator applications, whereas lower cost films used in the past have proven to be considerably less effective in several of these areas. While polyester PET films meet all of the above conditions critical to separator applications, polyester PEN films have improved thermal, chemical, and mechanical properties for harsh environment applications, filling the performance gap between PET and high-performance polyimide films.

Tekra offers Mylar® and Melinex® PET films and Kaladex® PEN films from DuPont Teijin Films™ as well as Teonex® PEN films from Toyobo Film Solutions Limited™ in a variety of thicknesses for lithium-ion battery separator and insulation applications in electric vehicles. These films are also ideal for flexible printed circuitry, bus bars, capacitors, motors, transformers, fuel cells, cables, wiring, and more. Some common film types include:

Contact a Tekra representative today to find a custom solution for your next application.

Electric Vehicle at a Battery Charging Station

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