1. What Is Mylar?
First, let's clarify the material itself. Mylar is a special insulating film exclusively used for batteries, made of polyethylene terephthalate (PET), a tough polymer material.
Its core properties can be summarized in three phrases: high tensile strength, superior electrical insulation, and excellent heat resistance.
In plain terms: This film is robust enough to withstand mechanical impacts during cell casing insertion. Electrically, it fully isolates the bare jelly roll from the metal shell. It also boasts outstanding thermal stability and will not fail under temperature rises generated during battery operation.
2. Mandatory Reasons for Mylar Wrapping
Mylar wrapping is a critical process step prior to loading jelly rolls into aluminum shells, serving three core protective purposes.
Purpose 1: Insulation Protection to Prevent Short Circuits
Without any separator between the bare jelly roll and the aluminum shell, contact between the negative electrode and the aluminum shell will instantly pull down the shell potential. This may cause minor corrosion at best, or severe internal short circuits at worst.
Mylar film acts as this separating barrier, completely cutting off direct contact between the negative electrode and the shell - its most fundamental function.
Purpose 2: Prevent Corrosion of the Aluminum Shell
Aluminum shell corrosion stems from electrochemical reactions that form aluminum-lithium alloys. This corrosion only occurs when two conditions are met simultaneously: electronic short-circuiting (conductive contact between the negative electrode and shell) and ionic short-circuiting (lithium ions making contact with the aluminum shell).
Mylar insulation blocks the electronic conductive path, while its full-wrap structure isolates the ionic path. With both triggering conditions eliminated, corrosion cannot occur.
Purpose 3: Physical Protection for Bare Jelly Rolls
Mechanical friction is inevitable during shell loading, alongside compression and impact forces from press-fitting. The electrodes and separators of bare jelly rolls are fragile. Without this buffer layer, they are prone to scratches or compressive deformation. Mylar functions as a physical protective layer here.
3. Supplementary: Mylar vs. Alternative Solutions
Alternative insulating solutions are documented for reference, which explains why Mylar has become the mainstream choice.
Mylar Film vs. Insulating Plastic Brackets
Advantages of Mylar
Lightweight and low-cost, helping improve the battery's energy density.
Disadvantages of Mylar
Higher requirements for production processes; jelly roll positioning is harder to control, increasing difficulty during shell insertion.
Advantages of Insulating Brackets
Excellent structural stability and simpler manufacturing processes, which help fix the jelly roll shape.
Disadvantages of Insulating Brackets
Extra weight reduces energy density and brings higher material costs.
Special Design Exceptions
Certain cell designs (such as the small π P7+ jelly roll) use insulating adhesive tape instead of Mylar. This is usually due to limited structural space that prevents thermal welding of Mylar film.
Conclusion
Mylar wrapping is an indispensable safety process for prismatic lithium-ion batteries. As a lightweight PET insulating film, Mylar creates a reliable insulating barrier between the jelly roll and aluminum shell. It fundamentally avoids battery failure caused by internal short circuits and shell corrosion induced by aluminum-lithium reactions, thus safeguarding battery safety and service life.
