Why does the polymer separator determine the safety of lithium batteries?

In modern life, lithium-ion batteries are almost everywhere: mobile phones, laptops, electric vehicles, energy storage devices... They can 

operate efficiently and stably not only because of the advancements in electrode materials, but also because of a frequently overlooked 

"backstage hero" - the polymer separator. The separator is a seemingly ordinary thin film in the structure of lithium batteries, but it plays 

a decisive role in safety. Many battery accidents, such as bulging, short circuits, and fires, are often closely related to the performance of 

the separator. So, why can the polymer separator determine the safety of lithium batteries?

First, it is necessary to understand the basic function of the diaphragm. The diaphragm is a porous film sandwiched between the positive 

and negative electrodes. Its core tasks are two: one is to physically prevent direct contact between the positive and negative electrodes 

to avoid short circuits; the other is to allow lithium ions to pass through smoothly, maintaining the electrochemical reaction of the battery. 

It can be said that the diaphragm has to "block people" and also "allow vehicles to pass". If the diaphragm is damaged, has abnormal 

pore size, or insufficient thermal stability, it will directly lead to short circuits or battery failure, which is the most dangerous situation for 

the battery.

Why can polymer separators perform such tasks? Currently, the mainstream lithium battery separators mostly use polyethylene (PE), 

polypropylene (PP), or multi-layer composite membranes. Their common feature is that they have good mechanical strength and 

chemical stability, and can remain undamaged in the electrolyte for a long time while maintaining structural stability. The surface of the 

separator has abundant and uniform micropores, which are like toll booths on a highway, allowing lithium ions to pass through quickly, 

while potential "dangerous factors" such as metal lithium dendrites are difficult to pass through smoothly. This "selective passage" 

ensures the stability of the battery during normal operation.

However, the true test of the value of the barrier layer lies in its performance under extreme conditions. Lithium batteries will heat up 

rapidly inside when overcharged, short-circuited externally, or exposed to high temperatures. If the barrier material does not have 

sufficient thermal stability, the pore structure may collapse or melt, thereby losing its isolation function and causing thermal runaway. 

To address this, polymer barrier membranes have been designed with a so-called "shutdown mechanism". For example, PE will 

contract and its pores will close at around 130°C, preventing lithium ions from continuing to flow, which is equivalent to automatically 

applying a "brake" to the battery and preventing the danger from escalating further. While PP has a higher melting point, it can 

maintain the structure to some extent and delay the failure of the battery. This multi-layer composite structure is often referred to as 

a "safety insurance" and is the most crucial contribution of the barrier layer in terms of safety.

In addition to the thermal shutdown function, the strength and toughness of the diaphragm also directly affect whether the battery 

can withstand mechanical shocks. In electric vehicles, when the cell is subjected to external force compression in a collision or 

vibration environment, if the diaphragm is prone to tearing, the positive and negative electrodes may come into contact instantly, 

causing a short circuit. High-quality polymer diaphragms are often processed through stretching, coating, and other techniques, 

and possess higher puncture resistance and mechanical strength, providing a physical barrier for the battery.

In recent years, as the energy density of batteries has been continuously increasing, the role of the separator has become even 

more crucial. High energy density means that more energy is stored in a given volume, and once it gets out of control, the risks 

are also greater. Therefore, the industry is constantly developing new polymer separators. For example, by applying ceramic 

coatings to improve the separator's heat resistance and wettability, it can maintain structural stability even at high temperatures; 

or by introducing nanofibers to enhance the separator, improving its ionic conductivity while also ensuring mechanical strength. 

These improvements are all aimed at ensuring the safety of batteries in more demanding application environments.

If the positive electrode material determines the upper limit of battery energy, and the negative electrode material determines the 

potential of cycle life, then the separator is the "bottom line" of battery safety. It is like an invisible firewall, silently safeguarding 

the operation of the battery. Once the separator fails, the battery will be exposed to the most dangerous state instantly. That's why 

battery manufacturers are extremely cautious when choosing separators. They not only need to examine the thickness, porosity 

and air permeability, but also conduct strict tests for heat resistance, pressure resistance, chemical compatibility, etc.

In summary, although the polymer separator is just a seemingly ordinary film, it plays a crucial role in the safety of lithium batteries. 

Through multiple performance aspects such as physical isolation, ion conduction, thermal shutdown, and resistance to mechanical 

damage, it jointly builds the safety defense line of the battery. With the rapid development of new energy vehicles and energy 

storage systems, the innovation and upgrading of separator technology will become another focus of competition in the battery 

industry. It can be said that whoever masters more advanced and reliable polymer separator technology will be able to gain the 

dual advantages of safety and performance in the future battery market.