When we complain about our phones being charged three times a day
and worry about electric vehicle range anxiety, few people realize that the thin membrane inside lithium batteries may be the key to determining these experiences. This seemingly inconspicuous component, like a city wall separating the positive and negative poles to prevent short circuits, and like a sieve allowing lithium ions to freely shuttle, can be called the “invisible guardian” inside the battery. Today, let’s talk about what black technologies are hidden in membranes made of different materials? How do they quietly change our electricity usage?
1、 Polyolefin diaphragm: the traditional veteran’s “balancing technique”
In the early stages of the popularization of lithium batteries, polyolefin materials became the “pioneer” of the separator industry with mature processes. This type of material is mainly divided into polypropylene (PP), polyethylene (PE), and their composite PP/PE/PP structures, each seeking a unique balance point on the performance balance.
Polypropylene (PP) separator is like a “tough guy”, with outstanding mechanical strength among its peers. Even if there is slight deformation inside the battery during charging and discharging, it can maintain structural stability. What’s even more rare is its high temperature resistance. When the battery temperature rises, the size change rate of PP separator is much lower than other materials, which builds the first line of defense for battery safety. However, this’ tough guy ‘also has a weakness – low porosity leads to low lithium ion passage efficiency, directly affecting the fast charging ability of the battery, which is also one of the reasons for the slow charging of early lithium batteries.
Polyethylene (PE) separators take a “flexible route”, with higher porosity and smoother lithium ion transport channels, which can significantly improve the charging and discharging speed of batteries. More importantly, its “self destruct” safety design: when the battery temperature exceeds 130 ℃, the PE material will quickly melt and close the pores, cutting off ion conduction like a gate and preventing thermal runaway from the root. But this kind of “flexibility” also comes at a cost – its mechanical strength is weak, and long-term use is prone to damage due to compression, which in turn poses a safety hazard.
In order to balance the advantages and disadvantages of both, engineers have developed a PP/PE/PP three-layer composite membrane. The outer layer of PP provides mechanical support, while the middle layer of PE is responsible for overheating protection, as if installing a “double insurance” for the battery. But this composite structure also brings new problems: multi-layer stacking leads to complex preparation processes, an increase in cost of about 30%, and the interlayer structure may hinder lithium ion transport, resulting in slightly inferior performance during high rate charging and discharging.
2、 Cellulose membrane: the “penetration advantage” of environmentally friendly new recruits
When environmental protection and performance requirements collide, cellulose membranes, as a new recruit, quietly emerge. It is made from plant fibers and naturally has a very high porosity – dense honeycomb structures can be seen under an electron microscope, with a porosity more than 20% higher than traditional PP membranes.
This structural advantage brings two significant benefits: first, the electrolyte wettability is greatly improved, and lithium ions are like driving on a highway, with significantly reduced transmission resistance; Secondly, the low-temperature performance is more stable. In an environment of -20 ℃, the capacity retention rate of batteries using cellulose separators is about 15% higher than that of PP separators. For northern users, this means that electric vehicle range will no longer experience a cliff like decline in winter.
But the “weakness” of cellulose materials is also very obvious: insufficient mechanical strength. Pure cellulose separators are easily scratched by electrode plates during battery assembly, so the improved solution of “cellulose+reinforcing agent” is now widely used. For example, adding nanocellulose fibers to form a mesh support can retain the advantage of high porosity and increase the tensile strength by 1.8 times.
3、 Aramid diaphragm: a “safety fortress” in extreme environments
In the field of power batteries, aramid separators can be called “special forces”. This material, made from aromatic polyamide as raw material, inherently possesses the “superpower” of high temperature resistance and corrosion resistance – it can maintain stability in environments above 200 ℃, and even strong acid-base electrolytes cannot corrode its structure.
In the puncture test, the performance of the aramid diaphragm is amazing: when the steel needle simulates the puncture of foreign objects inside the battery, the traditional PP diaphragm will instantly rupture, causing a short circuit, while the aramid diaphragm can rely on its own high-strength fibers to block the further invasion of the steel needle, and obtain power outage time for the battery management system. That’s also why high-end electric vehicle battery packs tend to use aramid separators.
However, the cost of this’ special forces soldier ‘is not high: the preparation process requires special spinning techniques, and the cost is 5-8 times that of PP membranes. Moreover, its dense molecular structure leads to a low porosity, requiring special stretching processes to manufacture channels to meet the requirements of lithium ion transport. At present, aramid membranes are mainly used in high-end fields such as aerospace and military, and the civilian market is still waiting for cost breakthroughs.
4、 Ceramic coated membrane: armor piercing traditional membranes
If traditional membranes are “cloth jackets”, then ceramic coated membranes are “armored warriors”. Engineers apply a layer of nano ceramic particles (usually alumina or zirconia) on the surface of PP or PE membranes, instantly transforming the performance of the membranes.
This ceramic coating brings at least three major improvements: first, high temperature resistance. The melting point of ceramic particles is as high as 1800 ℃, which can effectively prevent the diaphragm from shrinking at high temperatures; The second is puncture resistance, as the hard shell formed by the coating increases the tear resistance of the diaphragm by 40%; The third is electrolyte affinity. The porous structure of ceramics can adsorb more electrolyte, which is equivalent to building a “fast track” for lithium ions.
Actual testing shows that batteries coated with ceramic have a capacity retention rate 12% higher than ordinary separators after 500 charge and discharge cycles; In the squeezing experiment, the probability of short circuit decreased to 1/5 of the original. However, this type of “armor” requires extremely high process requirements, and the coating thickness error needs to be controlled within 1 micron, otherwise it will affect the lithium ion pass rate.
5、 Future Diaphragm: Breaking through the ‘Performance Ceiling’
With the rise of new technological routes such as solid-state batteries and sodium ion batteries, separator materials are also undergoing new changes. At present, scientists in the laboratory are trying to introduce graphene into membrane manufacturing – this single-layer carbon atom material can maintain ultra-high strength and provide excellent ion conduction channels. Preliminary tests show that its lithium ion mobility is three times that of aramid membranes.
Another research hotspot is the “intelligent response” diaphragm: by adding temperature sensitive materials to the coating, the diaphragm can not only close at high temperatures, but also automatically restore conductivity after the temperature drops, solving the limitations of traditional diaphragm “one-time protection”. If this technology matures, the safety of batteries will achieve a qualitative leap.
From mobile phone range to electric vehicle safety, from cost control to extreme environmental adaptability, every iteration of diaphragm materials is quietly rewriting the performance boundaries of lithium batteries. Perhaps one day in the future, when we no longer worry about charging, we should thank this silently dedicated ‘invisible guardian’. Article source: Lianjing Composite Materials
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