Text/Peach Lee
Batteries we are not strangers, our mobile phones, electrical energy and other electronic products will use it, and with the continuous development of new energy electric vehicles, batteries have a new development, and gradually began to replace the status of fuel, becoming the source of power for electric vehicles. People who pay attention to new energy vehicles may have a lot of understanding of the parameters of the power battery, but how much do you know about the interior of the battery? Today we will continue to talk about another important component of the battery interior - the diaphragm.
So what is the separator of the battery? The diaphragm is a microporous membrane used to separate the positive and negative pole plates inside the lithium-ion battery, and is a polymer functional material with a nanoscale microporous structure. Its main function is to prevent short circuits due to contact between the two poles and ensure that lithium ions can normally pass through the microporous channel during charging and discharging to ensure that the battery is working properly. Its performance determines the interface structure of the battery, internal resistance, etc., directly affecting the capacity of the battery, cycling and the safety performance of the battery, in view of its importance, the industry also calls it the "third electrode".
At present, the market penetration rate of new energy vehicles continues to increase, people pay more attention to it, the safety of the battery is the most important point, and with the advancement of battery technology, our requirements for the separator are naturally higher, so what conditions need to be met as a separator?
First of all, it must have electronic insulation, to ensure the mechanical isolation of the positive and negative electrodes, while to have good thermal stability and automatic shutdown protection performance, heat shrinkage should be small, to avoid short circuits leading to thermal runaway; secondly, there must be a certain pore size and porosity, to ensure lower resistance and higher ion conductivity, the space stability and flatness requirements are very high; the third is to ensure high mechanical stability on the basis of reducing its thickness as much as possible, reduce internal resistance; finally, because it is in the electrolyte, Therefore, it must also be resistant to electrolyte corrosion, strong chemical stability, and at the same time have good electrolyte infiltration and strong ability to absorb liquid and moisturize.
Usually the diaphragm is a single layer of PP (polypropylene), PE (polyethylene), there are also double layers or PP/ PE / PP three-layer composite diaphragm. The shading temperatures of PE film, PP film and PP/PE/PP film are 130~133°C, 156~163°C and 134~135°C, and the rupture temperatures are 139°C, 162°C and 165°C, respectively. PE with low melting point plays a role in closing the hole when the temperature is low, and PP can maintain the shape and mechanical strength of the diaphragm to prevent positive and negative contact, so the safety of PP/PE/PP composite film is better than that of single-layer film.
However, PP/PE/PP composite film has a large high temperature shrinkage rate, and the strength and safety performance need to be improved. Polyethylene terephthalate (PET) nonwoven diaphragms, new polyimide and polyamide diaphragms, organic/inorganic composite films, and ceramic-coated diaphragms improve strength, high temperature stability and safety performance.
At present, the production method of diaphragm is mainly divided into dry method and wet method. The dry method is to mix the raw materials of the diaphragm and the film-forming additives, form the structure of the sheet crystal by melt extrusion, and then annealing the process to obtain the dry diaphragm, which is mainly used for polypropylene (PP) diaphragm.
Wet method is the use of the principle of thermal phase separation, the plasticizer such as paraffin oil and polyolefin resin mixed and melted to form a uniform mixture, insulation for a certain period of time with solvents from the film extraction, so as to produce a mutual penetration of sub-micron size microporous film materials, mainly used for the manufacture of polyethylene (PE) diaphragm.
Either way of making it is good or bad, and it cannot be perfect. Wet process film pore size is small and uniform, thinner film, but large investment, complex process, environmental pollution; dry process is relatively simple, high added value, environmental friendly, but the pore size and porosity is difficult to control, the product is difficult to thin.
Because the diaphragm formed by the simple dry and wet method cannot fully meet the needs of the industry, the current common practice is to coat other materials on top of the base film, and the coating film with improved performance greatly improves the thermal stability of the wet diaphragm, which has obvious performance advantages.
Write at the end:
The introduction of the diaphragm is simply said here, want to deeply understand what this is a moment and a half is also difficult to say clearly, here is mainly to introduce to you its related concepts, so that everyone has a basic understanding, I hope to help you.