As a key material for lithium batteries, the battery separator acts as an electronic barrier to prevent direct contact between the positive and negative electrodes, allowing lithium ions to pass freely in the electrolyte. At the same time, the separator plays a vital role in ensuring the safe operation of the battery. .
China's lithium battery separator industry is in the stage of rapid development, wet diaphragm has gradually become the mainstream technical route, but at the same time the overall technical level of domestic diaphragm and the international first-line company technology level is still a big gap.
In the field of technology development, the traditional polyolefin diaphragm can not meet the current demand of lithium batteries, high porosity, high thermal resistance, high melting point, high strength, good wettability to the electrolyte is the future development direction of lithium-ion batteries.
As a key material of lithium batteries, the separator plays an electronic isolation role, preventing direct contact between the positive and negative electrodes, allowing lithium ions to pass freely in the electrolyte. At the same time, the separator plays a vital role in ensuring the safe operation of the battery.
In special cases, such as accidents, punctures, battery abuse, etc., partial damage of the diaphragm may occur, resulting in direct contact between the positive and negative electrodes, causing a severe battery reaction causing the battery to explode.
Therefore, in order to improve the safety of the lithium-ion battery and ensure the safe and smooth operation of the battery, the diaphragm must meet the following conditions:
1. Chemical stability: Does not react with electrolytes, electrode materials
2. Wetting: It is easy to infiltrate with electrolyte and does not stretch, does not shrink
3. Thermal stability: withstand high temperature, with high fuse isolation
4. Mechanical strength: good tensile strength to ensure the strength and width of the automatic winding
5. Porosity: Higher porosity to meet the needs of ion conduction
Currently, commercially available lithium battery separators are mainly microporous polyolefin separators based on polyethylene (PE) and polypropylene (PP). These separators are excellent in mechanical properties and excellent in terms of low cost. Chemical stability and electrochemical stability are widely used in lithium battery separators.
However, due to the lyophobic surface and low surface energy of the polyolefin material itself, the permeability of the separator to the electrolyte is poor, which affects the cycle life of the battery.
In addition, since the heat distortion temperature of PE and PP is relatively low (PE has a heat distortion temperature of 80 to 85 ° C and PP is 100 ° C), the separator may undergo severe heat shrinkage when the temperature is too high, so such separators are not suitable for high temperature environments. Under the use, the traditional polyolefin diaphragm can not meet the requirements of today's 3C products and power batteries.
In response to the development needs of lithium-ion battery technology, researchers have developed various new lithium-ion battery materials based on traditional polyolefin separators.
The nonwoven membrane is oriented or randomly arranged by a non-woven method to form a web structure, and then chemically or physically strengthened to form a film, which has good gas permeability and liquid absorption rate.
Natural materials and synthetic materials have been widely used in the preparation of non-woven fabrics. Natural materials mainly include cellulose and its derivatives. Synthetic materials include polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and poly Vinylidene-hexafluoropropylene (PVDF-HFP), polyamide (PA), polyimide (PI), aramid (meta-aramid, PMIA; para-aramid PPTA), etc.
Polyethylene terephthalate
Polyethylene terephthalate (PET) is a material with excellent mechanical properties, thermodynamic properties and electrical insulation properties. The most representative product of PET separators is based on PET separator developed by Degussa, Germany. Particle coated composite film, which exhibits excellent heat resistance, with a closed cell temperature of up to 220 ° C.
PET separator before charge and discharge cycle (a) after (b) SEM image
Xiao Qizhen et al. (2012) produced a PET nanofiber membrane by electrospinning. The nanofiber membrane produced has a three-dimensional porous network structure, such as (b), the average diameter of the fiber is 300 nm, and the surface is smooth.
The melting point of the electrospun PET membrane is much higher than that of the PE film, which is 255 ° C, the maximum tensile strength is 12Mpa, the porosity is 89%, the liquid absorption rate is 500%, much higher than the Celgard diaphragm on the market, and the ionic conductivity reaches 2 .27×10-3Scm -1, and the cycle performance is also better than the Celgard diaphragm. The PET membrane porous fiber structure remains stable after 50 cycles of the battery, as shown in (a).
Polyimide
Polyimide (PI) is also one of the polymers with good comprehensive properties. It has excellent thermal stability, high porosity, and good high temperature resistance. It can be used at -200~300 °C for a long time.
Miao et al. (2013) fabricated a PI nanofiber separator by electrospinning. The membrane has a degradation temperature of 500 ° C, which is 200 ° C higher than that of the conventional Celgard diaphragm. As shown in the figure below, aging and heat shrinkage do not occur at 150 ° C.
Secondly, due to the strong polarity of PI, the wettability of the electrolyte is good, and the manufactured separator exhibits excellent liquid absorption rate. The electrospinning PI diaphragm has lower impedance and higher impedance than the Celgard diaphragm. Rate performance, 0. 2C charge and discharge after 100 laps, the capacity retention rate is still 100%.
(a) Celgard, PI 40 μm, 100 μm diaphragm 150 ° C before treatment (a, b, c) after (d, e, f) heat shrinkage; (b) magnification test
Meta-aramid
PMIA is an aromatic polyamide with a metabenzamide type branch on its skeleton and a thermal resistance of up to 400 °C. Due to its high flame retardancy, the separator using this material can improve the safety of the battery.
In addition, due to the relatively high polarity of the carbonyl group, the separator has a higher wettability in the electrolyte, thereby improving the electrochemical properties of the separator.
In general, PMIA separators are manufactured by non-woven methods, such as electrospinning, but due to problems with non-woven membranes themselves, such as large pore sizes can cause self-discharge, which affects the safety and electrochemical performance of the battery. To some extent, the application of non-woven membranes is limited, and the phase inversion method has commercial prospects due to its versatility and controllability.
SEM and pore size distribution of PMIA diaphragm
The Zhu Baoku team of Zhejiang University (2016) produced a sponge-like PMIA diaphragm by the phase inversion method. As shown in the figure, the pore size distribution is concentrated, 90% of the pore diameter is below the micron, and the tensile strength is 10.3Mpa.
The PMIA separator manufactured by the phase inversion method has excellent thermal stability, and there is still no obvious mass loss when the temperature rises to 400 ° C. The separator does not shrink after being treated at 160 ° C for 1 h.
Also due to the strong polar functional group, the contact angle of the PMIA membrane is small, only 11. 3 °, and the sponge-like structure makes it absorb liquid quickly, which improves the wetting property of the separator, so that the activation time of the battery is reduced, and the stability of the long cycle is stabilized. Sexual improvement.
The ionic conductivity of the membrane produced by the phase inversion method is as high as 1.51mS ̇cm, because the porous structure of the sponge-like structure of the PMIA membrane is interconnected.-1.
Polyparaphenylene benzobisazole
The new polymer material PBO (polyphenylene benzobisazole) is an organic fiber with excellent mechanical properties, thermal stability and flame retardancy. Its matrix is a linear chain structure polymer below 650 °C. Does not decompose, has ultra high strength and modulus, is ideal for heat and impact resistant fiber materials.
Because the surface of PBO fiber is extremely smooth and physicochemically inert, the fiber morphology is difficult to change. PBO fiber is only soluble in 100% concentrated sulfuric acid, methanesulfonic acid, fluorosulfonic acid, etc. PBO fiber after strong acid etching The fibrils on the top will be peeled off from the trunk to form a filament shape, which improves the specific surface area and interfacial bond strength.
(a) PBO fibrils; (b) PBO nanofiber membrane structures
Hao Xiaoming et al. (2016) After dissolving PBO fibrils with a mixed acid of methanesulfonic acid and trifluoroacetic acid to form nanofibers, PBO nanoporous separators were prepared by phase inversion method. The fiber morphology is as shown above.
The ionic conductivity is 2. 3×10. The ultimate strength of the diaphragm is 525 MPa, the Young's modulus is 20 GPa, the thermal stability is up to 600 ° C, the diaphragm contact angle is 20°, and the contact angle of the Celgard 2400 diaphragm is 45°.-4S·cm -1, performs better than the commercial Celgard 2400 diaphragm under the 0. 1C cycle.
Due to the difficult manufacturing process of PBO fibrils, there are only a handful of companies producing excellent PBO fibers worldwide, and all of them use monomer polymerization. The production of PBO fibers is difficult to apply in the field of lithium battery separators due to the need for strong acid treatment.
The YoungMooLee team of Hanyang University (2016) used TRI (hydroxypolyimide) nanoparticles to prepare TR-PBO nanofiber composite membranes by thermal rearrangement. In addition to the high strength and high heat resistance of the PBO material itself, the separator. In addition, the pore size distribution is more concentrated, the pore size is smaller, and it does not need to be prepared under strong acid and alkali conditions.
