The changes brought by 4 new lithium battery separator materials to lithium batteries
As a key material for lithium batteries, the battery separator plays a role in isolating electrons, preventing direct contact between the positive and negative electrodes and allowing lithium ions in the electrolyte to pass freely. At the same time, the separator also plays a vital role in ensuring the safe operation of the battery. . my country's lithium battery separator industry is in a stage of rapid development, and wet separators have gradually become the mainstream technical route. However, at the same time, there is still a large gap between the overall technical level of domestic separators and the technical level of international first-tier companies.
In the field of technology development, traditional polyolefin separators can no longer meet the current needs of lithium batteries. High porosity, high thermal resistance, high melting point, high strength, and good wettability to electrolyte are the development direction of lithium-ion batteries in the future.
As a key material for lithium batteries, the separator plays a role in electronic isolation, preventing direct contact between the positive and negative electrodes and allowing lithium ions in the electrolyte to pass freely. At the same time, the separator also plays a vital role in ensuring the safe operation of the battery.
Under special circumstances, such as accidents, punctures, battery abuse, etc., the separator may be partially damaged and cause direct contact between the positive and negative electrodes, which may trigger a violent battery reaction and cause the battery to catch fire and explode.
Therefore, in order to improve the safety of lithium-ion batteries and ensure the safe and smooth operation of the battery, the separator must meet the following conditions:
1. Chemical stability: does not react with electrolytes and electrode materials
2. Wettability: easy to wet with electrolyte and does not stretch or shrink
3. Thermal stability: withstands high temperatures and has high fuse isolation
4. Mechanical strength: good tensile strength to ensure that the strength and width remain unchanged during automatic winding
5. Porosity: Higher porosity to meet the needs of ionic conductivity
Currently, the commercialized lithium battery separators on the market are mainly microporous polyolefin separators based on polyethylene (PE) and polypropylene (PP). This type of separator relies on its low cost, good mechanical properties, and excellent It is widely used in lithium battery separators due to its advantages such as chemical stability and electrochemical stability.
However, due to the lyophobic surface and low surface energy of the polyolefin material itself, this type of separator has poor wettability to the electrolyte, affecting the cycle life of the battery.
In addition, since the heat deformation temperatures of PE and PP are relatively low (the heat deformation temperature of PE is 80-85°C and PP is 100°C), the separator will undergo severe thermal shrinkage when the temperature is too high, so this type of separator is not suitable for use in high-temperature environments. Under such conditions, traditional polyolefin separators cannot 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 battery separator materials based on traditional polyolefin separators. Non-woven separators use non-woven methods to orient or randomly arrange the fibers to form a fiber mesh structure, and then use chemical or physical methods to reinforce the membrane to form a film, so that it has good air permeability and liquid absorption rate.
Natural materials and synthetic materials have been widely used in the preparation of non-woven membranes. Natural materials mainly include cellulose and its derivatives. Synthetic materials include polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), polyvinylidene fluoride (PVDF), Vinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyamide (PA), polyimide (PI), aramid (meta-aramid, PMIA; para-aramid PPTA), etc.
1
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 a composite membrane developed by Germany's Degussa company, which is based on PET separators and coated with ceramic particles. It exhibits excellent heat resistance, with a closed cell temperature as high as 220°C.
Xiao Qizhen of Xiangtan University and others (2012) used electrospinning method to prepare PET nanofiber separators. The manufactured nanofiber separators have a three-dimensional porous network structure, the average fiber diameter is 300nm, and the surface is smooth.
The melting point of electrospun PET separator is much higher than that of PE film, which is 255℃, the maximum tensile strength is 12Mpa, the porosity reaches 89%, the liquid absorption rate reaches 500%, which is much higher than the Celgard separator on the market, and the ionic conductivity reaches 2 .27×10-3Scm-1, and the cycle performance is also better than that of the Celgard separator. The porous fiber structure of the PET separator remains stable after 50 cycles of battery cycling, as shown in (a).
2
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, and can be used for a long time at -200 to 300°C.
Miao et al. (2013) used electrospinning to create a PI nanofiber separator. The degradation temperature of the separator is 500°C, which is 200°C higher than the traditional Celgard separator. As shown in the figure below, aging and thermal shrinkage will not occur under high temperature conditions of 150°C.
Secondly, due to the strong polarity of PI and its good wettability to the electrolyte, the manufactured separator exhibits excellent liquid absorption rate. The PI separator made by electrospinning has lower impedance and higher rate performance than the Celgard separator. The capacity retention rate is still 100% after 100 cycles of charge and discharge at 0.2C.
(a) Heat shrinkage of Celgard, PI 40μm and 100μm separators before (a, b, c) and after (d, e, f) treatment at 150°C; (b) Rate test
3
meta-aramid
PMIA is an aromatic polyamide with meta-aniline branched chains on its skeleton and has a thermal resistance of up to 400°C. Due to its high flame retardant properties, separators using this material can improve the safety performance of batteries.
In addition, due to the relatively high polarity of the carbonyl group, the separator has higher wettability in the electrolyte, thereby improving the electrochemical properties of the separator.
Generally speaking, PMIA separators are manufactured by non-textile methods, such as electrospinning. However, due to the problems inherent in non-textile separators, such as larger pore sizes, self-discharge will affect the safety performance and electrochemical performance of the battery. This limits the application of non-textile separators to a certain extent, but the phase inversion method has commercial prospects due to its versatility and controllability.
The Zhu Baoku team of Zhejiang University (2016) manufactured a sponge-like PMIA separator through the phase inversion method, as shown in the figure. The pore size distribution is concentrated, 90% of the pore sizes are below microns, and the tensile strength is as high as 10.3Mpa.
The PMIA separator manufactured by the phase inversion method has excellent thermal stability. There is no obvious mass loss when the temperature rises to 400°C. The separator does not shrink after being treated at 160°C for 1 hour.
Also due to the strong polar functional groups, the contact angle of the PMIA separator is small, only 11.3°, and the sponge-like structure allows it to absorb liquid quickly, which improves the wetting performance of the separator, reduces the activation time of the battery, and stabilizes long cycles. sexual enhancement.
In addition, due to the interconnected porous structure inside the sponge-like structure of the PMIA separator, lithium ions can be transmitted smoothly within it, so the ionic conductivity of the separator manufactured by the phase inversion method is as high as 1.51mS˙cm-1.
4
polyparaphenylenebenzodiazole
The new polymer material PBO (polyphenylenebenzodiazole) is an organic fiber with excellent mechanical properties, thermal stability, and flame retardancy. Its matrix is a linear chain structure polymer that does not decompose below 650°C. It has ultra-high strength and modulus and is an ideal heat-resistant and impact-resistant fiber material.
Because the PBO fiber surface is extremely smooth and physically and chemically inert, the fiber morphology is difficult to change. PBO fiber is only soluble in 100% concentrated sulfuric acid, methylsulfonic acid, fluorosulfonic acid, etc. After strong acid etching, the fibrils on the PBO fiber will peel off from the main trunk, forming a split filament morphology, which improves the ratio Surface area and interfacial bond strength.
(a) PBO fibrils; (b) PBO nanofiber membrane structure
Hao Xiaoming et al. (2016) used a mixed acid of methanesulfonic acid and trifluoroacetic acid to dissolve PBO fibrils to form nanofibers, and then prepared a PBO nanoporous separator through a phase inversion method.
The ultimate strength of the separator can reach 525Mpa, the Young's modulus is 20GPa, the thermal stability can reach 600°C, the contact angle of the separator is 20°, which is smaller than the 45° contact angle of the Celgard2400 separator, and the ionic conductivity is 2.3×10- 4S·cm-1, which performs better than the commercial Celgard2400 separator under 0.1C cycle conditions.
Due to the difficult manufacturing process of PBO fibrils, there are only a handful of companies around the world that produce high-quality PBO fibers, and they all use monomer polymerization. The produced PBO fibers require strong acid treatment and are difficult to apply in the field of lithium battery separators.
Hanyang University YoungMooLee team (2016) used HPI (hydroxypolyimide) nanoparticles to prepare a TR-PBO nanofiber composite separator through thermal rearrangement. In addition to the high strength and high heat resistance of the PBO material itself, the separator In addition to the advantages, 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.
