The diaphragm plays two main roles in lithium-ion batteries. First, the diaphragm material needs to have good insulation and a certain strength to avoid direct contact between the positive and negative electrodes in the battery, and can effectively prevent short circuits caused by punctures such as burrs and dendrites, and ensure that there is no significant dimensional change under sudden high temperature conditions, thereby ensuring the safety of the battery. Second, the porous structure of the diaphragm can provide a good migration channel for lithium ions, ensuring stable and efficient operation of the battery.
As the "third electrode" of lithium-ion batteries, the separator is a key material to ensure the safety of the battery system and affect the battery performance. It needs to have high strength, heat resistance, flame retardancy, high porosity, uniformity and good wettability.
At present, most lithium-ion battery separators use polyolefin microporous membranes based on polypropylene (PP) and polyethylene (PE). Their low melting point (PP is 165°C, PE is 135°C) and softening temperature make the battery prone to thermal runaway due to membrane shrinkage, especially in the case of overcharge, overdischarge and high-power charging and discharging, which may cause the battery to catch fire or explode.
Polyolefin diaphragm
In addition, PP and PE are non-polar polymers with poor electrolyte wettability, which leads to large internal resistance of the battery. In addition, their low porosity (about 40%) leads to low ionic conductivity, which seriously limits the high-rate performance of the battery and makes it difficult to meet the needs of high-current rapid charging and discharging of the battery. Although modification based on traditional polyolefin separators can improve the heat-resistant wettability of the separator, it cannot solve the current problems faced by the separator, nor can it meet the market demand for high-performance separators.
In order to improve the safety of lithium batteries and meet market demand, the development of a new generation of high-performance polymer separators is a problem that needs to be solved urgently. With the continuous advancement of science and technology, the research on heat-resistant polymer separators has also made further progress. This article summarizes different types of heat-resistant separators and introduces their performance, and also looks forward to the future development of heat-resistant high-performance separators.
1.Heat-resistant diaphragm performance
The performance of lithium-ion battery separators is crucial to the safety of the battery system and the improvement of electrochemical performance, and should meet the following requirements:
(1)Appropriate thickness and excellent dimensional stability. Usually, the thickness of lithium-ion battery separators is 20~25μm. The thickness of the separator is closely related to the dimensional stability and should be considered comprehensively.
(2) The porosity is high and the pores are uniform. The pore size of the diaphragm should be larger than the diameter of lithium ions and smaller than the diameter of the active material. High porosity can more effectively promote the absorption and penetration of electrolyte by the diaphragm and improve the conductivity of ions.
(3) Excellent mechanical properties can ensure the safety of the battery and prevent lithium dendrites from piercing the separator and causing battery short circuit.
(4) Good wettability can reduce interfacial resistance. The diffusion time of the electrolyte in the diaphragm, the degree of adsorption, or the contact angle between the electrolyte and the diaphragm all reflect the wettability of the diaphragm.
(5) Excellent chemical stability. The diaphragm cannot react with the electrode material. It can exist stably in the electrolyte and effectively block the positive and negative electrodes, ensuring the normal and efficient operation of the lithium battery.
(6) Excellent heat resistance and flame retardancy. Lithium batteries may experience thermal runaway under long-term use or under extreme temperatures. Excellent heat resistance and flame retardancy can prevent further deterioration and play a role in extinguishing fires.
2.Heat-resistant polymer diaphragm
At present, the polymers of heat-resistant membranes include PEEK, PET, polyamide, PVDF , PI, etc. [16, 26]. The above materials all have excellent mechanical properties, thermal stability and chemical stability, and can be prepared into membranes by electrospinning to ensure their high porosity, and can be used as candidate materials for high-performance membranes.
2.1 PEEK diaphragm
PEEK is an aromatic polymer with excellent heat resistance and chemical stability. At the same time, the polar oxygen atoms and carbon-oxygen double bonds in the PEEK polymer have strong interactions with carbonate electrolytes, which can ensure that the diaphragm has excellent wettability.
PEEK film
Li et al. used the phase inversion method to develop a sponge-like porous PEEK membrane with good thermal stability and high porosity (78%). The high porosity and excellent wettability of the membrane to the electrolyte ensured a high liquid absorption rate of the membrane (251%). The good wettability of the membrane was conducive to the transmission of lithium ions, resulting in higher ionic conductivity and improved rate performance of lithium batteries. The battery exhibited excellent discharge capacity (124.1 mAh/g) at 5C.
In addition, Li et al. used the prepared fluorinated PEEK as the raw material to prepare the spinning solution. The nanofiber membrane prepared by electrospinning had a very high porosity (88%). The presence of trifluoromethyl increased the proportion of polar groups, making the membrane exhibit excellent liquid absorption rate (559%) and good wettability, reducing the internal resistance of the battery, greatly improving the ionic conductivity of the membrane (3.12Ms/cm), and the membrane also had high mechanical properties (27.7MPa) and good thermal stability, which enhanced the safety of the battery.
2.2 PET diaphragm
PET has good mechanical properties, excellent thermal stability and good electrical insulation. Coating inorganic nanoparticles on the diaphragm prepared by it can further enhance the comprehensive properties of the diaphragm, such as heat resistance and wettability.
PET film
For example, Xie et al. used a dip coating method to coat two inorganic materials, SiO2 and Al2O3, on a PET diaphragm to form a uniform ceramic coating, which gave the diaphragm a special pore structure and a high porosity. In addition, the two inorganic nanoparticles have a good affinity with the electrolyte, which improves the wettability of the diaphragm and thereby increases the ionic conductivity of the diaphragm. The capacity retention rate (93.9%) of the diaphragm after 100 cycles is also excellent, and it still maintains a high capacity (82.7 mAh/g) at a current of 10C.
In addition, the PET nanofiber membrane prepared by electrospinning by Hao et al. has high tensile strength (12MPa), good elongation and excellent thermal stability. The membrane prepared by electrospinning has high porosity (89%) and high liquid absorption rate (484%), which can promote the efficient and stable migration of lithium ions and improve the ion conductivity. The battery assembled with PET membrane has better electrochemical stability and higher discharge capacity than the battery assembled with Celgard membrane, and the battery can operate more efficiently and stably.
2.3 Meta-aramid (PMIA) separator
The main chain of PMIA molecules is composed of aromatic rings and amide groups, and there is a very strong hydrogen bond network between its molecules. It is a high-performance material with high heat resistance, high flame retardancy, high mechanical strength, and high electrical insulation. PMIA membranes are mostly prepared by electrospinning, which can increase the specific surface area of
PMIA membranes and improve the applicability of the material. In addition, adding inorganic particles to the nanofiber membrane can further enhance the heat resistance of the membrane.
Meta-aramid paper
Jeon et al. prepared meta-aramid nanofiber membrane by electrospinning, and then prepared a diaphragm by coating Al2O3 particles on the nanofiber membrane, which made the diaphragm have better thermal stability and chemical stability. At the same time, Al2O3 has a high dielectric constant and good wettability with polar electrolytes, which can reduce the charge transfer resistance, improve the discharge capacity and cycle stability of the battery, and has a relatively high discharge capacity (232mAh/g) at a 1C rate.
In addition, Xiao et al. also used electrospinning to prepare PMIA and PMIA-(polyurethane) PU nanofiber membranes. The PMIA-PU nanofiber membrane prepared by electrospinning has a high porosity as a diaphragm, and the carbonyl groups in the molecular structure of PMIA and PU have a higher compatibility with the electrolyte, which synergistically enables the diaphragm to have an extremely high liquid absorption rate (maximum 843.52%), thereby enhancing the ionic conductivity of the diaphragm. At the same time, the diaphragm also has strong mechanical properties and thermal stability.
2.4 Polybenzimidazole (PBI) membrane
PBO is a chain aromatic polymer composed of aromatic heterocycles and benzene rings, which has excellent mechanical properties, thermal stability, dimensional stability and chemical stability.
Lee et al. prepared hydroxyl copolymer polyimide ( HPI ) nanoparticles by redeposition method and coated them on HPI nanofiber membrane prepared by electrospinning, and finally prepared a composite membrane by thermal rearrangement. They investigated the effect of particle shape on the membrane performance. The membrane showed excellent thermal stability at 490°C. The good wettability of the membrane can improve the ion transfer efficiency. In addition, since nanoparticles with ascidian structures have better electrochemical properties than nanoparticles with spherical structures at high temperatures, the battery assembled with TR-PBO nanocomposite membranes showed excellent high power density performance.
Hao et al. stripped Zylon ultrafine fibers ( PBO fibers ) into PBO ultrafine fibers with a diameter of 2~10nm, and then woven them to obtain PBO microporous membranes with pore sizes between 5~25nm. The high strength of PBO fibers and the interaction between nanofibers give the membrane higher mechanical properties (elastic modulus of 20GPa, ultimate strength of 525MPa), and the membrane can be used for a long time below 600℃, which can effectively improve the safety performance of the battery.
2.5 PVDF diaphragm
Fluorine-based polymers such as PVDF can be used as candidate materials for lithium battery separators because of their good chemical and electrochemical stability, and the presence of their β-crystalline phase is beneficial to improving the affinity between the separator and the electrolyte.
PVDF diaphragm
Wu et al. prepared PVDF/PAN blended porous membranes by thermally induced phase separation (TIPS). PAN usually has higher toughness and strength than PVDF. The addition of PAN greatly improves the thermal stability (stable at 300°C) and tensile strength of the membrane. Compared with the commercial Celgard2400 membrane, the membrane has higher ion transfer efficiency and good cycle performance after assembling the battery. However, the addition of PAN will reduce the pore size and porosity of the membrane, thereby reducing the ionic conductivity of the membrane and affecting the electrochemical performance. It can be adjusted according to different needs.
Widiyandari et al. prepared PVDF nanofiber membrane by electrospinning and immersed it in SiO2 sol to prepare PVDF/SiO2 composite membrane. The addition of SiO2 improved the membrane porosity, thermal stability and mechanical strength. SiO2 has good affinity with electrolyte, which can further improve the wettability of the membrane. Compared with pure PVDF membrane, the battery capacity assembled with PVDF membrane with SiO2 added was significantly improved after 6 cycles.
2.6 Polybenzimidazole (PBI) separator
PBI is an aromatic heterocyclic polymer with excellent mechanical properties and heat resistance. It can still maintain good mechanical and electrical properties above 400°C. The polar nitrogen atoms in the PBI molecules are positively compatible with the electrolyte, giving the diaphragm better wettability.
Liu et al. prepared polyarylether benzimidazole (OPBI) nanofiber separator by electrospinning, which showed excellent thermal stability. The separator had no dimensional shrinkage at 200°C and began to degrade at 550°C. The rich nitrogen atoms and polar ether bonds in OPBI gave the separator good wettability, making it easier for lithium ions to migrate, reducing battery resistance and improving battery performance.
In addition, Sun et al. prepared a PBI microporous membrane by wet pore formation, which had no dimensional change at 300°C, and the skeleton stability of the polymer in the air could be maintained at 545°C. It also showed excellent flame retardancy and self-extinguishing properties in the ignition test. The interaction between PBI and the electrolyte ester bond can increase the compatibility between the diaphragm and the electrolyte, thereby improving the wettability of the diaphragm, and thus enhancing the electrochemical performance of the battery. The battery performance test showed that the discharge capacity of the battery assembled with the PBI microporous membrane at 0.1C was as high as 157.1mAh/g, and the discharge capacity retention rate at 5C was 84%. PBI's excellent flame retardancy, wettability, and heat resistance all prove that it can be used as a candidate material for lithium battery diaphragms.
2.7 Polyphenylene sulfide (PPS) diaphragm
PPS is a special engineering plastic with super heat resistance and chemical stability. Its decomposition temperature is about 450℃. It can be used for a long time within 200℃. It is also resistant to corrosion from most solvents.
PPS film
In order to solve the problem of large and uneven pore size in PPS non-woven fabrics, Chen et al. uniformly coated PVDF and nano-SiO2 on the surface of PPS non-woven fabrics to prepare a composite diaphragm. After the coating was evenly covered on the surface of the PPS non-woven fabric, the composite diaphragm formed a relatively curved three-dimensional porous structure, which can promote the diaphragm to absorb and store more electrolyte. Studies have shown that the diaphragm has a high porosity (55.7%), high wettability and excellent thermal dimensional stability at 250°C. After 100 cycles, the capacity retention rate (66.34%) is higher than that of the commercial diaphragm (61.03%).
In addition, Kim et al. used plasma-assisted mechanochemical (MP) treatment to evenly disperse SiO2 in the PPS matrix, and then prepared a PPS porous membrane by etching away SiO2 with an HF acid solution. The porous membrane has a uniform pore size, a good pore structure, and a pore structure on its surface, which makes the prepared diaphragm have high porosity, good wettability, excellent mechanical properties and thermal stability (no dimensional deformation at 250°C), thereby improving the ion transmission efficiency of the diaphragm. The PPS diaphragm with excellent mechanical properties and uniform pore size distribution can effectively inhibit the growth of lithium dendrites.
2.8 PI diaphragm
PI is a high-performance polymer containing aromatic heterocycles with excellent thermal stability, chemical stability and mechanical properties. The most commonly used method for preparing PI membranes is electrospinning technology. The PI nanofiber membrane prepared by electrospinning has the advantages of high porosity and fast ion transmission efficiency. At the same time, it has PI's excellent heat resistance, mechanical properties and good wettability with electrolytes, which can improve the safety, charge and discharge rate and cycle performance of batteries.
PI nanofiber membrane
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