Characterization and Application of Nanomaterials (2022) Volume 5 Issue 1 doi:10.24294/can.v5i1.1429 46 Review Article Research progress on the electrospinning nanofiber lithium-ion bat- tery separators Yan Zhang, Bin Yu*, Mengzhen Zhai, Xiaohan Wang School of Textiles, Henan University of Engineering, Zhengzhou, Henan 450007, China. E-mail: zijieyb@163.com ABSTRACT Electrospinning nanofiber membrane has the advantages of wide raw materials, large specific surface area, and high porosity. It is an ideal separator material for lithium-ion batteries. This paper first introduces two common electro- spinning nanofiber diaphragms: polymer, polymer, and inorganic composite, and then focuses on the modification methods of composite modification, blending modification, and inorganic modification, as well as the methods of elec- trospinning nano modified polyolefin diaphragm. Finally, the development direction of the electrospinning lithi- um-ion battery separator has prospected. Keywords: Electrospinning Nanofibers; Lithium-ion Battery; Modification; Polyolefin; Separator ARTICLE INFO Received: 7 December 2021 Accepted: 16 January 2022 Available online: 10 February 2022 COPYRIGHT Copyright © 2022 Yan Zhang, et al. EnPress Publisher LLC. This work is li- censed under the Creative Commons Attribu- tion-NonCommercial 4.0 International Li- cense (CC BY-NC 4.0). https://creativecommons.org/licenses/by-nc/4 .0/ 1. Introduction Lithium-ion batteries have the advantages of high specific capac- ity and long service life. They are widely used in portable electronic equipment, new energy vehicles, energy storage of large power grids, and other fields[1]. As the main component of the lithium-ion battery, the separator can effectively prevent the bad condition of short cir- cuit between the positive and negative electrodes due to direct contact, and allow the lithium-ion battery to move rapidly between the positive and negative electrodes, so as to achieve the purpose of charging and discharging[2]. Nowadays, most commercial separators are polyolefin mi- croporous separators. Although they have certain advantages in cost and processing technology, there are still many deficiencies in liquid retention, thermal stability, electrolyte wetting and ion permeability[3]. In order to improve the above shortcomings, researchers have done a lot of work in the preparation of nanofiber porous lithi- um-ion battery separators by electrospinning technology. Electrospin- ning technology is one of the most effective methods to prepare nano- fibers at present. Its products have the advantages of wide raw materials, large specific surface area, and high porosity. It has been widely used in the fields of battery diaphragm, filtration and adsorp- tion, medical supplies, and fashion[4–6]. At present, polyimide (PI), polyacrylonitrile (PAN), polyethylene terephthalate (PET), polyvinyl- idene fluoride (PVDF), polymethylmethacrylate (PMMA), polyvinyli- dene fluoride hexafluoropropylene (PVDF-HFP), and other materials are often used to prepare electrospinning nanofiber lithium battery separator[7–8]. This paper mainly introduces the research progress of 47 electrospun lithium-ion battery membrane from three aspects: electrospinning nanofiber membrane material, electrospinning membrane modification and polyolefin membrane modification by electro- spinning. 2. Electrospinning nanofiber sepa- rator 2.1 Polymer nanofiber separator Priya et al.[9] prepared PAN porous nanofiber separator by optimizing the electrospinning process parameters as lithium-ion battery diaphragm. The fiber diameter distribution is 880 ~ 1260 nm. The obtained pan nanofiber membrane has good me- chanical properties and high porosity, and the ionic conductivity is much higher than that of Celgard diaphragm. Xu Caidi et al.[10] dissolved heated PVDF in DMF through electrospinning method to prepare spinning stock solution and PVDF nano- fiber separator. According to relevant tests, it can be concluded that when the concentration of PVDF is 12%, the spinning rate is better; when the voltage is 14 kV, the diameter of PVDF nanofiber diaphragm spun under electrospinning is better, the spindle is less and evenly distributed, which meets the rele- vant standards. Although common organic polymer nanofiber materials show good electrochemical properties, their heat resistance is relatively poor. High temper- ature resistant polymers can be used to make up for this deficiency. Ye et al.[11] dissolves PEEK with two chloroacetic acid (DCA) at 180 degrees Celsius. After coating the PEEK solution in the nanofiber shell of the spinnable polymer PBS by coaxial elec- trospinning, the shell core structure nanofibers are obtained, and then the core shell fibers are treated at low temperature to convert PEEK solution into gel state. PEEK nanofibers were obtained by removing the shell. After heat treatment at different tempera- tures for 2 hours, it was found that PEEK separator could maintain a good shape even at a high temper- ature of 370 ℃, and the shrinkage rates along H and T directions were 3.1% and 2.8% respectively. Xiao Ke[12] used the advantages of aramid nanofibers such as high melting point, good thermal stability and good affinity for electrolyte to obtain PMIA based nanofiber films with good multiple properties by using meta aramid (PMIA) and poly- urethane (PU). The results show that electrospin- ning nanofiber membrane has high porosity. PMIA PU Blend Membrane with high tensile strength (>15.79 MPa) is produced by introducing PU and nano cobweb structure, and its ionic conductivity can be as high as 1.38 mS/cm; the introduction of PMIA makes the blend membrane have excellent thermal stability, flame retardancy and excellent electrolyte wettability, which improves the safety performance of lithium battery to a certain extent. 2.2 Polymer and inorganic composite nano- fiber separator A common method for preparing polymer and inorganic composite nanofiber separators is to dope inorganic nanoparticles[13]. Peng Shujing et al.[14] added SiO2 nanoparticles into PVDF solution to make composite membrane. For PVDF separator and Cel-gard 2400 separator, the composite separa- tor has better charge discharge performance and battery cycle performance. Through relevant tests, it can be seen that its performance is the best when 5% SiO2 is added. Zhang Zhixiong[15] pre- pared PVA-SiO2 nanofiber composite separator by electrospinning technology. The separator has a unique three-dimensional network pore structure, and the porosity is as high as 73%; the porous structure and surface polar functional groups make the composite membrane have better electrolyte wettability. Its liquid absorption rate is as high as 405%, which is more than 3 times that of PP sepa- rator. Its ionic conductivity is as high as 1.81 mS/cm. After heat treatment at 170 ℃ for 0.5 h, the composite separator has no obvious thermal shrinkage and shows good thermal dimensional sta- bility. Ding Jun[16] prepared PI and TiO2 composite nanofiber films by electrospinning. The addition of TiO2 reduces the average fiber separator, increases the porosity and improves the electrochemical per- formance of the composite nanofiber separator, which makes its battery performance better than that of PI nanofiber separator. Li Lin et al.[17] used 48 electrospinning technology to blend PVDF and nano-TiO2, and nano-TiO2 particles played an aux- iliary role in the process of pore formation. Ac- cording to the relevant experiments, when the mass ratio of PVDF to TiO2 is 22:3, the porosity of the composite separator is improved; the liquid absorp- tion rate is 20 times higher than that of commercial PP separator, with excellent performance and obvi- ous improvement effect after compounding. The prepared composite membrane has good porosity and provides more and better channels for the passage of Li+. Due to its superior battery per- formance, the charge and discharge performance of lithium-ion batteries with PVDF and TiO2 compo- site membrane can be tested. Compared with the doping process, the inor- ganic nanoparticles produced by the electrostatic spraying method can be directly attached to the positive and negative electrodes of the polymer, and the inorganic composite separator can be directly attached to the surface of the polymer and inorganic composite separator, so as to further optimize the separator performance[18]. Jiao Xiaoning et al.[19] made a sandwich structure of polymer and inorgan- ic composite separator by spraying electrostatic spraying technology between two layers of nano- fibers. The test results show that the composite sep- arator has an excellent liquid absorption rate, ther- mal dimensional stability, and electrochemical stability. 3. Modification of electrospinning nanofiber separator Although electrospinning nanofiber diaphragm has certain advantages, it also has some problems, such as low strength and electrochemical perfor- mance to be further improved. Therefore, research- ers at home and abroad have done a lot of research on the modification of electrospinning nanofiber separator in order to obtain separator materials with better performance[20]. 3.1 Composite modification Zhao Jianmeng[21] prepared PVDF, PMMA and PVDF multilayer composite separators by electro- spinning technology. Compared with single-layer separators, the tensile strength of multilayer sepa- rators was improved, and the electrochemical po- tential was stable at 5.2 V, meeting the needs of lithium-ion batteries. Chen et al.[22] prepared inter- leaved and disordered polyimide (PI) and polyvi- nylidene fluoride Hexafluoride (PVDF-HFP) com- posite fiber separators by cross spinning process. The separator not only combines the advantages of the two materials but also melts part of PVDF-HFP through a special hot rolling treatment to realize the adhesion between fibers and improve the strength of the separator. Although the multi-layer composite modified electrospinning separator optimizes the strength and electrochemical properties of the separator to some extent, there is no strong interaction between the composite separator layers, and the battery perfor- mance may be affected by the separation of swell- ing and other effects after the separator absorbs the electrolyte[23]. 3.2 Blending modification Gopalan et al.[24] mixed PVDF and PAN by electrospinning to prepare lithium-ion battery sepa- rator. It was found that compared with pure PVDF separator, the affinity of mixed polymer membrane to electrolyte was better. When the mass fraction of PAN reached 25.0%, the membrane had higher liq- uid absorption rate, and the ionic conductivity at room temperature was as high as 7.80 mS/cm. When the mass fraction of PAN is 0.5%, the aver- age diameter of the prepared separator fiber is dif- ferent from that of PVDF separator fiber. At the same time, the fibers in the separator are entangled and knotted, which is very helpful to improve the electrochemical performance of polymer electro- lyte. Yang et al.[25] the preparation of PAN@PVDF-HFP composite fiber separator with core-shell structure is carried out by coaxial elec- trospinning technology. PVDF-HFP and PAN were used as shell and core materials respectively to study the structure, surface morphology, porosity and thermal properties of core-shell fiber separator. Compared with the traditional commercial porous PE separator, PAN@PVDF-HFP fiber composite 49 separator has better porosity, thermal stability and electrochemical performance. Blending modification uses the complementary effect of polymer properties to optimize the perfor- mance of electrospinning separator in some aspects. However, due to the large property differences be- tween complementary polymers, it is still a difficult problem to select the appropriate solvent for the preparation of blended polymer solution, and this research still has certain development possibility. 3.3 Inorganic modification In the polymer doped with inorganic particles, not only polymer and inorganic composite nano- fiber separator can be prepared, but also electro- spinning nanofiber separator can be modified. In- organic nanoparticles have good thermodynamic stability. The composite nanofiber separator pre- pared by doping inorganic particles into polymer can effectively improve the thermal size and elec- trochemical properties of the separator. In order to disperse nano-SiO2 particles, Wang Yuan et al.[26] used PVDF-HFP as the basic material to prepare SiO2 and PVDF-HFP composite lithium-ion battery separator by electrospinning. It is found that the addition of SiO2 significantly improves the liquid absorption rate and electrochemical performance of the separator. The fibers obtained by electrospin- ning are not only small in diameter, but also loosely and irregularly stacked. Therefore, the porosity of the fiber separator can reach 87%, the liquid ab- sorption rate of the prepared composite separator can reach 620%, and the ionic conductivity at room temperature can reach 2.92 mS/cm. The discharge specific capacity of the button battery assembled by the composite separator is 175 mAh/g, and the bat- tery capacity retention rate is 92% after 100 cycles, showing excellent cycle performance. Wang Zhenyu[27] formed a SiO2 inorganic layer on the surface of PEI nanofibers by in-situ growth method on the basis of the prepared polyetherimide (PEI) nanofiber film. The introduction of SiO2 en- sures the three-dimensional storage space of the electrolyte, which provides more space for the in- ternal storage of SiO2 groups, and it is found that the introduction of SiO2 has a good affinity for the electrolyte. Compared with commercial separator, PEI-SiO2 separator has better electrolyte affinity and heat resistance. The composite nanofiber separator prepared by mixing inorganic metal materials and polymers can not only make the composite separator material take into account the advantages of metal materials and polymers, but also improve the mechanical proper- ties and thermal stability of the membrane. Wang Ya et al.[28] made films with PI and Ag composite nanofibers. The test shows that the dielectric con- stant of the composite film is 6 times that of the pure PI film. The relevant experiments show that the PI nanofiber separator modified by nano Ag has excellent mechanical properties, thermal stability and better dielectric constant. The modified PI lith- ium battery separator also has other advantages. 4. Electrospinning modified poly- olefin separator Due to the limitation of its processing tech- nology, the liquid absorption rate and liquid reten- tion performance of commercial polyolefin separa- tor are poor, while electrospinning nanofiber membrane has a unique three-dimensional network structure. Electrospinning nanofibers can be depos- ited on one or both sides of polyolefin separator to prepare composite diaphragm, so as to improve the thermal stability, wettability and liquid retention of diaphragm, and to improve the electrochemical performance of the battery[29]. Lee et al.[30] deposit- ed polyvinylidene fluoride, chlorotrifluoroethylene and aluminum oxide composite nanofibers on both sides of polyolefin separator to prepare three-layer composite separator. The results show that the pre- pared three-layer composite separator has good thermal dimensional stability and good battery cy- cle performance. In order to improve the adhesion between pol- yolefin separator and polyvinylidene fluoride nano- fibers, Liang Yinzheng[31] treated the surface of polyolefin separator under the condition of argon atmospheric pressure plasma, which improved the adhesion between nanofibers and polyolefin sepa- rator and reduced the delamination of composite separator. Wang Yuan[32] analyzed the wettability, 50 thermal stability, mechanical properties and elec- trochemical properties of PVDF modified polyeth- ylene lithium ion battery separator. The results showed that PVDF modified PE separator showed better mechanical properties, the tensile strength and elongation at break of the separator were improved accordingly, and the modified sepa- rator also showed excellent ion conductivity battery charge discharge cycle and rate performance. 5. Expectation To sum up, the development of lithium ion spinning equipment should focus on reducing the production efficiency of lithium-ion separator; sec- ond, continuously improve the mechanical and electrochemical properties of existing separator materials through modification technology; third, continuously develop new materials to meet the needs of lithium-ion battery separator, such as high temperature resistant and low-cost power lithi- um-ion battery separator materials. 6. 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