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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.30 No.4 pp.228-241
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2020.30.4.228

Progress in Composite Polymer Membrane for Application as Separator in Lithium Ion Battery

Seok Hyeon Oh*, Rajkumar Patel**†
*Nano Science and Engineering, Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Incheon, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 21983, South Korea
**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 21983, South Korea
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr, https://orcid.org/0000-0002-3820-141X)
July 7, 2020 ; August 3, 2020 ; August 4, 2020

Abstract


Separators, which produces physical layer between a cathode and anode, are getting enormous attention as the quality of the separator determines the performance of lithium ion batteries (LIBs). Porous membranes based on polyethylene (PE) and polypropylene (PP) are generally utilized as the separator of LIBs because of their high electrochemical stability and suitable mechanical strength. However, low thermal resistance and wettability of PE and PP membranes limited the potential of LIBs. Operating at the temperature exceeding the melting point of membranes, the separators change their structures which lead to short circuit of LIBs. Low wettability of the separators corresponds to low ionic conductivity which increases the cell resistance. To overcome these weaknesses of PE and PP separators, different types of separator were prepared by co-electrospinning, applying coating layer, forming core shell around membrane, and papermaking method. The synthesized separator greatly enhanced the heat resistance and wettability of separator and mechanical properties like flexibility and tensile strength. In this review different type of polymer membrane used as separator in lithium ion battery are discussed.


separator , lithium ion battery , composite polymer membrane , nanofiber , electrospinning

리튬 이온 전지의 분리막으로 사용하기 위한 복합 고분자 막의 동향

오 석 현*, 파텔 라즈쿠마**†
*연세대학교 언더우드국제대학 융합과학공학부 나노과학공학
**연세대학교 언더우드 국제대학 융합과학공학부 에너지환경과학공학

초록


리튬 이온 전지의 양극과 음극 사이에 물리적인 층을 만들어주는 분리막은 분리막의 품질에 따라 리튬 이온 전지 의 성능을 결정함에 따라 많은 관심을 받고 있다. 일반적으로 전기화학적 안정성과 적절한 역학적 강도를 갖고 있는 폴리에 틸렌과 폴리프로필렌으로 구성된 다공성 막이 리튬 이온 전지의 분리막으로 사용된다. 하지만 폴리에틸렌과 폴리프로필렌의 낮은 열 저항성과 젖음성으로 인해 리튬 이온 전지의 잠재력을 충분히 끌어내지 못한다. 녹는점 이상의 온도에 도달하게 되 면 분리막의 구조가 변형되고 리튬 이온 전지는 단락된다. 분리막의 낮은 젖음성은 낮은 이온전도도와 부합하고, 이는 전지의 저항을 상승시킨다. 이러한 폴리에틸렌과 폴리프로필렌 분리막의 단점을 극복하고자 이중 전기방사방법, 코팅 층 도포 방법, 코어 셸 구조 형성 방법, 제지법 등 여러 가지 방법들이 연구되었다. 언급된 방법들로 합성된 분리막들은 열 저항성과 젖음성 이 크게 향상되었고 유연성과 인장 강도 같은 역학적 특성도 향상되었다. 본 리뷰 논문에는 각기 다른 방법으로 형성된 리튬 이온 전지의 분리막에 대해서 다루고 있다.


    1. Introduction

    Lithium ion batteries (LIBs) are the most common energy storage for electronic devices. As the desire for a better power source increased, a lot of researchers have been studied to enhance the performance of LIBs. The most important feature of LIBs to be utilized is separator of LIBs. Separators provide physical gap between a cathode and anode of LIBs to prevent electron flow, which prevent internal short circuit of LIBs and allow rapid migration of lithium ions through pores of separators. Because of the role of separator, enhancement of separators may influence the performance and safety of LIBs significantly[1-14].

    The most widely used materials of the separator are polyethylene (PE), and polypropylene (PP) due to their characteristics. They have electrochemical stability, proper mechanical strength, acceptable cost, and good thermal shut down property. Thermal shut down property limits thermal runaway at excessively high temperature. However, both PE and PP suffer from several disadvantages. Low thermal resistance due to their low melting temperature (PE about 130°C, PP around 165 °C) may cause the cathode and anode to interfere each other, which would lead to extensive flow of current [15]. Having relatively low melting temperature, LIBs become more vulnerable to explosion when short circuit occur at higher temperature[11]. Hydrophobic nature and low porosity of PE and PP separators restrict ion migration and compatibility with the polar electrolytes in LIBs that may decrease the battery performance due to higher cell resistance[15].

    When the temperature of LIBs increases over the melting point of the separator, separators start deformation which causes short circuit and they cannot be further utilized. In order to benefit both safety and performance of LIBs, enhancing thermal stability and wettability of a separator was crucial mission. Therefore, a lot of techniques were applied to make a separator with high thermal stability and wettability while maintaining durable electrochemical stability and mechanical strength. In this review, an overview of enhancements of separators through nanofibrous membrane, polyvinylidene fluoride (PVDF) based separators, coated membrane, core shell membrane, and heat resistant membrane were observed. The schematic representation of membrane separator in lithium ion battery is presented in Fig. 1. Schematic diagram with graphite as anode material is showed in Fig. 2[14].

    2. Separator

    Optimizing the porosity and thickness of the electrodes in LIBs may improve the performance[16]. Graphite can be used as anode, but it is only recommended for high discharge rates. Replacing graphite, Li4Ti5O12 can function as an anode for rapid charge Li-ion systems. The separators from Celgard Inc. were prepared by a dry process, and the separators from DSM Solutech were prepared by a wet process. Liquid electrolyte was prepared by mixing dimethyl carbonate (DMC) and ethylene carbonate (EC). The diameter of pores was computed by mercury intrusion and extrusion, and the distribution of pore diameter was calculated with Laplace equation. The permeability coefficient B was calculated by using Darcy’s law. The pore diameter was calculated by using Kozeny constant which depends on the tortuosity and shape of pores and B value. It was observed that low porosity leads to low conductivity, but microporous structure of separator might affect the conductivity. The tortuosity affects the conductivity by impacting on the microporous structure. The effect of separator morphology is important for performance of battery. Xu et al. reported the fabrication of cellulose/ polysulfonamide composite membrane separator by simple paper making process[17]. Composite membrane with better electrolyte wettability and higher thermal stability outperform commercially available PP separator (Fig. 2). Summary of the performance of different separators in the lithium ion battery is presented in Table 1.

    2.1. Membrane based on nanofiber

    Nonwoven polyacrylonitrile (PAN) was synthesized by and electrospinning technique[18]. The nonwoven PAN nanofiber was combined with a nonwoven composite separator which is charged with silica or alumina powder by the hot roll press technique. The morphology of the surface of nonwoven composite separator was determined with scanning electron microscopy (SEM). The average size of the pore and its distribution was obtained by bubble point method. The amount of the ceramic powder is the factor which controls the size of pore in a nonwoven composite separator, and nonwoven PAN nanofiber. The charge-discharge performance of both nonwoven composite separators (CNS) was stable because of their pore size and pore distribution being small and narrow. The thermal shrinkage of the polyolefin membrane and CNS were compared via area difference after exposure of heat in a hot oven.

    A poly(amic acid) (PAA) solution was prepared through condensation polymerization. PAA nanofibrous membrane was synthesized by electrospinning and dried for 12 hours in laminar flow cabinet to form polyimide (PI) nanofibrous membrane[19]. Then, the PI nanofibrous membrane was immersed into KOH solution for surface alkaline etching which provides active sites for TiO2+ ions to attach. The etched PI nanofibrous membrane was dipped into TiOSO4 solution and dried at room temperature. Thermal treatment of 300°C for 2 hours was applied to the dried membrane to switch TiO(OH)2 nanoshell into TiO2 nanoshell, and form TiO2@PI nanofibrous membrane. The surface morphology was observed with field-emission scanning electron microscope (FE-SEM) image. TiO2 nanolayer was deposited uniformly and compactly on the surface of PI nanofibers. The thickness of nanofiber increased when the complexation time in TiOSO4 solution increased. TiO2@PI membranes (5% weight loss at 452°C) had higher thermal resistance than both etched PI membrane (5% weight loss at 371°C) and, PP separator (5% weight loss at 272°C). TiO2@PI membrane (86.2~77.8% porosity, 455~320% electrolyte uptake varying by time of immersion from 0.5~1.5 h) had lower porosity and electrolyte uptake than pristine PI membrane (95% porosity, 620% electrolyte uptake) but had higher porosity and electrolyte uptake than PP separator (41% porosity, 92% electrolyte uptake). Contact angle (CA) test with water and the membrane indicated that TiO2 nanoshell improved the wettability of the membrane. Thermal stability of PI membrane (little prolongation above 350°C) was enhanced when TiO2 nanoshell was applied (no prolongation until 400°C). TiO2@PI membrane could not be ignited as well. TiO2@PI membrane was able to roll, twist, and fold which indicate that the flexibility was excellent.

    Electrospinning method provides most effective structure which has large surface area and well-interconnected pores[20]. PI has superior mechanical, chemical properties and thermal resistance up to 500°C. PAA solution was extruded using electrospinning method and the thickness of nanofiber was controlled by changing the time of electrospinning. The synthesized PI nanofiber was dried 24 hours in vacuum and calcined to form PI nanofiber mats. PI nanofiber mats were dipped into the coating solution which include Al2O3 nano powders and poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP). As-synthesized PI separator was observed with fourier transform infrared spectroscopy (FTIR) spectrum which confirmed that PI phase successfully occurred after imidization process. Using FE-SEM images, the diameter of PI nanofibers (300 nm) was measured. Al2O3 nanoparticles homogeneously coved the pores of PI separator. Wettability of the separator was improved due to hydrophilicity of Al2O3 layer and PVDF-HFP binder. A drop of electrolyte was casted on the PP (27.86°) and PI (22.09°) separators and Al2O3-PI separator (18.41°) prove that coating of Al2O3 layer improved the wettability of the separator. The thermal stability of PP (150°C) and PI (200°C) separator indicated that PI separator is more stable in heat than PP separator. Cycle performance of PP, PI and Al2O3-PI separator was evaluated by measuring the capacity retention ratios of each (86.87, 94.94, and 95.56% respectively).

    Electrospinning technique produces continuous nonwoven fibers of desired diameter between nanometers to micrometers[21]. Pyromellitic dianhydride (PMDA) and 4,4’-oxydianiline (ODA) are processed through polycondensation in N,N-dimethylacetamide (DMAc) to produce the polyimide, PAA precursor. Electrospinning was performed to synthesize PAA nanofiber membranes. The thickness can be controlled with the deposition time. FTIR data was used to confirm that imidization of PAA was completed. The diameter of fiber and morphology were observed with SEM. The thermal resistance was investigated with differential scanning calorimetry (DSC). PI nanofiber was more thermally stable without shrinkage and color change. Each separator’s wettability was observed with contact angle measurements. Nonwoven PI nanofiber had excellent wettability since the contact angle decreased rapidly. Since nonwoven PI nanofiber have small diameter, porous structure, and high wettability for an electrolyte, it shows higher capacity, better performance. It is observed that thinner membrane has less resistance.

    19 wt% of PAN was dissolved in N,N-dimethylformamide (DMF) uniformly by magnetic stirring for 24 h[22]. Electrospinning method was applied to the solution to provide porous PAN micro/nanofiber membranes. The surface morphologies of the membranes with different weight ratio of PAN and water were studied with FE-SEM images. It was found out that the best sample was the membrane with 9 wt% PAN and 7 wt% water. The average diameter of nanofibers was about 700~800 nm. The bright spots at the transmission electron microscopy (TEM) image indicated the systematic porous structure. The porosity in the structure of membranes fabricated by electrospinning is affected by the relative humidity (RH) level. Increased water content accelerated solidification which increased the size of fiber diameter and porosity. Although the PAN separator (35 m) was thicker than PP separator (25 m), the porosity of PAN separator (83%) was superior to PP separator (45%) which may lead to higher ionic conductivity. The tensile strength of PAN membrane (about 14.4 MPa) was lower than that of PP separator (about 129.0 MPa) due to structural nature of the membrane and higher porosity. DSC was measured to compare the thermal stability of PAN and PP separator. PAN separator (no apparent peak under 300 °C) was thermally more stable than PP separator (endothermic peak at around 166°C). When the separators were placed in a hot oven with 200°C for 45 minutes, porous PAN membrane only shrank 15% while PP separator shrank about 95%. The wettability of the PAN separator was excellent due to its porous structure, higher porosity, and existence of nitrile groups which has greater affinity with the liquid electrolyte. The wettability was further observed with the contact angle of a droplet of an electrolyte and the surface of a membrane. The contact angle of PAN nanofibrous membranes (30.3°) was much less than that of PP separator (92.5°). The electrolyte uptake value of PAN nanofibrous membrane (650%) was about 3 times higher than that of PP separator (218%).

    Controlling the pore size is essential to maintain high cycling performance because a separator cannot limit the formation of the lithium dendrites when the size of pores is too large[23]. Poly(p-phenylene terephthamide) (PPTA) nanofibers could limit polyphenylene sulfide (PPS) membrane’s pore size. CaCl2 and methoxypolyethylene (mPEG) were dissolved at n-methyl-2-pyrrolidone (NMP) and the solution was chilled at 0°C. p-phenylene diamine (PPD) was dissolved in the solution by stirring for 30 minutes, and then terephthaloyl chloride (TPC) was added with stirring at 2500 rad/min for 5 minutes. Excess NMP was added and the mixture was replaced into deionized water, forming ANFs suspension. To fabricate ANFs/PPS composite membrane, PPS nonwoven was beaten in PFI beater and it was mixed with ANFs suspension while sodium dodecyl sulfate (SDS) and polyethylene oxide (PEO) were added and stirred to make a homogeneous slurry. ANFs/PPS composite membrane was prepared by filtering the slurry. The surficial morphology and structure of pores were observed with SEM images. The pure PPS membrane had too large pores. ANFs/PPS membrane synthesized by papermaking method had uniform layer of aramid nanofibers when the ratio of ANFs/PPS was 15 and 20%. This membrane had three-dimensional structure of pores which transmit lithium ions and suppress the growth of lithium dendrites. Electrolyte uptake of composite membrane increased when ANFs component increased from 0% to 15%, but there was negligible improvement when ANFs component increased from 15 to 20%. This phenomenon happened because the excess of ANFs component causes decrease of pore size and electrolyte uptake. Electrolyte uptake of 15% ANFs/ PPS membrane (240.7%) was 2.7 times better than Celgard 2400 separator (89.5%). 15% ANFs/PPS separator had superior wettability than Celgard 2400 separator. 15% ANFs/PPS separator absorbed liquid electrolyte in 30 seconds and the contact angle reached 38° after 5 seconds, while Celgard 2400 separator maintained a droplet at the surface and the contact angle was 61° after 60 seconds. Due to water-liking amide groups in ANFs and good connection between pores, wettability of composite membrane was excellent. ANFs/PPS separator had much lower tensile strength (9.8 MPa) than Celgard 2400 separator (141 MPa) because of aramid nanofibers which are tightly connected each other. DSC and thermogravimetric analyzer (TGA) data was measured to check thermal stability of the separators. DSC data indicated that 15% ANFs/PPS separator absorbed energy at 285°C, whereas Celgard 2400 separator absorbed energy at 165°C. Celgard 2400 started to shrink as temperature increased and almost completely contracted at 200°C, but ANFs/PPS separator remained its dimension up to 200°C. Because of the strong structure of PPS fiber and highly heat resistant ANFs, 15% ANFs/PPS separator had higher melting point and more stable at heat. ANFs/PPS separator had self-extinguishing property as well.

    The composite membranes of PAN and polyurethane (PU) were synthesized by electrospinning method with multiple needle design[24]. The SEM images of PAN/PU membranes showed an interconnected three-dimensional structure. The diameter of fibers was in between 390 and 500 nm. The porosity of PAN/PU membranes (80~ 92%) were compared with Celgard and SiO2/nylon 6, 6 membranes (70~77%). The electrolyte uptake of PAN/ PU with 10~12 wt% membranes (714.50, 776.09%) were about 10 time better than Celgard membrane (76.18%). PAN/PU membranes owned superior flexibility which can be folded approximately 180°. Comparing the thermal stability of Celgard and PAN/PU membrane, both PAN/PU membranes with 10 and 12 wt% of PAN showed higher heat resistance at 170°C. Also, when the membranes were exposed to fire, Celgard membrane was ignited instantly while PAN/PU membranes did not catch fire. DSC data indicated that PAN/PU membranes had endothermic peaks around 275°C while Celgard membrane had endothermic peaks around 135~165°C. Ionic conductivity of 10 (1.30 mS cm-1), 12 wt% (2.07 mS cm-1) membranes were better than Celgard membrane (0.30 mS cm-1) due to the pore structure of membranes.

    Poly(vinyl alcohol-co-ethylene) (PVA-co-PE) nanofibers were synthesized. In weight ratio of 20 : 80, PVA-co-PE and cellulose acetate butyrate (CAB) were blended with twin-screw extruder[25]. Continuous yarns of the PVA-co-PE nanofibers were produced by the CAB matrix extraction. Then, surface-cleansed poly(ethylene terephthalate) (PET) was coated with PVA-co-PE nanofibers to form PET nonwoven sandwiched between two nanoscaled porous PVA-co-PE nanofibrous membranes (NFs/PET/NFs). The thickness of NFs/PET/NFs separator was about 44 m with 4 m thickness PAV-co-PE nanofibrous membrane on both sides. It was observed from the SEM images that PP separator had needle- like pores with low porosity. The SEM image of PET nonwoven indicated that PET nonwoven has 78% of void which is excessive porosity which might lead to nonuniform distribution of current and self-discharge. In case of the NFs/PET/NFs separator, macro-sized pores were homogeneously covered with both top and bottom layer of PVA-co-PE nanofibrous membrane. The wettability of NFs/PET/NFs separator was excellent due to highly hydrophilic PVA-co-PE layers due to rich functional hydroxyl groups. The performance of lithium ion battery is greatly related to the wettability of separator. The change of dimension of NFs/PET/NFs separator was almost zero at 150°C when PP separator shank about 33%.

    2.2. PVDF based membrane

    To enhance the safety of the battery, a cathode and an anode requires to be physically isolated. Applying inorganic layers can reduce the mechanical decomposition and thermal shrinkage[26]. Both sides of a PE separator is coated with the inorganic ceramic layer consisted of polymeric binders (PVDF-HFP) and nano-sized alumina (Al2O3). The ceramic coating layer was synthesized by following method. Al2O3 nanoparticles and PVDF-HFP were dissolved in acetone. Al2O3 powders were vigorously mixed with the solution. The characteristics of synthesized separators were examined by Gurley densometer, FE-SEM, measuring the change of area. A PE separator is coated by casting the coating solution. By the phase inversion, the acetone part of the solution gradually evaporates and solidifies. As more water takes more proportion of the solution, the size of pore of PVDF-HFP coating layers increases. When the amount of water increased in the solution, the Gurley values of separators decrease and air permeability increases. It is observed that the Gurley values and water content is not linearly proportional. The most important factor for this is to know where the initial condition of the solution belongs, either miscible or immiscible. The area change of ceramic composite separators were measured to compare their thermal shrinkage. As the tested temperature increases, the shrinkage difference of ceramic composite separators and the pristine PE separator was dramatic. Especially, the separator with miscible coating solution shrinks less than the separator with immiscible coating solution. On the other hand, the immiscible coating solution exhibits higher C-rate capability than the miscible coating solution.

    Polyolefin-based separators have low thermal resistance, and applying ceramic powders lower the shrinkage due to heat and strengthen the separators[27]. PE separator is coated with the ceramic coating layer which is consisted with PVDF-HFP binders and SiO2 nanoparticles (NPs) on both sides. PVDF-HFP was dissolved in acetone, and SiO2 was added and mixed vigorously by bead-milling. Then PE separator was dipped into the solution to form the coating layer on both sides. the surface of the coated PE separator was observed with field FE-SEM. The porosity of the coating layer was calculated with the density. The morphology was comparable to the self-assembly NP arrangement. The porosity is affected by the size of SiO2 powder. And the porosity of coating layer affects the ionic conductance. Thermal resistance was observed by comparing the dimension change of separators. The thermal shrinkage of the separator is affected by the amount of SiO2 particles containing in the coating layer. The composite separator with 40 nm SiO2 powder seemed to have higher discharge capacity then the composite separator with 530 nm SiO2 powder due to the ohmic polarization.

    2.3. Composite nanofiber membrane

    TiO2 nanoparticles were dissolved in toluene under nitrogen gaseous environment to make TiO2-NH2. TiO2-NH2 was mixed with toluene and triethylamine [28]. Then 2-bromopropionyl bromide and toluene were added into the solution slowly for an hour while stirring, forming TiO2-Br. TiO2-Br was mixed at methanol, and CuBr and N,N,N’,N’’,N’’’-pentamethyldiethylenetriamine (PMDETA) was added into the mixture under nitrogen gaseous environment. Then, 2-hydroxyethyl methacrylate (HEMA) was added to the solution to form TiO2-HEMA after purification and drying. PVDF-HFP solution and PI solution were alternately injected into four cartridges for co-electrospinning. Then, thermal calendaring was applied to the composite membrane to enhance the mechanical property. The change of absorption peak of FTIR indicated that preparation of functionalized TiO2 (f-TiO2) nanoparticles was successful. The analysis of FE-SEM image demonstrated that the average diameter of PI fiber decreased as 1 and 2% f-TiO2 nanoparticles were attached due to nanoparticles’ repulsive force. However, the average fiber diameter increased when 3% f-TiO2 nanoparticles were added because nanoparticles agglomerated. The porosity of fiber increased with addition of 1 and 2% f-TiO2 nanoparticles and decreased with addition of 3% f-TiO2 nanoparticles. The composite membrane was thermally stable regardless of the existence of nanoparticles.

    Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) membrane modified with Sb2O3 was prepared by electrospinning method[29]. Morphology of the fibrous membrane was characterized by FE-SEM and TEM. SEM analysis indicated that PVDF-CTFE fiber (PCF) has smooth surface, while SPCF nanofiber which is PCF with some Sb2O3 nanoparticles has rougher surface. The rougher surface of membranes effectively enhances the mechanical properties. Compared to PE membranes which is nonpolar, PCF and SPCF membranes can be wetted with the electrolyte faster. Microscale combustion calorimeter (MCC) can examine the behaviors of the membranes during combustion. Sb2O3 nanoparticles which has affinity towards electrolyte components enhance electrolyte wettability. Sb2O3 nanoparticles improve Lewis acid and base interactions. PE separator shrinks about 90% when exposed to heat while nanocomposite membrane has high thermal stability. Due to better electrochemical stability and ionic conductivity lithium ion battery shows better cycling stability and rate capability.

    In order to improve the PP nonwoven membranes, blending-type composite separator (CS) dissolved in poly(methyl methacrylate) (PMMA) solution and sandwich- like composite separator (nano-CS) with PMMA nanoparticles introduced on the surface were investigated. PMMA nanoparticles were produced by emulsifier-free emulsion polymerization. PP nonwoven fabrics were dipped in the coating solution to synthesize composite separator[30]. PVDF-HFP and PMMA were dissolved in DMF for the homogeneous coating solution. To dry the coating layer, it was placed under ambient temperature for 0.5 h and under 60°C for 12 h at vacuum. nano-CS were synthesized by dip coating PP nonwoven fabrics in the solution in which PVDF-HFP was dissolved in a mixed solvent of DMF and acetone. To dry the coating layer, it was placed under ambient temperature for 0.5 h and under 60°C for 12 h at vacuum. After the drying process, PP nonwoven fabrics with coating layer of PVDF-HFP was immersed into the solution of PMMA colloid, and then vacuum dried at 80°C for 12 h. The morphology and structure of pores could be observed from SEM images. The proportion of pores at the surface and size of pores decreased due to the addition of PMMA which interact with PVDF-HFP matrix. Larger pore size enhances electrolyte uptake but having too large pore size may cause difficulty of holding electrolyte. The best porosity was measured to be 77.9% of porosity which uptakes 212% of electrolyte from CS with 20% of PMMA content [CS(0.2)] and 75.3% of porosity which uptakes 202% of electrolyte from nano-CS. Thermal shrinkage of CS(0.2) and nano-CS were 19 and 12% at 140°C whereas that of pristine PVDF-HFP was 29% at 140°C. Comparing to pristine PVDF-HFP membrane, CS and nano-CS separator had better mechanical strength. The flexibility of CS separator was reduced but nano-CS separator maintained good flexibility. The cyclic performance of pristine PVDF-HFP, CS, and nano-CS separator were observed. Discharge capacity was 131, 138, 152 mAh g-1, capacity retention was 90, 97, 99% after 50 cycles for pristine PVDF-HFP, CS, and nano-CS respectively. Luo et al. reported PVDF-HFP/PE based composite membrane separator prepared by solvent liberation method[31]. Porous structure generated by evolution of solvent results in better performance in lithium ion battery (Figs. 3, 4).

    2.4. Coated membrane

    Radio-frequency (RF) magnetron sputtering is a technique which deposits uniform thin film of inorganic or metallic layers. Al2O3 layer was coated on the PE separator by using RF magnetron sputtering[14]. The surface morphology of PE separator was evaluated with SEM images. The SEM image after 40 min of sputtering indicated that Al2O3 successfully attached on PE separator. The thickness of PE separator changed negligibly after sputtering. Change of the Gurley numbers from 271 to 310 represented that the permeability of the separator decreased after sputtering. The wettability of separator dramatically improved as hydrophilic layer of Al2O3 was coated on PE separator. The thermal stability was investigated at 140 and 150°C. The coated separator endured very well as the dimension change was almost negligible while the bare separator shrank about 73.3% at 140°C.

    The cellulose membrane with 25 m thickness was prepared through papermaking process and dip-coated with polymeric lithium tartaric acid borate salt (PLTB) coating solution[32]. The morphology of the cellulose nonwoven was observed with FE-SEM. The diameter of fibers was between 0.2 and 2.0 m and the size of pores were greater than 2 m. The size of pore decreased to about 1 m when PLTB was applied to the membrane. The decrease of pore size decreases the chance of micro short-circuit and lithium dendrite growth. The attachment of PLTB at PVDF-HFP layer through dip-coating was confirmed with FTIR. The spectra of the single ion polymer electrolyte cellulose nonwoven separator (SPEC) indicated PLTB@PVDFHFP’s typical peaks which confirms that SPEC separator was successfully coated with PLTB. The X-ray diffraction (XRD) pattern was measured to observe the homogeneity of PLTB in PVDF-HFP. The homogeneous layer of PLTB improves the ionic conductivity. The exothermic peak of PP separator and SPEC separators were observed with DSC. PP separator exhibited peak at 160.4°C and SPEC separator did not exhibit an obvious peak until 300°C. Thermal shrinkage of PP and SPEC separators were measured after exposing them to 150°C for 0.5 h and concluded that PP separator shrank significantly while SPEC separator did not. The porosity of PP separator was 55% and that of the SPEC separator was 70%. SPEC separator had better ionic conductivity than PP separator due to higher porosity.

    To overcome PAN’s low chemical stability in organic solvents, a modified PAN/silica-aerogel (M-PSA) composite separator is studied. M-PSA separator is fabricated in three steps[33]. PAN nonwoven was fabricated through electrospinning precursor solution which is consisted of PAN powder and DMF/acetone in 7/3 ratio. Nitrile group of the PAN nonwoven was hydrolyzed with amino and carboxylate by solution of NaOH and H2O. This step produces modified PAN (M-PAN) nonwoven. Then, it was immersed in solution of ammonia water/ethyl alcohol for 10 minutes. M-PAN nonwoven was switched to silica sol 15 seconds and immersed in ethanol. When ethanol dries, M-PSA separator is produced. The diameter of PAN and M-PSA fiber was observed with SEM images. PAN fiber’s diameter was about 160 nm and M-PAN fiber’s diameter did not change significantly after hydrolysis. It was observed that silica-aerogel layer fully covered the M-PSA separator uniformly. The thickness of the cross section was about 115 m. The M-PSA separator was highly flexible that it could be folded and twisted. FTIR illustrated that the peak of nitrile group at PAN got weaker at M-PAN and M-PSA separator and acyl amino group was remained. When PAN nonwoven dissolved and shrank in EC/PC and EC/DMC, M-PSA was chemically stable in both EC/PC and EC/DMC. M-PSA showed high wettability since its contact angle with EC/PC, EC/DMC and diglyme was small. M-PSA separator remained stable until 297.3°C without clear weight loss. M-PSA separator had no clear change at 250°C and almost no shrinkage at 280°C. M-PSA separator became yellower at 350°C but the size of the separator was preserved. M-PSA separator applied batteries indicated long cycling life and high rate of ion diffusion.

    2.5. Core shell membrane

    Applying polysulfonamide (PSA) could enhance mechanical strength and thermal stability, and applying PVDF-HFP could enhance anodic stability and wettability[ 34]. PSA@PVDF-HFP composite nonwoven separator was synthesized via coaxial electrospinning. Using DMAc as the solvent, PSA and PVDF-HFP were dissolved and stirred vigorously. To perform the coaxial electrospinning, 15 wt% PSA solution and 25 wt% PVDF-HFP solution were filled in two chambers. Pumping PSA solution produced the core and PVDF-HFP solutions produced the shell. The thickness of synthesized fiber was 70 m and the composite nonwoven was pressed mechanically to form 40 m thickness. After quenching in liquid nitrogen, PSA@PVDF-HFP composite nonwoven’s cross-section image was taken with FE-SEM. Random arrangement of numerous nanofibers with diameter of 300 nm was observed and the size of pore was between 0.5 and 1.5 m. The distribution of pore size indicated that 90% of pore dimensions were between 750 nm and 1250 nm. This structure and uniformity of pore sizes enhance electrolyte uptake and prevent lithium dendrites growth. PSA@PVDF-HFP composite nonwoven separator (75%) had higher porosity than PP separator (55%). PSA@ PVDF-HFP composite nonwoven separator (22 s) had lower Gurley value than PP separator (235 s). Comparing the contact angle with liquid electrolyte droplets, PSA@ PVDF-HFP composite nonwoven separator (65°) had smaller contact angle than PP separator (85°). The interconnection of nanofibers in PSA@PVDF-HFP composite nonwoven separator leaded to better wettability. The DSC analysis indicated that the melting temperature of PP was 165°C and PSA@PVDF-HFP was over 300°C, and the latter had high thermal resistance due to its high melting temperature. PVDF-HFP was dissolved in a solution of acetone and DMF (1 : 1 ratio) and it was vigorously mixed for 10 h forming a slurry [35]. Electrospinning technique was applied to the slurry to form PVDF-HFP nonwoven membrane. Then, synthesized PVDF-HFP membrane was immersed into a poly-dopamine (PDA) solution for 48 h to form PVDF-HFP-PDA composite membrane. The surface change from PVDF-HFP to PVDF-HFP-PDA membranes were observed with SEM images. PVDF-HFP-PDA membrane maintained the interconnected structure of the pores while only a fine PDA film layer was added on the surface. The average diameter difference of PVDF-HFP (217.4 nm) and PVDF-HFP-PDA (257.4 nm) was 40 nm, and the decrease of average micropore diameter was from 429.8 nm to 387.7 nm. The cross-section image of PVDF-HFP-PDA membrane demonstrated that PDA homogeneously grew both surface and inner fibers as well. This homogeneously distributed PDA contributes to stable electric current of the batteries and diminishes the chance of forming metal Li. FTIR spectra was measured to confirm the attachment of phenolic hydroxyl and the amine groups at the surface. The thermal shrinkage of PVDF-HFP and PVDF-HFP-PDA membranes were compared with PP separator. While PP separator shrank about 10% at 140 °C, both PVDF-HFP and PVDF-HFP-PDA membranes had no obvious area changes. PVDF-HFP-PDA can endure high temperature due to the PDA coating layer whose melting temperature is above 230°C. The SEM images of annealed PVDF-HFP and PVDF-HFP-PDA membranes were investigated. The nanofibers of PVDF-HFP nonwoven membrane were bent and deformed under 160°C and significantly shrank at 170°C, whereas PVDF-HFP-PDA membrane remained almost no change at 160°C and had about 15% of shrinkage at 170°C. The wettability of the membrane was observed with the contact angle between a droplet of liquid electrolyte and the surface of membranes. While the contact angle of PP separator was 43.0°, the contact angle of PVDF-HFP and PVDF-HFP-PDA membrane rapidly decreased to 0°C. Although PVDF-HFP-PDA membrane (72.8%) had lower porosity than PVDF-HFPPDA membrane (77.7%), the former had higher electrolyte uptake capacity (254%) than the latter (206%), and PP separator only had 62.9% of electrolyte uptake capacity. Wei et al. reported PAN and poly(butylene succinate) (PAN@PBS) core shell membrane separator prepared by coaxial electrospinning method[36]. It showed higher electrochemical performance than Celgard separator (Fig. 5).

    2.6. Heat resistant membrane

    PSA polymers have superior heat resistance which can be applied to enhance the heat resistance of the separator. PSA/PP composite nonwoven (PSAP) was synthesized through the melt-blown spinning step[37]. The raw materials of PP and phenoxy polyphosphazene were heated at 200°C in screw extruder. When the poly-mers were melted, ultra-fine fibers were extracted from the spinneret holes. The extracted fibers were rolled by hot calendaring to form PP nonwoven. Then, PP nonwoven was dipped into the homogeneous PSA solution, which was prepared by dissolving PSA into DMAc. The morphology of PSAP separator was observed from SEM images. The pore diameter of PSAP separator was distributed well and was about 1 m. PSAP separator can prevent the lithium dendrites growth and absorb more electrolyte due to the interconnection of microporous structure of PSAP and its lyophilic nature. The porosity of PSAP separator (65%) was lower than that of PP separator (90%) due to the coating layer of PSAP separator, but it was higher than that of microporous polypropylene (MPP) separator (35%) and PSAP separator possessed enough porosity to hold sufficient electrolyte for ion transportation. The electrolyte uptake of PSAP was superior to MPP because PSAP separator owns higher affinity with the liquid electrolyte Introducing PSA coating layer successfully improved thermal resistance of the separator because PSAP separator shrank negligible about while MPP separator shrank about 50% at 150°C. Limiting oxygen index (LOI) of PSAP and MPP separator were compared. PSAP separator had higher LOI value (25%) than MPP separator (18%) and the difference could be visualized by setting fire on them. PSAP separator extinguished fire while MPP separator caught fire in 3 seconds. Due to superior electrolyte uptake and interconnection of porous structure of PSAP, the ionic conductivity of PSAP separator was 4 times higher at 20°C and 3 times higher at 60°C than MPP separator. Cycling stability of PSAP (90%) and MPP (0%) separator after 30 cycles at 120 °C indicated that PSAP separator had higher resistance to heat. Liu et al. reports a poly(ether ether ketone) (PEEF) polymer membrane with high strength and heat resistant property[38]. It is thermally stable up to 300 °C (Figs. 6, 7).

    3. Conclusions

    A lot of research has been conducted to enhance the heat resistance and wettability of the commercial polyolefin separators while sustaining their electrochemical stability and mechanical strength. PI membranes whose melting temperature is over 500°C were synthesized by electrospinning method. Co-electrospinning combined strengths of PVdF-HFP and PI nonwoven membrane. Immersing membranes in coating solution to form core shell membrane improved affinity with electrolyte. PSA/PP composite membrane which was synthesized by melt-blown spinning step exposed superior thermal resistance. This review discussed about composite membrane used as separator in lithium ion battery.

    Figures

    MEMBRANE_JOURNAL-30-4-228_F1.gif

    Schematic presentation of lithium ion battery with composite separator.

    MEMBRANE_JOURNAL-30-4-228_F2.gif

    Typical SEM images of (a) commercial PP separator, (b) cellulose/PSA composite membrane without exercising pressure, (c and d) cellulose/PSA composite membrane with the pressure of 20 MPa (Reproduced with permission from Xu et al., 17, Copyright 2014, American Chemical Society).

    MEMBRANE_JOURNAL-30-4-228_F3.gif

    (a) Photograph of as-reported composite separator. (b and c) SEM images of composite separators at different magnifications (Reproduced with permission from Luo et al., 31, Copyright 2014, American Chemical Society).

    MEMBRANE_JOURNAL-30-4-228_F4.gif

    PVDF (a) XRD and (b) FTIR patterns of PVDF-HFP/PE composite separator. (c) Thermogravimetry (TG) curves of phase-inversion and composite separators. (d) Stress-strain curves of phase-inversion, PVDF-HFP@ acetone, PVDF-HFP@NMP, and composite separators. (e) Thermography photographs through infrared radiations (Reproduced with permission from Luo et al., 31, Copyright 2014, American Chemical Society).

    MEMBRANE_JOURNAL-30-4-228_F5.gif

    SEM images of (a) PAN@PBS-0.3, (b) PAN@ PBS-0.4, and (c) PAN@PBS-0.5 membranes after rolling (the insets are their morphologies before rolling). TEM images of (d) PAN@PBS-0.3, (e) PAN@PBS-0.4, and (f) PAN@PBS-0.5 coaxial nanofiber. Laser scanning confocal microscope (LSCM) of PAN@PBS-0.3 coaxial nanofiber in (g) mixed field, (h) green fluorescence, (i) red fluorescence, and (j) bright field (Reproduced with permission from Wei et al., 36, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-30-4-228_F6.gif

    Schematic illustration of the fabrication of PEEK porous separator. The insets represent the mesoscopic morphology from DPD simulation and the transportation of lithium ion in the separator (Reproduced with permission from Liu et al., 38, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-30-4-228_F7.gif

    Electrochemical properties of the PE- and PEEK-based LiFePO4/separator-electrolyte/Li cells: (a) cycle performance of the cells at 2C from 2.5 to 4.2 V, (b) charge and discharge voltage curves of the cells at the 1st and 200th cycle, (c) C-rate performance of the cells, (d) opencircuit voltage (OCV) change in the cells from 20 to 165 °C (Reproduced with permission from Liu et al., 38, Copyright 2020, American Chemical Society).

    Tables

    Summary of Separator Membrane

    PAN: polyacrylonitrile, PVdF-HFP: poly(vinlidene fluoride-hexafluoropropylene), PSA: polysulfonamide, PAA: poly(amic acid), PVDF: polyvinylidene fluoride, PI: polyimide, PET: poly(ethylene terephthalate), PVA-co-PE: poly(vinyl alcohol-co-ethylene), PP: polypropylene, PU: polyurethane, PLTB: polymeric lithium tartaric acid borate salt, PDA: polydopamine, PPS: polyphenylene sulfide, PVDF-CTFE: poly(vinylidene fluoride-co-chlorotrifluo- roethylene).

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