Separators For Lithium-ion Batteries Research Articles

Nanodiamond-Enhanced Nanofiber Separators for High-Energy Lithium-Ion Batteries.

Current lithium-ion battery separators made from polyolefins such as polypropylene and polyethylene generally suffer from low porosity, low wettability, and slow ionic conductivity and tend to perform poorly against heat-triggering reactions that may cause potentially catastrophic issues, such as fire. To overcome these limitations, here we report that a porous composite membrane consisting of poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers functionalized with nanodiamonds (NDs) can realize a thermally resistant, mechanically robust, and ionically conductive separator. We critically reveal the role of NDs in the polymer matrix of the membrane to improve the thermal, mechanical, crystalline, and electrochemical properties of the composites. Taking advantages of these characteristics, the ND-functionalized nanofiber separator enables high-capacity and stable cycling of lithium cells with LiNi0.8Mn0.1Co0.1O2 (NMC811) as the cathode, much superior to those using conventional polyolefin separators in otherwise identical cells.

Electrospun PVDF-HFP/PAN bicomponent nanofibers as separators in lithium-ion batteries with high thermal stability and electrolyte wettability

Electrospun PVDF-HFP/PAN bicomponent nanofibers as separators in lithium-ion batteries with high thermal stability and electrolyte wettability

An upgraded polymeric composite with interparticle chemical bonding microstructure toward lithium-ion battery separators with enhanced safety and electrochemical performances

A composite separator of SiC/PVDF-HFP was synthesized for lithium-ion batteries with high thermal and mechanical stabilities. Benefiting from the nanoscale, high hardness, and melting point of SiC, SiC/PVDF-HFP with highly uniform microstructure was obtained. This polarization caused by barrier penetration was significantly restrained. Due to the Si-F bond between SiC and PVDF-HFP, the structural stability has been obviously enhanced, which could suppress the growth of lithium (Li) dendrite. Furthermore, some 3D reticulated Si nanowires are found on the surface of Li anode, which also greatly inhibit Li dendrites and result in irregular flakes of Li metal. Especially, the shrinkage of 6% SiC/PVDF-HFP at 150 °C is only 5%, which is notably lower than those of PVDF-HFP and Celgard2500. The commercial LiFePO4 cell assembled with 6% SiC/PVDF-HFP possesses a specific capacity of 157.8 mA h g−1 and coulomb efficiency of 98% at 80 °C. In addition, the tensile strength and modulus of 6% SiC/PVDF-HFP could reach 14.6 and 562 MPa, respectively. And a small deformation (1000 nm) and strong deformation recovery are obtained under a high additional load (2.3 mN). Compared with PVDF-HFP and Celgard2500, the symmetric Li cell assembled with 6% SiC/PVDF-HFP has not polarized after 900 cycles due to its excellent mechanical stabilities. This strategy provides a feasible solution for the composite separator of high-safety batteries with a high temperature and impact resistance.

Grafting Polyethyleneimine-Poly(ethylene glycol) Gel onto a Heat-Resistant Polyimide Nanofiber Separator for Improving Lithium-Ion Transporting Ability in Lithium-Ion Batteries.

To improve the lithium-ion transporting ability in lithium-ion batteries, a high-performance polyimide-based lithium-ion battery separator (PI-mod) was prepared by chemically grafting poly(ethylene glycol) (PEG) onto the surface of a heat-resistant polyimide nanofiber matrix with the assistance of amino-rich polyethyleneimine (PEI). The resulted PEI-PEG polymer coating exhibited unique gel-like properties with an electrolyte uptake rate of 168%, an area resistance as low as 2.60 Ω·cm2, and an ionic conductivity up to 2.33 mS·cm-1, which are 3.5, 0.10, and 12.3 times that of the commercial separator Celgard 2320, respectively. Meanwhile, the heat-resistant polyimide skeleton can effectively avoid thermal shrinkage of the modified separator even after 200 °C treatment for 0.5 h, which ensures the safety of the battery working under extreme conditions. The modified PI separator possessed a high electrochemical stability window of 4.5 V. Compared with the batteries from the commercial separator Celgard 2320 and the pure polyimide matrix, the assembled coin cell with the PI-mod separator showed much better rate capabilities and capacity retention due to the high electrolyte affinity of the PEI-PEG polymer coating. The developed strategy of using the electrolyte-swollen polymer to modify the thermal-resistant separator network provides an efficient way for establishing high-power lithium-ion batteries with good safety performance.

Graphene-Based Materials for the Separator Functionalization of Lithium-Ion/Metal/Sulfur Batteries.

With the escalating demand for electrochemical energy storage, commercial lithium-ion and metal battery systems have been increasingly developed. As an indispensable component of batteries, the separator plays a crucial role in determining their electrochemical performance. Conventional polymer separators have been extensively investigated over the past few decades. Nevertheless, their inadequate mechanical strength, deficient thermal stability, and constrained porosity constitute serious impediments to the development of electric vehicle power batteries and the progress of energy storage devices. Advanced graphene-based materials have emerged as an adaptable solution to these challenges, owing to their exceptional electrical conductivity, large specific surface area, and outstanding mechanical properties. Incorporating advanced graphene-based materials into the separator of lithium-ion and metal batteries has been identified as an effective strategy to overcome the aforementioned issues and enhance the specific capacity, cycle stability, and safety of batteries. This review paper provides an overview of the preparation of advanced graphene-based materials and their applications in lithium-ion, lithium-metal, and lithium-sulfur batteries. It systematically elaborates on the advantages of advanced graphene-based materials as novel separator materials and outlines future research directions in this field.

Sustainable lithium-ion battery separators based on cellulose and soy protein membranes

The food industry produces millions of tons of natural by-products. Through this study, we followed an environmentally friendly strategy using discards, such as soy protein isolate (SPI) from soya oil production and marine cellulose (Cell) from the agar industry, in order to achieve added-value applications. In particular, this work focuses on the development of membranes based on soy protein and cellulose, and their validation as battery separator membranes toward sustainable energy storage systems. SPI membranes with Cell show excellent compatibility with the electrolyte based on physical interactions. These physical interactions favor the swelling of the membranes, reaching swelling values of 1000% after three days in the liquid electrolyte. The membranes are thermally stable up to 180 °C. After being subjected to the liquid electrolyte, it is observed that the microstructure of the membranes change, but the porous structure is maintained, while the materials remain easy to handle. The ionic conductivity value, lithium transference number and battery performance in cathodic half-cells are ∼ 5.8 mS.cm−1, 0.77, and 112 mAh.g−1 at 1C-rate, respectively. Overall, considering environmental issues and circular economy, it is proven that it is possible to obtain more sustainable high-performance lithium-ion batteries based on waste materials.

Ultrafast self-assembly of supramolecular hydrogels toward novel flame-retardant separator for safe lithium ion battery

Traditional polyolefin separators for lithium-ion batteries (LIBs) often experience limited thermal stability and intrinsic flammability, resulting in great safety risks during their usage. Therefore, it is highly important to develop novel flame-retardant separators for safe LIBs with high performance. In this work, we report a flame-retardant separator derived from boron nitride (BN) aerogel with a high BET surface area of 1127.3 m2 g−1. The aerogel was pyrolyzed from a melamine-boric acid (MBA) supramolecular hydrogel, which was self-assembled at an ultrafast speed. The in-situ evolution details of the nucleation-growth process of the supramolecules could be observed in real-time using a polarizing microscope under ambient conditions. The BN aerogel was further composited with bacterial cellulose (BC) to form a BN/BC composite aerogel with excellent flame-retardant performance, electrolyte-wetting ability and high mechanical property. By using the BN/BC composite aerogel as the separator, the developed LIBs exhibited high specific discharge capacity of 146.5 mAh g−1 and excellent cyclic performance, maintaining 500 cycles with a capacity degradation of only 0.012% per cycle. The high-performance flame-retardant BN/BC composite aerogel represents a promising candidate for separators not only in LIBs but also in other flexible electronics.

Ultra-thin ceramic coated separator for high energy density lithium-ion battery:In-depth analysis on Al2O3 nano particles penetration into the structure pore

The energy density and power of lithium-ion batteries (LIBs) are undoubtedly essential to fuel the satisfying pursuit of next-generation energy storage systems. However, to ensure the safety of LIBs, a micrometer-thick ceramic coating layer (CCL) is coated on the separator by a conventional slurry process, which reduces the energy density and performance of LIBs. For this purpose, a ceramic-coated separator (CCS) fabricated by sputtering has attracted attention because it can secure thermal stability and performance while minimizing the CCL thickness to nanometers. Nevertheless, the analysis of why a CCL with only nanometer thickness could improve the properties of the separator still needs to be investigated. Theoretically, it could be suggested that sputtered nano ceramic particles could penetrate the internal micropore structure of the separator, but no experiments were conducted to identify this. In this study, depth profiling using time-of-flight secondary ion mass spectrometry was conducted to confirm the distribution of sputtered nano ceramic particles in the internal structure of the separator depending on the porosity. The surface composition of the separator changed by the plasma generated during the sputtering process was observed by X-ray photoelectron spectroscopy. In addition, to investigate the effect of the nm-CCL on the properties and electrochemical performance of the separator, we compared it with commercial slurry CCS and single/double-sided ceramic sputtering samples.

Improved electrochemical properties of janus composite membranes obtained by modification of PEEK/nanocellulose on polyethylene for lithium-ion batteries

To solve the disadvantages of polyethylene (PE) separator for lithium-ion batteries (LIBs), such as low polarity, poor electrolyte wettability, and severe thermal dimensional shrinkage, the composite solution of poly(ether ketone) (PEEK) and carbon nanoparticles, which was obtained by the carbonization of nanocellulose (NCC) in the sulfuric acid solution, were applied to modify the PE separator to prepare Janus composite separator (PE/PC) with high porosity. PE/PC exhibited high dimensional thermal stability with a shrinkage rate of 53% after 0.5 h of treatment at 150 °C, and a higher electrolyte loading rate (196.3%) than PE. Accordingly, it also presented excellent battery performance. The discharge capacity of the cell using PE separator was 148 mAh g−1 after 100 cycles, while that with PE/PC separator were 155 mAh g−1 after 200 cycles. The initial efficiency of discharge-charge of PE/PC with 2% NCC (PE/PC2) was as high as 97.6%. This should be related to the high ion conductivity (1.21 × 10−3 S cm−1). Furthermore, the carbon nanoparticles effectively provided abundant secondary oxidation reaction sites for the active material between the separator and cathode, allowing the free diffusion of Li+, thus also reducing the charge transfer resistance and improving the battery performance.

Enhancing the β Phase of Poly(vinylidene fluoride) Nanofibrous Membranes for Thermostable Separators in Lithium-Ion Batteries

It is challenging to simultaneously control the phase and the finer nanofiber shape to prepare a thinner poly(vinylidene fluoride) (PVDF)-based separator. Here, an ultrafine β-PVDF nanofibrous membrane was fabricated by combining the melt electrospinning and phase-changing techniques. Temperature and electrostatic fields were introduced simultaneously to boost the β phase of PVDF. Meanwhile, phase-sacrificial polylactic acid (PLA) was employed to further reduce the diameter of the melt-electrospun fibers, which can significantly reduce the separator thickness and increase the specific surface area, addressing the main limitation of the application of nanofiber membranes in commercial separators. Furthermore, the strong interactions between the −C–F bonds in PVDF and the −C═O bonds in PLA further enhanced the regular TTTT structural β phase. Besides, SiO2 and Al2O3 ceramic nanoparticles were codeposited on the β-PVDF nanofiber membrane by magnetron sputtering, which not only improves the thermostability but also prevents coating layer depowdering and thickness increase. The lithium-ion battery (LIB) with such a composite separator showed an ion conductivity of 2.242 mS/cm and a high thermostability of 150 °C. This work elucidates the controlling mechanism of the crystalline phase of PVDF-based nanofibrous membrane and provides encouraging guidance for constructing a functional layer of battery separators.

Preparation and Performance of a PU/PAN Lithium-Ion Battery Separator Based on a Centrifugal Spinning Method

The diaphragm is a key component of the lithium-ion battery and largely determines its performance. Currently, commercial diaphragms suffer from poor thermal stability, low porosity, and low liquid absorption rate. In this study, we prepared a polyurethane/polyacrylonitrile (PU/PAN) lithium-ion battery diaphragm using a centrifugal spinning method with PU as the main substrate and PAN as the additive. The results showed that the PU/PAN nanofiber diaphragm prepared by centrifugal spinning had a 3D porous structure, and when using 18% PU:PAN = 7:3, the porosity of the fiber diaphragm was 83.9%, the liquid absorption rate was 493%, and the ionic conductivity was 1.79 mS/cm. The battery system had good electrochemical performance and thermal stability, with an electrochemical stability window of 5.2 V. The diaphragm did not shrink when heated at 160 °C. In a lithium-ion battery system with lithium iron phosphate (LiFePO4) as the cathode material, the capacity remained at 147.1 mAh/g after 50 cycles at a 0.2 C rate, with a capacity retention rate of 95.8%. This indicated excellent cycle stability and a multiplicative performance with good application potential.

Synthesis of PVDF-SiO2 nanofiber membrane using electrospinning multi-jet vertical on conveyor collector as a lithium-ion battery separator

Research on synthesis of PVDF-SiO2 nanofiber membrane by electrospinning multi-jet vertical method on conveyor collector as a lithium-ion battery separator was systematically investigated. Various voltages of 15 kV, 16 kV, and 17 kV and types of PVDF such as Aldrich, Solvay, and both mixtures were used for nanofiber membrane synthesis. The nanofiber membrane was synthesized for 4 hours with a distance between the tip of the needle, and the collector was 10 cm, with a flow rate of 2.6 mL/hour and a collector speed of 37 rpm. Furthermore, the results were tested for the physicochemical properties of the nanofiber. Solution B (PVDF Solvay) treated with 17kV has the largest porosity percentage value of membrane, which was 84.659%. The optimum shrinkage ratio test was from solution A (PVDF Aldrich) with 15 kV, 16 kV, and 17 kV voltages. The smallest average diameter nanofiber was obtained from solution B (PVDF Solvay) with a voltage of 15kV. The FTIR spectrum was not affected by voltages variations. In addition, the nanofiber membrane was assembled in the battery using Aldrich, Solvay, and mixed PVDF types. The battery was then tested for the electrochemical properties of the lithium-ion battery. Solution B (Solvay) has the optimum charge-discharge results showing a specific charge of 107.472 mAh/g and a specific discharge of 77.990 mAh/g.

Superior thermal stability of PVA/cellulose composite membranes for lithium-ion battery separators prepared by impregnation method with noncovalent cross-linking of intermolecular multiple hydrogen-bonds

Commercial polymer separators in lithium-ion batteries (LIBs) usually have low thermal stability and electrolyte wettability, which can degrade battery performance, especially safety. Here, the novel PVA/cellulose composite membranes (P-CMs) are successfully prepared by noncovalent cross-linking of intermolecular multiple hydrogen-bonds and used as the separator for LIBs. P-CMs show superior thermal degradation (300 °C), and they are heat treated in the tube furnace at 300 °C for 30 min, The P-CMs exhibit negligible dimension change. The 3D-network porous structure produced by phase separation is more suitable for Li+ transport. After 200 cycles at 0.5C, the cell retains 92 % of its capacity retention. Cross-linking structure with abundant polar groups(-OH) improves the electrolyte affinity of the P-CMs. In the LiPF6 electrolyte system, the plentiful polar hydroxyl groups in the P-CMs can interact with the F atoms in the PF6− anionic by means of HF hydrogen bonds, thereby enhancing the affinity of the separator with the electrolyte. In addition, Noncovalent cross-linking of intermolecular multiple hydrogen-bonds reduces free hydroxyl group quantity and improves the interfacial compatibility of P-CMs, and decreases the impedance of LIBs. This study offers a new strategy for the fabrication of superior thermal stability in LIBs.

Boosting Thermal and Mechanical Properties: Achieving High-Safety Separator Chemically Bonded with Nano TiN Particles for High Performance Lithium-Ion Batteries.

Currently, the commercial separator (Celgard2500) of lithium-ion batteries (LIBs) suffers from poor electrolyte affinity, mechanical property and thermal stability, which seriously affect the electrochemical performances and safety of LIBs. Here, the composite separators named PVDF-HFP/TiN for high-safety LIBs are synthesized. The integration of PVDF-HFP and TiN forms porous structure with a uniform and rich organic framework. TiN significantly improves the adsorption between PVDF-HFP and electrolyte, causing a higher electrolyte absorption rate (192%). Meanwhile, XPS results further demonstrate the tight link between PVDF-HFP and TiN due to the existence of TiF bond in PVDF-HFP/TiN, resulting in a strong impediment for the puncture of lithium dendrites as a result of the improved mechanical strengths. And PVDF-HFP/TiN can effectively suppress the growth of lithium dendrites by means of uniform lithium flux. In addition, the excellent heat resistance of TiN improves the thermal stability of PVDF-HFP/TiN. As a result, the LiFePO4 ||Li cells assembled PVDF-HFP/TiN-12 exhibit excellent specific capacity, rate performance, and capacity retention rate. Even the high specific capacity of 153 mAh g-1 can be obtained at the high temperature of 80°C. Meaningfully, a reliable modification strategy for the preparation of separators with high safety and electrochemical performance in LIBs is provided.

Heat-resistant Al2O3 nanowire-polyetherimide separator for safer and faster lithium-ion batteries

• Al 2 O 3 NWs exhibit ultrahigh modulus, superior insulation and chemical stability. • Al 2 O 3 NWs-PEI membrane was prepared by non-solvent-induced phase separation method. • The produced membrane exhibits excellent electrochemical properties. • Superior thermal and flame resistance of produced membrane makes it safer for LIBs. Poor heat/flame-resistance of polyolefin (e.g., polyethylene and polypropylene) separators and high flammability of organic electrolytes used in today's lithium-ion batteries (LIBs) may trigger rare yet potentially catastrophic safety issues. Here, we mitigate this challenge by developing a heat-resistant and flame-retardant porous composite membrane composed of polyetherimide (PEI) and Al 2 O 3 nanowires (NWs). The membranes are fabricated based on an industrially scalable non-solvent-induced phase separation process, which results in an intimately interconnected porous network of Al 2 O 3 NWs and PEI. The produced composite membranes exhibit excellent flexibility, thermal stability, and flame-retardancy. Importantly, the composite membranes exhibit minimal thermal shrinkage and superior tensile strength (16 MPa) at temperatures as high as 200°C, significantly exceeding the performance of conventional polyolefin separators. Compared with commercial separators, their superior wettability and higher ionic conductivity (by up to 2.4 times) when filled with the same electrolyte, larger electrolyte uptake (∼190 wt.%), as well as improved cycle and rate performance demonstrated in LiNiMnCoO 2 (NCM)-based LIBs make them attractive choices for a variety of electrochemical energy storage devices.

UV-photopolymerized cellulose acetate-acrylate membranes for lithium-ion battery separator

Cellulose acetate (CA) is attractive for use as a separator in lithium-ion batteries (LIBs). However, the dissolubility in the liquid electrolyte limits its applications. Herein, we would report a cellulose acetate (CA) cross-linked film for the separator of lithium-ion battery. First, methacrylate modified cellulose acetate (M-CA) was prepared by esterification between CA and methylacryloyl chloride. Subsequently, an insoluble polymer membrane was obtained through the UV-induced polymerization of M-CA, poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and trihydroxymethylpropyl trimethylacrylaye (TMPTMA). The results show that these polymer membranes have good thermal and chemical stability due to the cross-linked polymer matrix. When the amount of TMPTMA is 20% of the molar amount of PEGMEMA, not only has the obtained gel polymer electrolyte (GPE) a suitable ionic conductivity of 4.17 mS cm−1 and an excellent lithium ion transfer number of 0.80, but also it has discharge capacity of the half battery composed of LiFePO4/GPE/Li 120 mAh g−1 at 2 C. This study demonstrated a great possibility that the high-performance and biodegradable GPE could be applied in LIBs. In addition, there are many other methacrylate monomers that can be polymerized with M-CA to prepare various GPEs, which will open a new avenue for the application of CA in GPEs.

Construction of PMIA@PAN/PVDF-HFP/TiO2 coaxial fibrous separator with enhanced mechanical strength and electrolyte affinity for lithium-ion batteries

Poly(m-phthaloyl-m-phenylenediamine) (PMIA) is promising as the separator in lithium-ion batteries (LIBs) for its excellent thermostability, insulation and self-extinguishing properties. However, its low mechanical strength and poor electrolyte affinity limit its application in LIBs. In this work, a new PMIA@polyacrylonitrile-polyvinylidene fluoride hexafluoropropylene-titanium dioxide (PMIA@PAN/PVDF-HFP/TiO2) composite fibrous separator with a coaxial core-shell structure was developed by combining coaxial electrospinning, hot pressing, and heat treatment techniques. This separator not only inherits the exceptional thermostability of PMIA, showing no evident thermal shrinkage at 220 °C, but also reveals improved mechanical strength (29.7 MPa) due to the formation of firm connections between fibers with the melted PVDF-HFP. Meanwhile, the massive polar groups in PVDF-HFP play a vital role in improving the electrolyte affinity, which renders the separator a high ionic conductivity of 1.36 × 10−3 S/cm. Therefore, the LIBs with PMIA@PAN/PVDF-HFP/TiO2 separators exhibited excellent cycling and rate performance at 25 °C, and a high capacity retention rate (76.2%) at 80 °C for 200 cycles at 1 C. Besides, the lithium metal symmetric battery assembled by the separator showed a small overpotential, indicating that the separator had a role in inhibiting lithium dendrites. In short, the PMIA@PAN/PVDF-HFP/TiO2 separator possesses a wide application prospect in the domain of LIBs.

Electrospun Cellulose Nanofiber Membranes as Multifunctional Separators for High Energy and Stable Lithium-Sulfur Batteries

Although Li–S batteries (LSB) are one of the most promising electrochemical energy storage technologies, their practical applications are limited by their rapid capacity decay and uncontrolled lithium dendrite formation. In addition to the well-known role of a separator in lithium-ion batteries, LSB separators must perform additional functions. Using a facile electrospinning method, we developed an eco-friendly separator prepared from natural cellulose with an interconnected fibrous and porous structure rich in polar oxygen-containing functional groups. These polar functional groups enhance electrolyte wettability, polysulfide adsorption, and lithiophilicity, thus boosting LSB performance. A cellulose separator with a thickness (22 μm) comparable to that of a commercial polypropylene (Celgard) separator delivers an initial discharge capacity of 1458 mAh·g−1 at 0.1C with a high sulfur utilization of 87%, including a high reversible discharge capacity of 1091 mAh·g−1 for 100 cycles, exhibiting a 1.8-times greater capacity retention than those of Celgard-containing LSBs. In addition, an excellent rate capability of 908 mAh·g−1 can be achieved at a high rate of 1C. These intriguing characteristics indicate that these separators could replace conventional synthetic polymer-based separators for commercial LSBs.

Coaxial electrospun core-shell lithium-ion battery separator with flame retardant and thermal shutdown functions

The preparation of flame-retardant encapsulated electrospun core-shell fibrous membrane (ECSFM) is the key to ensure both high performance and safety of battery separator. However, there are lacks of simulation analysis on the thermal deformation behaviour of core-shell fibers, and stable, controllable, and batch preparation method of ECSFM, which makes it difficult to efficiently prepare ECSFM with higher safety. Here, the thermal deformation process of core-shell fiber was simulated to analyze the core and shell ratio with the best flame retardancy and thermal shutdown performance. The results showed that a high ratio of shell layer would make it difficult for the flame retardant in core layer to spill out in time, causing insufficient flame retardancy, while the low ratio would lead to the insufficient thermal shutdown performance of the ECSFM. The ECSFM was prepared by coaxial electrospinning with the optimal proportion of shell and core layer, in which the shell layer was polyvinylidene fluoride (PVDF) and the core layer was triphenyl phosphate (TPP), and the optimized one (TPP@PVDF) had shutdown temperature (177 °C), a membrane rupture temperature (227 °C), and a short self-extinguishing time (1.82 s), while the unoptimized one could not thermal shutdown and had longer self-extinguishing time (2.13 s). Moreover, the batch preparation of TPP@PVDF was realized by sheath focusing multi-nozzle coaxial electrospinning, the output increased by 20.33 times compared with single coaxial electrospinning, and the comprehensive performance was better than commercial battery separator. This work provides a convenient method for the rapid fabrication of safe and high-performance battery separators.

Aging of lithium-ion battery separators during battery cycling

The separator is a core component of lithium-ion batteries, and its service life impacts the electrochemical performance and device safety. This study reports the performance of aluminum oxide ceramic-coated polyethylene separators (PE-Al2O3 separators) before and after aging. During lithium-ion battery cycling, degradation products from the electrolyte and graphite were found at the separator surface, leading to a decrease in porosity and ionic conductivity and an increase in the internal resistance of the separator. The mechanical properties of the separators gradually deteriorated with the increase in current rates and the number of cycles. After 200 cycles at 1.5C, the tensile strength of the aging separator decreased by about 6.70 % compared with the fresh separator. The contact angle between the separator and the electrolyte increased from 15.2° to 24.4°, over the first 200 cycles at 1.5C, and the wettability deteriorated with the aging of the separator. In addition, on aging, the separator maintained a stable thermal decomposition temperature and showed stable chemical properties.