Separator For High-performance Lithium-ion Batteries Research Articles

Thermally-stable and flame-retardant metal-organic framework-based separator for high-performance lithium-ion batteries

Safety hazards hinder the development of lithium-ion batteries (LIBs), in which the failure of separator is a key point. However, commercial polyolefin separators display poor thermal stability and flame-retardant property. Aramid nanofiber (ANF) is a promising material for safer separators in batteries due to its excellent thermal stability and remarkable flame retardancy, but the pure ANF separator often shows low ionic conductivity because of the tightly packed fibers. Metal-organic framework (MOF) is a promising candidate for increasing the porosity and ionic conductivity of ANF separator due to its ultrahigh porosity and ultrahigh specific area. Herein, a thermally-stable and high-performance composite separator is fabricated by combining MOF pore-forming agent and ANF network skeleton. The MOF/ANF1composite separator with 10 wt% ANF shows excellent thermal-stability (melting-point above 300 °C and decomposition temperature above 500 °C) and remarkable flame-retardant property. And it exhibits high ionic-conductivity of 1.01 mS·cm−1, contributed to the superior long-term cycling performance of the coin cell with the capacity of 147.3 mAh·g−1 after 200 cycles at 1 C, almost 25.4 % higher than that of the coin cell with PP separator for LIBs. The strategy of this research provides new ideas for the design of high-safety LIBs.

Lithium-ion battery separators based-on nanolayer co-extrusion prepared polypropylene nanobelts reinforced cellulose

Low-dimensional nanomaterial separators are considered prime candidates for next-generation lithium-ion battery separators due to their higher porosity and thinner thickness. For polyolefin materials, there is no suitable solvent for solution electrospinning, the low yield of melt electrospinning, and the large size of melt-blown fibers make it challenging to efficiently prepare polyolefin low-dimensional nanomaterials. Moreover, polyolefin materials have been criticized for poor electrolyte wettability and insufficient thermal stability. Herein, polypropylene (PP) nanobelts (NBs) are prepared via a high-efficient nanolayer co-extrusion technique as supporting skeleton to suppress the shrinkage of natural cellulose separators during dehydration effectively, and the cellulose fibers have optimized pore size to prevent internal short circuits. The results indicate that the as-prepared PPNBs/cellulose composite separator (PPNBs/CS) with sandwich structure has superior porosity (78.4%) and ionic conductivity (1.04 mS cm−1). Meanwhile, the thermal stability, electrolyte uptake, and tensile strength are all significantly improved due to the synergies between PPNBs and cellulose fibers. More importantly, the LiFePO4||PPNBs/CSs||LM half cells exhibit better rate-performance and cycling durability than Celgard® 2400. Similarly, the NCM811||LM half cells and NCM811||graphite full cells assembled from PPNBs/CS-1/2 also exhibit good electrochemical performance. This work opens a new approach for easy batch preparation of high-performance lithium-ion battery separators.

Cellulose microspheres enhanced polyvinyl alcohol separator for high-performance lithium-ion batteries

Separators typically play an important role in enhancing the electrochemical performance of lithium-ion batteries (LIBs), while the preparation of separators suffers good electrochemical performance and high stability. In this study, regenerated porous cellulose microspheres (RCM) were innovatively fabricated and the biodegradable RCM/Polyvinyl alcohol (PVA) separators were successfully prepared through simple mixing and solvent substitution. Interestingly, the RCM with rich carboxylic groups, not only function as nanofillers that increases the strength properties of the three-dimensional porous network, but also enhances Li+ transfer (due to the COO− and Li+ interactions), resulting in outstanding Li+ transference number (0.54) of the RCM/PVA separator. In addition, the RCM/PVA separator shows excellent thermal stability and high liquid absorption rate (481.25 %). The LiFePO4/3 % RCM-HCl/PVA/Li cell displayed a high discharge capacity of 152.6 mAh·g−1 after 200 cycles at 1C. This work provides a new light on the fabrication of biodegradable separators for LIBs via a novel and cost-effective strategy.

In situ mineralized Ca3(PO4)2 inorganic coating modified polyethylene separator for high-performance lithium-ion batteries

• Flower-like calcium phosphate coating is successfully formed on the surface of modified PE separator. • The obtained composite separator exhibits stronger affinity to electrolyte and absorbs more liquid electrolyte. • CaP@PE separator possess an improved ionic conductivity (0.52 mS cm −1 ) and lithium-ion transference number (0.36). • Cell assembled with CaP@PE separator exhibits excellent cycle performance and rate performance. The organic–inorganic composite separator possesses great thermal stability and electrolyte wettability, which is normally prepared via the slurry containing binder. However, this preparation is involved with large amounts of organic solvent that is harmful to health. In this paper, we design a novel method that combines the crosslinking technology and biomimetic mineralization process to prepare a Ca 3 (PO 4 ) 2 inorganic coating modified polyethylene separator (CaP@PE). The obtained composite separator exhibits stronger affinity to electrolyte, and its porous coating structure can store more liquid electrolyte, thus the ionic conductivity is promoted from 0.27 mS cm −1 to 0.52 mS cm −1 and the lithium-ion transference number is increased from 0.26 to 0.36. Compared with PE separator, CaP@PE separator shows better thermal stability at high temperature. Due to the improved ionic transport performance and reduced charge transfer impedance, LiCoO 2 /Li half-cell employing CaP@PE separator displays superior cycle stability and capacity retention ability after 150 cycles at a current density of 1C. Even at a high rate of 5C (7.5 mA cm −2 ), the cell with CaP@PE separator still exhibits a discharge capacity of 0.80 mAh. This work provides a promising separator to optimize the electrochemical performance and safety performance in lithium-ion battery.

Improving Surface Functionality, Hydrophilicity, and Interfacial Adhesion Properties of High-Density Polyethylene with Activated Peroxides.

Polyolefins have had limited application in advanced technologies due to their low surface energy, hydrophobicity, and weak interfacial adhesion with polar coatings. Herein, we propose the use of transition metals at their lowest oxidation state and inorganic peroxides to improve the functionality, surface free energy, hydrophilicity, and adhesion properties of high-density polyethylene (HDPE). Among the nine combinations of transition metals and peroxides used in this study, the combination of Co(II) and peroxymonosulfate (PMS) peroxide was the most effective for surface modification of HDPE, followed closely by the combination of Ru(III) and PMS. After chemical treatment, HDPE's surface functionality, composition, and energy were analyzed via Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurements. Hydroxyl, carbonyl, and carboxylic acid functional groups were detected on the surface, which explained the improved hydrophilicity of the modified HDPE surface; the contact angle of HDPE with DI water decreased from 94.31 to 51.95° after surface treatment. To investigate the effect of HDPE's surface functionality on its interfacial properties, its adhesion to a commercial epoxy coating was measured via pull-off strength test according to ASTM D54541. After only 20 min of surface treatment with Co(II)/PMS solution, the adhesion strength at the interface of HDPE and the epoxy coating increased by 193%, confirming the importance of polyolefins' surface functionality on their interfacial adhesion properties. The method outlined herein can improve HDPE's surface functionality by introducing sulfate radicals. It improves HDPE's hydrophilicity and adhesion properties without requiring strong acids or time-consuming pre- or post-treatment processes. This process has the potential to increase the use of polyolefins in various industries, such as for protective coatings, high performance lithium-ion battery separators, and acoustic sensors.

Hierarchically Porous Silica Membrane as Separator for High-Performance Lithium-Ion Batteries.

Commercial polymeric separators in lithium-ion batteries (LIBs) typically suffer from limited porosity, low electrolyte wettability, and poor thermal and mechanical stability, which can degrade the battery performance especially at high current densities. Here, the design of hierarchically porous, ultralight silica membranes as separator for high-performance LIBs is reported through the assembly of hollow mesoporous silica (HMS) particles on the cathode surface. The rich mesopores and large cavity of individual HMS particles provide low-tortuosity pathways for ionic transport, while simultaneously serving as electrolyte reservoir to further boost the electrochemical kinetics. Moreover, benefiting from their inorganic and hierarchically porous nature, such HMS separators display better electrolyte affinity, thermal stability, and mechanical strength than commercial polypropylene (PP) separators. As a demonstration, LIBs with a LiFePO4 cathode coated with HMS separators exhibit exceptional rate capability and cycling stability, outperforming LIBs with PP and Al2 O3 -modified PP separators as well as separators made of solid silica particles.

Vapor-induced phase inversion of poly (m-phenylene isophthalamide) modified polyethylene separator for high-performance lithium-ion batteries

A separator plays a vital role in the performance and safety of lithium-ion batteries. The modification of the commercial polyolefin separators is the most practical way to overcome the low thermal stability and electrolyte wettability problems. Herein, we propose a simple vapor-induced phase inversion method to fabricate poly (m-phenylene isophthalamide) (PMIA) modified polyethylene (PE) separator (referred to PE@PMIA). During the phase inversion process, the porous PMIA layers are formed on both sides of the PE separator and the thickness of the whole modified separator is only 13 μm. Due to the introduction of the PMIA protective layers with high heat-resistant, the obtained PE@PMIA displays enhanced thermal stability, showing no dimension shrinkage after heat treatment at 150 °C for 30 min. The PE@PMIA separator also exhibits excellent electrolyte affinity, enhanced mechanical property, and excellent lithium dendrite resistance for batteries' superior safety. The assembled LiFePO4/Li cells using PE@PMIA separators show great cycling performances and good rate capability. These results demonstrated that the PE@PMIA separators are promising for high-performance lithium-ion batteries and the vapor induced phase inversion method could be extended to fabricate other modified separators.

High performance of polyacrylonitrile/[Mg[sbnd]Al]-layered double hydroxide composite nanofiber separators for safe lithium-ion batteries

There is a great need for a thermally stable separator to meet the application requirements in high-power lithium batteries. In this paper, Polyacrylonitrile (PAN) /[MgAl]-layered double hydroxide (LDH) composite nanofiber membranes were produced by electrospinning mixed solution which is prepared by dispersing different content of LDH particles into PAN. The results of Energy Disperse Spectroscopy (EDS) indicates that special LDH has been successfully compounded into nanofibers. The morphology of PAN/LDH nanofiber composite membranes by Scanning Electron Microscope (SEM) shows that it has a distinctive three-dimensional porous structure, among them, the PAN/15%LDH nanofiber composite membrane exhibits high porosity (87%), high electrolyte uptake (1053%), high ionic conductivity (4.25 mS cm−1), low interface resistance (106 Ω) and wide electrochemical stability window (5.4 V). In addition, high-temperature performance tests show that PAN/15%LDH membranes have no thermal shrinkage at 200 °C. More importantly, the overpotential of Li/ PAN/15%LDH/ Li symmetrical batteries is very low during the cycle, which can inhibit the growth of lithium dendrites to a certain extent. These advantages indicate that PAN/LDH composite nanofiber membranes are a promising candidate for high-performance lithium-ion battery separators.

Membranes Made from Electrospun Polyacrylonitrile Nonwoven Fibers with Uniform Diameter for Lithium-Ion Battery Separators

Membranes were prepared from polyacrylonitrile (PAN) fibers with uniform diameter for lithium-ion battery separators using optimized electrospinning parameters. In this work, the coefficient of variation (CV) is proposed as one of the indexes to evaluate the uniform consistency of fiber diameter. The most uniform fibrous membrane was compared with a control group that had different uniformity. The effects of the uniformity of the fiber diameter were studied in terms of the membrane porosity, electrolyte absorption, mechanical tensile strength, and thermal stability. The membranes were assembled into coin cells with lithium cobaltite cathodes and graphite anodes, and the performance of the cells was tested. As the uniformity of the fiber diameter increased, the ionic conductivity improved and the interface resistance decreased. The proposed separator improves the discharge capacity (181 mAh·g−1 at the 0.2C rate), C-rate cycling performance, and Coulombic efficiency (99.64 %), which indicates that the uniform membrane could be a candidate for high-performance lithium-ion battery separators.

A novel polymer-modified separator for high-performance lithium-ion batteries

In this study, hyperbranched polybenzimidazole (HBPBI) was synthesized and coated on polyethylene membrane surface with terephthaldehyde (TPA) cross-linker to obtain high-performance CHBPBIE separator for application in lithium ion battery (LIB). The introduction of benzimidazole, amino and carboxyl groups to TPA cross-linked HBPBI (CHBPBI) could improve the wettability, electrolyte uptake, heat resistance and thermal stability of CHBPBIE separator. The absorption of the liquid electrolyte is increased from 90% for the control separator to 181% for CHBPBIE separator. More importantly, the cross-linked coating structure can greatly improve the ionic conductivity of CHBPBIE separator, and it is 0.77 mS cm−1 at 25 °C, which is increased by 10.6 times compared with that of the control PE separator. The LIB half cells assembled with CHBPBI modified separator shows 73% increase in power capability at 7 C rate and a discharge capacity retention rate of 91.1% after 400 cycles at 2 C rate. Thus, the modified separator has the potential for use in high-performance LIB.

Covalently‐Bonded Poly(vinyl alcohol)‐Silica Composite Nanofiber Separator with Enhanced Wettability and Thermal Stability for Lithium‐Ion Battery

AbstractCovalently‐bonded poly(vinyl alcohol)‐silica (PVA‐SiO2) composite nanofiber membranes are fabricated via sol‐gel electrospinning and their physical and electrochemical properties are investigated for application as separators in lithium‐ion batteries (LIBs). Experimental results demonstrate the PVA‐SiO2 membranes possess a unique three‐dimensional interconnected porous structure and display higher porosity (73%), better electrolyte affinity, higher electrolyte uptake (405%) and lower thermal shrinkage compared to commercial polypropylene (PP) membranes. In addition, batteries using PVA‐SiO2 composite nanofiber membranes as separators exhibit enhanced ionic conductivity (1.81 mS cm−1), superior cycling stability and C‐rate performance than those using PP separators. These findings reveal that the PVA‐SiO2 composite nanofiber membrane might be a promising alternative to commercial polyolefin separator for high performance LIBs.

A high-strength PPESK/PVDF fibrous membrane prepared by coaxial electrospinning for lithium-ion battery separator

Electrospinning fibrous membranes have attracted a great deal of attention because of their advantages, including uniform pore size, large ratio surface area, and high porosity. For extended application in lithium-ion battery, it is essential to further improve their electrochemical, mechanical, and thermal properties. In this work, a new poly (phthalazine ether sulfone ketone) (PPESK)/polyvinyli-denefluoride (PVDF) core/shell fibrous membrane was fabricated via the coaxial electrospinning technique, followed by hot press. The PPESK/PVDF membrane hot pressed at 160°C exhibits excellent comprehensive performance, including large porosity (80%), high electrolyte uptake (805%), and excellent thermal stability (at 200°C). Moreover, due to the improved bonding effect derived from the solidification of the PVDF shell layer after the hot press, the mechanical property of the membrane is effectively enhanced. The electrochemical tests also indicate that the PPESK/PVDF membrane shows larger ionic conductivity and lower interfacial resistance when compared with commercial microporous polypropylene separator. In addition, simulated cells assembled with the PPESK/PVDF membrane present superior discharge capacity, stable cycle performance, and excellent rate capability. Therefore, the hot-pressed coaxial PPESK/PVDF fibrous membrane has the potential to be a promising candidate as the separator for high-performance lithium-ion battery.

Modified polypropylene/cotton fiber composite nonwoven as lithium-ion battery separator

The polypropylene (PP) fiber is modified with grafting and coating. The modified PP/Cotton fiber composite nonwoven is prepared with cotton and PP fiber via wet nonwoven papermaking method. Properties of composite nonwoven including morphology, electrolyte uptake, mechanical strength, thermal stability, ionic conductivity and the influence to battery performance are characterized. As a result, The modified PP/Cotton fiber composite nonwoven shows a better performance such as electrolyte uptake (180%), tensile strength (1.6529 kN m−1), size unchanged at 170 °C, ionic conductivity (1.76 mS cm−1), discharge capacities (169.9 mAh·g−1) and cycling performance (92%). Hence, the modified PP/Cotton fiber composite nonwoven is a promising separator for high-performance lithium-ion battery.

Facile fabrication of multilayer separators for lithium-ion battery via multilayer coextrusion and thermal induced phase separation

Polypropylene (PP)/polyethylene (PE) multilayer separators with cellular-like submicron pore structure for lithium-ion battery are efficiently fabricated by the combination of multilayer coextrusion (MC) and thermal induced phase separation (TIPS). The as-prepared separators, referred to as MC-TIPS PP/PE, not only show efficacious thermal shutdown function and wider shutdown temperature window, but also exhibit higher thermal stability than the commercial separator with trilayer construction of PP and PE (Celgard® 2325). The dimensional shrinkage of MC-TIPS PP/PE can be negligible until 160 °C. In addition, compared to the commercial separator, MC-TIPS PP/PE exhibits higher porosity and electrolyte uptake, leading to higher ionic conductivity and better battery performances. The above-mentioned fascinating characteristics with the convenient preparation process make MC-TIPS PP/PE a promising candidate for the application as high performance lithium-ion battery separators.

A phase inversion based sponge-like polysulfonamide/SiO 2 composite separator for high performance lithium-ion batteries

Abstract In this work, a sponge-like polysulfonamide (PSA)/SiO 2 composite membrane is unprecedentedly prepared by the phase inversion method, and successfully demonstrated as a novel separator of lithium-ion batteries (LIBs). Compared to the commercial polypropylene (PP) separator, the sponge-like PSA/SiO 2 composite possesses better physical and electrochemical properties, such as higher porosity, ionic conductivity, thermal stability and flame retarding ability. The LiCoO 2 /Li half-cells using the sponge-like composite separator demonstrate superior rate capability and cyclability over those using the commercial PP separator. Moreover, the sponge-like composite separator can ensure the normal operation of LiCoO 2 /Li half-cell at an extremely high temperature of 90 °C, while the commercial PP separator cannot. All these encouraging results suggest that this phase inversion based sponge-like PSA/SiO 2 composite separator is really a promising separator for high performance LIBs.

On the Relevance of the Polar β-Phase of Poly(vinylidene fluoride) for High Performance Lithium-Ion Battery Separators

Separator membranes based on poly(vinylidene fluoride), PVDF, poly(vinylidene fluoride-co-trifluoroethylene), PVDF-TrFE, poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, and poly(vinylidene fluoride-co-chlorotrifluoroethylene), PVDF-CTFE, were prepared by a solvent casting method using N,N-dimethylformamide (DMF) as solvent. In all cases, the same polymer/solvent ratio and solvent evaporation temperature were used. For all membranes, a porous microstructure is achieved with a degree of porosity larger than 50%. The β-phase content as well as degree of crystallinity were different for each membrane, which were lower for the copolymer membranes when compared with PVDF. On the other hand, the observed ionic conductivity values, electrolyte uptake, tortuosity, and MacMullin number were similar for all membranes. The electrochemical performance of the separator membranes was evaluated in a Li/C–LiFePO4 half-cell configuration showing good cyclability and rate capability for all membranes. Among all ...

Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators

Inorganic nanoparticles are frequently adopted to modify polymer porous membranes, aiming to fabricate high performance lithium-ion battery separators. However, the loading of inorganic nanoparticles is usually low, which limits their effects on the performance of modified polymer porous membranes. This study reports novel hybrid membranes with highly loaded silica nanoparticles of 67.5 wt% as a matrix. Good strength and flexibility of hybrid silica membranes are ensured by interpenetrated polymer nanofibers as a mechanical skeleton. The hybrid silica membranes have excellent dimensional stability at 200 °C, and high liquid electrolyte uptake and resultant ionic conductivity. The coin cells with hybrid silica membranes as separators show superior discharge capacity and cyclic performance even at a high temperature of 120 °C.

Hierarchical Polyamide 6 (PA6) Nanofibrous Membrane with Desired Thickness as Separator for High-Performance Lithium-Ion Batteries

Polymeric nanofibrous separators have drawn a great attention due to their unique tortuous pores for preventing the growth of lithium dendrites and hence ensuing high safety and cycling stability. Various nonwoven mats with strong mechanical strength are typically served as substrate materials for nanofibrous membranes. However, the overall thickness (>40 μm) of most nanofibrous separator is far beyond the ideal dimension (25∼30 μm), thus severely reducing the energy densities of batteries. In present work, a hierarchically structured polyamide 6 (PA6) nanofibrous separator (designated as PA6/PET/PA6) with a desired thickness of 30 μm has been successfully developed using an ultra-thin poly (ethylene terephthalate) (PET) nonwoven (18 μm) as the substrate. Due to the flatness and hydrophobic nature, surface modifications of PET nonwoven have been attempted to improve the adhesion force between PET and PA6 membranes by NaOH solution treatment. The resultant PA6/PET/PA6 separator exhibits lower thermal shrinkage, higher electrolyte affinity and ionic conductivity, as well as superior rate capabilities and cycling performance compared with commercial PP separator, suggesting a promising candidate for practical application in high-rate LIBs.

Hollow mesoporous silica sphere-embedded composite separator for high-performance lithium-ion battery

A high-performance composite separator based on hollow mesoporous silica spheres, poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), and poly(ethylene terephthalate) nonwoven was explored as lithium-ion battery separator via a dip coating technique followed by a phase separation wet process. Systematical investigations including morphology characterization, porosity measurement, electrolyte wettability, thermal shrinkage testing, and cell performance examination are carried out. It is demonstrated that the unique mesoporous structures of silica spheres endow the composite separator with well-defined microporous structure. Owing to the preferable microstructure and relatively polar constituents, the composite separator exhibits higher porosity, superior electrolyte wettability, and outstanding thermal stability. Based on the above advantages, the composite separator shows better electrochemical performances, such as the discharge C-rate capability and cycling performance, as compared to the commercialized PE separator. This research presents a simple approach to prepare high-performance separator, which is demonstrated to be a good candidate for lithium-ion batteries.

Porous cellulose diacetate-SiO2 composite coating on polyethylene separator for high-performance lithium-ion battery

The developments of high-performance lithium ion battery are eager to the separators with high ionic conductivity and thermal stability. In this work, a new way to adjust the comprehensive properties of inorganic-organic composite separator was investigated. The cellulose diacetate (CDA)-SiO2 composite coating is beneficial for improving the electrolyte wettability and the thermal stability of separators. Interestingly, the pore structure of composite coating can be regulated by the weight ratio of SiO2 precursor tetraethoxysilane (TEOS) in the coating solution. The electronic performance of lithium ion batteries assembled with modified separators are improved compared with the pristine PE separator. When weight ratio of TEOS in the coating solution was 9.4%, the composite separator shows the best comprehensive performance. Compared with the pristine PE separator, its meltdown temperature and the break-elongation at elevated temperature increased. More importantly, the discharge capacity and the capacity retention improved significantly.