Polyolefin Separators Research Articles

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.

Fabricating a high performance composite separator with a small thickness for lithium ion batteries

Separator is a porous membrane that physically separates the cathode and anode, yet ensures the lithium ion transport between these two electrodes. This work attempts to develop a high performance composite separator that can also provide a low cost potential when produced at a large scale. For this purpose, a non-woven reinforced macroporous polyaramid separator with a thickness comparable to mainstream commercial products was developed to achieve improved separator mechanical performance and uniformity. The separator consists of a polymer phase with a uniform porous structure and a non-woven reinforcement. The non-woven structure made using a wet-laid process provides the increased separator mechanical strength, while the macroporous polyaramid phase modified with nano and submicron ceramic particles optimizes the pore structure for improved battery electrochemical performance. Compared with the commercial polyolefin separator, this developed separator showed superior mechanical and physical robustness at elevated temperatures. The separator wettability by the standard liquid electrolyte was also much improved. A stable cycle performance and an improved rate capability have been observed when the separator was evaluated in full coin cells.

An AlOOH-coated polyimide electrospun fibrous membrane as a high-safety lithium-ion battery separator

To improve the safety of lithium-ion batteries (LIBs), an AlOOH-coated polyimide (API) fibrous membrane as an inorganic composite separator is developed via an electrospinning technique and a subsequent blade-coating process. Benefiting from the good thermostability of polyimide and the flame-retarding property of AlOOH, the API separator shows excellent thermal stability and flame retardance. Compared with commercial polyolefin separator, the API separator exhibits superior electrolyte wettability and electrochemical stability. The Li||LiCoO2 cell using the API separator exhibits the enhanced cycling performance and rate capability. Thus, the as-prepared API separator is promising as a new kind of inorganic composite separator for advanced LIBs with high safety.

A comparative study of thermal runaway of commercial lithium ion cells

Thermal runaway of three lithium ion cells (“A” - NCA/Graphite, “B” - LFP/Graphite, “C” - NCA/LTO) at 0%, 50%, and 100% state of charge (SOC) is studied by Accelerating Rate Calorimetry (ARC). Thermal behaviour of harvested positive and negative electrodes at three SOC (0%, 50%, and 100%) is analyzed using Differential Scanning Calorimetry (DSC). Thermal stability of recovered separators is also investigated by DSC. Harvested electrodes and separators are studied alone and in contact with a liquid electrolyte. The thermal behaviour of each component and its contribution is quantified and thoroughly discussed. A crucial negative impact of the state of charge and presence of highly flammable liquid electrolyte on the thermal instability of the investigated cells and “electrode - electrolyte” systems is clearly revealed. Among studied cells, LiFePO4/Graphite one is the safest due to intrinsic thermal stability of lithium iron phosphate LiFePO4 based cathode and despite the fact of using a microporous polyolefin separator with limited thermal stability.

In Situ Polysulfide Detection in Lithium Sulfur Cells.

Lithium sulfur batteries promise significant improvements in specific energy compared to Li-ion, but are limited by capacity fade upon cycling. Efforts to improve durability have focused on suppressing the solubility of intermediate polysulfides in the electrolyte. Here we describe an in situ electrochemical polysulfide detection method based on the cyclic volatmmetric response. The voltammetric peaks correlate with increased discharge, consistent with increased polysulfide species in the electrolyte as demonstrated by prior literature measurements using spectroscopic methods. We verified that adding metal sulfide species to the sulfur cathode and ceramic-coatings on the polyolefin separator result in reduced polysulfide concentration, consistent with improved cycle life reported earlier. Further, the use of highly concentrated electrolytes produces no detectable dissolved polysulfide species. Future advances in Li/S technology could utilize this method to determine the polysulfide contents in the electrolyte, and thus quantify the efficacy of the sulfur-sequestering strategies.

Roll-to-Roll Functionalization of Polyolefin Separators for High-Performance Lithium-Ion Batteries

Modified polyolefin separators fabricated via a roll-to-roll system exhibit markedly improved compatibility with lithium ion battery electrolytes. Zwitterionic molecules containing a perfluorophenyl azide functional group were synthesized and covalently bound to the surface of commercial polyolefin separators via UV-activated photochemistry. A roll-to-roll prototype system was constructed allowing for the functionalization of large areas of separator under ambient conditions at low cost. Lithium-ion battery cells containing the modified separators exhibit superior electrochemical performance using a common commercial electrolyte. The modified separators, both monolayer PE and trilayer PP/PE/PP, are wetted instantly upon contact with liquid electrolytes lacking linear carbonates. These electrolytes have been designed for use in batteries with advanced thermal stability properties and/or higher voltage windows, which have previously been hindered by incompatibility with commercial trilayer polyolefin separa...

Silicone modified polypropylene separator for high temperature lithium ion battery applications

The poor thermal stability of polyolefin separator greatly limits the output performance and safety performance of a lithium ion battery at high temperature. Herein, we report a novel silicone grafted polyolefin separator prepared by a simple solution process and its lithium ion battery performance. Despite its low coating thickness, the grafted silicone coating significantly improved the thermal stability of polyolefin separator. In addition, the silicone grafted separator shows enhanced electrolyte affinity. Due to these favorable features, the novel separator exhibits enhanced battery performance at a high temperature of 80 °C. Our results suggest that the silicone grafted separator could be potentially used in practical battery applications.

Electrochemical performance and thermal stability of the electrospun PTFE nanofiber separator for lithium‐ion batteries

ABSTRACTSeparator is a very important set of lithium‐ion batteries. At present, low porosity and poor thermal stability are two major disadvantages of separator. In this work, we first apply electrospinning method to prepare the Polytetrafluoroethylene (PTFE) nanofiber separator, which has the advantages of electrospinning method and PTFE materials. The effect of the PTFE nanofiber separator is investigated by scanning electron microscope, Capillary Flow Porometer, thermogravimetric–differential scanning calorimeter, linear sweep voltammeter, AC impedance, and charge/discharge cycling tests. The results demonstrate that the PTFE nanofiber separator has a special fiber structure made from PTFE particles gathering one by one along the fibers. Moreover, the PTFE nanofiber separator exhibits several advantages, including suitable pore diameter, uniform pore size distribution, high porosity, and electrolyte uptake, which enhance the ionic conductivity. The thermal stability of the PTFE nanofiber separator is much better than that of the conventional polyolefin separator. The Li/LiCoO2 cell equipped with PTFE nanofiber separator exhibits excellent rate performance and first charge–discharge specific capacity of 142 and 131 mA h g−1, respectively, accompanied by relatively stable cycle performance at 0.2 C rate. It is supposed to be a candidate for application in lithium‐ion batteries. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46508.

Oxidatively Stable Fluorinated Sulfones Electrolytes for High Voltage High Energy Li-Ion Batteries

Developing a high-voltage enabling electrolyte is extremely critical for the success of the next generation high-energy density lithium-ion battery especially for electric vehicles. Material scientists have developed new cathode materials with improved specific capacity[1,2] and operating voltage (5 V vs. Li+/Li).[3,4] However, designed for a 4V-class lithium-ion chemistry, the conventional electrolyte suffers from oxidation instability on the charge cathode/electrolyte interface at high charging voltages which leads to severe transition metal (TM) dissolution and rapid capacity fading. The voltage instability of electrolyte becomes the bottleneck for the extensive application of the high voltage cathode materials. Thus, new electrolytes with elevated voltage stability has been widely explored.[5-8] Here we report a new class of fluorinated electrolytes comprising novel fluorinated sulfones including ((trifluoromethyl)sulfonyl)ethane (FMES), 1-((trifluoromethyl)sulfonyl)propane (FMPS) and 2-((trifluoromethyl)sulfonyl)propane (FMIS).[9] These compounds have been synthesized via new synthetic routes and evaluated as electrolyte materials under high voltage operation. The results indicate that sulfones with an α-trifluoromethyl group possess enhanced oxidative potential and reduced viscosity as compared to their non-fluorinated counterparts. The α-fluorinated sulfones also show enhanced wetting ability with polyolefin separator. A facile synthesis method for a reported sulfone 1,1,1-trifluoro-3-(methylsulfonyl)propane (FPMS) was also developed. With the new reaction method, large quantity of material was obtained and comprehensive electrochemical properties of FPMS as high voltage electrolyte was evaluated. Unlike the α-fluorinated sulfones, the γ-fluorinated sulfones resemble the property of the non-fluorinated ones.

M-N-Doped Carbon Nanofibers Modified Separators for Lithium-Sulfur Batteries

A novel lightweight, multi-functional layer comprised of 1D porous metal-embedded nitrogen-doped carbon nanofibers (M-N-C) and 2D graphene sheets is introduced to modify a commercial polyolefin separator for using in lithium-sulfur batteries. Benefiting from the high electrical conductivity and strong lithium polysulfides (LiPSs) chemical adsorption ability of M-N-C, the porous intercalation-structured modified layer exhibits excellent ability to suppress LiPSs shuttling while maintaining high lithium-ion transport, and the modified layer functions as a highly conductive upper current collector to reutilize active sulfur species. The cell assembled with the modified multifunctional separator shows a high rate capability and excellent cycling stability. These exciting results indicate that this multifunctional membrane could be a promising separator candidate for Li-S batteries.

A paper-supported inorganic composite separator for high-safety lithium-ion batteries

A paper-supported inorganic composite (PIC) separator is prepared by spraying Al2O3 particles onto both surfaces of a commercial paper substrate. The as-prepared PIC separator shows excellent thermal stability without any shrinkage at 130 °C for 30 min, superior wettability towards the electrolyte, with a contact angle of 0°, and high ionic conductivity compared with that of the conventional polyethylene (PE) separator. The graphite|LiCoO2 full-cells with the PIC separator exhibit good cycling and rate performance. In the nail penetration tests, the pouch cell assembled with the PIC separator shows high safety characteristics. Given the advantages of the PIC separator, it is a good potential replacement for the polyolefin separators used in high-safety lithium-ion batteries.

Polyphenylene sulfide nonwoven-based composite separator with superior heat-resistance and flame retardancy for high power lithium ion battery

In this study, a new alternative nonwoven based on engineering plastic polyphenylene sulfide (PPS) was explored as the support to construct high performance and safety separator for the first time. By the method of physical coating polymer poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and inorganic nanoparticle SiO2 on the support, the designed composite separator was successfully obtained for lithium ion battery. Systematic investigations ranging from physical properties, thermal properties to electrochemical performances were carried out. It was found that compared with commercialized polyolefin separator, PPS nonwoven-based composite separator possessed higher porosity, air permeability, improved electrolyte wettability and electrolyte uptake, thus being helpful for lithium ion transfer between electrodes and increasing the ionic conductivity. These behaviors accordingly endowed battery with superior discharge capacity at various discharge current rates from 0.2 C to 2 C. Moreover, the composite separator was observed to exhibit excellent dimensional stability even after thermal treatment at 250 °C and present good flame retardant ability. The afore-mentioned outstanding performances of PPS nonwoven-based composite separator would shed light on the development of high power lithium ion battery.

Improving the Electrochemical Performance of Polypropylene Separator through Instantaneous Photo-Induced Functionalization

The intrinsically low electrolyte affinity of polyolefin separators limits their electrochemical performance in lithium ion battery. Most of the traditional modification methods which improve the polyolefin separator's electrolyte affinity either rely on the use of thick coating layer that may decrease energy density of the battery or involve multiple laborious steps, making them less competitive for practical applications. Herein, we report a photo-induced functionalization that could significantly improve the electrolyte affinity of the polypropylene separator within 3 minutes. We find that the electrolyte saturated functionalized separator shows improved ionic conductivity, increased lithium ion transfer number, and thus excellent charge-discharge performance. Our results not only demonstrate the high potential of the functionalized separator, but also suggest that the photo-induced functionalization could be used as a general strategy to endow polyolefin separator with task-specific properties required for different battery applications.

Designing Polyolefin Separators to Minimize the Impact of Local Compressive Stresses on Lithium Ion Battery Performance

In this paper, we study how polyolefin separators respond to compressive stress, which can occur during battery operation as active particles lithiate, expand, and push into and deform the separator. We use real microstructures of polyethylene and polypropylene separators acquired with focused ion beam scanning electron microscopic (FIB-SEM) tomography and simulate how these structures deform under compressive strain. After validating our mechanical simulations, we characterize how the microstructural properties of the separators change as a function of compressive strain and simulate the influence these changes in microstructure have on the lithium ion transport through the separator. We find that a given compressive strain negatively impacts the microstructure of polyethylene separator more than that of a polypropylene separator. To understand the origins of the different response to compressive strains in polyethylene and polypropylene separators, we use a network-based analysis to assess the type of mechanical deformations occurring in the separator membrane and show that it is the combination of material properties and structure that are responsible for the greater stability of polypropylene. This work highlights how the structure of a separator plays an important role in its mechanical robustness and prevention of cell degradation.

Symmetric Lithium Sulfide – Sulfur Cells: A Method to Study Degradation Mechanisms of Cathode, Separator and Electrolyte Concepts for Lithium-Sulfur Batteries

Novel cathode, electrolyte and separator concepts are expected to improve the capacity retention and cycle life of lithium-sulfur batteries by avoiding the dissolution of polysulfide-intermediates into the electrolyte or preventing their diffusion to and reaction with the lithium metal anode. However, their intrinsic evaluation is obscured by ill-defined degradation mechanisms of the lithium counter electrode due to electrolyte and polysulfide consumption during reformation of the solid electrolyte interface. Herein, we introduce symmetric lithium sulfide – sulfur cells to study the “cathode" specific degradation mechanisms of the cathode/electrolyte/separator-system in order to differentiate cathode-, separator, and electrolyte and lithium metal anode degradation. Two identical carbon/sulfur-composite cathodes, either of them one at first lithiated versus lithium, are assembled in a coin cell versus each other. The virtue of this test cell design is demonstrated evaluating prime shuttle-suppression concepts, namely a Nafion-coated polyolefin separator and a sparingly polysulfide-solvating electrolyte. We conclude that symmetric lithium sulfide – sulfur cells provide a valuable methodology to analyze the intrinsic capacity decay and cycle life of cathodes for lithium-sulfur cells with limited solvation and/or blocking of polysulfides, and it is an essential strategy to distinguish cathode from lithium anode degradation.

The polypropylene membrane modified by an atmospheric pressure plasma jet as a separator for lithium-ion button battery

The plasma modification of the polypropylene (PP) membrane as a separator in lithium-ion battery (LIB) has been reported in this paper. Specifically, a commercial PP separator was modified by an atmospheric pressure glow discharge plasma jet (APGD-PJ) with the acrylic acid (AA) monomer. The morphology, electrolyte uptake and retention, the pore size distribution, and the electrochemical properties of the modified PP separator were analyzed. The experimental results indicate that the AA polymer (PAA) was uniformly coated on the surface of the PP membrane, and the contact angle of modified PP was reduced from 112° to 39° using He/O2/AA plasma and the electrolyte uptake of the modified PP separator increased to above 300% of the value of the unmodified one. Compared with the original PP, the cells assembled with the PP separator modified with Ar/O2/AA or He/O2/AA plasma showed higher capacities and better C-rate capacity. The excellent performances of assembled LIB endow the APDG-PJ as a promising protocol for modifying polyolefin separators for LIB applications.

Fast assemble of polyphenol derived coatings on polypropylene separator for high performance lithium-ion batteries

Improving the wettability of liquid electrolyte to polyolefin separators plays a significant role in the fabrication of high performance lithium-ion batteries. Herein, we report a facile surface coating method to enhance the wetting capacity of commercially available polypropylene (PP) separator. Natural polyphenol tannic acid (TA) and sodium periodate are selected as the coating precursor and inorganic trigger, respectively. Coating formation is initiated by the absorption of tannin molecules on the separator surface and then triggered by a single contact (5min) with sodium periodate solution. After being modified, wettability of the separator can be significantly enhanced without damaging its original advantage properties, which accordingly resulted in higher electrolyte uptake and better interfacial compatibility. Furthermore, the LiCoO2/graphite full cells assembled with the modified separator displays an excellent cycle stability with coulombic efficiency exceeding 99.9% and superior rate performance. The ease, low cost and scalability of this coating process, combine with the general surface binding affinity of polyphenol, making this surface modification technique suitable to upgrade other inert substrates for various applications.

Robust polyetherimide fibrous membrane with crosslinked topographies fabricated via in-situ micro-melting and its application as superior Lithium-ion battery separator with shutdown function

With more and more applications of Lithium-ion batteries (LIBs) in electronics, Polyolefin (PO) separators, including Polyethylene (PE) and Polypropylene (PP) separators, cannot meet the increasing requirements of high performance LIBs. Separators with good thermal stability and high electrochemical performance are in great demand. In this paper, Polyetherimide (PEI) fibrous membrane separators with crosslinking network morphologies have been successfully fabricated via electrospinning and in-situ micro-melting technique. The in-situ micro-melting process converts the loose and weak PEI nonwoven membrane to the compact and robust crosslinked fibrous membrane, and then improves the tensile strength of the membrane from 4MPa to 21MPa. The crosslinking structure greatly reduces the risk of fibrous membrane disassembling into the loose nonwovens during long-term battery cycling, and consequently improves the stability of LIBs. The results of Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) show that the crosslinked PEI fibrous membrane presents excellent thermal stability (5% weight loss at about 522.2°C) and thermal dimensional stability (without shrinkage below 240°C), better than Celgard-2400 separator. The LIB with this separator shows higher capacity (113.3mAhg−1, 5C) than that with Celgard-2400 separator (95.2mAhg−1, 5C). More importantly, this PEI separator exhibits thermal shutdown function at 260°C, which provides LIB with a safe guarantee under high temperature. All above-mentioned properties make crosslinked PEI fibrous membrane a good candidate as a separator of LIB.

New Separators in Lithium/Sulfur Cells with High-Capacity Cathodes

Lithium-sulfur cells exhibit poor cycle life, due to the ‘polysulfide shuttle’ enabled by the dissolution of sulfur reduction products in organic electrolyte. Different strategies have been implemented to reduce the shuttle effects, including novel cathode designs to sequester polysulfides within the cathode, new electrolytes with reduced polysulfide solubility, and blocking layers to hinder polysulfide migration towards the anode. However, these are less successful in the case of high-energy Li/S cells with cathodes featuring substantial sulfur loadings, as proportionately greater polysulfide dissolution rapidly degrades performance. Additionally, the electrolyte/sulfur ratio is a limiting factor in high-energy cells, indicating that a minimum polysulfide solubility is required. Herein we describe the benefits of modified separators with the objective of blocking polysulfide migration. Specifically, we demonstrate improved discharge performance in laboratory Li/S cells with ceramic-coated separators, including single-sided and double-sided Al2O3-coated polyolefin material from commercial sources. We also developed a new AlF3 coating on a polyolefin separator that showed even greater improvements: higher initial specific capacity (∼800 mAh/gS), coulombic efficiency (>96%), and cycle life (>100 cycles). Detailed analytical study via SEM and EDS of separators harvested from cycled cells indicated an approximately uniform sulfur distribution across the ceramic-coated separator. Finally, concentrated electrolytes, which reportedly minimize polysulfide diffusion and lithium dendrite growth, were tested and shown to yield high coulombic efficiency although with low sulfur utilization.

Mechanically Reinforced PVdF/PMMA/SiO2 Composite Membrane and Its Electrochemical Properties as a Separator in Lithium‐Ion Batteries

AbstractCompared with commercial polyolefin separators, the poor mechanical performance of electrospun polymeric membranes limits their usage as battery separators. Herein, poly(methyl methacrylate) (PMMA) and SiO2 nanoparticles were introduced into electrospun poly(vinylidene fluoride) (PVdF) membranes to form a PVdF/PMMA/SiO2 nonwoven membrane. A hot‐pressing method controlled the thickness of the electrospun membranes and improved their mechanical performance further. SEM tests show that PMMA partly melts in the composite membrane, which bonds neighboring electrospun fibers to reinforce the mechanical strength of the membrane. Uniformly distributed SiO2 nanoparticles on the electrospun fibers could supply higher resistance to mechanical impact. As a result, the composite membrane shows a high tensile strength (32.69 MPa) and high elongation at breakage (137.50 %). Differential scanning calorimetry and hot oven tests indicate that the composite membrane has excellent thermal stability. Furthermore, the addition of PMMA and SiO2 can decrease the crystallinity of PVdF and further improve the absorption of liquid electrolyte. According to the results of electrochemical tests, the composite membrane exhibits higher ionic conductivity (4.0×10−3 S cm−1) and lower interfacial resistance than those of the Celgard separator. The lithium‐ion cell assembled from the composite membrane exhibits more stable cycle performance, higher discharge capacity (158 mA h g−1), and excellent capacity retention.