Polyolefin Separators Research Articles

Bi-functional Janus all-nanomat separators for acid scavenging and manganese ions trapping in LiMn2O4 lithium-ion batteries

LiMn2O4-based lithium-ion batteries (LIBs) have attracted considerable attention due to their cost advantage, but industrial use has been hindered by severe capacity deterioration during cycling with LiPF6-based carbonate electrolytes, especially under high-temperature (55 °C) cycling conditions. Herein, a bifunctional, Janus-faced, inorganic-organic hybrid separator is demonstrated to ensure long-term cycling reversibility of LiMn2O4||graphite LIBs. The monolithic all-nanomat separator is constructed by a bottom layer of poly (m-phenylene isophthalamide) nanofibers by electrospinning and a top layer of interpenetrating hydroxyapatite/bacterial cellulose fibers. Benefiting from such a unique Janus configuration, the as-formed separator exhibits synergistic coupling in functions, simultaneously scavenging trace acid and mitigating the dissolution/deposition of Mn2+ ions in batteries. Consequently, the cells with the Janus separator achieve enhanced capacity retention compared with traditional cells with the commercial polyolefin separator over 100 cycles at 55 °C. Furthermore, bulk structure and surface chemistry characterizations confirm that irreversible structural change in the LiMn2O4 cathode could be avoided, and the amount of manganese on the graphite anode from the cycled cells with the Janus separator is suppressed. This work demonstrates that a composite Janus all-nanomat separator offers an unexpected route towards highly durable LiMn2O4-based LIBs.

A flame-retardant, high ionic-conductivity and eco-friendly separator prepared by papermaking method for high-performance and superior safety lithium-ion batteries

The separator plays a significant role in influencing electrochemical performances and monitoring battery safety in lithium-ion batteries. Nevertheless, suffered by commercial polyolefin separator intrinsic drawbacks of intolerable electrolyte affinity, low ionic conductivity, poor thermal stability and terrible flammability with large thermal contribution, to develop an advanced separator that meet the above requirements is imminent. In this work, an environmentally friendly bacterial cellulose and attapulgite composite separator (BA@ATP) with low cost, high ionic conductivity and excellent flame retardant performance is designed by papermaking method. With strong adsorption energy with electrolyte, the BA@ATP separator exhibits extremely high electrolyte uptake and ionic conductivity, giving rise to an enhanced specific discharge capacity with higher capacity retention and an excellent rate capability with low voltage polarization at high current density. In addition, the advanced separator displays self-extinguishing characteristic after ignition, and extremely low heat and flue gas contribution can be obtained in the combustion state, indicating promoted battery safety. After being assembled into the pouch cell, the cell with BA@ATP separator exhibits a higher self-heating temperature and lower heat release rate (HRR) and total heat release (THR) values when burning. Overall, the advanced separator has great potential to replace the polyolefin separator for attaining high performance safety batteries.

Influence of electrolyte presence on the vibration behavior of a Li-ion pouch cell and on the mechanical characteristics of its incorporated [formula omitted]-coated polyolefin separator

Quality improvement of battery crash simulation models is essential for realistic mechanically induced short-circuit prediction in modern electric vehicles, where the battery is part of the load path. In this paper, the electrolyte influence on the mechanical behavior of a commercial pouch cell and its incorporated Al2O3-coated separator is investigated. To mechanically characterize the separator, both compression and tensile tests were performed. Modal analyses were performed to determine the damping behavior and dynamic stiffness of a normally conditioned cell (wet) and a cell without electrolyte (dry). In the tensile tests, a slight anisotropy and an increase in stiffness, triggered by higher test speeds, were observed. The wet specimens soaked in substitute electrolyte mixture showed longer elongation to failure with a stiffer response than the dry specimens under tensile loading. In the compression tests, exactly the opposite mechanical response of wet and dry specimens was measured. From the modal analyses, a stiffening of the dry cell could be determined. Based on the component tests, a finite element model of the separator was developed in LS-DYNA, enabling prediction of the deformation behavior of the separator under mechanical loads — this can be further used in high-fidelity cell simulation models.

High performance polyimide-based separator for 4.5V high voltage LiCoO2 battery with superior safety

Polyolefin separator has poor thermal stability which leads to short circuit and thermal runaway of lithium ion batteries(LIBs) about 100 °C. Here, we used polyimide (PI) as the core material and poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) as the shell material to prepare PI/PVDF-HFP composite separator by coaxial electrospinning. Compared with Celgard 2400, the PI/PVDF-HFP separator not only demonstrates negligible thermal shrinkage at 200 °C and excellent flame retardancy, but also shows higher porosity and electrolyte uptakes as well as superior wettability, which are conducive to reduced interfacial impedance and improved ion conductivity. The 4.5 V high-voltage LiCoO2/Li cell with PI/PVDF-HFP separator exhibits an initial discharge specific capacity of 181 mAh g−1 and a capacity retention ratio of 95% after 300 cycles, and it also shows superior C-rate capability. The accelerating rate calorimetry (ARC) test manifests that the onset temperature of thermal-runaway of the 4.5 V LiCoO2/Li cell with PI/PVDF-HFP separator is 190 °C, indicating that PI/PVDF-HFP separator possesses superior safety.

Ultra-robust polyimide nanofiber separators with shutdown function for advanced lithium-ion batteries

Safety issues have significantly impeded the development of modern lithium-ion batteries (LIBs), and fabricating advanced separators has been considered as a feasible solution to improve the security of LIBs. Here, we present an advanced polyimide (PI) nanofiber membrane (referred to as PIE) with ultrahigh strength and the shutdown function by optimizing precursor viscosity and nanofibers microstructure. Increasing chain length of the PI backbone and in -situ bonding technique can markedly promote mechanical properties of PI nanofiber separator (strength of pristine PI nonwoven >60 MPa, strength of modified PI nonwoven >90 MPa), which is a new record so far among current PI nanofiber membranes. The present PIE separator displayed superior wettability, heat-resistant, and ion conductivity compared to the commercial Celgard separator. Importantly, the excellent flame resistance and high-temperature shutdown function of PIE separators ensure the security of LIBs. Based on our rational separator design, the PIE separators possessed higher LIB capability (111.3 mAh g −1 at 10 C) than the Celgard separator (90.8 mAh g −1 at 10 C). Moreover, the PIE separator-based LIBs can run stably at 1 C and 5 C for 100 cycles. The present work demonstrated the likelihood of replacing the commercial polyolefin separator with the advanced PI nanofiber membranes. • Advanced PIE separators are prepared by increasing chain length of PI backbone and using in -situ bonding technique. • The strength of as-fabricated PI separators present a highest record so far (pristine PI nonwoven >60 MPa, modified PI nonwoven >90 MPa). • The excellent thermostability, flame resistance, and shutdown function can ensure the safety of LIBs. • PIE separators exhibit superior LIB capability and cycling stability.

Boosting safety and performance of lithium-ion battery enabled by cooperation of thermotolerant fire-retardant composite membrane and nonflammable electrolyte

• Thermotolerant and fire-retardant nano-CaCO 3 -based membrane was designed. • LIBs with nonflammable electrolyte-immersed membrane possess superior cyclability. • LIBs exhibit better safety by preventing short circuit and reducing heat release. • Lithium ion full battery shows good long cycling stability at 120 °C under 2C. Developing nonflammable electrolytes is regarded as a convenient strategy to solve the combustion problem of lithium-ion batteries (LIBs), but its influences are limited by the severe shrinkage and high combustibility of polyolefin separators at elevated temperatures. Herein, an intrinsically thermotolerant and fire-resistant nano-CaCO 3 -based composite membrane (CPVH) is designed rationally. The CPVH membrane presents superior thermal stability (barely visible shrinkage at 300 °C), excellent nonflammability and extremely low heat release (∼11% as much as PP). Moreover, the alkaline nano-CaCO 3 can neutralize hydrofluoric acid that inevitably exists in LiPF 6 -based electrolytes, guaranteeing the long-term stability of interfacial layers. Noticeably, LiFePO 4 /Li battery with CPVH membrane delivered a discharge capacity of 133.6 mAh g −1 at 0.5C, along with a capacity retention of 93.0% and a coulombic efficiency of 99.9% after 650 cycles. Additionally, harsh safety tests of the battery demonstrate stable circuit safety at a high temperature of 150 °C and a significantly low heat release during thermal runaway. Significantly, the LiFePO 4 /CPVH/graphite full battery showed good cycling stability with a capacity retention of 78.4% and an average coulombic efficiency of 98.0% at 120 °C and 2C after 100 cycles. By combining CPVH composite membrane and nonflammable electrolyte, both safety and cycling performance of LIBs were improved completely.

Robust Room‐Temperature Sodium‐Sulfur Batteries Enabled by a Sandwich‐Structured MXene@C/Polyolefin/MXene@C Dual‐functional Separator

Room-temperature sodium-sulfur (RT-Na-S) batteries are attracting increased attention due to their high theoretical energy density and low-cost. However, the traditional RT-Na-S batteries assembled with glass fiber (GF) separators are still hindered by the polysulfide shuttle effect and sodium dendrite growth, limiting the battery's capacity and cycling stability. Here, a facile and effective method toward commercial polyolefin separators for constructing stable RT-Na-S batteries is presented. By coating commercial polypropylene membrane with core-shell structured MXene@C nanosheets, a powerful dual-functional separator with improved electrolyte wettability that can inhibit polysulfide migration and induce uniform sodium disposition is developed. More importantly, the modified separator can also accelerate the conversion kinetics of sodium polysulfides. Benefiting from these characteristics, the as-prepared RT-Na-S battery exhibits a remarkably enhanced capacity (1159 mAh g-1 at 0.2 C) and excellent cycling performance (95.8% of capacity retention after 650 cycles at 0.5 C). This study opens a promising avenue for the development of high-performance Na-S batteries.

Thermal-triggered fire-extinguishing separators by phase change materials for high-safety lithium-ion batteries

High-energy lithium-ion batteries face significant challenges at abuse conditions, where thermal runaway is easily triggered and always accompanied with fire and explosion. Here we report a novel separator design that simultaneously absorbs thermal energy within the cell and improves fire safety. A thermo-responsive composite separator is fabricated by coating the commercial polyolefin separator with ceramic silica microcapsules that are internally encapsulated with both a phase-change material (PCM) and a flame retardant. As a “smart gatekeeper”, the PCM can not only store heat within the cell to minimize the temperature rise, but control the thermal-stimuli release of the flame retardant. In particular, the electrochemical performance of batteries is not sacrificed, taking advantages of the elaborate encapsulation of flame retardant and PCM inside a protective silica microcapsule and avoiding the direct dissolution of them into the electrolyte. Upon thermal stimuli, the solid-liquid transition of PCM occurs to increase molecular mobility and release the flame retardant. The low melting point of PCM could facilitate the prompt release of flame retardant at the early stage of thermal runaway, avoiding short circuit caused by thermal inertia. The PCM-based heat-regulated release system for flame retardant effectively reduces the safety risks caused by thermal runaway, which holds great promise as multifunctional separators for high-safety lithium-ion batteries.

Silkworm Cocoon Layer with Gradient Structure as Separator for Lithium‐Ion Battery

Structural design of separator is becoming a strategy as attractive as material innovation for improving lithium‐ion batteries (LIBs) performance. Silkworm cocoon displays a very unique multilayer gradient structure in the thickness direction, but how the gradient structure in every single layer affects its separator properties is still unknown. Herein, the cocoon is initially peeled into various layers, and the differences in the composition, porosity, and packing density of each layer confirm the multilayer gradient structure of the cocoon. The thermal, puncture, wetting, and electrochemical properties of every layer are then investigated, and the relationship between structure and property is further analyzed. The performance is greatly dependent on the number of peeled layers and the layer position in the cocoon. Particularly, the puncture strength of the layered cocoon is better than the polyolefin separator and increases from the inner to the outer layer, while the wettability decreases. Moreover, the electrochemical performance of the cocoon layer is better than the unlayered cocoon with larger capacity retention. Therefore, it is hoped that such a study on the gradient structure and property of cocoon layers could provide inspiration for the separator design research focusing on the high safety and high performance for LIBs development.

A High Performance Polyacrylonitrile Composite Separator with Cellulose Acetate and Nano-Hydroxyapatite for Lithium-Ion Batteries.

The traditional commercial polyolefin separators suffer from high-temperature thermal shrinkage, low electrolyte wettability and other issues. In order to improve the overall performance of the separators, electrostatic spinning technology was applied to obtain PAN nanofiber separators with an average diameter of 320 nm. Then cellulose acetate (CA) resin and nano-hydroxyapatite (HAP) were introduced to fabricate the PAN/CA/HAP composite separators through the constant temperature hot pressing and dip-coating crafts. The composite separator has a good thermal stability, with no significant dimensional change after a constant temperature treatment of 200 °C for 35 min. The electrolyte uptake rate of the PAN/CA/HAP-1.0 composite separator reaches 281%, which exhibits an efficient ionic conductivity. At the same time, it also attains a tensile strength of 11.18 MPa, which meets the requirement for separator use. Button cells assembled from PAN/CA/HAP-1.0 composite separators have an excellent rate of performance (160.42 mAh·g−1 at 0.2 C) and cycle capability (157.6 mAh·g−1 after 50 cycles at 0.5 C). The results support that lithium-ion batteries assembled with PAN/CA/HAP-1.0 composite separators will exhibit higher safety stability and better electrochemical performance than that of polyolefin separators, with a very immense potential for application.

Morphology optimizing of polyvinylidene fluoride (PVDF) nanofiber separator for safe lithium‐ion battery

AbstractCommercial polyolefin separator cannot guarantee the safety of lithium‐ion batteries at high temperatures due to their poor thermal stability. All kinds of nanofiber membranes with high thermal stability become the focus. In this paper, we have prepared PVDF nanofiber membrane by electrospinning method and have studied the relationship between the electrochemical performance and nanofiber morphology in detail. It is found that the morphology of PVDF is transformed from ball‐like to various nanofiber with increasing the concentration of spinning solution. As a result, the PVDF nanofiber membrane (24 wt%‐HP) prepared from spinning solution with the concentration of 24 wt% is composed of coarse nanofiber and fine nanofiber, which exhibits better electrochemical properties, such as cycling stability (300 cycles) and ionic conductivity (1.65 mS cm−1). LiCoO2/Li cells based on PVDF nanofiber separators have higher discharge capacity even at the current density of 5 C‐rate, superior than the cells based on PE separator. It is inspiring for high‐energy‐density battery safety to use this PVDF nanofiber separator.

Talcum-doped composite separator with superior wettability and heatproof properties for high-rate lithium metal batteries

Separator is supposed to own outstanding thermal stability, superior wettability and electrolyte uptake, which is essential for developing high-rate and safe lithium metal batteries (LMBs). However, commercial polyolefin separators possess poor wettability and limited electrolyte uptake. For addressing this issue, we put forward a composite separator to implement above functions by doping layered-silicate (talcum) into polyvinylidene fluoride (PVDF). With significant improvement of electrolyte absorption benefiting from the strong adsorption energy values (-1.64 ∼ -1.70 eV) between talcum and the electrolyte in lithium metal batteries, PVDF/Talcum (PVDF/TM) composite separator owns a small contact angle and superior electrolyte uptake. PVDF/TM composite separator with 10 wt% talcum (T-10) owns a tiny contact angle of 8°, while those of polypropylene (PP) and PVDF are 48° and 20° with commercial electrolyte. Moreover, the addition of thermotolerant talcum endows the T-10 composite separator with great thermostability, whose thermal shrinkage is only 5.39% at 150 °C for 0.5 h. The cell with LiFeO4 cathode and the T-10 composite separator reaches 91.7 mAh/g in discharge capacity at 4.8 mA/cm2 (10 C), far superior to that with pure PVDF separator (56.3 mAh/g) and PP (51.4 mAh/g).

A Critical Review on Materials and Fabrications of Thermally Stable Separators for Lithium‐Ion Batteries

AbstractLi‐ion batteries nowadays are widely used as energy storage systems owing to their high power and energy densities. However, the safety of Li‐ion batteries has increasingly become a grave concern, and how to effectively mitigate or avoid the thermal runaway of Li‐ion batteries, particularly during fast charging/discharging, is a critical issue. It has been pointed out that among various factors governing the safety of Li‐ion batteries, the thermal stability of separators plays an important role. In this review, a critical discussion is reported on recent research and development of thermally stable separators, including surface modified polyolefin separators, novel polymer separators, inorganic structured separators, and functional smart separators, for improving the safety of Li‐ion batteries, summarizing different techniques, processes, and materials being researched and developed for these separators for lithium‐ion batteries. Finally, suggestions are provided on future directions to develop thermally stable separators with respect to the safety of next‐generation Li‐ion batteries.

Poly(ethylene oxide)-ethylene carbonate solid binary electrolyte with higher conductivity, lower operating temperature and fully impregnated separator for all solid-state lithium ion batteries

Poly(ethylene oxide) (PEO) solid electrolyte suffers from the low conductivity and the high operating temperature owing to the cation is trapped in the highly crystalline domains and enmeshed in amorphous domains. Therefore, a tactic was attempted by forming the solid binary electrolyte with the solid cosolvent, ethylene carbonate (EC). X-ray diffraction revealed the crystallinity of the electrolyte progressively degraded at room temperature as EC was added, eventually disappeared, accompanying the endothermic peak continuously shifted to the lower temperature and then vanished in differential scanning calorimetry. Fourier transform infrared spectroscopy also indicated the surroundings around PEO chains mutated after addition of 50% EC (in EO ratio). More importantly, the conductivity was significantly improved ten times after 60% EC was added, still three times at 25 °C than the pristine electrolyte at 60 °C, which was further corroborated with cyclic voltammogram of LiFePO4 micro electrode. Based on the rate discharge, no less than 60% EC content was recommended for the application in the medium rate (<2C) at 25 °C. Additionally, another barrier, the poor wettability of polyolefin separator with the solid state electrolyte was also overcome completely by the facile surface modification.

Simultaneously Blocking Chemical Crosstalk and Internal Short Circuit via Gel-Stretching Derived Nanoporous Non-Shrinkage Separator for Safe Lithium-Ion Batteries.

The separator, an ionic permeable and electronic insulating membrane between cathode and anode, plays a crucial role in the electrochemical and safety performance of batteries. However, commercial polyolefin separators not only suffer from inevitable thermal shrinkage at elevated temperature, but also fail to inhibit the hidden chemical crosstalk of reactive gases such as O2 , leading to often reported thermal runaway (TR) and hence preventing large-scale implementation of high-energy-density lithium-ion batteries. Herein, a nanoporous non-shrinkage separator (GS-PI) is fabricated via a novel gel-stretching orientation approach to eliminate TR. In situ synchrotron small angle X-ray scattering during heating clearly shows that the as-prepared thin GS-PI separator exhibits superior mechanical tolerance at high temperature, thus effectively preventing internal short circuit. Meanwhile, the unique nanoporous structure design further blocks chemical crosstalk and the associated exothermic reactions. Accelerating rate calorimetry tests reveal that the practical 1 Ah LiNi0.6 Co0.2 Mn0.2 O2 (NCM622)/graphite pouch cell using GS-PI nanoporous separator show a maximum temperature rise (dT/dtmax ) of only 3.7°C s-1 compared to 131.6°C s-1 in the case of Al2 O3 @PE macroporous separator. Moreover, despite the reduced pore size, the GS-PI separator demonstrates better cycling stability than conventional Al2 O3 @PE separator at high temperature without sacrificing specific capacity and rate capability.

Dendrite-suppressing separator with high thermal stability by rod-like ZnO coating for lithium batteries

The application of lithium (Li) metal as an anodic electrode for potential use in Li batteries has of late attracted much attention. This is because of its higher theoretical energy capacity (3860 mAh g−1) and negative potential (−3.04 V vs. standard hydrogen electrode (SHE)). However, Li metal anode possesses its own problem and chief being the formation of Li dendrites. Acuminate Li dendrites can pierce through the polyolefin separator causing the battery to short and this can potentially lead to fire. To buttress this problem, we synthesized and applied rod-like zinc oxide (ZnO) nanoparticles as a coating material on separators for suppressed dendrite formation in Li metal batteries. The cells with the coated separator showed improved electrochemical performance with an 83.4% capacity retention compared to the pristine separator with 65.5% after 100 cycles. This was due to suppressed dendrite formation. These results show that rod-like ZnO nanoparticle is a potential candidate for use as a coating material for separators in Li metal batteries.

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.

Manufacturing Processes of Microporous Polyolefin Separators for Lithium-Ion Batteries and Correlations between Mechanical and Physical Properties

Rechargeable lithium-ion batteries (LIBs) have emerged as a key technology to meet the demand for electric vehicles, energy storage systems, and portable electronics. In LIBs, a permeable porous membrane (separator) is an essential component located between positive and negative electrodes to prevent physical contact between the two electrodes and transfer lithium ions. Among several types, microporous polyolefin membranes have dominated the commercial separator market for LIBs operated with liquid electrolytes, favored for their chemical and electrochemical stability, high mechanical strength, uniform pore size, and inexpensive manufacturing and materials cost. In this review, we summarize the principles and theoretical background underlying conventional manufacturing processes and newly emerging microporous polyolefin separators. Based on their mechanical and physical properties, as collected from the literature, we introduce a number of processing type-dependent characteristics and universal correlations among their properties. This will provide a macroscopic view on the subject and a guideline for the development of next-generation separators.

Inhibition of Lithium Dendrite Formation in Lithium Metal Batteries via Regulated Cation Transport through Ultrathin Sub‐Nanometer Porous Carbon Nanomembranes (Adv. Energy Mater. 29/2021)

Lithium Metal Batteries In article number 2100666, Andrey Turchanin, Leela Arava and co-workers incorporate a nanometer thick two-dimensional carbon nanomembrane onto a commercial polyolefin separator to inhibit the propagation of lithium dendrites in lithium metal batteries. The sub-nanometer porous membrane regulates the lithium-ion transport, creating a uniform lithium flux that prevents dendrite nucleation. Cover art by Ilia Ghiasedin.

High-efficient multifunctional electrochemical membrane for lithium polysulfide redox flow batteries

Redox flow batteries (RFBs) based on lithium polysulfide (Li-PS) chemistry present great opportunities for large-scale energy storage and electric vehicles because of their use of abundant raw materials and their higher energy density compared with traditional flow batteries. However, to successfully implement Li-PS RFBs, issues related to the crossover of PS species through a membrane separator must be resolved. In this work, we demonstrate a facile method for fabricating a novel multifunctional electrochemical membrane (mECM) consisting of an organic ion exchange membrane reinforced with a porous carbon nanotube layer and a boron nitride layer. This rational design endows the membrane with remarkable ion selectivity and dimensional stability in organic electrolyte, leading to a greatly enhanced Li+/PS ion selectivity, which exceeds that of a commercial polyolefin separator (i.e., Celgard 2325) by three orders of magnitude. A Li-PS RFB with the mECM exhibited stable electrochemical performance (0.05% capacity decay per cycle after 40 cycles) with 78% capacity retention over 100 cycles at 0.75C, while a reference cell with a Celgard 2325 membrane rapidly lost its capacity (0.33% capacity decay per cycle and 33% capacity at 100 cycles). Our results strongly suggest that the mECM with its high Li+/PS ion selectivity is a promising membrane separator for developing high-performance Li-PS RFB systems.