Commercial Polyethylene Separator Research Articles

Ceramic Nanoparticle-Decorated Melt-Electrospun PVDF Nanofiber Membrane with Enhanced Performance as a Lithium-Ion Battery Separator.

Designing a composite separator that can withstand high temperature, deliver high capacity, and offer fast charge–discharge capability is imperative for developing a high-performance lithium-ion battery. Here, a series of ceramic nanoparticle-coated nanofiber membranes, including Al2O3/poly(vinylidene fluoride) (PVDF), SiO2/PVDF, and Al2O3/SiO2/PVDF, were prepared by melt-electrospinning and magnetron sputtering deposition. Among all of these composite separators, Al2O3/SiO2/PVDF showed several advantages including excellent thermal stability (no dimensional shrinkage at temperature up to 130 °C and an onset degradation temperature of 445 °C) and superb electrolyte compatibility (340% electrolyte uptake). In addition, the β phase of the fibrous PVDF membrane as well as the presence of polar ceramic nanoparticles on the fiber surface can synergistically improve the ion conductivity to 2.055 mS/cm at room temperature, which is more than 8 times higher than that of the commercial polyethylene (PE) separator. Performance of these ceramic nanoparticle-coated separators in a lithium-ion battery demonstrated an improved discharge capacity of 161.5 mAh/g and more than 84.3% capacity retention rate after 100 cycles. The ceramic nanoparticle-coated PVDF separators also maintained 58.4% capacity at a high current density of 8C, which is better than the 49.8% capacity for the commercial PE separator. Therefore, the ceramic nanoparticle-coated PVDF membrane proves to be a promising separator for a high-power and more secure lithium-ion battery.

Highly safe lithium-ion batteries: High strength separator from polyformaldehyde/cellulose nanofibers blend

As a pivotal part of lithium-ion batteries, separator is supposed to have high strength, thermal stability and excellent wettability. In this study, polyformaldehyde/cellulose nanofibers blend separators are firstly prepared via thermally induced phase separation to improve the performance, especially the safety of LIBs. The polyformaldehyde/cellulose blend separators show considerable tensile strength (116 MPa) and Young's modulus (6.07 GPa) owing to the high crystallinity and high performance of polyformaldehyde and cellulose. In particular, benefiting from their abundant polar groups and highly porous structure, the polyformaldehyde/cellulose blend separators possess high electrolyte uptake (412%) and small contact angle (19°) as compare to the commercial polyethylene separator (115%, 58°). Moreover, the thermal treatment tests indicate that the polyformaldehyde/cellulose blend separators are thermally stable at as high temperature as 180 °C. And the polyformaldehyde/cellulose blend separators show higher ionic conductivity (1.39 mS cm−1) and lower interface resistance (49.5 Ω) than polyethylene separator (0.76 mS cm−1, 136.4Ω). As expected, the LiFePO4/Li cell with polyformaldehyde-20-cellulose blend separators exhibit the best stable cycling performance and improved rate performance than PE separator, especially at high rate. In summary, the polyformaldehyde/cellulose blend separators are promising new kind of separator for LIBs with high safety and enhanced performance.

Novel Poreless Inorganic Separator for Blocking Polysulfide Shuttle Effect in Lithium Sulfur Battery

Lithium ion batteries (LIBs) have been used in portable electronic devices due to high energy density, rechargeable characteristics, and tiny memory effect. However, advanced portable electronic device, electric vehicles (EVs), and large scale energy storage system (EES) require high specific capacity (energy density), light weight, and low cost LIBs. One of the important factors in determining the specific capacity of LIBs is the cathode active material. During the decades, general cathode active materials (lithium, cobalt, nickel, etc.) have some issue of rapid price rise as the demand increment for cathodic materials of EVs and EES. Especially, cobalt, which is the most essential cathode active material, the Democratic Republic of the Congo (DRC) accounts for more than 50% of the world's production. Cobalt supply is very unstable due to civil war in the DRC and exploitation of labor in the process of cobalt production. Therefore, a new cathode active material is necessary. Lithium sulfur (Li-S) batteries use an elemental sulfur as cathode and lithium metal as anode. The elemental sulfur is one of the most abundant materials on earth and has a high theoretical capacity. The theoretical capacity of 1675 mAh/g is up to seven times higher than that of LIBs. And elemental sulfur also has high theoretical gravimetric energy density of 2500 Wh/kg which is over four times higher than the ideal state of common LIBs. Additionally, sulfur is a stable and cost affordable material. Hence, the Li-S battery has attracted attention as a next-generation battery, and much researches have been reported. However, there are some problems that commercialization. The first problem is the dissolution of the polysulfide. The intermediate lithium polysulfides which was formed during the charge and discharge process are soluble in the typical organic electrolytes for LIBs. This phenomenon is called “polysulfide shuttle effect”. This leads to continuous cathode active material consumption, eventually the capacity is faded during cycle. The second problem is the safety and stability of the lithium metal used as the anode. Lithium metal surface control is a necessary step not only for the stability of Li metal anode, but also for a high capacity with a high coulombic efficiency. In this study, we developed poreless poly (vinylidene fluoride-co-hexafluoropropene) (PVdF-HFP) coated anodic aluminum oxide (PAAO) separator to blocking polysulfide shuttle effect and to control lithium metal interface. PAAO was prepared by coating PVdF-HFP solution on anodic aluminum oxide (AAO). AAO provides a highly ordered vertical pore structure and AAO has less tortuosity than conventional separator. The vertical pore structure of AAO evenly distributes the Li ion flux in battery system, so proposed separator effectively prevents the ununiformed growth of lithium. It is also mechanically, chemically, and thermally stable. The vertical aligned pores of AAO are filled with PVdF-HFP which has been used as gel polymer electrolyte, offer high lithium ion conductivity with conventional electrolyte. PVdF-HFP selectively passes lithium ions and effectively blocks dissolution of the polysulfide into the anode region. Consequently, the proposed system conserved high specific capacity with high coulombic efficiency during the cycle. Prepared PAAO showed high specific capacity (∼850 mAh/g, 0.2C), high coulombic efficiency (>98%), and long cycle life (>100 cycles, 0.5C). Reference polyethylene separator shoed low specific capacity (∼550 mAh/g, 0.2C), low coulombic efficiency (~95%) and fast capacity fade during cycle. Compared with a commercial polyethylene separator and bare AAO separator, PAAO separator showed better initial capacity, coulombic efficiency, and electrochemical stability. The suggested PAAO separator effectively block the polysulfide shuttle effect and improve the poor cyclability of the LI-S system. It suggests that this novel poreless separator can be a possible candidate of the powerful separator for LI-S batteries.

Lithium ion battery separator with high performance and high safety enabled by tri-layered SiO2@PI/m-PE/SiO2@PI nanofiber composite membrane

In the current study, a novel needleless electro-spinneret based on the Von Koch curve of Fractal Theory is developed to prepare electrospun membrane. A sandwich-like composite separator is elaborately designed with two layers of SiO2 nanoparticles doped polyimide membranes being the sheath layers and a single layer of ethylcellulose modified PE membrane being core layer towards its application for separators in high-performance lithium-ion batteries. This tri-layered composite separator combines both thermal shutdown function at low temperature and thermal runaway function at high temperature, where ethylcellulose is used to enhance the interfacial affinity and strength between commercial polyethylene separator and the electrospun polyimide membrane. The two sheath layers of electrospun membrane prepared by SiO2 nanoparticles doped polyimide impart the separator with high thermal stability and high-temperature runaway function at, and the single core layer of ethylcellulose modified polyethylene membrane provides high mechanical property and low-temperature shutdown function. The resultant tri-layered composite membrane is of high potential as lithium-ion battery separators for electrically powered vehicles.

Enhancement of electrochemical performance of lithium-ion battery by single-ion conducting polymer addition in ceramic-coated separator

Single-ion conducting polymers have been widely reported in the literature as solid polymer electrolytes, but their low ionic conductivity has limited industrial applications at ambient temperature. Here, we employed a perfluoroalkyl sulfonamide-based single-ion conducting polymer-lithiated poly(perfluoroalkylsulfonyl)imide (LiPFSI) to promote the migration of free Li-ions and diminish cell polarization in lithium-ion batteries. After blending with Al2O3 powder, the LiPFSI/Al2O3 composite was coated on a commercial polyethylene separator. Adding the high surface energy of Al2O3 particles and the exceptional ionic conductivity of LiPFSI resulted in a LiPFSI/Al2O3 composite-coated separator with excellent wettability and low impedance. A LiFePO4/Li half-cell with this separator showed a highly improved charge–discharge cyclability up to 130 mAh/g that maintained 98% retention of the original reversible capacity after 220 charge–discharge cycles at a high current rate of 2 C (1 C = 170 mAh/g). Even at a high rate of 5 C, the cell capacity could be maintained above 100 mAh/g. Herein, we present a simple and effective method to optimize the separator with the LiPFSI/Al2O3 composite and thus improve the high rate charge–discharge performance of Li-ion batteries.

Improving Ionic Conductivity and Lithium-Ion Transference Number in Lithium-Ion Battery Separators.

The microstructure of lithium-ion battery separators plays an important role in separator performance; however, here we show that a geometrical analysis falls short in predicting the lithium-ion transport in the electrolyte-filled pore space. By systematically modifying the surface chemistry of a commercial polyethylene separator while keeping its microstructure unchanged, we demonstrate that surface chemistry, which alters separator-electrolyte interactions, influences ionic conductivity and lithium-ion transference number. Changes in separator surface chemistry, particularly those that increase lithium-ion transference numbers can reduce voltage drops across the separator and improve C-rate capability.

Communication—Technique for Visualization and Quantification of Lithium-Ion Battery Separator Microstructure

Separators play an important role in lithium-ion battery operation; however, no comprehensive studies of their microstructure and its impact exist. To enable such studies, we present a simple method for separator microstructure visualization and quantification based on focused-ion-beam scanning electron microscopic tomography. Here, we use this approach to visualize a sample of commercial polyethylene separator, calculate its directional effective transport parameters, and explain the impact of these values on battery performance. We further extend this technique to visualize metallic deposition within the separator, which could facilitate the study of lithium plating and dendritic growth.

Preparation and electrochemical performance of ZrO2 nanoparticle-embedded nonwoven composite separator for lithium-ion batteries

To improve the safety of lithium-ion batteries, a high performance composite separator based on ZrO2 nanoparticles and poly (ethylene terephthalate) nonwoven was successfully prepared. Systematical investigations including morphology characterization, porosity measurement, electrolyte wettability, thermal shrinkage testing, and cell performance examination were carried out. The results show that ZrO2 nanoparticles and nonwoven substrate endow the composite separator with outstanding thermostability, which may lead to improved safety of the batteries. Meanwhile, the composite separator possesses unique porous structure formed between ZrO2 particles. Owing to the relatively polar constituents and well-developed microstructure, the composite separator exhibits higher air permeability and porosity, as well as superior electrolyte wettability compared to the commercialized polyethylene separator. Based on the above advantages, this composite separator shows better electrochemical properties, such as the discharge C-rate capability and cycling performance, as compared to the polyethylene separator. This research presents a simple approach to prepare high performance separator, which is demonstrated to be a good candidate for lithium-ion batteries.

Battery performance of PMMA-grafted PE separators prepared by pre-irradiation grafting technique

Poly(methyl methacrylate)-grafted polyethylene (PE-g-PMMA) separators were prepared by pre-irradiation grafting technique of methyl methacrylate onto a commercial polyethylene separator. The prepared separators were characterized by using charge/discharge (C/D) cycling test, AC impedance, and thermal stability analyses. Thermal shrinkage (TS) of the PE-g-PMMA separators decreased with an increasing degree of grafting up to 70% above which it was saturated. The PE-g-PMMA separators showed a better oxidation stability on the anode up to 5 V and a better cycle life performance than the original PE separator. These characteristics make the prepared separators suitable for applications in high voltage secondary lithium batteries.