Li-ion Battery Separators Research Articles

Comprehensive crosslinking strategy using fluorinated azide to enhance the thermal stability of ceramic-coated separators for Li-ion batteries

Enhancing the thermal and mechanical stabilities of ceramic-coated separators (CCSs) is required for high-safety lithium-ion batteries (LIBs). As both the thickness and areal mass of ceramic coating layers (CCLs) have reduced, various strategies have been developed to overcome the limitations of conventional ceramics and polymeric binders. In this regard, we suggest a comprehensive photocrosslinking approach to form fully crosslinked CCLs using a new fluorinated azide binder that can readily react with aliphatic functional groups in any component of CCSs. When a small amount of azide binder is added to a CCL slurry consisting of Al2O3 particles and a poly(acrylic acid) (PAA) binder, short UV irradiation can lead to inter- and intramolecular crosslinking points within the polyethylene (PE) separator, between the PE separator and the PAA binder, and within the PAA binders in the CCL. Consequently, the thermal stability and the adhesive strength of crosslinked CCS (X-CCS) can be significantly improved compared to the control CCS without azide. Moreover, a LiNi0.6Co0.2Mn0.2O2/graphite cell with X-CCS exhibited a comparable cycle life to that of the PE separator and non-crosslinked CCS. Furthermore, because UV illuminators are easily added to conventional fabrication processes, this strategy is a simple but effective approach for advancing high-safety LIBs.

Recent progress of advanced separators for Li-ion batteries

Recent progress of advanced separators for Li-ion batteries

Enhancing CA-based separators with thermo-responsive ionic liquids: A path to eco-friendly membrane production and multifaceted applications

We synthesized a temperature-responsive ionic liquid, [N4444][SS], and incorporated it into an environmentally friendly cellulose acetate (CA)-based battery separator. A pore was formed in the battery separator by [N4444][SS], which pierced a plasticized part due to water pressure. Varying drying temperatures during membrane fabrication revealed that the CA/[N4444][SS] membrane dried at 50 °C exhibited greater thickness and a smaller average pore size, resulting in an asymmetric internal structure. Despite the asymmetry, this membrane demonstrated significantly higher water flux and a lower Gurley value compared to the membrane dried at 25 °C, indicating minimal tortuosity and low resistance within the internal pores. Thermal behavior analysis through TGA and DSC, as well as FT-IR spectroscopy, confirmed that [N4444][SS] remains within the CA matrix, forming coordinative bonds. The findings suggest that the CA/[N4444][SS] membrane, when used as a Li-ion battery separator, could enhance Li-ion transport properties and conductivity. Moreover, the recyclability of the IL in the membrane fabrication process contributes to a more environmentally friendly approach.

One-pot simple and highly thermally stable PI-film separators for Li-ion batteries

Separators play a vital role in the battery industry, particularly Li-ion battery separators, and should be thermally and chemically stable to prevent internal short circuits for safety and ionic conductivity. In this study, a polyimides-film separator (PI-FS) was synthesized using a simple one-pot method, and its thermal and chemical stabilities were studied. The results of this study were compared with those of Celgard 2400 and PP-PE-PP separators. The PI-FS exhibited superior thermal stability in the EC-DMC electrolyte than in the EC-DEC and other commercial separators.

Tuneable and efficient manufacturing of Li-ion battery separators using photopolymerization-induced phase separation

In an effort to increase the thermomechanical stability of lithium-ion battery separators, thermoset membranes (TM) are a viable alternative to commercial polyolefin separators. We present an efficient and scalable method...

Projection Micro Stereolithography of Skin Layered Microporous Polymer Membranes As a Separator for Li-Ion Batteries

Commercial liquid electrolyte lithium-ion batteries (LIBs) traditionally incorporate microporous polymer membranes, or separators, to isolate electrode contact and enable ionic transport. Although they do not actively participate in cell chemistry, well-designed separators can influence cell performance and safety, and thus, are essential components of a cell. Current separator manufacturing processes include "wet" or "dry" mechanical stretching , phase inversion, and evaporation-induced phase separation. However, these processes have limited morphological control of geometry and layered structures. Additive manufacturing techniques have been used to rapidly prototype materials that have both high customizability and fine microstructural control. We investigate hexanediol diacrylate (HDDA) as a polymer system using projection micro-stereolithography, a layer-by-layer photopolymerization technique, to develop separators with controlled microstructures for Li-ion batteries. We successfully fabricate a unique polymer architecture of alternating nanometer thin skin layers and microporous network. The fine control of printing parameters (i.e., printing platform, skin layers, and exposure time), results in tunable porosity and morphology. This unique approach to manufacturing LIB separators provides hierarchical homogeneity for more uniform Li-ion transport. It’s potential as a LIB separator is explored by characterizing its electrolyte wettability, thermal tolerance, electrochemical properties, and mechanical stability. Finally, lithium symmetric cells using the HDDA microporous membranes are evaluated against Celgard 2325 separators. This work provides a promising new route for developing separators using additive manufacturing techniques to enable high performance Li-ion batteries.

Polysiloxane-Coated PI Nonwoven Separators with Higher Thermal and Electrochemical Stability for Lithium Ion Battery Application

Polyolefin separators are widely used for most of commercial application owing to their excellent mechanical properties, electrical insulation, and internal porous structures. The raw polyolefin materials have economic viability and possess facile mechanical extension suitable for processing as a film [1]. However, the inherent thermal property of polyolefin inevitably results in the severe thermal deformation of the separators upon the elevation of LiB temperature that has been pointed out as a major cause of fire in LiBs with the possibility of short circuits caused by lithium dendrites [2]. There have been trials to secure cycle characteristics by preventing LiB separators from shrinkage. But, even the coated polyolefin separator couldn’t have thermal stability at high temperatures above 180 oC, since the polyolefin was the base film material.Many studies have been performed to achieve high temperature stability with engineering plastics, including polyimide (PI) [1]. As PI has superior thermal stability, it has been widely applied as nonwoven separators. PI nonwoven fabric separators exhibit stability usually above 200 oC. As PI nonwoven separators have also high ionic conductivity and good electrolyte wettability, it is therefore expected to show excellent rate capability and cyclability [3]. Despite those advantages, however, PI nonwoven separators still have several issues to resolve before application of them to LiBs. One of the problems is the existence of large pores [4]. Pores over a certain size is thought to render particles in cathode active materials pass through or Li dendrites penetrate upon overcharging. Then, it can result in leakage current and internal short circuit eventually. Various studies such as particle-coating or ceramic-coating have been studied in order to figure out this problem [26].Other than pore-size distribution or existence of large pores, it must be the electrochemical stability of PI nonwoven separator and the electrochemical cell performance with the separator at high voltage that needs to be studied and examined for the potential application to LiBs [3]. In accordance with the strong demand on high power LiB development, study on the electrochemical resistance of materials and components is highly required [5]. As PI has polar chemical structure in contrast to non-polar chemical structure of polyolefin, it would be worthy to investigate the electrochemical nature and stability under high current or high voltage. In this study, a PI nonwoven separator is modified by coating of polysiloxane to improve its electrochemical stability and properties as well as its porous structure. As the coated PI nonwoven separator should be stable up to 200 oC due to the robust PI frame and the modification via polysilicon-coating is rather simple, the feasibility of this chemical approach is demonstrated and studied including its electrochemical behaviors. Ionic conductivity, electrolyte wettability, and gas permeability of the modified separator is also examined as a set of tests for LiB application. It was electrochemically stable during LSV even at 5 V vs Li+/Li. The polysiloxane-coating maintains or improves the excellent thermal and electrical stability of the PI nonwoven separators. The full cell test demonstrated that the polysiloxane-coating enabled a cyclability of 98.6% after 100 cycles, while the PI nonwoven could not be charged due to an internal short circuit.[1] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci, 7, 2014, 3857.[2] X. Zhang, E. Sahraei, K. Wang, Li-ion Battery Separators, Mechanical Integrity and Failure Mechanisms Leading to Soft and Hard Internal Shorts, Sci. Rep., 6, 2016, 32578.[3] Z. Lu, F. Sui, Y. Miao, G. Liu, C. Li, W. Dong, J. Cui, T. Liu, J. Wu, C. Yang, Polyimide separators for rechargeable batteries, J. Energy Chem., 58, 2021, 170.[4] G. Dong, B. Liu, G. Sun, G. Tian, S. Qi, D. Wu, TiO2 nanoshell@polyimide nanofiber membrane prepared via a surface-alkaline-etching and in-situ complexation-hydrolysis strategy for advanced and safe LIB separator, J. Membr. Sci., 577, 2019, 249.[5] Y. Xiang, J. Li, J. Lei, D. Liu, Z. Xie, D. Qu, K. Li, T. Deng, H. Tang, Advanced Separators for Lithium-Ion and Lithium-Sulfur Batteries: A Review of Recent Progress, ChemSusChem, 9, 2016, 1. Figure 1

Multifunctional Separator for Li-Ion Batteries Enabling Acid Scavenging and Long Cyclability

The membrane separator is indispensable in lithium-ion batteries (LIBs), electrically isolating the electrodes while hosting liquid electrolyte for facile ion transport. Microporous polyolefin materials, consisting of polyethylene (PE) and/or polypropylene (PP), represent the state-of-the art in separator technology. They offer sufficient mechanical properties, chemical inertness, effective pore sizes <100 nm, porosity ranging from 30-78%, and relatively low costs. However, safety issues associated with thermal shrinkage above 70℃ and poor electrolyte wettability are significant limitations. Replacing these conventional polyolefin separators with multifunctional microporous membranes provides opportunities to address these limitations and other challenges in lithium ion batteries. LiMn2O4 cathodes, for example, attract widespread interest due to the abundance and low cost of Mn, especially in comparison to Co, and reasonable energy densities. However, these cathodes do not provide the calendar and cycle life of the layered oxide cathodes, especially at elevated temperatures (>50°C), due to dissolution of Mn and structural instabilities.1 One of the underlying reasons is the presence of HF in commercial LiPF6-carbonate electrolytes; HF will attack and dissolve the Mn, leading to severe capacity decay, as shown in Figure 1.2 Chemically active multifunctional separator can scavenge HF from the electrolyte, improving the stability of LiMn2O4, but they must be fabricated by low cost, scalable processes without compromising other separator properties.Herein, a multifunctional separator will be described, wherein poly(4-vinylpyridine) (P4VP) is incorporated into separators prepared by polymerization induced phase separation (PIPS). In PIPS, a homogenous solution of difunctional monomers and porogen is polymerized; the difunctional monomers polymerize and crosslink while the porogen phase separates into nanoscale domains. Our previous study employed PIPS to prepare highly porous and thermally stable separators, exhibiting comparable ion transport to conventional Celgard 2500 Polypropylene (PP) separators and negligible thermal shrinkage at 150 °C (digital images in Figure 1).3 Moreover, when incorporated into LIB half cells, these separators enable better electrochemical performance than Celgard 2500 separators due to their reduced thicknesses and superior electrolyte uptake. In the study, P4VP is integrated with the PIPS process to scavenge both HF and PF5 (precursor to HF) through acid-base interactions. A thin layer of P4VP coating was fabricated via initiated chemical vapor deposition (iCVD) process and makes full use of high porosity of separators due to the conformal nature of the coating, increasing the surface area to scavenge the acids. The P4VP-coated PIPS separator enables better capacity retention in Mn-based cathode materials, relative to cells using inert polyolefin separators. The scavenged concentration of HF and transition metal will be compared to validate the HF scavenging property. Post-mortem analysis of the Li anode also confirms the suppression of Mn dissolution and deposition on the anode at elevated temperatures (50°C).

Polycarbonate/Polypropylene-conjugated Melt-blown Nonwovens: Thermal Stability and Performance as Li-ion Battery Separators

Polycarbonate/Polypropylene-conjugated Melt-blown Nonwovens: Thermal Stability and Performance as Li-ion Battery Separators

Zeolite membrane separators for fire-safe Li-ion batteries – Effects of crystal shape and membrane pore structure

The state-of-the-art lithium-ion batteries (LIB) use carbonate-based liquid electrolytes and polymeric separators which pose safety issues including fire-hazard. The use of fire-retarding salt-concentrated electrolytes can address the safety issues of LIBs, but these electrolytes have high viscosity and low wettability with polymeric separators making it difficult to fabricate high performance LIBs with the electrolytes. Here we report development of zeolite membrane separators with tailored pore structure and surface chemistry for preparation of fire-safe LIBs using salt-concentrated electrolytes. The zeolite membrane separators are made of zeolite particles with a specific zeolite structure, Si/Al ratio, and different particle shapes, coated on cathode by a scalable blade coating method. Intermediate pore, pure silica MFI-type zeolite (silicalite) was selected as the fire-proof material for membrane separators which also offer high wettability for the fire-retarding salt-concentrated electrolyte of lithium bis(fluorosulfonyl)imide (LiFSI) in tri-methyl phosphate solvent (TMP) solvent. Two silicalite membrane separators of similar pore size but different porosity and tortuosity were coated on cathode (LiNi0.5Mn0.3Co0.2O2 or NMC) using plate-shaped silicalite particles of low aspect ratio (LAR) and high aspect ratio (HAR). Both zeolite separators show significantly better wettability for the LiFSI/TMP electrolyte than the commercial polypropylene (PP) separator allowing fabrication of high-performance NMC/LiFSI-TMP/graphite cells with the non-combustible silicalite separator. The new fire-safe zeolite-membrane-based LIBs with the fire-retarding electrolyte exhibit excellent charge and discharge characteristics and cycle performance (similar to the state-of-the-art, but fire-unsafe LIBs with LiPF6-carbonate electrolyte and polymer separator). The membrane separators made of HAR plate-shaped zeolite showing low interparticle porosity and high tortuosity perform better than the membrane made of LAR zeolite particles, due to a more uniform lithium-ion flux from both interparticle pores and intraparticle zeolitic pores at the separator/anode interface.

Synthesis and Characterization of Polymer Electrolyte Membrane Based on Cellulose-Chitosan-Alginate as Li-Ion Battery Separator

The current commercial Gel Polymer Electrolyte (GPE) products are generally made of synthetic and non-biodegradable materials. In addition, some of these polymers require toxic reagents and complex synthesis processes. The purpose of this research is to manufacture GPE membrane products using biodegradable raw materials, a combination of Hydroxy Ethyl Cellulose (HEC), Carboxymethyl Chitosan (CMCs), and Sodium Alginate (SA) with lithium salt as the electrolyte source. The methods start from the fabrication/synthesis of biodegradable GPE membranes in various compositions, then LiOH is added as an electrolyte source and glutaraldehyde as a crosslinking agent using a solution casting technique. The mechanical membrane testing (tensile strength and elongation) and characterization were carried out using XRD, SEM, and FTIR. Based on mechanical tests carried out, variations in HEC 50%: SA 50% has the highest tensile strength value of 81.4255 MPa and the lowest elongation value of 11.68%. The results of XRD analysis in the presence of a typical peak in the HEC: SA variation was 11.56º, which could affect the strength of the electrolyte-polymer gel membrane (GPE). The results of SEM analysis proved that the HEC: SA variation has a porous morphology that can affect the ion absorption capacity in lithium-ion battery applications. The results of FTIR analysis proved that there are functional groups S=O, CH, CO, NH, OH, and COC in the three membranes (SA, CMCs, and HEC).

Surface Modification of Polyethylene Separator for Li-Ion Batteries via Imine Formation

Porous polyethylene (PE) polymer film is a representative material for separators in Li-ion batteries (LIBs). Although PE separators are widely used in commercial LIBs owing to several advantages such as chemical stability, low cost, and good processability, the material still suffers from poor wettability to organic liquid electrolytes and low thermal stability. In this study, a facile surface modification method via the nucleophilic addition of amine in a solution is proposed. To improve the electrolyte wettability and thermal stability of the PE separators, a carbonyl group (C=O) was introduced onto the surface by pretreatment. Then, an imine functional group (R–N=C) was created by the conversion reaction between the carbonyl and amine groups (R-NH2) in a poly(ethylene glycol) bis(amine) solution. By including the formed imine group onto the surface, the surface-modified PE separator exhibited enhanced wettability and thermal stability. The chemical surface treatment transformed the PE separator from hydrophobic to hydrophilic, resulting in the increased ionic conductivity of the electrolyte and rate capability of the cells without any negative effect on the physical and electrochemical properties of the separator.

Engineering high transport plastic separators for next-generation Li-ion batteries

Engineering high transport plastic separators for next-generation Li-ion batteries

Applications of Soft Dendritic Colloids in Li-Ion Batteries with Advanced Structure-Derived Performance

Adoption of electric vehicles and increasing demands of consumer electronics require electrochemical energy storage devices with high capacity and rate capabilities. Supply-chain and material-cost concerns as well as charge-rate limitations associated with irreversible lattice changes motivates the replacement of mixed-metal oxide cathodes in lithium-ion batteries (LIBs). Electrochemically active polymeric materials have emerged as promising candidates to replace metal oxides due to their high tunability and increased capacity. However, many of these materials suffer from poor intrinsic electronic conductivity and may be soluble into the electrolyte, causing irreversible capacity fade. Nanostructuring and composite formation are two polymer processing methods that can be leveraged to combat these drawbacks. The introduction of new nanostructured and nanocomposite cell components can decrease diffusion length scales, increase mechanical stability, and allow for higher electronic conductivity in polymer electrodes. Herein, we utilize a new class of polymeric materials called soft dendritic colloids (SDCs) as a platform for creating such nanocomposites. These fibrillar polymeric particles, formed via turbulent solvent-nonsovent induced phase separation, have hierarchical morphology, large aspect ratios and demonstrate pronounced adhesion and network-forming behavior. SDC-based materials have shown impressive results as nonwoven polyvinylidene difluoride separators in Li-ion batteries. Herein, we propose and present their use as Li-ion battery electrodes with advanced structural properties, derived from electroactive polymeric materials.

Polymer-in-ceramic flexible separators for Li-ion batteries

Polymer-in-ceramic flexible separators for Li-ion batteries

Sepiolite and ZIF-67 co-modified PAN/PVdF-HFP nanofiber separators for advanced Li-ion batteries

Electrospun PAN/PVdF-HFP membranes have the potential to be used as separators for Li-ion batteries owing to their good mechanical properties and high chemical stability. However, the application of PAN/PVdF-HFP separators has been hampered by their poor electrochemical performances. In this study, semi-aligned PAN/PVdF-HFP nanofiber separators have been fabricated by an electrospinning technology. Sepiolite and ZIF-67 co-modification was employed to enhance the physical properties of the PAN/PVdF-HFP separators. The test cells with the as-prepared composite separator showed better electrochemical performance than the commercial and raw PAN/PVdF-HFP separators.

Sophisticated 3D microstructural reconstruction for numerical analysis of electrolyte imbibition in Li-ion battery separator and anode

A simplified approach to visualize and create high-fidelity three-dimensional (3D) microstructure reconstruction of Li-ion battery (LIB) separator (Polyethylene - PE) is proposed here, for comprehending the relationship between transport properties (Electrolyte saturation, Residual gas phase, effective transport coefficient - δ, and Macmullin number - Nm) and microstructure characteristics. In the construction of Li-ion battery, since electrolyte filling in separator and electrode is a decisive step that primarily governs the storage capacity, battery life cycle and safety, herein the study was extended to identify the quantity of residual gas in the separator/electrode microstructure for facilitating better wetting and infilling of electrolyte into the electrode and separator. Moreover, the effect of structural and transport properties with different graphite content (50%, 60%, and 70%) in the electrode were investigated followed by contact angle (CA) effect with electrolyte in the separator and graphite is presented. From the results of the study, it is claimed that the graphite contents of anode and contact angle of electrolytes explicitly impact the transport properties, in addition the conductivity of the electrolyte diminishes with respect to the microstructure of separator and anode.

A multiscale study on the effect of compression on lithium-ion battery separators

Separators play a vital role within Li-ion batteries. Separators allow rapid Li-ion transport while isolating the cathode and anode to prevent electrical short circuits. Herein, the effects of compression on the mechanical and transport properties of the separator were investigated using a multiscale approach. The microscale domain of the separator was stochastically reconstructed based on the scanning electron microscopy images of a Celgard 2400 membrane. The reconstructed model was then imported to perform explicit dynamic simulations to investigate the mechanical behavior of the separator at different compression ratios. A pore-scale model was employed to calculate the effective transport properties of the deformed models. This was then used in a 3D macroscopic multiphysics model to evaluate the impact of compression on the battery performance. The simulation results revealed the effect of compression on the separator's mechanical properties and battery performance from the microscale to macroscale level. The compression of the separator was found to adversely influence the charging performance of the Li-ion battery. When the compression ratio reaches 40 %, the charging performance of the battery decreased significantly. The present study demonstrates a multiscale approach for investigating the effect of compression on Li-ion battery separators. This is a powerful method for optimizing separators in the future.

Novel fast ionic conductor ceramic composite separator for high-performance safe Li-ion power batteries

The ceramic composite separators coated with silica or alumina particles have been used in power batteries due to their better electrolyte wettability and better thermal stability compared with bare polymer separators. However, these oxide ceramics are Li+ ion insulators, which increase internal resistance and hinder the improvement of rate capability of batteries. Herein, we report a strategy to further improving the performance of lithium-ion batteries (LIBs) by using fast ionic conductor ceramic composite separator as an alternative to traditional ceramic coated separators. Lithium lanthanum titanate (LLTO), a fast ionic conductor with excellent room temperature bulk conductivity, are coated on the common polyethylene (PE) separators. Our results demonstrate that such a novel LLTO-coated separator possess excellent electrolyte wettability and thermal stability; and the assembled NCM523/graphite lithium-ion pouch cells with LLTO-coated separator show better rate capability and cyclic performance with 88.7% capacity retention after 1000 cycles at room temperature compared with the pouch cells with Al2O3-coated separators. The fast ionic conductor ceramic composite separators will be a potential competitor to the next-generation novel separators for high-performance Li-ion power batteries.

Toward stretchable batteries: 3D-printed deformable electrodes and separator enabled by nanocellulose

The needs for stretchable batteries surge as wearable and epidermal electronics emerge. The development of stretchable batteries, however, remains a grand challenge, as the battery components are intrinsically brittle and fracture easily under mechanical loading. Existing efforts to increase the stretchability of battery components often involve complex fabrication processes and thus are not viable for scalable and cost-effective manufacturing. To address this challenge, herein a facile yet effective strategy is developed to fabricate stretchable electrodes and separator for Li-ion batteries using extrusion-based 3D printing of active materials mixed with nanofibrillated cellulose. The resulting electrodes and separator can achieve reversible stretchability of 50%. After 50 stretching cycles, the resistance of the electrodes under 50% stretch only increases by 3%. The origin of the exceptional mechanical and electrical performances of the 3D-printed battery components is twofold: (i) excellent deformability enabled by the 3D-printed serpentine structure at the component level; (ii) the robust nanoscale structure due to the high aspect ratios of nanofibrillated cellulose and carbon nanotubes and the strong interactions between nanofibrillated cellulose and carbon nanotubes or among the individual cellulose fibers at the material structure level. The facile 3D printing of the patterned electrodes/separator leads to low-cost manufacturing of high-performance stretchable Li-ion batteries, demonstrating its promising potential to enable stretchable energy storage devices for wearable and epidermal electronics.