Separators For Lithium-ion Batteries Research Articles

Estimating the Permeability of the Ceramic Coating on Lithium-Ion Battery Separators via the Ideal Laminate Theory

In this study, a simple method for quantifying the porous nature based on the permeability of the thin ceramic coating on microporous polyolefin flims used as separators in lithium-ion batteries is demonstrated. The air permeability of the ceramic coating was determined via the ideal laminate theory (ILT), which is widely accepted for multilayered films or well-defined laminates. This study mainly aims at introducing an ILT-based resistors-in-series model with the Gurley method and its potential applications in battery separators. We investigated a few previously reported examples of ceramic-coated separators to compare the air permeability of various ceramic coatings along with their base polyolefin films.

Highly porous and thermally stable zeolitic imidazolate framework-8/aramid nanofibers composite separator for lithium-ion batteries

Currently, the inferior electrolyte wettability and poor thermal stability of conventional polyolefin separators have impeded the development of high-performance lithium-ion batteries (LIBs). Herein, a novel zeolitic imidazolate framework-8 (ZIF-8)/aramid nanofibers (ANFs) composite separator was fabricated via in-situ assembling ZIF-8 on ANFs followed by filtration process. The ZIF-8 particles prevented the dense packing of ANFs and significantly improved the porosity from 23.8% (in pure ANFs separator) to 61.6% (in composite separator). Combining the well-developed porous structure and exceptional electrolyte affinity, the as-prepared 30%ZIF-8/ANFs composite separator (with 30 wt% of ZIF-8) exhibited high electrolyte uptake (224.0%) and good electrolyte wettability, which brought about superior ionic conductivity (1.37 mS cm−1), excellent rate performance and cycling stability. Furthermore, compared to commercial polypropylene separator, the 30%ZIF-8/ANFs separator showed suitable mechanical strength, outstanding thermal resistance and flame retardancy, which ensured higher safety during battery operation. Accordingly, the 30%ZIF-8/ANFs composite separator with both enhanced performance and high safety is a promising candidate for next-generation LIBs.

A Single-Layer Composite Separator with 3D-Reinforced Microstructure for Practical High-Temperature Lithium Ion Batteries.

Incorporation of ceramic materials into separators has been frequently applied in both research and industry to improve the overall high-temperature performances of lithium ion batteries. However, inorganic ceramic particles tend to form aggregation in separators and even fall off in the separator matrix due to the inferior combination between ceramic particles and polymer matrix, giving rise to a decrease in separator porosity and thus the degradation of battery performances. Herein, a single-layer core-shell architecture is designed to reinforce the polymer matrix through encircling Al2 O3 particles by poly(vinylidene fluoride) with strong inter-molecular interaction. The 3D-reinforced microstructure effectively improves pore distribution and thermal stability to resist the dimensional deformation at high temperatures, thus giving rise to a high Coulombic efficiency of 99.16% and 87.5% capacity retention after 500 cycles at 80°C for LiFePO4 /Li batteries. In particular, the excellent performances of the proposed separator microstructure are confirmed with a thickness value of commercial separators. This work provides a promising strategy to fabricate a core-shell structural composite separator for stable lithium ion batteries at high temperatures.

Understanding the Accelerated Sodium-Ion-Transport Mechanism of an Interfacial Modified Polyacrylonitrile Separator

Sodium-ion batteries (SIBs) are considered as the most attractive candidates for large-scale energy storage systems owing to their low cost and abundant reserves. They need a special separator with rapid Na+ transport for a satisfactory power performance. Herein, an alkaline-solution-treated electrospun polyacrylonitrile separator with an in situ chemical-grafted graphene oxide (denoted as M-PAN-GO) is prepared via regulating the interfacial group. The increased amount and variety of oxygen-containing polar groups −COO–, −COOH–, and −OH in the M-PAN-GO separator not only increase the number of active sites for bonding and transferring Na+ but also produce more H-bonds that impede the migration of PF6–. The obtained M-PAN-GO separator displayed accelerated transmission of Na+ and Li+, and the Na+ transference number is larger than that of Li+ owing to a more efficient binding that can be formed between oxygen-containing functional groups and Na+. DFT calculations demonstrate that a greater binding energy is favorable to binding and promoting ion transport in a certain range. Along with the M-PAN-GO separator’s desolvation capability, high Na+ transference number (0.70) and ion conductivity (4.53 mS cm–1) of the M-PAN-GO separator in 1 M NaPF6 EC DMC electrolyte are obtained. The Na|Na3V2(PO4)3 cells assembled with the M-PAN-GO separator have a superior polarization voltage of 0.76 V rather than 0.97 V at 4 C in SIBs. This work provides a potential high-performance separator for SIBs and lithium-ion batteries.

Structure–Performance Relationships of Li-Ion Battery Fiber-Based Separators

Lithium-ion battery separators are receiving increased consideration from the scientific community. Many research efforts trend toward creating high-performance fiber-based battery separators with a small and uniform pore size to maximize ionic conductivity and cell discharge capacity. Here, we show that not only the pore size but also the pore size distribution has an important effect on these electrochemical properties. In this work, we studied nonwoven membranes fabricated from a single polymer, poly(vinylidene fluoride) (PVDF), with different pore sizes and pore size distributions using three different techniques (meltblowing, electrospinning, and shear spinning). We evaluate their performance as separators in Li-ion cells. Although meltblowing is commonly employed to produce commercial microfibers/nanofibers, electrospinning has been studied mostly in the academic literature. Shear spinning is an emerging method to fabricate nanofibrous material where, for this study, the morphology of the resulting PVDF membranes may be controlled from fibrous-like to nano-sheet-like with subsequent effects on the electrochemical properties. We show that the smaller the pore size and the wider the pore size distribution, the higher are the electrolyte uptake and ionic conductivity of the mats, resulting in improved in-use discharge capacity and rate capability of Li/LiCoO2 cells.

High safety and electrochemical performance electrospun para-aramid nanofiber composite separator for lithium-ion battery

Separators have a significant impact on the electrochemical performance and safety of lithium-ion batteries (LIBs). To date, it is still a great challenge to prepare a single separator that can meet all the requirements for LIBs. Herein, a porous composite membrane based on para-aramid nanofibers (ANFs) via introducing poly (ethylene oxide) (PEO) is successfully prepared by electrospinning and applied in LIB separators. The obtained ANFs/PEO separators are found to exhibit excellent morphology and high porosity, which promote a high electrolyte uptake. Therefore, a battery with the ANFs/PEO separator shows better electrochemical performance to that of the Celgard 2400 separator. More importantly, the tensile strength of ANFs/PEO separators after heat treatment reach 41.52 MPa while maintaining a porosity of 75.85%. At the same time, the thermal stability of ANFs/PEO separators is outstanding, and there is almost no thermal shrinkage at 200 °C for 1 h. Compared with Celgard 2400 separators, ANFs/PEO separators exhibit better electrochemical performance and thermal stability, as well as good mechanical strength and flexibility, achieving a balance between electrochemical performance and safety of LIBs. These results demonstrate that the ANFs/PEO separator with comprehensive performance is promising for high-performance LIBs.

Safety and cycling stability enhancement of cellulose paper-based lithium-ion battery separator by aramid nanofibers

Commercial polyolefin separators suffer from poor electrolyte wettability and low thermal stability, which give rise to unsatisfied electrochemical performance and severe safety hazards for lithium-ion batteries (LIBs). Herein, a novel paper-based composite separator composed of electrolyte-affinitive cellulose fibers (CFs) and thermally stable aramid nanofibers (ANFs) was successfully fabricated through the traditional papermaking method. The ANFs as a functional nanofiller played crucial roles in improving the defects of pure CF separator including large-sized pores, low mechanical strength and high flammability. Specifically, the CF/ANF composite separator with 20 wt% ANFs (CF/ANF-20) possessed narrow micropores, satisfied tensile strength (33 MPa), excellent thermal resistance (without dimensional shrinkage up to 200 °C) and flame retardancy, which greatly enhanced the safe operation of battery. Moreover, inheriting from the highly porous structure and exceptional electrolyte affinity of CF separator, the CF/ANF-20 composite separator exhibited appropriate porosity and superior electrolyte wettability, which brought about a high electrolyte uptake (157%), thus endowing it with better ionic conductivity (0.75 mS cm−1) and lower interfacial resistance compared to commercial polypropylene separator. Accordingly, the LiFePO4/Li half cells using CF/ANF-20 separator delivered outstanding rate capability and stable cycling performance. All results indicate that the CF/ANF-20 separator shown great balance between the electrochemical performance and safety is an intriguing candidate for advanced LIBs.

Enhancing the safety and cyclic performance of lithium-ion batteries using heat resistant and wettable separator based on covalent organic framework and polybenzimidazole

The separator, an indispensable part of lithium-ion batteries (LIBs), plays a vital role related to the safety and electrochemical performance of these batteries because its thermal stability, mechanical properties, and ionic conductivity directly influence the performance and safety of LIBs. This work presents the preparation of a novel LIBs separator through the introduction of a covalent organic framework (COF) into poly(arylene ether benzimidazole) (OPBI). COFs are emerging crystalline porous organic polymers with large specific surface area and facile functionalization, which can promote electrolyte penetration. OPBI is a type of polymer known for its excellent thermal stability, inherent flame retardancy, and strong mechanical properties. Therefore, compared with the polypropylene separator, the obtained OPBI@COF separator presented an enhanced electrolyte uptake rate (428%), higher ionic conductivity (1.214 mS cm−1), excellent thermal stability, and first-rate flame retardancy. The specific discharge capacity of LiFePO4/Li cell assembled with the OPBI@COF20 separator is 148.3 mAh g−1, and the cell capacity retention reached 98.98% after 200 cycles at 0.5C. More interestingly, the OPBI@COF separator enhanced the safety of LIBs, even after a long run time, by suppressing lithium dendrite growth. Thus, OPBI@COF separator can be considered as an effective and highly stable LIB separator.

Aluminum Diethylphosphinate-Incorporated Flame-Retardant Polyacrylonitrile Separators for Safety of Lithium-Ion Batteries.

Herein, we developed polyacrylonitrile (PAN)-based nanoporous composite membranes incorporating aluminum diethylphosphinate (ADEP) for use as a heat-resistant and flame-retardant separator in high-performance and safe lithium-ion batteries (LIBs). ADEP is phosphorus-rich, thermally stable, and flame retardant, and it can effectively suppress the combustibility of PAN nanofibers. Nanofibrous membranes were obtained by electrospinning, and the content of ADEP varied from 0 to 20 wt%. From the vertical burning test, it was demonstrated that the flame retardancy of the composite membranes was enhanced when more than 5 wt% of ADEP was added to PAN, potentially increasing the safety level of LIBs. Moreover, the composite membrane showed higher ionic conductivity and electrolyte uptake (0.83 mS/cm and 137%) compared to those of commercial polypropylene (PP) membranes (Celgard 2400: 0.65 mS/cm and 63%), resulting from interconnected pores and the polar chemical composition in the composite membranes. In terms of battery performance, the composite membrane showed highly stable electrochemical and heat-resistant properties, including superior discharge capacity when compared to Celgard 2400, indicating that the PAN/ADEP composite membrane has the potential to be used as a heat-resistant and flame-retardant separator for safe and high-power LIBs.

Paperator: The Paper-Based Ceramic Separator for Lithium-Ion Batteries and the Process Scale-Up Strategy

Paperator: The Paper-Based Ceramic Separator for Lithium-Ion Batteries and the Process Scale-Up Strategy

Stable fouling resistance of polyethylene (PE) separator membrane via oxygen plasma plus zwitterion grafting

Low-cost and fouling resistant microfiltration/ultrafiltration membranes for wastewater treatment has been a challenging task when balancing the membrane cost and performance stability. Herein, a low-cost polyethylene (PE) lithium-ion battery separator was explored as a potential solution. To tackle the intrinsic hydrophobic character which incurs severe fouling, a novel cascade process of plasma treatment plus zwitterion grafting was designed: first oxygen plasma to endow oxygen-containing groups for hydrophilicity and supply “grafting sites”, and grafting zwitterion 3-[[3-(trimethoxysilane)-propyl] amino] propane-1-sulfonic acid (TMAPS) for robust fouling resistance. The membranes were analyzed by water contact angle, Fourier-transform Infrared Spectroscopy (FTIR), Energy Dispersive Spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS). Challenged by two typical foulants, e.g., humic acid (HA) and bovine serum albumin (BSA), the zwitterion grafted membrane showed the best antifouling performance. Oxygen plasma treated PE membrane presented a better performance towards HA than to BSA. The Resistance-in-Series model revealed that the plasma-treated membrane showed improvement in reversible resistance but not in irreversible resistance. Migration of hydrophobic PE segment to the surface due to the low glass transition temperature was attributed to the low fouling resistance of the plasma-treated membrane. However, the zwitterion-grafted membrane led to stabilized hydrophilicity and inhibited the hdrophobic-hydrophobic interaction between the membrane and foulant, thus resulting to significantly improved flux recovery and low irreversible fouling rate. This study clarified that oxygen plasma alone is not sufficient to provide sufficient fouling resistance, but grafting with zwitterion provides stable hydrophilicity and fouling resistance.

A newly-developed heat-resistance polyimide microsphere coating to enhance the thermal stability of commercial polyolefin separators for advanced lithium-ion battery

Polymer separators with high-temp dimensional stability is strongly demanded for high-safety lithium-ion batteries (LIBs). In this study, a novel polyimide (PI) microsphere coating slurry was prepared and then plated on Celgard polypropylene (PP) separator, producing PP@PI microsphere composite separator with excellent thermal stability, flame retardancy, high liquid retention capacity and high ionic conductivity by a simple blade-coating process. The results show that the thermal stability and liquid retention capacity of the PP@PI microsphere composite separator are significantly improved. Compared with the pure commercial PP separator which sharply shrank at 150 °C, the PI microsphere composite separator shows negligible shrinkage over 150 °C. The obtained PP@PI microsphere composite separators exhibited excellent thermal stability and fire-resistance properties. At the same time, the peel strength of composite separator reached 138.6 N·m−1, with excellent adhesion. The surface density of PP@PI microsphere composite separator is 16.62 g·m−2, which is lower than 30.22 g·m−2 of ceramic coated composite separator. Electrochemical characterizations indicate that the discharge capacity and capacity retention rate of the battery assembled with PP@PI microsphere composite separator are significantly higher than those assembled with the pure PP separator. The half-cell assembled with this composite separator shows a high capacity of 144.3 mAh·g−1 at 5C, which is much higher than the 117.8 mAh·g−1 of the commercial PP separator equipped battery. Moreover, the lithium-ion batteries assembled with the PP@PI microsphere composite separator show excellent cycle stability after 200 cycles at 1C. More importantly, the lithium-ion battery assembled with PP@PI microsphere composite separator has excellent safety at high temperature. These features demonstrate the potential of PP@PI microspheres composite membrane as high-security separator for LIBs.

Advanced separators for lithium-ion batteries

The separator technology is a major area of interest in lithium-ion batteries (LIBs) for high-energy and high-power applications such as portable electronics, electric vehicles and energy storage for power grids. Separators play an essential part that physically prevents direct contact between positive and negative electrodes while acting as an electrolyte reservoir to transport lithium ions. The characteristics of different separators would directly affect the performance under cell abuse; hence separators are crucial for battery safety. This paper introduces the characteristics of separators, means to improve traditional commercial polymeric separators and novel materials for separators. Other novel high-performance separators are also briefly discussed in this paper. Insights from this paper illustrate that various strategies could enhance the performance of separators, and better performance and safety can be achieved in separators in high-energy lithium-ion batteries.

High-performance separator for lithium-ion battery based on dual-hybridizing of materials and processes

• The composite separator had high mechanical strength after heat treatment. • The composite separator had high porosity and electrolyte uptake after hydrolysis. • After heat treatment and hydrolysis, the composite separator had a complete profile at 200 °C. • After heat treatment and hydrolysis, the composite separator had a satisfactory performance. A preparation method for a lithium-ion battery separator was developed based on the dual hybridizing of materials and processes. This preparation method aimed to prepare a new composite separator by electrospinning various polymer materials with different properties. In addition to the characteristics of each material, multiple corresponding processes were used for post-treatment of the composite separator to enhance the comprehensive performance of the separator. The prepared composite separator polyacrylonitrile (PAN) / polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) / Polyvinyl Pyrrolidone (PVP) allows the use of various raw materials. PAN with high thermal stability is suitable as a framework to support the separator structure. PVDF-HFP are binders that can crosslink the fibers after heat treatment (HT), thereby increasing the tensile strength of the separator threefold. PVP, which is easily soluble in water, was used as a pore-forming agent. After hydrolysis (HD), the fiber of the separator formed a porous structure with high porosity, high electrolyte absorption, and high ionic conductivity. After a heat treatment combined with hydrolysis (HT-HD) to improve the mechanical strength and porosity, the composite separator showed excellent thermal dimensional stability. Its profile integrity was maintained, even at 200 °C. The HT-HD separator produced outstanding electrochemical performance. Hence, the HT-HD separator with comprehensive performance has potential applications in lithium-ion batteries.

Mechanically robust, nonflammable and surface cross-linking composite membranes with high wettability for dendrite-proof and high-safety lithium-ion batteries

The separator plays a pivotal role to guarantee the safety and improve the electrochemical properties of lithium batteries. However, poor thermal stability and nonpolar surface of commercial polyolefin separator seriously restricted the development of the high-performance lithium battery. A "hydrophilic and cross-linking" strategy is proposed here to modify and enhance porous flame retardant poly(arylene ether nitrile) (PEN) polymer membrane to obtain high safety and heat resistant lithium batteries. The resultant PEN@PDA-PEI composite separator exhibits three-dimensional porous structure, superior thermal stability (no shrinkage up to 200 °C). Furthermore, the abundant polar groups (cyano group, amino group, hydroxyl group, etc.) from the composite membrane endow it with super electrolyte affinity (the contact angle is 0°) and the highly enhanced electrolyte uptake (from 400% to 618%). It is worth noting that the introduction of the PDA-PEI cross-linked structure on the surface of PEN membranes significantly improves the mechanical properties of PEN porous membrane. The composite membrane presents good lithium metal interface compatibility and high ionic conductivity (1.5 mS cm −1 ). As expected, the LiFePO 4 /Li battery based on PEN@PDA-PEI separator displays better rate and cycle performance than that of commercial polyolefin separator at elevated temperature. Significantly, the surface high-polarity and uniform pore structure of composite separators prevent the growth of lithium dendrite in cycle term, resulting an interesting 3D spherical morphology on Li metal anode. This work provides a new strategy for the preparation of high-performance and high-safety lithium-ion battery separator, it also paves a facile surface crosslinking strategy reinforce the mechanical strength of various porous membranes. • PDA-PEI crosslinking structure is used to modify the flame retardant PEN membrane for Li metal batteries. • Crosslinking structure with abundant polar groups improves the electrolyte affinity and mechanical strength of PEN separator. • The PEN@PDA-PEI separator shows good thermal and shrinkage stability at 200 °C. • Li ion distribution can be well regulated due to the uniform pore structures and the polar groups on PEN@PDA-PEI. • Batteries with PEN@PDA-PEI separator maintains stable cycling performance at high temperature.

A porous, mechanically strong and thermally stable zeolitic imidazolate framework-8@bacterial cellulose/aramid nanofibers composite separator for advanced lithium-ion batteries

Currently, the commercial polyolefin-based separators suffer from inferior electrolyte wettability and poor thermal resistance, leading to unsatisfied electrochemical performance and severe safety hazards for lithium-ion batteries (LIBs). Herein, a high-performance composite membrane composed of zeolitic imidazolate framework-8@bacterial cellulose (ZIF-8@BC) matrix and aramid nanofibers (ANFs) filler was developed as the LIB separator via the facile in-situ synthesis and subsequent filtration process. The as-prepared ZIF-8@BC/30%ANFs composite separator (with 30 wt% of ANFs) showed high tensile strength (70.7 MPa), outstanding thermal resistance and flame retardancy, which significantly enhanced the safety of LIBs during operation. Moreover, due to the well-developed porous structure and exceptional electrolyte affinity, the ZIF-8@BC/30%ANFs composite separator exhibited good electrolyte wettability and high electrolyte uptake (267.8%), which brought about superior ionic conductivity (1.60 mS cm−1) compared to commercial polypropylene separator. These synergistic advantages eventually endowed the battery with excellent rate capability and cycling performance. Accordingly, the ZIF-8@BC/30%ANFs composite separator is a promising candidate for next-generation LIBs with both high safety and enhanced performance.

Research progress on the electrospinning nanofiber lithium-ion battery separators

Electrospinning nanofiber membrane has the advantages of wide raw materials, large specific surface area, and high porosity. It is an ideal separator material for lithium-ion batteries. This paper first introduces two common electrospinning nanofiber diaphragms: polymer, polymer, and inorganic composite, and then focuses on the modification methods of composite modification, blending modification, and inorganic modification, as well as the methods of electrospinning nano modified polyolefin diaphragm. Finally, the development direction of the electrospinning lithium-ion battery separator has prospected.

Synthesis and application of nano-organosilicon coating through cyclonic plasma deposition on a polymeric separator for lithium-ion batteries

Synthesis and application of nano-organosilicon coating through cyclonic plasma deposition on a polymeric separator for lithium-ion batteries

Controllable Coaxial Coating of Boehmite on the Surface of Polyimide Nanofiber Membrane and Its Application as a Separator for Lithium‐Ion Batteries

With the continuous development of lithium‐ion batteries (LIBs), the electrochemical performance and safety of LIBs have become increasingly challenged recently. Therefore, research on high‐performance LIB separators has gradually attracted widespread attention. As an important component of lithium batteries, the diaphragm plays an important role in battery safety. Improving battery separator performance is an important way to improve batteries’ safety and electrochemical performance. Herein, to prepare a new type of LIB separator, a preparation strategy in which polyimide (PI) nanofibers can be coaxially encapsulated by the inorganic boehmite (AlOOH) ceramics through an adsorption and complexation process is developed. This strategy involves immersing partially imidized PI nanofiber membranes into anhydrous aluminum chloride ethanol solution and dilute ammonia, where the adsorption−complexation and hydrolysis processes are completed. During the hydrolysis process, the PI nanofibers are evenly coated with AlOOH. The amount of AlOOH can be controlled easily by adjusting the concentration of the anhydrous aluminum chloride ethanol solution. After being encapsulated with AlOOH, the separator exhibits better mechanical properties, wettability, thermal stability, cycling durability, and a high capacity, along with faster ion transport speed. All excellent features prove that the PI@AlOOH composite nanofiber membrane is an advanced high‐performance LIB separator.

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.