Polyolefin Separators Research Articles

Anionic metal-organic framework modified separator boosting efficient Li-ion transport

Low lithium ion (Li+) mobility and thermal stability of polyolefin separator are crucial obstacles that limit the utilization of Li metal electrodes in the age of rapid acceleration toward high-energy-density rechargeable batteries. Herein, we report anionic metal-organic frameworks (MOFs) modified separator to overcome the barriers. By grafting –SO3− group onto the Universitetet i Oslo (UIO) structure, the anionic MOFs nanoparticles are synthesized (noted as UIO-SOLi). The UIO-SOLi is adhered on the surface of Celgard 2400 via polyacrylic acid (PAA) to fabricate the modified separator (UIOSOL@PP). The MOFs coating optimizes the wettability and thermal stability of the pristine separator. Negatively charged moieties –SO3− within the pore surface electrostatically repel anions while facilitate the transport of Li+, thus achieving a high Li+ transference number (tLi+) of 0.82. The effective Li+ transport relieves the concentration polarization, regulates the Li+ flow and realizes the uniform deposition of lithium. By implementing the UIOSOL@PP separator, the LiFePO4|Li cells deliver superior rate performance and cycling lifespan that renders a high discharge capacity retention of 155 mAh g−1 over 600 cycles at 1C. These results indicate that anionic MOFs modified separator is a promising strategy to modulate Li+ flow and assist with efficient long-term operation of lithium metal batteries.

High‐Wettability Composite Separator Embedded with in Situ Grown TiO2 Nanoparticles for Advanced Sodium‐Ion Batteries

The separator, as an important inner part of the sodium‐ion battery (SIB), has a significant impact on the electrochemical performance and security of the battery. However, conventional polyolefin separators are inapplicable for SIBs due to their poor wettability to liquid electrolytes and unsatisfactory heat resistance. To address these problems, a novel polyethylene (PE)‐ hydroxyethyl cellulose (HEC)‐TiO2 composite separator modified on the PE matrix is proposed and successfully prepared by a multistep synthesis procedure of HEC coating and TiO2 in situ self‐growth, while almost maintaining the initial separator thickness. Compared with conventional PE separators, this composite separator possesses remarkable wettability which benefits from the introduction of a polar HEC‐TiO2‐incorporated coating. Besides, thanks to a significant improvement in wettability, the separator presents high electrolyte uptake of up to 186.5% and an extraordinary ionic conductivity of 0.342 mS cm−1. As expected, a Na|Na3V2(PO4)3 battery with the PE‐HEC‐TiO2 separator exhibits a reversible capacity of 99.0 mAh g−1 and a capacity retention of 94.8% after 1000 cycles at 5 C with a steady Coulombic efficiency of nearly 100%. These brilliant performances convincingly make it a promising separator for advanced SIBs with high reversibility, high capacity, and long life.

Electrolyte-philic and thermal-resistant polyimide separator enhances the performance of flexible silicon/carbon nanofibers for lithium-ion batteries

It is still challenging to construct a stable silicon/electrolyte interface to inhibit the mechanical fracture of Si particles and repeated formation of a solid electrolyte interface layer. The separator is a vital component in lithium-ion batteries, however, commercial polyolefin separators suffer from low thermal stability and poor electrolyte wettability. Herein, a polyimide separator is reported to achieve a stable interface of silicon anode due to its high polarity, outstanding electrolyte absorption, and reduction of side reactions. The binder-free self-standing silicon carbon (Si@C) nanofiber composite is designed by a facile electrospinning method and subsequent pyrolysis, where the nanosized Si particles are confined in polyimide-derived carbon nanofibers to form a bamboo-like morphology with good flexibility. Such flexible nanofiber film with cross-linked 3D network structure is important to increase energy density of batteries, mitigate the growth of new interfaces, and avoid the known issues of side reaction during the electrochemical operation. Encouragingly, the 40 %-Si@C composite illustrates an improved lithium storage property with high reversible capacity of 1819.7 mAh g−1 at 0.2 A g−1, excellent rate capability and cycling stability, demonstrating superiority of the film electrode and polyimide separator. This work provides new insights for stabilizing the interface electrochemistry of alloy anodes for the development of next-generation flexible energy storage devices.

Fabrication and Investigation of PE-SiO2@PZS Composite Separator for Lithium-Ion Batteries.

Commercial polyolefin separators exhibit problems including shrinkage under high temperatures and poor electrolyte wettability and uptake, resulting in low ionic conductivity and safety problems. In this work, core–shell silica-polyphosphazene nanoparticles (SiO2@PZS) with different PZS layer thicknesses were synthesized and coated onto both sides of polyethylene (PE) microporous membranes to prepare composite membranes. Compared to pure silica-coated membranes and PE membranes, the PE-SiO2@PZS composite membrane had higher ionic conductivity. With the increase in the SiO2@PZS shell thickness, the electrolyte uptake, ionic conductivity and discharge capacity gradually increased. The discharge capacity of the PE-SiO2@PZS composite membrane at 8 C rate was 129 mAh/g, which was higher than the values of 107 mAh/g for the PE-SiO2 composite membrane and 104 mAh/g for the PE membrane.

Design Considerations for Practical Li-S Battery Components for Electric Aviation

The development of high-energy, high-safety, and high-power batteries beyond electric automobile requirements is vital for future electric aviation applications. The transition to non-volatile solid-state electrolytes (SSE) promises many advantages over traditional flammable liquid electrolytes and may also be an enabling technology for next generation chemistries. However, significant manufacturing challenges must be overcome before the adoption of such technology. This study focuses on advancement of previous electrolyte development to produce sulfide-polymer composites with densified thicknesses between 20-30 microns using a tape-casting technique with an elastomer binder [1]. Parameters such as binder type, binder loading, and filler loading are modified to determine their impact on electrochemical performance and mechanical stability. The composites were determined to retain reasonable ionic conductivity with improved flexibility and scalability critical for practical manufacturing of such cells. Films were produced 10-15 times thinner than comparable bulk powder electrolytes and within the range of commercial polyolefin separators (25 micron) used in commercial liquid containing lithium-ion cells. Composite conductivities were maintained above 0.2mS/cm, which holds promise for future electric aviation applications. Processing techniques are investigated to further improve electrochemical performance.[1] Donald A. Dornbusch, Rocco P. Viggiano, John W. Connell, Yi Lin, Vadim F. Lvovich. Practical considerations in designing solid state Li-S cells for electric aviation, Electrochimica Acta, 2021, 139406, ISSN 0013-4686, https://doi.org/10.1016/j.electacta.2021.139406.

Separator Dependency on Cycling Stability of Lithium Metal Batteries Under Practical Conditions

Development of practical lithium (Li) metal batteries (LMBs) remains challenging despite promises of Li metal anodes (LMAs), owing to Li dendrite formation and highly reactive surface nature. Polyolefin separators used in LMBs may undergo severe mechanical and chemical deterioration when contacting with LMAs. To identify the best polyolefin separator for LMBs, this study investigated the separator‐deterministic cycling stability of LMBs under practical conditions, and redefined the key influencing factors, including pore structure, mechanical stability, and chemical affinity, using 12 different commercial separators, including polyethylene (PE), polypropylene (PP), and coated separators. At extreme compression triggered by LMA swelling, isotropic stress release by balancing the machine direction and transverse direction tensile strengths was found to be crucial for mitigating cell short‐circuiting. Instead of PP separators, a PE separator that possesses a high elastic modulus and a highly connected pore structure can uniformly regulate LMA swelling. The ceramic coating reinforced short‐circuiting resistance, while the cycling efficiency degraded rapidly owing to the detrimental interactions between ceramics and LMAs. This study identified the design principle of separators for practical LMBs with respect to mechanical stability and chemical affinity toward LMAs by elucidating the impacts of separator modification on the cycling performance.

Core-Shell Structured Gel Polymer Electrolyte with Single-Ion Conducting and Thermal Stability Bifunction for Lithium-Ion Batteries

The commercial electrolyte of lithium-ion batteries (LIBs) is usually liquid state supplemented by a porous polyolefin separator to prevent short circuit. Recent years, polymer electrolyte shows great potential for its flexibility and enhanced safety. And novel single-ion conductor gel polymer electrolyte (SIGPE) could effectively suppress the anion polarization by immobilizing anions on the polymer bone. Further enhancing the thermal stability of SIGPE plays a significant role for the safety of the LIBs. Herein, a novel bifunctional core–shell structured composite was prepared, consisting of thermal stable polyimide as core and a composite single-ion conductor polymer. The relations of fiber morphology with spinning parameters and heat-treatment temperature were studied carefully. As a result, the composite core–shell structured membrane shows excellent high dimensional thermal stability up to 190 °C. Moreover, when the charged half battery was stored at 150 °C, the core–shell structured SIGPE cell shows excellent potential stability while ordinary cell quickly short out, greatly enhancing the safety of the LIBs. The ionic conductivity of the plasticized gel electrolyte could reach 1.7*10−5 S cm−1 with a Li-ion transference number of 0.76. The assembled LiFePO4 battery shows stable cycle performance and remained 85% capacity after 100 cycles.

Upgrading the Properties of Ceramic-Coated Separators for Lithium Secondary Batteries by Changing the Mixing Order of the Water-Based Ceramic Slurry Components

Developing uniform ceramic-coated separators in high-energy Li secondary batteries has been a challenging task because aqueous ceramic coating slurries have poor dispersion stability and coating quality on the hydrophobic surfaces of polyolefin separators. In this study, we develop a simple but effective strategy for improving the dispersion stability of aqueous ceramic coating slurries by changing the mixing order of the ceramic slurry components. The aqueous ceramic coating slurry comprises ceramics (Al2O3), polymeric binders (sodium carboxymethyl cellulose, CMC), surfactants (disodium laureth sulfosuccinate, DLSS), and water. The interaction between the ceramic slurry components is studied by changing the mixing order of the ceramic slurry components and quantitatively evaluating the dispersion stability of the ceramic coating slurry using a Lumisizer. In the optimized mixing sequence, Al2O3 and DLSS premixed in aqueous Al2O3-DLSS micelles through strong surface interactions, and they repel each other due to steric repulsion. The addition of CMC in this state does not compromise the dispersion stability of aqueous ceramic coating slurries and enables uniform ceramic coating on polyethylene (PE) separators. The prepared Al2O3 ceramic-coated separators (Al2O3–CCSs) exhibit improved physical properties, such as high wettability electrolyte uptake and ionic conductivity, compared to the bare PE separators. Furthermore, Al2O3–CCSs exhibit improved electrochemical performance, such as rate capability and cycling performance. The half cells (LiMn2O4/Li metal) comprising Al2O3–CCSs retain 90.4% (88.4 mAh g−1) of initial discharge capacity after 150 cycles, while 27.6% (26.4 mAh g−1) for bare PE. Furthermore, the full cells (LiMn2O4/graphite) consisting of Al2O3–CCSs exhibit 69.8% (72.2 mAh g−1) of the initial discharge capacity and 24.9% (25.0 mAh g−1) for bare PE after 1150 cycles.

Thermostable and nonflammable polyimide/zirconia compound separator for lithium-ion batteries with superior electrochemical and safe properties

Separators are applied to segregate cathode and anode, and provide ion transport channels in lithium-ion batteries (LIBs). Nevertheless, present commercial polyolefin separators represent high thermal shrinkage and inferior electrolyte wettability, seriously limiting wider development of LIBs. In this work, we prepared zirconia (ZrO2) nanolayer encapsulated polyimide (PI) nanofiber compound separator through in-situ polar adsorption and hydrolysis strategy. The obtained PI/ZrO2 compound separator has superior thermal stability, electrolyte wettability and flame retardance in comparison with polypropylene (PP) separator. The shrinkage ratio of prepared PI/ZrO2 compound separator is 0 even at 300 °C, while the PP separator significantly shrank at 160 °C. Furthermore, the ionic conductivity of PI/ZrO2 separator reaches up to 1.32 mS cm−1, far higher than 0.34 mS cm−1 of PP separator. Besides, the coin batteries of LiNi0.8Co0.1Mn0.1O2 (NCM811)/electrolyte-separator/lithium (Li) assembled with PI/ZrO2 compound separator exhibit enhanced rate performance, high discharge capacity retention rate of 88.3% after 100 cycles at 1C and excellent battery safety performance even at 140 °C. Thus, combined with its advantages, such as preparation, thermostability, electrolyte wettability, electrochemical property and safety, the PI/ZrO2 compound separator exhibits promising prospect in the application of commercial LIBs.

Enhancing Cellulose‐Based Separator with Polyethyleneimine and Polyvinylidene Fluoride‐Hexafluoropropylene Interpenetrated 3D Network for Lithium Metal Batteries

AbstractAside from the electrolyte, a separator is another important component in lithium‐based batteries that has a direct impact on the safety feature and electrochemical performances. To overcome the thermal shrinkage and poor electrolyte affinity of commonly used polyolefin separators, cellulose‐based separators are appealing due to their abundant polar functional groups, thermal stability, and environmental friendliness, especially for large‐sized and high‐energy‐density batteries. Herein, a porous three‐dimensional (3D) network of polymer cellulose‐based separator (denoted as PIC) modified with polyethyleneimine (PEI) and polyvinylidene fluoride‐hexafluoropropylene (PVDF‐HFP) was prepared using a non‐solvent induced phase separation approach. The lithium metal batteries consisting of a PIC separator can deliver a specific capacity of up to 114 mAh g−1 even at a high C‐rate of 8 C (1.36 A g−1) after 300 cycles. Such superior performances of the lithium metal batteries can be attributed to the good wetting ability (390 % electrolyte absorption) and high ionic conductivity (0.754 mS cm−1) of the as‐prepared PIC separator. More importantly, the introduction of polyethyleneimine as a cross‐linking agent significantly improves the mechanical strength of the separator, promotes the uniform deposition of lithium, and compatibility with high voltage (4.4 V) cathode materials LiNi0.8Mn0.1Co0.1O2. This work demonstrates a new strategy for the separator design for high‐performance lithium metal battery applications.

In Situ Thermal Safety Aspect of the Electrospun Polyimide-Al2O3 Separator Reveals Less Exothermic Heat Energies Than Polypropylene at the Thermal Runaway Event of Lithium-Ion Batteries.

Polyimide-Al2O3 membranes are developed as a direct alternative to current polyolefin separators by the electrospinning technique and their chemical structures confirm the carbonyl group with the presence of asymmetric and symmetric stretching and bending vibrations at 1778, 1720, and 720 cm-1 and stretching vibration at 1373 cm-1 for the imide group. Porous nanofiber architecture morphology is realized with a nanofiber thickness of ∼200 nm and shows an ultrasmooth surface and >1 μm pore size in the architecture, built with the chemical constituents of carbon, nitrogen, aluminum, and oxygen elements. The galvanostatic cycling study of the Li/PI-Al2O3/LiFePO4 lithium cell delivers stable charge-discharge capacities of 144/143 mAh g-1 at 0.2 C and 110/100 mAh g-1 at 1 C for 1-100 cycles. The fabricated MCMB/PI-Al2O3/LiFePO4 lithium-ion full-cell reveals less charge transfer resistance of Rct ∼ 25 Ω and yields stable charge-discharge capacities of 125/119 mAh g-1. The thermogravimetric curve for the PI-Al2O3 separator discloses thermal stability up to 525 °C, and the differential scanning calorimetric curve shows a straight line until 300 °C and depicts high thermal stability than the PP separator. In situ multimode calorimetry analysis of the MCMB/PP/LiFePO4 full-cell showed a pronounced exothermic peak at 225 °C with a higher released heat energy of 211 J g-1 at the thermal runaway event, while the MCMB/PI-Al2O3/LiFePO4 full-cell revealed an almost 8-fold less exothermic released heat energy of 25 J g-1 than the Celgard polypropylene separator, which was because the MCMB anode and LiFePO4 cathode can be mechanically isolated without any additional separator's melting and burning reactions, as a fire-suppressant separator for lithium-ion batteries.

A degradable membrane based on lignin-containing cellulose for high-energy lithium-ion batteries

Traditional polyolefin separator has the defects of non-degradation and poor wettability with the electrolyte. In this paper, we have designed the porous membrane based on the lignin-containing cellulose nanofibers (LCNFs) from the unbleached pulp by a facile method. In the designed membrane, the amorphous lignin can effectively adjust the dispersion of the cellulose to form a uniform mesoporous structure. And the abundant oxygen-containing functional groups both in cellulose and lignin can promote the affinity of the membrane with the electrolyte. Finally, the prepared the LCNF porous membrane shows a high electrolyte uptake of 276 % and high ionic conductivity of 1.86 mS cm−1. As the separator for LIBs, the batteries with high-voltage Li3V2(PO4)3 as cathode (up to 4.8 V), the batteries displayed 183 mAh g−1 with the energy density of 669 Wh kg−1, indicating the good application potential in high-energy LIBs. More importantly, the LCNF membrane made from the unbleached pulp has low cost and it can degraded in soil only after 16 days, which can largely reduce environment pollution of the LIBs. This research provided an efficient and low-cost method to fabricate a membrane with good biodegradability and high electrochemical performance for LIBs.

Bifunctional separator with high thermal stability and lithium dendrite inhibition toward high safety lithium-ion batteries

Coating inorganic ceramic particles on commercial polyolefin separators has been considered as an effective strategy to improve thermostability of separator. However, the introduction of the coating layer could induce pore blockage on the surface of the polyolefin separator. Herein, a ceramic composite layer that consists of alumina nanoparticles (n-Al2O3) and halloysite nanotubes (HNTs) is designed to modify the polyethylene (PE) separator (the modified separator is denoted as AH-PE). The HNTs with hollow nanotubular structure construct a light skeleton and provide fast ion transport channels while Al2O3 particles function as heat-resistant fillers to inhibit the shrinkage of the separator at elevated temperatures. The total thickness of AH-PE separator is only 14 µm. Consequently, the mass increment of AH-PE separator decreases from 5 g/m2 to 3.5 g/m2, and the Gurley value reduces by 23%, compared with Al2O3 coated PE separator (A-PE). Due to the synergistic effects of Al2O3 and HNTs, AH-PE separator exhibits highly improved thermal stability (almost no shrinkage at 170 °C for 30 min), high Li+ transference number (up to 0.47), and long cycle life of 450 h for Li|Li cells. Moreover, the LiFePO4/Li cells assembled with AH-PE separators demonstrate improved rate capability and safety performance.

Multifunctional robust aerogel separator towards high-temperature, large-rate, long-cycle lithium-ion batteries

Separators is indispensable for the normal operation of lithium-ion batteries (LIBs). However, the widely used commercial polyolefin separators have some inherent deficiencies such as poor thermotolerance, high inflammability and inferior electrolyte wettability, which restrict their further applications of the advanced and safe batteries. Herein, we design a novel thermotolerant (a shrinkage percentage of 0% at 300°C) and flame retarded aerogel separator consisting of aramid nanofibers (ANFs). Because of its high porosity (86.5%±6.1%) and excellent electrolyte uptake (695%), the ANFs aerogel separator has an ionic conductivity of 1.04 mS/cm and a high lithium-ion transference number (0.67), which can endow LIBs with outstanding rate performance and superior cycling performance. Specifically, the ANFs aerogel separator-based batteries possess a discharge specific capacity of 102 mAh/g with a capacity retention of 90.7% and a Coulombic efficiency of 99.3% after 600 cycles at 5 C. In addition, under an operated temperature of 90 ℃, the battery with ANFs aerogel separator can still conduct the very steady charge-discharge, presenting a capacity retention of 90.1% and a Coulombic efficiency of 99.6% after 200 cycles at 3 C. Accordingly, the separator can probably serve as a potential candidate for application to advanced and safe LIBs.

A tailored ceramic composite separator with electron-rich groups for high-performance lithium metal anode

Ceramic composite separator is more competitive than traditional polyolefin separator in the field of power supply for superior thermal stability and wettability of liquid electrolyte. In this study, we develop a two-steps method that modifies SiO2 with acrylamide (AM) by grafting process and prepare a functional SiO2 composite separator (PE/SiO2-AM). This kind of composite separator exhibits similar size-shrinkage (8.8%) to that of PE/SiO2 composite separator at the tested temperature, lower than 18.1% of bare PE separator, and is electrochemically stable below 4.5 V (vs. Li/Li+). In addition, the Li-symmetric cells employing PE/SiO2-AM composite separator have the lowest overpotential and an improved lithium-ion transference number of 0.44. It is demonstrated from DFT calculation that the electron-rich species of imide group is able to uniformly disperse lithium-ion flux at the interface of electrolyte/lithium anode, and contributes to lithium-ion transport process. After assembling the LiCoO2/Li half cells, the cell with PE/SiO2-AM composite separator displays better cycle performance and higher discharge capacity when compared with other separators. Therefore, functional ceramic separator would be more attractive for next-generation lithium metal battery with high energy density.

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.

Electrospun poly(ionic liquid) nanofiber separators with high lithium-ion transference number for safe ionic-liquid-based lithium batteries in wide temperature range

Ionic-liquid electrolytes (ILEs) promise high-safety lithium-metal batteries (LMBs) because of their wide electrochemical stability window and nonflammability. However, existing commercial polyolefin separators for LMBs are incompatible with ILEs owing to poor wettability and thermal stability. Also, conventional ILE/separator systems suffer from the inherent sluggish Li+ transport kinetics. Here we report on a novel highly Li+-conductive poly(ionic liquid) nanofiber membrane (PILNM) separator. It exhibits nonflammability, excellent thermal stability, and remarkable wettability with ILEs, achieving high ionic conductivity (3.81 mS cm−1) and significantly improved Li+ transference number (0.47). Benefiting from desirable interactions between the charged PILNM and ions of the ILE, the first solvation shell of [lithium-bis(trifluoromethane)sulfonimidex]1−x ([Li-TFSIx]1−x) complex is regulated. Fast Li+ transport and dendrite-free lithium metal anode are achieved. The PILNM combined with the ILE enables Li||LiFePO4 cells with enhanced capacities and rate capability in a wide temperature range from 25 °C to 120 °C. Notably, the thermal safety of Li|PILNM/ILE|LiFePO4 pouch cells at elevated temperatures has been further verified by adiabatic accelerating rate calorimetry. This work provides a new strategy to design and fabricate functionalized, thermally durable separators towards high-safety ILE-based LMBs.

Enhancement of Li+ ion kinetics in boehmite nanofiber coated polypropylene separator in LiFePO4 cells

Boehmite (γ-AlO(OH)) nanofibers (length 90–140 ​nm; dia. ∼15 ​nm) synthesized by low temperature (180 ​°C) hydrothermal method and coated onto both sides of surface modified polypropylene. Ceramic coated composite separator (ALO-PP) shows enhanced thermal and dimensional stability over uncoated polypropylene (PP). Optimum coating thickness is found to be 8 ​μm in each side. Electrochemical properties are investigated by assembling 2032-type coin cells using LiFePO4 as cathode against Li/Li+ in half-cell and Mesocarbon microbeads (MCMB) in full-cell configuration respectively. It is found that composite separator results in improved cell kinetics due to better electrolyte uptake and formation of a Li+ ions buffer reservoir facilitating fast ion transport at high current rates. About 16% higher capacity is observed for ALO-PP over PP at a current rate of C/5. Present results show that nanostructured boehmite could be a promising coating material for surface modification of polyolefin separators not only with improved safety but also with enhanced rate performance.

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.

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.