In Situ Armoring: A Robust, High-Wettability, and ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 2978−2988

In Situ Armoring: A Robust, High-Wettability, and Fire-Resistant Hybrid Separator for Advanced and Safe Batteries Lushi Kong,†,∥ Yu Wang,§,∥ Hongsheng Yu,† BingXue Liu,‡ Shengli Qi,*,† Dezhen Wu,† Wei-Hong Zhong,§ Guofeng Tian,† and Jie Wang† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China China Automotive Battery Research Institute Co., Ltd, Beijing 100088, China § School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164, United States ‡

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S Supporting Information *

ABSTRACT: Development of nonflammable separators with excellent properties is in urgent need by next-generation advanced and safe energy storage devices. However, it has been extremely challenging to simultaneously achieve fire resistance, high mechanical strength, good thermomechanical stability, and low ion-transport resistance for polymeric separators. Herein, to address all these needs, we report an in situ formed silica@silica-imbedded polyimide (in situ SiO2@(PI/SiO2)) nanofabric as a new high-performance inorganic−organic hybrid separator. Different from conventional ceramics-modified separators, this in situ SiO2@(PI/SiO2) hybrid separator is realized for the first time via an inverse in situ hydrolysis process. Benefiting from the in situ formed silica nanoshell, the in situ SiO2@(PI/SiO2) hybrid separator shows the highest tensile strength of 42 MPa among all reported nanofiber-based separators, excellent wettability to the electrolyte, good thermomechanical stability at 300 °C, and fire resistance. The LiFePO4 half-cell assembled with this hybrid separator showed a high capacity of 139 mAh·g−1@5C, which is much higher than that of the battery with the pristine PI separator (126.2 mAh·g−1@5C) and Celgard-2400 separator (95.1 mAh·g−1@5C). More importantly, the battery showed excellent cycling stability with no capacity decay over 100 cycles at the high temperature of 120 °C. This study provides a novel method for the fabrication of high-performance and nonflammable polymeric−inorganic hybrid battery separators. KEYWORDS: inorganic−organic hybrid nanofabric, polyimide, fire resistance, separator, battery safety providing the pathways for fast ionic transport.4,5 To ensure these functions in the device, demands for advanced properties of separators are numerous and increasing. Primarily owing to the superiorities in mechanical properties and electrochemical stability, porous polymeric membranes still dominate the world of battery separators. The most classic separators are porous polyolefin films, either polyethylene (PE), polypropylene (PP), or a combination of both in a double-layer or triple-layer structure.4,6,7 However, polyolefin separators are well known for their disadvantages in thermal stability and wettability with electrolytes due to their low melting points (135 °C for PE, 165 °C for PP) and nonpolar characteristics.8 Thermal instability may lead to the failure of the separator, and thus the safety risks such as fire and explosion at high temperatures.9,10 Moreover, the poor wettability and low porosity (ca. 40%) of polyolefin separators are the other two factors limiting the ion-transport and so high C-rate capability. These issues of conventional polyolefin-based separators have been critical

1. INTRODUCTION High energy and power densities, excellent rate capability, long cycle stability, and device safety under harsh service conditions have been the primary requirements for the next-generation of energy storage systems (ESS).1 In particular, the safety of ESS is playing a more and more critical role in practice, as they are so closely and widely used in our daily life through electronic devices (e.g., smartphones, digital cameras, and laptops), electric vehicles, aircraft, etc.2,3 Unfortunately, safety concerns remain one of the most urgent and challenging issues for most of the ESS, such as lithium-ion batteries (LIBs). For example, the Boeing 787 grounding event in 2013 and the recall of Galaxy Note7 in 2016 mainly owed to the battery safety issues. It is well known that primarily two flammable components fundamentally control these safety issues in the device, that is, the organic liquid electrolytes (OLEs) and the polymeric separators. Therefore, the solutions to address the safety concerns have been focused on research and development of electrolytes and separators. It is well known that there are two primary functions for a battery separator: blocking the electron transport between the two electrodes even under harsh compression conditions and © 2018 American Chemical Society

Received: October 10, 2018 Accepted: December 13, 2018 Published: December 13, 2018 2978

DOI: 10.1021/acsami.8b17521 ACS Appl. Mater. Interfaces 2019, 11, 2978−2988

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ACS Applied Materials & Interfaces

Figure 1. Surface armoring of nanofabric-based separator by in situ silica nanocoating. (a) Schematic of the structure for the in situ SiO2@(PI/ SiO2) hybrid separator. (b) Comparison of significant separator properties between the in situ SiO2@(PI/SiO2) hybrid separator and conventional polyolefin separator. (c) Schematic of the fabrication strategy for the in situ SiO2@(PI/SiO2) hybrid separator by combining electrospinning and inverse in situ hydrolysis.

separators showed a high degradation temperature of ∼500 °C. The polarity/hydrophobicity of the PI materials endows the resulting nanofiber separators better electrolyte wettability than Celgard separators, which facilitates the high electrolyte uptake. Electrochemical data showed that the PI nanofiber had a higher initial discharge capacity, and better rate capability than the Celgard separator. To further improve the performance of the PI nanofiber separator, researchers paid attention to the PI−ceramic composite nanofiber separator. A sandwiched Al2O3-coated PI nanofiber separator mat was reported by Lee et al.30 In 2015, Liang et al. prepared the Al2O3/SiO2−PI composite nanofiber mat by dip-coating Al2O3 and SiO2 nanoparticles on both sides of the PI nanofiber membrane.31 In addition, Shayapat et al. reported the preparation of PI−Al2O3 and PI−SiO2 composite nanofiber membranes by adding the Al2O3 or SiO2 nanoparticles into the polyamic acid (PAA) solution for electrospinning followed by thermal imidization.32 Because of the incorporation of hydrophilic Al2O3 or SiO2, the wettability and thermal stability of the PI nanofiber membranes were improved. The oxide coating can help stabilize the pore structure of PI nanofiber membranes. However, there are still various challenging issues, and more critical efforts are highly in need. The surface coating can inevitably increase the overall thickness and even block some of the pores of the separators, which can lead to an additional loss in ion-transport capability. The stability of coating need be considered during long-term use of the battery. More importantly, the low mechanical strength of PI

obstacles for the development of safe and high-power LIB systems, which are in critical need, especially for electric vehicles. In the past decades, tremendous efforts have been made in addressing the above issues of conventional polymeric separators.11,12 A conventional strategy is the surface coating using inorganic particles (e.g., SiO2,13−15 Al2O32) to build up an inorganic protection layer, or an inorganic/polymer/ inorganic triple-layer film, through atomic layer deposition,2 dip-coating,16 or blade-coating method.17 Electrolyte wettability and thermal stability of the separators were improved via these methods generating the inorganic coating with excellent hydrophilicity and thermal stability. The other strategy for improving the thermal stability and wettability of a separator is the fabrication of nonwoven separators made of polymers or inorganic materials with excellent thermal stability and wettability with liquid electrolyte.18 Various polymer materials, including poly(vinylidene difluoride) (PVDF),19,20 poly(ethylene terephthalate),21,22 polyacrylonitrile (PAN),23 poly(butylene terephthalate),24 and polyethersulfone25 have been reported in literature. Inorganic nonwoven fabrics, such as traditional glass fibers,26 ZrO2 fiber,27 and hydroxyapatite nanowires,28 have also been studied as fire-resistant separators. Among these new polymeric separators, aromatic polyimide (PI) nanofiber nonwoven has been considered as a promising solution. Miao et al. first studied the application of PI nanofiber nonwovens as LIB separators.29 Because of the excellent thermal stability of PI materials, the PI nanofiber 2979


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sample was measured by GH-01 digital thickness meter. Thermal gravimetric analysis (TGA) was conducted from 50 to 900 °C at 10 °C·min−1 in the air on a TA Q50 TGA analyzer. Dynamic mechanical thermal analysis (DMA) was performed under N2 atmosphere on a TA Q800 system from 50 to 450 °C at a heating rate of 10 °C·min−1. Thermal mechanical analysis (TMA) data were collected on the TA Q800 instrument in TMA mode at 5 °C·min−1 under N2 with a constant tensile load of 0.02 N. The wettability of the samples was tested by a contact angle analyzer (OCA20, DATAPHYSICS). The mechanical properties were evaluated by a CMT-400 universal tensile tester. The porosity was determined by the typical weighing method using 1-butanol as the liquid.29 The wet weight of the membranes was measured after wiping the excess n-butanol. The dry weight of the membranes was measured after a vacuum drying process. The difference between dry weight and wet weight of the separator is the weight of n-butanol absorbed by the separator. The porosity of the membranes was calculated according to eq 1

nanofiber membranes is still a big concern for their practical application in LIBs.25,33 In this work, we report an inorganic−organic hybrid separator with significant fire resistance, high mechanical properties, and excellent wettability to electrolytes. These excellent performances leading to the novel separator are realized via fabricating through a new inverse in situ hydrolysis, which has never been reported. The PI nanofabric is coated and protected by a silica nanolayer as illustrated in Figure 1a. In this unique nanofabric structure, the PI core can provide good thermal stability and mechanical properties including high strength and flexibility. The SiO2 nanoshell endows the nanofabric with a superhydrophilic surface for excellent wettability to electrolytes and a strong mechanical bonding among nanofibers to improve the mechanical properties of the nanofabric. More significantly, the SiO2 nanoshell acts as a protection layer for achieving improved not only thermal stability but also fire-resistant properties, which have rarely been realized in any polymeric separators to the authors’ knowledge. Benefiting from this unique inorganic−organic hybrid structure, one can comprehensively create a new property, fire resistance, for polymeric separators via the inorganic silica coating, as illustrated in Figure 1b, which was drawn based on the performance comparison in Section 3. In other words, all excellent properties of silica, including significant fire resistance, outstanding thermal stability, high surface hardness, and superhydrophilicity, can be integrated into the hybrid separator, which is fundamentally different from the conventional ceramic coating of polymeric separators.

porosity = [(Wb/ρb )/(Wb/ρb + Wp/ρp )] × 100%

(1)

where Wb and Wp are the weights of the n-butanol and the dry separator and ρb and ρp are the densities of n-butanol (0.809 g·cm−3) and the polyimide (1.27 g·cm−3), respectively. Liquid electrolyte uptakes35 of the separators were measured by immersing the membranes in the mixed solution (ethyl carbonate/ dimethyl carbonate/diethyl carbonate (EC/DMC/DEC) = 1:1:1, v/ v) for 2 h. Liquid-electrolyte-soaked membranes were weighed quickly after removing the redundant surface solution using wipes. The electrolyte uptake was then calculated according to eq 2

uptake = [(W1 − W0)/W0] × 100%

(2)

where W1 and W0 are the weights of the electrolyte-soaked membranes and the dry membranes, respectively. 2.3. Electrochemical Measurements. The ionic conductivities of the PI nonwovens sandwiched between two stainless steel plates were measured by electrochemical impedance spectroscopy using an electrochemical workstation (CHI660C, Shanghai Chenhua) operated at the amplitude of 5 mV within the 1 Hz−100 kHz frequency range.36,37 From the impedance spectrum, the bulk resistance of the electrolyte-saturated polymer membrane (Rb) was first obtained. The ionic conductivity of the separator was then calculated from the following eq 3

2. EXPERIMENTAL SECTION 2.1. Material Preparation. According to our previous publication,34 polyamic acid (PAA) solution (12 wt % in solid) was synthesized via condensation polymerization by first dissolving 4,4′oxydianiline (1.8407 g) in dimethylformamide (30 mL) and then adding pyromellitic dianhydride (2.0252g) gradually. After being stirred at ambient temperature for 2 h, a yellow viscous resin solution was prepared. The inherent viscosity of the synthesized PAA was measured to be 2.81 dL·g−1 at 35 °C. Subsequently, to control the silica loading in hybrid nanofabrics, a series of tetraethoxysilane (TEOS) 1.4, 2.3, 3.4, and 4.5 mL were added into 30 mL of the viscous PAA resin, respectively, and stirred vigorously for 2 h to yield a series of homogeneous PAA/TEOS mixture precursors. The mixture resins were then electrospun into the PAA/TEOS nanofiber nonwovens via an electrostatic spinning apparatus equipped with a flat-tip stainless steel needle (inner diameter, 0.5 mm) as the spinneret. The solution was supplied from a syringe pump at a constant feed rate of 1.2 mL·h−1. An 18−20 kV high voltage was applied between the needle, and a grounded aluminum collector was placed 20 cm from the needle. The as-prepared PAA/TEOS nanofiber nonwovens were then placed in a laminar flow cabinet for around 12 h to evaporate the remaining solvent, followed by thermal curing over 1 h to 280 °C and held constant for 0.5 h to convert the PAA into PI. After that, the nonwoven membranes were treated to be fully wet by spraying a solution of hydrochloric acid/ethanol/deionized water (1/ 45/75, mol/mol), followed by keeping in a vacuum oven at 60 °C for 10 h to complete the hydrolysis of TEOS. The in situ SiO2@PI/SiO2 hybrid separators with different silica loading were finally obtained after a further thermal treatment at 300 °C for 2 h. 2.2. Characterization. An S-4700 field-emission scanning electron microscopy (SEM, Hitachi) and an H-800 transmission electron microscopy (TEM, Hitachi) were employed to observe the SEM and TEM morphologies of the samples, respectively. For the observation of the cross section of the nanofibers, the sample was embedded in epoxy resin and quenched in liquid nitrogen. The cross section of the sample was observed by SEM. The thickness of the

σ = d /(R b·S)

(3)

where σ is the conductivity, Rb is the bulk resistance, d is the thickness of the membrane, and S is the area of the symmetrical electrode. Coin-type cells (CR2025) in a half-cell configuration were assembled in an argon-filled dry glovebox, where Li foil was used as the anode, LiFePO4 as the cathode, and 1.0 M LiPF6 in ethyl carbonate/diethyl carbonate/dimethyl carbonate (EC/DEC/DMC, 1:1:1, v/v/v) as the electrolyte. The LiFePO4 electrode was prepared by spreading the mixture of active material (LiFePO4 powders), carbon black, and poly(vinylidene difluoride) (PVDF) (75:15:15, w/ w/w) on Al foil and drying at 120 °C for 12 h under vacuum. The LiFePO4 loading in the electrode is 6 mg·cm−2. Battery performance, including charge−discharge curves, capacities, Coulombic efficiency at various C rates (0.1C, 0.2C, 0.5C, 1C, 2C, and 5C) and cycling stability were measured at 25 and 120 °C in the half-cell using a LAND-CT2001A battery cycler (LANHE, Wuhan, China) within the voltage range of 2.0−4.2 V (vs Li+/Li).

3. RESULTS AND DISCUSSION 3.1. In Situ SiO2@(PI/SiO2) Hybrid Separator. To fabricate this unique in situ SiO2@(PI/SiO2) hybrid separator, we developed a novel synthesis technique via a combination of polymer chemistry, polymer processing, and in situ hydrolysis, as shown in Figure 1c. First, the polyamic acid (PAA) nanofabric containing tetraethyl orthosilicate (TEOS) was prepared by electrospinning the solution containing a mixture 2980


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Figure 2. Structure of the in situ SiO2@(PI/SiO2) hybrid separator. SEM images of (a) the pristine PI nanofibers and (b) in situ SiO2@(PI/SiO2) hybrid nanofibers. (c) Illustration of preparation of the cross section of in situ SiO2@(PI/SiO2) hybrid nanofiber for SEM. (d) SEM images of the cross section of in situ SiO2@(PI/SiO2) hybrid nanofibers embedded in epoxy matrix. (e) Illustration of the structure of the in situ SiO2@(PI/ SiO2) hybrid nanofiber. (f) XPS spectra of the pristine PI separator and the in situ SiO2@(PI/SiO2) hybrid separator. (g) TGA curves of the pristine PI separator and in situ SiO2@(PI/SiO2) hybrid separator.

smooth surface morphology. The in situ SiO2@(PI/SiO2) hybrid nanofibers were tightly and uniformly covered by a thin layer of particles. More images and the size distributions of the two types of nanofabrics can be found in Figures S1 and S2. Figures 2c−e and S3 show a hybrid structure of the in situ SiO2@(PI/SiO2) hybrid nanofiber. The SiO2 nanoparticle layer is the shell and the SiO2 nanoparticles-imbedded polyimide is the core. This unique core−shell structure confirmed the migration of TEOS in turn. Otherwise, the hybrid nanofiber would be a SiO2-particles-imbedded PI nanofiber instead of PI/SiO2 nanofiber with a core−shell structure. The imbedded SiO2 was converted from the residue TEOS inside of the nanofiber. Two possible reasons caused the hydrolysis of the residue TEOS. First, the water diffusion in the inside of the nanofiber occurred because the formed SiO2 layer on the fiber surface is a particles layer that contains gaps for water diffusion. Second, the water produced during the imidization of PAA can facilitate the hydrolysis of TEOS. X-ray photoelectron spectroscopy (XPS) was used to characterize the elements in the hybrid nanofiber membrane to study the composition of the shell and the fiber, as displayed in Figure 2f. The coating of a SiO2 layer onto the PI nanofibers was confirmed by the existence of apparent peaks of Si 2s and Si 2p in the XPS survey scan. The XPS data indicate that the atomic content of silicon and oxygen increases substantially, whereas the content of carbon and nitrogen decreases due to the SiO2 coating (PI: C 1s 75.72%, N 1s 6.49%, O 1s 17.05%, Si 2p 0.74%; PI/SiO2: C 1s 66.86%, N 1s 4.32%, O 1s 22.69%, and Si 2p 6.13%). Attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR) characterization was carried out on the nanofabric to investigate the chemical structure change during the preparation as shown in Figure S4. In the ATR-FTIR spectra, the characteristic absorption of polyamic acid appeared at 1716 cm−1 (Vco of −COOH),

of PAA and TEOS. Then, the PAA/TEOS nanofiber membrane underwent a thermal treatment, during which the PAA was converted into PI and TEOS migrated from inside to the surface of the nanofiber. The thermal treatment temperature (280 °C) is much higher than the boiling point of TEOS (168 °C). It seems that the high thermal treatment temperature may evaporate the TEOS agent. However, several factors can help realize the TEOS coating on the PI nanofiber surface. First, the TEOS is miscible with PAA. It acts as a plasticizer for the PAA or PI during the thermal treatment. This fact can increase the boiling point of TEOS and suppress its evaporation. Second, we believe the high-temperature thermal treatment was a driving force to induce the TEOS migration from the inside to the nanofiber surface, which finally helped the formation of uniform TEOS coating on the nanofiber surface. Third, the time of thermal treatment was controlled carefully. Overall, because the diffusion or evaporation of TEOS was slowed down by the PAA or PI matrix, the TEOS coating may form during the cooling process and not the thermal treatment process. After that, the PI/ TEOS nanofabric was subjected to in situ hydrolysis. The TEOS was hydrolyzed into Si(OH)4 first and formed a shell cover over the PI nanofiber. In the last step, the nanofabric was further thermally treated and the Si(OH)4 shell was converted into the SiO2 shell. Finally, an in situ SiO2@(PI/SiO2) hybrid separator was successfully fabricated. For more details, please see Section 2. The microstructures of this in situ SiO2@(PI/SiO2) hybrid separator were studied by SEM, X-ray photoelectron spectroscopy (XPS), and TGA. The results are shown in Figure 2. Figure 2a,b shows the SEM images of the pristine PI nanofibers and SiO2@(PI/SiO2) hybrid nanofibers, respectively. As shown, the diameters of pristine PI nanofibers are in the range 200−300 nm and the nanofibers show a rather 2981


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Figure 3. Mechanical properties and wetting behavior of the in situ SiO2@(PI/SiO2) hybrid separator. (a) Stress−strain curves of the pristine PI and in situ SiO2@(PI/SiO2) hybrid separator (the cartoon illustrates the mechanism for the mechanical reinforcement by the silica coating). (b) SEM and TEM images of the in situ SiO2@(PI/SiO2) hybrid separator showing the silica bonding among different fibers. (c) The influence of SiO2 loading on storage modulus of the in situ SiO2@(PI/SiO2) hybrid separator. (d) Demonstration of the excellent flexibility of the in situ SiO2@(PI/ SiO2) hybrid separator under different conditions: rolled, twisted, and folded, respectively. (e) Contact angle testing by electrolyte for Celgard2400, pristine PI separator, and the in situ SiO2@(PI/SiO2) hybrid separator, respectively. (f) Illustration of the wetting process for the three types of separators and digital photos showing the liquid electrolyte absorption ability.

1651 cm−1 (amide 1), and 1549 cm−1 (amide ll). After thermal treatment and in situ hydrolysis, the PAA was converted into PI. The absorbance peaks at 1780, 1720, 1380, and 720 cm−1 correspond to CO symmetric stretching, CO asymmetric stretching, C−N stretching, and CO bonding, respectively, indicating the existence of PI core. Meanwhile, the TEOS was converted into SiO2. The peaks corresponding to the siloxane (Si−O−Si) group (1079 cm−1) were observed obviously. The content of SiO2 in the hybrid nanofiber is about 10.9 wt %, which was obtained by TGA analysis as shown in Figure 2g. More TGA curves derived from the in situ SiO2@(PI/SiO2) hybrid separators prepared with different formulas can be found in Figure S5. From the TGA data, one can also find that the thermal stability was gradually improved from 511 to 561 °C with the increasing silica loading in the in situ SiO2@(PI/ SiO2) hybrid separator. All the above results confirm the unique silica@PI hybrid structure. 3.2. Mechanical Properties and Wettability of the In Situ SiO2@(PI/SiO2) Hybrid Separator. The mechanical properties of battery separator are the primary factors contributing to the device safety. In particular, high mechanical strength and good flexibility are critical for battery separators. It is well known that separators based on nonwoven fabrics are generally concerned by their weakness in mechanical properties mainly due to the weak interactions among different fibers. Here, the first contribution from the in situ formed silica nanoshell to the in situ SiO2@(PI/SiO2) hybrid separator is a significant improvement in mechanical properties. As shown in Figure 3a, the tensile strength of the nanofabric was remarkably increased from 8 to 42 MPa with the help of the silica nanoshell. This improved mechanical strength can meet the requirements for practical application in LIBs.36 Importantly, the storage modulus of the nanofabric was also substantially improved by five times from 77 to 391 MPa, which is beneficial

for maintaining the structural integrity under harsh compression condition.38,39 This significant improvement in mechanical properties is mostly due to the silica bonding at the contact area among different fibers, which works in a way similar to chemical cross-linking, as illustrated in Figure 3a. Indeed, the SEM and TEM images in Figure 3b clearly show some bonding and connections among different fibers. It is believed that the unique in situ technique for the fabrication of the silica nanoshell is the key to form strong silica bond among different fibers. The silica coating also improved the hardness of the PI nanofibric. Figure 3c shows that the storage modulus of the in situ SiO2@(PI/SiO2) hybrid separators increased significantly with the increasing SiO2 loading. At 300 °C, the in situ SiO2@(PI/SiO2) hybrid separator still shows a high storage modulus of 297 MPa, much better than that of pristine PI nanofabric. More DMA data are shown in Figure S6. Additionally, benefitting from the unique inorganic@polymer hybrid structure, the in situ SiO2@(PI/SiO2) hybrid separator also shows excellent flexibility. It can be rolled, twisted, and folded without damages, as shown in Figure 3d. Electrolyte wettability of battery separator is another critical property affecting the ion-transport resistance and electrolyte penetration during cell assembly. Polyolefin separators usually show undesirable wettability with polar liquid electrolytes owing to their intrinsically nonpolar nature and low surface energies. To investigate the influence of the silica coating on the surface properties of the nanofabric, contact angle tests using the electrolyte (EC/DEC/DMC, 1:1:1, v/v/v) for the pristine PI nanofabric, in situ SiO2@(PI/SiO2) hybrid separator and Celgard-2400 separator were performed, and the results are shown in Figure 3e. The contact angle of the in situ SiO2@(PI/SiO2) hybrid separator is only 11°, which is smaller than that of either the pristine PI nanofabric (16°) or Celgard-2400 separator (44°). The hydrophilic surface and 2982


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Figure 4. Thermomechanical stability and fire resistance of the in situ SiO2@(PI/SiO2) hybrid separator. (a) Thermomechanical property of the in situ SiO2@(PI/SiO2) hybrid separator as compared with pristine PI and Celgard-2400 separators. (b) Digital photos showing thermal shrinkage behavior for the Celgard-2400 separator at different temperatures. (c) Digital photos showing good dimension stability for both PI and in situ SiO2@(PI/SiO2) hybrid separators up to 300 °C. (d−f) Digital photos showing flammability of the Celgard-2400, pristine PI, and in situ SiO2@(PI/SiO2) hybrid separators.

the in situ SiO2@(PI/SiO2) hybrid separator still retains a low ion-transport resistance and gives rise to a slightly lower ion conductivity than the PI nanofabric. The uniformity of in situ SiO2@(PI/SiO2) hybrid separator was characterized by the thickness, area density, porosity, electrolyte uptake, and ionic conductivity, as shown in Table S2. The results show that the in situ SiO2@(PI/SiO2) hybrid separator has a uniform thickness. The repeatability of area density, porosity, electrolyte uptake, and ionic conductivity is good. 3.3. Thermomechanical Stability and Fire Resistance of the In Situ SiO2@(PI/SiO2) Hybrid Separator. In addition to improving mechanical properties and electrolyte wettability, the silica coating also significantly improved the thermomechanical stability and, more importantly, endowed the PI nanofabric with excellent fire resistance. Thermomechanical stability is a very critical property for battery separators, especially true for high-power and high-energydensity battery applications. As the “third electrode” of a battery, a thermally and mechanically stable separator can notably improve the safety when the battery is undergoing thermal runaway. Here, we further studied the thermomechanical stability of the in situ SiO2@(PI/SiO2) hybrid separator by thermal mechanical analysis (TMA). As displayed in Figure 4a, the in situ SiO2@(PI/SiO2) hybrid separator is thermomechanically stable up to 371 °C, ca. 40 and 230 °C higher than PI and Celgard-2400 separators, respectively. This result indicates that the in situ SiO2@(PI/SiO2) hybrid separator can work as a high-performance separator with good mechanical properties even at a high temperature of 371 °C, whereas Celgard-2400 will experience a fast mechanical failure only around 145 °C, which is the softening point of polypropylene. The influence of silica loading on thermal stability of the in situ SiO2@(PI/SiO2) hybrid separator is

porous structures of the in situ SiO2@(PI/SiO2) hybrid separator also notably improve the speed of the electrolyte absorption, as shown in Figure 3f. It was found that the liquid electrolyte spread much faster for the in situ SiO2@(PI/SiO2) hybrid separator as compared with the other two control samples, pure PI nanofabric, and Celgard-2400 separator. In about 5 s, the liquid electrolyte was completely absorbed by the in situ SiO2@(PI/SiO2) hybrid separator. The liquid electrolyte spreading speed is a good indicator of the interactions between the liquid electrolyte and the nanofabric. The above results demonstrate that the silica coating endows the nanofabric with a hydrophilic surface, which can notably improve the wettability of liquid electrolyte. The specific parameters for the in situ SiO2@(PI/SiO2) hybrid separators are summarized and compared with the other two control samples in Table S1. Porosity and the electrolyte uptake were calculated to determine the electrolyte retention ability. As shown in Table S1, the porosity of the in situ SiO2@(PI/SiO2) hybrid separator is 73%, which is much higher than that of the Celgard-2400 separator (41%) and lower than that of the PI nanofabric (85%). The lower porosity of in situ SiO2@(PI/ SiO2) hybrid separator than the PI nanofabric can be attributed to the hydrolysis process. During the fabrication, the nanofiber mat was wetted by spraying a solution of hydrochloric acid/ethanol/deionized water for hydrolysis. Because of the capillary force, the wetted nanofiber mat shrank and the porosity of nanofiber mat decreased consequently. The decrease of porosity results in the slight decrease in electrolyte uptake. The electrolyte uptake of the in situ SiO2@(PI/SiO2) hybrid separator (296%) is much higher than that of the Celgard-2400 separator (80%) and lower than that of the PI separator (332%). Although the SiO2 coating slightly reduces the porosity and electrolyte uptake capability, 2983


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Figure 5. Electrochemical performance of the in situ SiO2@(PI/SiO2) hybrid separator tested at room and high temperatures. (a) Charge− discharge curves at 0.1C at 25 °C. (b) C-rate capability at 25 °C. (c) Cycling performance with a high current density of 1C at 25 °C. (d) Cycling performance with a high current density of 1C at 120 °C. (e, f) Illustration of separator/electrode contact and its contribution to lithium ion transport for pristine PI separator and in situ SiO2@(PI/SiO2) hybrid separator, respectively.

solution for this is to develop fire-resistant battery components, mainly electrolyte and separators. Wang et al. reported a new type of fire-extinguishing OLE based on a common flameretardant agent (trimethyl phosphate) to improve the safety of batteries.40 Here, for nonflammable batteries, the next step is to develop fire-resistant separators as presented in this study. We have conducted ignition tests with a flame for the three types of separators, including Celgard-2400, pristine PI, and in situ SiO2@(PI/SiO2) hybrid separator, as shown in Figure 4d−f. It can be clearly observed that the Celgard-2400 separator shows no fire resistance and is highly flammable. The pristine PI separator shows much-improved fire resistance with self-extinguished behavior as compared with Celgard-2400, but it generates heavy smoke. In contrast, the in situ SiO2@(PI/ SiO2) hybrid separator cannot be even ignited, indicating excellent fire resistance and nonflammability. 3.4. Electrochemical Performance of the In Situ SiO2@(PI/SiO2) Hybrid Separator. To study the electrochemical performance of the in situ SiO2@(PI/SiO2) hybrid separator, half-cells with the configuration of LiFePO4/ separator/Li were assembled and the performance was tested at both room temperature and a high temperature of 120 °C.

shown in Figure S7. The thermal stability increased with increasing the silica loading in the in situ SiO2@(PI/SiO2) hybrid separator. The advantage in the thermal and dimensional stability of the in situ SiO2@(PI/SiO2) hybrid separator is further demonstrated in Figure 4b,c. It can be seen that the Celgard-2400 separator shows a huge thermal shrinkage around 150 °C and becomes even transparent at an elevated temperature around 200 °C, indicating a completed destruction of the separator structures mainly due to the melting of the components. In contrast, the pristine PI and the in situ SiO2@(PI/SiO2) hybrid separator do not show any shrinkage or color change from room temperature up to 300 °C. The above results indicate that PI-based fabric separators show a much better thermomechanical stability as compared with a traditional polymeric separator, and importantly, the silica nanocoating can further improve the thermomechanical stability of PI nanofabric. Fire resistance is another critical property enabled by the silica nanocoating. It is well known that traditional liquid electrolytes and polymeric separators are highly flammable. Once the thermal runaway happens, the battery will be at the risk of explosion or catching fire. Therefore, the fundamental 2984


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stability of cathode. In contrast, the cell with the PP separator only lasted for ca. 3 cycles at 120 °C due to the failure of the separator, and the Coulombic efficiency decreased and became unstable. Consequently, the cell was basically internally shorted after a few cycles. The above battery studies indicate that the in situ SiO2@(PI/SiO2) hybrid separator can comprehensively improve the electrochemical performance in a wide range of temperature. Even at a high temperature of 120 °C, which is around the dangerous level for conventional batteries, cells assembled with in situ SiO2@(PI/SiO2) hybrid separator still showed the highest capacity with excellent cycling stability among the three samples studied here. Limited by the liquid electrolyte, we did not investigate fire resistance of the whole battery. However, the significant improvement in both safety and performance at high temperatures has been successfully demonstrated by Figure 5d. More battery performance comparisons at room temperature are shown in Figures S9− S11. Figure S12 shows that with the increasing silica loading, the capacity was improved, especially at a high rate (5C). Figure 5e,f illustrates the possible mechanism for the performance improvement by in situ SiO2@(PI/SiO2) hybrid separator as compared with the PI separators, especially at high C rate. The main reason is that the rigid silica coating facilitates the Li+ transport at the interface between the nanofabric separator and electrodes. The smooth PI fiber may lead to tight contact and inactive interfaces when compressed with surfaces of the hard cathode or soft Li metal, which will block the Li+ transport through these interfaces. In contrast, the in situ SiO2@(PI/SiO2) hybrid separator shows rough and hard surface for the fibers. When compressed with the two electrodes, it will give rise to porous contact and interfaces that can provide channels for Li+ transport. As a result, there is more surface or interface area for Li+ transportation, which improves the battery performance, especially at high C rates. The above results indicate that the surface mechanics of separator and electrodes are critical factors affecting the battery performance, which is very instructive for the design of advanced battery materials. Table 1 summarizes our work and the reported studies on oxide-modified polymeric separators used in Li-ion batteries. Unlike the conventional methods, such as electrospinning/dipcoating or blend electrospinning, we combined electrospinning and inverse in situ hydrolysis to prepare in situ SiO2@(PI/ SiO2) hybrid separator. Benefitting from the organic/inorganic hybrid structure of every single nanofiber, the in situ SiO2@(PI/SiO2) hybrid separator possesses excellent fire resistance, which has rarely been achieved for polymeric separators. In addition, the in situ formed SiO2 shell fused together and provided strong bonding between nanofibers, which significantly improved the tensile strength of the separator. Specifically, the tensile strength is 42 MPa, which is the highest among fiber-based separators. The thermal stability of the PI separator is further enhanced, and it can maintain the good mechanical properties at 300 °C for at least 1 h. The in situ SiO2@(PI/SiO2) hybrid separator has neither color change nor thermal shrinkage after the treatment at 300 °C for 1 h, which is much better than that for all other oxidemodified PI separators and polymeric separators as listed in the table. The battery with the in situ SiO2@(PI/SiO2) hybrid separator also shows much higher capacity (150 mAh·g−1) at 2C than that of all other separators.

The Nyquist plots (see Figure S8) show that the lithium ion transport resistance of the symmetric cell with the in situ SiO2@(PI/SiO2) hybrid separator is slightly higher than that of PI nanofabric but much lower than that of Celgard-2400. This result can be explained by the much higher porosity and electrolyte uptake of the PI-based nanofabrics as summarized in Table S1. Figure 5a shows the charge−discharge profiles of the cells assembled with the pristine PI, in situ SiO2@(PI/ SiO2) hybrid, and Celgard-2400 separators. All of the half-cells exhibit stable charge−discharge profiles at 25 °C and 0.1C. The cell with the Celgard-2400 separator, pristine PI, and in situ SiO2@(PI/SiO2) hybrid separators delivered initial discharge capacities of 155.5, 164.1, and 164.5 mAh·g−1, respectively, indicating that the PI-based nanofabrics can improve the discharge capacity even at a low C rate of 0.1C. This is because the electrolyte uptake of SiO2/PI is higher than that of Celgard separator. The more electrolyte the separator contains, the higher efficiency the Li+ transportation is. It can help to take full advantage of electrode. Figure 5b compares the rate capabilities of the in situ SiO2@(PI/SiO2) hybrid separator with Celgard-2400 and pristine PI. Overall, the PIbased nanofabrics show a much improved C-rate capability as compared with Celgard-2400. More interestingly, the cell with the in situ SiO2@(PI/SiO2) hybrid separator exhibits a much better C-rate performance even compared with the PI nanofabric separator, especially at higher C-rates, such as 2C and 5C. Specifically, at 1C, the discharge capacity of the cell with the SiO2@(PI/SiO2) separator is ca. 95% of the discharge capacity at 0.1C. This capacity retention is higher than those of the other two samples (PI: 90%, Celgard-2400: 86%). When the current rate reaches 5C, the discharge capacity of the in situ SiO2@(PI/SiO2) hybrid separator is still as high as 84% of the discharge capacity at 0.1C. In contrast, for Celgard-2400 and PI separators, the retention is only ca. 62 and 77%, respectively. The improvement in rate capability further explains the advantage of silica coating in improving the Liions transport, which may be related to the excellent wettability with electrolyte as shown in Figure 3f, or the rough surface morphology of in situ SiO2@(PI/SiO2) hybrid nanofiber as shown in Figure 2b. The cycle performance with a high C-rate of 1C at both room temperature and high temperature (120 °C) was further investigated for the in situ SiO2@(PI/SiO2) hybrid separator and compared with the other two samples. Figure 5c shows the results of cycle stability test at 25 °C. Overall, all the three types of cells showed capacity retention of almost 100% and Coulombic efficiency of the three cells is stable at about 99.8% after 100 cycles. In this case, the advantage of using PI-based separators is only the high specific capacity. However, when the test was performed at high temperatures (e.g., 120 °C), as shown in Figure 5d, the advantages of using PI separators, especially in situ SiO2@(PI/SiO2) hybrid separator, become more significant. Specifically, the cells with the PI-based separators delivered very stable capacity over 100 cycles at 120 °C, and the in situ SiO2@(PI/SiO2) hybrid provides a higher capacity as compared with the pristine PI nanofabric. The Coulombic efficiency of the two cells is still stable at about 99.8%. In Figure 5c,d, the capacities using SiO2/PI separator at 25 and 120 °C are almost identical because of the good thermal stability of separator and electrode. As shown in Figures S13 and S14, the SEM images and ATR-FTIR spectra show that the morphology and composition of cathode do not change at both 25 and 120 °C, indicating good thermal 2985


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14

3.5

22

NA

NA

NA electrospinning SiO2−Nylon 6,6 composite

SiO2-coated PAN

SiO2-coated PVDF

electrospinning and dipcoating electrospraying and electrospinning sol−gel and elecrospinning

EC: ethyl carbonate; DEC: diethylene carbonate; DMC: dimethyl carbonate; EMC: ethyl methyl carbonate. bAfter exposure to 300 °C for 1 h, the separator shows no thermal shrinkage. cNA: not available.

Li|LiFePO4 1 M LiPF6 in EC/DEC (1:1 v/v)

Li|LiFePO4 1 M LiPF6 in EC/EMC (1:1 v/v)

graphite|Li(Ni0.5 Co0.2Mn0.3)O2/LiMn2O4 1 M LiPF6 in EC/DEC (1:1 v/v) Li|LiFePO4 1 M LiPF6 in EC/EMC (1:1 v/v) NA NA

200 °C for 30 min 150 °C for 30 min 150 °C for 30 min 150 °C for 30 min

NA NA

250 °C for 1 h

NA NA

4. CONCLUSIONS A unique SiO2@(PI/SiO2) hybrid structure is successfully fabricated as a fire-resistant and high-performance battery separator. The separator does not only improve the electrochemical performance but also the safety at especially high temperatures for the batteries. To achieve the excellent comprehensive performance of the novel in situ SiO2@(PI/ SiO2) hybrid nanostructure, a facile fabrication strategy was invented by combining electrospinning and inverse in situ hydrolysis. Armed with this inorganic shell, the PI nanofabric showed significant improvements in many aspects, and importantly, excellent fire resistance. In particular, the tensile strength increased by more than five times from 8 to 42 MPa, and tensile modulus increased by five times from 77 to 391 MPa due to the silica bonding among the nanofibers. The wettability to electrolytes, thermomechanical stability, and nonflammability were much advanced as compared with that of the bare PI nanofabrics. For battery performance, the LiFePO4 half-cell assembled with this hybrid separator showed a high capacity of 139 mAh·g−1@5C, which is much higher than that of the battery with a bare PI separator (126.2 mAh· g−1@5C) and Celgard-2400 separator (95.1 mAh·g−1@5C). More importantly, the battery showed a high capacity and excellent cycling stability with no capacity decay over 100 cycles at the high temperature of 120 °C. These advanced battery performances reveal that the in situ SiO2@(PI/SiO2) hybrid separator can provide fast ion transport with excellent performance stability over a wide temperature range, which is critical for both electrochemical performance and battery safety. Therefore, this study does not only bring up a novel method on the fabrication of inorganic−organic hybrid nanofabrics but also leads to an important strategy on the design of fire-resistant and high-performance polymeric separators for the development of durable energy storage systems with high energy and power densities, and importantly, nonflammability.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17521. SEM images of the pristine PI nanofibers and fiber diameter distribution, SEM images of the SiO2@(PI/ SiO2) nanofibers and fiber diameter distribution, crosssectional SEM image of the SiO2@(PI/SiO2) nanofiber, ATR-FTIR spectra of the pristine PAA nanofibers, PAA + TEOS nanofibers, pristine PI nanofibers, and SiO2@(PI/SiO2) nanofibers, TGA curves of the PI nanofibers and SiO2@(PI/SiO2) nanofibers derived from PAA/TEOS nanofibers with various PAA/TEOS ratios, DMA data of the PI nanofibers and SiO2@(PI/ SiO2) nanofibers derived from PAA/TEOS nanofibers with various PAA/TEOS ratios; physical properties of the pristine PI, in situ SiO2@(PI/SiO2) hybrid separator and Celgard-2400 separator; uniformity of the in situ SiO2@(PI/SiO2) hybrid separator, TMA curves of the PI nanofibers and in situ SiO2@(PI/SiO2) hybrid separator with different SiO2 loadings; the Nyquist plots of the symmetric cell with the pristine PI, in situ SiO2@(PI/SiO2) hybrid separator, and Celgard-2400 separators; battery performance of in situ SiO2@(PI/ SiO2) hybrid separator with different SiO2 loadings;

a

44

43

42

30

41

our work 31

160 (0.5C); 150 (2C) 140 (0.5C); 125 (2C) 102 (0.5C); 98 (2C) 135 (0.5C); 130 (2C) 160 (0.5C); 138 (2C) 158 (0.5C); 125 (2C) 148 (0.5C); 118 (2C) Li|LiFePO4 1 M LiPF6 in EC/DEC/DMC (1:1:1 v/v/v) Li|LiFePO4 1 M LiPF6 in EC/DMC/EMC (1:1:1 v/v/v) Li|LiMn2O4 1 M LiPF6 in EC/DEC (1:1 v/v) 300 °C for 1 h 42

nonflammable NAc

electrospinning and inverse in situ hydrolysis electrospinning and dipcoating electrospinning

in situ SiO2@(PI/SiO2) hybrid nanofabric SiO2/Al2O3-side-coated PI nanofabric PI−SiO2 composite nanofabric Al2O3 side-coated PI

in situ formation of uniform silica coating on all nanofibers SiO2−Al2O3/PI-fabric/SiO2−Al2O3 sandwiched membrane SiO2-embedded PI nanofiber membrane Al2O3/PI nanofibers/Al2O3 sandwiched membrane SiO2-nanoparticles-coated PVDF nanofiber membrane SiO2-coated PAN nanofiber membrane SiO2-embedded Nylon 6,6 nanofiber membrane

method fabric-based separators

structure

fire resistance

Table 1. Comparison of Modified Polymeric Separators Used in Li-Ion Batteries

tensile strength (MPa)

thermal dimensional stability

b

anode|cathode electrolyte

a

discharge capacity (mAh·g−1)

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comparison of battery performance at different C rates for in situ SiO2@(PI/SiO2) hybrid separators with different SiO2 loadings, SEM images of interface of cathode after cycling at (a) 25 °C and (b) 120 °C, ATRFTIR spectra of interface of cathode after cycling at 25 °C and 120 °C (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shengli Qi: 0000-0001-8871-7951 Wei-Hong Zhong: 0000-0002-1232-4147 Author Contributions ∥

L.K. and Y.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China [973 Program, 2014CB643604]; the National Natural Science Foundation of China [51673017]; the Natural Science Foundation of Jiangsu Province [BK20140006] and Changzhou Sci & Tech Program [CZ20150001]; National Natural Science Foundation of China [21404005]; Natural Science Foundation of Jiangsu Province [BK20150273].

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