Electrospun Nanofibers for Lithium-Ion Batteries

Electrospun Nanofibers for Lithium-Ion Batteries

CHAPTER ELECTROSPUN NANOFIBERS FOR LITHIUM-ION BATTERIES 22 Yunyun Zhai1, Haiqing Liu1, Lei Li1, Jianyong Yu2, Bin Ding2 College of Biological, Che...

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Yunyun Zhai1, Haiqing Liu1, Lei Li1, Jianyong Yu2, Bin Ding2 College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, China1; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China2

22.1 INTRODUCTION TO LITHIUM-ION BATTERIES Energy is one of the most important topics of this century. The increasing global energy crisis and environmental pollution have driven scientists and engineers to develop highly efficient and environmentally friendly methods of creating and storing renewable energy (Wang et al., 2015; Liu et al., 2010; Goodenough and Park, 2013). A battery is an electrochemical device that stores electrical energy as chemical energy in its anode and cathode during the charging process, and releases the energy when needed as electrical output during discharge (Lu et al., 2016). Batteries based on lithium-ion intercalation have become very important since the introduction of commercial lithium-ion batteries (LIBs) by Sony in 1991, as they show great promise as an appealing power source in a wide variety of applications (Etacheri et al., 2011), such as portable electronic devices, energy storage systems, electric vehicles, and hybrid electric vehicless. The main components of a LIB are the anode, cathode, separators, and electrolytes. Charge and discharge occur by a redox process in which lithium (Li) ions shuttle between electrodes, and the charge capacity depends on how much Li can be incorporated into the electrode materials (Choi et al., 2012), as shown in Fig. 22.1 (Etacheri et al., 2011). The electrochemical reactions occurring in the current generation of LIBs lithium cobalt oxide (LiCoO2 system) can be described as follows: LiCoO2 þ C!Li1x CoO2 þ Lix C During the charge process, Li ions are removed from the cathode and inserted into the anode, while the electrons are transported from the cathode to the anode through an external circuit (Choi et al., 2012; Etacheri et al., 2011). The characteristics of LIBs, such as long cycle life, high energy densities, high operational voltage, low self-discharge rate, and no memory effect, have attracted interest for academic research and industrial application (Armand and Tarascon, 2008). Many advances in LIB technology would not have been possible without the development of nanocomposites and nanometer-thick coatings to optimize ionic and electronic conduction pathways and block undesired and irreversible side reactions (Lu et al., 2016). The advantageous features of controllable fiber diameter, high porosity, high specific surface area, and interconnected pore structure endow electrospun nanofibers with high electronic and ionic Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00022-4 Copyright © 2019 Elsevier Inc. All rights reserved.

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FIGURE 22.1 Schematic presentation of the most commonly used lithium-ion battery based on graphite anodes and LiCoO2 cathodes. Lithium-ions migrate back and forth between the anode and cathode through the electrolyte-filled separator upon discharging/charging; electrons doing so similarly, through the outer electrical circuit. Reprinted with permission from Etacheri, V., Marom, R., Elazari, R., Salitra, G., Aurbach, D., 2011. Challenges in the development of advanced Li-ion batteries: a review. Energy & Environmental Science 4, 3243e3262. Copyright © 2011, Royal Society of Chemistry.

conductivity, which are beneficial for enhancing the cyclability and rate capability (Dong et al., 2011; Ding and Yu, 2014). Moreover, if necessary the nanofibers can be further functionalized to control the property (i.e., electrolyte affinity, pore size, and thermal stability) better, and thus improve the performance of the membrane-based cell. These characteristics make electrospun nanofibers well suited for assembly of LIBs. This chapter gives a detailed overview of recent advances of electrospun nanofibers in LIBs.

22.2 ELECTROSPUN NANOFIBER ANODES 22.2.1 CARBON NANOFIBER-BASED ANODES Carbon nanofiber anodes have attracted much attention over the years. Polymer nanofiber-based anodes are prepared by electrospinning and subsequent carbonization at high temperature (e.g., 1000 C) to obtain carbon nanofibers. To fabricate carbon nanofiber anodes using the electrospinning technique, various polymers such as polyacrylonitrile (PAN) (Ji et al., 2009), cellulose (Deng et al., 2013), poly(4-vinyl)pyridine (PVP) (Wang et al., 2012), polycarbosilane (Rose et al., 2010), and polyvinylidene fluoride (PVdF) (Yang et al., 2011) can be used as nanofiber templates and carbon precursors. Among these precursors, PAN is widely used although it requires an additional oxidation step at a low temperature (e.g., 250e300 C) to stabilize and maintain the fibrous morphology of the

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spun PAN nanofibers (Ji et al., 2009; Ji and Zhang, 2009; Peng et al., 2016). By a judicious combination of electrospinning and thermal treatment, Kim and his coworkers fabricated dimensionally thin, mechanically tough, electrically conductive web-based electrodes comprising interconnected PANbased carbon nanofiber anodes with diameters in the range of 200e300 nm (Kim et al., 2006). Owing to the mechanical toughness, large accessible surface area, and relatively good electrical conductivity, the PAN-derived carbon nanofibers exhibited reversible capacity of 450 mAh g1 and high rate capability of 350 mAh g1 at a charge current of 100 mA g1 (Kim et al., 2006). Benefiting from unique structural features of short pathways and fast kinetics for both Li ions and electrons, carbon nanofiber anodes fabricated from PAN/PPy displayed high reversible capacity (454 mAh g1 after 50 cycles) and relatively good rate capability (Ji et al., 2010). Electrospun carbon nanofiber anodes can also be made from carbon nanofiber composite. Yu et al. prepared novel [email protected] nanoparticles encapsulated in bamboo-like hollow carbon nanofibers by pyrolysis of tributyltin (TBT) (core)/PAN (sheath) nanofibers through a coaxial electrospinning method, as shown in Fig. 22.2A. Transmission electron microscopy (TEM) confirmed the formation of

FIGURE 22.2 (A) Preparation of [email protected] nanoparticles encapsulated in hollow carbon nanofibers. (B) Bright-field zero-loss filtered elastic TEM micrograph of the pyrolyzed nanofibers obtained by calcining the composite in Ar/H2 at 1000 C for 5 h. (C) Elemental mapping of the nanofibers showing the chemical distribution of carbon (blue) and tin (yellow). (D) HRTEM and SAED (inset) images of the edge of an isolated [email protected] nanoparticle encapsulated in a hollow carbon nanofiber. Reprinted with permission from Yu, Y., Gu, L., Wang, C., Dhanabalan, A., Van Aken, P.A., Maier, J., 2009. Encapsulation of [email protected] carbon nanoparticles in bamboo-like hollow carbon nanofibers as an anode material in lithium-based batteries. Angewandte Chemie International Edition 48, 6485e6489. Copyright © 2009, Wiley-VCH.

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a coreeshell structure (Fig. 22.2B), and element mapping, high-resolution transmission electron microscopy (HRTEM), and selected area electrondiffraction (SAED) investigations demonstrated the presence of single-crystalline metallic tin and graphitic carbon (Fig. 22.2C and D). Owing to the specific structure, the prepared anodes exhibited a high reversible capacity of 737 mAh g1 after 200 cycles at 0.5 C and a reversible discharge capacity as high as 480 mAh g1 at 5 C rate (Yu et al., 2009). The hollow graphitic carbon nanospheres in amorphous carbon nanofibers used as anode materials show high reversible capacity (750 mAh g1 and 1.1 Ah cm3), excellent high-rate performance (300 mAh g1 at a rate of 8.2 C), and good cycling stability (Chen et al., 2012b). The electrospun carbonesilicon (Si) composite nanofibers exhibit large reversible capacity up to 1240 mAh g1 and stable capacity retention even after 40 cycles, and ex situ scanning electron microscopy reveals that the fibrous morphology can effectively prevent the electrode from mechanical failure due to the large volume expansion during Li insertion in Si (Wang et al., 2010). Carbonization temperature has proved to be a key factor for electrospun carbon nanofiber anodes, since it affects their morphology and electrochemical performance. Yu et al. prepared tin nanoparticle-dispersed carbon (Sn/C) nanofibers by stabilizing electrospun SnCl2/PAN fibers and subsequently carbonizing at different temperatures; the Sn/C nanofibers produced at 700 and 850 C had the highest charge (785.8 and 811 mAh g1) and discharge (1211.7 and 993 mAh g1) capacities due to not only the combined advantages of the Sn (high Li storage capacity) and carbon (stable cycling) matrix but also the unique feature of reticular nanofiber geometries (Yu et al., 2010). Electrospun carbon nanofiber anodes loaded with metal oxide have attracted much attention as anode materials in LIBs because of their high theoretical capacity, long cycle life and high recharging rates. Owing to the elimination of Li4Ti5O12 aggregates and the formation of a carbon-based fiber structure, which provides short pathways for both Li ions and electrons, the Li4Ti5O12/C fibers exhibit even higher capacity, greater rate performance, and smaller electrode polarization (Guo et al., 2011). Titanium dioxide (TiO2)ecarbon composite nanofibers were prepared by thermal pyrolysis and oxidization of electrospun titanium (IV) isopropoxide/PAN nanofibers, and exhibited highly reversible capacity of 206 mAh g1 up to 100 cycles at current density of 30 mA g1 with high coulombic efficiency of nearly 100% (Yang et al., 2012). Similarly, the characteristics of abundant inner spaces, high electric pathway, and high structural stability made the charge transfer resistance of the carbon nanofiber/ Mn3O4 much smaller than that of Mn3O4 powder. Carbon nanofiber/Mn3O4 coaxial nanocables delivered an initial capacity of 1690 mAh g1 and maintained a high reversible capacity of 760 mAh g1 even after 50 cycles at 100 mA g1 without any obvious capacity fading (Park and Lee, 2015).

22.2.2 SILICON-BASED NANOFIBER ANODES Si is an exceptional anode material due to its extraordinary lithiation capacity of 3579 mAh g1, but during Li insertion and extraction Si undergoes volume expansion and shrinkage, which undermines the advantage of Si’s high capacity. To overcome these problems, mesoporous Si nanofibers (m-SiNFs) were fabricated. An aqueous solution of sacrificial polymer template polyacrylic acid (PAA) and colloidal SiO2 nanoparticles was firstly electrospun to obtain SiO2 nanofiber webs; after heat treatment at 500 C for 1.5 h the polymer template was removed, which gives rise to SiO2 nanofibers; finally, the prepared SiO2 nanofibers were reduced by Mg powder at 650 C under an Ar atmosphere, and the MgO formed during the reduction process was removed by etching with HCl (Fig. 22.3A) (Lee et al., 2013). Due to the formation of mesoporous and interconnected nanostructures, the cells fabricated using these

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FIGURE 22.3 (A) Schematic illustration showing the synthesis of mesoporous silicon nanofibers (m-SiNFs). (B) Rate capability of the m-SiNFs electrode measured at a series of current rates. Reprinted with permission from Lee, D.J., Lee, H., Ryou, M-H., Han, G-B., Lee, J-N., Song, J., Choi, J., Cho, K.Y., Lee, Y.M., Park, J-K., 2013. Electrospun three-dimensional mesoporous silicon nanofibers as an anode material for high-performance lithium secondary batteries. ACS Applied Materials & Interfaces 5, 12005e12010.). Copyright © 2013, American Chemical Society.

m-SiNFs exhibited a reversible capacity as high as 2846.7 mAh g1 at 0.1 A g1, a stable capacity retention of 89.4% at 2 A g1 over 100 cycles, and a rate capability of up to 36 A g1 (1214.0 mAh g1) (Fig. 22.3B). Amorphous Si (a-Si) has been intensively studied as one of the most attractive candidates for high-capacity and long-cycle-life anodes in LIBs primarily because of its reduced volume expansion characteristic (w280%) compared to crystalline Si anodes (w400%) after full Liþ insertion. Hence Si-based multicomponent amorphous alloy Si60Sn12Ce18Fe5Al3Ti2 nanofibers with a graphene wrapping layer were fabricated, as shown in Fig. 22.4A. Due to the role of the graphene shell in minimizing and stabilizing the SEI layers, Si60Sn12Ce18Fe5Al3Ti2 nanofibers wrapped in reduced graphene oxide (rGO) demonstrated remarkable cycle performance with 569.77 mAh g1 at the 2000th cycle (99.93% capacity retention), which was approximately 200 mAh g1 higher than that of Si60Sn12Ce18Fe5Al3Ti2 nanofibers throughout the cycling range (Fig. 22.4B). Moreover, rGO-wrapped Si60Sn12Ce18Fe5Al3Ti2 nanofibers exhibited better rate capabilities, with discharge capacity of 255 mAh g1 at 10 C (Fig. 22.4C) (Jung et al., 2015).

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FIGURE 22.4 (A) Schematic illustrations of the synthesis of the graphene-wrapped Si60Sn12Ce18Fe5Al3Ti2 nanofibers. (B) Comparative charge/discharge cycling property of the Si60Sn12Ce18Fe5Al3Ti2 nanofibers and Si60Sn12Ce18Fe5Al3Ti2 [email protected] at a current density of 0.5 C. (C) Rate capability tests at rates varying from 0.05 to 10 C. Reprinted with permission from Jung, J-W., Ryu, W-H., Shin, J., Park, K., Kim, I-D., 2015. Glassy metal alloy nanofiber anodes employing graphene wrapping layer: toward ultralong-cycle-life lithium-ion batteries. ACS Nano 9, 6717e6727. Copyright © 2015, American Chemical Society.

22.2.3 METAL OXIDE NANOFIBER ANODES Tin dioxide (SnO2) has attracted tremendous interest because of its high capacity of 781 mAh g1. It is reported that uniform polycrystalline SnO2 nanofibers consisting of SnO2 nanoparticles bonded in an orderly way along the uniform nanofiber were obtained by thermal pyrolysis and oxidization of electrospun tin (II) 2-ethylhexanoate/PAN nanofibers in air. This special structure not only facilitated electrolyte diffusion and charge transfer but also alleviated the mechanical strain from volume changes during the charge/discharge process. The SnO2 nanofibers delivered a specific capacity of 446 mAh g1 after 50 cycles at 100 mA g1 and excellent rate capability of 477.7 mAh g1 at 10.0 C (Yang et al., 2010). Hematite (a-Fe2O3) is an appealing anode material owing to its high theoretical capacity of 1007 mAh g1, low cost, environmental friendliness, and high resistance to corrosion (Chaudhari and Srinivasan, 2012). Interconnected hollow-structured a-Fe2O3 nanofibers were synthesized by calcining electrospun Fe (acac)3ePVP composite fibers. Owing to the interconnected hollow structure and large aspect ratio, the a-Fe2O3 fiber anodes exhibit a high reversible capacity of 1293 mAh g1 after 40 cycles at 60 mA g1 and excellent rate capability of 1001 mAh g1 at 800 mA g1 (Fig. 22.5) (Chaudhari and Srinivasan, 2012). Similarly, TiO2 has a theoretical capacity of 336 mAh g1 and its major voltage plateau lies above 1.5 V, which renders the material resistant to electrolyte reaction and subsequent dendritic SEI layer formation. Hence TiO2 is considered one of the most promising anode candidates for improving the LIBs

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FIGURE 22.5 (A) Cycling performance of the a-Fe2O3 hollow fibers at 60 mA g1. (B) Rate capability of a-Fe2O3 hollow fibers at different current density. Reprinted with permission from Chaudhari, S., Srinivasan, M., 2012. 1D hollow a-FE2O3 electrospun nanofibers as high performance anode material for lithium ion batteries. Journal of Materials Chemistry 22, 23049e23056. Copyright © 2015, Royal Society of Chemistry.

safety. TiO2 nanofibers were prepared by electrospinning a PVPetitanium (IV) isopropoxide precursor solution and subsequently calcinating it in various atmospheres to achieve anataseerutile mixed-phase crystallites with and without a carbon coating. It was found that argon-calcined TiO2 fibers exhibited the highest reversible galvanostatic capacity of 250 mAh g1 at 0.1 C rate, which was 27% higher than air-calcined fibers. The improvement was attributed to the alternative Li migration pathway provided by extra oxygen vacancies and the facilitation of electron conduction (Qing et al., 2015). Luo et al. synthesized porous ZnCo2O4 nanotubes with diameters of 200e300 nm and lengths up to several millimeters by an easy single-nozzle electrospinning strategy combined with subsequent heating treatment (Luo et al., 2012). The prepared ZnCo2O4 nanotubes exhibited reversible capacity of 794 mAh g1 at a current density of 2000 mA g1 after 30 cycles, making them a promising candidate for LIB anodes (Luo et al., 2012). Moreover, it was reported that a simple and efficient approach for the synthesis of one-dimensional a-Zn2V2O7 nanofibers was electrospinning followed by an annealing process (Luo et al., 2015). Owing to the short ion diffusion path and continuous electron transportation provided by the nanofiber architectures, mesoporous structures, and relatively large specific surface area, prepared a-Zn2V2O7 nanofibers had a reversible capacity of 708 mAh g1 after 100 cycles at a current density of 50 mA g1 and an excellent rate capability of 311 mAh g1 at a current density of 2000 mA g1 (Luo et al., 2015).

22.3 ELECTROSPUN NANOFIBER CATHODES 22.3.1 LITHIUM TRANSITION METAL OXIDE NANOFIBER CATHODES Commercial LIBs usually use Li cobalt oxide (LiCoO2) cathodes because of its easy preparation and good cycling life. Electrospun LiCoO2 nanofiber has been used to enhance the specific area and liquid

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diffusion rate. It was found that the addition of vapor-grown carbon fiber (VGCF) in the precursor solution could decrease the diameter of the LiCoO2 wire structure due to the effect of VGCF suppressing the crystal growth of LiCoO2 particles. Although the resulting materials showed the reversible Li-ion insertion/extraction reaction, the interfacial resistance increases resulted in poor cyclability and lower rate capability, especially for LiCoO2 nanofibers fabricated with VGCF (Mizuno et al., 2012). To enhance the stability and cycling performance, coreeshell LiCoO2eMgO nanofibers were prepared by coelectrospinning combined with a solegel process. The MgO coating on the LiCoO2 effectively avoided impedance growth by protecting the surface from passive surface film formation during cycling, thus the core-shell LiCoO2eMgO nanofibers had a higher capacity retention of 90.0% after 40 cycles compared to the uncoated LiCoO2 nanofibers (52% retention) (Gu et al., 2007). LiFePO4 is gaining significant attention because of its relatively low cost, acceptable operating potential (3.4 V vs. Liþ/Li), high theoretical capacity (170 mAh g1), environmental benignity, and safety. However, its low electronic and ionic conductivity lead to low rate capability and high impedance in LIBs. To avoid these limitations, LiFePO4/C submicrofibers were prepared by electrospinning a LiNO3, Fe(NO3)3$9H2o, Nh4H2PO4, citric acid, and PVP precursor solution and calcinating in a flow gas mixture of H2 (10 vol%) and Ar (90 vol%). The LiFePO4 submicrofibers are covered by an amorphous carbon layer with a thickness of about 2.6 nm, which may improve the electronic conductivity (Chen et al., 2012a). Moreover, Leng et al. prepared composite fibers of LixFe0.2Mn0.8PO4/C (x ¼ 1.05, 1.1, 1.2) by electrospinning the precursor solution and then calcinating. Li1.2Fe0.2Mn0.8PO4/C nanofibers prepared with a polyethylene glycol precursor displayed the highest capacity of 174 mAh g1 at 0.05 C and the best cycling property, which can be ascribed to the porous microstructure and the stable olivine structure of LixFe0.2Mn0.8PO4/C composite fibers (Leng et al., 2016). Spinel LiMn2O4 is highly promising cathode materials because of its low cost, high power density, and environmentally benign nature, but it suffers capacity fading during chargeedischarge cycles. Wang et al. prepared porous LiMn2O4 nanofibers of 150 nm diameter and 20 mm length by electrospinning. The network-structured LiMn2O4 nanofibers exhibited much lower degradation due to the suppressed Mn3þ disproportionation process (Zhou et al., 2014a). Doping with Ni had solved the rapid capacity fading problem at higher charging rates of manganese spinel LiMn2O4 by maintaining the oxidation state of Mn to 4þ which had a comparatively smaller ionic radius than Mn3þ (Haridas et al., 2016). Owing to the fast Li-ion transport provided by the presence of uniform nanosized grains and substantial amounts of conductive Mn3þ (Fig. 22.6A), the LiNi0.5Mn1.5O4 (LNMO) caterpillar structures based cell exhibited stable cyclability of 118 mAh g1 at 1 C rate after 100 cycles with retention of 98.3% (Fig.22.6B) (Haridas et al., 2016). Recently, Li-rich cathode materials xLi2MnO3$(1-x)LiMO2 (M ¼ Ni, Co, Mn, etc.) have attracted attention due to their high capacity of more than 250 mAh g1 and high operating voltage of 3.5 V versus Li/Liþ. It was reported that the process of electrospinning Li1.2Ni0.17Co0.17Mn0.5O2 leads to the formation of an effective conducting nanofiber with improved intercalation kinetics. The electrospun Li1.2Ni0.17Co0.17Mn0.5O2 nanofibers showed higher initial discharge capacity of 256 mAh g1 and better rate capability compared to a coprecipitated Li1.2Ni0.17Co0.17Mn0.5O2 particle sample (Min et al., 2013).

22.3.2 TRANSITION METAL OXIDE NANOFIBER CATHODES Among transition metal oxides, vanadium pentoxide (V2O5) has drawn significant interest over the decades for its high theoretical capacity of 510 mAh g1. However, the electrochemical performance

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FIGURE 22.6 (A) Cyclic voltammetry plot of LNMO powder and LNMO caterpillar rate. (B) Cyclability of the cell with LNMO powder and LNMO caterpillar at 1 C rate. Reprinted with permission from Haridas, A.K., Sharma, C.S., Rao, T.N., 2016. Caterpillar-like sub-micron LiNi0.5Mn1.5O4 structures with site disorder and excess Mn3þ as high performance cathode material for lithium ion batteries. Electrochimica Acta 212, 500e509. Copyright © 2016, Elsevier.

of bulk V2O5 has been restricted by the slow Li-ion diffusion rate and low ionic conductivity. To overcome these problems, electrospun V2O5-based nanofibers have been widely investigated. An rGO and V2O5 (GVO) nanowire composite was prepared by wet mixing electrospun V2O5 nanofibers and rGO. As shown in Fig. 22.7A, electrospun V2O5 nanofibers were firstly prepared from a solution of vanadyl acetylacetonate and poly(vinylpyrolidone) dissolved in N,N-dimethylformamide (DMF); then V2O5 nanofibers were ground to nanowire shape and fully dispersed in ethanol, after which rGO was placed in the solution; and finally slowly evaporation of the solvent and drying in a vacuum produced GVO composite (Pham-Cong et al., 2014). Because of the introduction of highly conductive rGO, compared to pure V2O5 nanowires the GVO composite with 1 wt% rGO exhibited higher initial discharge capacity of 225 mAh g1 and 60 cycles discharge capacity of 125 mAh g1 between 2.0 and 4.0 V at 0.2 C rate (Fig. 22.7B and C). Single crystalline b-Ag0.33V2O5 nanorods with self-limited aggregation were prepared by electrospinning followed by a hydrothermal process (Wu et al., 2013). Due to the ultrafine crystallinity and nanomorphology induced via the electrospinning technique, the prepared b-Ag0.33V2O5 nanostructures showed a high initial capacity of 250 mAh g1 and improved cycling stability, with a capacity loss of only 1 mAh g1 per cycle after 30 runs (Wu et al., 2013). Moreover, porous V2O5 nanotubes have been synthesized by a simple electrospinning technique followed by an annealing process using a low-cost inorganic vanadium source. Due to the high Li diffusion rate caused by the hollow interconnected porous structure and the existence of pyrolysis carbon, the prepared porous V2O5 nanotubes displayed good cycling performance with a capacity retention of 97.4% after 200 cycles at 50 C (Li et al., 2015).

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FIGURE 22.7 (A) Schematic illustration for fabrication of nanofibers and synthesis route of the GVO composite. (B) Initial charge-discharge curves and (C) cyclability of the cell using V2O5 nanowires and GVO composite. Reprinted with permission from Pham-Cong, D., Ahn, K., Hong, S.W., Jeong, S.Y., Choi, J.H., Doh, C.H., Jin, J.S., Jeong, E.D., Cho, C.R., 2014. Cathodic performance of V2O5 nanowires and reduced graphene oxide composites for lithium ion batteries. Current Applied Physics 14, 215e221. Copyright © 2014, Elsevier.

22.4 ELECTROSPUN NANOFIBER SEPARATORS Among major battery components, separators play an important role in preventing electronic contact between the cathode and anode electrodes to avoid short circuit, and also enable the transport of ionic charge carriers between electrodes (Zhang, 2004, 2007). Currently, polyolefin separators are widely

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used in LIBs because of their characteristics of high mechanical strength, good electrochemical stability, and small pore size (Zhang, 2004). However, their intrinsic limitations, such as poor thermal stability and sluggish ionic transport (Fang et al., 2013), urgently need to be overcome. Electrospun nanofiber separators have been widely studied for their high porosity and high ionic conductivity. This section describes recent advances in the area of electrospun nanofiber separators for LIBs.

22.4.1 ELECTROSPUN POLYMER NANOFIBER SEPARATORS 22.4.1.1 Electrospun PVdF-Based Nanofiber Separators Electrospun PVdF nanofibers have attracted significant attention because of the high dielectric constant and excellent anodic stability of PVdF (Choi et al., 2004; Kim et al., 2004). X-ray diffraction (XRD) and Fourier transform (FT)-Raman suggest that electrospun PVdF nanofibers have a mixed-crystal structure comprising both Form II (a-type) and Form III (g-type) (Choi et al., 2007). Due to their nonwoven state without interface bonding, electrospun PVdF nanofibers possess poor mechanical strength, so thermal treatment was used to enhance their mechanical properties (Choi et al., 2004; Liang et al., 2013). After heat treatment at 160 C for 2 h, the liquid electrolytesoaked PVdF nanofibers showed a mechanical strength of 9.7 MPa, a high ionic conductivity of 1.35 mS cm1 at room temperature, a good electrochemical stability up to 4.8 V versus Liþ/Li, and a relatively low interfacial resistance of 93.5 U cm2 (Liang et al., 2013). The crystalline part of PVdF hinders the migration of Liþ ions, hence batteries with PVdF-based electrolytes exhibit lower charge/discharge capacities and poor C-rate values. To overcome these drawbacks, other polymers (PAN (Gopalan et al., 2008), PEO (Prasanth et al., 2014), and poly(methyl methacrylate) (PMMA) (Li et al., 2011; Zhou et al., 2013a) et al.) were introduced to prepare PVdF-based composite nanofibers. The PVdF/PAN nanofibers exhibited a high conductivity of 7.8 mS cm1 at 25 C and were electrochemically stable up to 5.1 V (Gopalan et al., 2008). However, the PVdF-based fibrous membranes had low thermal stability with a melting point of 172 C, so there would be a safety problem in high-power LIBs. To resolve the problem, Ding et al. fabricated sandwich-structured PVdF/PMIA/PVdF (V/M/V) nanofibers using a sequential electrospinning technique (Fig. 22.8A) (Zhai et al., 2014). Due to the introduction of the high-strength PMIA (poly(m-phenylene isophthalamide) fibers and the formation of a bonding structure between the adjacent layers, the V/M/ V composite membranes exhibited a robust tensile strength of 13.96 MPa (Fig. 22.8B) and showed no dimension shrinkage after exposure to 180 C for 1 h (Fig. 22.7C and D). Moreover, the fusion of the interconnected pores in the PVdF layers could endow the V/M/V nanofibers with thermal shutdown like that of polyolefin trilayer separators, which is beneficial for improving the safety performance of LIBs. Two important copolymers of PVdF, P(VdF-HFP) (Li et al., 2007; Zhou et al., 2014b; Raghavan et al., 2014) and P(VdF-CTFE) (Croce et al., 2011), have been developed for gel polymer electrolytes in LIBs. Copolymer components were introduced to reduce the crystallinity of the PVdF chain and thus increase the ionic conductivity. The electrospun P(VdF-HFP) nanofibers demonstrated high electrolyte uptake of 600% and high ionic conductivity of 2.8 mS cm1 (Zhou et al., 2014b). Srinivasan reported that P(VdF-HFP)ebutanedinitrile (BDN) composite nanofibers displayed higher ionic conductivity and better rate capability because of the addition of BDN (Shubha et al., 2014). It was also reported that the mechanical integrity of trilayer P(VdF-HFP)/PVC/P(VdF-HFP) nanofibers was

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(A)

(B)

(C)

(D)

FIGURE 22.8 (A) FE-SEM image of the cross-section view of V/M/V composite membranes. (B) Stress-strain curves of the as-prepared electrospun PVdf, V/M/V and PMIA nanofibers. Photographs of electrospun nanofibers and Celgard separator (C) before and (D) after thermal treatment at 180 C for 1h. Reprinted with permission from Zhai, Y., Wang, N., Mao, X., Si, Y., Yu, J., Al-Deyab, S.S., El-Newehy, M., Ding, B., 2014. Sandwich-structured PVdF/PMIA/PVdF nanofibrous separators with robust mechanical strength and thermal stability for lithium ion batteries. Journal of Materials Chemistry 2, 14511e14518. Copyright © 2014, Royal Society of Chemistry.

enhanced by the introduction of PVC (Angulakshmi and Stephan, 2014). However, the mechanical strength of 1.8 MPa still cannot satisfy the demand of practical applications (Angulakshmi and Stephan, 2014), so there is a great need for further enhancement of the mechanical strength of electrospun P(VdF-HFP)-based nanofibers.

22.4.1.2 Electrospun PAN-Based Nanofiber Separators PAN has many good characteristics, like high ionic conductivity, thermal stability, and good compatibility with Li electrodes, which could minimize dendrite growth during the chargeedischarge cycling process (Kim et al., 2014; Raghavan et al., 2011). Interactions between the nitrile (-CN) groups in PAN and Liþ ions in the electrolyte also contribute to enhanced ionic conductivity (Carol et al., 2011; Rao et al., 2012). Mechanical strength was improved by the addition of trimethylolpropane triacrylate (TMPTA), and PANeacrylate (1/0.5) nanofibers showed the highest ionic conductivity of up to 5.22 mS cm1 due to the swelling behavior of fibers and good affinity with liquid electrolytes (Kim et al., 2014). However, excess TMPTA results in a distorted morphology and low ionic conductivity (Kim et al., 2014). It was found that the coreeshell structured PANePMMA nanofibers

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FIGURE 22.9 (A) Impedance spectra of SS/separators/SS cells. (B) Linear sweep voltammogram of Li/separators/SS cells. (C) Initial interface impedance spectra of Li/separators/Li cells based on core-shell PAN-PMMA and pure PAN nanofibers. (D) Cyclic stability of the Li/LiCoO2 cells using different membranes up to 50 cycles. Reprinted with permission from Bi, H., Sui, G., Yang, X., 2014. Studies on polymer nanofibre membranes with optimized coreeshell structure as outstanding performance skeleton materials in gel polymer electrolytes. Journal of Power Sources 267, 309e315. Copyright © 2014, Elsevier.

present high ionic conductivity, good interface stability, and compatibility with Li electrodes, and thus exhibit excellent cycle performance compared to cells with PAN nanofibers and a commercial Celgard membrane (Bi et al., 2014), as shown in Fig. 22.9. Thermal stability was higher for PANePMMAe polystyrene (PS) nanofibers with higher PAN content, and ionic conductivity was improved by increasing PMMA and PS content (Prasanth et al., 2012).

22.4.1.3 Electrospun Polyimide-Based Nanofiber Separators

Polyimide (PI), a high-performance engineered plastic of outstanding thermal stability (>500 C) and excellent mechanical strength, can effectively avoid the short circuits caused by shrinkage at high temperatures over 150 C. Moreover, polar solvents can be strongly coordinated within the polymer

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FIGURE 22.10 Digital pictures of static contact angles of (A) Celgard membrane and (B) PI nanofibers. (C) Cyclability of the cell with PI nanofibers at 0.2 C rate. Reprinted with permission from Miao, Y-E., Zhu, G-N., Hou, H., Xia, Y-Y., Liu, T., 2013. Electrospun polyimide nanofiber-based nonwoven separators for lithium-ion batteries. Journal of Power Sources 226, 82e86. Copyright © 2013, Elsevier.

chains due to the PI electron donor and acceptor groups, which greatly enhance the ability of PI-based nanofiber separators to retain electrolytes (Miao et al., 2013). Usually, PI nanofibers are prepared by electrospinning the poly(amic acid) precursor followed by an imidization process (Ding et al., 2012; Miao et al., 2013). Miao et al. (2013) synthesized poly(amic acid) via the polycondensation of pyromellitic dianhydride and 4,40 -oxydianiline, and then electrospun and thermal imidized the output to obtain PI nanofibers (Miao et al., 2013). The resultant PI nanofibers showed better wettability for the electrolyte compared to a Celgard membrane due to the close polarity between the polar PI and the highly polar liquid electrolyte (Fig. 22.10A and B), thus a cell based on PI nanofiber demonstrated stable cyclability with capacity retention of almost 100% after 100 cycle at 0.2 C rate (Fig. 22.10C) (Miao et al., 2013). PI/PET composite nanofibers showed only about 2% thermal shrinkage at 180 C under an air atmosphere, and a cell with PI/polyethylene terephthalate (PET) composite nanofibers showed more stable cycling performance, with capacity retention ratios of 87.5% after 50 cycles and better rate capabilities than cells a Celgard membrane, indicating that they are promising separator candidates for LIBs (Ding et al., 2012). The onset temperature of degradation for the PI nanofibers was over 600 C and the electrospun PI nanofibers showed excellent flame retardancy, demonstrating great advantages in safety for LIBs (Wang et al., 2014).

22.4.1.4 Others Polymer Nanofibers Polyester (such as PET and poly(butylene terephthalate) (PBT)) could be used as the separator material in LIBs due to its good thermal stability, resistance to shrinkage, good mechanical properties, and excellent electronic insulation. The impedance measurements of electrospun PBT separators under conditions that mimic a battery environment showed thermal stability to 210 C, making them potentially much more abuse tolerant than polyolefin separators (Orendorff et al., 2013). The maximum tensile strength of PET nanofibers was 12.0 MPa with an elongation at break

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of 42%, which are beneficial in the fabrication of LIBs. A differential scanning calorimetry (DSC) test demonstrated that PET nanofibers gave a single endothermic peak around 255 C for melting, meaning that the PET nanofibers can withstand a high temperature; this makes them suitable for separator materials in LIBs in higher temperature applications (Hao et al., 2013). Coreeshell structured electrospun [email protected] nanofibers displayed high porosity of 75%, high ionic conductivity of 1.97 mS cm1, and superior electrolyte wettability, which are beneficial to improve the rate capability and cycle performance of LIBs (Zhou et al., 2013b). The obtained discharge capacity of a cell using [email protected] composite nanofibers after 100 cycles was 105 mAh g1, and the discharge capacity was 103 mAh g1 at 4 C rate and 85 mAh g1 at 8 C rate (Zhou et al., 2013b). To solve the thermal shrinkage, flammability, and wettability problems of conventional polyolefin separators, nanonet-structured poly(m-phenylene isophthalamide)epolyurethane (PMIAePU) nanofibers with enhanced thermostability and good wettability for LIBs were fabricated via a one-step electrospinning technique (Xiao et al., 2015). Due to the introduction of PMIA, the prepared PMIAePU membranes demonstrated improved thermostability (180 C) and excellent wettability with liquid electrolytes. A Li/LiFePO4 cell using PMIAePU nanofibers exhibited equal cycling stability and better rate capability compared with a cell using a Celgard membrane. Recently, x-polyethylene glycol diacrylate (x-PEGDA) coated polyetherimide (PEI)/ PVdF nanofibers were obtained by a simple combination of dip coating and free radical polymerization of PEGDA on the electrospun PEI/PVdF fiber membranes (Zhai et al., 2016). Due to the good affinity of PEI and PEGDA with liquid electrolytes, the x-PEGDA coated PEI/PVdF nanofibers were endowed with good wettability, high electrolyte uptake of 235.6%, and high ionic conductivity of 1.38 mS cm1. A Li/LiFePO4 cell based on x-PEGDA coated PEI/PVdF nanofibers displayed comparable cycling stability and better rate capability than a cell based on a Celgard membrane (Fig. 22.11A and B).

FIGURE 22.11 (A) Cyclabilities and (B) rate capabilities of Li/LiFePO4 cells assembled with Celgard membrane, PEI/PVdF and x-PEGDA coated PEI/PVdF nanofibers. Reprinted with permission from Zhai, Y., Xiao, K., Yu, J., Ding, B., 2016. Closely packed x-poly(ethylene glycol diacrylate) coated polyetherimide/poly(vinylidene fluoride) fiber separators for lithium ion batteries with enhanced thermostability and improved electrolyte wettability. Journal of Power Sources 325, 292e300. Copyright © 2016, Elsevier.

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22.4.2 ELECTROSPUN POLYMER/INORGANIC NANOFIBER SEPARATORS Inorganic nanoparticles have been used to fabricate separators for LIBs because they not only enhance the ionic conductivity, thermal stability, and compatibility with Li metal but also prevent dimensional changes by thermal deformation at high temperature because of their intrinsic heat resistance (Raja et al., 2014; Choi and Lee, 2011). Various ceramic fillers have been used for preparing the electrospun polymer/inorganic separators, including aluminum oxide (Al2O3) (Lee et al., 2014), silica (SiO2) (Yanilmaz et al., 2014; Jung et al., 2009), TiO2 (Zhou et al., 2013a), and Li aluminum titanium phosphate (LATP) (Liang et al., 2011). This subsection describes recent advances in the area of preparing electrospun polymer/inorganic nanofiber separators for LIBs.

22.4.2.1 Directly Adding Inorganic Nanoparticles to a Polymer Solution Directly electrospinning a polymer solution containing inorganic nanoparticles is the simplest method to obtain polymer/inorganic nanofibers. The Li-ion-conducting material LATP was synthesized by a citric acid-assisted solegel method, and dispersed in PAN solutions to prepare electrospun LATP/PAN nanofibers (Liang et al., 2011). It was found that as the LATP content increased, the electrospun LATP/ PAN composite nanofibers had higher electrolyte uptake, higher Li-ion conductivity, better electrochemical stability, and lower interfacial resistance with Li electrodes (Liang et al., 2011). Due to this characteristic, a Li/LiFePO4 cell using 15 wt% LATP/PAN composite nanofibers had stable cycling performance and slightly higher capacity retention ratio (91%) after 50 cycles than cells using a Celgard separator (84%) and PAN fiber-based nanofibers (88%) (Liang et al., 2011). Introduction of SiO2 nanoparticles (NPs) helped to tune the pore structure of SiO2ePEIePU nanofibers (Zhai et al., 2015b). Gradually increased wrinkles, nanoprotrusions, and SiO2 NPs were clearly visible on the fiber surface (Fig. 22.12AeE), and the average pore size decreased from 1.81 to 1.58 mm as the content of SiO2 NPs increased (Fig. 22.12F) (Zhai et al., 2015b). Compared with a cell based on a Celgard membrane, a Li/LiFePO4 cell based on SiO2ePEIePU nanofibers exhibited better cyclability and rate capability not only at room temperature but also at an elevated temperature of 60 C.

22.4.2.2 Dip Coating Inorganic Nanoparticles Another method for introduction of inorganic NPs is dip coating them on to electrospun nanofibers. Thin Al2O3 overlayers were coated on both sides of a PI separator via a dip-coating process, and the excellent electrolyte wettability gave the prepared Al2O3-coated PI separators more stable capacity cyclability of 95.53% retention after 200 cycles at 1 C and better rate capabilities (78.91% retention at 10 C) compared to a bare PI separator (68.65% retention at 10 C) and a commercial Celgard separator (18.25% retention at 10 C) (Lee et al., 2014). Some of the pores in the SiO2/PEIePU membranes were filled with SiO2 NPs connected by PVdF-HFP binders (Fig. 22.13A), thus decreasing the average pore size (Fig. 22.13B), which was beneficial for resolving thermal abuse tolerance and restraining the microshorting caused by Li dendrites (Zhai et al., 2015a). Although the SiO2 coating reduced porosity and electrolyte uptake, the SiO2/PEIePU membranes had higher ionic conductivity, which may be due to the Lewis acid base interactions between the SiO2 NPs and the polar groups of electrolytes (Zhai et al., 2015a).

22.4.2.3 In Situ SoleGel Method Solegel is another effective way to obtain inorganic NPs. Electrospun PVdFePMMA nanofibers with in situ TiO2 were prepared by adding acetic acid glacial and tetrabutyl titanate with vigorous stirring

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(B)

(C)

(D)

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FIGURE 22.12 FE-SEM images of the as-prepared SiO2-PEI-PU nanofibers containing different SiO2 contents (A) 0 wt%, (B) 2 wt%, (C) 5 wt%, (D) 8 wt%, and (E) 11 wt%. (F) Pore size distributions of the as-prepared SiO2-PEI-PU nanofibers. Reprinted with permission from Zhai, Y., Xiao, K., Yu, J., Yang, J., Ding, B., 2015b. Thermostable and nonflammable silicapolyetherimide-polyurethane nanofibrous separators for high power lithium ion batteries. Journal of Materials Chemistry 3, 10551e10558. Copyright © 2015, Royal Society of Chemistry.

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FIGURE 22.13 (A) FE-SEM images of the as-prepared SiO2/PEI-PU nanofibers. (B) Pore size distributions of Celgard membrane and as-prepared SiO2/PEI-PU nanofibers. Reprinted with permission from Zhai, Y., Xiao, K., Yu, J., Ding, B., 2015a. Fabrication of hierarchical structured SiO2/polyetherimide-polyurethane nanofibrous separators with high performance for lithium ion batteries. Electrochimica Acta 154, 219e226. Copyright © 2015, Elsevier.

(Zhou et al., 2013a). The PVdFePMMA nanofibers with in situ TiO2 possessed higher porosity, higher electrolyte uptake, and higher ionic conductivity. The partial swelling of the in situ TiO2 nanofibers with a large surface area significantly contributed to the increase in the electrochemical stability of the electrolyte solution, although ionic conduction occurred mainly through the entrapped liquid electrolytes in the pore structure. Thus the swollen phase of the nanofibers probably included complexes such as the associated VdFeLiþ groups, which is beneficial for enhancing the electrochemical stability of PVdFePMMA nanofibers with in situ TiO2. The high anodic stability of PVdFePMMA nanofibers with in situ TiO2 renders them very suitable for applications in LIBs (Zhou et al., 2013a). Thermoplastic polyurethane (TPU)ePVdF nanofibers with in situ SiO2 were fabricated by the hydrolysis and condensation reactions of tetraethoxysilane (TEOS). Electrospun TPUePVdF nanofibers with 3% in situ TiO2 showed high ionic conductivity of 4.8 mS cm1 with electrochemical stability up to 5.4 V versus Liþ/Li at room temperature. The reduction of crystallinity values seen in DSC data may result from the partial inhibition effect of SiO2 addition on polymer crystal formation (Wu et al., 2011). PVdF/SiO2 composite nanofibers were fabricated by electrospinning a mixed solution of PVdF and SiO2 sol prepared by hydrolyzing TEOS with ethanol (Zhang et al., 2014). Elemental analysis and TEM confirmed the successful introduction of SiO2, which provided substantial improvement in thermal shrinkage. Prepared PVdF/SiO2 composite nanofibers exhibited more stable cycle performance, a higher discharge capacity of 159 mAh g1, and higher capacity retention of almost 100%, which makes them promising candidates for separators in high-performance LIBs.

22.4.2.4 Other Method PMMA-grafted TiO2 (PMMA-g-TiO2) was synthesized by atom transfer radical polymerization and then electrospun with PVdF to fabricate composite nanofibers (Cui et al., 2013). The FT-IR spectrum

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and TEM images indicated that the PMMA was grafted successfully on to the surface of TiO2, and the electrochemical stability window and ionic conductivity of the polymer electrolyte were enhanced due to the increased amorphous content of PVdF and the excellent affinity between the liquid electrolytes and PMMA-g-TiO2.

22.5 CONCLUSIONS AND OUTLOOK Resolving the critical worldwide energy issue is an urgent global problem due to the increasing concern about the sustainable development of energy, economy, and society, which is closely related to efficient and versatile storage and consumption of energy. Nanofibers may play a key role in addressing these issues because of their unique structures and high surface area to volume ratios. Among the many nanofiber fabrication methods, electrospinning has attracted significant attention because of its ability to produce nanofibers from a wide variety of polymers. In addition, the required manufacturing hardware and operation parameters can be easily reconfigured to produce fibers with a variety of structures, such as coreeshell, hollow, or aligned fibers. The characteristics of electrospun nanofibers, such as controllable fiber diameter, remarkable specific surface area, high porosity, and interconnected porous structure, endow them with shorter diffusion pathways, faster Li intercalation kinetics, good mechanical strengths, high surface areas and porosities, mitigated charge transfer resistance, and enhanced ionic conductivities, which are beneficial for the development of high-power LIBs. Although better performance has been shown by nanofibers when used as the electrodes and separators of LIBs, several aspects should be taken into consideration. First, the specific surface area and pore structures of the nanofibers should be further improved to promote the cyclability and rate capability; second, the mechanical strength of nanofibers should be enhanced (especially in the fragile inorganic electrospun nanofibers) to meet the demands of practical application; and third, the costs have to be reduced before they can be exploited in commercial applications. Continuing laboratory research should shed light on forthcoming electrospinning products for LIBs.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51503079, 51103063 and 21177049), the Program for Science and Technology of Zhejiang (No. 2017C31071), and the Program for Science and Technology of Jiaxing (No. 2016AY13008).

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