Encyclopedia
Electrospinning spinning techniques enables spinning continuous nanofibers with typical diameters in the range of some ten to some hundred nanometers. Rechargeable or secondary batteries are electrochemical power sources commonly utilized in portable devices such as camcorders, mobile phones, laptops, and electric transportations. In general, batteries are comprised of one or more electrochemical cells. Positive electrode (cathode), negative electrode (anode), porous separator membrane, and ionic conductive electrolyte are the essential components for fabrication of each electrochemical cell, electrospun fibers could enhanced various characteristics of electrolytic cells.
- batteries
- Electrospun Nanofibers
1. Electrospinning Process
The electrospinning technique has been identified as a versatile and highly efficient method for fabrication of continuous nanofibers from polymer solution or melt. Generally, an electrospinning set-up contains a high voltage power supplier, a feeding pump, a spinning apparatus (such as spinneret), and a rotational/constant collector. In electrospinning procedure, ultrathin fibers are fabricated in an electrical field created between the collector and spinneret by applying a high voltage. The applied voltage, the distance between the spinneret and collector, the feeding rate, the spinneret inner diameter, and the collector speed belong to the common electrospinning parameters which can influence morphology of the electrospun membranes as well as fiber diameters. Fiber orientation, pore size distribution, and membrane porosity are significant morphology features of the electrospun membranes. Apparently, the morphology characteristics and fiber diameter should be tuned to obtain the most appropriate electrospun fibrous structure for the considered application [1][2][3]. As an example, fiber orientation may lead to production of an electrochemical biosensor with high sensitivity, while it can reduce ionic conductivity of an electrospun electrolyte applicable in lithium ion batteries [4][5].
Optimum electrospinning parameters should be determined for each polymer system as they vary from polymer to polymer. Overall, increasing the applied voltage up to a critical point causes formation of finer fibers due to more stretching of the electrospinning jet resulting from higher repulsion forces in it. On the other hand, thicker or beaded electrospun fibers can be obtained by exceeding the critical voltage as a result of higher velocity as well as smaller size of the Taylor cone [6]. The electrospinning distance should be recognized regarding the evaporation rate and deposition time of the electrospinning jet to gain a uniform membrane. A short electrospinning distance may lead to formation of ribbon-like nanofibers with large diameters, whereas thick fibers are fabricated through a high electrospinning distance [7]. In addition, a critical flow rate is essential to obtain homogenous and beadless electrospun fibers. By increasing the feeding rate beyond the critical point, defective fibers with high diameters in a wide range and large pore sizes may be obtained [8]. Moreover, the receiver type and collector speed mainly influence the fibers’ orientations and so the pore sizes. As the collector speed increases, fibers with higher orientation and less porosity and pore sizes are obtained [9].
Besides the apparatus adjustments, features of the polymer solution or melt influence the membrane morphology and fibers’ diameter. The role of polymer solutions on various features of the electrospun membrane depends on their concentration, viscosity, and conductivity. Enhancement of the solution concentration up to a critical point provides more entanglement between the polymer chains which results in formation of beadless fibers with higher uniformity. Nevertheless, beaded and defective fibers can be obtained beyond the critical point due to drying of the polymer solution on the applied spinneret tip. Notably similar trends have been observed for the viscosity impact on the obtained electrospun fibers by numerous researchers. Furthermore, the electrospinning process highly depends on the Coulomb forces between the electrical field and accumulated charges on the solution surface. Therefore, a polymer solution with very low conductivity cannot be electrospun due to lack of charges which are essential for formation of the Taylor cone. In contrast, polymer solutions with conductivity beyond the critical point cannot also be processed because of spreading of the fibers in the electrospinning environment [10][11].
2. Electrospun Nanofibers in Batteries
6 compound in the anode material. Apparently, the reverse behavior takes place in the discharge step [12][13]. Over the past decades, most research in advanced development of LIBs has emphasized the use of electrospun fibers for fabrication of versatile and highly efficient components [14][15]. Recent progresses in the fabrication of electrospun cathode, anode, separator, and electrolyte are provided in the following section.
Figure 1. Schematic showing the intercalation mechanism in Li-ion batteries through charge and discharge. Reprinted from [16], with permission from Elsevier.
2.1. Electrospun Cathode Materials
Electrochemical performance of the batteries, such as potential window and storage capacity, is mostly affected by the cathode material. In fact, the number of extracted lithium ions from the cathode electrode determines the battery capacity. The energy could be stored in the cathode materials through two various conversion and intercalation techniques. In the conversion mechanism, lithium insertion and extraction are associated with changes in the crystalline structure of the applied cathode material, while the cathode structure acts as a host in the intercalation mechanism. So, the lithiation/delithiation can reversibly occur in the intercalation cathode materials. Notably, low electron conduction as well as high volume expansion have been reported as challenges linked with the conversion cathode materials. Therefore, the intercalation cathode structures have received more attention from numerous researchers. Among various types of intercalation structures (transition metal oxides, chalcogenides, and poly anions), transition metal oxides and poly-anionic compounds have displayed superior characteristics such as higher energy storage and greater operating voltage, while development of chalcogenide materials has been influenced by their irreversible structure [17][18].
2
2
2
1−x
2
+
2
−1
−1
2
2
0.5
0.5
2
4+
−1
2
+
x
y
1−x−y
2
1/3
1/3
1/3
2
4
2
4
+
3+
0.5
1.5
4
2
4 spinel oxide structure [20][21].
4
3−
4
4 are of the well-known poly-anionic cathode materials. This could be linked with their great power capability and proper structural stability. Nevertheless, low conductivity of the aforementioned materials has restricted their applications [22][23].
Synthesis of highly efficient cathode materials is considered as a key building block toward progress of energy storage systems with high power and proper capacity in the future. Numerous researchers have illustrated great potential of the electrospun structures as cathode material of the Li-ion batteries. Besides storage capacity, cycling stability is considered as an important parameter for determination of the efficiency and capability of a designed cathode material. Cycling durability is measured by calculation of the storage capacity in various cycles. Apparently, a more ideal battery structure would be obtained through increment of the cycling stability. Commercial LIBs normally show cycling stability during first 300 to 500 cycles (or about 2 to 3 years) [24][25]. As an example, electrospun LiCoO
2
−1
2
−1) [26]. Notably, the enhanced capacity of the nanofibrous structures could also be further improved through modification of the electrospun fibers by coating methods. In addition, electrospun LiM
2
4 structures provide faster diffusion of the lithium ions as well as the promoted cycling stability. In fact, the highly porous structure of the electrospun fibers leads to reduction of the degradation rate during charging/discharging processes [27][28]. Moreover, presence of the carbon nanofibers in the poly-anionic compounds such as LiFePO
4 compensates the poor ionic conductivity of this category of cathode materials and causes approaching a more appropriate rate capability [29]. The most recent approaches in fabrication of electrospun fibers applicable as cathode materials are summarized in
Table 1.
| Material | Capacity (mAh·g | −1 | ) | Cycling Stability | Autor (Year) | Ref. |
|---|
| Li | 2 | CoTi | 3 | O | 8 | /TiO | 2 | 82 at 0.1 C | 83% after 25 cycles | Kap et al. (2020) | [30] | [99] |
| LiFePO | 4 | nanocrystals/carbon nanofibers (CNFs) | 152 at 0.5 C | 98.2% after 500 cycles | Cao et al. (2020) | [31] | [100] | |||||
| V | 2 | O | 5 | /GO | 342 at 0.5 C | 80% after 20 cycles | Ahmadian et al. (2020) | [32] | [101] | |||
| Li | 2 | MnTiO | 4+z | 210 at 0.1 C | 95.3% after 100 cycles | Vu et al. (2020) | [33] | [102] | ||||
| LiFe | 0.8 | Mn | 0.2 | PO | 4 | /C | 169.9 at 0.1 C | 160 after 200 cycles | Chen et al. (2020) | [34] | [103] | |
| LiFe | 0.4 | Mn | 0.6 | PO | 4 | /CNFs | 133.5 at 1 C | 138.8 after 100 cycles | Yang et al. (2020) | [35] | [104] |
2.2. Electrospun Anode Materials
+
−1 along with 100% discharge/charge efficiency have been recorded for this anode structure, whereas most of the electrolyte solvents (e.g., ethylene carbonate (EC), propylene carbonate (PC), and so on) are decomposed between 0.5 and 0.7 V, resulting in the formation of a solid–electrolyte interface (SEI) layer. It is worth noting that proper ionic conductivity, low electrical conductivity, and great stability are the essential characteristics of the ideal SEI layer. Overall, poor capacity is a major drawback associated with the graphite anode materials [36][37].
+ ions. Therefore, various studies have been devoted to fabrication of carbonaceous nanomaterials. PAN is the most common precursor for synthesis of electrospun carbon fibers. This could be linked with the simple fabrication procedure, proper mechanical characteristics, and affordable cost. However, environmental concerns associated with DMF, an essential solvent for dissolving PAN polymer, has led to investigation for new precursor resources including lignin, polyvinyl alcohol (PVA), and many more [36][37]. Kim et al. [38] reported a large capacity of 450 mAh·g
−1 derived from the electrospun PAN nanofibers. In another attempt, Chen et al. [39] introduced a large capacity of 1150 mAh·g
−1
−1 for a hollow CNT/CNF composite. In addition, Culebras et al. [40] claimed a high capacity of 611 mAh·g
−1 after 500 cycles for a CNF mat obtained from lignin/polylactic acid (PLA) precursor. Moreover, Nan et al. [41] revealed a large capacity of 841 mAh·g
−1
+ ions’ spaces. Further, it causes easier interaction between electrode and electrolyte components by reduction of the distances between ions and electronics [42].
3
4
2
x
y
x
y
x
y
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−1
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−1
4
+ ions. In order to suppress volume changes of the conversion-reaction-based anode structures, fabrication of porous nanomaterials has received wide attention. Existence of pores in such materials manages the volume changes during lithiation/dilithiation processes through providing sufficient spaces for extraction and contraction of the applied anode material. In addition, combination of these materials with the carbonaceous structures has been claimed as another effective method for control of the volume changes [36][37]. Zhang et al. [43] reported a large capacity of 835 mAh·g
−1
−1
3
4
−1 after 100 cycles [44]. Moreover, electrospun NiO fibers have exhibited a discharge capacity of 784 mAh·g
−1
−1 [45].
−1
−1 (Si). The major drawback linked with this materials are volume changes during insertion and extraction of the lithium ions, which could be suppressed by size reduction of the applied particles as well as combination with the carbonaceous materials [36][37]. Jang et al. [46] revealed a discharge capacity of 560 mAh·g
−1
−1
−1 after 100 cycles was claimed for the Si/CNF three dimensional structure [47]. Furthermore, a SnS/CNF composite membrane presented 648 mAh·g
−1
−1 after 500 cycles [48].
Table 2.
[72][73], and atomic layer deposition [74] are significant modification methods. In such procedures, a material is introduced into the electrospun separator, which results in improvement of its final properties. Direct electrospinning of the combination of two polymer solutions (e.g., PAN/PU [75], PAN/Lignin [76], PSA/PVDF-HFP [77], and so on) or filler-loaded polymer solution (e.g., PAN/SiO
2 [78], PI/Al
2
3 [79], Nylon6,6/TiO
2 [80], etc.) leads to the fabrication of composite separator membranes with enhanced hydrophilicity and heightened thermal stability. Besides the role of the polymer type, the morphology of the electrospun fibers also influences the obtained electrochemical behavior. As an example, fabrication of finer fibers results in increment of electrolyte uptake and so enhancement of the ionic conductivity. Therefore, the morphology of the electrospun fibers should be tuned to approach appropriate electrospun separators with proper electrochemical characteristics [81]. A summary of the recent progresses in the fabrication of electrospun separators is provided in
Table 3.
| Material | Porosity (%) | Tensile Strength (MPa) | Electrolyte Uptake (%) | Ionic Conductivity (mS·cm | −1 | ) | Autor (Year) | Ref. |
|---|
| PAN | 67.7 | 11.3 | 478.2 | 1.97 | Dong et al. (2020) | [82] | [151] | ||||
| PAN/PBS | 59.3 | 7.66 | 665 | 2.1 | Wei et al. (2020) | [83] | [152] | ||||
| PVA/ZrO | 2 | 78 | 14.5 | 350 | 2.19 | Xiao et al. (2020) | [84] | [153] | |||
| /CNF | |||||||||||
| PI/Al | 2 | O1635 at 1 A·g | −1 | Liu et al. (2020) | 3 | 81 | [52 | - | ] | [121] | |
| 912 | - | Iaritphun et al. (2020) | [ | 85 | ] | [154] | Sn | 4 | P | 3 | /CNF |
| PVDF-HFP/SiO | 2 | 710 | Ran et al. (2020) | [ | 89.7 | 53 | 5 |
2.4. Electrospun Electrolyte
+
+ ions between them. All-solid-state electrolytes are generally synthesized based upon polymeric structures and inorganic solid materials [91][92].
4
4
+
+ ions are transported between the electrodes through polymer chain local motions or hopping mechanism. So, reduction of the glass transition temperature as well as increment of the amorphous regions are key solutions for enhancement of the ionic conductivity of the polymer films [92][93]. Therefore, introduction of particulate fillers (such as SiO
2 [94], Al
2
3 [95], TiO
2 [96], and many more) and plasticizer molecules (such as EC, PC, etc.) into the polymer matrix have been reported as influential methods for enhancement of the ionic conductivity. Particulate fillers placed between polymer chains of the utilized polymer matrix cause reduction of the crystalline phases. Thus, the polymer chains would be able to move easily and so accelerate transportation of the Li
+ ions. In addition, inserted fillers enhance ion pair dissociation of the applied lithium salts, which obviously influences the ionic conductivity [94][95][96]. In 2017, Freitag et al. reported higher conductivity of the electrospun solvent-free electrolytes in comparison with that of the casted ones. Based on this research, PEO/SN/LiBF
4
−1 [97]. In a similar research, they showed a high ionic conductivity of 0.1 mS·cm
−1
4 membrane [98]. Higher ionic conduction of the solvent-free electrospun structures compared with that of the solution-casted membranes are linked with two main issues. First, small pores between the electrospun fibers are excellent pathways for transportation of the Li
+ ions. Second, fast evaporation of the solvent during electrospinning procedure does not allow the polymer chains and lithium salts to form polymer/salt crystalline regions. So, concentration of the free lithium ions increases in the electrospun membranes, leading to enhancement of the ionic conductivity [4][99]. It is worth noting that electrochemical behavior of the electrospun mats highly depends on the morphology of the fabricated fibers. Based on the obtained results, ionic conductivity could be enhanced by reduction of the average fiber diameter to an optimum range. This may be linked with formation of tiny pores and so more ideal pathways for fast transportation of the Li
+ ions. Nevertheless, further decrement in average diameter of the fabricated fibers could result in reduction of the ionic conductivity. This trend is attributed to formation of more crystalline regions in the structure of finer fibers as well as superior density of the electrospun fibrous mats containing thinner fibers [4][100]. A comparison between ionic conductivity of the electrospun and solution-casted electrolytes with similar chemical compositions is provided in
Table 4.
| Material | Fabrication Method | Ionic Conductivity (mS·cm | −1 | ) | Author (Year) | Ref. |
|---|
| PEO/PC/LiClO | 4 | Casting | 1.7 × 10 | −3 | Banitaba et al. (2019) | [101] | [170] | |||||||||
| Electrospinning | 5 × 10 | −2 | ||||||||||||||
| PEO/Li(TFSI) | Casting | 1 × 10 | −3 | Walk et al. (2018) | [102] | [171] | ||||||||||
| Electrospinning | 4.4 × 10 | −3 | ||||||||||||||
| ] | [ | 122 | ] | |||||||||||||
| 483 | - | Xu et al. (2020) | [ | 86 | ] | [155] | P/CNF | Alloying reaction | 730 at 0.1 A·g | −1 | Liberale et al. (2020) | [54] | [123] | |||
| PVDF-HFP/PI | 85.9 | 9.76 | 483.5 | 1.78 | Cai et al. (2020) | [87] | [156] | Si/PCNF | 1033 at 5 A·g | −1 | Tian et al. (2020) | |||||
| PVDF-HFP/LAGP | [ | 55 | ] | [124] | ||||||||||||
| - | - | 215 | 3.18 | Liang et al. (2021) | [ | 88] | [157] | SnP | 0.94 | /CNF | Conversion/Alloying reactions | 750 at 0.1 A·g | −1 | Yadav et al. (2020) | [56] | [125] |
| SnSe/CNF | 405 at 1 A·g | −1 | Xia et al. (2020) | [57] | [126] | |||||||||||
| SnSe/N-doped CNF | 460 at 0.2 A·g | −1 | Shaji et al. (2020) | [58] | [127] |
2.3. Electrospun Separator
+ ions between the electrodes, along with retaining the liquid electrolyte are apparent responsibilities of this crucial element. Regarding the role of a separator part in LIBs, an ideal separator must provide sufficient ionic conductivity, wettability, and permeability. In addition, dimensional, thermal, and electrochemical stabilities are other vital characteristics of an appropriate separator. Porous PP or PE membranes are common structures utilized as separators in LIBs. However, poor conductivity as well as low wettability are the most well-known downsides associated with these kinds of separators. Among various techniques applied for the fabrication of ideal separators, electrospun membranes have revealed more appealing features. The highly porous structure of the nanofibrous mats, interconnected pores, and large surface-to-volume ratios of the electrospun fibers provide proper wettability and permeability for separators [59][60].
The electrospun separators are mainly divided into four classes, including: monolayer, multilayer, modified, and composite membranes. Monolayer separators are mainly fabricated from one polymeric precursor such as PVDF [61], polyimide (PI) [62][63], PAN [64][65], and so on, while multilayer membranes are obtained by sequential fabrication of various polymer precursors. In this method, appropriate advantages of the various polymers (such as thermal stability, dimensional stability, electrochemical performance, etc.) could be attained in one separator membrane. PVDF/poly(m-phenylene isophthalamide) (PMIA) [66], PVDF/polyethylene terephthalate (PET) [67], PVDF/PI [68], and polysulfonamide (PSA)/PET [69] are some of the reported multilayer electrospun separators. Post-treatment of the electrospun fibrous membranes is a great technique for modification and improvement of various characteristics. Dip-coating [70][71], in situ polymerization
| PEO/EC/LiClO | |||||||||||
| 4 | |||||||||||
| Casting | 8 × 10 | −3 | Banitaba et al. (2020) | [100] | [169] | ||||||
| Electrospinning | 1.72 × 10 | −1 | |||||||||
| PEO/EC/LiClO | 4 | /Al | 2 | O | 3 | Casting | 4.4 × 10 | −3 | Banitaba et al. (2019) | [103] | [172] |
| PVDF/TPP/CA | 90 | 6.9 | 301 | 4.4 | Chen et al. (2020) | [89] | [158] | ||||
| Electrospinning | 5.9 × 10 | −2 | PAN/HCNFs@PVDF/UiO-66 | 77.61 | 24.77 | 570.97 | 1.59 | Fa et al. (2021) | [90] | [159] |
+
10
2
12
−1 at room temperature. Nevertheless, the existence of rare and expensive elements in the structure of inorganic solid materials, along with low flexibility, has restricted their practical usage. To overcome the aforementioned obstacles, several researchers have suggested applying electrospun inorganic solid materials as fillers in the polymeric membranes [104][105][106]. So, a high ionic conductivity of 0.25 mS·cm
−1
6.4
3
2
0.2
12 fillers [104]. In addition, ionic conductivity of a PAN-based casted film was enhanced up to 0.24 mS·cm
−1
0.33
0.557
3
Figure 3) [105]. Moreover, Liu et al. [106] have reported that dispersion of well-oriented ceramic nanowires instead of random nanowires in a host polymer matrix could cause more ionic conductivity resulting from faster transportation of the Li
+
Figure 2. Schematic illustration of the synthesis of ceramic nanowire-filled polymer-based composite electrolytes, together with the comparison of possible lithium-ion conduction pathway in nanowire-filled and nanoparticle-filled composite electrolytes, and illustration of the electrode. Reprinted with permission from [105]. Copyright (2015) American Chemical Society.
