# Electrospun Cellulose Nanofiber Membranes as Multifunctional Separators for High Energy and Stable Lithium-Sulfur Batteries

Authors: Dongjoo Park; Sangbaek Park; Dong-Wan Kim
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Although Li–S batteries (LSB) are one of the most promising electrochemical energy storage technologies, their practical applications are limited by their rapid capacity decay and uncontrolled lithium dendrite formation. In addition to the well-known role of a separator in lithium-ion batteries, LSB separators must perform additional functions. Using a facile electrospinning method, we developed an eco-friendly separator prepared from natural cellulose with an interconnected fibrous and porous structure rich in polar oxygen-containing functional groups. These polar functional groups enhance electrolyte wettability, polysulfide adsorption, and lithiophilicity, thus boosting LSB performance. A cellulose separator with a thickness (22μm) comparable to that of a commercial polypropylene (Celgard) separator delivers an initial discharge capacity of 1458 mAh·g−1 at 0.1C with a high sulfur utilization of 87%, including a high reversible discharge capacity of 1091 mAh·g−1 for 100 cycles, exhibiting a 1.8-times greater capacity retention than those of Celgard-containing LSBs. In addition, an excellent rate capability of 908 mAh·g−1 can be achieved at a high rate of 1C. These intriguing characteristics indicate that these separators could replace conventional synthetic polymer-based separators for commercial LSBs.


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## Body

## 1. Introduction

Lithium-ion batteries (LIBs) are gaining popularity owing to increased regulations concerning fuel economy and initiatives to increase the proportion of electric vehicles in use [1, 2]. This expanding market for batteries has also increased demand for battery separators. Currently, polyolefin-based polymers, such as semi-crystalline polyethylene or polypropylene, are the main separators used in LIBs [3]. To date, several different types of separators have been studied in attempts to improve battery performance and stability, for example, modified microporous membranes, which have high surface areas comparable to those of polyolefin-based separators, composite membranes containing nanosized inorganic particles, nonwoven mat membranes having fibrous structures, gel/solid polymer electrolyte membranes, and solid ceramic electrolyte membranes [4]. Ideally, next-generation separators must satisfy four criteria: safety, stability, high performance, and low cost. Safety and stability of separators are dependent on their electrochemical stability, electrical insulation, mechanical properties, and thermal stability (heat shrinkage). The performance-related characteristics include porosity, pore size, ionic conductivity when wetted by electrolytes, and wettability [3–6]. Finally, commercial separators are manufactured through high-cost compression and stretching processes, and this high cost must be addressed in the development of next-generation separators [4].Great efforts have been made to develop alternative technologies to LIBs, and the lithium-sulfur battery (LSB) is one of the most promising new electrochemical energy storage technologies [7]. Crucially, elemental sulfur is earth-abundant, nontoxic, and low cost and has a high theoretical specific capacity of 1675 mAh·g-1 and a high gravimetric energy density of 2600 Wh·kg-1 (250–300 Wh·kg-1 for LIBs) [8, 9]. Therefore, LSBs are attracting attention as next-generation batteries with higher power and energy densities, less pollution, and lower costs than LIBs [10]. However, because of the insulating nature of sulfur, as well as the large volumetric expansion and shuttling of soluble lithium polysulfides (LPSs) during battery operation, the practical application of LSBs has been limited to date [7, 11–13], and numerous studies attempting to resolve this issue have been carried out, for example, the modification of the surface of Celgard separators to adsorb polysulfide or the insertion of an interlayer to aid the utilization of sulfur and prevent the dissolution of polysulfides [14–17]. In addition, intense research into LSB separators has been carried out, and methods such as controlling the pore size or functional groups by modifying the Celgard surface [18–21] or using nonflammable polymers [22], synthetic polymers [23–27], and bacterial cellulose [28–32] as separators have been studied. As has been the case for LIB separators [33–38], the development of LSB separators requires a holistic approach, and advances in the following four key areas must be made. First, the separator must adsorb polysulfides and act as a host for these species. Currently, LSBs suffer from severe polysulfide loss; thus, many approaches have been used to increase the reactivity of polysulfide [14]. Secondly, the separator should be easily wetted by the electrolyte, especially because LSBs must contain a small volume of electrolyte to prevent the dissolution of the polysulfides. Despite this small amount of electrolyte, the separator must have excellent ion conductivity [39]. Thirdly, Li dendrite formation must be suppressed. In more detail, LSBs use lithium metal as the anode, which causes dendrite formation, adversely affects the battery lifespan, and causes safety concerns because of short-circuiting. Thus, methods to achieve uniform Li deposition in LSBs are required [40]. Finally, the separator must be reasonably priced because it accounts for approximately 20% of the total battery cost (for comparison, the cathode active materials represent 36% of the total cost). Therefore, if the separator cost can be reduced, the overall battery production costs can be correspondingly reduced [16]. However, achieving improvements in all four areas simultaneously is challenging.Cellulose is one of the most abundant natural polymers on Earth; it is inexpensive, sustainable, biodegradable, renewable, and contains several oxygen-containing functional groups that have an affinity toward polar solvents, such as polar organic electrolytes [41, 42]. In addition to strong intra-/interchain hydrogen bonds that provide thermostability and chemical inertness, natural cellulose possesses amorphous regions that do not form hydrogen bonds and can facilitate ion transport [29–31]. Based on these characteristics, cellulose separators are a promising alternative to commercial polyolefin separators in LIBs [43–45]. As a result, recently, cellulose-based separators have also been studied for use in LSBs [36, 46–48]. For instance, Pavlin et al. described the production of a porous cellulose separator for LSBs by Yu et al. using a commercial skincare mask [29]. In a recent pioneering study, nanofibrillated cellulose (NFC) separators prepared using the casting method were reported to exhibit several useful properties making them suitable for use in LSBs, including biodegradability, ease of modification, and chemical durability [30].Electrospinning is a facile method for manufacturing separators. Notably, electrospinning can be employed to vary the separator thickness and generate interconnected open-pore structures with high permeability and large surface-to-volume ratios [49]. For example, a cellulose/PVdF-HFP composite was prepared by electrospinning and applied as a renewable and superior heat-resistant separator for LIBs [33, 50]. Kang et al. also reported electrospun cellulose acetate/PVDF membranes for polymer LIBs [51]. In addition, Leng et al. proposed nanocellulose-containing heterolayered nanomaterials with terpyridine-functionalized cellulose nanofibrils as the top layer and electrospun PVP/PAN as the bottom layer for LIBs [52]. Inspired by these studies, we hypothesized that electrospun cellulose nanofibers could be used to prepare a separator that fulfills the above criteria for application in LSBs. Despite the many reports of cellulose-based separators in both LIBs and LSBs [4, 5, 53–56], the functions and behavior of electrospun cellulose separators in LSBs remain poorly understood.Therefore, in this study, we employed electrospinning to prepare cellulose nanofibers for use as a separator material in high-energy, stable LSBs. Using electrospinning, the cellulose separator thickness was easily adjusted to 22, 40, or 55μm without affecting other variables, allowing for a systematic evaluation of the LSB performance with respect to separator thickness. Interestingly, the LSB containing an electrospun cellulose nanofiber separator with an optimal thickness (22 μm) achieved a higher initial capacity (1458 mAh·g−1 at 0.1C), rate capability (908 mAh·g−1 at 1C), and a more stable cyclic performance (1091 mAh·g−1 after 100 cycles at 0.1C) than that containing a commercial Celgard separator. Moreover, a comprehensive characterization of electrospun cellulose nanofiber separators with varying thicknesses revealed that the enhanced LSB performance can be attributed to the synergistic effects of the abundant surface hydroxyl groups and the unique interconnected fibrous structure, enhancing the separator wettability/affinity for the electrolyte, increasing LPS adsorption, and suppressing Li dendrite growth. Considering that a glass fiber (GF) membrane with a porous structure similar to that of an electrospun separator must be thick (260 μm) for LSB application [57], thin electrospun cellulose layers with advanced surface morphologies are promising alternatives to conventional separators for LSBs.


## 2. Experimental Section

### 2.1. Preparation of Cellulose Nanofiber-Based Separators

The interconnected cellulose nanofiber separators were fabricated by electrospinning. First, cellulose acetate (3.0 g, Mw ≈30,000, Sigma–Aldrich) was dissolved in 6 ml of acetone (Sigma–Aldrich) and N,N-dimethylacetamide (6 ml, DMAc, Kanto Chemical Inc., Japan) and stirred for 4 h. Secondly, as-spun cellulose sheets were deacylated in 0.05 m NaOH (1 l) for 24 h. The resultant cellulose sheets were washed several times with deionized water and then dried in an oven at 50°C. The electrospinning system for the fabrication of cellulose-based separators for LSB is limited to humidity below 10% and performed at a constant temperature (25±3°C). To control the thickness of the cellulose separators, the precursor solution volume was used as a variable; the cellulose acetate solution was limited to a volume of 3, 6, and 10 ml and subjected to a post-spinning deacetylation process, resulting in samples with a thickness of approximately 22, 44, and 55 μm, which were designated C22, C44, and C55, respectively.


### 2.2. Characterization

The morphology (FiguresS1 and S2) and thickness of the electrospun films were analyzed using field-emission scanning electron microscopy (FE-SEM, SU-70, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM2100F, JEOL, Japan). The functional groups were examined using Fourier transform infrared spectroscopy (FTIR, Spectrum GX1, Perkin Elmer, USA). Surface chemical analysis was performed using X-ray photoelectron spectroscopy (XPS, Kα+, Thermo Fisher Scientific, USA). Contact angles were measured using a contact angle meter (Phoenix 300, SEO, Korea). Polysulfide adsorption ability was qualitatively analyzed using a UV–vis spectrometer (UV-2450, Shimadzu, Japan). A 0.25 M Li2Sn stock solution was prepared by stirring sublimed sulfur (0.12 g) and Li2S (0.0345 g) in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1 : 1, v/v, 15 ml), followed by magnetic stirring at 70°C for 6 h. The stock solution was diluted to 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm for the polysulfide adsorption test. Following the visible adsorption test, each solution was then filtered using a PTFE syringe filter (Whatman, 0.2 μm, Germany) and investigated using a UV–vis spectrophotometer.


### 2.3. Electrochemical Measurements

Typical CR2032-type cells were utilized for all the analyses. The sulfur cathode was fabricated using a slurry of 80% sulfur (approximately 325 mesh, Alfa Aesar)–carbon (Ketjenblack EC-600D) composite, 5% sodium carboxymethyl cellulose (CMC; Sigma-Aldrich), 5% styrene–butadiene rubber (SBR; BM-400B, ZEON), and 10% carbon black (Super P, Alfa Aesar) in deionized water. The “sulfur–carbon composite” contained 73% sulfur (FigureS3), as determined by thermogravimetric analysis (TGA, SDT Q600, TA Instruments, USA). The sulfur loading was 1.8±0.2 mg·cm−2. For the electrolyte, 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI; Alfa Aesar) in DOL (Sigma-Aldrich) and DME (Sigma-Aldrich) (1 : 1, vol/vol) with 1 wt.% LiNO3 (Sigma–Aldrich) was used. The galvanostatic charge/discharge curves were measured using a battery tester system (WBCS 3000, WonATech, Korea). Electrochemical impedance spectroscopy (EIS) was tested at an AC amplitude of 5 mV in the frequency range of 1 MHz to 0.01 Hz using an electrochemical test system (Ivium-n-Stat, Ivium Technologies, Netherlands).


## 2.1. Preparation of Cellulose Nanofiber-Based Separators

The interconnected cellulose nanofiber separators were fabricated by electrospinning. First, cellulose acetate (3.0 g, Mw ≈30,000, Sigma–Aldrich) was dissolved in 6 ml of acetone (Sigma–Aldrich) and N,N-dimethylacetamide (6 ml, DMAc, Kanto Chemical Inc., Japan) and stirred for 4 h. Secondly, as-spun cellulose sheets were deacylated in 0.05 m NaOH (1 l) for 24 h. The resultant cellulose sheets were washed several times with deionized water and then dried in an oven at 50°C. The electrospinning system for the fabrication of cellulose-based separators for LSB is limited to humidity below 10% and performed at a constant temperature (25±3°C). To control the thickness of the cellulose separators, the precursor solution volume was used as a variable; the cellulose acetate solution was limited to a volume of 3, 6, and 10 ml and subjected to a post-spinning deacetylation process, resulting in samples with a thickness of approximately 22, 44, and 55 μm, which were designated C22, C44, and C55, respectively.


## 2.2. Characterization

The morphology (FiguresS1 and S2) and thickness of the electrospun films were analyzed using field-emission scanning electron microscopy (FE-SEM, SU-70, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM2100F, JEOL, Japan). The functional groups were examined using Fourier transform infrared spectroscopy (FTIR, Spectrum GX1, Perkin Elmer, USA). Surface chemical analysis was performed using X-ray photoelectron spectroscopy (XPS, Kα+, Thermo Fisher Scientific, USA). Contact angles were measured using a contact angle meter (Phoenix 300, SEO, Korea). Polysulfide adsorption ability was qualitatively analyzed using a UV–vis spectrometer (UV-2450, Shimadzu, Japan). A 0.25 M Li2Sn stock solution was prepared by stirring sublimed sulfur (0.12 g) and Li2S (0.0345 g) in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1 : 1, v/v, 15 ml), followed by magnetic stirring at 70°C for 6 h. The stock solution was diluted to 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm for the polysulfide adsorption test. Following the visible adsorption test, each solution was then filtered using a PTFE syringe filter (Whatman, 0.2 μm, Germany) and investigated using a UV–vis spectrophotometer.


## 2.3. Electrochemical Measurements

Typical CR2032-type cells were utilized for all the analyses. The sulfur cathode was fabricated using a slurry of 80% sulfur (approximately 325 mesh, Alfa Aesar)–carbon (Ketjenblack EC-600D) composite, 5% sodium carboxymethyl cellulose (CMC; Sigma-Aldrich), 5% styrene–butadiene rubber (SBR; BM-400B, ZEON), and 10% carbon black (Super P, Alfa Aesar) in deionized water. The “sulfur–carbon composite” contained 73% sulfur (FigureS3), as determined by thermogravimetric analysis (TGA, SDT Q600, TA Instruments, USA). The sulfur loading was 1.8±0.2 mg·cm−2. For the electrolyte, 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI; Alfa Aesar) in DOL (Sigma-Aldrich) and DME (Sigma-Aldrich) (1 : 1, vol/vol) with 1 wt.% LiNO3 (Sigma–Aldrich) was used. The galvanostatic charge/discharge curves were measured using a battery tester system (WBCS 3000, WonATech, Korea). Electrochemical impedance spectroscopy (EIS) was tested at an AC amplitude of 5 mV in the frequency range of 1 MHz to 0.01 Hz using an electrochemical test system (Ivium-n-Stat, Ivium Technologies, Netherlands).


## 3. Result and Discussion

As depicted in Figure1(a), the electrospun cellulose nanofiber-based membrane exhibits an interconnected porous structure, wherein the pores represent the interstices between the nanofibers. The membrane separator features abundant porosity, excellent permeability, and a high surface-area-to-volume ratio owing to the interconnected one-dimensional nanofibers. Figures 1(b) and 1(c) illustrate the interconnected porous and fibrous cellulose structure, with the cellulose nanofibers having an average diameter of approximately 320 nm. Figure 1(d) depicts the cross-sectional SEM images of electrospun cellulose separators having thicknesses of 22, 40, and 55 μm, denoted by C22, C40, and C55, respectively. Next, FTIR spectroscopy and XPS analyses were conducted to investigate the surface characteristics and functional groups in the separators (Figures 1(e)–1(g)). Figure 1(e) illustrates the FTIR spectra of the cellulose separators and a Celgard separator, which was employed as a control sample. The spectral bands of the Celgard separator can be ascribed to carbon–carbon multiple bonds, as depicted in Figure 1(e), which is typical of the polypropylene structure of Celgard. In contrast, the spectral bands of the cellulose separators can be attributed to various carbon–oxygen bonds [58], including intermolecular O–H bending (3360 cm−1), C–H stretching (2890 cm−1), C=O stretching (1590 cm−1), C–H deformation (1375 cm−1), C–O–C asymmetric stretches (1157 cm−1), C–O vibrations (1060–1015 cm−1), and C–H deformation (895–892 cm−1) [59, 60].Figure 1
(a) Schematic of the electrospun cellulose nanofiber separator structure and properties. (b) SEM image of electrospun cellulose separators and (c) magnification of a cellulose fiber. (d) Magnified cross-sectional images of C55, C40, and C22 separators. (e) FTIR spectra of Celgard and cellulose separators. XPS (f) C 1 s and (g) O1s spectra.
(a)(b)(c)(d)(e)(f)(g)Figure1(f) and Figure S4 illustrate the deconvoluted C 1 s XPS peaks of the Celgard and cellulose separators. The peak at 285 eV represents the C–C/C–H bonds in the polypropylene molecules in Celgard [61, 62]. In contrast, the C 1 s spectra of the cellulose separators exhibit bands at binding energies of approximately 288.6 (O–C=O), 287.6 (C–C=O), 286.2 (C–O), and 284.6 eV (C–H in C–C bonds) [63, 64]. The deconvoluted O1s spectra of all samples are depicted in Figure 1(g). No peaks were observed in the O1s spectrum of the Celgard separator, whereas those of the cellulose separators exhibited a peak at 532.4 eV (C–O–C/C–OH) [64]. As observed in Figures 1(e)–1(g), the cellulose separator comprises polar oxygen-containing functional groups, including carbonyl (C=O), carboxyl (–COO–), and hydroxyl (–OH) groups. The abundant oxygen-containing polar functional group represents a Lewis base with an electron pair and a strong affinity for the Lewis acid of the terminal lithium atom of lithium polysulfide, enabling the immobilization of lithium polysulfide on the cellulose surface [65]. In addition, numerous surface polar functional groups have a strong affinity for the polar organic electrolyte [66], improving the wettability of the cellulose. Moreover, the excellent wettability of the oxygen-containing polar functional groups to the polar electrolyte contributes to the formation of a stable solid electrolyte interphase (SEI) layer and lowers the electrolyte depletion rate occurring during battery reaction, thereby inhibiting lithium dendritic growth [67]. Therefore, a cellulose separator comprising a greater number of polar oxygen functional groups than polypropylene-based Celgard with low pore density, limited pore size, and limited surface functional groups could adsorb lithium polysulfide, inhibiting the indiscriminate dissolution of sulfur intermediates, lithium dendritic growth, and high wettability to polar organic electrolytes (Figure 1(a)).Subsequently, the electrochemical performance of cells containing Celgard, C22, C40, and C55 separators was compared (Figure2). The electrolyte quantity is adjusted to 20 μl (14.2 μl·mgS−1) for C22, 30 μl (21.2 μl·mgS−1) for Celgard and C40, and 40 μl (28.3 μl·mgS−1) for C55. The Celgard-containing cell yielded specific capacitances of 1409, 618, 540, and 501 mAh·g−1 at 0.1C, 0.2C, 0.5C, and 1C, respectively (Figure 2(a)). Interestingly, the C22-containing cell yielded specific capacitances of 1458, 1126, 991, and 908 mAh·g−1 at 0.1C, 0.2C, 0.5C, and 1C, respectively, which exceed over the entire range of C-rates than those of the Celgard-containing sample (Figure 2(b)). Additionally, the C22-containing cell preserved its original capacity well, whereas the Celgard-containing cell exhibited a significant capacity loss throughout the first five cycles (Figures 2(a) and 2(b)). As porosity increases, the tortuosity also increases per the Bruggeman relationship [53], which might result in lithium-ion diffusion limitations that degrade the separator membrane performance [53]. The pore size distributions of cellulose separators are wider than those of Celgard in Figure S5 and Table S1, which can offset the increase in tortuosity following an increase in porosity. Moreover, the cellulose separator thickness strongly impacted cell performance than that caused by porosity, which recorded a higher specific capacity and stable operation. This is attributed to the thin separator layer, which does not significantly restrict lithium-ion mobility [53]. Next, the cell cycle performances were measured at 0.1C, as depicted in Figure 2(e). The reversible capacities after 100 cycles were 625, 1091, 920, and 843 mAh·g−1 for the Celgard-, C22-, C40-, and C55-containing cells, respectively, corresponding to capacity retentions of 41%, 74%, 75%, and 68%, respectively. Furthermore, the cellulose-containing cells displayed low attenuation rates of 0.26% (C22), 0.25% (C40), and 0.3% (C55) per cycle, whereas the Celgard-containing cell exhibited a rapid capacity loss of 0.59% per cycle. The initial coulombic efficiencies are 97.7, 91, 88, and 88% for Celgard, C22, C40, and C55, respectively, which after 100 cycles exhibit the values of 97.8, 98.6, 98.2, and 97%, respectively. In the initial cycle, Celgard exhibited better coulombic efficiency than the cellulose separator; however, after 100 cycles, the coulombic efficiency changed to the sequence of C22 > C40 > Celgard ≥ C55. From the perspective of the Bruggeman relationship, the cellulose porosity is approximately 1.5 times higher than that of Celgard, which increases internal resistance in the initial cycle, inhibiting lithium-ion mobility [68]. However, as the cycle progresses and the electrolyte sufficiently permeates into the battery, the resistance and porosity decrease, allowing the lithium ions to move smoothly through the wide pores of the cellulose separator. And this phenomenon was analyzed in depth through EIS analysis in Figure S7 which is fitted by the equivalent circuit model in Figure S6. In the C22-modified cell, which recorded the best performance, the Rct (6.4 Ω) value in the OCV state decreased to 5.7 Ω after 10 cycling at 0.1C as shown in Table S3. Therefore, it was reconfirmed that the high porosity of cellulose activates the mobility of lithium ions, thereby gradually reducing the internal resistance of the lithium-sulfur battery.Figure 2
Galvanostatic charge/discharge curves at various rates for LSBs containing (a) Celgard, (b) C22, (c) C40, and (d) C55 separators, and (e) cycling performance at 0.1C for LSBs containing Celgard, C22, C40, and C55 separators.
(a)(b)(c)(d)(e)To optimize the cell performance considering the electrolyte quantity for each cellulose separator thickness, five charge/discharge cycles in the presence of different electrolyte quantities were measured (Figure3). Sulfur loading was fixed at 1.8±0.2mg·cm−2. When 10 μl of electrolyte (7.1 μl·mgS−1) was utilized for the cell with the C22 separator, the first discharge capacity equaled 1478 mAh·g−1, but overcharging had occurred (Figure 3(a)). In contrast, when 30 μl of electrolyte (21.2 μl·mgS−1) was employed, an excellent first discharge capacity was obtained (1529 mAh·g−1), which eventually decreased by 28% over five cycles to 1090 mAh·g−1 (Figure 3(c)). Thus, we inferred that the optimal amount of electrolyte was 20 μl (14.2 μl·mgS−1) for the C22-containing cell (Figure 3(b)). For the C40-containing cell, a low initial discharge capacity of 661 mAh·g−1 was obtained for 30 μl of electrolyte (Figure 3(d)). In contrast, using 40 μl of electrolyte (28.3 μl·mgS−1) resulted in an initial discharge capacity of 1232 mAh·g−1, which was significantly reduced by 30% to 860 mAh·g−1 over five cycles (Figure 3(f)). Thus, the optimal amount of electrolyte was 30 μl for the C40-containing cell (Figure 3(e)). For the C55-containing separator, the first discharge capacity was excellent (1251 mAh·g−1) when 30 μl of electrolyte was used, which rapidly decreased during cycling and resulted in overcharging (Figure 3(g)). On using 50 μl of electrolyte (35.4 μl·mgS−1), a low initial discharge capacity was obtained (Figure 3(i)). Thus, we determined that the optimal amount of electrolyte was 40 μl for the C55-containing cell (Figure 3(h)). Therefore, if the electrolyte quantity is insufficient, overcharging occurs or a low specific capacity is obtained. This can be attributed to the lack of electrolyte resulting in the deterioration of both the electrode and electrolyte [69]. However, when the electrolyte quantity was very high, the capacity decreased rapidly (approximately 30% of the initial discharge capacity over five cycles) because the large quantity of electrolyte caused the dissolution of S-species [55]. In summary, the optimal electrolyte–sulfur ratios for the C22, C40, and C50-containing cells were determined to be 14.2, 21.2, and 28.3 μl·mgS−1, respectively. These results are expected because the space wherein the electrolyte permeates increases with the increasing thickness of the cellulose separator. Interestingly, the C22-modified cell not only exhibited the highest performance but also required only 14.2 μl·mgS−1, much less than the Celgard-modified cell (21.2 μl·mgS−1), owing to the abundant polar functional groups on the cellulose separator surface. All cellulose separators demonstrated enhanced performance stability compared to Celgard, which was commonly observed in both C22 with a lower electrolyte/sulfur ratio, C40 with an identical electrolyte/sulfur ratio, and C55 with a higher electrolyte/sulfur ratio. Therefore, it is concluded that the cellulose separator can deliver enhanced cell performance than the commercial polypropylene separator by Celgard, regardless of the thickness and electrolyte quantity, following optimization of electrolyte volume according to thickness.Figure 3
Optimization of the electrolyte quantities (a–c) in a Celgard-containing cell (low dosage (14.2μl·mgS−1), optimal dosage (21.2 μl·mgS−1), and high dosage (28.3 μl·mgS−1)), (d–f) C22-containing cell (low dosage (7.1 μl), optimal dosage (14.2 μl), and high dosage (21.2 μl)), (g–i) C40-containing cell (low dosage (14.2 μl·mgS−1), optimal dosage (21.2 μl·mgS−1), and high dosage (28.3 μl·mgS−1)), and (j–l) C55-containing cell (low dosage (21.2 μl·mgS−1), optimal dosage (28.3 μl·mgS−1), and high dosage (35.4 μl·mgS−1)).
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)The separator wettability for the organic electrolyte (1 M LiTFSI in 1 : 1 (vol/vol) DOL and DME with 1 wt.% LiNO3) was investigated using contact angle measurements (Figure 4(a)). For Celgard, the contact angle was 36.8°, whereas those for C22, C40, and C55 were all approximately 0° (Figure 4(a)). The high contact angle on the Celgard separator results from its intrinsic hydrophobicity and low surface energy owing to its hydrocarbon structure [66], whereas the flat contact angles of the cellulose separators can be ascribed to the abundant polar oxygen-containing functional groups in the cellulose nanofibers, as confirmed by FTIR and XPS analyses (Figures 1(e) and 1(g)) and the highly porous and interconnected fibrous structure [70], as observed in the porosity measurements (Figure S5 and Table S1). Generally, separator wettability is determined by the time required for a quantity of electrolyte to be adsorbed by the separator and the contact angle between the separator and electrolyte [71]. Lengthy diffusion times and large contact angles indicate poor wettability. Because wettability relates to the surface energies and intermolecular interactions of a solid and liquid [6], poor compatibility between the separator surface and electrolyte leads to high internal cell resistances and poor rate capabilities [71]. Compared to Celgard, the cellulose separators rapidly adsorbed the electrolyte and exhibited low contact angles, indicating excellent wettability and low interfacial energy between the separator and electrolyte; consequently, they are anticipated to yield enhanced rate capabilities when applied to LSBs [6].Figure 4
(a) Electrolyte contact angles on Celgard, C22, C40, and C55 separators. (b) Electrolyte reservoir capability of each separator. (c) Photographs of 0.5, 1.0, 1.5, 2.0, and 2.5 mm Li2S6 solutions before and after the immersion of separators for 6 h. (d) Comparison of UV–vis spectra of Li2S6 solutions before and after immersion of separators for 6 h. (e) Absorption peaks corresponding to S42− species and S3∗− radical. (f) Adsorption of sulfur species in 0.5, 1.0, 1.5, 2.0, and 2.5 mm Li2S6 solutions at 0 h and after 6 h. (g) Thermal shrinkage of Celgard, C22, C40, and C55 separators.
(a)(b)(c)(d)(e)(f)(g)Next, the electrolyte reservoir capability of each separator was evaluated by immersing them in a 10 ml of the electrolyte-filled vial for 10 min. This vial was placed in a dry vial with a closed lid to prevent electrolyte evaporation during weighing, and the mass of the electrolyte adsorbed by the separators was estimated by measuring the weights prior to and following immersion. Celgard exhibited the lowest electrolyte reservoir capability (340%), whereas those of C22, C40, and C55 were 988%, 703%, and 575%, respectively (Figure4(b) and Table S2). This result suggests that the unique polar surface functional groups and interconnected open-pore structures of the electrospun cellulose enhance the electrolyte reservoir capability and provide a high affinity with the electrolyte. Particularly, LSBs require a separator material with a high electrolyte affinity/reservoir to decrease polysulfide dissolution by reducing the required electrolyte quantity [5]. As previously discussed, owing to the excellent wettability and high electrolyte reservoir of the C22 separator, the cell containing this separator solely required 20 μl of electrolyte, lower than that of the conventional Celgard-containing cell (30 μl). Therefore, the extent of polysulfide dissolution into the electrolyte was reduced, resulting in a higher capacity and improved capacity retention rate as compared to that of the Celgard-containing cell (Figure 2). Furthermore, the cellulose separator retained its shape without collapsing after being soaked in the organic electrolyte for one month, as shown in Figure S8.The improved capacity retention can also be attributed to the high lithium polysulfide adsorption of the electrospun cellulose. Therefore, to test the ability of the cellulose separators to adsorb lithium polysulfides (FigureS9), visible adsorption tests were performed. As depicted in Figure 4(c), the color of the Li2Sn solution faded after the cellulose separator was immersed for 6 h, exhibiting lithium polysulfide adsorption ability, whereas the colors of the Li2Sn solutions with Celgard separators exhibited no observable color fading, suggesting a weak polysulfide adsorption ability. To further investigate the adsorption ability of lithium polysulfide, the amount of adsorbed sulfide was obtained using different concentrations of Li2S6 (0.5 to 3 mm) and measuring the UV–vis peak intensities prior to and following adsorption (Figures 4(d) and 4(e)). The absorption peaks corresponding to the S42− and S3∗− radicals exhibit a relaxation of absorbance intensity associated with cellulose separators. As the separator thickness increased, the reduction in absorbance was remarkable [72]. The relaxation of the peaks corresponding to the S42− and S3∗− radicals indicates the spontaneous reaction of lithium polysulfides with abundant cellulose surface polar groups, leading to a decrease in the concentration of Li2Sn solutions. Furthermore, the experimental adsorption capacity was calculated using the following:
(1)qe=I0−IfmCSVwhere qe (mg·g−1) represents the adsorbed amount of polysulfides, I0 and If represent the initial and final absorption intensities of lithium polysulfides at wavelengths of 320 and 425 nm, respectively, CS (mg·l−1) represents the polysulfide solution concentration, V (l) is the volume of the Li2Sn solution, and m (g) is the mass of the separator [73]. The outcomes are illustrated in Figure 4(f), where the Celgard separator exhibits a maximum inactive adsorption capacity of ~0.07 mg·g−1, and cellulose separators exhibit maximum active adsorption capacities 0.34 mg·g−1, 0.46 mg·g−1, and 1.54 mg·g−1 to the max with C22, C40, and C55, respectively. Lithium polysulfide is anchored using the Lewis acid–base reaction with numerous polar oxygen-containing functional groups on the cellulose surface, and sulfur intermediates can be confined by the dense morphology of electrospinning-based cellulose [20, 65]. Therefore, as depicted in Figure 4(f), with the thickening of the cellulose membrane, the surface chemical reaction and dense morphology improved the capability for sulfur species. Furthermore, the thermal resistance of the electrospun cellulose was tested by heating it at 160°C for 10 min. Figure 4(g) shows the photographs of the Celgard, C22, C40, and C55 separators after 0 (upper figure) and 10 min (lower figure) heating. When using the Celgard separator, shrinkage occurred immediately after it was exposed to heat, and it formed a rod-like shape after 10 min. These changes are attributed to the stretching process employed during Celgard fabrication. Thus, when Celgard is exposed to heat, shrinkage occurs in the direction of stretching employed during fabrication [59]. In contrast, the cellulose separators did not change shape on heating. Notably, during electrospinning, a random structure of interconnected nanofibers is produced which solidifies on the collector [40], and no stretching is observed, unlike Celgard. Therefore, no preferential shrinkage in any direction is observed. Because the thermally induced contraction of separators is a problem that can threaten the safety of batteries during overheating [3], the electrospun cellulose separator could improve the safety of both LSBs and LIBs.To confirm the contribution of cellulose separators in preventing lithium dendrite formation, images of the lithium metal anode were obtained following cycling. As illustrated in Figure5(a), the lithium anode from the cell with the Celgard separator exhibits an irregular structure resulting from lithium dendrite growth. Various models have been proposed to explain the growth mechanism of lithium dendrite, but the currently accepted model reveals that lithium dendritic growth results from the unstable electrodeposition of lithium ions [74]. Lithium anode exhibits a very high chemical reactivity and reacts with a liquid electrolyte to form the SEI layer that possesses ion conductivity but is electrically insulated on the anode surface [74]. Typically, the developed SEI layer is uniform and heterogeneous, causing a “tip-effect” to promote disordered lithium nucleation and additional lithium dendrite growth owing to nonuniform current distribution [75]. The unstable SEI layer causes continuous exhaustion of the electrolyte, and lithium is discharged from lithium dendrites exhibiting a lower impedance during discharging, with some lithium being trapped in the SEI layer (“dead Li”), which becomes electrically inactive [75]. In this repeated process, the lithium anode endures significant volumetric expansion, threatening battery stability [76]. Therefore, forming a stable SEI layer during the commencement of the battery reaction can be an effective method to prevent lithium dendrite formation and electrolyte depletion. The Celgard separator possesses poor surface polarity, resulting in low electrolyte wettability, causing an uneven lithium-ion distribution and deposition on the anode surface [77, 78]. In comparison, the morphologies of the lithium metal anodes in the cells incorporating cellulose separators (Figures 5(b)–5(d)) imply decreased dendritic formation, which could be one of the reasons for the higher capacity retention of the corresponding LSBs. As previously discussed, cellulose-based separators (such as lignocellulose and bacterial cellulose) [29, 30, 33, 36, 46–48, 50, 51, 79] contain numerous oxygen-containing functional groups, and these abundant polar groups (such as –OH) and lithiophilic polar groups (–C=O and COOH) promote electrolyte permeation and well-distributed lithium-ion flux [78]. Specifically, in LSBs, the polysulfide dissolved in the electrolyte is regarded as a solute and is easily reduced by chemical or electrochemical reactions with the highly reactive lithium metal anode [80, 81]. Consequently, the active polysulfides are depleted by side reactions with the fresh lithium, resulting in the rapid degradation of the sulfur cathode [81]. Concurrently, the formation of a solid electrolyte interphase (SEI) during the lithium deposition/stripping process impairs the reactivity of the lithium anode and reduces performance [82]. Therefore, LSBs necessitate dual-function separators that can enhance lithium-ion distribution and prevent the loss of active sulfur species. Because cellulose separators not only enhance lithium-ion mobility (Figures S10 and S11) owing to their porous, well-interconnected 3D morphology but also exhibit a high adsorption ability for lithium polysulfides (Figures 3(d)–3(f)) and the formation of a stable SEI layer during initial cell operation owing to the polar oxygen-contained functional groups, the extent of lithium dendrite formation during LSB cycling was reduced. Notably, as the separator thickness increased, the extent of lithium dendrite formation decreased significantly, suggesting an enhanced suppressive effect with the increasing number of surface polar functional groups. The influence of the polar functional groups on the surface of the cellulose separator can also be confirmed in the analysis of lithium-ion conductivity (Figure S11). The Celgard separator recorded the lowest 0.21 mS·cm-1, and the cellulose separator recorded higher lithium-ion conductivity with increasing thickness, respectively, 0.44, 1.73, and 3.01 mS·cm-1. In addition, as a result of evaluating the lithium-ion mobility of the C55 separator that formed the smoothest dendrite surface, an excellent value of 0.81 was recorded, 1.26 times higher than that of Celgard (0.64), as shown in Figure S12. Furthermore, to evaluate the improvement of lithium stability, a lithium stripping and plating test was conducted on the control group C55 separator with Celgard separator, which exhibited the best lithium dendritic growth inhibition within a cellulose separator. In the cellulose separator, a current density of 98 mV was observed in the initial cycle (Figure S12b), 51 mV in the middle part (Figure S12c), and 41 mV in the last part (Figure S12b), suggesting that the current density gradually stabilized. In contrast, the Celgard separator exhibited high current densities of 104, 202, and 106 mV (Figures S12b, S12c, and S12d, respectively) and unstable curves. The formation of lithium dendrites can be classified into two types: dendrites that protrude toward the separator, causing a short circuit of the cell, and fibrous lithium dendrites, including dead lithium, formed in bulk lithium [83]. For lithium dendrites causing a short circuit, the cell voltage dropped to approximately 0 V following Ohm’s law after a sudden voltage drop during the Li stripping and plating process [83]. Fibrous lithium dendrites do not short-circuit the cell, but the unstable SEI layer promotes electrolyte depletion and increases cell impedance, thereby threatening cycling stability [84]. Therefore, the higher voltage polarization and unstable plating/stripping voltage of the Celgard separator as compared to those of the cellulose membrane imply that rapid buildup of SEI accumulation/impedance is in progress in the Celgard-modified cell. Meanwhile, the cellulose separator initially reacted at a relatively high current density and gradually decreased the current density, indicating that the excellent electrolyte wettability of the separator and its surface comprising lithium-friendly functional groups can hinder SEI accumulation/impedance. Overall, the thin cellulose separators exhibit excellent electrolyte wettability and capacity retention compared to those of reported LSBs containing cellulose-based separators (Table 1) and reported LSBs with electrospun separators (Table 2).Figure 5
Potential mechanisms through which the cellulose separators stabilize Li metal anode. Surface morphologies of Li metal anode after 50 cycles of galvanostatic cycling at 0.1C with (a) Celgard, (b) C22, (c) C40, and (d) C55 separators. Schematic of the effects of the (e) Celgard and (f) cellulose separators on Li deposition.
(a)(b)(c)(d)(e)(f)Table 1
Comparison of electrolyte reservoir capabilities and performance of the separators.
No.MaterialMethodThickness (μm)Electrolyte wettabilityThermal stabilityCell performance (initial capacity)Ref.This work∗CelluloseElectrospinning22Contact angle 0°, intake 988%160°C1458 mAh·g-1 @ 0.1C∗1Cellulose nanofiber separatorVacuum filtration20820 mAh·g-1 @ 0.1C202Cellulose-based porous membraneMask pack301322 mAh·g-1 @ 0.2C213Nanofibrillated cellulose hydrogelHydrogel42940 mAh·g-1 @ 0.1C224CNT-coated cellulose separatorVacuum filtration331409 mAh·g-1 @ 0.1C235Functionalized bacterial cellulose separatorVacuum filtration25Intake 135%140°C1200/1033 mAh·g-1 @ 0.1C24Table 2
Comparison of the performance of LSBs containing different separators.
No.MaterialThickness (μm)Electrolyte wettabilityThermal stabilityCell performance (initial capacity)Ref.This work∗Cellulose22Contact angle 0°, intake 988%160°C1458 mAh·g-1 @ 0.1C∗1PAN@ammonium polyphosphate separator70Contact angle 0°200°C1380 mAh·g-1 @ 0.2C192rGO–PVDF/PVDF composite nanofiber membrane135Intake 380%80°C1380 mAh·g-1 @ 0.1C233PAN/poly(acrylic acid) composite nanofiber separator30Contact angle 0o, intake 1600%150°C1200 mAh·g-1 @ 0.1C244PAN/SiO2 nanofiber separator68Intake 330%980 mAh·g-1 @ 0.1C255PAN/GO nanofiber membrane68Intake 303%1000 mAh·g-1 @ 0.1C266PVDF/poly(4-styrenesulfonic acid) membraneContact angle 24°1200 mAh·g-1 @ 0.1C27In addition, promising performance compared to those of reported cellulose-based separators was also observed when electrospun cellulose separators were employed in LIBs, as depicted in FigureS13 and Table 3. Therefore, we believe that the electrospun cellulose separators reported herein are highly promising alternatives to synthetic polymer separators for applications in lithium-based batteries. And then, recently, technologies that can prevent failures and fires that may occur during the operation of battery-based devices have been actively researched by combining AI and ICT technologies [84–86]. If the various benefits and disadvantages of the electrospinning-based cellulose separator for lithium-sulfur batteries are analyzed in more detail through the above technologies, it is expected that the future of cellulose-based separators for lithium-sulfur batteries can be further advanced.Table 3
Comparison of the performance of LIBs containing cellulose separators.
No.MaterialMethodThickness(μm)Electrolyte wettabilityThermal stabilityCell performance (initial capacity)Ref.This work∗CelluloseElectrospinning22Contact angle 0°, intake 988%160°CLFP 167 mAh·g-1 @ 0.2C∗1Commercial rice paper100LFP 140 mAh·g-1 @ 0.2C372Cellulose powderHomogenizer, vacuum filtration19150°C, 0.5 hLCO 138 mAh·g-1 @ 0.2C333Cellulose nonwovenElectrospinning25Intake 340%200°C, 0.5 h50Cellulose/PVDF-HFPElectrospinning, hot-pressing27Contact angle 55°, intake 280%200°C, 0.5 hLCO 121 mAh·g-1 @ 0.2C4Cellulose acetate, PVDF-HFPElectrospinningContact angle 55.4°, intake 355%, 200°C, 1 hLCO 138 mAh·g-1 @ 0.2C385Cellulose pulp, sodium alginate, flame retardant, silicaVacuum filtration, hot press40Intake 270%150°CLFP 129 mAh·g-1 @ 0.2C346Cellulose pulpGrinding machine, papermaking machine40Intake 275%200°C, 0.5 hLCO 125 mAh·g-1 @ 0.2C35Cellulose/polydopamineGrinding machine, papermaking machine, immersion40Intake 200%200°C, 0.5 hLCO 130 mAh·g-1 @ 0.2C7Bleached softwood pulpBeater, roll to roll14431.2–335%LFP 155 mAh·g-1 @ 0.2C348Cellulose acetate, PVDFElectrospinning27Intake 768%165°CLCO 204 mAh·g-1 @ 0.2C399Cellulose nanofibril, PVP, PAN, terpyridineElectrospinning≈20150°C, 0.5 hLMO 102 mAh·g-1 @ 0.2C6610Cladophora cellulose powderVacuum filtration35Contact angle 23°150°C, 0.5 hLFP 154 mAh·g-1 @ 0.2C3211Bacterial cellulose, PEGHomogenizer, roll to roll12200°C, 1 h3612Bleached softwood pulpBeater, wet membrane12Contact angle 35°160°C, 2 hLFP 165 mAh·g-1 @ 0.2C3813Lyocell fibersPFI milling, beater19.4NCM811 186 mAh·g-1 @ 0.2C35


## 4. Conclusion

In summary, the abundant surface polar groups, interconnected pores, and fibrous structure of cellulose were utilized to fabricate separators for use in LSBs. The unique 3D morphology of the separator was prepared using a facile electrospinning method, which was inherently rich in polar (especially –OH) and lithiophilic polar (–C=O and COOH) groups. By adjusting the cellulose separator thickness during electrospinning without altering other factors, a systematic analysis of the effects of the separator thickness on LSB performance was successfully conducted. An assembled cell containing a 22-micrometer–thick cellulose separator (C22) delivered a high specific discharge capacity of 1458 mAh·g−1 at 0.1C, where the discharge capacity was 1.8 times greater than that of the Celgard-modified cell after 100 cycles and exhibited suppressed Li dendrite formation. The excellent performance of LSB and LIB cells containing the C22 separator can be attributed to the good electrolyte wettability, adsorption of S-species, and suppression of Li dendritic growth resulting from the abundant oxygen-containing functional groups and unique 3D morphology of the cellulose nanofiber network. Considering that these multifunctional electrospun cellulose nanofiber membranes not only satisfy the basic criteria (safety, stability, performance, and cost) for conventional LIBs but also provide essential functions for LSBs (effective polysulfide adsorption, good wettability, and suppression of Li dendrites), they are promising materials to resolve the chronic issues preventing the widespread application of LSBs.



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*Source: 1541858-2023-03-21.xml*