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Electrospun Flexible Cellulose Acetate-based Separators for Sodium-ion Batteries with Ultra-long Cycle Stability and Excellent Wettability: The Role of Interface Chemical Groups Weihua Chen, Lupeng Zhang, Chuntai Liu, Xiangming Feng, Jianmin Zhang, Linquan Guan, Li-Wei Mi, and Shizhong Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06706 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

Electrospun Flexible Cellulose Acetate-based Separators for Sodium-ion batteries with Ultra-long cycle stability and Excellent Wettability: The Role of Interface Chemical Groups Weihua Chena∗, Lupeng Zhanga, Chuntai Liub, Xiangming Fenga, Jianmin Zhanga, Linquan Guana, Liwei Mic∗, Shizhong Cuic a College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450001, PR China ∗

E-mail: [email protected]

b National Engineering and Research Center for Adv. Polymer Processing Technology, Zhengzhou University, Zhengzhou, 450001, PR China c Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou, 450007, PR China ∗

E-mail: [email protected]

Abstract: Na-ion battery is one of the best technologies for large-scale applications depending on almost infinite and widespread sodium resources. However, the state-of-the-art separators can not meet the engineering needs of large-scale sodium-ion batteries to match the intensively investigated electrode materials. Here, a kind of flexible modified cellulose acetate separator (MCA) for sodium-ion batteries was synthesized via electrospinning process and subsequent optimizing the interface chemical groups by changing acetyl to hydroxyl partly. Upon the rational design, the flexible MCA separator exhibits high chemical stability and excellent wettability (contact angles nearly 0o) in electrolytes (EC/PC, EC/DMC, Diglyme and Triglyme). Moreover, the flexible MCA separator shows high onset temperature of degradation (over 250 oC) and excellent thermal stability (no shrinkage at 220 oC). Electrochemical measurements, importantly, show that the Na-ion batteries with flexible MCA separator exhibit ultra-long cycle life (93.78%, 10000 cycles) and high rate capacity (100.1 mAh g-1 at 10 C) in Na/Na3V2(PO4)3 (NVP) half cell (2.5-4.0 V) and good cycle performance (98.59%, 100 cycles) in Na/SnS2 half cell (0.013 V), respectively. Moreover, the full cell (SnS2/NVP) with flexible MCA separator displays the capacity of 98 mAh g-1 and almost no reduction after 40 cycles at 0.118 A g-1. Thus, this work provides a kind of flexible modified cellulose acetate separator for Na-ion batteries with great potential for practical largescale applications. Key words: Na-ion batteries, flexible cellulose acetate-based separator, stability, wettability, interface chemical groups

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1.Introduction

Sodium-ion batteries (SIBs) have gradually become the focus of researches1-7 because of rich sodium resources, low cost and similarly electrochemical mechanism to lithium-ion batteries,8-11 and it is expected to make outstanding contributions for large-scale energy storage systems (ESSs).12-13 And, electrode materials have attracted extensive attentions and a large number of achievements have been made, such as anode materials (carbon, FeS2, silicon-based, Sn-based, Ti-based compounds, and so on),14-20 and cathode materials (Na3V2(PO4)3, Na(Li1/3Mn2/3)O2 and Na3V2(PO4)2O2F).21-23 However, the available separators have a long way to meet the engineering needs of sodium-ion batteries technology. As we all known, separators play the important role of preventing contact of the cathode and anode and permitting ion transport in coordination with electrolyte inside the batteries24-26, which strongly affects on batteries’ performance, such as the thermal safety,27-29 mechanical safety,30-31 rate performance and cycle life.32-33 Thus, synergistic development of separator for sodium-ion batteries is solidly necessary. Up to now, the few reported investigations about separators for the sodium-ion batteries including: the glass fiber separator, composite separator and organic polymer nonwoven. As the usually used separator, though glass fiber separator has the advantages of good thermal stability, and rich porosity, a critical disadvantage of mechanical strength blocks its large-scale applications. The composite separator was, always, fabricated by coating ceramic or polymeric materials on polyolefin separator, such as polypoly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate),34 SiO2,35 and ZrO2.36 Though this method improves the performance of separators, the ability to enhance wettability and thermal stability of the separators is relatively poor. In addition, polyvinylidene fluoride (PVDF) nonwoven shows ordinary wettability and thermal stability owing to inherent defect.37 As the most representative derivatives of cellulose, cellulose acetate (CA) has the advantages of good thermal stability, environmental protection and biological


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safety,38 and it also has much applications on filtration and energy field.39, 40, 51 However, no cellulose acetate is directly used as separator for sodium ion batteries because of the dissolution in electrolytes (EC/PC). In the present paper, the cellulose acetate nonwoven is synthesized firstly and modified, subsequently, via optimizing the interface chemical groups by changing acetyl to hydroxyl partly to achieve the goal that the modified flexible cellulose acetate separator not only had well chemical stability but also good affinity with electrolytes (EC/PC). Fortunately, it also exhibited well thermal stability (no shrinkage at 220 oC for 5 h). Moreover, the electrochemical performance of the modified flexible cellulose acetate separators were investigated in Na/NVP half cell, Na/SnS2 half cell and SnS2/NVP full cell, and they all showed superior long-cycle and high-C rate performance. Thus, this work provides a kind of flexible modified cellulose acetate separator for Na-ion batteries with great potential for practical large-scale applications. 2. Experimental section 2.1. Preparation of the modified flexible cellulose acetate separator

Electrospinning process was carried out using a syringe with a spinneret having a diameter of 0.1 cm at an applied voltage of 20 kV. The cellulose acetate solution was fed at a speed of 0.8 mL h-1 with a distance of 20 cm between tip of the needle and Al foil collector. The cellulose acetate nonwovens were developed by electrospinning technique, and the cellulose acetate (CA) nonwovens with different fiber morphology were developed by controlling the solutions of concentration and solvent volume ratio (DMF/acetone). The CA nonwovens were detached from Al foil collector and subsequently are further immersed in 0.06 mol/L sodium hydroxide/water solution for 0, 1, 3, 10 h at ambient temperature to explore the effects of modification time for chemical stability of separators. The solutions of CA were obtained via dissolving CA powder in


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DMF/acetone (3:7) with solid content of 12 wt.%. The flexible MCA-1 separator (100 µm) MCA-2 separator (150 µm) and MCA-3 separator (200 µm) were obtained by firstly electrospinning solutions (8, 12 and 16 mL) then further immersed in 0.06 mol/L sodium hydroxide/water solution for 3 h. The modified flexible cellulose acetate separators were washed by ethanol and overnight dried at 60 oC to remove the residual ethanol. 2.2. Characterization The morphology of the separators was obtained by using Scanning Electron Microscopy (SEM, ZEISS Merlin Compact). The chemical composition of the cellulose acetate nonwovens and the modified flexible cellulose acetate separators were confirmed by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50). At the same time, stability experiments of CA separator and flexible MCA-1 separator in electrolyte were carried out. The mechanical property of flexible MCA-1 separators was evaluated by stretching testing machine. The contact angles of the flexible MCA-1 separator with a series of electrolytes (EC/PC, EC/DMC, Diglyme and Triglyme) were measured by using a contact angle measurement instrument (Dataphysics OCA20). Thermogravimetric (TG) analysis and differential scanning calorimeter (DSC) were conducted under air flow from 80 to 450 oC at a heating rate of 5 oC min-1. To examine the thermal shrinkage characteristic, the thermal shrinkage experiment of the obtained membranes was determined by measuring dimensional change. The bulk resistances of the obtained separators were measured by AC impedance spectroscopy of stainless steel/separator /stainless steel using CHI 604e electrochemical workstation at the frequency range of 100-0.01 Hz. The thermal shrinkage of the obtained separators was calculated according the following equation: Thermal shrinkage (%) = (S0 - S)/S0 × 100%

(1)

S0 and S are the areas of separator before and after the heat treatment. The electrolyte uptake (EU) was calculated as follows:


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EU (%) =(W-W0)/W0 × 100%

(2)

W0 was the weights of the initial separator, and W is the weights of separators after the immersion in the liquid electrolyte. The porosity of the obtained separator was calculated by calculating the absorbed nbutanol volume, which is equal to the porous volume of the separator, i.e.: Porosity (%) = (∆m/ρ)/V

(3)

∆m is the mass difference of the separator between after and before the absorption with n-butanol, ρ is the density of the n-butanol, V is the volume of the separator and n-butanol. 2.3. Electrochemical measurements. The electrochemical performance of the flexible separators measured in Na/NVP half cells (2025): NVP cathode

materials

were

synthesized

according

to

published

article.41

NVP,

super-p

and

polyvinylidenedifluoride (PVDF) were added into N-methyl-2-pyrrolidine (NMP) at the mass ratio of 80:10:10 and fully mixed to obtain uniform slurry. The obtained uniform slurry was coated onto aluminum foil and dried at 60 oC for 12 h, and tailored into discs with a diameter of 13 mm. The electrolyte was composed of 1.0 M NaClO4 and a 1:1 (v/v) solution of ethylene carbonate (EC) – propylene carbonate (PC) with 5 wt. % fluoroethylene carbonate (FEC) additive. The flexible MCA-1, MCA-2 and MCA-3 separators were tested as battery separators. Na metal was used as anode. The sodium ion batteries were assembled in an argon-felled glove box. The cycle voltammetry (CV) tests were performed by CHI 604e electrochemical workstation at a scanning rate of 0.1 mV s-1. The rate capability tests of the batteries with flexible MCA-1, MCA-2 and MCA-3 separators were carried out by Battery Testing System (Neware Technology Limited) under different current densities with the potential window of 2.5 to 4.0 V. To investigate the cycle performance of the batteries assembled with the flexible MCA-1 separator, the galvanostatic charge-discharge tests were also examined.


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The electrochemical performance of the obtained separator measured in Na/SnS2 half cells (2025): SnS2 materials were synthesized according to published article.42 SnS2, carbon black and carboxymethyl cellulose were added in water at the mass ratio of 80:10:10 and fully mixed to obtain uniform slurry. The obtained uniform slurry was coated onto copper foil and dried at 60 oC for 12 h, and tailored into discs with a diameter of 13 mm. The electrolyte was composed of 1.0 M NaClO4 and a 1:1 (v/v) solution of ethylene carbonate (EC)–propylene carbonate (PC) with 5 wt.% fluoroethylene carbonate (FEC) additive. The flexible MCA-1 separators were tested as battery separators. Na metal was used as anode. The sodium ion batteries were assembled in an argon-felled glove box. The cycle voltammetry (CV) tests were performed by CHI 604e electrochemical workstation at a scanning rate of 0.1 mV s-1. The rate capability tests of the batteries with flexible MCA-1 separators were carried out by Battery Testing System (Neware Technology Limited) under different current densities with the potential window of 0.01 to 3.0 V. To investigate the cycle performance of the batteries assembled with the flexible MCA-1 separator, the galvanostatic charge-discharge tests were also examined. The electrochemical performance of the obtained separator measured in SnS2/NVP full cells (2025): The full sodium-ion batteries were fabricated using the NVP cathode, flexible MCA-1 separators and the charged SnS2 anode, and excess amount of SnS2 were used. The as-prepared SnS2 anode was charged and discharged at a current density of 0.5 A g-1 for three cycles. The full batteries were reassembled in an argon-felled glove box. The galvanostatic charge-discharge tests were examined at a potential range of 0.8-2.7 V. 3. Results and discussion Modified flexible cellulose acetate separator for sodium-ion batteries was synthesized via electrospinning process and subsequently regulating amounts of acetyl. More details are presented in Scheme 1: By utilizing electrospinning techniques, the cellulose acetate solution could form nonwovens with randomly arranged smooth fibers. Then, cellulose acetate nonwovens were immersed in sodium hydroxide/water


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Scheme 1. Synthetic process of the modified flexible cellulose acetate (MCA) separators.

solution to control the contents of acetyl. Figure S1(a)-(c) presented SEM images of nonwovens obtained from different solvent volume ratio (DMF and acetone, 7:3, 5:5, 3:7) at 10 wt.%. As shown in the SEM images, there is no fiber in Figure S1(a) and a lot of fibers in Figure S1(c), indicating better fiber when the content of acetone increased. According to Figure S1(d)-(e), it’s obvious that the beads decreased and the fiber became smoother when the concentration of cellulose acetate increased. The Figure S1(f) presented that the nanofibers were smooth and little of defects obtained from the 12 wt.% concentration and a 3:7 volume ratio (DMF and acetone), which were used in following investigation. Figure 1(a)-(d) showed SEM images of the cellulose acetate nonwovens immersed in sodium hydroxide/water solution for 0, 1, 3 and 10 h, respectively. Obviously, the fibers became the rougher with time increasing. Meanwhile, the diameter of nanofibers was uniformly around 100 nm. The images (Figure 1(e)) revealed that the modified flexible cellulose acetate separators had high flexibility under different bending conditions. Figure 1(f) showed the chemical stability of separators with different modification time (0, 1, 3 and 10 h). Obviously, the separators with modification time (0 h and 1 h) would dissolve in electrolyte, and the separators with modification time (3 h and 10 h) could stably exist. The dynamic dissolution processes of separators (0 h and 3 h) were presented in Video S1. Lots of acetyl groups of the separators with modification time (0 h and 1 h) were found. However, moderate amounts of acetyl groups and little


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Figure 1. (a)-(d) SEM images of the cellulose acetate nonwovens immersed in sodium hydroxide/water solution for 0, 1, 3 and 10 h; (e) High flexibility of the modified flexible cellulose acetate separators under different bending conditions; (f) Stabilities of the modified flexible cellulose acetate separators (immersed in sodium hydroxide/water solution for 0, 1, 3 and 10 h) in electrolyte (EC/PC (volume ratio 1:1)); (g) FTIR spectra.

acetyl group were observed in the separators (3 h and 10 h). Therefore, the separators with modification time (0 h and 1 h) would dissolve in electrolyte (EC/PC) according to the “like dissolve like”. Approach of controlling amounts of acetyl of CA nonwovens to fabricate steady separators is proved to be effective and successful. FTIR spectra (Figure 1(g)) were carried out to confirm functional groups contents of separators with different modification time (0, 1, 3 and 10 h). As for the obtained separator with modification time (0 h), peaks at 3700-3200, 1743, 1370, 1233 and 1045 cm-1 were observed, which are attributed to hydroxyl stretching vibration, carbonyl group stretching vibration, methyl deformation vibration, acetyl ester bond stretching vibration and primary alcohol stretching vibration, respectively. The data of FTIR spectra is reasonable, and the similar phenomenon is discovered in the previous paper.50


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After immersion in the 0.06 M sodium hydroxide/water solution for 1 and 3 h, all above peaks of the flexible MCA-1 separator were still preserved but the peak intensity changed. Peak intensity of carbonyl group, methyl and acetyl ester bond decreased, at the same time peak intensity of hydroxyl and primary alcohol increased. These changes of peak intensity were explained by moderately reduced acetyl by sodium hydroxide/water solution. Peaks at 3700-3200, 1045 cm-1 were observed when the modification time increased to 10 h. Contact angle test and electrochemical performance of this separator were showed in Figure S(3), which showed ordinary property. Apart from the chemical stability, the superior wettability performance of separator is highly needed for batteries. In this work, the wettability performance of the flexible MCA-1 separator was evaluated by contact angle measurements. As shown in Figure 2(a), the contact angles of the flexible MCA-1 separators with EC/PC, EC/DMC, Diglyme and Triglyme were almost 0°. These results proved excellent wettability of flexible MCA-1 separator for above solutions, and they could be firstly explained by the high surface energy (High surface energy is explained by the highly specific surface area and porosity).43, 44

In addition, there was strong interaction between the flexible MCA-1 separator and electrolyte owing to

moderate contents of hydrophobic functional group (acetyl) in the modified cellulose acetate main chain. According to above two reasons, the flexible MCA-1 separator had good affinity with EC/PC, EC/DMC, Diglyme and Triglyme solutions and exhibited superior wettability. Due to rough structure of nanofibers and abundant porosity (porosity is up to 87%) of the whole separator, the electrolyte uptake rate of the flexible MCA-1 separator was up to 517.6 wt.% (according to equation 2). A small quantity of separator can store much electrolyte, indicating that the weight of whole battery have the very small increase when the flexible MCA-1 separator is used. Combing high porosity and high electrolyte uptake, the flexible MCA-1 separator was expected to have impressive rate performance in sodium-ion batteries.


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Figure 2. (a) Contact angle images of flexible MCA-1 separator with EC/PC (volume ratio 1:1), EC/DMC (volume ratio 1:1), Diglyme, Triglyme. (b) Stress-strain curve of the flexible MCA-1 separator; (c) TG and DSC curves of flexible MCA-1 separator; (d) thermal stability of flexible MCA-1 separator at 220 oC for 0, 1, 2, 5 h, respectively.

Safety characteristic of the separator is the essential property for batteries. The mechanical strength measurement of flexible MCA-1 separator (Figure 2(b)) demonstrated that the maximum tensile strength of flexible MCA-1 separator was up to 11 MPa. Moreover, the 8.8% deformation was observed at the breakpoint, indicating that the flexible MCA-1 separator had good performance in elongation. By comparison, the mechanical strength of the conventional glass fiber separator was also examined and the maximum tensile strength was only 0.6 MPa (Figure S(2)). The flexible MCA-1 separator exhibited high tensile strength and big deformation, suggesting high safety of flexible MCA-1 separator applied to sodium ion batteries. Well thermal characteristic of separator should be also important for batteries. TG, DSC and thermal shrinkage experiment were carried out to investigate thermal property of the obtained flexible MCA-1 separators. As was shown in Figure 2(c), it was easily found that the onset temperature of degradation of flexible MCA-1 separator exceeded 250 oC. At the same time, a dramatic fall was about 355 oC. And DSC curve of flexible MCA-1 separator had a widely endothermal peak at 355 oC, which well corresponded to the TG curve of the flexible MCA-1 separator. Figure 2(d) showed the thermal shrinkage of flexible MCA-1 separator after thermal treatment at 220 oC for 0, 1, 2, 5 h, respectively, and


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no shrinkage was observed. The above experimental results lead to the conclusion that flexible MCA-1 separator has excellent thermal stability with high decomposition temperature and good dimension stability at high temperature, indicating that the flexible MCA-1 separator could effectively isolate cathode and anode at higher temperature. It can be confirmed that the flexible MCA-1 separator deliver an excellent thermal stability to ensure safety property of sodium-ion batteries when the flexible MCA-1 separator is applied to the batteries.

Figure 3. (a)-(c) The electrochemical performance of Na/NVP half-cells with flexible MCA-1, MCA-2 and MCA-3 separators: CV curves (5th); Charge-discharge curves; C-rate capability. (d), (e) and (g) the electrochemical performances of sodium batteries with flexible MCA-1separator. (f) Comparison of electrochemical performance of the flexible MCA-1 separator in this work with reported related separator.


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Electrochemical performances, an essential performance in featuring separator, were firstly examined in Na/NVP half cell (2.5-4.0 V) and Na/SnS2 half-cell (0.01-3 V). Figure 3(a) presented the cyclic voltammetry of Na/NVP half cells with flexible MCA-1, MCA-2 and MCA-3 separators at the scanning rate of 0.1 mV s-1, and the peak potential differences were 0.372, 0.388 and 0.445 V, respectively. Obviously, the batteries composed of flexible MCA-1 separator showed the lowest peak potential difference, and this could be explained by the bulk resistance of flexible MCA-1 separator. The bulk resistances of flexible MCA-1, MCA-2 and MCA-3 separators (Figure S(4)) were 1.62, 2.08 and 2.82 Ω, respectively, indicating that the lowest polarization of battery were corresponding to the lowest bulk resistance of separator.45 As depicted in Figure 3(b), the discharge voltage platform of flexible MCA-1 separator was 3.32 V and the charge voltage platform was 3.41 V. By contrast, the batteries of flexible MCA-2 and MCA-3 separators were measured and demonstrated a trend of increased charge voltage platform and decreased discharge voltage platform. And the phenomenon is clearly corresponding to the result of the cyclic voltammetry. In addition, the rate performances of the batteries with the three separators were further investigated in Figure 3(c). It could be seen that the flexible MCA-1 separator presented rate capacities of 112.9, 111.9, 108.8, 105.2 and 99.6 mAh g-1 at the rate of 0.5, 1, 2, 5 and 10 C, and the capacity restored to 112.8 mAh g-1 when the current density was returned to 0.5 C. However, with the increase of charge/discharge rate of the batteries composed of MCA-2 and MCA-3 separators, the notable fading of capacity was observed. The findings indicated that the thinner separator (MCA-1) showed better rate performance, and the similar phenomenon was discovered in the previous paper.46 Comparison with performance of MCA-1 separator and conventional glass fiber separator was shown in Figure S9. Figure 3(d) showed the charge/discharge curves of flexible MCA-1 separator in Na/NVP halfcell and Na/SnS2 half-cell at different current densities. As expected, stable charge/discharge voltage platform and high capacity were obtained at current densities of 0.5, 1, 2, 5 and 10 C. With the increasing current density, the charge/discharge capacity had a slight decrease and the difference of charge/discharge voltage platform had a slight expansion. The similar phenomenon was discovered in Na/SnS2 half cell with flexible MCA-1 separator. And the charge/discharge curves and rate performance of Na/SnS2 half


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cell with flexible MCA-1 separator were showed in Figure S(6). Such better rate performance of batteries with flexible MCA-1 separator should be attributed to the abundant porosity, the low bulk resistance and good affinity with electrolyte,48 which are good for fast ions diffusion in batteries. The cycle performances of the batteries with the flexible MCA-1 separator were also investigated. After 100 cycles, the capacity retention of the Na/SnS2 half-cell with flexible MCA-1 separator was found to be 98.59% in Figure 3(e). As shown in Figure 3(g), the reversible capacity of batteries with flexible MCA-1 separator at 10 C was up to 100.1 mAh g-1, corresponding to the great capacity retention rate of 93.78% (93.87 mAh g-1) after 10000 cycles, and the coulombic efficiency (calculated by discharge/charge capacity) remained at ~100%. The cycle performance of the batteries with flexible MCA-1 separator also exhibited superior cycle performance at a rate of 1 C for 1000 cycles (Figure S(5)). Na/NVP half cells with MCA-1 separator were also measured at 0 and 60 oC, 56 and the results were shown in Figure S8. The cycle performance of the battery with flexible MCA-1 separator was compared to some reported articles about separator, indicating ultra-long cycle performance. More characterizations and details of comparison were presented in Table S1. The desirable cycling performances of the batteries with the flexible MCA-1 separator could be explained as follows: First of all, as the typical Na+ superionic conductor, Na3V2(PO4)3 has good electronic conductivity and high ionic conductivity. Moreover, the flexible MCA-1 separator shows superior chemical stability and affinity for electrolyte (EC/PC), which is suit for the batteries with long-life and excellent rate performance. 47, 49 Therefore, Na/NVP half-cell with MCA-1 separator exhibited superior electrochemical performance. These results imply that the flexible MCA-1 separator is stable enough to withstand high and low voltages and exhibits well electrochemical performance. Following the surprisingly electrochemical performances of the half cells with flexible MCA-1 separator, SnS2/NVP full cell with flexible MCA-1 separator was more deeply tested. The full sodium-ion cells with flexible MCA-1 separator were schematically illustrated in Figure 4(a). Upon the CV curves of Na/NVP half cell and Na/SnS2 half cell with flexible MCA-1 separator in Figure 4(b), the voltage


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window for this full cell was set between 0.8 and 2.7 V. Figure 4(c) showed the charge/discharge curves of SnS2/NVP full cell with an initial capacity of 101 mAh g-1 (based on the weight of cathode). Figure 4(d) exhibited cycle performance (the first 40 cycles) of the full cell with flexible MCA-1 separator at 0.118 A g-1, and the capacity (98 mAh g-1) had almost no reduction. High coulombic efficiency of the full cells was obtained, demonstrating good reversibility of the full cells. The successful assembling of SnS2/NVP full cell has a big promotion for practical application of the modified flexible cellulose acetate separator. Electrochemical performances of flexible MCA-1 separator for advanced lithium-ion batteries were also measured and shown in Figure S7. Surprisingly, good cycling performance and high ion conductivity (1.02 mS cm-1) were obtained.

Figure 4. (a) Schematic illustration of SnS2/NVP full cell with flexible MCA-1 separator; (b) CV curves (5th) of Na/SnS2 and Na/NVP half cell (The current of CV curves was operated by normalization according to I=i/ip, in which, i is the actual current and ip is maximal peak current ); (c) Charge-discharge curves of SnS2/NVP full cell with the flexible MCA-1 separator at 0.118 A g-1; (d) Cycle stability of SnS2/NVP full cell with the flexible MCA-1 separator at 0.118 A g-1.

4. Conclusion


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In summary, the modified flexible cellulose acetate separators were designed and successfully synthesized via firstly electrospinning process and subsequently regulating amounts of acetyl. The modified flexible cellulose acetate separator (MCA-1) exhibited well chemical stability comparing to cellulose acetate nonwoven, superior wettability (contact angles nearly 0o) for electrolytes (EC/PC, EC/DMC, Diglyme and Triglyme). The superior wettability could be explained by high surface energy and moderate contents of hydrophobic groups (acetyl) in flexible MCA-1separator. The flexible MCA1separator also exhibited high onset temperature of degradation and excellent thermal stability. The good chemical stability, porous structure, high electrolyte uptake rate and superior wettability ensured the ion transport efficiency. The Na/NVP half cell with MCA-1 showed lowest polarization owing to the low bulk resistance. As expected, high rate capability (100.1 mAh g-1 at 10 C) and superior cycle performance (up to 10000 cycles) of the Na/NVP half cell with flexible MCA-1 separator were obtained. Analogically, Na/SnS2 half-cells with flexible MCA-1 separator exhibited good cycle performance and fast kinetics (2 A g-1). Moreover, SnS2/NVP full cell with flexible MCA-1 separator exhibited good cycle performance. This work provided a new approach to improve the chemical stability and wettability of separators, and the separator exhibits superior electrochemical performance and a wide voltage window in sodium-ion batteries. The modified MCA separators have strong potential for commercialization. Supporting Information SEM image of the obtained separators, stress-strain curve of the glass fiber separator and its electrochemical performance, contact angle images of the separator after immersion in sodium hydroxide/water solution for 10 h and its electrochemical performance. physical resistance of liquid electrolyte-soaked separators, cycle stability of Na/NVP half cell with the flexible MCA-1 separator at 1 C under 25, 0 and 60 oC, the electrochemical performance of Na/SnS2 half cell with the flexible MCA-1 separator, comparison of characterization and electrochemical performance of the flexible MCA-1 separator in this work with reported related separator.


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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21771164, 21671205, 11572290), Scientific and Technological Innovation talents in colleges and universities in Henan Province (No. 15HASTIT003), 2017 post-doctoral fund of Henan province and Innovation Training Project of Zhengzhou University (2017cxcy030). References 1 Lee, M.; Hong, J.; Lopez, J.; Sun, Y.; Feng, D.; Lim, K.; Chueh, W.C.; Toney, M. F.; Cui, Y.; Bao, Z. Universal Quinine Electrodes for Long Cycle Life Aqueous Rechargeable Batteries. Nat. Mater, 2017, 16, 841-848. 2 Qiu, S.; Xiao, L.; Sushko, M. L.; Han, K. S.; Shao, Y.; Yan, M.; Liang, X.; Mai, L.; Feng, J.; Cao, Y.; Ai, X.; Yang, H.; Liu, J. Manipulating Adsorption-Insertion Mechanisms in Nanostructured Carbon Materials for High-Efficiency Sodium Ion Storage. Adv. Energy Mater., 2017, 7, 1700403. 3 You, Y.; Yu, X.; Yin, Y.; Nam, K.-W.; Guo, Y.-G. Sodium Iron Hexacyanoferrate with High Na Content as a Na-rich Cathode Material for Na-ion Batteries. Nano Res, 2015, 8, 117-128. 4 Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-type Nax[Fe1/2Mn1/2]O2 Made from Earth-abundant Elements for Rechargeable Na Batteries. Nat. Mater, 2012, 11, 512-517. 5 Zhang, B.; Dugas, R.; Rousse, G.; Rozier, P.; Abakumov, A.- M.; Tarascon, J.-M. Insertion Compounds and Composites Made by Ball Milling for Advanced Sodium-ion Batteries. Nat. Commun., 2016, 7, 10308. 6 Fang, Y.; Xiao, L.; Ai, X.; Cao, Y.; Yang, H. Hierarchical Carbon Framework Wrapped Na3V2 (PO4)3 as a Superior High-Rate and Extended Lifespan Cathode for Sodium-Ion Batteries. Adv. Mater., 2015, 27, 5895-5900. 7 Guo, J.; Yang, Y.; Liu, D.; Wu, X.; Hou, B.; Pang, W.; Huang, K.; Zhang, J.; Su, Z. A Practicable Li/Na-Ion Hybrid Full Battery Assembled by a High-Voltage Cathode and Commercial Graphite


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