Improving the performance of paper-based separator for lithium-ion batteries application by cellulose fiber acetylation and metal-organic framework coating

Abstract

Paper-based separator for lithium-ion battery application has attracted great attention due to its good electrolyte affinity and thermal stability. To avoid the short circuit by the micron-sized pores of paper and improve the electrochemical properties of paper-based separator, cellulose fibers were acetylated followed by wet papermaking and metal-organic framework coating. Due to the strong intermolecular interaction between acetylated cellulose fibers and N,N-dimethylformamide, the resulting separator exhibited compact microstructure. The zeolitic imidazolate framework-8 coating endowed the separator with enhanced electrolyte affinity (electrolyte contact angle of 0°), ionic conductivity (1.26 mS·cm^–1), interfacial compatibility (284 Ω), lithium ion transfer number (0.61) and electrochemical stability window (4.96 V). The assembled LiFePO₄/Li battery displayed an initial discharge capacity of 146.10 mAh·g^–1 at 0.5 C with capacity retention of 99.71% after 100 cycles and good rate performance. Our proposed strategy would provide a novel perspective for the design of high-performance paper-based separators for battery applications.

Graphical abstract

Keywords

paper-based separators / lithium-ion batteries / acetylation / metal-organic framework coating

1 Introduction

Lithium-ion batteries (LIBs) have been applied in electric vehicle, electronics and other fields due to their high energy density and long lifetime [1−3]. With the development of LIBs for bigger size and higher energy density, their safety issues have attracted ever-increasing attention. It is reported that the thermal runaway from the internal short circuit caused by the shrinkage of commercial polyolefin separators under high temperatures would result in fire or explosion of LIBs [4−7]. Additionally, the polyethylene/polyethylene separators show poor electrolyte wettability, which severely affects the electrochemical properties of assembled LIBs [8−10]. Accordingly, separators with superior thermal stability and electrolyte affinity are highly desirable for safe and high-performance LIBs [11].

Under this background, paper-based separators have gained great research interest due to the abundant polar hydroxyl groups of cellulose fibers and porous structure, which endow separators with excellent thermal stability and electrolyte absorption [12−14]. However, the assembled LIBs may suffer from short circuit by the micron-sized pores of paper [15]. To solve this problem, various methods, including blending or coating paper by inorganic nanoparticles [16] or polymers [17], mixing with cellulose nanofibers (CNFs) [9], pulp refining [15], and chemically modifying cellulose fibers [18], have been developed.

To further improve the cycling property of paper-based separators, additional modification should be performed. Metal-organic frameworks (MOFs) are composed of metal nodes and organic linkers by coordination bonds with high porosity and surface areas [15−20]. Sun et al. [21] synthesized zeolitic imidazolate framework-67 (ZIF-67) on the surface of CNFs to solve the problems of inhomogeneous pore size and pore distribution of pure CNF membranes. The assembled battery displayed discharge capacity retention of 88.41% after the 100th cycle at 0.5 C, which was better than that of CNF separator under the same condition (83.27%). Huang et al. [22] prepared bacterial cellulose (BC)/ZIF-67 composite separator to enhance the electrolyte retention capacity of the separator. The discharge capacity retention of the LIB using BC or BC/ZIF-67 separator was 89.63% or 91.41% after 100 cycles at 0.2 C, respectively. Even though, the application of MOF in paper-based separators for LIBs has been rarely reported and the performance of the derived separators needs to be enhanced.

Based on the above discussion, in this work, zeolitic imidazolate framework-8 (ZIF-8) was dip-coated on the paper that was made by acetylated cellulose fibers to improve the performance of paper-based separator. The working mechanism of fiber acetylation and MOF coating was systematically investigated. The ZIF-8-coated acetylated cellulose paper (ZIF-8@ACP) separator displayed good electrolyte affinity, high ionic conductivity, and superior interface compatibility. The assembled LiFePO₄/Li cell exhibited good cycling performance.

2 Experimental

2.1 Materials

Bleached bagasse pulp was acquired from Guangxi Yongkai Liujing Paper Group Co., Ltd. (Nanning, China). Acetyl chloride (98%) and 2-methylimidazole (98%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Zinc nitrate hexahydrate (99%) was obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Liquid electrolyte (1 mol·L^–1 LiPF₆ in ethylene carbonate (EC) /dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, weight ratio)) was provided by Zhuhai Saiwei Technology Co., Ltd. (Zhuhai, China). All other chemicals were of analytical grade and purchased from Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2 Preparation of ACP separator

The pulp with a consistency of 0.5 wt % was defibered by disintegrator (2800 r·min^–1, AG 04, Estanit, Germany) for 20 min to get an SR number of 25°. Subsequently, the slurry was filtered followed by drying at 40 °C for 24 h. The dried pulp (9 g) was dispersed in N,N-dimethylacetamide (600 mL) with the addition of anhydrous lithium chloride (10 g). The mixture was then heated at 140 °C for 0.5 h. When the temperature of the above suspension dropped to 80 °C, acetyl chloride with a molar ratio of 6 to that of dry pulp was added with pyridine (a molar ratio of 3 to that of the acetyl chloride) as catalyst. The mixture was reacted at 80 °C for 1 h followed by dropping into ethanol (600 mL) and filtration. The crude sample was purified by the process of dispersion in the ethanol and filtration. Finally, the acetylated pulp was dispersed in N,N-dimethylformamide (DMF) followed by filtration and drying at 80 °C under a pressure of –954 ± 4 bar for 10 min. The resulting sample was named as ACP separator. Cellulose paper (CP) separator was made from pristine pulp as a reference.

2.3 Preparation of ZIF-8@ACP separator

ZIF-8 was prepared as reported with a few modifications [23]. Briefly, zinc nitrate hexahydrate (4.0 g) was dissolved in methanol (400 mL) followed by the addition of 2-methylimidazole (4.4 g) and triethylamine (2 mL). After reaction for 12 h, the resulting suspension was centrifuged. The crude product was purified by washing with methanol for 5 times. ACP separator was then dipped into the DMF suspension of ZIF-8 (a concentration of 2 wt %) for 10 min followed by drying at 80 °C under a pressure of –954 ± 4 bar for 10 min. The coated paper was named as ZIF-8@ACP separator.

2.4 Material characterization

The chemical structure was characterized by attenuated total reflectance-Fourier infrared spectrometer (FTIR, TENSOR II, Bruker, Germany). The degree of substitution (DS) of acetylated cellulose fibers was determined by titration [24]. The crystallinity was measured by X-ray diffractometer (XRD, MINFLE600, Japan) with Cu target, 2θ from 5° to 50°, and a scanning speed of 0.02 °·s^–1. Scanning electron microscope (SEM, SU8010, Netherland) was used to observe the morphology of the separator at an acceleration voltage of 5 kV. Transmission electron microscope (TEM, Hitachi HT7700, Japan) was used to observe the morphology of ZIF-8 at 100 kV. The stress-strain curves of the separator (1 cm × 6 cm) were obtained by an electronic universal material testing machine (LS1, AMETEK, USA) with a loading force of 50 N and a stretch speed of 1 mm·min^–1. The thermal stability of the separator was investigated by differential scanning calorimetry (DSC, 200PC) from 100 to 220 °C at a heating rate of 10 °C·min^–1 under N₂ flow. The wettability was measured by contact angle instrument (DSA 100, KRUSS, Germany) at RT with deionized water or liquid electrolyte solvent (EC:DMC:DEC = 1:1:1, weight ratio) as droplet, and images were taken after 15 s. The liquid electrolyte uptake ratio was measured by immersing the separator into liquid electrolyte until saturation. The porosity of separators was calculated by Eq. (1).

(1)$\mathrm{P}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{W_2-W_1}{\rho \times \mathrm{V}}\times 100\mathrm{\%},$

where, W₁ and W₂ represent the weight of the separator before and after absorption of n-butanol, respectively. ρ is the density of n-butanol, and V is the volume of the separator.

2.5 Electrochemical measurements

An electrochemical working station (PGSTAT302N, Autolab, Switzerland) was used to measure the ionic conductivity, lithium ion transference number, the interfacial compatibility, and electrochemical stability window of the separator according to our previous work [20]. Linear sweep voltammetry (LSV) was performed at RT with the potential ranging from 2.5 to 6.0 V and a sweep rate of 5 mV·s^–1. Before measurement, the cells were aged for 24 h to ensure that the electrolytes were completely penetrated into the electrodes. The cycling and rate performance of the coin cells were investigated by a battery test system (LAND CT 2001A, China). The cathode piece was prepared by mixing LiFePO₄, polyvinylidene fluoride and Ketjen Black in N-methylpyrrolidone at a weight ratio of 90:5:5. The mass loading of LiFePO₄ in the LiFePO₄/Li battery cathode was 0.64 mg·cm^–2. The mixture was thoroughly ground in an agate mortar and coated onto a piece of aluminum foil by using a spatula. The coated aluminum foil was vacuum-dried at 60 °C overnight. The dried cathode sheet was finally cut into a disc with a diameter of 14 mm.

3 Results and discussion

3.1 Chemical structure

A novel composite paper-based separator was prepared by dip-coating ZIF-8 nanoparticles onto the ACP separator (Fig.1). The design idea was based on that the swelling of acetylated cellulose fibers and coated ZIF-8 nanoparticles tended to reduce the pore size and improve the electrochemical properties of the paper-based separator [18].

Fig.1 Schematic illustration of the preparation of ZIF-8@ACP separator.

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Fig.2 shows the FTIR spectra of CP, ACP, ZIF-8@ACP separators and ZIF-8 nanoparticles. The CP separator displayed the characteristic peaks of the cellulose backbone, including the stretching vibration of –OH, C–H, C–O–C, and C–O groups at 3327, 2894, 1163, and 1026 cm^–1, respectively [18]. After acetylation of cellulose fibers, a new peak appeared at 1740 cm^–1 in the ACP separator, which was ascribed to the stretching vibration of C=O groups [25]. Based on the titration method, the calculated DS value of acetylated cellulose fibers was 1.8. It should be mentioned that this DS value is optimal for cellulose fibers in consideration of the balance between the strength of the derived separator and its ionic conductivity [26]. As for the ZIF-8 nanoparticles, the peaks at 3135, 2929, and 1584 cm^–1 belonged to the aromatic and aliphatic C–H stretching vibration of the imidazole ring and C=N groups, respectively [27]. According to the TEM image (Fig. S1, cf. Electronic Supplementary Material, ESM), the size of ZIF-8 nanoparticles was about 100 nm. When the ZIF-8 nanoparticles were dip-coated onto the ACP separator, obvious C=N signal appeared at 1584 cm^–1 in the ZIF-8@ACP separator.

Fig.2 FTIR spectra of CP, ACP, ZIF-8@ACP separators and ZIF-8 nanoparticles.

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Concerning the crystal structure, the CP separator displays the typical XRD pattern of cellulose I with the main diffraction signals at 2θ of 14.9°, 16.3°, 22.5°, and 34.2° (Fig.3), which was assigned to the diffraction planes of (101), (10ī), (002), and (040), respectively [28,29]. As for the ACP separator, there were a significant decrease and broadening in the intensities of the signals at the 101 and 002 planes and a decrease in that at the 040 plane. The crystallinity indices of the CP and ACP separators were calculated to be 53.52% and 40.12%, respectively. The decreased crystallinity would be beneficial for the ion conduction [24]. The peaks at 7.4°, 10.5°, 12.8°, 14.8°, 16.5°, and 18.1° for the ZIF-8 nanoparticles were related to the crystal planes of (011), (002), (112), (022), (013), and (222), respectively [23]. Furthermore, it could be seen that the crystal signals of the ACP separator and ZIF-8 nanoparticles both appeared in the ZIF-8@ ACP separator.

Fig.3 XRD spectra of CP, ACP, ZIF-8@ACP separators and ZIF-8 nanoparticles.

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3.2 Morphology

The morphology of separators for LIBs affected both their safety performance and electrolyte uptake ratio. The SEM images of CP, ACP and ZIF-8@ACP separators and energy dispersive spectrometer (EDS)-mapping image of Zn element in the ZIF-8@ACP separator are shown in Fig.4. The surface of the CP separator consisted of intertwining cellulose fibers with a lot of micron-sized pores (Fig.4(a)). When the pulp was acetylated and dispersed in the DMF, the resulting ACP separator exhibited compact microstructure due to the strong intermolecular interaction between the acetylated cellulose fibers and the DMF (Fig.4(b)) [30]. This dense structure of the ACP separator would prevent the short circuits of the LIBs [31]. As illustrated in Fig.4(c–e), ZIF-8 nanoparticles were homogeneously dispersed on the surface of the ACP separator with some aggregates, which endowed the ZIF-8@ACP separator with more compact structure. Interestingly, the porosity of the ACP separator after the surface coating of ZIF-8 nanoparticles increased from 9% to 33%, which could be attributed to the high surface area and abundant pore structure of the ZIF-8 nanoparticles [20]. The increased porosity would play a key role in the electrolyte uptake of the separator to obtain a good ionic conductivity [21]. Although the porosity of the ZIF-8@ACP separator seemed to be a little low, abundant pores appeared on its surface after electrolyte absorption. This phenomenon could be attributed to the molecular interaction between the acetylated cellulose fibers in the electrolyte and the carbonate solvents by the “like dissolves like” principle, leading to the swelling of the acetylated cellulose fibers [32]. The swelling of the ZIF-8@ACP separator in the electrolyte would also be beneficial for the electrolyte retention to improve its electrochemical properties [24].

Fig.4 SEM image of (a) CP, (b) ACP and (c, d) ZIF-8@ACP separators; (e) EDS-mapping image of Zn element in the ZIF-8@ACP separator; (f) SEM image of ZIF-8@ACP separator after electrolyte absorption.

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3.3 Mechanical strength

Mechanical strength of the separator is desirable for the assembly and cycling performance of LIBs [33]. Fig.5 shows the stress-strain curves of the CP, ACP, ZIF-8@ACP and Celgard separators. The maximum tensile strength of the CP separator was 7.63 MPa due to the inter- and intra-hydrogen bonds among abundant hydroxyl groups of cellulose fibers. The ACP separator displayed a tensile strength of 13.41 MPa. The enhanced mechanical strength could be due to its more compact morphology as seen in Fig.4(b). Since the coating of ZIF-8 nanoparticles on the separator would affect the fiber interaction, the tensile strength of the ZIF-8@ACP separator was 11.22 MPa. In addition, it was reported that the acetylation improved the fiber flexibility [25], which endowed the ACP and ZIF-8@ACP separators with higher strain in comparison with that of the CP separator. Furthermore, the Celgard separator had a tensile strength of 79.46 and 6.32 MPa in the machine direction and transversal direction, respectively, due to a uniaxial stretching in the preparation process. Although our paper-based separator displayed lower mechanical strength than that of the Celgard separator, it still meets the application requirement [18].

Fig.5 Stress-strain curves of CP, ACP, ZIF-8@ACP and Celgard separators.

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3.4 Thermal stability

During the working process of LIBs, the separator should be thermally stable to undergo the generated heat [11]. The photographs and area shrinkage percentages of the ACP, ZIF-8@ACP and Celgard 2500 separators treated at different temperatures are shown in Fig.6(a) and Fig.6(b). The ACP and ZIF-8@ACP separators displayed remarkable thermal stability from 75 to 175 °C, while the Celgard separator suffered from severe shrinkage when the temperature was higher than 125 °C. The area shrinkage of the Celgard separator achieved over 90% at 175 °C. As displayed in Fig.6(c), a clear endothermic peak appeared at 167 °C in the Celgard separator, which was related to its melting point [34]. Instead, the ACP and ZIF-8@ACP separators displayed no thermal decomposition up to 220 °C. All these results indicated the high thermal stability of the ACP and ZIF-8@ACP separators.

Fig.6 (a) Photographs and (b) area shrinkage percentages of ACP, ZIF-8@ACP and Celgard separators treated at different temperatures for 0.5 h; (c) DSC curves of ACP, ZIF-8@ACP and Celgard separators.

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3.5 Ionic conductivity

Ionic conductivity plays a key role in the battery performance, which mainly depends on the electrolyte wettability and uptake ratio [16,35]. The wetting capability of the CP, ACP, ZIF-8@ACP, and Celgard separators were evaluated by contact angle measurements. As shown in Fig.7(a), the CP separator displayed strong water absorption due to its abundant hydroxyl groups. After acetylation, the water contact angle of the generated ACP separator increased to 89° due to the low surface energy of the attached nonpolar acetyl groups and its compact microstructure as shown in the SEM image (Fig.4(b)) [36]. When ZIF-8 nanoparticles were coated onto the ACP separator, the ZIF-8@ACP separator displayed a contact angle of 131°. This phenomenon could be ascribed to the formation of nano-/micro-structure on the ACP surface, where the embedded air bubbles would hold the above water droplet [37]. As for the Celgard separator, it had a contact angle of 108° due to its nonpolar alkyl chains. Additionally, if liquid electrolyte was dropped on the surface, the contact angles of the CP, ACP, ZIF-8@ACP and Celgard separators were 11°, 41°, 0°, and 62°, respectively. Obviously, the CP separator displayed better electrolyte wetting property than the Celgard separator due to its abundant polar hydroxyl groups and micron-sized pores [38]. Although it could be supposed that the ACP separator with ester groups might have more affinity to the electrolyte with carbonate solvents due to the “like dissolves like” principle [39], its denser microstructure endowed it with higher electrolyte contact angle in comparison with that of the CP separator. Interestingly, the rich pore structure of the ZIF-8 coating made the ZIF-8@ACP separator with the best electrolyte affinity [22].

Fig.7 (a) Contact angles, (b) electrolyte uptake and (c) EIS plots of CP, ACP, ZIF-8@ACP and Celgard separators.

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Fig.7(b) demonstrates the electrolyte uptake ratio of the CP, ACP, ZIF-8@ACP and Celgard separator, which were 61%, 112%, 162%, and 77%, respectively. Although the ACP separator had poorer electrolyte wetting capacity in comparison with that of the CP separator, it displayed higher electrolyte uptake due to the swelling properties of acetylated cellulose fibers in the electrolyte based on the “like dissolves like” principle [39]. Thanks to the excellent electrolyte affinity, the ZIF-8@ACP separator achieved the highest electrolyte absorption percentage. The slightly higher electrolyte uptake ratio of the Celgard separator compared with that of the CP separator could be attributed to its higher porosity [34].

In the following, the ionic conductivity of separators was measured by electrochemical impedance spectroscopy (EIS), and the according Nyquist plots are displayed in Fig.7(c). It could be seen that the bulk resistance of the CP, ACP, ZIF-8@ACP and Celgard separators were 21.09, 6.09, 6.12, and 1.71 Ω, respectively, and the calculated ionic conductivity were 0.24, 0.82, 1.26, and 0.64 mS·cm^–1, respectively. In comparison with the Celgard separator, the higher ionic conductivity of the ACP and ZIF-8@ACP separators could be due to the increased separator thickness by the swelling of acetylated fibers in the electrolyte.

3.6 Interfacial compatibility

The interfacial compatibility between the separator and the electrode is an important factor to affect the cell cycling performance [18], which is measured by the interfacial resistance of the separator in the assembled Li/separator/Li cell. The alternating current (AC) impedance plots of CP, ACP, ZIF-8@ACP and Celgard separators are displayed in Fig.8, and the semicircle of the plot represents the interface resistance. The interface resistance values of the CP, ACP, ZIF-8@ACP and Celgard separators were 506, 1270, 284, and 526 Ω, respectively. The best interfacial compatibility between the ZIF-8@ACP separator and the Li metal anode could be explained by its best flexibility, electrolyte wettability and highest electrolyte uptake ratio, which would facilitate the Li⁺ ion transport [40]. Additionally, it should be mentioned that although the thicker ZIF-8@ACP separator displayed higher bulk resistance in comparison with that of the Celgard separator, the MOF coating on our separator ameliorated the poor contact interface between the Li metal anode and the separator due to its high surface wettability and wetting speed [41], resulting in better interfacial compatibility.

Fig.8 Nyquist plots of CP, ACP, ZIF-8@ACP and Celgard separators in assembled Li/separator/Li cells at room temperature.

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3.7 Li⁺ ion transfer number

The Li⁺ ion transfer number ($t_{{\mathrm{L}\mathrm{i}}^{+}}$) is a crucial parameter to the Li⁺ ion transport. Figure S2 (cf. ESM) displays the direct current polarization curves and AC impedance spectra of the CP, ACP, ZIF-8@ACP and Celgard separators before and after polarization, and the corresponding current and resistance values are listed in Tab.1. The $t_{{\mathrm{L}\mathrm{i}}^{+}}$ values of the CP, ACP, ZIF-8@ACP and Celgard separators were 0.35, 0.53, 0.61 and 0.31, respectively. The enhanced Li⁺ ion transfer capacity of the ACP separator in comparison with that of the CP separator could be due to the stronger molecular interaction between the attached ester groups and Li⁺ ions, which would facilitate the dissociation of the lithium salts [24]. Since the coated ZIF-8 nanoparticles had rich metal ion sites to coordinate with the anions in the lithium salts, more Li⁺ ions could be dissociated for a higher $t_{{\mathrm{L}\mathrm{i}}^{+}}$ value [41].

Tab.1 Lithium ion transference number of CP, ACP, ZIF-8@ACP and Celgard separators at room temperature

Sample	I0/10−5A	IS/10−5A	R0/Ω	RS/Ω	$t_{{\mathrm{L}\mathrm{i}}^{+}}$
CP separator	1.75	1.18	508.13	531.11	0.35
ACP separator	0.68	0.55	1257.12	1421.65	0.53
ZIF-8@ACP separator	2.88	2.40	287.32	316.87	0.61
Celgard separator	1.86	1.58	518.56	560.34	0.31

3.8 Electrochemical stability window

The electrochemical stability window of the separator determines the working voltage of the battery [16], which is measured by LSV. Since the normal running voltage of the LiFePO₄/separator/Li cell is between 2.5 and 4.2 V, a stable LSV curve at a voltage lower than 4.5 V is required to ensure the normal charge and discharge of LIBs [34]. As shown in Fig.9, the decomposition of the electrolyte in the cell assembled by the ZIF-8@ACP separator took place at 4.96 V, which was higher than that of the CP (4.81 V) and ACP separators (4.89 V). This could be due to the decreased amount of reactive hydroxyl groups by the acetylation and the good electrochemical stability of the ZIF-8 coating [32,42].

Fig.9 LSV curves of CP, ACP, ZIF-8@ACP and Celgard separators at a scan rate of 5 mV·s–1.

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3.9 Cell performance

The cell performance reflects the practical application possibility of the separator. The charge/discharge curves of the first cycle of the LIB assembled by the CP, ACP, ZIF-8@ACP and Celgard separators at 0.5 C are displayed in Fig.10(a). The according discharge capacity of the cells were 141.20, 142.31, 146.10, and 140.41 mAh·g^–1, respectively. After 100 cycles at 0.5 C, the capacity retention of the cells using the CP, ACP, ZIF-8@ACP and Celgard separators were 94.89%, 97.06%, 99.71%, and 97.08%, respectively (Fig.10(b)). The best cycling performance of the ZIF-8@ACP separator-assembled LIB was associated with its best ion conduction capacity, interfacial compatibility and electrochemical stability [43]. Fig.10(c) shows the rate performance of the cells by using the CP, ACP, ZIF-8@ACP and Celgard2500 separators at current density of 0.2, 0.5, 1 and 2 C. Notably, the ZIF-8@ACP separator-based cell exhibited the best invertibility among other separators when the discharge current density recovered from 2 to 0.2 C, which also had excellent discharge capacity at the same current density. The good rate performance was related to the highest ion conductivity and lowest interfacial resistance of the ZIF-8@ACP separator [44]. Additionally, the ZIF-8@ACP separator displayed the best comprehensive properties compared with the reported paper-based separators [18,21,44−48].

Fig.10 (a) Initial charge-discharge curves; (b) cycle performance at 0.5 C; (c) rate capability at 0.2, 0.5, 1 and 2 C of CP, ACP, ZIF-8@ACP and Celgard separators; (d) capacity retention at 0.5 C and ionic conductivity of CF/ANF-20 separator [45], CS(10:1) separator [46], CFS/PPS-1/1 separator [47], ECM12 separator [44], ZIF-67@CNF separator [21], RCF-45 separator [48], PSF separator [18], and ZIF-8@ACP separator (this work).

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4 Conclusions

In summary, a novel paper-based separator was fabricated by dip-coating ZIF-8 nanoparticles on the acetylated cellulose fiber-derived paper. Compared with unmodified CP, the ZIF-8@ACP separator displayed more compact microstructure, which would avoid the short circuit of the assembled battery. The separator showed good mechanical strength (11.22 MPa) and thermal stability (up to 220 °C). The strong molecular interaction between the acetylated cellulose fibers and the carbonate solvents in the electrolyte as well as the abundant pore structure in the ZIF-8 coating endowed the separator with good electrolyte affinity to achieve an ionic conductivity of 1.26 mS·cm^–1. Due to its enhanced flexibility and electrolyte uptake, the ZIF-8@ACP separator exhibited an interfacial resistance of 284 Ω. Thanks to the abundant metal ion sites in the ZIF-8 coating and its good electrochemical stability as well as the decreased amount of reactive hydroxyl groups in the cellulose fibers by acetylation, a high Li⁺ ion transfer number (0.61) and wide electrochemical stability window (4.96 V) were obtained by the ZIF-8@ACP separator. The assembled LiFePO₄/Li battery displayed better capacity retention (99.71% after 100 cycles at 0.5 C) and rate performance in comparison with that of commercial Celgard separator, indicating the application prospect of our designed ZIF-8@ACP separator.

References

[1] Zhang Y , Wang Z . Review on cellulose paper-based electrodes for sustainable batteries with high energy densities. Frontiers of Chemical Science and Engineering, 2023, 17(8): 1010–1027

[2] Sheng J , Tong S , He Z , Yang R . Recent developments of cellulose materials for lithium-ion battery separators. Cellulose, 2017, 24(10): 4103–4122

[3] Wang J , Yang J , Shen L , Guo Q , He H , Yao X . Synergistic effects of plasticizer and 3D framework towards high-performance solid polymer electrolyte for room-temperature solid-state lithium batteries. ACS Applied Energy Materials, 2021, 4(4): 4129–4137

[4] Liu X , Qin M , Sun W , Zhang D , Jian B , Sun Z , Wang S , Li X . Study on cellulose nanofibers/aramid fibers lithium-ion battery separators by the heterogeneous preparation method. International Journal of Biological Macromolecules, 2023, 225: 1476–1486

[5] Wang Z , Shen L , Deng S , Cui P , Yao X . 10 μm-thick high-strength solid polymer electrolytes with excellent interface compatibility for flexible all-solid-state lithium metal batteries. Advanced Materials, 2021, 33(25): 2100353

[6] Xu F , Deng S , Guo Q , Zhou D , Yao X . Quasi-ionic liquid enabling single-phase poly(vinylidene fluoride)-based polymer electrolytes for solid-state LiNi_0.6Co_0.2Mn_0.2O₂||Li batteries with rigid-flexible coupling interphase. Small Methods, 2021, 5(7): 2100262

[7] Wang Z , Guo Q , Jiang R , Deng S , Ma J , Cui P , Yao X . Porous poly(vinylidene fluoride) supported three-dimensional poly(ethylene glycol) thin solid polymer electrolyte for flexible high temperature all-solid-state lithium metal batteries. Chemical Engineering Journal, 2022, 435(3): 135106

[8] Song C , Gao C , Peng Q , Gibril M , Wang X , Wang S , Kong F . A novel high-performance electrospun of polyimide/lignin nanofibers with unique electrochemical properties and its application as lithium-ion batteries separators. International Journal of Biological Macromolecules, 2023, 246: 125668

[9] Pei H , Chen J , Liu H , Zhang L , Hui H , Li Z , Li J , Li X . Ionic conductivity enhanced by crown ether bridges for lithium-ion battery separators. Applied Surface Science, 2023, 608: 155030

[10] Feng G , Li Z , Mi L , Zheng J , Feng X , Chen W . Polypropylene/hydrophobic-silica-aerogel-composite separator induced enhanced safety and low polarization for lithium-ion batteries. Journal of Power Sources, 2018, 376: 177–183

[11] Wang N , Liu W , Liao H , Li Z , Chen Y , Zeng G . Pure cellulose nanofiber separator with high ionic conductivity and cycling stability for lithium-ion batteries. International Journal of Biological Macromolecules, 2023, 250: 126078

[12] Zhang L , Sun X , Hu Z , Yuan C , Chen C . Rice paper as a separator membrane in lithium-ion batteries. Journal of Power Sources, 2012, 204: 149–154

[13] Leijonmarck S , Cornell A , Lindbergh G , Wagberg L . Single-paper flexible Li-ion battery cells through a paper-making process based on nano-fibrillated cellulose. Journal of Materials Chemistry A: Materials for Energy and Sustainability, 2013, 1(15): 4671–4677

[14] Zolin L , Destro M , Chaussy D , Penazzi N , Gerbaldi C , Beneventi D . Aqueous processing of paper separators by filtration dewatering: towards Li-ion paper batteries. Journal of Materials Chemistry A: Materials for Energy and Sustainability, 2015, 3(28): 14894–14901

[15] Wang Y , Luo J , Chen L , Long J , Hu J , Meng L . Effect of fibrillated fiber morphology on properties of paper-based separators for lithium-ion battery applications. Journal of Power Sources, 2021, 482: 228899

[16] Wang Z , Xiang H , Wang L , Xia R , Nie S , Chen C , Wang H . A paper-supported inorganic composite separator for high-safety lithium-ion batteries. Journal of Membrane Science, 2018, 553: 10–16

[17] Zhang Z , Zhou M , Yu J , Cai J , Yang Z . Poly(vinylidene fluoride) modified commercial paper as a separator with enhanced thermal stability and electrolyte affinity for lithium-ion battery. Energy & Environmental Materials, 2021, 4(4): 664–670

[18] Li Z , Wang W , Liang X , Wang J , Xu Y , Li W . Fiber swelling to improve cycle performance of paper-based separator for lithium-ion batteries application. Journal of Energy Chemistry, 2023, 79(4): 92–100

[19] Joseph L , Jun B M , Jang M , Park C M , Muñoz-Senmache J C , Hernández-Maldonado A J , Heyden A , Yu M , Yoon Y . Removal of contaminants of emerging concern by metal-organic framework nanoadsorbents: a review. Chemical Engineering Journal, 2019, 369: 928–946

[20] Riccò R , Liang W , Li S , Gassensmith J , Caruso F , Doonan C , Falcaro P . Metal-organic frameworks for cell and virus biology: a perspective. ACS Nano, 2018, 12(1): 13–23

[21] Sun X , Xu W , Zhang X , Lei T , Lee S , Wu Q . ZIF-67@cellulose nanofiber hybrid membrane with controlled porosity for use as Li-ion battery separator. Journal of Energy Chemistry, 2021, 52(1): 170–180

[22] Huang Q , Zhao C , Li X . Enhanced electrolyte retention capability of separator for lithium-ion battery constructed by decorating ZIF-67 on bacterial cellulose nanofiber. Cellulose, 2021, 28(5): 3097–3112

[23] Zheng S , Zhu X , Ouyang Y , Chen K , Chen A , Fan X , Miao Y , Liu T , Xie Y . Metal-organic framework decorated polymer nanofiber composite separator for physiochemically shielding polysulfides in stable lithium-sulfur batteries. Energy & Fuels, 2021, 35(23): 19154–19163

[24] Wang W , Li Z , Huang H , Li W , Wang J . Facile design of novel nanocellulose-based gel polymer electrolyte for lithium-ion batteries application. Chemical Engineering Journal, 2022, 445: 136568

[25] Abd El-Monaem E M , Omer A M , Khalifa R E , Eltaweil A S . Floatable cellulose acetate beads embedded with flower-like zwitterionic binary MOF/PDA for efficient removal of tetracycline. Journal of Colloid and Interface Science, 2022, 620: 333–345

[26] LiZ. Preparation and performance studies of acylated paper-based separators for lithium-ion battery application. Dissertation for the Master’s Degree. Nanning: Guangxi University, 2023

[27] Hu Y , Kazemian H , Rohani S , Huang Y , Song Y . In situ high pressure study of ZIF-8 by FTIR spectroscopy. Chemical Communications, 2011, 47(47): 12694–12696

[28] Yang Z , Wang W , Shao Z , Zhu H , Li Y , Wang F . The transparency and mechanical properties of cellulose acetate nanocomposites using cellulose nanowhiskers as fillers. Cellulose, 2013, 20(1): 159–168

[29] Ajkidkarn P , Manuspiya H . Novel bacterial cellulose nanocrystals/polyether block amide microporous membranes as separators. International Journal of Biological Macromolecules, 2020, 164: 3580–3588

[30] Zhu Y , Xiao S , Li M , Chang Z , Wang F , Gao J , Wu Y . Natural macromolecule based carboxymethyl cellulose as a gel polymer electrolyte with adjustable porosity for lithium ion batteries. Journal of Power Sources, 2015, 288: 368–375

[31] Li M , Wang X , Yang Y , Chang Z , Wu Y , Holze R . A dense cellulose-based membrane as a renewable host for gel polymer electrolyte of lithium ion batteries. Journal of Membrane Science, 2015, 476: 112–118

[32] Yang J , Zhao X , Zhang W , Ren K , Luo X , Cao J , Zheng S , Li W , Wu X . “Pore-hopping” ion transport in cellulose-based separator towards high-performance sodium-ion batteries. Angewandte Chemie International Edition, 2023, 62(15): e202300258

[33] Xing J , Li J , Fan W , Zhao T , Chen X , Li H , Cui Y , Wei Z , Zhao Y . A review on nanofibrous separators towards enhanced mechanical properties for lithium-ion batteries. Composites Part B: Engineering, 2022, 243: 110105

[34] Tan L , Li Z , Shi R , Quan F , Wang B , Ma X , Ji Q , Tian X , Xia Y . Preparation and properties of an alginate-based fiber separator for lithium-ion batteries. ACS Applied Materials & Interfaces, 2020, 12(34): 38175–38182

[35] Zhan X , Zhang J , Liu M , Lu J , Zhang Q , Chen F . Advanced polymer electrolyte with enhanced electrochemical performance for lithium-ion batteries: effect of nitrile-functionalized ionic liquid. ACS Applied Energy Materials, 2019, 2(3): 1685–1694

[36] Qin C , Wang W , Li W , Zhang S . Reactive water vapor barrier coatings derived from cellulose undecenoyl esters for paper packaging. Coatings, 2020, 10(11): 1032

[37] Kim M , Kim H , Lee K , Kim D . Frosting characteristics on hydrophobic and superhydrophobic surfaces: a review. Energy Conversion and Management, 2017, 138: 1–11

[38] Zhang J , Yue L , Kong Q , Liu Z , Zhou X , Zhang C , Xu Q , Zhang B , Ding G , Qin B . . Sustainable, heat-resistant and flame-retardant cellulose-based composite separator for high-performance lithium ion battery. Scientific Reports, 2014, 4(1): 3935

[39] Minoofar P , Hernandez R , Chia S , Dunn B , Zink J , Franville A . Placement and characterization of pairs of luminescent molecules in spatially separated regions of nanostructured thin films. Journal of the American Chemical Society, 2002, 124(48): 14388–14396

[40] Wang J , Wang C , Wang W , Li W , Lou J . Carboxymethylated nanocellulose-based gel polymer electrolyte with a high lithium ion transfer number for flexible lithium-ion batteries application. Chemical Engineering Journal, 2022, 428(3): 132604

[41] Zhang J , Zhang Z , Wu T , Luo X . MOF particles (UiO-66 and UiO-66(Ce))/cellulose nanocomposite separators with regulating ion transport controllably for lithium battery. Journal of Electroanalytical Chemistry, 2023, 946: 117708

[42] Song X , Xu X , Liu J . Application of metal/covalent organic frameworks in separators for metal-ion batteries. ChemNanoMat: Chemistry of Nanomaterials for Energy, Biology and More, 2023, 9(10): e202300246

[43] Zhao X , Wang W , Huang C , Luo L , Deng Z , Guo W , Xu J , Meng Z . A novel cellulose membrane from cattail fibers as separator for Li-ion batteries. Cellulose, 2021, 28(14): 9309–9321

[44] Sheng J , Chen T , Wang R , Zhang Z , Hua F , Yang R . Ultra-light cellulose nanofibril membrane for lithium-ion batteries. Journal of Membrane Science, 2020, 595: 117550

[45] Zhang S , Luo J , Du M , Hui H , Sun Z . Safety and cycling stability enhancement of cellulose paper-based lithium-ion battery separator by aramid nanofibers. European Polymer Journal, 2022, 171: 111222

[46] Zeng X , Liu Y , He R , Li T , Hu Y , Wang C , Xu J , Wang L , Wang H . Tissue paper-based composite separator using nano-SiO₂ hybrid crosslinked polymer electrolyte as coating layer for lithium ion battery with superior security and cycle stability. Cellulose, 2022, 29(7): 3985–4000

[47] Zhua C , Zhang J , Xua J , Yin X , Wu J , Chen S , Zhu Z , Wang L , Wang H . Facile fabrication of cellulose/polyphenylene sulfide composite separator for lithium-ion batteries. Carbohydrate Polymers, 2020, 248: 116753

[48] Wang Y , Liu X , Sheng J , Zhu H , Yang R . Nanoporous regenerated cellulose separator for high-performance lithium ion batteries prepared by nonsolvent-induced phase separation. ACS Sustainable Chemistry & Engineering, 2021, 9(44): 14756–14765

Competing interests

The authors declare that they have no competing interests.

Acknowledgements

This work was supported by the Basic Ability Improvement Project for Young Teachers in Guangxi Colleges and Universities (Grant No. 2023KY0396), Doctoral Research Start-up Fund of Nanning Normal University (Grant No. 602021239438), the Scientific and Technological Project of Henan Province (Grant No. 232102230071), the Key Scientific Research Projects of Henan Higher Education Institutions (Grant No. 23B550006) and Foundation of Central Laboratory of Xinyang Agriculture and Forestry University (Grant No. FCL202110).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-024-2495-0 and is accessible for authorized users.

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