Aramid Nanobers Reinforced Cellulose Paper-Based Lithium-Ion Battery Separator with Improved Safety and Cycling Stability

Commercial polyolen separators with poor electrolyte wettability and inferior thermal stability have hampered the development of advanced lithium-ion batteries (LIBs) due to their unsatised electrochemical performance and severe safety hazards. Herein, a novel paper-based composite separator composed of electrolyte-anitive cellulose bers (CFs) and thermally stable aramid nanobers (ANFs) was successfully fabricated through the traditional papermaking method. It was found that the incorporation of ANFs played crucial roles in improving the defects of pure CF separator such as large-sized pores, low mechanical strength and high ammability. Specically, the CF/ANF composite separator with 20 wt.% ANFs (CF/ANF-20) possessed narrow micropores, satised tensile strength (33MPa), excellent thermal resistance (without dimensional shrinkage up to 200 °C) and ame retardancy, greatly enhancing the safe operation of battery. In addition, beneting from the highly porous structure and exceptional electrolyte anity of CF separator, the CF/ANF-20 composite separator exhibited appropriate porosity and superior electrolyte wettability, which brought about a high electrolyte uptake (157%), thus endowing it with better ionic conductivity (0.75 mS cm −1 ) and lower interfacial resistance than that of commercial polypropylene (PP) separator. Accordingly, the LiFePO4/Li half cells using CF/ANF-20 separator delivered outstanding rate capability and stable cycling performance. All results indicate that the CF/ANF-20 separator with great balance between the electrochemical performance and safety is an intriguing candidate for advanced LIBs.


Introduction
Nowadays, rechargeable lithium-ion batteries (LIBs) have been recognized as the most reliable energy storage devices for portable electronic products and electronic vehicles by reason of their high energy density, long lifetime and environmental friendliness (Costa et al. 2020;Manthiram 2017). As a key element of LIB, the separator provides a porous ion-transport channel while separating the cathode and anode to prevent internal short-circuits, thus directly affecting the cell performance and safety. (Jana et al. 2018;Zhai et al. 2021). Conventionally, the widely commercialized LIB separators are microporous polyole n membranes comprised of polyethylene (PE) or polypropylene (PP) due to their uniform pore size, high mechanical strength and good electrochemical stability (Costa et al. 2019). However, these separators still encounter the drawbacks of low porosity and bad electrolyte wettability, which damage the electrolyte storage and the transport of Li + , thus resulting in low ionic conductivity and inferior battery performance. In addition, the poor thermostability of polyole n separators would cause internal shortcircuits at elevated temperatures and eventually lead to the explosion of LIBs (Cha et al. 2018; Waqas et al. 2019). Therefore, considerable efforts have been made to develop advanced separators that can satisfy the increasing demand of high-performance LIBs (Zhang et al. 2021a).
Cellulose paper-based separators, as one of the promising alternatives of polyole n separators, have so far garnered considerable attention owing to their intriguing properties, such as high porosity, good electrolyte wettability and excellent chemical resistance ( Furthermore, cellulose materials combining the features of natural abundance, sustainability and low cost are more attractive for separator application in contrast to the petroleum-based polyole n materials due to the exhausted fossil fuel and severe environmental problems ). However, normal cellulose paper is inappropriate for direct use as the LIB separator due to its relatively large-sized pores and poor mechanical strength, which can bring about serious self-discharge and other safety risks caused by the formation of lithium dendrites . With the aim to address these problems, numerous endeavors have been exerted. For example, some inorganic ceramic particles such as Al 2 O 3 and SiO 2 have been adopted as the coating layer or additives to minimize the large pores of cellulose paper separator Xu et al. 2017). Unfortunately, restricted by the poor adhesion of ceramic particles which are readily detached from the paper substrate, these paper-supported inorganic composite separators were not bene cial for the long-term practical operation of LIBs. Recently, Lv et.al fabricated a composite cellulose paper (CCP) separator combined with cellulose nano brils (CNFs) through the ltration process (Lv et al. 2021). The favorable length to width ratio and nanoscale of CNFs was used to ll the large pores created by the networks of cellulose bers (CFs). It was found that the CCP separator showed narrow pore size distribution, superior electrolyte wettability and enhanced mechanical strength, yielding good working stability for the LiCoO 2 /Li batteries. Disappointingly, the pure cellulose paper separator is highly ammable, which is unfavorable for mitigating safety concerns when LIBs suffer from thermal runaway under harsh conditions (Zhang et al. 2021b). In this case, the scienti c community has paid much attention to the development of ame-retardant cellulose paper separators.
Incorporating ame retardants into cellulose pulps during the homogeneous mixing process has been considered an e cient method to enhance the ame retardance of cellulose paper separators (Zhang et al. 2014). Moreover, the ammability of cellulose paper separators can be improved by compounding cellulose bers with suitable re-resistant materials, such as hydroxyapatite nanowires, xonotlite nanowires and calcium alginate (Jia et al. 2020;Li et al. 2017;Tan et al. 2020). However, these modi cation strategies provide limited improvement in the mechanical properties of cellulose paper separators. Therefore, the pursuit of high-performance cellulose paper separators with appropriate pore size, superb mechanical strength and enhanced ame retardance remains an ongoing challenge.
As a novel one-dimensional nanomaterial, aramid nano bers (ANFs) possess exceptional mechanical properties and thermal resistance, allowing them to be promising material for the design of highperformance battery separators (Yang et al. 2020a Meanwhile, ANF paper was also developed as LIB separator, which had substantial advantages in the electrolyte wettability, mechanical property and ame retardance compared to the commercial polyole n separators (Patel et al. 2020). Nevertheless, these separators still rely on the consumption of large amounts of petroleum-based materials, which inevitably gives rise to environmental concerns and relatively high costs. Thus, this makes us to envision the possibility of introducing ANFs into the sustainable and cost-effective cellulose paper separator.
Herein, inspired by the advantages of cellulose and ANFs in LIB separator applications, a novel paperbased composite separator (CF/ANF) composed of cellulose bers and aramid nano bers was fabricated through the facile papermaking method. Given that highly porous and electrolyte-a nitive cellulose paper bene ts the absorption of liquid electrolyte and promotes the migration of Li + , cellulose bers were deliberately selected for the construction of separator backbone. Besides, based on the high aspect ratio and polar aramid component, the introduced aramid nano bers were expected to optimize the relatively large pores and insu cient mechanical and thermal properties of pure cellulose paper separator. The porous structure, mechanical strength, thermal stability, electrolyte wettability and electrochemical performance of the as-prepared CF/ANF separator were systematically evaluated. It was demonstrated that the CF/ANF composite paper with high safety and excellent electrochemical performance could be a promising alternative for advanced LIB separators.

Preparation of ANFs suspension
A stable ANFs suspension was obtained from the deprotonation and dissociation of PPTA bers according to Yang's method (Yang et al. 2011). Brie y, 0.3 g KOH was rstly dissolved in 4 ml deionized water to obtain the uniform KOH solution. Then, 0.2 g PPTA bers, 100 ml DMSO and the resultant KOH solution were mixed and magnetically stirred for about 4h at room temperature to form the homogeneous dark red solution of ANFs/DMSO. Afterwards, 200 ml deionized water was injected into the solution under vigorous stirring, making the occulent ANFs fully precipitated. The following puri cation and separation of ANFs were achieved by repeatedly washing the colloidal ANFs with alcohol and deionized water through vacuum assisted ltration. Finally, the puri ed ANFs were diluted with deionized water to reach a homogeneous ANFs/H 2 O suspension (0.5 mg/mL).

Preparation of the CF/ANF paper-based composite membranes
The schematic illustration for the preparation of CF/ANF composite membranes was shown in Fig. 1. Firstly, cellulose paper was smashed by a PFI beater for 10000 revolutions to yield the brillated cellulose bers pulp. Next, the obtained cellulose bers pulp was fully dispersed in deionized water at a concentration of 0.5 mg/mL with vigorous magnetic stirring. Then, at a xed basis weight of 25 g/m 2 , cellulose bers and ANFs suspensions were mixed with the proportion of 9:1, 8:2 and 7:3 (w/w), respectively, and subsequently ltrated under vacuum to fabricate a series of wet composite membranes.
Finally, all the wet membrane samples were pressed in a paper presser at 0.4 MPa for 5 min and then dried under vacuum at 105°C for 10 min. The prepared CF/ANF composite membranes were correspondingly denoted by CF/ANF-10, CF/ANF-20 and CF/ANF-30, according to the percent of ANFs contents. Meanwhile, pure cellulose membrane without the introduction of ANFs was also prepared at the same procedure and named as CF.

Characterization
The microstructure of ANFs in water was investigated by transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN, USA). The surface morphologies of the membranes were observed via scanning electron microscope (SEM, HITACHI-S4800, Japan) at an accelerating voltage of 5.0 kV. Fourier transform infrared spectroscopy (FTIR) spectrometer (VECTOR22, Bruker, German) was used to determine the chemical components of the membranes. The pore size distribution of the membranes was measured by using automatic mercury porosimeter. The porosity (P) of the membranes was determined by immersing them in n-butanol for 2 h and then calculated by the following Eq. (1): where Δm is the weight deviation between the saturated and dry membrane, ρ b and V s are the density of n-butanol (0.81 g/mL) and the volume of dry membrane, respectively.
Likewise, the electrolyte uptake (EU) of the membranes was measured according to the following Eq. (2): where M dry , M wet are the weight of the membranes before and after soaking in the liquid electrolyte for 2 h.
The mechanical properties of the membranes (30 mm × 10 mm) were tested by a universal tensile tester (AI-7000-NGD, Goodtechwill, China) at a drawing speed of 5 mm min − 1 .
To evaluate electrolyte wettability of the membranes, electrolyte wetting behavior was observed by dripping electrolyte droplet (10 µL) on membrane surface. Optical contact angle measuring device (OCA20, Dataphysics, Germany) was used to record the static contact angles of water and electrolyte droplets (5 µL). The capillary absorption height was further tested by immersing one end of the membrane sample in electrolyte for 1 h.
Thermal properties of the membranes were investigated through thermogravimetric analyzer (STA449F3, NETZSCH, Germany) from 20 to 600 ℃ at 10 ℃/min under the nitrogen gas atmosphere and differential scanning calorimeter (STA449F3, NETZSCH, Germany) from 20 to 300 ℃ at 10 ℃/min under the nitrogen gas atmosphere. Thermal shrinkage behavior was measured by comparing the dimensional change of the membranes after heat treatment (25, 120, 160 and 200°C) in an oven for 0.5 h. Combustion test was carried out to evaluate the ame retardancy of the membranes.

Electrochemical Measurements
The ionic conductivity and interfacial resistance of the separators were evaluated through electrochemical impedance spectroscopy (EIS) measurements in a frequency region of 0.1 to 10 5 Hz. For the ionic conductivity test, a symmetrical SS/separator/SS cell was assembled by placing the electrolytesoaked separator between two stainless steel (SS) electrodes. The ionic conductivity (σ, mS cm − 1 ) was calculated according to the following Eq. (3): where d (cm), S (cm − 2 ) and R b (Ω) are the thickness, e cient area and bulk resistance of separator sample, respectively.
Similarly, a symmetrical Li/separator/Li cell was assembled to assess the interfacial resistance between separator and Li electrode. These EIS and LVS measurements were fully conducted on an electrochemical workstation (CHI 760e, CH Instruments) at room temperature.
To evaluate the charge/discharge performance, half-cells (2032-type) were assembled by sandwiching the electrolyte-soaked separator (PP, CF and CF/ANF-20, respectively) between a LiFePO 4 cathode and a lithium foil anode. All the cells were assembled in an argon-lled glove box (Lab 2000, O 2 and H 2 O < 0.1 ppm) and tested on a LAND battery instrument (CT3001A, Wuhan LAND Electronic., Co, Ltd, China) under a cut-off voltage range from 2.5 to 4.2 V.
The rate capability was investigated at different charge/discharge current densities varied from 0.2 to 3.0 C (0.2 C, 0.5 C, 1 C, 2 C and 3 C). Moreover, cycling performance was appraised at 25°C with a xed charge/discharge current density of 0.5 C/0.5 C.

Results And Discussion
Morphology and structure Figure 2 displayed the surface morphologies of ANFs and the membrane samples. It could be observed from Fig. 2(a) that the as-prepared ANFs were homogeneously distributed and the mean ber diameter was approximately 36 nm (Fig. S1). As shown in Fig. 2(b), the commercial PP membrane presented a uniform and elliptic pore structure with an average pore size of about 77 nm (Fig. S2) due to the uniaxial stretching technology during the manufacturing process (Lee et al. 2014). By contrast, extremely large and irregular pores were seen in the CF membrane (Fig. 2c). This structure was bene cial to the fast transportation of Li + between the cathode and anode, yielding good rate performance for the battery. However, the large-sized pores of CF membrane easily caused self-discharge and internal short-circuits arising from lithium dendrite growth or the migration of electrode particles (Boateng et al. 2021;Pan et al. 2017). Thus, aramid nano bers with different ratios were introduced to regulate the pore size of CF membrane through the facile ltration process. As illustrated in Figs. 2(d-f), the uniformly distributed ANFs connected and intertwined with the relatively loose cellulose bers, constructing more nanosized pores for the CF/ANF membranes. The pore size distribution of these membranes examined here was depicted in Fig. 3(a). Apparently, the pore size values of CF membrane decreased signi cantly with the addition of ANFs. As ANFs ratio increased to 20 wt.%, the obtained CF/ANF-20 membrane displayed smaller pore size (average pore size around 410 nm) and narrower pore size distribution than that of CF and CF/ANF-10 membranes, making it more suitable for serving as the LIB separators.
The chemical composition of CF/ANF composite membranes was identi ed by FTIR spectroscopy in Fig. 3(b). It was seen that the pure ANF membrane showed several unique characteristic peaks around Besides, the strong absorption peak at 1010 cm − 1 could be ascribed to the C-O stretching vibration in C-O-C or C-OH groups (Nabipour et al. 2019). Upon the addition of ANFs, the characteristic peaks of ANFs were clearly detected in all the composite membranes. Moreover, the intensity of these peaks tended to increase as the ANFs content increases. These results fully con rm that different ratios of ANFs have been successfully introduced into the CF membrane matrix.
It is well known that high porosity is required for LIB separators to store liquid electrolyte and provide more channels for the transfer of Li + , thus enabling superior electrochemical properties (Sheng et al. 2020). The porosity and electrolyte uptake of the membranes were calculated and depicted in Fig. 3(c).
The CF membrane showed the highest porosity (59.6%) and electrolyte uptake (223.6%), which were nearly 1.5 and 2.4 times higher than that of PP membrane, respectively. These characteristics were believed to originate from its interconnected and macroporous structure as seen in the SEM image.
However, the addition of ANFs ratios from 10-30% had a negative impact on the porosity of CF membrane, which might be ascribed to the fact that excessive ANFs could ll or even block the pores of CF membrane, resulting in dense morphology and reduced porosity. Meanwhile, the electrolyte uptakes among the CF/ANF composite membranes were observed to decrease in a similar trend with the porosity.
Remarkably, the CF/ANF-30 membrane had a relatively low electrolyte uptake (96.5%) due to its lowest porosity (40.2%), which would increase the internal resistance of battery when acting as the separator (Huang et al. 2021).
Mechanical properties LIB separators are supposed to possess high mechanical properties that can withstand the high tension from cell assembly and the growth of lithium dendrites formed during prolonged cycling (Mao et al. 2021). The tensile strength and modulus of the membranes were surveyed via the tensile test. As seen in Fig. 3(d), the CF membrane showed a weak tensile strength of 7 MPa due to the incompact connection of cellulose bers. In contrast, the tensile strength of CF/ANF membranes signi cantly increased along with the addition of ANFs, and the CF/ANF-30 membrane possessed the highest tensile strength (48 MPa).
Although PP membrane had a high tensile strength of 111 MPa along the machine direction (Fig. S3), its tensile strength at transverse direction was only recorded as 15 MPa, much lower than that of CF/ANF composite membranes due to the uniaxial stretching technology (Hao et al. 2020). Besides, the introduction of ANFs also showed a reinforcing effect on Young's modulus of CF membrane, which was more advantageous relative to PP membrane (Fig. S4). According to previous literatures, high Young's modulus was conducive to sustain mechanical integrity and avoid the rupture of separators when an unexpected collision happened (Liu et al. 2018; Zhu et al. 2020). The improved mechanical properties of CF/ANF membranes not only stemmed from the uniform dispersion of ANFs in CF membrane matrix, but also the effective interaction between ANFs and cellulose bers through hydrogen bonds. It was worth noticing that the CF/ANF-20 membrane possessed superior tensile strength than most of other cellulosebased separators (Table S1), which would guarantee the safe operation of LIBs under rigorous conditions.
From a practical perspective, an ideal LIB separator should possess the critical parameters including small and uniform pore size, high porosity and electrolyte uptake, robust mechanical strength, good electrolyte wettability as well as excellent thermal stability to enable superior battery performance (Deimede and Elmasides 2015). As listed in Table 1, the obtained CF/ANF-20 membrane with tailored pore size, appropriate porosity and mechanical strength, which was more hopeful to achieve the trade-off between high safety and electrochemical performance of LIB, was thus chosen as the separator for further investigations. Electrolyte wettability and thermal stability Good electrolyte wettability is in critical need for LIB separator to accelerate the battery assembly process and promote e cient Li + transport between the electrodes (Huang et al. 2019). As shown in Fig. 4(a), after dropping equal volume of liquid electrolyte on each separator surface, the electrolyte droplet did not well diffuse on PP separator for a long time, while the entire CF and CF/ANF-20 separators were rapidly wetted within 30 s. The contact angles of water and liquid electrolyte (LE) were also measured and displayed in Fig. 4(b). It was clearly found that the water contact angle of PP separator was up to 104.6°d ue to the lyophobic nature of PP. Likewise, a high LE contact angle of 43.9° was recorded for PP separator, indicating its poor electrolyte wettability. In contrast, the CF separator contained numerous polar groups (hydroxyl groups), endowing it with the smallest water and LE contact angles. A slight increase of contact angles was observed for the CF/ANF-20 separator, which mainly resulted from the decrease of free hydrophilic groups in cellulose and ANFs due to the formation of abundant hydrogen bonds ). Furthermore, the electrolyte wettability of various separators was quantitatively evaluated through the electrolyte immersion-height measurements. As seen from Fig. 4(c), when soaking in the liquid electrolyte for 1 h, the nal immersion heights of CF and CF/ANF-20 separator samples were about 12 mm and 7 mm, respectively, which were both much higher than that of PP separator (3 mm). The above results explicitly demonstrated the excellent electrolyte wettability of CF-based separators, which was attributed to their highly porous structure as well as the strong a nity of cellulose and ANFs with the polar electrolyte mixture (Fig. 4(d)).
High thermal stability is another pivotal parameter for LIB separator, which can ensure the safety and lifetime of battery since local heating may take place under rigorous conditions (e.g., battery overheating, overcharging, and overcurrent) (Feng et al. 2020;Wen et al. 2019). The TG and DSC measurements were carried out to evaluate the thermal stability of the separators. As shown in Fig. 5(a), the weight of PP separator sharply dropped at approximately 350°C due to the degradation of polyole n backbone, and a weight loss of 100% was reached for the PP separator when the temperature elevated to 500°C (Tan et al. 2020). For the CF separator, two distinct stages of weight loss were observed. The rst stage was a slight weight loss of water moisture (before 200°C). The second stage from 250 to 350°C was a major weight loss, arising from the degradation of cellulose backbone ). Additionally, one minor weight loss occurred at about 520°C for the CF/ANF-20 composite separator, corresponding to the degradation of ANFs polymer backbone ). Meanwhile, the remaining weight of CF and CF/ANF-20 separators was respectively determined to be 13.8 and 30.5 wt. % at 500°C, indicating that the addition of ANFs could help to enhance the thermal stability of CF separator. Furthermore, it was observed from DSC curves (Fig. 5(b)) that the endothermic peak of PP separator appeared at about 165°C, which could be assigned to the melting of PP and caused the shutdown of micropores . In contrast, no obvious endothermic peak was recorded below 300°C for the CF-based separators, demonstrating the high stability in the pore structure of CF-based separators at high temperatures.
Since the shape change of separators will cause the physical contact between cathode and anode, thus leading to internal short-circuits of battery. Therefore, the dimensional thermal stability of different separators was also investigated through the thermal shrinkage tests. As shown in Fig. 5(c), the dimension of PP separator was inclined to decrease with the elevation of temperature. Particularly, the PP separator suffered from great shrinkage after heat treatment at 160°C for 0.5 h, and its thermal shrinkage rate reached almost 100% at 200°C. In sharp contrast, no apparent dimensional change was observed for the CF and CF/ANF-20 separators when the heating temperature approached 200°C. This behavior was attributed to the strong skeleton provided by cellulose bers and outstanding thermal resistance of ANFs, which enabled the CF-based separators to resist thermal shrinkage substantially (Yang et al. 2020b). Non ammability is also a critical necessity for high-performance LIB separators because it can prevent the battery from catching re or exploding when thermal runaway happens (Zhang et al. 2021b). It could be found from Fig. 5(d) that the PP and CF separators were rapidly ignited and burnt out as soon as exposed to a ame. This phenomenon was consistent with previous literatures ( (Fig. 6(b)). The interconnected porous structure, excellent electrolyte wettability and uptake, which could boost the transport of Li + between electrodes, should be responsible for the superior ionic conductivity of CF membrane. However, the ionic conductivity of CF/ANF composite membranes decreased with the increase of ANFs content, which might be explained from the aforementioned blocking function of ANFs in composite membranes. In particular, the ionic conductivity of CF/ANF-30 membrane was inferior to that of PP membrane due to its dense structure, which greatly limited its application in high-performance LIBs. Additionally, the interfacial resistance between the separator and Li electrode was also investigated by assembling Li-symmetrical batteries. Figure 6(c) showed the corresponding Nyquist plots of different separators, where the semicircle diameter in the middle frequency represented the interfacial resistance. It was obviously found that the interfacial resistance of PP, CF and CF/ANF-20 separator was approximately 310 Ω, 250 Ω and 280 Ω, respectively.
The better interfacial compatibility of CF-based separators was mainly attributed to their excellent electrolyte wettability and absorption behavior, which improved the interfacial contact with Li electrode, thus facilitating Li + diffusion at the separator/electrode interface (Liu et al. 2020b).
The linear sweep voltammetry (LSV) tests were carried out to evaluate the electrochemical stability window of separators, which was crucial for determining the operation voltage of LIBs. As seen in . In contrast, the initial discharge capacity of battery containing PP separator was only 153.2 mA h g − 1 . Moreover, a downward trend was observed for the discharge capacity of all batteries with the increase of discharge current density as shown in Fig. 7(a). Particularly, the corresponding discharge capacities of batteries based on CF, CF/ANF-20 and PP separators decreased to 109.9 mA h g − 1 , 98.7 mA h g − 1 and 83.4 mA h g-1 at a high rate of 3 C. This phenomenon revealed the capacity loss caused by the increased ohmic polarization effect and serious over potential (i.e., IR drop) at high current density (Yang et al. 2021). Notably, the battery with CF separator showed the highest discharge capacity at all C-rates. This might be ascribed to the superior ionic conductivity and lowest interfacial resistance of CF separator, which could slow down ohmic polarization degree in LIBs (Fig. S6) ). Furthermore, the capacity divergency between the cells with CF/ANF-20 separator and PP separator became more striking as the discharge rate increased, suggesting better rate capability of CF/ANF-20 separator than that of PP separator ( Fig. 7(b)).
More attractively, when the discharge current returned to 0.2 C, the discharge capacity of batteries could recover to their initial values, implying the good reversibility of cells using these separators.
The cycling performance of batteries with three separators were tested at a constant charge/discharge current density of 0.5C/0.5C at 25°C. As shown in Fig. 7(c), although the battery with CF separator initially displayed a high discharge capacity of 149.8 mA h g − 1 , its nal discharge capacity was only maintained at approximately 92.1 mA h g − 1 after 100 cycles, indicative of a capacity retention of 61.5%. As a comparison, the discharge capacity retention of the battery with CF/ANF-20 separator was about 89.6% (decreased from 145.4 mA h g − 1 to 130.3 mA h g − 1 ), which was also higher than that of PP separator (82.8%). Additionally, the coulombic e ciency was found to be nearly 100% for the battery using CF/ANF-

Conclusions
In this study, a novel ANFs reinforced cellulose paper with controllable pore structure, superior electrolyte wettability and high thermal stability was successfully developed as an advanced LIB separator. It could be found that the obtained CF/ANF-20 composite separator (with ANFs content of 20%) alleviated the risks of pure cellulose separator including large pore sizes, poor mechanical strength and high ammability which severely threaten the safety of LIBs. In comparison with the commercial PP separator, the CF/ANF-20 separator exhibited higher electrolyte uptake and superior electrolyte wettability, thus giving preferable ionic conductivity (0.75 mS cm − 1 ), lower interface resistance and better electrochemical stability. Meanwhile, the CF/ANF-20 separator could effectively prevent thermal runaway due to its excellent thermal resistance (without dimensional shrinkage up to 200°C) and ame retardancy. These advantages were ultimately con rmed to endow the LiFePO 4 /Li half cells using CF/ANF-20 separator with outstanding rate capability and good cycling performance (discharge capacity retention up to 89.6% after 100 cycles). All results convincingly prove that the CF/ANF-20 separator present great potential application in high-performance and safe LIBs.

Supplementary Files
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