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 fiber 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 beneficial 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 nanofibers with different ratios were introduced to regulate the pore size of CF membrane through the facile filtration process. As illustrated in Figs. 2(d-f), the uniformly distributed ANFs connected and intertwined with the relatively loose cellulose fibers, 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 significantly 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 identified by FTIR spectroscopy in Fig. 3(b). It was seen that the pure ANF membrane showed several unique characteristic peaks around 3300 cm− 1 (N-H stretching vibration), 1650 cm− 1 (C = O stretching vibration in amide), 1540 cm− 1 (N-H bending vibration) and 1510 cm− 1 (C = C stretching vibration in aromatic ring) (Liu et al. 2020a). With regard to pure CF membrane, the broad absorption peak at 3200–3600 cm− 1 corresponded to stretching vibration of O-H groups. The peak at 2891 cm− 1 was assigned to the stretching vibration of C–H bonds. 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 confirm 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 fill 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 fibers. In contrast, the tensile strength of CF/ANF membranes significantly 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 fibers through hydrogen bonds. It was worth noticing that the CF/ANF-20 membrane possessed superior tensile strength than most of other cellulose-based 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.
Table 1
Physical properties of the PP, CF and CF/ANF composite membranes.
| Thickness /µm | Mean pore size /µm | Porosity /% | Electrolyte uptake/% | Tensile strength/MPa |
PP | 25 | 0.077 | 41.0 | 92.3 | 15 |
CF | 45 | 1.76 | 59.6 | 223.6 | 7 |
CF/ANF-10 | 42 | 0.90 | 54.3 | 185.1 | 21 |
CF/ANF-20 | 40 | 0.41 | 49.5 | 157.4 | 34 |
CF/ANF-30 | 37 | 0.32 | 40.2 | 96.5 | 48 |
Electrolyte wettability and thermal stability
Good electrolyte wettability is in critical need for LIB separator to accelerate the battery assembly process and promote efficient 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° due 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 (Luo et al. 2019). 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 final 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 affinity 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 polyolefin 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 first 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 (Chen et al. 2020). 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 (Luo et al. 2019). 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 (Xu et al. 2019). 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 fibers and outstanding thermal resistance of ANFs, which enabled the CF-based separators to resist thermal shrinkage substantially (Yang et al. 2020b). Nonflammability is also a critical necessity for high-performance LIB separators because it can prevent the battery from catching fire 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 flame. This phenomenon was consistent with previous literatures (Jia et al. 2020; Zhang et al. 2014). However, the CF/ANF-20 separator exhibited excellent self-extinguishing ability and even remained intact as a result of the high thermal stability of polyaramid nature (Liu et al. 2021; Patel et al. 2020). These results indicate that the CF/ANF-20 separator with superior thermal stability and flame-retardant capability is a promising candidate for highly safe LIBs.
Electrochemical performance of the separators
The ionic conductivity and interfacial resistance are regarded as two key factors of separator that greatly affects the performance of LIBs (Arora and Zhang 2004). Figure 6(a) showed the Nyquist plots measured by EIS at room temperature, where the intercept of the real axis corresponded to the bulk resistance (Rb) of separators. It was observed that the Rb were 3.40 Ω, 1.15 Ω, 2.30 Ω, 2.72 Ω and 5.72 Ω for the PP, CF, CF/ANF-10, CF/ANF-20 and CF/ANF-30 membranes, respectively. Then based on Eq. (3), the ionic conductivity of the membranes was calculated to be 0.38 mS cm− 1, 2 mS cm− 1, 0.93 mS cm− 1, 0.75 mS cm− 1 and 0.33 mS cm− 1, respectively (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 Fig. 6(d), no obvious oxidation decomposition potential was observed up to 5.5 V vs Li/Li+ for the CF separator, while the electrolyte-soaked CF/ANF-20 separator began to decompose at approximately 4.8 V vs Li/Li+, indicating that the introduction of ANFs resulted in inferior electrochemical stability due to the Lewis base aramid component (Yang et al. 2020b). However, the electrochemical stability window of CF/ANF-20 separator was still broader than that of PP separator (4.3 V vs Li/Li+) due to its superior affinity with carbonate electrolyte. These results suggest that the CF-based separators with high electrochemical stability are qualified for applications in high-voltage LIBs (Wen et al. 2017).
Battery performance
Battery performance was further tested in a LiFePO4/separator/Li system, which could effectively evaluate the long-term stability of separators in LIBs (Liu et al. 2020b). The discharge rate capability of batteries with different separators was firstly investigated at various current densities ranging from 0.2 C to 3 C. Fig. S5 displayed the initial charge/discharge curves of LiFePO4/Li batteries assembled with CF, CF/ANF-20 and PP separators at 0.2 C. It could be found that the batteries with CF and CF/ANF-20 separators delivered higher discharge capacities of 162.9 mA h g− 1 and 159.4 mA h g− 1, respectively, which were 95.8% and 93.8% of the theoretical capacity of LiFePO4 (170 mA h g− 1). 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) (Zhu et al. 2020). 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 final 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 efficiency was found to be nearly 100% for the battery using CF/ANF-20 separator during 100 cycles, but was relatively low at first cycle due to the generation of solid electrolyte interfacial (SEI) layer (Xu et al. 2019). The severe capacity degradation and poor cycling performance of CF-based battery mainly arose from the non-homogeneous pores of the CF separator, through which a mass of lithium dendrites could be formed as a consequence of the undistributed deposition of Li+, therefore leading to the depletion of liquid electrolyte and formation of thick SEI layer (Lv et al. 2021; Zhang et al. 2015). Moreover, the thick SEI layer increased the interfacial resistance of battery (Fig. 7(d)), which further caused severe polarization effect and deteriorated the cycling stability of battery (Fig. S7). Fortunately, with the optimized pore size, high electrolyte adsorption and superior mechanical strength, the CF/ANF-20 separator was able to provide rapid and uniform ionic transport, as well as suppress the growth of lithium dendrites, thus endowing the battery with better cycling performance during long-term running (Du et al. 2019; Tan et al. 2020).