A Fluorinated‐Polyimide‐Based Composite Nanofibrous Separator with Homogenized Pore Size for Wide‐Temperature Lithium Metal Batteries

Lithium (Li) is known for excellent theoretical specific capacity and most negative electrochemical potential, while still restricted by the irregular lithium dendrites and safety risks in practical applications of lithium metal batteries (LMBs) due to thermal runaway. Herein, a fluorinated‐polyimide (F‐PI)‐based composite nanofibrous separator containing poly(vinylidene fluoride) (PVDF) component, which is designed as PVDF/F‐PI, is developed via a facile electrospinning strategy for wide‐temperature LMBs. First, abundant polar trifluoromethyl (–CF3) groups in F‐PI create an electronegative environment to facilitate rapid Li‐ions (Li+) transport. Meanwhile, the PVDF component, acting as both the physical linker between F‐PI nanofibers and the regulator of homogenized pore size, simultaneously improves the mechanical properties and homogenizes the Li+ flux on the electrode surface. Therefore, a steady circulation of 2400 h is achieved for the symmetric cell using PVDF/F‐PI separator, which still displays a stable cycle life with a low voltage polarization of 15 mV in 1000 h even under 60 °C. Therefore, the fluorinated‐PI‐based composite nanofibrous separator with high ionic conductivity and uniform pore structure offers a practical method for design of functionalized separators in wide‐temperature LMB applications.

Lithium (Li) is known for excellent theoretical specific capacity and most negative electrochemical potential, while still restricted by the irregular lithium dendrites and safety risks in practical applications of lithium metal batteries (LMBs) due to thermal runaway. Herein, a fluorinated-polyimide (F-PI)-based composite nanofibrous separator containing poly(vinylidene fluoride) (PVDF) component, which is designed as PVDF/F-PI, is developed via a facile electrospinning strategy for wide-temperature LMBs. First, abundant polar trifluoromethyl (-CF 3 ) groups in F-PI create an electronegative environment to facilitate rapid Li-ions (Li þ ) transport. Meanwhile, the PVDF component, acting as both the physical linker between F-PI nanofibers and the regulator of homogenized pore size, simultaneously improves the mechanical properties and homogenizes the Li þ flux on the electrode surface. Therefore, a steady circulation of 2400 h is achieved for the symmetric cell using PVDF/F-PI separator, which still displays a stable cycle life with a low voltage polarization of 15 mV in 1000 h even under 60°C. Therefore, the fluorinated-PI-based composite nanofibrous separator with high ionic conductivity and uniform pore structure offers a practical method for design of functionalized separators in wide-temperature LMB applications. transport channels for lithium ions (Li þ ) and homogenize Li þ flux through the proper design of physical and chemical structures to hinder dendritic growth. On the other hand, the separator can physically establish a mechanical barrier between the anode and cathode without significantly increasing battery's weight or volume, preventing chemical energy stored in the rechargeable battery being converted into heat. [29,32] Nevertheless, the commercial polyolefin separators are still afflicted with low porosity, weak liquid electrolyte affinity, inferior thermal stability, and flammability. The separator composed of polyethylene or polypropylene shrinks significantly as the internal temperature of the battery increases to its melting point, which easily results in the short-circuit risks. [29] Therefore, developing functional separators possessing superior flame-retardant properties and thermal resistance is of vital importance. [33] As a typical high-performance engineering polymer, polyimide (PI) is considered as an ideal candidate for battery separator applications due to its low dielectric coefficient, superior chemical stability, mechanical properties, and excellent thermal stability. [34][35][36][37][38] With the combination of the electrospinning technique, nanofibrous membranes based on PI are endowed with high porosity and excellent electrolyte wettability, which contribute to largely improved battery performance. [39][40][41][42][43][44][45][46] However, high porosity of electrospun nanofibrous separators is usually accompanied by poor mechanical strength. Meanwhile, the large pore size of the separators with uneven distribution could lead to dendrite growth and soft short circuit inside batteries, which severely hinder large-scale applications of electrospun nanofibrous separators. [47,48] Herein, a novel fluorinated-polyimide (F-PI)-based composite nanofibrous separator containing poly(vinylidene difluoride) (PVDF) component, which is denoted as PVDF/F-PI, has been prepared by a facile electrospinning technique of the composite solution of PVDF and fluorinated polyamic acid (F-PAA), followed by the subsequent imidization treatment. First, the F-PI component offers a negative electric environment owing to the introduction of trifluoromethyl (-CF 3 ) groups, which effectively promotes the rapid Li þ transfer. Moreover, the -CF 3 group is profit for increasing the free volume of polyimide, thereby enhancing the thermostability and fire retardancy which are prerequisites for the applicability of LMBs in a wide-temperature range. On this basis, the PVDF component, serving as both the physical linker between F-PI nanofibers and the regulator of the homogenized pore size, can provide higher mechanical properties for the composite separator and realize uniform lithium deposition on the anode surface simultaneously. Consequently, the symmetric cell with the PVDF/F-PI separator finally achieves steady cycling within 2400 h in the long-term deposition/stripping process. Even under the condition of 60°C, the cell with PVDF/F-PI exhibits an outstanding cycle stability in 1000 h with a low voltage polarization of about 15 mV, which is much better than the cell with commercial Celgard. Therefore, the flame-retardant fluorinatedpolyimide-based composite nanofibrous separator with high ionic conductivity and well-distributed pore structure holds great potentials for applications in high-energy and wide-temperature LMBs.

Morphology and Structural Features of PVDF/F-PI Separators
As illustrated in Figure 1a, the PVDF/F-PI composite nanofibrous separator was obtained by the polycondensation of 4,4 0 -(hexafuoroisopropylidene) diphthalic anhydride (6FDA) and 4,4 0 -diaminodiphenyl ether (ODA), followed by a further mixture with PVDF at a mass ratio of 2:1 for subsequent electrospinning and thermal imidization. For comparison, the nanofibrous separators of pure F-PI and polyimide derived from pyromellitic dianhydride (PMDA) monomer (denoted as PMDA-PI) are also obtained. Compared with the 2D planar structure of Celgard, both the F-PI and PMDA-PI separators show a 3D nanofibrous structure (Figure 1b and S1, Supporting Information), which is beneficial for promoting Li þ transmission. On this basis, the PVDF/F-PI composite nanofibrous separator, whose thickness is 50 μm ( Figure S2, Supporting Information), exhibits a strong physical adhesion structure between the adjacent nanofibers owing to the melting of PVDF during the heat treatment process (Figure 1c). The corresponding energy-dispersive X-ray spectroscopy (EDS) mappings indicate that all the elements are evenly distributed within the composite separator (Figure 1d), which demonstrates the uniformity of the PVDF and F-PI components in the composite separator. Besides, the morphology and pore size of PVDF/F-PI composite separators were further optimized by tuning the PVDF content, which indicates a much smaller and more homogenized pore size of about 1.48 μm for the PVDF/F-PI separator with a PVDF mass ratio of 50 wt% compared to that (4.25 μm) of the pure F-PI (Figure 1e,f, S3 and S4, Supporting Information).
Fourier-transform infrared (FTIR) spectroscopy was used to demonstrate chemical constitution of F-PI and PVDF/F-PI separators ( Figure 1g). For the F-PI nanofibrous separator, characteristic peaks located at 1785 and 1725 cm À1 correspond with the asymmetric and symmetric stretching vibrations of C═O bond in the imide ring, while the peaks at 1376 and 722 cm À1 represent the stretching and bending vibrations of C─N bond, respectively. Besides, the absorption wavelength within 1300-1100 cm À1 belongs to the stretching vibration of C─F bond in 6FDA. All the above absorption peaks of F-PI can be found in the PVDF/F-PI composite nanofibrous separator correspondingly. In addition, peaks at 3300-3000 cm À1 in PVDF/F-PI are assigned to the unsaturated stretching vibrations of C─H bond while peaks at 883 and 821 cm À1 represent the β-phase crystal structures of C─F bond in PVDF, proving the successful synthesis of PVDF/F-PI. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical constitutions of the two separators ( Figure S5, Supporting Information). As displayed in Figure S5b, Supporting Information, the composition ratios of F-PI and PVDF/F-PI separators are consistent with the theoretical calculation results before and after adding PVDF at a specific proportion, which further demonstrates the successful synthesis of the PVDF/F-PI composite separator.
The mechanical properties of separators are evaluated through the tensile and puncture tests. As shown in Figure 2a,b, the F-PI nanofibrous separator possesses stronger tensile modulus and www.advancedsciencenews.com www.small-structures.com puncture resistance compared to the traditional PMDA-PI.
In particular, the PVDF/F-PI composite nanofibrous separator achieves largely improved Young's modulus (83.7 MPa) and puncture strength (276.7 N m À1 ) simultaneously, which can be attributed to the strong adhesion among overlapped fibers contributed by the melting of PVDF as mentioned above ( Figure S6, Supporting Information). Then, electrolyte affinity of various separators is measured by contact angle tests. Obviously, all the polyimide-based separators are completely infiltrated by ether electrolyte and the contact angle becomes 0°after soaking for only 10 s, while it still retains 55°for Celgard separator (Figure 2c and S7, Supporting Information). On the one hand, the excellent electrolyte wettability is contributed by the 3D network structure of electrospun nanofibrous membranes. On the other hand, plentiful polar groups (-CF 3 ) in 6FDA can endow the F-PI-based nanofibrous separators with increased affinity toward the polar organic electrolytes. It is known that the heat resistance of membranes is critical to the safety of LMBs. Benefiting from the macromolecular backbone containing heterocyclic imide rings and aromatic benzene rings of polyimide, [27] all the PI-based separators display excellent thermal stability without any change during the heating process from 50 to 200°C in Figure 2d, in sharp contrast with the severe shrinkage of Celgard separator as the temperature exceeds 125°C. Combined with the thermogravimetric (TG) analysis ( Figure S8, Supporting Information), the superior thermal stability of polyimide can be successfully proven. In addition, the flame resistance of fluorinated-polyimide-based nanofibrous

Ionic Conductivity and Cyclic Stability of PVDF/F-PI Separators
The ionic conductivity is then obtained to evaluate the Li þ transmission capacity through the separators in Figure 3a. The PVDF/ F-PI separator obviously represents a relative high value (1.05 mS cm À1 ) in contrast to those of Celgard (0.38 mS cm À1 ) and PMDA-PI (0.99 mS cm À1 ) in virtue of its remarkable network structure and abundant -CF 3 groups. Moreover, the linear sweep voltammetry (LSV) measurements reveal that PVDF/F-PI exhibits a wider electrochemical stability window (Figure 3b), showing great potential applicability in high-voltage batteries.
For the sake of exploring practical applications of the fluorinated separators in LMBs, all the samples are first assembled in Li||Cu half cells. In Figure 3c, the Li||Cu cell using PVDF/F-PI separator presents slowest decay in the Coulombic efficiency (%98%) and a longer stable circulation when the current density is 1.0 mA cm À2 over the cells based on other separators. Calculated based on the charge-discharge voltage curves (Figure 3d and S10, Supporting Information), the overpotential of the Li||Cu cell utilizing PVDF/F-PI experiences a decline from 18.8 to 14.0 mV after 100 cycles, showing an obvious superiority to other comparative samples (Figure 3e). Furthermore, the Li anode of the cell utilizing PVDF/F-PI maintains a uniform and smooth appearance after 100 cycles while the Li foils coupled with other separators display severe surface cracking or dendrite growth after long-term cycles in the scanning electron microscope (SEM) images (Figure 3f-i and S11, Supporting Information), confirming that PVDF/F-PI with homogenized pore structure can facilitate the uniform Li deposition during cycling.
To further discuss the superiority of PVDF/F-PI during the cyclic process of LMBs, Li||Li cells using different separators are assembled. As the -CF 3 polar groups can reduce the charge-transfer impedance (R ct ) between anode and electrolyte and promote the Li-ion transfer, the cell using the F-PI separator possesses lower overpotential and longer cycles than those of PMDA-PI and Celgard separators. Moreover, the cell equipped with PVDF/F-PI exhibits a steady circulation of 2400 h at a current density of 1 mA cm À2 and an areal capacity of 1 mAh cm À2 (Figure 4a), although accompanied by a relatively high overpotential possibly due to the slight drop in ionic conductivity. This result largely gives the credit to the PVDF component, which effectively enhances the physical connection between F-PI nanofibers and successfully endows the PVDF/F-PI separator with well-distributed pore size. Hence, a more uniform Li deposition is allowed on the electrode surface with much higher cycling stability in the long plating/stripping process. Moreover, the improvement of mechanical properties derived from the reinforced nanofibrous structure contributes to the long-term cycle performance. Similar outcomes are observed in the cell with PVDF/F-PI separator under a current density of 2 mA cm À2 with a fixed capacity of 2 mAh cm À2 in Figure 4b. Besides, the cell with PVDF/F-PI maintains a steady cycle life for over 550 h, while the voltage hysteresis of other samples increases within 200 cycles (Figure 4c). All the samples exhibit a significant drop in the R ct after 25 cycles, with the cell fit with PVDF/F-PI  separator showing the lowest R ct (1.1 Ω), proving the advantage of the composite separator for battery applications (Figure 4d,e and S12, Supporting Information).
In addition, to further demonstrate the effect of PVDF/F-PI on the interface between the anode and separator, XPS measurement was employed to investigate the component of the SEI layer formed on the Li metal anode after 250 cycles ( Figure S13, Supporting Information). As shown in Figure S13a, Supporting Information, the abundant C─F bonds supplied by the PVDF/F-PI separator could react with Li metal anode to in situ form the amorphous carbon and crystalline LiF, thus creating a dense and robust LiF-rich SEI layer. Owing to the low-Li þ diffusion energy and high Young's modulus, LiF has been widely reported as an excellent inhibitor of dendrite growth, which is profit for the formation of a stable separator/anode interface. [49] Furthermore, the C 1s spectra of Li metal anode ( Figure S13b, Supporting Information) demonstrate that the component of RCO 3 (16.8%) caused by the decomposition of electrolyte is relative fewer than the Li anode coupled with Celgard separator (35.9%) in recent reports, [49] indicating that the composite separator can reduce the side reactions between the Li metal and electrolyte. Furthermore, the morphology and structure of the cycled composite separator were characterized. As displayed in Figure S14, Supporting Information, the PVDF/F-PI separator maintains the original porous structure with tangled fibers after 50 cycles, although with a slight decrease in the average pore size due to the swelling caused by absorbing electrolyte ( Figure S15, Supporting Information). The porosity of the cycled PVDF/F-PI separator also slightly decreases while it is still much higher than that of Celgard, which verifies the advantage of PVDF/F-PI in guaranteeing rapid ion transport ( Figure S16, Supporting Information). Moreover, the characteristic peaks of PVDF/F-PI could be clearly observed in the cycled composite separator in Figure S17, Supporting Information, which coincide with those that occur before cycling (Figure 1g), further proving the excellent stability of PVDF/F-PI during the cyclic process.
Subsequently, the potential application of PVDF/F-PI is measured in the LMB taking LiFePO 4 (LFP) as the cathode. Compared with the noticeable drop in the specific discharge capacity of the cells with Celgard and PMDA-PI separators, the cell utilizing F-PI demonstrates a more consistent capacity retention, while the cell based on the PVDF/F-PI separator possesses higher capacity retention at 1C rate (Figure 5a). Even at the rate of 2C, the cell using PVDF/F-PI still shows an initial capacity    of 130.6 mAh g À1 with a highest capacity retention after 1000 cycles (Figure 5b). The specific discharge capacity of Li|| LFP cells at various C rates are measured to further prove the superiority of PVDF/F-PI separator (Figure 5c). The cell utilizing PVDF/F-PI reveals a higher specific capacity throughout compared with other samples and could be restored to 159 mAh g À1 as current rate decreases from 4C back to 0.5C (Figure 5d and S18, Supporting Information). In addition, all the samples are applied in Li||LiCoO 2 full cells under the conditions of higher LiCoO 2 (LCO) loading and voltage. The cell fit with PVDF/F-PI displays a very stable cycle performance within 100 cycles, while other samples experience severe capacity degradation (Figure 5e and S19, Supporting Information), which further proves the potential application of PVDF/F-PI separators in the field of high-voltage batteries.

Practical Applications of PVDF/F-PI Separator for Wide-Temperature LMBs
To reveal the influence of thermal stability and flame resistance on the optimization of battery performance, the cycle stability of batteries is further characterized under higher temperature. As shown in Figure S20, Supporting Information, the Li||Li cell using PVDF/F-PI maintains a steady cycling of more than 1000 h at 40°C, with an overpotential as low as 18 mV. In addition, the Li||Cu cell equipped with PVDF/F-PI keeps steady cycles for about 120 cycles with a Coulombic efficiency of over 99% ( Figure S21, Supporting Information). The separator also provides outstanding rate performance in the Li||LFP full cell with a restored capacity of 147.3 mAh g À1 as the C-rate decreases from 4C back to 0.5C ( Figure S22, Supporting Information). As the operation temperature reaches 60°C, the Li||Li symmetric cell based on Celgard shows increasing polarization after 100 cycles and completely is damaged after 600 h (Figure 6a). By comparison, the cell with PVDF/F-PI separator keeps a stable cycle life within 1000 h and the voltage hysteresis also presents a more gradual trend (Figure 6b). Figure 6c,d further confirms that the Li||LFP cell fit with PVDF/F-PI retains a good rate performance under the condition of 60°C, while the cell with Celgard fluctuates strongly in the beginning and the specific discharge capacity drops quickly as current rate increases. Even as the temperature increases to 80°C, the Li||Li cell with PVDF/F-PI separator still displays a stable cycle life within 250 h ( Figure S23, Supporting Information). In addition, the symmetric cells with PVDF/F-PI and Celgard separators are applied in a lowtemperature environment for further comparison. As shown in Figure S24, Supporting Information, the Li||Li cell based on PVDF/F-PI separator keeps very steady for over 250 h under À15°C while the cell with Celgard experiences serious voltage polarization at the beginning. Moreover, the PVDF/F-PI separator reveals better cyclic stability in the symmetric cell compared to other lately published works ( Figure 6e and Table S1, Supporting Information). Even under a wider-temperature range, the cell with PVDF/F-PI presents a much longer cycling stability which largely exceeds the performance reported by other studies at room temperature. [50][51][52][53][54][55][56]

Conclusion
In conclusion, a novel fluorine-functionalized PVDF/F-PI composite nanofibrous separator has been fabricated through a simple electrospinning strategy combined with  subsequent imidization. On the one hand, the abundant electron-withdrawing groups of -CF 3 in PVDF/F-PI offer an electronegative environment, thereby promoting the fast transfer of Li þ . In addition, the fluorinated groups are considered as the main reasons for excellent flame resistance and selfextinguishing properties of the separator, enabling the possible application of LMBs under a wide operating temperature. On the other hand, the incorporation of PVDF not only promotes the uniform Li deposition by homogenizing the pore size distribution, but also allows the electrospun nanofibrous separator with better mechanical properties. Consequently, the Li||Li cell with PVDF/F-PI separator demonstrates a steady circulation of 2400 h at a current density of 1 mA cm À2 and an areal capacity of 1 mAh cm À2 . Even under a higher temperature of 60°C, the cell with PVDF/F-PI exhibits an outstanding cycle performance after 1000 h accompanied by an overpotential of about 15 mV, which is much better than the cell with commercial Celgard. Therefore, the fluorinated composite nanofibrous separator provides a simple and practical method for applications of functional separators in high-performance LMBs.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.