Functionalized polar Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane assisted polyimide nanofiber composite membrane with excellent ionic conductivity and wetting mechanical strength towards enhanced lithium-ion battery

Journal Pre-proof Functionalized polar Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane assisted polyimide nanofiber composite membrane with excellent ionic conductivity and wetting mechanical strength towards enhanced lithium-ion battery Nanping Deng, Lu Wang, Yong Liu, Chongli Zhong, Weimin Kang, Bowen Cheng PII:

S0266-3538(19)32892-1

DOI:

https://doi.org/10.1016/j.compscitech.2020.108080

Reference:

CSTE 108080

To appear in:

Composites Science and Technology

Received Date: 15 October 2019 Revised Date:

14 February 2020

Accepted Date: 16 February 2020

Please cite this article as: Deng N, Wang L, Liu Y, Zhong C, Kang W, Cheng B, Functionalized polar Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane assisted polyimide nanofiber composite membrane with excellent ionic conductivity and wetting mechanical strength towards enhanced lithium-ion battery, Composites Science and Technology (2020), doi: https://doi.org/10.1016/ j.compscitech.2020.108080. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Functionalized polar Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane assisted polyimide nanofiber composite membrane with excellent ionic conductivity and wetting mechanical strength towards enhanced lithium-ion battery Nanping Denga,#, Lu Wanga, b,#, Yong Liua, b, Chongli Zhonga, b, Weimin Kanga, b,*, Bowen Chenga, b,* a

State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint

Research on Separation Membranes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b

School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China

Abstract: Based on the urgent demand of excellent wetting mechanical strength electrospun nanofiber separators and some doping compounds which are beneficial to enhance electrochemical performance for Li-ion cell, in this study, a natty nanofiber composite membrane based on heat-resistant polyimide (PI) matrix via introducing polar Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane (OCP-POSS) nanoparticles for the first time is resoundingly prepared by electrospinning technology. The hybrid PI/OCP-POSS separator (HOPS separator) with 3.5 wt.% OCP-POSS nanoparticles presents significant improvement in wetting mechanical strength (11.8 MPa), eminent electrolyte retention rate (1025 %) together with a tendency of gelation phase and still exerts laudable high thermal stability. Meanwhile, the battery with the HOPS separator exhibits preferable electrochemical stability window (5.2 V) and exceedingly excellent ionic conductivity (2.8×10−3 S·cm-1). The most extraordinary is that the capacity retention reaches up to 81.45 % after 100 cycles at 0.2 C and 91.96 % in C-rates test, while the pristine PI separator merely achieves to 56.03 % and 75.90 %, respectively. In addition, the modified separator possesses more stable cycling properties than PP separator (Celgard 2400) at 1 C and 2 C rate, respectively, which all fully signifies that the heat-resistant HOPS separator has great application prospects in high-performance and advanced-safety lithium-ion battery. Keywords: Lithium-ion battery; Electrospinning PI nanofibers composite membrane; Polar OCP-POSS;

Excellent ionic conductivity and

wetting mechanical

strength;

electrochemical performance

*Corresponding author. E-mail: [email protected] (W. Kang), [email protected] (B. Cheng) # Nanping Deng and Lu Wang are regarded as first joint authors. 1

Enhanced

■ INTRODUCTION In the recent decade, the increasing worldwide energy demands and substantial environmental deteriorations make the vigorous development of green and renewable energy [1]. Compared with traditional fossil fuel, lithium-ion battery has become the ideal electrochemical energy storage system by virtue of its high specific energy, environmental friendliness and long cycle life [2, 3]. Battery separator mainly acts to block the cathode and anode electrodes from direct contacting to avoid the internal short circuit and allows the favoring transmission of lithium ions. Although separator does not participate in the internal electrochemical reaction, its structure and property have an important influence on battery performance [4-6]. By the virtue of excellent chemical stability, outstanding corrosion resistance and prominent mechanical strength, polyolefin membrane is the preferred leader for battery separator [7]. However, there still are some unavoidable defects such as weak thermal stability, poor electrolyte wettability and low porosity [8]. Fortunately, multitudinous strategies have been applied to overcome the above mentioned problems [9, 10]. Electrospinning method has the advantage to directly fabricate nanofiber membranes from a spinning solution [11], which can form a three-dimensional network structure to accelerate the absorption and retention of electrolyte, furtherly improving the ionic conductivity. For further developing high performance lithium-ion battery, electrospinning polymer membranes have been adopted by to replace the traditional polyolefin microporous separators, such as Polyacrylonitrile (PAN) [12], Polyurethane (TPU) [13], Polyvinylidene fluoride (PVDF) [14] etc. Liu et al. prepared a novel electrospun PU@GO separator with large interconnected pores for lithium-ion battery. The special three-dimensional structure made the separator possess high porosity. More importantly, the separator can effectively promote the ionic transmission and electrolyte-uptake capacity [15]. Among them, for the past few years, thanks to the excellent thermal stability, excellent wettability of electrolyte and chemical corrosion resistance, Polyimide (PI) membrane has been used as a high performance separator for lithium-ion battery. Miao et al. fabricated a high thermally stable membrane as lithium-ion battery separators by using polyimide as raw material. The excellent electrolyte wettability caused by the similar polarity of polyimide with liquid electrolyte endowed the lower resistance and higher capacity for the assembled batteries [16]. However, the general pure PI electrospun nanofiber membrane with large pore sizes, especially for the separator with thin 2

thickness, may cause micro-short circuits of a battery due to the large apertures. The disadvantage at present maybe limit the large-scale application of heat-resisting polyimide matrix for lithium ion batteries. So diverse modified PI separators including some structure designs have been studied to furtherly improve the performance of the batteries [17-18]. For example, Shayapat et al. introduced inorganic nanoparticles of SiO2 or Al2O3 into pure polyimide by electrospinning as lithium-ion separator. By virtue of the hydrophilic SiO2 or Al2O3, both separators assembled with inorganic nanoparticles both presented the improved electrolyte uptake capability and ionic conductivity when compared with pure polyimide membranes [19]. Hou et al. fabricated hierarchical 3D micro/nano-architecture of polyaniline nanowires wrapped-on polyimide nanofibers for lithium-ion cell separators. The assembled batteries with the prepared separator showed excellent rate capability and stable cycling performance [20]. Polyhedral oligomeric silsesquioxane (POSS) possessing three-dimensional cage-like structure belongs to hybrid organic-inorganic nanoparticle materials. By virtue of the unique structure, it has attracted widespread interest for researchers. It is considered as the smallest possible silica particle with the minuscule diameter range from 1 to 3 nm. More importantly, the special structure of POSS makes it not only possess the advantages of the inorganic particles such as excellent rigidity and good chemical stability, but also has the organic attributes of flexibility, dielectric and excellent compatibility with polymer matrix. In recent years, various kinds of POSS have attracted considerable attention in many applications, such as low dielectric materials [21], proton exchange membranes [22] and so on. In the battery fields, POSS nanoparticles has been used to improve the performance of lithium-ion batteries by modified electrolytes or coated commercial polyolefin membrane. Liu et al. selected POSS as polymer matrix, and successfully prepared a porous gel polymer electrolyte by phase inversion process. The interaction of the strongly polar groups of -C≡N from PAN structure was diminished by the introduction of POSS nanoparticles, making the more flexible structure. And the prepared polymer electrolyte exhibited the lower crystallinity, better pore structure and more superior electrolyte uptake [23]. Chi et al. produced a molecular-level ZrO2/POSS multilayer-modified PE separator for lithium ion battery, which dramatically enhanced the breaking strength and improved the electrochemical performance which could decrease the concentration gradient in polarization process [24]. However, when the drying breaking strengths of some battery separators are excellent, the wetting breaking strength of some 3

cell separators still need to be studied. (The definition of drying and wetting breaking strength are the breaking strength of separator before and after immersing in the electrolyte) The Octa(γ-chloropropyl) polyhedral oligomeric silsesquioxane(OCP-POSS) belongs to a kind of POSS family. As shown in Fig.S1, the core of OCP-POSS consists of the rigid inorganic nucleus connected by Si-O bonds (0.3~0.4 nm diameter of nanopore), surrounded by the eight vertices link with flexible polar organic groups (-Propyl chloride). Last few years, our team mainly researched the functionality of simple nonpolar Octaphenyl-POSS or SiO2 on PVDF and PMIA separators for lithium batteries [25, 26], which verified that the nonpolar POSS with special structure can facilitate the ionic transmission and improve cycle stability. However, the wetting mechanical strength of these membranes after immersing in liquid electrolyte have an extraordinarily rapid decline when compared with drying mechanical strength of raw membranes. More importantly, it's worth mentioning that polar POSS nanoparticles has not been used in lithium-ion batteries by modified separators and few research was explored on electrospun technology with polar OCP-POSS nanoparticles with special functions in separators especially PI membrane for lithium-ion batteries. Therefore, based on previous research and a problem easily neglected about the wetting breaking strength of electrospinning membrane as battery separator, in this work, combining the underlying structural advantages of polar OCP-POSS nanoparticles and the outstanding heat resisting of PI matrix, a novel separator with the excellent merits of thermal stability, electrolyte affinity, wetting breaking strength and cycling performance for lithium-ion batteries was fabricated for the first time by electrospinning with a homogenous mixed PAA solution accompanied with polar OCP-POSS nanoparticles, followed by the imidization reaction at sustained slight hot-pressed conditions. The physical properties of the hybrid PI/OCP-POSS nanofiber separators (HOPS separators) and the electrochemical properties of the assembled lithium-ion batteries with the HOPS separators were investigated. By virtue of the heat tolerance PI and well dispersed OCP-POSS nanoparticles, the prepared membranes showed excellent thermal stability. Remarkably, the membranes exhibited significantly improvement of electrolyte wettability, wetting breaking strength and ionic conductivity. More importantly, due to the incorporation of OCP-POSS nanoparticles and PI matrix, the electrochemical stability of batteries was enhanced greatly. Therefore, the results convincingly ensure the potential value of the novel designed separators for the exploitation of high safety and stable electrochemical performance for lithium-ion batteries. 4

■ EXPERIMENTAL SECTION The comprehensive introduction of the preparation process of the HOPS membrane, material features, and the cell assembling process and electrochemical properties testing of the Li-ion cell assembled with the prepared membrane are presented in the Supporting Information. The graphic illustration of preparation of the HOPS separator and the assembly processes of battery using the prepared membrane were showed in Fig.1.

Fig. 1. Schematic representation of preparation of the HOPS separators and coin cell assembly.

3. Results and discussion Fig. 2(a~d) presented the micrographs of pristine PI and the prepared various HOPS nanofiber membranes. Apparently, the interconnected 3D network structures were observed in these membranes. Moreover, due to the natural affinity between the organic polymer and groups on the vertices of OCP-POSS, the smooth fibers of the HOPS membranes can be seen clearly. And the average fiber diameter of pure PI was around 600 nm. However the fiber diameter had been greatly reduced to around 100 nm for these HOPS membranes with the introduced of OCP-POSS as presented in Fig. S2, which were attributed to synergistic effect of the lowered viscosity [27], and the improved solution conductivity by virtue of the existence of silicon and oxygen elements in OCP-POSS nanoparticles [28] (as shown in Fig. S3). In addition, with the increase in the content of OCP-POSS, the fiber diameter decreased firstly and then increased. The fiber diameter (AFD) of pure PI membranes was 615.89 nm and the various HOPS membranes were only 150.45 nm, 140.44 nm and 187.13 nm, respectively. Due to the introduction of OCP-POSS nanoparticles, the viscosity of PAA/OCP-POSS solution (Fig. S3) was lower than that of PAA solution. It can be ascribed to the 5

introduced OCP-POSS nanoparticles which can make the polymer form a slip effect between segments and segments. The solution conductivity increased firstly and then decreased. It is due to too much accretion which makes the intense mutual attraction between ions and ions. In addition, the collision chance of ions is increased, so the effective concentration of ions reduces, making the solution conductivity decreased [29]. Although the S3 spinning solutions has higher conductivity and lower viscosity when compared with S1, the dielectric permittivity of the S3 solution was higher than that of S1 spinning solutions, which was because that the dielectric constant of nanoparticles is usually much larger than that of pure polymer solution including the internal polarization of nanoparticles [30]. However, the dielectric constant of the larger spinning solution will make it more difficult for the fiber to be drawn longer, thus making the diameter of the spinning fiber bigger. More importantly, the excessive doping of nanoparticles will also have a certain impact on the electrospinning process and diameter of the spinning fibers. Furthermore, some agglomerated nanoparticles are easy to agglomerate on the fiber surface, which also makes the average fiber diameter become biger and biger. Based on these reasons, the fiber diameter of S3 is slightly larger than that of S1. Meanwhile, from the SEM and their corresponding EDS-mapping images presented in Fig. S4, it could be observed the existence and equidistribution of the polar OCP-POSS nanoparticles in the prepared nanofiber membrane, which the main reasons are attributed to the below: Accordingly, POSS are organic-inorganic fillers which easily dispersible in a polymer matrix by virtue of their three-dimensional, nanometer-sized and confined cage-shaped core/shell structures [31]. Meanwhile, the eight vertices of POSS can link with various organic substituents [32], bringing the well compatibility with the polymeric matrix. As shown in Fig. 2(i), XPS is conducted to further verify the successful introduction of OCP-POSS in the membranes. Compared with the pure PI membrane, the appearance of the two new characteristic peak of Cl and Si at around 200 eV and 100 eV intensities in all prepared HOPS membranes corresponds to the characteristic peak intensity of the OCP-POSS [33]. The obtained results indicate that the successfully doping of OCP-POSS into the membranes furtherly. The typical stress-strain contrasting curves of pure PI membrane and the various HOPS membranes before and after immersing in liquid electrolyte are presented in Fig. 2(j). For the samples before immersing in the electrolyte, the strength of all the HOPS membranes increased significantly after the addition of OCP-POSS nanoparticles and the S2 membrane possessed the 6

excellent mechanical properties reaching up to 19.67 MPa when compared with pure PI (10.21 MPa). The reasons of the improved mechanical properties can be mainly attributed to the drastically decreased fiber diameter with the introduced of OCP-POSS and high distribution homogeneity of OCP-POSS nanoparticles in polymer matrix caused by the good interface compatibility between polymer matrix and OCP-POSS [34]. When the membranes were tolerated appropriate force, the finer diameter for fibers can increase the contact point, entangled force and friction between fibers and fibers and furtherly improve the mechanical property [35]. On the other hand, the addition of OCP-POSS nanoparticles plays a role as a reinforced medium in the PI matrix. However, the S3 membranes presented lower tensile strength than that of the S2 membranes. This result is according to the reduced entanglement and contact area between fibers and fibers because of the increased fiber diameter of S3. In addition, due to the cooperation area occupied by nanoparticles, the excessive OCP-POSS nanoparticles adding caused the instability of the integral membranes [36]. Meanwhile, the wetting breaking strength of the prepared samples after immersing in the electrolyte also are rather important, which represents the actual situation of battery separator during the process of charge-discharge process. As presented in Fig. 2(j), the breaking strength of pure PI dropped to 6.8 MPa and the S2 separator decreased to 11.8 MPa to a certain degree. It means that the strength of the membranes doped with OCP-POSS all have some descend after the immersion treatment of liquid electrolyte, but the membranes with adequate OCP-POSS still presented higher breaking strength than that of pristine PI. It is widely acknowledged that the mechanical property of separators plays a significant factor for the daily batteries application and fabrication [37], thus the breaking strength of HOPS membranes can satisfy the basic demand of the battery cycles. The obtained results not only showed that the prepared separator can form gelation, but also explained that the breaking strength value of these prepared membranes still can meet the fundamental strength of actual application. Furthermore, the breaking elongation of all composite membranes improved significantly. This appears to be a function of the formative gel phase. It diminished the intermolecular forces and increased relative molecule slippage when electrolytes fully absorbed in these electrospun membranes [38].

7

Fig. 2. The SEM images of pure PI and the HOPS membranes (a: S0, b: S1, c: S2 and d: S3); The SEM images of pure PI and the HOPS membranes after immersing in electrolyte for 8 h (e: S0, f: S1, g: S2 and h: S3); The XPS spectra of pure PI and the HOPS membranes(i); The typical stress-strain contrast curves of pure PI and the HOPS membranes before and after immersed in electrolyte (The solid line and dashed line represents the drying and wetting breaking strength namely, the strenth before and after immersed in electrolyte, respectively) (j).

Fig. S5. (a) shows the photos of the prepared HOPS membranes, pure PI membranes and PP separator (Celgard 2400) membranes before and after thermal treatment at 130 °C, 170 °C, 210 °C and 250 °C for 1 h, respectively. All of the HOPS membranes and pure PI membrane don’t exhibit any dimension shrinkage when compared to PP separator (Celgard 2400) membranes at each temperature stage, however, PP separator (Celgard 2400) membrane becomes obviously plicate at 170 °C. The testing results present that the thermal shrinkage resistance of the HOPS membranes and pure PI membranes are more excellent than that of PP separator (Celgard 2400) membrane. Correspondingly, Fig. S5 (b) presents the thermal gravimetric analysis curves of these membranes in nitrogen atmosphere. Apparently, the thermal decomposition temperature of all the HOPS separators was much higher than that of PP separator (Celgard 2400). Moreover, the similar curves of composite membranes and pure PI membrane also suggested the introduction of OCP-POSS still made the modified membranes as well excellent thermal stability as the pristine PI. Meanwhile, when the temperature increases to 600 °C, the PP separator (Celgard 2400) separator appears a large weight loss of about 96.2 %, however, the HOPS separators just only has a weight loss of about 8

45.0 %. It mainly attributed to the inherent thermal resistance of PI itself with plentiful stable aromatic heterocyclic ring structure. And indispensably, the inorganic nanoparticles (OCP-POSS) possess high thermal stability and present uniform distribution in PI fibers on the membrane. Usually, the short circuit phenomenon of the assembled battery with polyolefin separators will cause the temperature elevated, and the short circuit area become larger and larger, resulting in explosion combustion finally. It's worth mentioning that the obtained hybrid polyimide membranes can endure the heat to avoid continuous deterioration from the discharging by short-circuiting. The electrolyte uptake behavior of separators plays an important role in the transmission of ions in the charge-discharge process of lithium-ion batteries, and excellent electrolyte uptake property can accelerate ion transmission and improve the electrochemical performances of the assembled battery furtherly. As shown in Fig. 3, the contact angles of the HOPS membranes were measured to evaluate the wettability of liquid electrolyte. All samples quickly absorbed the electrolyte within 1s, but the difference is that the contact angle of S1, S2, S3 membranes is 19°, 18° and 19°, respectively. While the contact angle of pure PI is much higher at 35°. After 6 s, when compared with the contact angle of 14° for pure PI, all the HOPS membranes had fully absorbed the electrolyte accompanied with the contact angle of about 0°. It is obviously that the PI membranes with the introduced of OCP-POSS nanoparticles had faster electrolyte absorption ability and possessed more excellent wettability of liquid electrolyte. These results can be attributed to the similar polarity of PI itself and electrolyte [39], the smaller fiber diameter and higher porosity of well interconnected electrospun HOPS nanofibers than that of pure PI nanofibers.

Fig.3. The contact angle of one drop liquid electrolyte on pure PI and the HOPS membranes within 1 sec (a: S0, b: S1, c: S2 and d: S3); The contact angle of pure PI and the HOPS membranes within 6 sec (e: S0, f: S1, g: S2 and h: S3).

9

The liquid electrolyte retention rates of pure PI and the HOPS membranes are demonstrated in Fig. 4. It is obviously displayed that the liquid retention ratio of all the HOPS membranes were improved compared with pure PI membranes. It mainly was ascribed to two aspects: On the one hand, the appearance of slenderer fiber of the HOPS membranes can tremendously increase the surface contact area between electrolyte and three-dimensional network fibers than PI membrane [40]. On the other hand, as shown in Fig. 2(e~h), the forming of gel phase and surface adsorption for electrolyte of the HOPS membranes were more distinct than that of PI membranes after immersing in electrolyte. It means that the introduced OCP-POSS particles can increase the compatibility of separators with electrolyte, furtherly improve the liquid electrolyte retention capacity [41]. In particular, the formation of gel condition and the distinct surface adsorption of electrolyte can protect the cells from the leakage of electrolyte. Meanwhile, the liquid electrolyte retention capacity decreased slightly of S3 membranes compared with that of S2 membranes. It mainly caused by the decreased surface contact area between electrolyte and fibers, which corresponded to the increased average diameter of membranes as showed in Fig. S2. As shown in Fig. 4 (b), the porosity of S1, S2, S3 separators were 88.12 %, 90.90 % and 87.38 %, respectively, which are higher than that of PI separators (75.28 %). It furtherly verified that higher porosity and smaller diameters of electrospun membranes can ameliorate the electrolyte uptake capacity and improve the compatibility with electrolyte solutions [42]. Therefore, the modified polyimide membranes with OCP-POSS nanoparticles can greatly improve the wettability of electrolyte and electrolyte retention capacity, making it has a considerable applied value to accelerate the migration of lithium ions and improve the electrochemical performances. As we all know, the compatibility of the prepared membrane to liquid electrolyte as a function of time is rather important to understand the interfacial properties. So the liquid electrolyte uptake capacity of pure PI and the HOPS membranes with the time changes which directly reflect the compatibility of the membrane to liquid electrolyte were tested. The testing results are presented in Fig. 4. (c). From the figure, the abilities of absorbing and preserving liquid electrolyte for these membranes increases gradually and then stabilized. More importantly, the absorbing and preserving capacity of S2 membranes were the strongest among these samples at regular intervals. All in all, the excellent compatibility of the prepared HOPS nanofiber membranes to liquid electrolyte presented the prominent interfacial properties of the membranes [43]. 10

Fig. 4. (a)The liquid electrolyte retention rate of pure PI and the HOPS membranes; (b) The porosity of pure PI and the HOPS nanofiber membranes and (c) Electrolyte uptake capacity of pure PI and HOPS membranes with time changes.

The electrochemical impedance spectroscopy (EIS) of the assembled batteries (Fig. 5(a)) with pristine PI and the HOPS separators soaked electrolyte was researched to assess the compatibility of hybrid separators with electrode materials. The spans of the semicircles correspond to the contact resistance between the battery separators and the electrode material, and the starting point on the left side of the semicircle corresponds to the ionic conductivity or ion conduction resistance of the electrolyte in the separator [16]. Apparently, the contact resistance value of S2 membrane keeps the minimum, which is lower than that of S1 and S3. It's worth mentioning that the interfacial resistance of the assembled batteries with all the HOPS separators were found reduced significantly compared with that of pure PI separators. It is ascribed to the elevated ability of absorbing electrolyte and the apparent surface adsorption of electrolyte after introducing the OCP-POSS inorganic nanoparticles. As shown in Fig. 5 (b), the bulk resistances (Rb) of the polymer electrolytes can be obtained from the intersection of the oblique line with the Z’ axis [44]. The bulk resistance (Rb) and ionic 11

conductivity of pure PI and these HOPS separators are listed in Table S1. Compared with the pristine PI membrane, the HOPS separators all presented upper ionic conductivity accompanied with the decreased ionic resistance. It is mainly attributed to the addition of OCP-POSS, which can reduce the AFD of fibers and improve the electrolyte uptake ability of nanofibrous membranes to expedite the migration of lithium-ions between electrodes. More importantly, as showed in Fig. 5 (c), besides the normal transmission of lithium ions in the HOPS nanofibers pores just like the pure PI fibers, the functional -Cl groups with unshared electron pairs in OCP-POSS can reduce the transmission paths of lithium ions and accelerate the transport rate of lithium ions on the surface of the HOPS separator [45]. Meanwhile, the special hollow structure of OCP-POSS and the homogeneous membranes interface with electrolyte also can accelerate the lithium ion transportation between electrodes synergistically. In addition, the obtained S2 membranes presented the supreme ionic conductivity (2.8×10-3 S/cm) compared with the S3 membrane (2.6×10-3 S/cm), which corresponded to the lower liquid electrolyte retention capacity caused by the improved AFD of S3 membranes. As shown in Fig. 5 (d), the linear sweep voltammetry (LSV) test was applied to assess the electrochemical stability of pure PI and these prepared HOPS separators and the corresponding electrochemical stability windows value of pure PI and the HOPS separators were shown in Table. S1. The decomposition voltages of all separators doped with OCP-POSS nanoparticles are higher than that of pure PI (4.6 V), which signifies the introduced OCP-POSS can availably ameliorate the electrochemical stability and satisfy the practical safety application in lithium ion batteries, which mainly was attributed to the greatly reduced AFD and the formation of more homogeneous interface structure caused by introducing the OCP-POSS nanoparticles. More importantly, the safety problem of lithium ion battery has become a hot spot of people concern due to the related accidents frequently happens [46]. As a heat-resisting material, polyimide has been widely used in electrodes and separators, but it still imperfect. As predicted, through introducing the OCP-POSS nanofibers into pristine PI separators, the electrochemical widow can be improved greatly, which means the HOPS separators will have tremendous superiority in whole fields of safety lithium ion batteries. To sum up, the reduced interfacial impedance and improved ionic conductivity can make the migration of the lithium ions more active between electrodes, which can greatly improve the cycle stability and reduce the active material loss. 12

Fig. 5. (a) The interfacial resistances of the lithium ion cells with pure PI and the HOPS separators; (b) The impedance spectra of pure PI and the HOPS separators; (c) A schematic model for transmission of solvated Li+ in the HOPS separator and pure PI separator, respectively; (d) The electrochemical window plots of pure PI and HOPS separators.

Fig. S6 shows the cyclic voltammetry of the assembled batteries with pure PI, PP separator (Celgard 2400) and the HOPS separators. All the different types of separators have a pair of similar strong oxidation and reduction peaks around 3.9 V, which indicated the introduced OCP-POSS had no negative impact on the reaction potentials [47]. The oxidation peak intensity of the PP separator (Celgard 2400) attenuated sequentially and the position of redox peak position were not shifting for 3 cycles. The pure PI showed a more obvious variation of the peak intensity and position at every cycle. To the contrary, the curves of the HOPS separator have scarcely changed for each cycle. It furtherly verified that the electrospun PI membranes with the OCP-POSS nanoparticles can improve the electrochemical stability and compatibility of battery with separator. Fig. 6 (c) shows the cycling performance of Li/LiCoO2 batteries with pristine PI and the HOPS separators at 0.2 C. At the same time, the corresponding typical charge-discharge curves in the first cycle and 100th cycle were presented in Fig. 6 (a~b). It can be obviously observed that the assembled battery with S2 separators delivered the highest initial discharge capacity of 168.4 mAh•g-1, and all the HOPS separators exhibited much higher initial discharge capacity than that of 13

pure PI membrane, which mainly was attributed to the improved electrolyte wettability, the superior ionic conductivity and reduced interfacial resistance by introducing the OCP-POSS nanoparticles [48]. Additionally, due to the upper interfacial resistance and inferior ionic conductivity compared with S2 separator, the S3 separator showed a slightly lower discharge capacity. And more notably, the capacity retention of the pure PI, S1, S2, S3 separators is 56.03 %, 71.36 %, 81.45 % and 75.97 % after 100 cycles, respectively. It meant that the introduced OCP-POSS nanoparticles can effectively improve the reversibility of ion migration and enhance the cycling stability.

Fig.6. (a) The galvanostatic discharge-charge profiles of Li/separator activated by non-aqueous electrolyte/LiCoO2 batteries using PI and the HOPS separators in the first cycle; (b) Galvanostatic discharge-charge profiles of batteries using PI and HOPS separators in the 100th cycle; (c) The cycle performance of pure PI and HOPS separators at 0.2 C.

The rate performance of batteries with pure PI and the HOPS separators were shown in Fig. 7 (c). As we can see clearly, the discharge capacity of batteries with all HOPS separators were more excellent than that of pure PI membrane at each different rate. After the rate conversion from 0.2 C to 2 C, the discharge capacity decreased rapidly of all batteries, but it recovered moderately when the current density reverted to 0.2 C finally. It's worth noting that the capacity retention finally reached to 75.90 %, 79.25 %, 91.96 % and 85.14 % after the current density reverted to initial 0.2 C in the period of 60 cycles, corresponding to the S0, S1, S2, S3 separators, respectively. To its credit, the assembled battery with S2 separator possessed the highest reversible rate capacity, which was ascribed to the homogeneous interface structure, decent ionic conductivity and friendly 14

compatibility of the HOPS separators with electrodes after filling the OCP-POSS nanoparticles into pristine membrane. In addition, for researching the polarization at different C-rates, the discharge-charge profiles of batteries with pure PI and the HOPS separator with better proportion of OCP-POSS (S2) at testing rate of 0.2, 1 and 2 C are presented in Fig. 7 (a~b). It clearly presented that the polarization gap of pure PI changes more enormous than that of the HOPS separator with the increased current density, which indicates the modified separator can reduce polarization phenomenon in electrochemical reaction [49]. These results fully verified that the introducing of OCP-POSS can effectively strengthen the properties of pure electrospun PI separators and the HOPS separator equipped with excellent reversible rate capacity and cycle performance have great potential in lithium ion batteries.

Fig.7. (a) The galvanostatic discharge-charge profiles of Li/separator activated by non-aqueous electrolyte/LiCoO2 batteries with PI separator at rate of 0.2, 1 and 2 C; (b) Galvanostatic discharge-charge profiles of batteries with the HOPS separator at rate of 0.2, 1 and 2 C; (c) The high rate capacity of Li/separator activated by non-aqueous electrolyte/LiCoO2 batteries using PI and the HOPS separators at 0.2 C, 1 C, 1.5 C and 2 C.

To compare the performance of batteries with the HOPS separator and PP separator (Celgard 2400) at higher current density, the modified separator with best ratio of OCP-POSS (S2) were 15

chosen as the contrasting samples. Fig. S7 shows the cycling performance and Coulombic efficiency of Li/LiCoO2 batteries with PP separator (Celgard 2400), pristine PI and the HOPS separators at 1 C and 2 C, respectively. After 100 cycles, the capacity retention of batteries with PP separator (Celgard 2400), pristine PI and the HOPS separators are 48.33 %, 57.92 %, 73.28 % at 1.0 C, respectively. At 2.0 C, capacity retention of PP separator (Celgard 2400), pristine PI and the HOPS separators are 47.15 %, 33.47 %, 54.55 %, respectively. As seen from both Fig. S7 (a) and Fig. S7 (b), the Coulombic efficiency of the HOPS separator are more stable and higher than that of PP separator (Celgard 2400) and pure PI. The phenomenon reflects that the HOPS separator possesses higher capacity retention and charge-discharge efficiency than that of PP separator (Celgard 2400) separator at high current density, which was attributed to especially poor affinity of PP separator (Celgard 2400) with electrolyte, accelerated transfer rate of lithium ions and reduced polarization after the introduction of OCP-POSS nanoparticles for pure PI separator.

4. Conclusions In this work, a distinctive nanofiber membrane based on polyimide matrix via introducing OCP-POSS nanoparticles is successfully prepared by the electrospinning process. The hybrid PI/OCP-POSS separator (HOPS separator) with 3.5 wt.% OCP-POSS nanoparticles presents significant improvement in wetting mechanical strength (11.8 MPa), eminent electrolyte retention rate (1025 %) together with a tendency of gelation phase and still exerts laudable high thermal stability. In addition, the electrochemical testing verified that the addition of OCP-POSS nanoparticles can greatly improve the electrochemical performance. It exerts the higher electrochemical stability window of 5.2 V and a superior ionic conductivity of 2.8×10−3 S·cm-1. The most extraordinary is that the battery with the HOPS separator exhibited the highest initial specific capacity of 168.4 mAh g-1 at 0.2 C and the superior capacity retention up to 91.96 % after different C-rates. Besides, compared with PP separator (Celgard 2400), the modified HOPS separator with OCP-POSS possesses more stable cycling properties at high current densities of 1 C and 2 C. All in all, these tested results fully confirmed that the addition of OCP-POSS nanoparticles can bring a series of benefits for modified polyimide composite separators to enhance the battery performance. Therefore, the HOPS separators not only have huge potential for the next candidate separator for high performance lithium-ion battery, but also will open a new perspective for separator 16

modification by introducing integrated organic-inorganic compound in all lithium metal batteries. Acknowledgement The authors thank the National Natural Science Foundation of China (51673148, 51678411), China Postdoctoral Science Foundation Grant (2019M651047) and The Science and Technology Plans of Tianjin (No.17PTSYJC00040 and 18PTSYJC00180) for their financial support. References [1] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries. Nat. nanotechnol. 12 (2017) 194-206. [2] H. Yang, S. Liu, L. Cao, S. Jiang, H. Hou, Thin, Superlithiation of non-conductive polyimide toward high-performance lithium-ion batteries. J. Mater. Chem. A 6 (2018) 21216-21224. [3] J.M. Tarascon, M. Armand. Issues and challenges facing rechargeable lithium batteries. Nature 414 (2001) 359-367. [4] W. Zheng, Y. Zhu, B. Na, R. Lv, H. Liu, W. Li, H. Zhou, Hybrid silica membranes with a polymer nanofiber skeleton and their application as lithium-ion battery separators. Compos. Sci. Technol. 144 (2017) 178-184. [5] J. Zhang, Z. Liu, Q. Kong, C. Zhang, S. Pang, L. Yue, X. Wang, J. Yao, G. Cui, Renewable and superior thermal-resistant cellulose-based composite nonwoven as lithium-ion battery separator. Acs Appl. Mater. Inter. 5 (2013) 128-134. [6] D. Luo, M. Chen, J. Xu, X. Yin, J. Wu, S. Chen, L. Wang, H. Wang, Polyphenylene sulfide nonwoven-based composite separator with superior heat-resistance and flame retardancy for high power lithium ion battery. Compos. Sci. Technol. 157 (2018) 119-125. [7] L. Pan, H. Wang, C. Wu, C. Liao, L. Li, Tannic-acid-coated polypropylene membrane as a separator for lithium-ion batteries. Acs Appl. Mater. Inter. 7 (2015) 16003-16010. [8] L. Yue, Ya. Xie1, Y. Zheng, W. He, S. Guo, Y. Sun, T. Zhang, S. Liu, Sulfonated bacterial cellulose/polyaniline composite membrane for use as gel polymer electrolyte. Compos. Sci. Technol. 145 (2017) 122-131. [9] W. Yao, Q. Zhang, F. Qi, J.-N. Zhang, K. Liu, J.-P. Li, W.-X. Chen, Y.-P. Du, Y.-C. Jin, Y.-R. Liang, N.-L. Liu, Epoxy containing solid polymer electrolyte for lithium ion battery, Electrochim. Acta 318 (2019) 302-313. [10] Z. Wang, R. Pan, C. Ruan, K. Edström, M. Strømme, L. Nyholm, Redox-Active Separators for Lithium-Ion Batteries. Adv. Sci. 5 (2018) 1700663. [11] Z. Zhu, Z. Liu, L. Zhong, C. Song, W. Shi, F. Cui, W. Wang, Breathable and asymmetrically superwettable 17

Janus membrane with robust oil-fouling resistance for durable membrane distillation, J. Membr. Sci. 563 (2018) 602-609. [12] T.H. Cho, M. Tanaka, H. Onishi, Y. Kondo, T. Nakamura, H. Yamazaki, S. Tanase, T. Sakai, Battery performances and thermal stability of polyacrylonitrile nano-fiber-based nonwoven separators for Li-ion battery. J. Power Sources 181 (2008) 155-160. [13]

G.

Zainab,

X.

Wang,

J.

Yu,

Y.

Zhai,

A.-A.

Babar,

K.

Xiao,

B.

Ding,

Electrospun

polyacrylonitrile/polyurethane composite nanofibrous separator with electrochemical performance for high power lithium ion batteries. Mater. Chem. Phys. 182 (2016) 308-314. [14] J. Lu, Y. Liu, P. Yao, Z. Ding, Q. Tang, J. Wu, Z. Ye, K. Huang, X. Liu, Hybridizing poly(vinylidene fluoride-co-hexafluoropropylene) with Li6.5La3Zr1.5Ta0.5O12 as a lithium-ion electrolyte for solid state lithium metal batteries. Chem. Eng. J. 367 (2019) 230-238. [15] X. Liu, K. Song, C. Lu, Y. Huang, X. Duan, S. Li, Y. Ding, Electrospun PU@GO separators for advanced lithium ion batteries. J. Membr. Sci. 555 (2018) 1-6. [16] Y.-E. Miao, G.-N. Zhu, H. Hou, Y.-Y. Xia, T. Liu, Electrospun polyimide nanofiber-based nonwoven separators for lithium-ion batteries. J. Power Sources 226 (2013) 82-86. [17] Y. Ding, H. Hou, Y. Zhao, Z. Zhu, H. Fong, Electrospun polyimide nanofibers and their applications, Prog. Polym. Sci. 61 (2016) 67-103. [18] H. Hou, W. Xu, Y. Ding, The recent progress on high-performance polymer nanofibers by electrospinning, J. Jiangxi Norm. Univ.(Nat. Sci.) 42 (2018) 551-564. [19] J. Shayapat, O.H. Chung, J.-S. Park. Electrospun polyimide-composite separator for lithium-ion batteries. Electrochim. Acta 170 (2015) 110-121. [20] W. Ye, J. Zhu, X. Liao, S. Jiang, Y. Li, H. Fang, H. Hou, Hierarchical three-dimensional micro/nano-architecture of polyaniline nanowires wrapped-on polyimide nanofibers for high performance lithium-ion battery separators, J. Power Sources 299 (2015) 417-424. [21] Y.-J. Lee, J.-M. Huang, S.-W. Kuo, F.-C. Chang, Low-dielectric, nanoporous polyimide films prepared from PEO-POSS nanoparticles. Polymer 46 (2005) 10056-10065. [22] Z. Wu, S. Zhang, H. Li, Y. Liang, Z. Qi, Y. Xu, Y. Tang, C. Gong, Linear sulfonated polyimides containing polyhedral oligomeric silsesquioxane (POSS) in main chain for proton exchange membranes. J. Power Sources 290 (2015) 42-52. [23] B. Liu, Y. Huang, H.-J. Cao, L. Zhao, Y. Huang, A. Song, Y. Lin, X. Li, M. Wang, A novel porous gel 18

polymer electrolyte based on poly(acrylonitrile-polyhedral oligomeric silsesquioxane) with high performances for lithium-ion batteries. J. Membr. Sci. 545 (2018) 140-149. [24] M. Chi, L. Shi, Z. Wang, J. Zhu, X. Mao, Y. Zhao, M. Zhang, L. Sun, S. Yuan, Excellent Rate Capability and Cycle Life of Li Metal Batteries with ZrO2/POSS Multilayer-Assembled PE Separators. Nano Energy 28 (2016) 1-11. [25] Y.-F. Li, X.-M. Ma, N.-P. Deng, W. Kang, H.-H. Zhao, Z.-J. Li, B. Cheng, Electrospun SiO2/PMIA nanofiber membranes with higher ionic conductivity for high temperature resistance lithium-ion batteries. Fiber. Polym. 18 (2017) 212-220. [26] X.-Y. Song, W.-Q. Ding, B.-W. Cheng, J. Xing, Electrospun poly(vinylidene-fluoride)/POSS nanofiber membrane-based polymer electrolytes for lithium ion batteries. Polym. Composite. 38 (2015) 629-636. [27] M. M. A. Zadeh, M. Keyanpour-Rad, T. Ebadzadeh. Effect of viscosity of polyvinyl alcohol solution on morphology of the electrospun mullite nanofibres. Ceram. Int. 40 (2014) 5461-5466. [28] D. Fallahi, M. Rafizadeh, N. Mohammadi, B. Vahidi, Effect of applied voltage on surface and volume charge density of the jet in electrospinning of polyacrylonitrile solutions. Polym. Eng. Sci. 50 (2010) 1372-1376. [29] G.-K. Arumugam, S. Khan, P.-A. Heiden. Comparison of the Effects of an Ionic Liquid and Other Salts on the Properties of Electrospun Fibers, 2-Poly(vinyl alcohol). Macromol. Mater. Eng. 294 (2010) 45-53. [30] L. Gao, Q. Zhang, Effects of amorphous contents and particle size on the photocatalytic properties of TiO2 nanoparticles. Scripta Mater. 44 (2001) 1195-1198. [31] L. Wang, K. Zeng, S. Zheng, Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide) telechelics: synthesis and behavior of physical hydrogels, ACS Appl. Mater. Interfaces. 3 (2011) 898-909. [32] S. Subianto, M. K. Mistry, N. R. Choudhury, N. K. Dutta, R. Knott, Composite polymer electrolyte containing ionic liquid and functionalized polyhedral oligomeric silsesquioxanes for anhydrous PEM applications, ACS Appl. Mater. Interfaces. 1 (2009) 1173-1182. [33] Q. Hong, X. Ma, Z. Li, F. Chen, Q. Zhang, Tuning the surface hydrophobicity of honeycomb porous films fabricated by star-shaped POSS-fluorinated acrylates polymer via breath-figure-templated self-assembly. Mater. Design 96 (2016) 1-9. [34] F. Zhao, X. Bao, A.-R. Mclauchlin, J. Gu, C. Wan, B. Kandasubramanian, Effect of POSS on morphology and mechanical properties of polyamide 12/montmorillonite nanocomposites. Appl. Clay Sci., 47 (2010) 249-256. [35] B.-X. Fu, B.-S. Hsiao, S. Pagola, P. Stephens, H. White, M. Rafailovich, J. Sokolov, P.TMather, H. GJeon, S. 19

Phillips, J. Lichtenhan, J. Schwab, Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules. Polymer 42 (2001) 599-611. [36] M. Mehrasa, A.O. Anarkoli, M. Rafienia, N. Ghasemi, N. Davary, S. Bonakdar, M. Naeimi, M. Agheb, M. R. Salamat, Incorporation of zeolite and silica nanoparticles into electrospun PVA/collagen nanofibrous scaffolds: The influence on the physical, chemical properties and cell behavior. Int. J. Polym. Mater. 65 (2016) 457-465. [37] S. Kalnaus, Y. Wang, J.-A. Turner. Mechanical behavior and failure mechanisms of Li-ion battery separators. J. Power Sources 348 (2017) 255-263. [38] Z. Li, T.-T. Cao, Y. Zhang, Y. Han, S. Xu, Z. Xu, Novel lithium ion battery separator based on hydroxymethyl functionalized poly(ether ether ketone). J. Membr. Sci. 5407 (2017) 422-429. [39] H. Wang, T. Wang, S. Yang, L. Fan, Preparation of thermal stable porous polyimide membranes by phase inversion process for lithium-ion battery. Polymer 54 (2013) 6339-6348. [40] L Wang, Z Wang, Y Sun, X Liang, H Xiang, Sb2O3 modified PVDF-CTFE electrospun fibrous membrane as a safe lithium-ion battery separator. J. Membr. Sci. 572 (2019) 512-519. [41] J. Liu, X. Wu, J. He, J. Li, Y. Lai, Preparation and performance of a novel gel polymer electrolyte based on poly(vinylidene fluoride)/graphene separator for lithium ion battery. Electrochim. Acta 235 (2017) 500-507. [42] X. Ma, P. Kolla, R. Yang, Z. Wang, Y. Zhao, A. L. Smirnova, H. Fong, Electrospun polyacrylonitrile nanofibrous membranes with varied fiber diameters and different membrane porosities as lithium-ion battery separators. Electrochim. Acta 236 (2017) 417-423. [43] J.-F. Zhang, C. Ma, Q.-B. Xia, J.-T. Liu, Z.-P. Ding, M.-Q. Xu, L.-B. Chen, W.-F. Wei, Composite electrolyte membranes incorporating viscous copolymers with cellulose for high performance lithium-ion batteries. J. Membr. Sci. 497 (2016) 259-269. [44] X. Huang, N. Ellison, Fabricating a high performance composite separator with a small thickness for lithium ion batteries. Compos. Sci. Technol. 168 (2018) 346-352. [45] C.-E. Lin, H. Zhang, Y.-Z. Song, Y. Zhang, J.-J. Yuan, B.-K. Zhu, Carboxylated polyimide separator with excellent lithium ion transport properties for a high-power density lithium-ion battery. J. Mater. Chem. A 6 (2018) 991-998. [46] B. Luo, X.Wang, Ho. Wang, Z. Cai, L. Li, P(VDF-HFP)/PMMA flexible composite films with enhanced energy storage density and efficiency. Compos. Sci. Technol. 151 (2017) 94-103. [47] M. He, X. Zhang, K Jiang, J. Wang, Y. Wang, Pure Inorganic Separator for Lithium Ion Batteries. ACS Appl. Mater. Inter. 7 (2015) 738-742. 20

[48] C. Liao, S. Wu, Pseudocapacitance behavior on Fe3O4-pillared SiOx microsphere wrapped by graphene as high performance anodes for lithium-ion batteries. Chem. Eng. J. 355 (2019) 805-814. [49] J. Fu, Q. Lu, D. Shang, L. Chen, Y. Jiang, Y. Xu, J. Yin, X. Dong, W. Deng, S. Yuan, A novel room temperature POSS ionic liquid-based solid polymer electrolyte. J. Mater. Sci., 53 (2019) 8420-8435.

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HIGHLIGHTS ·A novel heat-resistant PI/OCP-POSS separator was prepared by electrospinning. ·The additive amount of polar OCP-POSS effecting on PI membrane was investigated. ·The HOPS membrane had outstanding excellent ionic conductivity and wetting mechanical strength.

· The separator achieved higher capacity retention and better rate capability for lithium-ion battery.

1

Dear Editor of Composites Science and Technology We wish to draw the attention of the editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. We warrant that the article is the authors' original work, hasn't received prior publication and isn't under consideration for publication elsewhere. We also confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the corresponding author is the sole contact for the editorial process. Bowen Cheng is responsible for communicating with the other authors about progress and submission of revisions. All the authors listed have approved the revised manuscript. Finally, the authors declare no competing financial interests. Thank you and best regards. Yours sincerely, Nanping Deng, Lu Wang, Yong Liu, Chongli Zhong, Weimin Kang, Bowen Cheng

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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