Robust and thermal-enhanced melamine formaldehyde–modified glassfiber composite separator for high-performance lithium batteries

Accepted Manuscript Title: Robust and thermal-enhanced melamine formaldehyde–modified glassfiber composite separator for high-performance lithium batteries Author: Qingfu Wang PII: DOI: Reference:

S0013-4686(15)30489-8 http://dx.doi.org/doi:10.1016/j.electacta.2015.09.077 EA 25710

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

30-6-2015 14-9-2015 14-9-2015

Please cite this article as: Qingfu Wang, Robust and thermalenhanced melamine formaldehydendashmodified glassfiber composite separator for high-performance lithium batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.09.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Robust and thermal-enhanced melamine formaldehyde–modified glassfiber

composite

separator

for

high-performance

lithium

batteries Qingfu Wang ab * a. Qingdao University of Science and Technology, Qingdao 266042, P. R. China. b. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China. * Correspondence should be addressed to Qingfu Wang ([email protected])

Abstract The composite separator of melamine formaldehyde resin coated glass microfiber membrane was prepared for high performance lithium ion battery. It was demonstrated that this composite membranes possessed a significantly enhanced tensile strength and a modified porous structure, compared with that of pristine glass microfiber membrane. Impressive improvements in thermo-stability, with no shrinkage at an elevated temperature of 150 °C. Meanwhile, such composite membrane presented a favorable wettability and remarkable electrochemical stability in commercial liquid electrolyte. In addition, the battery test results of LiCoO2/graphite cells proved the composite membrane was a promising separator with an improved cycling performance and rate capability. The cycle performance of LiFePO4/Li cells at the elevated temperature of 120 °C demonstrated their excellent safety characteristic as separator in LIB, indicating the composite membrane was a potential separator candidate for high power battery. Key words: melamine formaldehyde, glassfiber, composite membrane, thermal stability, lithium batteries

1

1. Introduction Lithium ion battery (LIB) has been considered as a promising candidate for high power source in vehicles (EV) and hybrid vehicles (HEV).[1-2] However, current lithium batteries pose a major risk as fire hazard, most of which are caused by the internal short-circuit, which is derived from the separator poor thermal-stability, especially at harsh condition of heating, abuse and overcharge. In LIB, the separator prevents physical contact of the electrodes while enabling free ionic transport and isolating electronic flow.[3] Therefore, the separator shoud a) tolerant for chemical and electrochemical interaction from the electrolyte and electrode materials, b) possesses superior thermal-stability, and c) mechanically strong to deal with collision and stretching during battery manufacturing. Now the commercial separators of LIB are porous membranes based on polyolefin-based separators, such as polypropylene (PP), polyethylene (PE) and PP/PE/PP trilayer membrane.[4] However, these separators have congenital deficiencies including inferior thermal-stability and poor wettability with liquid electrolyte.[5,6] Many efforts have been devoted to solve abovementioned issues, such as polymer coating and inorgnic ceramic nanoparticle depositing.[7-12] All these modified separators exhibited improved thermostability, but still existed some issues, either not security enough or relatively high cost[13]. Hence, it is significant to explore low cost and intrinsically thermostable material for separator. As we know, glass microfiber (GF) possesses excellent heat resisting property, [14] but the commercial membrane of glass microfiber can’t be suitable for LIB as separator attributing to poor mechanical strength and large pores (leading to severe self-discharge). Besides, melamine formaldehyde resin (hereafter abbreviated as MFR) is always considered to be an potential candidate material for secured separator because of its low thermal conductivity, low smoke toxicity, high nitrogen content, and non-formation of molten drops or non-contraction at high temperature.[15-17] More importantly, compared to PE or PP, the cost of raw materials including cheap glass microfiber[14], melamine and formaldehyde are much lower. In this study, we fabricated glassfiber membrane (denoted as GFM) by papermaking process, and the thickness (≈40 µm) was much thinner than commercial glass fiber mat (1 mm), which is favorable to shorten ion diffusion path and improve the ion transport ability. To compensate for the drawbacks of inferior mechanical strength and large pores, a modified composite separator was explored by melamine-formaldehyde resin coated the surface of thin glassfiber membrane. The experimental results demonstrated that this composite separator (abbreviated as GMF) is one of promising separator to significantly improve the safety of LIB, which due to 2

its good mechanical property, excellent dimensional themostability. 2. Material and methods 2.1 Material Glass microfiber (diameter: 1.0–3.5 μm) was supplied by Johns Manville (USA). Melamine and formaldehyde solution (37%, wt.%) was purchased from Sinopharm (China). Polyvinyl alcohol (PVA, n=1795) was purchased from Aladdin. Ethylene carbonate (EC), dimethyl carbonate (DMC) and lithium hexafluorophosphate (LiPF6) were commercially available and without further purification, the electrolyte containing of 1M LiPF6 in EC/DMC (1:1, v/v) solvents. Polypropylene (PP) separators (Celgard 2400) were acquired from Celgard, LLC.(USA). 2.2 Prepolymer synthesis Melamine and formaldehyde (1:6, mole) as raw materials, gently heated to 85 ºC under stirring firstly. After the blend solution became transparent and uniform, added 8% (wt, %) PVA aqueous solution (the mass ratio was referred to refs. [18]) and adjusted the pH value to 8 with LiOH solution (1 M). Subsequently, kept reacting in 85 ºC for 1 hrs, and then cooled transparent solution sample below 40 ºC quickly. The solid content was 42%. 2.3 Preparation of glass microfiber membrane (GFM) The membrane of glass microfiber was fabricated through a paper making process of suction filtration. The details can be found in literature

[19,20]

. Compared to the

commercial glass microfiber membrane (more than 1mm), the obtained glass microfiber film was 40μm. 2.4 Fabrication of composite separator Diluted the prepolymer solution with distilled water, the solid contents was controlled at 5 %, 10 %, 15 %, 20 % and 30 %. The didued prepolymer solution sample were transferred into porous glass-fiber mats by dip-coating process, and the thermal-cured of prepolymer of MFR was carried out at 100 °C for 3 hrs. Finally, put the composite membrane into 120 °C vacuum dry box for 8 hrs to cure deeply and removed the trace water. Herein, GF, GMF-5, GMF-10, GMF-15, GMF-20, GMF-30 were defined as the solid content of prepolymer at 0, 5 %, 10 %, 15 %, 20 % and 30 %, respectively. Correspondingly, the content of glass fiber in the composites was 100 %, 82.4 %, 71.5 %, 60.3 %, 47 % and 32.6%, respectively. The thickness of the composite separator in the range of 40-41 µm. 2.5 Electrodes fabrication Cathode slurry was prepared by mixing LiCoO2 or LiFePO4, carbon (super P) and 3

polyvinylidene fluoride (PVDF) binder (80:10:10, wt.%) in N-methyl-2-pyrrolidone (NMP). The cathode electrode was fabricated by casting the slurry onto aluminum foil current collector. Similarly, anode electrode was prepared by casting a slurry of graphite carbon and PVDF (90:10, wt. %) in NMP onto a copper foil current collector. The cathode and anode electrodes were vacuum dried prior to use. 2.6 Characterization The tensile properties of samples were tested by tensile-machine (AI-7000M, GOTECH). Tensile speed was 100 mm/min. SEM images of the films were taken by FE-SEM HITACHS4800 with operating voltage of 3.0 kV. The air permeability was obtained using a Gurley-type densometer[3,7] (4110N, Gurley) by measuring the time of 100 cc air to pass through the separator (1 square inch), the pressure was 4.88 inH2O. The porosity of composite membrane was determined using n-butanol uptake method and then calculated using Equation (1): Porosity =( m2/ρ2)/(m1/ρ1+ m2/ρ2) × 100%

Equation (1)

where m1 and m2 are the mass of composites and n-butanol, the ρ1 and ρ2 are the density of composite material and n-butanol. The electrolyte uptake was measured by the weight of composite membranes with a certain area before and after liquid electrolyte (1M LiPF6/EC:DMC (1/1, v/v) ) soaking for 8 hrs at 25°C, and then was calculated using Equation (2): Electrolyte uptake = (wb- wa)/wa × 100%

Equation (2)

where wa and wb are the mass of the membrane before and after immersion in electrolyte, respectively. Differential scanning calorimeter (Diamond-DSC, PerkinElmer) was used to evaluate the thermal properties of the samples. Samples were scanned from 50 °C to 300 °C at a heating rate of 10 °C min-1 under a nitrogen atmosphere. The chemical structure of the membranes was characterized by Fourier transform infrared spectroscopy (FT-IR, Bruker VERTEX 70). Electrochemical stability of composite membranes was evaluated by linear sweep voltammetry (LSV). The membrane was sandwiched between a stainless steel working electrode and a lithium metal electrode as both the counter and reference electrode. The Autolab PGSTAT 302N system was used to recordthe electrochemical stability and the voltage was swept at a scan rate of 1 mV/s in the range of 2.5 V- 5.0 V at 25 °C and 100 °C, the sweep rate was 10 mV s-1. 4

Ionic conductivity of membranes was determined by electrochemical impedance spectroscopy (EIS) using an Autolab PGSTAT 302N system. The composite electrolyte membranes were sandwiched between two stainless-steel plate electrodes and the spectra were recorded in a frequency range of 0.01 Hz–1 MHz with an AC amplitude of 20 mV at various temperatures. The bulk resistance of membranes was determined from the impedance spectrum. The ionic conductivity was calculated from Equation (3):

σ=L/RbA

Equation (3)

where Rb is the bulk resistance, L and A are the thickness and area of the composite membrane, respectively. The open circuit voltage (OCV) variation of cell as a function of time was measured by electrochemical working station (CHI600E, Chenhua China). The LiFePO4/Li cell charged up to 4 V before measurement. 3. Results and Discussion 3.1 Structure and morphology The MFR was synthesized via one-pot solution polycondensation method, the process as shown in scheme1. The prepolymer is consisted of hydroxymethylate formed by malemine and formaldehyde reaction. The prepolymer further to cure and form MFR with three-dimensional net structure. The morphology of membranes can be observed by scanning electron microscope (SEM). The SEM images of composite membranes with different solid contents modification are shown in Figure 1. As displayed in Figure S1b , the pristine glass microfiber membrane was consisted with crossed and stacked microfiber (average diameter was 3 µm), this kind of loose structure with large-sized and irregular pores is the typical feature of papermaking process. SEM images for glass fiber membrane coated with MFR are shown in Figure 1(a-d), is corresponding to prepolymer solid contents of 5 %, 15 %, 20 % and 30 %, respectively. It is observed that the MFR is transferred on the surface of glassfiber successfully. With the increasing of MFR contents, the more and more polymer fill into the pores among microfibers. It is clear that the polymer almost filled glass fiber mat fully and the porosity is very low when the solid content at 30 %. Besides, the inset of Figure 1b and Figure S2 display the SEM micrographs with high resolution. Obviously, in the part of larger-sized pores of the optimal GMF-15, the polymer is inevitable to penetrate into the gap of the fibers and coat on the internal fibers. So, it is necessary to note that, because of the loose structure with large-sized and irregular 5

pores of glass-fiber membrane, polymer is hard to be only coated on the surface of glass fiber membrane uniformly, some polymer would fill into the interior of membrane, especially in the part of larger-sized pores. Thus, it is reasonable to anticipate that, the polymer, either coated on the surface or filled into the interior of glass-fiber membrane, is favorable for preventing micro-short-circuit. The FTIR spectrum of composites is descripted as Figure 2. The vibrational peak frequencies and assignments are displayed in Table S1. The typical peaks situate at 1664, 1565 cm-1 are assigned to the triazine ring, the strong absorption intensity of C-O-C vibration band located at 1048 cm-1, indicating polycondensation reaction occurred between hydroxymethyl existed in MFR pre-polymer and PVA . 3.2 Mechanical property and physical parameters Figure 3a dipicts the variation of mechanical performance with the different solid contents. It is clear that, the tensile strength of pristine glass microfiber membrane is as low as 0.8 MPa, which is far away from the requirements of battery assembling. In the solid content range among 5 % and 30 %, the tensile strength increases with the rising of polymer concentrations, from 8.7 MPa to 21.2 MPa. It is demonstrated that incorporation of MFR is an effective strategy to enhance the mechanical strength. The tensile strength of Celgard 2400 can be seen in Figure 3b, it is found that the tensile strength of as-developped separator is much better than the transverse strength of PP separator (12 MPa) and lower than that of PP separator at the machine direction (120 MPa). PP separator was prepared via a uniaxially stretching technology. Different orientations at vertical and horizontal directions lead to the different tensile strength. However, GFM membrane was fabricated by papermaking process. The distribution of glass microfibers in GFM separator is uniform, thus, tensile strength is same at both vertical and horizontal directions. In addition, melamine resin is used for bonding glass microfibers, further enhancing the tensile strength of GFM separator. The porosity and Gurley value of composite membrane with different MFR concentrations are shown in Figure 3a (bottom inset). It can be found that with the increasing of MFR solid content, the porosity of composite membranes is decreasing, from 75% to 11 % among the MFR solid content of 0 and 30 %. While the Gurley value is increasing continuously, from 128 s cc-1 to 1210 s cc-1, which means that the air permeability become lower as the increasing of MFR concentration. The results are consistent with the SEM analysis. Meanwhile, it is noted that the thickness of composite membranes (top inset of Figure 3a) are vary gently (40 - 41 µm). Herein, the composite separator with the solid content of 15 % (GMF-15) is 6

determined to fabricate for batteries due to the proposity (43 %) closed to the commercial separator (42 %) and good tensile strength (16 MPa). The physical parameters comparision of GFM, GMF-15 and Celgard 2400 are summarized in Table 1. It is obvious that, except for the better porosity and Gurley value discussed above, the wettability and electrolytes uptake are vital for cells. As shown in Figure 4, it is very clear that the GMF-15 separator is quickly wetted with the electrolyte, where the electrolyte droplets easily spread over a wide area of the separator. In contrast, PP separator is not wetted as much and only electrolyte drop is observed on the surface with contact angle of 92 º, which is much higher than that of the GMF-15 separator (38º). Thus, the superior wettability and preferred liquid electrolytes uptake (280 %/120 %, GMF-15/Celgard2400) of GMF-15 indicate the as-developed separator could wet more easily in the electrolytes and retain the electrolytes more permanently than those of PP separators, which can facilitate the process of electrolytes filling in battery assembly and improve cycle life of the battery. Besides, though the GFM exhibits better parameters as Table 1 shown, the inferior mechanical strength is the main obstacle for its application in battery. 3.3 Thermal analysis Thermal stability property of separators is a vital aspect of battery safety characteristic.[3] Differential scanning calorimetry (DSC) curves of Celgard 2400 and GMF-15 are shown in Figure 5a. It suggested that, compared to the melting point at 162 °C, GMF-15 has no obvious melting point at the temperature range of 60 °C280 °C, which reveals the GMF-15 possesses preferred thermal stability. For further corroborating the thermo-stability of composite separator, thermal weight loss was measured by thermogravimetry. Thermogravimetry (TG) curves are shown in Figure 5b, the curves present that the decomposition temperature of PP separator is about 280 o

C, and GMF-15 exhibits no clear weight loss until the temperature of 350 oC. It is

deduced that the composite separator has superior heat-resistance against the commercial separator. The dimensional thermostability of separator is one of the most important factors of separator for lithium battery safety.[13] The thermal shrinkage images before and after storing in the oven at 150 oC for 1h are displayed in Figure 5c. Obviously, the Celgard 2400 separator reveals a significant shrinkage (about 30 %) after storage at 150 oC for 1 h, and the color change from white to translucent, whereas GMF-15 exhibits no obvious shrinkage after storage at 150 oC. Therefore, from the viewpoint of the dimensional thermostability, the resultant composite separator could provide 7

excellent safety characteristic for high power battery, even at the extreme conditions. 3.4 Electrochemical performance Ionic conductivity is considered as a key factor of electrolyte because relatively high value can ensure better rate capability of the battery.[21] The inset of Figure 6a shows the Nyquist plots of GMF-15 and PP separator with 1 M LiPF6/EC:DMC (1:1, v/v) liquid electrolyte (abbreviated as GMF-LE and PP-LE, respectively). The ionic conductivity was then calculated from the Rb values by employing the equation (3) : σ=L/RbA, where L was the thickness of the separator sample and A was the contact area between the separator and the electrode. It can be obtained that the ionic conductivity of GMF-15/liquid electrolyte was 0.52 mS cm-1, which was higher than that of PP separator (0.39 mS cm-1). Figure 4a displays the variation of ionic conductivities for GMF-15 and PP along with the rising temperature rise. The σ values for GMF-15 are higher than those for PP at the temperature range of 25 ºC to 80 ºC, The dependence of ionic conductivity on temperature can be reasonably fitted by Arrhenius formula: σ=Aexp(−Ea/RT), where A is the pre-exponential factor and Ea is the activation energy. The calculated Ea values are 11.8 kJ mol-1 and 13.9 kJ mol-1 for GMF-LE and PP-LE, respectively. That is, the movement of Li+ ions in the electrolyte with GMF-15 membrane is easier than that of PP separator. [22-24] It is obvious that such composite separator is very promising for applications in high-power LIB. The electrochemical stability of separator in liquid electrolyte ensure the reliability of LIB. The electrochemical window of membranes saturated with liquid electrolyte, was measured by linear sweep voltammetry (LSV) experiment at room temperature. As can be seen in Figure 6b, it is found that the electrochemical working window of PP/liquid electrolyte is about 4.5 V vs. Li+/Li, while that of GMF-15 is up to 4.6 V vs. Li+/Li. This result indicates that MFR separator possessed slightly wider electrochemical window, which makes it compatible with most of cathode materials such as LiCoO2 and LiFePO4. [22] The open circuit voltage (OCV) can be used to measure the insulation of the separator.[4] Herein, the LiFePO4/PC-LiBOB/Li cells with GMF-15, Celgard 2400 and GFM separator were full charged at 4.0 V, and the open circuit voltage profiles for the cells at room temperature for 10 hrs and at the elevated temperature of 150 °C for 30 minutes are displayed in Figure 7. Figure 7a dispicts that the cells with PP and GMF-15 separator exhibit excellent OCV retention performance (3.85 V and 3.78 V, respectivly), compared with GFM separator, which can be attribute to the large scale 8

pores and higher porosity ( as observed by the SEM images) of GFM leading to the poor insulation and self-discharge issue.[13] Subsequently, the cells, which charged up to 4.0 V, are exposed at 150 °C for 30 minutes, the OCV profiles are shown in Figure 7b, it is clear that, the OCV of the cell with PP separator fluctuates severely after 15 minutes, maybe due to internal short circuit by shrinking of separator. Though the OCV profile of cell with GFM separator has no show voltage fluctuation, it still drops dramatically at the elevated temperature, whereas the cell with GMF-15 exhibits gentle votage drop and possesses satisfied OCV retention. Hence, compared with the cells with PP and GFM separator, the cell with GMF-15 reveals a reliable OCV retention at room temperature as well as at the elevated temperature. 3.5 Battery performance To evaluate the battery performance, the LiCoO2/graphite cells with GFM, GMF-15 and Celgard 2400, as well as 1 M LiPF6 EC:DMC (1:1, v/v), prepared in Ar-filled glavebox. The cycle retention for 100 cycles of cells at a constant charge/discharge current density (0.5 C/0.5 C) and ambient temperature are shown in Figure 8a, it is clear that the cell with GMF-15 separator shows 95% capacity retention, whereas 92% and 82% capacity retention of the cell with PP and GFM separator, respectively, indicating that the cell with GMF-15 exhibits improved cycling performance against PP and GFM separator which can be abscribed to the higher liquid electrolyte uptake and easier wettability of GMF-15 separator. The rate capabitlity is essential for high power battery.[23] Figure 8b depicts the rate capability of cells at different discharged rate. It can be found that, compared to the discharge capacity of the cell with PP separator can achieve 140, 132, 125, 108 and 91 mAh g-1 at corresponding rates of 0.2 C, 0.5 C, 1 C, 2 C and 4 C, respectively, the cell with GMF-15 exhibits superior rate capacity (140, 132, 125, 108 and 91 mAh g-1 at the same rate), while the cell with GFM separator the capacity at the same rate reveals poor capacity at the higher current density, the reason may be attributed to the cells severe shelf-discharge behavior is caused by large scale pores and poor insulation of GFM.[13] To further clarify the safety characteristics of GMF-15 in battery, the LiFePO4/PC-LiBOB/Li cell with three different separators are prepared and investigated at the elevated temperature of 120 °C. The cycling performance can be seen in Figure 9, it can be found that, the cells with GMF-15 and GFM can operate normally, and the cell with GMF-15 reveals better cycling performance. According to the previous literatures,[24,25] the cell with PP separator can’t operate. This result 9

suggests that, compared to the PP separator can’t operate and GFM separator presents poor cycling performance, GMF-15 exhibits superior cycling stability at the elevated temperature and can be as a potential candidate for lithium battery with superior safety. 4. Conclusion The composite separator of glass microfiber and MFR is prepared successfully, which exhibits improving porous structure and mechanical strength. It is demonstrated that the composite separator possesses excellent liquid electrolyte wettability and electrolyte retention. It is important to note that the composite separator shows superior thermostability and no obvious shrinkage at 150 °C for 1 h, whereas the PP separator shrink severely. Moreover, the LiCoO2/graphite cells with composite separator shows improved cycling performance and rate capacity against to the PP and GFM separator. Besides, the cell with the composite separator offered a reliable battery performance at the elevated temperature of 120 °C, indicating the resultant membrane is a potential separator for high power battery. Acknowlagments This work was supported by Qingdao Institute of Bioenergy and Bioprocess Technology Director Technology Foundation. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652-657. [2] M.M. Thackeray, C. Wolverton, E.D. Isaacs, Electrical energy storage for transportation approaching the limits of, and going beyond, lithium-ionbatteries, Energy Environ. Sci. 5 (2012) 7854-7863. [3] S.S. Zhang, A review on the separators of liquid electrolyte Li-ion batteries, J. Power Sources 164 (2007) 351-364. [4] T.H. Cho, M. Tanaka, H. Ohnishi, Y. Kondo, M. Yoshikazua, T. Nakamura, T. Sakai, Composite nonwoven separator for lithium-ion battery: Development and characterization, J. Power Sources 195 (2010) 4272-4277. [5] J.M. Ko, B.G. Min, D.W. Kim, K.S. Ryu, K.M. Kim,Y.G. Lee, S.H. Chang, Thin-film type Li-ion battery, using a polyethylene separator grafted with glycidyl methacrylate, Electrochim. Acta 50 (2004) 367-370. [6] M.H. Ryou, Y.M. Lee, J.K. Park, J.W. Choi, Mussel-inspired polydopamine 10

-treated polyethylene separators for high-power li-ion batteries, Adv. Mater. 23 (2011) 3066-3070. [7] J.J. Zhang, L.P. Yue, Q.S. Kong, Z.H. Liu, G.L. Cui, A heat-resistant silica nanoparticle enhanced polysulfonamide nonwoven separator for high-performance lithium ion battery, J. Electrochem. Soc. 160 (2013) A769-A774. [8] H.J. Wang, T.P. Wang, S.Y. Yang, L. Fan, Preparation of thermal stable porous polyimide membranes by phase inversion process for lithium-ion battery, Polymer 54 (2013) 6339-6348. [9] X.S. Huang, D. Bahroloomi, X.R. Xiao, A multilayer composite separator consisting of non-woven mats and ceramic particles for use in lithium ion batteries, J. Solid State Electro-chem. 18 (2014) 133-139. [10] J. Ding, Y. Kong, P. Li, J.R. Yang, Polyimide/poly(ethyleneterephthalate) compo -site membrane by electrospinning for nonwoven separator for lithium-ion battery, J. Electrochem. Soc. 159 (2012) A1474-A1480. [11] J. Lee, C.L. Lee, K. Park, I.D. Kim, Synthesis of an Al2O3-coated polyimide nanofiber mat and its electrochemical characteristics as a separator for lithium ion batteries, J. Power Sources 248 (2014) 1211-1217. [12] W. Qi, C. Lu, P. Chen, L. Han, Q. Yu, R.Q. Xu, Electrochemical performances and thermal properties of electrospun poly(phthalazinone ether sulfone ketone) membrane for lithium-ion battery, Mater. Lett. 66 (2012) 239-241. [13] B. Zhang, Q.F. Wang, G.L. Cui, A superior thermostable and nonflammable composite membrane towards high power battery separator, Nano Energy 10 (2014) 277-287. [14] Y.S. Zhu, Y.P. Wu, Cheap glass fiber mats as a matrix of gel polymerelectrolytes for lithium ion batteries, Sci. Rep. 3 (2013) 3187. [15] C. Devallencourt, J.M. Saiter, A. Fafet, E. Ubrich, Thermogravimetry Fourier -transform infrared coupling investigations to study the thermal-stability of melamine-formaldehyde resin, Thermochim. Acta, 259 (1995) 143-151. [16] A. Kandelbauer, G. Wuzella, A. Mahendran, I. Taudes, P. Widsten, Model-free kinetic analysis of melamine–formaldehyde resin cure, Chem. Eng. J. 152 (2009) 556-565. [17] B. Bann, S.A. Miller, Melamine and derivatives of melamine, Chem. Rev. 58 (1958) 131-172. [18] Z.Sh. Hang, L.H. Tan, S.J. Ying, Preparation of melamine microfibers by reaction electrospinning, Materials Letters 65 (2011) 1079-1081. 11

[19] Sh.H.S. Yousfani, R.H. Gong, I. Porat, Manufacturing of fibreglass nonwoven webs using a paper making method and study of fibre orientation in these webs, Fibers & Textiles in Eastern Europe 2 (2012) 61-67. [20] X.S. Huang, Performance evaluation of a non-woven lithium ion battery separator prepared through a paper-making process, J. Power Sources 256 (2014) 96-101. [21] J.H. Kim, B.R. Min, J. Won, H.C. Park, Y.S. Kang, Phase behavior and mechanism of membrane formation for polyimide/DMSO/water system, J. Memb. Sci. 187 (2001) 47-55. [22] Y.S. Zhu, F.X. Wang, L.L. Liu, S.Y. Xiao, Z. Chang, Y.P. Wu, Composite of a nonwoven fabric with poly(vinylidene fluoride) as a gel membrane of high safety for lithium ion battery, Energy Environ. Sci. 6 (2013) 618-624. [23] L.P. Yue, J.J. Zhang, Z.H. Liu, G.L. Cui, A heat resistant and flame-retardant polysulfonamide/polypropylene composite nonwoven for high performance lithium ion battery separator, J. Electrochem. Soc. 6 (2014) A1032-A1038. [24] Y.S. Zhu, S.Y. Xiao, Y. Shi, Y.Q. Yang, Y.Y. Hou, Y.P. Wu, A composite gel polymer electrolyte with high performance based on poly(vinylidene fluoride) and polyborate for lithium ion batteries, Advanced Energy Materials 4 (2014) 9. [25] P. Raghavan, J. Manuel, X. Zhao, C. Nah, Preparation and electrochemical characterization of gel polymer electrolyte based on electrospunpolyacrylonitrile nonwoven membranes for lithium batteries, J. Power Sources 196 (2011) 6742-6749. [26] M. Ulaganathan, R. Nithya, S. Rajendran, S. Raghu, Li-ion conduction on nanofiller incorporated PVdF-co-HFP based composite polymer blendlectrolytes for flexible battery applications, Solid State Ionics 218 (2012) 7–12. [27] Q.F. Wang, B. Zhang, G.L. Cui, Heat-resistant and rigid-flexible coupling glass fiber nonwoven supported polymer electrolyte for high performance lithium ion batteries, Electrochim. Acta 157 (2015) 191-198. [28] J.J. Zhang, L.P. Yue, L.Q. Chen, Sustainable, heat-resistant and flameretardant cellulose-based composite separator for high-performance lithium ion battery, Sci. Rep. 4 (2014) 3935.

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Figures Scheme 1 A schematic illustration of composite separator fabrication process. Fig. 1 Typical SEM images of GMF membrane with different solid content of prepolymer, (a) 5 %, (b)15 %, (c) 20 %, (d) 30 %. Fig. 2 FTIR spectra of GMF-15 composite membrane Fig. 3 a) Mechanical properties of different solid content of MFR prepolymer, the inset (top left) is the variation of thickness as a function of MFR concentration, the inset (bottom right) are the variations of porosity and Gurley value as a function of MFR concentration. b) The tensile strength of Celgard 2400 separator in machine direction and transverse direction (the inset). Fig. 4 Contact angle images of GFM, GMF-15 and PP separators.b) the photographs of liquid electrolyte wettability of two different separators Fig. 5 (a) DSC curves of PP separator, and GMF-15, (b) TG profiles of GFM, GMF-15 and PP separator Fig. 6 (a) Effect of temperature on ionic conductivity of GMF-LE and PP-LE (temperature range: 25 °C-80 °C), the inset is the Nyquist plots of GMF-LE and PP-LE at 25 °C. (b) The linear sweep voltammograms of the GMF-LE and PP-LE. Fig. 7 The profiles of open circuit voltage for the LiFePO4/PC-LiBOB/Li cells with GMF-15, Celgard 2400 and GFM separator at (a) room temperature for 10 hrs, (b) 150 °C for 0.5 h. Fig. 8 (a) Cycle performance of LiCoO2/graphite cells at a charge/discharge current density of 0.5 C and (b) rate capability of LiCoO2/graphite cells using GMF-LE, PP-LE and GFM-LE at 25°C. Fig. 9 Cycle performance of LiFePO4/PC-LiBOB/Li cells at the temperature of 120 °C (0.5 C/0.5 C), the inset is the first charge/discharge profiles with GMF-15 and GFM separator, respectively.

13

Table

Table 1 Physical parameters of GFM, GMF-15 and Celgard 2400 separator Thickness (μm)

Porosity (%)

Gurley value (s cc-1)

GFM

40

75

128

240

32

GMF-15

40

43

420

280

38

25

42

610

120

92

Sample

Celgard 2400

14

Electrolyte Contact angle uptake (%) (º)

Scheme 1 A schematic illustration of composite separator fabrication process.

15

Fig. 1

16

Fig. 2

17

a

b Machine direction

Transverse direction

Fig. 3

a)

18

GFM

GMF-15

b)

Celgard 2400

GMF-15

Celgard 2400

Fig. 4

19

c

Fig. 5

20

Fig. 6

21

Fig. 7

22

Fig. 8

23

Fig. 9

24